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I.E.    SMITH 


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^    vA  LIBRARY   OF 
UNIVERSAL    LITERATURE 

2  N    F  O  U  R     PARTS 

(  Comprising  Science,  Biography,  Fiction   J 
and  the  Great  Orations 

PART   ONE—  SCIENCE  ) 

The  Principles   of  Chemistry 


(PART    FOUR) 


BY 

D.  MENDEL&EFF 


520959 


NEW    YORK 
P.  F.  COLLIER  AND  SON 

•  M  C  M  I  - 
28 


PRESS  OF 
P.  F.  COLLIER  &SON 


ALL    RIGHTS     RESERVED 


A   LIBRARY    OF 
UNIVERSAL    LITERATURE 


SCIENCE 

VOLUME  TWENTY-EIGHT 


BOARD  OF  EDITORS 


SCIENCE 

ANGELO    HEILPRIN,   author  of    "The   Earth  and   Its  Story,"  etc.; 

Curator  Academy  of  Natural  Sciences  of  Philadelphia. 
JOSEPH   TORREY,  JR.,  Ph.D.,  Instructor  in  Chemistry  in   Harvard 

University. 
RAY  STANNARD  BAKER,  A.B.,  author  of  "The   New   Prosperity," 

etc.;  Associate  Editor  of  McClure's  Magazine. 

BIOGRAPHY 

MAYO  W.  HAZELTINE,  A.M.,  author  of  "Chats  About  Books,"  etc.; 

Literary  Editor  of  the  New  York  Sun. 
JULIAN   HAWTHORNE,  author  of  "Nathaniel   Hawthorne  and   His 

Wife,"  "History  of  the  United  States,"  etc. 
CHARLES  G.   D.  ROBERTS,   A.B.,   A.M.,   author  of  "A  History  of 

Canada";    late   Professor  of  English  and    French    Literature, 

King's  College, 

FICTION 

RICHARD  HENRY  STODDARD,  author  of  "The  King's  Bell,"  etc.; 

Literary  Editor  of  the  New  York  Mail  and  Express. 
HENRY  VAN  DYKE,  D.D.,  LL.D.,  author  of  "Little  Rivers,"   etc.; 

Professor  of  English  Literature  at  Princeton  University. 
THOMAS  NELSON  PAGE,  LL.D.,  Litt.D.,  author  of  "Red  Rock,"  etc. 

ORATIONS 

HON.  HENRY  CABOT  LODGE,  A.B.,  LL.B.,  author  of  "Life  of  Daniel 
Webster,"  etc.;  U.  S.  Senator  from  Massachusetts. 

HON.  JOHN  R.  PROCTOR,  President  U.  S.  Civil  Service  Commission. 

MORRIS  HICKEY  MORGAN,  Ph.D.,  LL.D.,  Professor  in  Latin,  Har- 
vard University. 


PEINCIPLES    OF    CHEMISTEY 

(PART    FOUR) 


CHAPTER  XXI 

CHROMIUM,    MOLYBDENUM,   TUNGSTEN,   URANIUM,   AND   MANGANESE 

SULPHUR,  selenium,  and  tellurium  belong  to  the  uneven  series  of  the 
sixth  group.  In  the  even  series  of  this  group  there  are  known  chro- 
mium, molybdenum,  tungsten,  and  uranium  ;  these  give  acid  oxides 
of  the  type  RO3,  like  S03.  Their  acid  properties  are  less  sharply 
defined  than  those  of  sulphur,  selenium,  and  tellurium,  as  is  the  case 
with  all  elements  of  the  even  series  as  compared  with  those  of  the 
uneven  series  in  the  same  group.  But  still  the  oxides  CrO3,  MoO3, 
W0?,  and  even  U03,  have  clearly  defined  acid  properties,  and  form 
salts  of  the  composition  MO,nR03  with  bases  MO.  In  the  case  of  the 
heavy  elements,  and  especially  of  uranium,  the  type  of  Oxide,  U03, 
is  less  acid  and  more  basic,  because  in  the  even  series  of  oxides  the 
element  with  the  highest  atomic  weight  always  acquires  a  more  and 
•more  pronounced  basic  character.  Hence  UO3  shows  the  properties  of 
-a  base,  and  gives  salts  UO2X2.  The  basic  properties  of  chromium, 
"molybdenum,  tungsten,  and  uranium  are  most  clearly  expressed  in  the 
lower,  oxides,  which  they  all  form.  Thus  chromic  oxide,  Cr2O3,  is  as. 
distinct  a  base  as  alumina,  A12O3. 

Of  all  these  elements  chromium  is  the  most  widely  distributed 
and  the  most  frequently  used.  It  gives  chromic  anhydride,  CrO3,  and 
chromic  oxide,  Cr2O3 — two  compounds  whose  relative  amounts  of 
oxygen  stand  in  the  ratio  2:1.  Chromium  is,  although  somewhat 
rarely,  met  with  in  nature  as  a  compound  of  one  or  the  other  type. 
The  red  chromium  ore  of  the  Urals,  lead  chromate  or  crocoisite 
PbCrO4,  was  the  source  in  which  chromium  was  discovered,  by 
Vauquelin,  who  gave  it  this  name  (from  the  Greek  word  signifying- 
colour)  owing  to  the  brilliant  colours  of  its  compounds  ;  the  chrornates 
(salts  of  chromic  anhydride)  are  red  and  yellow,  and  the  chromic  salts 
(from  Cr2O3)  green  and  violet.  The  red  lead  chromate  is,  however,  a 
rare  chromium  ore  found  only  in  the  Urals  and  in  a  few  other  localities. 
Chromic  oxide,  Cr2O3,  is  more  frequently  met  with.  In  small  quantities 
it  forms  the  colouring  matter  of  many  minerals  and  rocks— for  example, 

276 


CHROMIUM,  MOLYBDENUM,   TUNGSTEN,   URANIUM,   ETC.      277 

of  some  serpentines.  The  commonest  ore,  and  the  chief  source  of  the 
chromium  compounds,  is  the  chrome  iron  ore  or  chromite,  which  occurs 
in  the  Urals l  and  Asia  Minor,  California,  Australia,  and  other 
localities.  This  is  magnetic  iron  ore,  FeO,Fe2O3,  in  which  the  ferric 
oxide  is  replaced  by  chromic  oxide,  its  composition  being  FeO,Cr2O3. 
Chrome  iron  ore  crystallises  in  octahedra  of  sp.  gr.  4-4  ,  it  has  a  feeble 
metallic  lustre,  is  of  a  greyish-black  colour,  and  gives  a  brown  powder. 
It  is  very  feebly  acted  on  by  acids,  but  when  fused  with  potassium 
acid  sulphate  it  gives  a  soluble  mass,  which  contains  a  chromic  salt, 
besides  potassium  sulphate  and  ferrous  sulphate.  In  practice  the 
treatment  of  chrome  iron  ore  is  mainly  carried  on  for  the  preparation 
of  chromates,  and  not  of  chromic  salts,  and  therefore  we  will  trace  the 
history  of  the  element  by  beginning  with  chromic  acid,  and  especially 
with  the  working  up  of  the  chrome  iron  ore  into  potassium  dichromate, 
K2Cr2O7,  as  the  most  common  salt  of  this  acid.  It  must  be  remarked 
that  chromic  anhydride,  CrO3,  is  only  obtained  in  an  anhydrous  state, 
and  is  distinguished  for  its  capacity  for  easily  giving  anhydro-salts 
with  the  alkalis,  containing  one,  two,  and  even  three  equivalents  of  the 
anhydride  to  one  equivalent  of  base.  Thus  among  the  potassium  salts 
there  is  known  the  normal  or  yellow  chromate,  K2CrO4,  which  corre- 
sponds to,  and  is  perfectly  isomorphous  with,  potassium  sulphate,  easily 
forms  isomorphous  mixtures  with  it,  and  is  not  therefore  suitable  for  a 
process  in  which  it  is  necessary  to  separate  the  salt  from  a  mixture 
containing  sulphates.  As  in  the  presence  of  a  certain  excess  of  acid, 
thedichromate,  K2Cr207  =  2K2CrO4  +  2HX  -  2KX  —  H2O,  is  easily 
formed  from  K2CrO4,  the  object  of  the  manufacturer  is  to  produce 
such  a  dichromate,  the  more  so  as  it  contains  a  larger  proportion  of  the 
elements  of  chromic  acid  than  the  normal  salt.  Finely-ground  chrome 
iron  ore,  when  heated  with  an  alkali,  absorbs  oxygen  almost  as  easily 
(Chapter  III.,  Note  7)  as  a  mixture  of  the  oxides  of  manganese,  with 
an  alkali.  This  absorption  is  due  to  the  presence  of  chromic  oxide, 
which  is  oxidised  into  the  anhydride,  and  then  combines  with  the 
alkali  Cr2O3  +  O3  =  2Cr03.  As  the  oxidation  and  formation  of  the 
chromate  proceeds,  the  mass  turns  yellow.  The  iron  is  also  oxidised, 
but  does  not  give  ferric  acid,  because  the  capacity  of  the  chromium  for 
oxidation  is  incomparably  greater  than  that  of  the  iron. 

A  mixture  of  lime  (sometimes  with  potash)  and  chrome  iron  ore 
is  heated  in  a  reverberatory  furnace,  with  free  access  of  air  and  at  a 

The  working  of  the  Ural  chrome  iron  ore  into  chromium  compounds  has  been 
firmly  established  in  Russia,  thanks  to  the  endeavours  of  P.  K.  Ushakoff,  who  con- 
etructed  large  works  for  this  purpose  on  the  river  Kama,  near  Elabougi,  where  as  much 
as  2,000  tons  of  ore  are  treated  yearly,  owing  to  which  the  importation  of  chromium  pre- 
parations into  Russia  has  ceased 


278  PRINCIPLES  OF  CHEMISTRY 

red  heat  for  several  hours,  until  the  mass  becomes  yellow  ;  it  then 
contains  normal  calcium-  chromate,  CaCrO4,  which  is  insoluble  in 
water  in  the  presence  of  an  excess  of  lime.1  bis  The  resultant  mass  is 
ground  up,  and  treated  with  water  and  sulphuric  acid.  The  excess  of 
lime  forms  gypsum,  and  the  soluble  calcium  dichromate,  CaCr2O7, 
together  with  a  certain  amount  of  iron,  pass  into  solution.  The 
solution  is  poured  off,  and  chalk  added  to  it ;  this  precipitates  the 
ferric  oxide  (the  ferrous  oxide  is  converted  into  ferric  oxide  in  the 
furnace)  and  forms  a  fresh  quantity  of  gypsum,  while  the  chromic  acid 
remains  in  solution — that  is,  it  does  not  form  the  sparingly-soluble 
normal  salt  (1  part  soluble  in  240  parts  of  water).  The  solution  then 
contains  a  fairly  pure  calcium  dichromate,  which  by  double  decom- 
position gives  other  chromates  ;  for  example,  with  a  solution  of  potassium 
sulphate  it  gives  a  precipitate  of  calcium  sulphate  and  a  solution  of 
potassium  dichromate,  which  crystallises  when  evaporated.2 

Potassium  dichromate,  K2Cr2O7,  easily  crystallises  from  acid  solu- 
tions in  red,  well- formed  prismatic  crystals,  which  fuse  at  a  red  heat 
and  evolve  oxygen  at  a  very  high  temperature,  leaving  chromic  oxide 
and  the  normal  salt,  which  undergoes  no  further  change  :  2K2Or2O7 
=  2K2Cr04  -f  Cr2O3  +  O3.  At  the  ordinary  temperature  100  parts 
of  water  dissolve  10  parts  of  this  salt,  and  the  solubility  increases  as 
the  temperature  rises.  It  is  most  important  to  note  that  the 
dichromate  does  not  contain  water,  it  is  K2CrO4  +  CrO3 ,  the  acid 
salt  corresponding  to  potassium  acid  sulphate,  KHS04,  does  not  exist. 
It  does  not  even  evolve  heat  when  dissolving  in  water,  but  on  the  con* 
trary  produces  cold,  i.e.  it  does  not  form  a  very  stable  compound  with 
water.  The  solution  and  the  salt  itself  are  poisonous,  and  act  as 
powerful  oxidising  agents,  which  is  the  character  of  chromic  acid  ill 
general.  When  heated  with  sulphur  or  organic  substances,  with 
sulphurous  anhydride,  hydrogen  sulphide,  &c.,  this  salt  is  deoxidised, 
yielding  chromic  compounds.2  bis  Potassium  dichromate  3  is  used  in  the 
arts  and  in  chemistry  as  a  source  for  the  preparation  of  all  other 

1  bu  But  the  calcium  chromate  is  soluble  in  water  tn  the  presence  of  an  excess  of 
chromic  acid,  as  may  be  seen  from  the  fact  that  a  solution  of  chromic  acid  dissolves 
lime. 

2  There  are  many  variations  in  the  details  of  the  manufacturing  processes,  and  these 
must  be  looked  for  in  works  on  technical  chemistry.    But  we  may  add  that  the  chromate 
may  also  be  obtained  by  slightly  roasting  briquettes  of  a  mixture  of  chrome  iron  and 
lime,  and  then  leaving  the  resultant  mass  to  the  action  of  moist  air  (oxygen  is  absorbed, 
and  the  mass  turns  yellow). 

»bb  The  oxidising  action  of  potassium  dichromate  on  organic  substances  at  thfe 
ordinary  temperature  is  especially  marked  under  the  action  of  light.  Thus  it  acts  on 
gelatin,  as  Poutven  discovered ;  this  is  applied  to  photography  in  the  processes  of  photo- 

For  Note  3  see  p.  279. 


CHKOMIUM,  MOLYBDENUM,  TUNGSTEN)  URANIUM,  ETC.      279 

chromium  compounds.  It  is  converted  into  yellow  pigments  by  means 
of  double  decomposition  with  salts  of  lead,  barium,  and  zinc.  When 
solutions  of  the  salts  of  these  metals  are  mixed  with  potassium 
dichroraate  (in  dyeing  generally  mixed  with  soda,  in  order  to  obtain 
normal  salts),  they  are  precipitated  as  insoluble  normal  salts ;  for 
example,  2BaCl2  +  K2Cr207  +  H2O  =  2BaCrO4  +  2KC1  +  2HC1.  It 
follows  from  this  that  these  salts  are  insoluble  in  dilute  acids,  but 
the  precipitation  is  not  complete  (as  it  would  be  with  the  normal  salt). 
The  barium  and  zinc  salts  are  of  a  lemon  yellow  colour  ;  the  lead  salt 
lias  a  still  more  intense  colour  passing  into  orange.  Yellow  cotton 
prints  are  dyed  with  this  pigment,  The  silver  salt,  Ag2CrO4,  is  of  a 
bright  red  colour. 

When  potassium  dichromate  is  mixed  with   potassium   hydroxide 

gravure,  photo-'lithography,  pigment  printing,  &c.  Under  the  action  of  light  this  gelatin 
is  oxidised,  and  the  chromic  anhydride  deoxidised  into  chromic  oxide,  which  unites  with 
the  gelatin  and  forms  a  compound  insoluble  in  warm  water,  whilst  where  the  light  has  not 
acted,  the  gelatin  remains  soluble,  its  properties  being  unaffected  by  the  presence  of 
chromic  acid  or  potassium  dichromate. 

5  Ammonium  and  sodium  dichromates  are  now  also  prepared  on  a  large  scale.  The 
sodium  salts  may  be  prepared  in  exactly  the  same  manner  as  those  of  potassium.  The 
normal  salt  combines  with  ten  equivalents  of  water,  like  Glauber's  salt,  with  which  it  is 
isomorphous.  Its  solution  above  30°  deposits  the  anhydrous  salt.  Sodium  dichromate 
crystals  contain  Na2Cr2O7,2H2O.  The  ammonium  salts  of  chromic  acid  are  obtained 
by  saturating  the  anhydride  itself  with  ammonia.  The  dichromate  is  obtained  by 
saturating  one  part  of  the  anhydride  with  ammonia,  and  then  adding  a  second  part  of 
anhydride  and  evaporating  under  the  receiver  of  an  air-pump.  On  ignition,  the  normal 
and  acid  salts  leave  chromic  oxide.  Potassium  ammonium  chromate,  NH4KCr04,  is 
obtained  in  yellow  needles  from  a  solution  of  potassium  dichromate  in  aqueous  ammonia ; 
it  not  only  loses  ammonia  and  becomes  converted  into  potassium  dichromate  when 
ignited,  but  also  by  degrees  at  the  ordinary  temperature.  This  shows  the  feeble  energy 
of  chromic  acid,  and  its  tendency  to  form  stable  dichromates.  Magnesium  chromate  is 
soluble  in  water,  as  also  is  the  strontium  salt.  The  calcium  salt  is  also  somewhat  soluble, 
but  the  barium  salt  is  almost  insoluble.  The  isomorphism  with  sulphuric  acid  is  shown 
in  the  chromates  by  the  fact  that  the  magnesium  and  ammonium  salts  form  double  salts 
containing  six  equivalents  of  water,  which  are  perfectly  Jsomorphous  with  the  corre- 
ponding  sulphates.  The  magnesium  salt  crystallises  in  large  crystals  containing  seven 
equivalents  of  water.  The  beryllium,  cerium,  and  cobalt  salts  are  insoluble  in  water. 
Chromic  acid  dissolves  manganous  carbonate,  but  on  evaporation  the  solution  deposits 
manganese  dioxide,  formed  at  the  expense  of  the  oxygen  of  the  chromic  acid.  Chromic 
acid  also  oxidises  ferrous  oxide,  and  ferric  oxide  is  soluble  in  chromic  acid. 

One  of  the  chromates  most  used  by  the  dyer  is  the  insoluble  yellow  lead  chiromate, 
PbCrO4  (Chapter  XVIII.,  Note  46),  which  is  precipitated  on  mixing  solutions  of 
PbX2  with  soluble  chromates.  It  easily  forms  a  basic  salt,  having  the  composition 
PbO,PbCrO4,  as  a  crystalline  powder,  obtained  by  fusing  the  normal  salt  with  nitre  and 
then  rapidly  washing  in  water.  The  same  substance  is  obtained,  although  impure  and 
in  small  quantity,  by  treating  lead  chromate  with  neutral  potassium  chromate,  especially 
on  boiling  the  mixture ;  and  this  gives  the  possibility  of  attaining,  by  means  of  these 
materials,  various  tints  of  lead  chromate,  from  yellow  to  red,  passing  through  different 
orange  shades.  The  decomposition  which  takes  place  (incompletely)  in  this  case  is 
as  follows:  2PbCrQ4-l-K2Cr04,=  PbCr04)PbO  +  K2Cr207— that  is,  potassium  dichromate 
19  formed  in  solution. 


280  PRINCIPLES  OF  CHEMISTRY 

or  carbonate  (carbonic  anhydride  being  disengaged  in  the  latter  case)  it 
forms  the  normal  salt,  K2Cr04,  known  as  yellow  chromate  of  potassium. 
Its  specific  gravity  is  2*7,  being  almost  the  same  as  that  of  the  dichro- 
mate. It  absorbs  heat  in  dissolving ;  one  part  of  the  salt  dissolves  in 
1'75  part  of  water  at  the  ordinary  temperature,  forming  a  yellow 
solution.  When  mixed  even  with  such  feeble  acids  as  acetic,  and  more 
especially  with  the  ordinary  acids,  it  gives  the  dichromate,  and  Graham 
obtained  a  trichromate,  K2Cr3O,0  =  K2CrO4,2CrO3,  by  mixing  a 
solution  of  the  latter  salt  with  an  excess  of  nitric  acid. 

Chromic  anhydride  is  obtained  by  preparing  a  saturated  solution  of 
potassium  dichromate  at  the  ordinary  temperature,  and  pouring  it  in  a 
thin  stream  into  an  equal  volume  of  pure  sulphuric  acid.4  On  mixing, 
the  temperature  naturally  rises ,  when  slowly  cooled,  the  solution 
deposits  chromic  anhydride  in  needle-shaped  crystals  of  a  red  colour 
sometimes  several  centimetres  long.  The  crystals  are  freed  from  the 
mother  liquor  by  placing  them  on  a  porous  tile.4  bls  It  is  very  important 
at  this  point  to  call  attention  to  the  fact  that  a  hydrate  of  chromic 
anhydride  is  never  obtained  in  the  decomposition  of  chromic  compounds, 

4'  The  Sulphuric  acid  should  not  contain  any  lower  oxides  of  nitrogen,  because  they 
reduce  chromic  anhydride  into  chromic  oxide.  If  a  solution  of  a  chromate  be  heated 
with  an  excess  of  acid — for  instance,  sulphuric  or  hydrochloric  acid — oxygen  or  chlorine 
is  evolved,  and  a  solution  of  a  chromic  salt  is  formed.  Hence,  under  these  circum- 
stances, chromic  acid  cannot  be  obtained  from  its  salts.  One  of  the  first  methods 
employed  consisted  in  converting  its  salts  into  volatile  chromium  hexafluoride,  CrF6. 
This  compound,  obtained  by  Unverdorben,  may  be  prepared  by  mixing  lead  chromate 
with  fluor  spar  in  a  dry  state,  and  treating  the  mixture  with  fuming  sulphuric  acid  in  a 
platinum-  vessel :  PbCrO4 + 3CaF2  +  4H2SO4  =  PbSO4  +  3CaS04  +  4H2O  +  CrF6.  Fuming 
sulphuric  acid  is  taken,  and  in  considerable  excess,  because  the  chromium  fluoride  which 
is  formed  is  very  easily  decomposed  by  water.  It  is  volatile,  and  forms  a  very  caustic, 
poisonous  vapour,  which  condenses  when  cooled  in  a  dry  platinum  vessel  into  a  red, 
exceedingly  volatile  liquid,  which  fumes  powerfully  in  air.  The  vapours  of  this 
substance  when  introduced  into  water  are  decomposed  into  hydrofluoric  acid  and 
chromic  anhydride  :  CrF6  +  8H2O  =  CrO5  +  6HF.  If  very  little  water  be  taken  the  hydro- 
fluoric acid  volatilises,  and  chromic  anhydride  separates  directly  in  crystals.  The 
chloranhydride  of  chromic  acid,  CrO2Cl2  (Note  6),  is  also  decomposed  in  the  same 
manner.  A  solution  of  chromic  acid  and  a  precipitate  of  barium  sulphate  are  formed  by 
treating  the  insoluble  barium  chromate  withi  an  equivalent  quantity  of  sulphuric  acid. 
If  carefully  evaporated,  the  solution  yields  crystals  of  chromic  anhydride.  Fritzsche 
gave  a  very  convenient  method  of  preparing  chromic  anhydride,  based  on  the  relation 
of  chromic  to  sulphuric  acid.  At  the  ordinary  temperature  the  strong  acid  dissolves 
both  chromic  anhydride  and  potassium  chromate,  but  if  a  certain  amount  of  water  is 
added  to  the  solution  the  chromic  anhydride  separates,  and  if  the  amount  of  water  be 
increased  the  precipitated  chromic  anhydride  is  again  dissolved.  The  chromic  anhy- 
dride is  almost  all  separated  from  the  solution  when  it- contains  two  equivalents  of 
water  to  one  equivalent  of  sulphuric  acid.  "Many  methods  for  the  preparation  of  chromic 
anhydride  are  based  on  this  fact. 

4bl»  They  cannot  be  filtered  through  paper  or  washed,  because  the  chromic 
anhydride  is  reduced  by  the  filter-paper,  and  is  dissolved  during  the  process  of 
washing. 


CHROMIUM,  MOLYBDENUM,  TUNGSTEN,  URANIUM,  ETC.  281 

but  always  the  anhydride,  Cr03.  The  corresponding  hydrate,  CrO4H2, 
or  any  other  hydrate,  is  not  even  known.  Nevertheless,  it  must  be 
admitted  that  chromic  acid  is  bibasic,  because  it  forms  salts  isomorphous 
or  perfectly  analogous  with  the  salts  formed  by  sulphuric  acid,  which  is 
the  best  example  of  a  bibasic  acid.  A  clear  proof  of  the  bibasicity  of 
CrO3  is  seen  in  the  fact  that  the  anhydride  and  salts  give  (when  heated 
with  sodium  chloride  and  sulphuric  acid)  a  volatile  chloranhydride, 
CrO2Cl2,  containing  two  atoms  of  chlorine  as  a  bibasic  acid  should.5 

5  Berzelius  observed,  and  Rose  carefully  investigated,  this  remarkable  reaction, 
which  occurs  between  chromic  acid  and  sodium  chloride  in  the  presence  of  sulphuric 
acid.  If  10  parts  of  common  salt  be  mixed  with  12  parts  of  potassium  dichromate,  fused, 
cooled,  and  broken  up  into  lumps,  and  placed  in  a  retort  with  20  parts  of  fuming  sul- 
phuric acid,  it  gives  rise  to  a  violent  reaction,  accompanied  by  the  formation  of  brown 
fumes  of  chromic  chloranhydride,  or  chromyl  chloride,  CrO2Cl2,  according  to  the  re- 
action :  Cr05  +  2NaCl  +  H2SO4  =  Na2SO4  +  H2O  +  Cr02Cl2.  The  addition  of  an  excess  of 
sulphuric  acid  is  necessary  in  order  to  retain  the  water.  The  same  substance  is  always 
formed  when  a  metallic  chloride  is  heated  with  chromic  acid,  or  any  of  its  salts,  in  the 
presence  of  sulphuric  acid.  The  formation  of  this  volatile  substance  is  easily  observed 
from  the  brown  colour  which  is  proper  to  its  vapour.  On  condensing  the  vapour  in  a 
dry  receiver  a  liquid  is  obtained  having  a  sp.  gr.  of  •  1:9,  boiling  at  118°,  and  giving  a 
vapour  whose  density,  compared  with  hydrogen,  is  78,  which  corresponds  with  the  above 
formula.  Chromyl  chloride  is  decomposed  by  heat  into  chromic  oxide,  oxygen,  and 
chlorine:  2CrO2Cl2  =  Cr2O3  +  2Cl2  +  O;  so  that  it  is  able  to  act  simultaneously  as  a 
powerful  oxidising  and  chlorinating  agent,  which  is  taken  advantage  of  in  the  investiga- 
tion of  many,  and  especially  of  organic,  substances.  When  reated  with  water,  this 
substance  first  falls  to  the  bottom,  and  is  then  decomposed  into  hydrochloric  and  chromic 
acids,  like  all  chloranhydrides :  CrO2Cl2  +  H2O  =  Cr05  +  2HC1.  When  brought  into  con- 
tact with  inflammable  substances  it  sets  fire  to  them ;  it  acts  thus,  for  instance,  on 
phosphorus,  sulphur,  oil  of  turpentine,  ammonia,  hydrogen,  and  other  substances.  It 
attracts  moisture  from  the  atmosphere  with  great  energy,  and  must  therefore  be  kept  in 
closed  vessels.  It  dissolves  iodine  and  chlorine,  and  even  forms  a  solid  compound  with 
the  latter,  which  depends  upon  the  faculty  of  chromium  to  form  its  higher  oxide, 
Cr207.  The  close  analogy  in  the  physical  properties  of  the  chloranhydrides,  CrO2Cl2  and 
802C12,  is  very  remarkable,  although  sulphurous  anhydride  is  a  gas,  and  the  corresponding 
oxide,  Cr02,  is  a  non-volatile  solid.  It  may  be  imagined,  therefore,  that  chromium  di- 
oxide (which  will  be  mentioned  in  the  following  note)  presents  a  polymerised  modification 
of  the  substance  having.the  composition  CrO2;  in  fact,  this  is  obvious  from  the  method 
of  its  formation. 

If  three  parts  of  potassium  dichromate  be  mixed  with  four  parts  of  strong  hydrochloric: 
acid  and  a  small  quantity  of  water,  and  gently  warmed,  it  all  passes  into  solution, 
and  no  chlorine  is  evolved ;  on  cooling,  the  liquid  deposits  red  prismatic  crystals,  known 
as  Peligot's  salt,  very  stable  in  air.  Thi's  has  the  composition  KCl,Cr03,  and  is  formed 
according  to  the  equation  K2Cr2O7  +  2HC1  =  2KCl,CrO3  +  H2O.  It  is  evident  that  this 
is  the  first  chloranhydride  of  chromic  acid,  HCr05Cl,  in  which  the  hydrogen  is  re- 
placed by  potassium.  It  if  decomposed  by  water,  and  on  evaporation  the  solution  yields 
potassium  dichromate  and  hydrochloric  acid.  This  is  a  fresh  instance  of  the  reversible 
reactions  so  frequently  encountered.  With  sulphuric  acid  Peligot's  salt  forms  chromyl 
chloride.  The  latte'r  circumstance,  and  tHe  fact  that  Geuther  produced  Peligot's  salt 
from  potassium  chromate  and  chromyl  chloride,  give  reason  for  thinking  that  it  is  a 
compound  of  these  two  substances .  2KCl,Cr05= K2CrO4  +  CrO2Cl2.  It  is  also  sometimes 
regarded  as  potassium  dichromate  in  which  one  atom  of  oxygen  is  replaced  by  chlorine — 
that  is,  K2Cr2O6Cl2,  corresponding  with  K2Cr2O7.  When  heated  it  parts  with  all  its 
chlorine,  and  on  further  heating  gives  chromic  oxide. 


282  PRINCIPLES  : OF  CHEMISTRY 

Chromic  anhydride  is  a  red  crystalline  substance,  which  is  converted 
into  a  black  mass  by  heat ;  it  fuses  at  190°,  and  disengages  oxygen 
above  250°,  leaving  a  residue  of  chromium  dioxide,  CrO.2,G  and,  on  still 
further  heating,  chromic  oxide,  Cr.2O3.  Chromic  anhydride  is  exceed- 
ingly soluble  in  water,  and  even  attracts  moisture  from  the  air,  but,  as 
was  mentioned  above,  it  does  not  form  any  definite  compound  with 
water.  The  specific  gravity  of  its  crystals  is  2'7,  and  when  fused  it  has 
a  specific  gravity  2-6.  The  solution  presents  perfectly  defined  acid 
properties.  It  liberates  carbonic  anhydride  from  carbonates  ;  gives 
insoluble  precipitates  of  the  chromates  with  salts  of  barium,  lead,  silver, 
and  mercury. 

The  action  of  hydrogen  peroxide  on  a  solution  of  chromic  acid  or  of 
potassium  dichromate  gives  a  blue  solution,  which  very  quickly  becomes 
colourless  with  the  disengagement  of  oxygen.  Barreswill  showed  that 
this  is  due  to  the  formation  of  a  perchromic  anhydride,  Cr2O7,  corre- 
sponding with  sulphur  peroxide.  This  peroxide  is  remarkable  from  the 
fact  that  it  very  easily  dissolves  in  ether  and  is  much  more  stable  in 
this  solution,  so  that,  by  shaking  up  hydrogen  peroxide  mixed  with  a 
small  quantity  of  chromic  acid,  with  ether,  it  is  possible  to  transfer  all 
the  blue  substance  formed  to  the  ether.6  bis 

With  oxygen  acids,  chromic  acid  evolves  oxygen  ;  for  example,  with 

6  This  intermediate  degree  of  oxidation,  CrO2,  may  also  be  obtained  by  mixing  sola- 
tions  of  chromic  salts  with  solutions  of  chromates.  The  brown  precipitate  formed 
Contains  a  compound,  Cr2O3,Cr03,  consisting  of  equivalent  amounts  of  chromic  oxide 
and  anhydride.  The' brown  precipitate  of  chromium  dioxide  contains  water.  The  same 
substance  is  formed  by  the  imperfect  deoxidation  of  chromic  anhydride  by  various  redu- 
cing agents.  Chromic  oxide,  when  heated,  absorbs  oxygen,  and  appears  to  give  the  same 
substance.  Chromic  nitrate,  when  ignited,  also  gives  this  substance.  When  this  sub- 
stance is  heated  it  first  disengages  water  and  then  oxygen,  chromic  oxide  being  left.  It 
corresponds  with  manganese  dioxide,  Cr2O5,CrO3  =  8CrO2.  Kriiger  treated  chromium 
dioxide  with  a  mixture  of  sodium  chloride  and  sulphuric  acid,  and  found  that  chlorine 
gas  was  evolved,  but  that  chromyl  chloride  was  not  formed.  Under  the  action  of  light, 
a  solution  of  chromic  acid  also  deposits  the  brown  dioxide.  At  the  ordinary  temperature 
chromic  anhydride  leaves  a  brown  stain  upon  the  skin  and  tissues,  which  probably  pro- 
ceeds from  a  decomposition  of  the  same  kind.  Chromic  anhydride  is  soluble  in  alcohol- 
containing  water,  and  this  solution  is  decomposed  in  a  similar  manner  by  light. 
Chromium  dioxide  forms  K2CrO4  when  treated  with  H2O2  in  the  presence  of  KHO. 

6  bi»  NOW  that  persulphuric  acid  H2S.2O8  is  well  known  it  might  be  supposed  that 
perchromic  anhydride,  Cr2O7,  would  correspond  to  perchromic  acid,  H2Cr2O8,  but  as  yet 
it  is  not  certain  whether  corresponding  salts  are  formed.  Pechard  (1891)  on  adding  an 
excess  of  H2O2  and  baryta  water  to  a  dilute  solution  of  CrO2  (8  grm.  per  litre),  observed 
the  formation  of  a  yellow  precipitate,  but  oxygen  was  disengaged  at  the  same  time  and 
the  precipitate  (which  easily  exploded  when  dried)  was  found  to  contain,  besides  an 
admixture  of  BaO2,  a  compound  BaCrO5,  and  this  =  BaO3  +  CrO3,  and  does  not  correspond 
to  perchromic  acid.  The  fact  of  its  decomposing  with  an  explosion,  and  the  mode  of  its 
preparation,  proves,  however,  that  this  is  a  similar  derivative  of  peroxide  of  hydrogen  t<L 
persulphuric.  acid  (Chapter  XX.) 


Cfi'KOMlUM,  MOLYBDENUM,   TUNGSTEN,   UKAN1UM,  ETC.      283 

Sulphuric  acid  the  following  reaction  takes  place  2CrO3 -f  3H2SO4 
=  Cr2(SO4)3  +  O3  4-  3H2O.  It  will  be  readily  understood  from  this  that 
a  mixture  of  chromic  acid  or  of  its  salts  with  sulphuric  acid  forms  art 
excellent  oxidising  agent,  which  is  frequently  employed  in  chemical 
laboratories  and  even  for  technical  purposes  as  a  means  of  oxidation. 
Thus  hydrogen  sulphide  and  sulphurous  anhydride  are  converted  into 
sulphuric  acid  by  this  means.  Chromic  acid  is  able  to  act  as  a  powerful 
oxidising  agent  because  it  passes  into  chromic  oxide,  and  in  so  doing 
disengages  half  of  the  oxygen  contained  in  it  :  2CrO3=Cr2O3-f  O3. 
Thus  chromic  anhydride  itself  is  a  powerful  oxidising  agent,  and  is 
therefore  employed  instead  of  nitric  acid  in  galvanic  batteries  (as  a 
depolariser),  the  hydrogen  evolved  at  the  carbon  being  then  oxidised, 
and  the  chromic  acid  converted  into  a  non-volatile  product  of  deoxida- 
tion,  instead  of  yielding,  as  nitric  acid  does,  volatile  lower  oxides  of 
offensive  odour.  Organic  substances  are  more  or  less  perfectly  oxidised 
by  means  of  chromic  anhydride,  although  this  generally  requires  the  aid 
of  heat,  and  does  not  proceed  in  the  presence  of  alkalis,  but  generally 
in  the  presence  of  acids .  In  acting  on  a  solution  of  potassium  iodide, 
chromic  acid,  like  many  oxidising  agents,  liberates  iodine  ;  the  reaction 
proceeds  in  proportion  to  the  amount  of  Cr03  present,  and  may  serve 
for  determining  the  amount  of  Cr03,  since  the  amount  of  iodine  liberated 
can  be  accurately  determined  by  the  iodometric  method  (Chapter  XX., 
Note  42).  If  chromic  anhydride  be  ignited  in  a  stream  of  ammonia,  it 
gives  chromic  oxide,  water,  and  nitrogen.  In  all  cases  when  chromic 
acid  acts  as  an  oxidising  agent  in  the  presence  of  acids  and  under  the 
action  of  heat,  the  product  of  its  deoxidation  is  a  chromic  salt,  CrX3, 
which  is  characterised  by  the  green  colour  of  its  solution,  so  that  the 
red  or  yellow  solution  of  a  salt  of  chromic  acid  is  then  transformed  into 
a  green  solution  of  a  chromic  salt,  derived  from  chromic  oxide,  Cr2O3, 
which  is  closely  analogous  to  A12O3,  Fe2O3,  and  other  bases  of  the  com- 
position R2O3.  This  analogy  is  seen  in  the  insolubility  of  the  anhydrous 
oxide,  in  the  gelatinous  form  of  the  colloidal  hydrate,  in  the  formation 
of  alums,7  of  a  >  volatile  chloride  of  chromium,  &c.7  bls 

7  As  a  mixture  of  potassium  dichromate  and  sulphuric  acid  is  usually  employed 
for  oxidation,  the  resultant  solution  generally"  contains  a  double  sulphate  of 'potas- 
sium and  chromium — that  is,  chrome  alum,  isomorphous  with  ordinary  alum— 
K2Cr2O.7  +  4H2SO4  +  20H2O  =  63-f  K2Cr2(SO4)4,24H20  or  2(KCr(804)ai12H2O).  It  is  pre- 
pared by  dissolving  potassium  dichromate  in  dilute  sulphuric  acid  ;  alcohol  is  then  added 
and  the  solution  slightly  heated,  or  sulphurous  anhydride  is  passed  through  it.  On  the 
addition  of  alcohol  to  a  cold  mixture  of  potassium  dichromate  and  sulphuric  acid,  the 
gradual  disengagement  of  pleasant-smelling  volatile  products  of  the  oxidation  of  alcohol, 
and  especially  of  aldehyde,  C2H40,  is  remarked.  If  the  temperature  of  decomposition 

For  Note  7  bis  see  p.  285. 


584  PRINCIPLES  OF  CHEMISTRY 

Chromic  oxide,  Cr203,  rarely  found,  and  in  small  quantities,  in  chrome^ 
ochre,  is  formed  by  the  oxidation  of  chromium  and  its  lower  oxides,  by  ' 

does  not  exceed  35°,  a  violet  solution  of  chrome  alum  is  obtained,  but  if  the  tempera- 
ture be  higher,  a  solution  of  the  same  alum  is  obtained  of  a  green  colour.  As  chrome 
alum  requires  for  solution  7  parts  of  water  at  the  ordinary  temperature,  it  follows  that  if 
a  somewhat  strong  solution  of  potassium  dichromate  be  taken  (4  parts  of  water  and  1& 
of  sulphuric  acid  to  1  part  of  dichromate),  it  will  give  so  concentrated  a  solution 
of  chrome  alum  that  on  cooling,  the  salt  will  separate  without  further  evaporation.  If 
the  liquid,  prepared  as  above  or  in  any  instance  of  the  deoxidation  of  chromic  acid, 
bo  heated  (the  oxidation  naturally  proceeds-  more  rapidly)  somewhat  strongly,  for  in- 
stance, to  the  boiling-point  of  water,  or  if  the  violet  solution  already  formed  be  raised  to 
the  same  temperature,  it  acquires  a  bright  green  colour,  and  on  evaporation  the 
same  mixture,  which  at  lower  temperatures  so  easily  gives  cubical  crystals  of  chrome 
alum,  does  not  give  any  crystals  whatever  If  the  green  solution  be  kept,  however,  for 
several  weeks  at  the  ordinary  temperature,  it  deposits  violet  crystals  of  chrome  alum. 
The  green  solution,  when  evaporated,  gives  a  non-crystalline  mass,  and  the  violet 
crystals  lose  water  at  100°  and  turn  green.  It  must  be  remarked  that  the  transition  of 
the  green  modification  into  the  violet  is  accompanied  by  a  decrease  in  volume  (Lecoq  de 
Boisbaudran,  Favre).  If  the  green  mass  formed  at  the  higher  temperature  be  evaporated 
to  dryness  and  heated  at  80°  in  a  current  of  air,  it  does  not  retain  more  then  6  equi- 
valents of  water.  Hence  Lb'wel,  and  also  Schrbtter,  concluded  that  the  green  and  violet 
modifications  of  the  alum  depend  on  different  degrees  of  combination  with  water,  which 
may  be  likened  to  the  different  compounds  of  sodium  sulphate  with  water  and  to  the 
different  hydrates  of  ferric  oxide. 

However,  the  question  in  this  case  is  not  so  simple,  as  we  shall  afterwards  see. 
Jf  ot  chrome  alum  alone,  but  all  the  chromic  salts,  give  two,  if  not  three,  varieties.  At 
least,  there  is  no  doubt  about  the  existence  of  two — a  green  and  a  violet  modification. 
The  green  chromic  salts  are  obtained  by  heating  solutions  of  the  violet  salts,  the  violet 
solutions  are  produced  on  keeping  solutions  of  the  green  salts  for  a  long  time.  The  con- 
version of  the  violet  salts  into  green  by  the  action  of  heat  is  itself  an  indication  of  the 
possibility  of  explaining  the  different  modifications  by  their  containing  different  propor- 
tions (or  states)  of  water,  and,  moreover,  by  the'green  salts  having  a  less  amount  of 
water  than  the  violet.  However,  there  are  other  explanations.  Chromic  oxide  is  a  base 
like  alumina,  and  is  therefore  able  to  give  both  acid  and  basic  salts.  It  is  supposed  that 
the  difference  between  the  green  and  violet  salts  is  due  to  this  fact.  This  opinion  of 
Kriiger  is  based  on  the  fact  that  alcohol  separates  out  a  salt  from  the  green  solution 
which  contains  less  sulphuric  acid  than  the  normal  violet  salt.  On  the  other  hand, 
Lb'wel  showed  that  all  the  acid  cannot  be  separated  from  the  green  chromic  salts  by 
suitable  reagents,  as  easily  as  it  can  be  from  the  same  solution  of  the  violet  salts ;  thus 
barium  salts  do  not  precipitate  all  the  sulphuric  acid  from  solutions  of  the  green  salts. 
According  to  other  researches  the  cause  of  the  varieties  of  the  chromic  salts  lies  in  a 
difference  in  the  bases  they  contain — that  is,  it  is  connected  with  a  modification  of  the 
properties  of  the  oxide  of  chromium  itself.  This  only  refers  to  the  hydroxides,  but  as 
hydroxides  themselves  are  only  special  forms  of  salts,  the  differences  observed  as  yet 
in  this  direction  between  the  hydroxides  only  confirm  the  generality  of  the  difference 
observed  in  the  chromic  compounds  (see  Note  7  bis). 

The  salts  of  chromic  oxide,  like  those  of  alumina,  are.  easily  decomposed,  give  basic 
and  double  salts,  and  have  .an  acid  reaction,  as  chromic  oxide  is  a  feeble  base.  Potas- 
sium and  sodium  hydroxides  give  a  precipitate  of  the  hydroxide  with  chromic  salts, 
CrXj.  The  violet  and  green  salts  give  a  hydroxide  soluble  in  an  excess  of  the 
reagent ;  but  the  hydroxide  is  held  in  solution  by  very  feeble  affinities,  so  that  it  is 
partially  separated  by  heat  and  dilution  with  water,  and  completely  so  on  boiling. 
In  an  alkaline  solution,  chromic  hydroxide  is  easily  converted  into  chromic  acid 
by  the 'action  of  lead  dioxide,  chlorine,  and  other  oxidising  agents.  If  the  chromic 
oxide  occurs  together  with  such  oxides  as  magnesia,  or  zinc  oxide,  then  on  precipitation 


CHEOMIUM,  MOLYBDENUM,  TUNGSTEN,  URANIUM,   ETC.       285 

the  reduction  of  chromates  (for  example,  of  ammonium  or  mercurio 
chromate)  and  by  the  decomposition  (splitting  up)  of  the  saline  corn- 
it  separates  out  from  its  solution  in  combination  with  these  oxides,  forming,  for  example, 
ZnO,Cr203.  Viard  obtained  compounds  of  Cr2O3  with  the  oxides  of  Mg,  Zn,  Cd,  &o.) 
On  precipitating  the  violet  solution  of  chrome  alum  with  ammonia,  a  precipitate  contain- 
ing  Cr2O3,6H20  is  obtained,  whilst  the  precipitate  from  the  boiling  solution  with  caustii 
potash  was  a  hydrate  containing  four  equivalents  of  water.  When  fused  with  borax  chromic 
salts  give  a  green  glass.  The  same  coloration  is  communicated  to  ordinary  glass  by  the 
presence  of  traces  of  chromic  oxide.  A  chrome  glass  containing  a  large  amount  of 
chromic  oxide  may  be  ground  up  and  used  as  a  green  pigment.  Among  the  hydratea 
of  oxide  of  chromium  Guignet's  green  forms  one  of  the  widely-used  green  pigments  which 
have  been  substituted  for  the  poisonous  arsenical  copper  pigments,  such  as  Schweinfurt 
green,  which  formerly  was  much  used.  Guignet's  green  has  an  extremely  bright  green 
colour,  and  is  distinguished  for  its  gre.xt  stability,  not  only  under  the  action  of  light  but 
also  towards  reagents  ;  thus  it  is  not  altered  by  alkaline  solutions,  and  even  nitric  acid 
does  not  act  on  it.  This  pigment  remains  unchanged  up  to  a  temperature  of  250°  ;  it 
contains  Cr203,2H02,  and  generally  a  small  amount  of  alkali.  It  is  prepared  by  fusing  8 
parts  of  boric  acid  with  1  part  of  potassium  dichromate ;  oxygen  is  disengaged,  and  a 
green  glass,  containing  a  mixture  of  the  boratesof  chromium  and  potassium,  is  obtained. 
When  cool  this  glass  is  ground  up  and  treated  with  water,  which  extracts  the  borio 
acid  and  alkali  and  leaves  the  above-named  chromic  hydroxide  behind.  This  hydroxide 
only  parts  with  its  water  at  a  red  heat,  leaving  the  anhydrous  oxide. 

The  chromic  hydroxides  lose  their  water  by  ignition,  and  in  so  doing  become  spon- 
taneously incandescent,  like  the  ordinary  ferric  hydroxide  (Chapter  XXII.).  It  re  not 
known,  however,  whether  all  the  modifications  of  chromic  oxide  show  this  phenomenon. 
The  anhydrous  chromic  oxide,  CroO5,  is  exceedingly  difficultly  soluble  in  acids,  if  it 
has  passed  through  the  above  recalescence.  But  if  it  has  parted  with  its  water,  or  the 
greater  part  of  it,  and  not  yet  undergone  this  self-induced  incandescence  (has  not  lost  a 
portion  of  its  energy),  then  it  is  soluble  in  acids.  It  is  not  reduced  by  hydrogen.  It  it 
easily  obtained  in  various  crystalline  forms  by  many  methods.  The  chromates  of  mer- 
cury and  ammonium  give  a  very  convenient  method  for  its  preparation,  because  when 
ignited  they  leave  chromic  oxide  behind.  In  the  first  instance  oxygen  and  mercury  are 
disengaged,  and  in  the  second  case  nitrogen  and  water  :  2Hg2Cr04  =  Cr2O3  +  05  +  4Hg  or 
(NH4)2Cr2O7  =  Cr2O5  +  4H2O  +  N2.  The  second  reaction  is  very  energetic,  and  the  mass 
of  salt  burns  spontaneously  if  the  temperature  be  sufficiently  high.  A  mixture  of  potas- 
sium sulphate  and  chromic  oxide  is  formed  by  heating -potassium  dichrbmate  with  an 
equal  weight  of  sulphur :  K2Cr2O7  +  S  =  K2SO4  +  Cr2O5.  The  sulphate  is  easily  extracted 
by  water,  and  there  remains  a  bright  green  residue  of  the  oxide,  whose  colour  is  more 
brilliant  the  lower  the  temperature  of  the  decomposition.  The  oxide  thus  obtained  ia 
used- as  a  green  pigment  for  china  and  enamel.  The  anhydrous  chromic  oxide  obtained 
from  chromyl  chloride,  Cr02Cl?,  has  a  specific  gravity  of  5'21,  and  forms  almost  black 
crystals,  which  give  a  green  powder.  They  are  hard  enough  to  scratch  glass,  and  have  a 
metallic  lustre.  The  crystalline  form  of  chromic  oxide  is  identical  with  that  of  the  oxide 
of  iron  and  alumina,  with  which  it  is  isomorphous. 

7  bis  The  most  important  of  the  compounds  corresponding  with  chromic  oxide  is  chromic 
chloride,  Cr2Cl6,  which  is  known  in  an  anhydrous  and  in  a  hydrated  form.  It  resembles 
ferric  and  aluminic  chlorides  in  many  respects.  There  is  a  great  difference  between 
the  anhydrous  and  the  hydrated  chlorides  ;  the  former  is  insoluble  in  water,  the  latter 
easily  dissolves,  and  on  evaporation  its  solution  forms  a  hygroscopic  mass  which  is  very 
unstable  and  easily  evolves  hydrochloric  acid  when  heated  with  water.  The  anhydrous 
form  is  of  a  violet  colour,  and  Wbhler  gives  the  following  method  for  its  preparation :  an 
intimate  mixture  is  prepared  of  the  anhydrous  chromic  oxide  with  carbon  and  organic 
matter,  and  charged  into  a  wide  infusible  glass  or  porcelain  tube  which  is  heated  in  a 
combustion  furnace ;  one  extremity  of  the  tube  communicates  with  an  apparatus  generat- 
ing chlorine  which  is  passed  through  several  bottles  containing  sulphuric  acid  in  orde* 


286  PEINCIPLES  OP  CHEMISTRY 

pounds  of  the  oxide  itself,  CrX3  of  Cr2X6,  like  alumina,  which  it 
resembles  in  forming  a  feeble  base  easily  giving  double  and  basic  salts, 
which  are  either  green  or  violet. 

to  dry  it  perfectly  before  it  reaches  the  tube.  On  heating  the  portion  of  the  tube  in 
which  the  mixture  is  placed  and  passing  chlorine  through,  a  slightly  volatile  sublimate  of 
chromic  chloride,  CrCl3  or  Cr2Cl6,  is  formed.  This  substance  forms  violet  tabular 
crystals,  which  may  be  distilled  in  dry  chlorine  without  change,  but  which,  however,  re- 
quire a  red  heat  for  their  volatilisation.  'These  crystals  are  greasy  to  the  touch  and  in- 
soluble in  water,  but  if  they  be  powdered  and  boiled  in  water  for  a  long  time  they  pass 
into  a  green  solution.  Strong  sulphuric  acid  does  not  act  on  the  anhydrous  salt,  or 
only  acts  with  exceeding  slowness,  like  tfater.  Even  aqua  regia  and  other  acids  do  not 
act  on  the  crystals,  and  alkalis  only  show  a  very  feeble  action.  The  specific  gravity  of 
the  crystals  is  2'99.  When  fused  with  sodium  carbonate  and  nitre  they  give  sodium 
chloride  and  potassium  chromate,  and  when  ignited  in  air  they  form  green  chromic  oxide 
and  evolve  chlorine.  On  ignition  in  a  stream  of  ammonia,  chromic  chloride  forms 
sal-ammoniac  and  chromium  nitride,  CrN  (analogous  to  the  nitrides  BN,A1N).  Mosberg  and 
Peligot  showed  that  when  chromic  chloride  is  ignited  in  hydrogen,  it  parts  with  one-third 
of  its  chlorine,  forming  chromous  chloride,  CrCl2 — that  is,  there  is  formed  from  a  com- 
pound corresponding  with  chromic  oxide,  Cr2O3,  a  compound  answering  to  the  suboxide, 
chromous  oxide,  CrO — just  as  hydrogen  converts  ferric  chloride  into  ferrous  chloride  with 
the  aid  of  heat.  Chromous  chloride,  CrCl2,  forms  colourless  crystals  easily  soluble  in 
water,  which  in  dissolving  evolve  a  considerable  amount  of  heat,  and  form  a  blue  liquid, 
capable  of  absorbing  oxygen  from  the  air  with  great  facility,  being  converted  thereby 
into  a  chromic  compound. 

The  blue  solution  of  chromous  chloride  may  also  be  obtained  by  the  action  of  metallic 
zinc  on  the  green  solution  of  the  hydrated  chromic  chloride  ;  the  zinc  in  this  case  takes 
up  chlorine  just  as  the  hydrogen  did.  It  must  be  employed  in  a  large  excess.  Chromic 
oxide  is  also  formed  in  the  action  of  zinc  on  chromic  chloride,  and  if  the  solution  remain 
for  a  long  time  in  contact  with  the  zinc  the  whole  of  the  chromium,  is  converted  into 
chromic  oxychloride.  Other  chromic  salts  are  also  reduced  by  zinc  into  chromous  salts, 
CrX2,  just  as  the  ferric  salts  FeX3  are  converted  into  ferrous  salts  FeX2  by  it.  The 
chromous  salts  are  exceedingly  unstable  and  easily  oxidise  and  pass  into  chromic  salts ; 
hence  the  reducing  power  of  'these  salts  is  very  great.  From  cupric  salts  theyseparate 
cuprous  salts,  from  stannous  salts  they  precipitate  metallic  tin,  they  reduce  mercuric 
salts  into  mercurous  and  ferric  into  ferrous  salts.  Moreover,  they  absorb  oxygen  from 
the  air  directly.  With  potassium  chromate  they  give  a  brown  precipitate  of  chromium 
dioxide  or  of  chromic  oxide,  according  to  the  relative  amounts  of  the  substances  taken  : 
CrO3  +  CrO  =  2CrO2  or  CrO5  +  3CrO  =  2Cr20;.  Aqueous  ammonia  gives  a  blue  precipi- 
t»te,  and  in  the  presence  of  ammoniacal  salts  a  blue  liquid  is  obtained  which  turns  red 
in  the  air  from  oxidation.  This  is  accompanied  by  the  formation  of  compounds  analo- 
gous to  those  given  by  cobalt  (Chapter  XXII.)  A  solution  of  chromous  chloride  with  a 
hot  saturated  solution  of  tedium  acetate,  C2H3Na02,  gives,  on  cooling,  transparent  red 
crystals  of  chromous  acetate,  C4H0CrO4,H2O.  This  salt  is  also  a  powerful  reducing 
agent,  but  may  be  kept  for  a  long  time  in  a  vessel  full  of  carbonic  anhydride, 

The  insoluble  anhydrous  chromic  chloride  Cr(M5  very  easily  passes  into  solution  in 
the  presence  of  a  trace  (0'004)  of  chromous  chloride  CrCl2.  This  remarkable  phe- 
nomenon was  observed  by  Peligot  and  explained  by  Lb'wel  in  the  following  manner: 
chromous  chloride,  as  a  lower  stage  of  oxidation,  is  capable  of  absorbing  both  oxygen 
and  chlorine,  combining  with  various  substances.  It  is  able  to  decompose  many 
chlorides  by  taking  up  chlorine  from  them ;  thus  it  precipitates  mercurous  chloride  from 
a  solution  of  mercuric  chloride,  and  in  so  doing  passes  into  chromic  chloride :  2CrCl2 
+  2HgCl2  =  Cr2Cl6  +  2HgCl.  Let  us  suppose  that  the  same  phenomenon  takes  place 
when  the  anhydrous  chromic  chloride  is  mixed  with  a  solution  of  chromous  chloride,. 
The  latter  will  then  take  up  a  portion  of  the  chlorine  of  the  former,  and  pass  into  a, 


CHROMIUM,   MOLYBDENUM,  TUNGSTEN,  URANIUM,  ETC.      287 

The  reduction  of  chromic  oxide— for  instance,  in  a  solution  by  zinc 
and  sulphuric  acid— leads  to  the  formation  of  chromous  oxide,  CrO,  and 

soluble  hydrate  of  chromic  chloride  (hydrochloride  of  oxide  of  chromium),  and  the 
original  anhydrous  chromic  chloride  will  pass  into  chromous  chloride.  The  cliromoua 
chloride  re-formed  in  this  manner  will  then  act  on  a  fresh  quantity  of  the  chromic 
chloride,  and  in  this  manner  transfer  it  entirely  into  solution  as  hydrate.  This  view  is 
confirmed  by  the  fact  that  other  chlorides,  capable  of  absorbing  chlorine  like  chromous 
chloride,  also  induce  the*olution  of  the  insoluble  chromic  chloride — for  example,  ferrous 
chloride,  FeCl2,  and  cuprous  chloride.  The  presence  of  zinc  also  aids  the  solution  of 
chromic  chloride,  owing  to  its  converting  a  portion  of  it  into  chromous  chloride^  The 
solution  of  chromic  chloride  in  water  obtained  by  these  methods  is  perfectly  identical 
with  that  which  is  formed  by  dissolving  chromic  hydroxide  in  hydrochloric  acid.  On 
evaporating  the  green  solution  obtained  in  this  manner,  it  gives  a  green  mass,  con- 
taining water...  On  further  heating  it  leaves  a  soluble  chromic  oxychloride,  and  when 
ignited  it  first  forms  an  insoluble  oxychloride  and  then  chromic  oxide ;  but  no  anhy- 
drous chromic  chloride,  Cr2Cl6)  is  formed  by  heating  the  aqueous  solution  of  chromic 
chloride,  which  forms  an  important  fact  in  support  of  the  view  that  the  green  solu- 
tion of  chromic  chloride  is  nothing  else  but  hydrochloride  of  oxide  of  chromium.  At 
100°  the  composition  of  the  green  hydrate  is  Cr2Cl6,9H2O,  and  on  evaporation  at  the 
ordinary  temperature  over  H2S04  crystals  are  obtained  with  12  equivalents  of  water ; 
the  red  mass  obtained  at  120°  contains  Cr2O3,4Cr2Cl6,24H2O.  The  greater  portion  of 
it  is  soluble  in  water,  like  the  mass  which  is  formed  at  150°  The  latter  contains 
Cr203,2Cr2Cl6,9H2O  =  S(Cr2OCl4,3H2O) — that  is,  it  presents  the  same  composition  as 
chromic  chloride  in  which  one  atom  of  oxygen  replaces  two  of  chlorine.  And  if  the 
hydrate  of  phromic  chloride  be  regarded  as  Cr2O3,6HCl,  the  substance  which  is  ob- 
tained should  be  regarded  as  Cra03,4HCl  combined  with  water,  H2O.  The  addition 
of  alkalis — for  example,  baryta— to  a  solution  of  chromic  chloride  immediately  produces 
a  precipitate,  which,  however,  re-dissolves  on  shaking,  owing  to  the  formation  of  one  of 
the  oxychlorides  just  mentioned,  which  may  be  regarded  as  basic  salts.  Thus  we  may 
represent  the  product  of  the  change  produced  on  chromic  chloride  under  the  influence 
of  water  and  heat  by  the  following  formulas  .  first  Cr203,6HCl  or  OoCl^SHoO  is  formed, 
then  Cr203)4HCl,H2O  or  Cr2OCl4,3H2O,  and  lastly  Cr2O3,2HCl,2H26  or  Cr202Cl2,3H2O. 
In  all  three  cases  there  are  2  equivalents  of  chromium  to  at  least  3  equivalents  of 
water.  These  compounds  may  be  regarded  as  being  intermediate  between  chromic 
hydroxide  and  chloride ;  chromic  chloride  is  Cr2Clo,  the  first  oxychloride  Cr2(OH)oClj, 
the  second  Cr2(OH)4Cl2,  and  the  hydrate  Cr2(OH)6 — that  is,  the  chlorine  is  replaced  by 
hydroxyl. 

It  is  very  important  to  remark  two  circumstances  in  respect  to  this .  (1)  That  the 
whole  of  the  chlorine  in  the  above  .compounds  is  not  precipitated  from  their  solutions 
by  silver  nitrate;  thus  the  normal  salt  of  the  composition  Cr2Clc,9H20  only  gives  up 
two-thirds  of  its  chlorine  ,  therefore  Peligot  supposes  that  the  normal  salt  contains  the 
oxychloride  combined  with  hydrochloric  acid:  Cr2Cl6  +  2H2O  =  Cr2O2Cl2,4HCl,  and  that 
the  chlorine  held  as  hydrochloric  acid  reacts  with  the  silver,  whilst  that  held  in  the 
oxychloride  does  not  enter  into  reaction,  just  as  we  observe  a  very  feebly-developed 
faculty  for  reaction  in  the  anhydrous  chromic  chloride;  and  (2)  if  the  green  aqueous 
solution  of  CrCl3  be  left  to  stand  for  some  time,  it  ultimately  turns  violet ;  in  this  form 
the  whole  of  the  chlorine  is  precipitated  by  AgN03,  whilst  boiling  re-converts  it  into  the 
green  variety.  Lowel  obtained  the  violet  solution  of  hydrochloride  of  chromic  oxide  by 
decomposing  the  violet  chromic  sulphate  with  barium  chloride.  Silver  nitrate  precipi- 
tates all  the  chlorine  from  this  violet  modification ;  but  if  the  violet  solution  be  boiled 
and  so  converted  into  the  green  modification,  silver  nitrate  then  only  precipitates  a  portion 
of  the  chlorine. 

Recoura  (1890-1893)  obtained  a  crystallohydrate  of  violet  chromium  sulphate, 
with  18  or  15  H2O  By  boiling  absolution  of  this  crystallohydrate,  he 


288  PRINCIPLES  OF  CHEMISTRY 

its  salts,  CrX2,  of  a  blue  colour  (see  Notes  7  and  7  b!s).     The  further 

converted  it  into  the  green  salt,  which,  when  treated  with  alkalis,  gave  a  precipitate 
of  Cr2O3,2H2O,  soluble  in  2H2S04  (and  not  3),  and  only  forming  the  basic  salt, 
Cr2(OH)2(SO4)2.  He  therefore  concludes  that  the  green  salts  are  basic  salts.  The 
cryoscopic  determinations  made  by  A.  Speransky  (1892)  and  Marchetti  (1892)  give  a 
greater  'depression1  for  the  violet  than  the  green  salts,  that  is,  indicate  a  greater 
molecular  weight  for  the  green  salts.  But  as  Etard,  by  heating  the  violet  sulphate  to 
100°,  converted  it  into  a  green  salt  of  the  same  composition,  but  with  a  smaller  amount 
of  H2O,  it  follows  that  the  formation  of  a  basic  salt  alone  is  insufficient  to  explain  the 
difference  between  the  green  and  violet  varieties,  and  this  is  also  shown  by  the  fact  that 
BaCl,  precipitates  the  whole  of  the  sulphuric  acid  of  the  violet  salt,  and  only  a  portion 
of  that  of  the  green  salt.  A.  Speransky  also  showed  that  the  molecular  electro- 
conductivity  of  the  green  solutions  is  less  than  that  of  the  violet.  It  is  also  known  that 
the  passage  of  the  former  into  the  latter  is  accompanied  by  an  increase  of  volume,  and, 
according  to  Recoura,  by  an  evolution  of  heat  also. 

Piccini's  researches  (1894)  throw  an  important  light  upon  the  peculiarities  of  the 
green  chromium  trichloride  (or  chromic  chloride) ;  he  showed  (1)  that  AgF  (in  contra- 
distinction to  the  other  salts  of  silver)  precipitates  all  the  chlorine  from  an  aqueous 
solution  of  the  green  variety  ;  (2)  that  solutions  of  green  CrCl3,6H2O  in  ethyl  'alcohol 
i  and  acetone  precipitate  all  their  chlorine  when  mixed  with  a  similar  solution  of  AgNO3  ; 
(8)  that  the  rise  of  the  boiling-point  of  the  ethyl  alcohol  and  acetone  green  solutions  of 
CrCl3,6H2O  (Chapter  VII.,  Note  27  bis)  shows  that  i  in  this  case  (as  in  the  aqueous  solu- 
tions of  MgSO4  and  HgCl2)  is  nearly  equal  to  1,  that  is,  that  they  are  like  solutions  of 
non-conductors ;  (4)  that  a  solution  of  green  CrCl3  in  methyl  alcohol  at  first  precipitates 
about  I  of  its  chlorine  (an  aqueous  solution  about  §)  when  treated  with  AgNO3,  but 
after  a  time  the  whole  of  the  chlorine  is  precipitated ;  and  (5)  that  an  aqueous  solution 
of  the  green  variety  gradually  passes  into  the  violet,  while  a  methyl  alcoholic  solution 
preserves  its  green  colour,  both  of  itself  and  also  after  the  whole  of  the  chlorine  has 
been  precipitated  by  AgNO5.  If ,  however,  in  an  aqueous  or  methyl  alcholic  solution 
only  a  portion  of  the  chlorine  be  precipitated,  the  solution  gradually  turns  violet. 
In  my  opinion  the  general  meaning  of  all  these  observations  requires  further  elucidation 
and  explanation,  which  should  be  in  harmony  with  the  theory  of  solutions.  Recoura, 
moreover,  obtained  compounds  of  the  green  salt,  Cr2(SO4)3,  with  1,  2,  and  3  molecules  of 
'H2SO4,  K2SO4,  and  even  a  compound  Cr2(SO4)3H2Cr04.  By  neutralising  the  sulphuric 
acid  of  the  compounds  of  Cr2(S04)3  and  H2SO4  with  caustic  soda,  Recoura  obtained  an 
.evolution  of  83  thousand  calorjes  per  each  2NaHO,  while  free  H2SO4  only  gives  30'8 
thousand  calories.  Recoura  is  of  opinion  that  special  cJiromo  sulphuric  acids,  for 
instance  (CrSO4)H2SO4  =  iCr2(SO4)3H2SO4,  are  formed.  With  a  still  larger  excess  of 
sulphuric  acid,  Recoura  dbtained  salts  containing  a  still  greater  number  of  sulphuric 
acid  radicles,  but  even  this  method  does  not  explain  the  difference  between  the  green  and 
violet  salts. 

These  facts  must  naturally  be  taken  into  consideration  in  order  to  arrive  at 
any  complete  decision  as  to  the  cause  of  the  different  modifications  of  the  chromic  salts, 
We  may  observe  that  the  green  modification  of  chromic  chloride  does  not  give  double 
salts  with  the  metallic  chlorides,  whilst  the  violet  variety  forms  compounds  Cr2Cl6,2RCl 
(where  R  =  an  alkali  metal),  which  are  obtained  by  heating  the  chromates  with  an  excess 
of  hydrochloric  acid  and  evaporating  the  solution  until  it  acquires  a  violet  colour.  As 
the  result  of  all  the  existing  researched  On  the  green  and  violet  chromic  salts,  it  appears 
to  me  most  probable  that  their  difference  is  determined  by  the  feeble  basic  character  of 
chromic  oxide,  by  its  faculty  of  giving  basic  salts,  and  by  the  colloidal  properties  of  its 
hydroxide  (these  three  properties  are  mutually  connected),  and  moreover,  it  seems  to  me 
that  the  relation  between  the  green  and  violet  salts  of  chromic  oxide  best  answers  to  the 
relation  of  the  purpureo  to  the  luteo  cobaltic  salts  (Chapter  XXII.,  Note  85).  This 
subject  cannot  yet  be  considered  as  exhausted  (see  Note  7). 

We  may  here  observe  that  with  tin  the  chromic  salts,  CrX3,  give  at  low  temperatures 


CHROMIUM.  MOLYBDENUM,  TUNGSTEN,   URANIUM,   ETC      289 

reduction8  of  oxide  of  chromium  and  its  corresponding  compounds 
gives  metallic  chromium.  Deville  obtained  it. (probably  containing 
carbon)  by  reducing  chromic  oxide  with  carbon,-  at  a  temperature  near 
the  melting  point  of  platinum,  about;  1750°,  but  the  metal  itself  does 
not  fuse  at  this  temperature.  Chromium  has  a  steel-grey  colour  and  is 
very  hard  (sp.  gr.  5*9),  takes  a  good  polish,  and  dissolves  in  hydro- 
chloric acid,  but  cold  dilute  sulphuric  and  nitric  acids  have  no  action 
upon  it.  Bunsen  obtained  metallic  chromium  by  decomposing  a  solution 
of  chromic  chloride,  Cr2Cl6,  by  a  galvanic  current,  as  scales  of  a  grey 
colour  (sp.  gr.  7'3).  Wohler  obtained  crystalline  chromium  by  igniting 
a  mixture  of  the  anhydrous  chromic  chloride  Cr2ClG  (see  Note  7  bis) 
with  finely-divided  zinc,  and  sodium  and  potassium  chlorides,  at  the 
boiling-point  of  zinc.  When  the  resultant  mass  has  cooled  the  zinc  may 

CrX2  and  SnX2,  whilst  at  high  temperatures,  on  the  contrary,  CrX2  reduces  the  metal 
from  its  salts  SnX2.  The -reaction,  therefore,  belongs  to  the  class  of  reversible  reac- 
tions (Beketoff). 

Poulenc  "obtained  anhydrous  CrF3  (sp.  gr.  S'78)  and  CrF2  (sp.  gr.  4'11)  by  the 
action  of  gaseous  HF  upon  CrCl2.  A  solution  of  fluoride  of-  chromium  is  employed  as  a> 
mordant  in  dyeing.  Recoura  (1890)  obtained  green  and  violet  varieties  of  Cr2Br6,6H2O. 
The  green  variety  can  only  be  kept  in  the  presence  of  an  excess  of  HBr  In  the  solution , 
If  alone  its  solution  easily  passes  into  the  violet  variety  with  evolution  of  heat. 

8  The  reduction  of  metallic  chromium  proceeds  with  comparative  ease  in  aqueoua 
solutions.  Thus  the  action  of  sodium  amalgams  upon  a  strong  solution  of  Cr2Cl6  gives 
(first  CrClj)  an  amalgam  of  chromium  from  which  the  mercury  may  be  easily  driven  off 
by  heating  (in  hydrogen  to  avoid  oxidation),  and  there  remains  a  spongy  mass  of  easily 
oxidizable  chromium.  Plaset  (1891),  by  passing  an  electric  current  through  a  solution  of 
chrome  alum  mixed  with  a  small  amount  of  H2S04  and  K2SO4,  obtained  hard  scales  of 
chromium  of  a  bluish- white  colour  possessing  great  hardness  and  stability  (under  the 
action  of  water,  air,  and  acids).  Glatzel  (1890)  reduced  a  mixture  of  2KC1  +  Cr2Clt;  by 
heating  it  to  redness  with  shavings  of  magnesium.  The  metallic  chromium  thus 
obtained  has  the  appearance  of  a  fine  light-grey  powder  which  is  seen  to  be  crystalline 
Under  the  microscope ;  its  sp.  gr.  at  16'  is  6'7284.  It  fuses  (with  anhydrous  borax)  only  at 
the  highest  temperatures,  and  after  fusion  presents  a  silver-white  fracture.  The  strongest 
magnet  has  no  action  upon  it. 

Moissan  (1893)  obtained  chromium  by  reducing  the  oxide  Cr3O3  with  carbon  in  the 
electrical  furnace  (Chapter  VIII.,  Note  17)  in  9-10  minutes  with  a  current  of  350  amperes  and 
60  volts.  The  mixture  of  oxide  and  carbon  gives  a  bright  ingot  weighing  100-110  grams. 
A  current  of  100  amperes  and^SO  volts  completes  the  experiment  upon  a  smaller  quantity 
.pf  material  in  15  minutes ;  a  current  of  80  amperes  and  50  volts  gave  an  ingot  of  10  grama 
In  80-40  minutes.  The  resultant  carbon  alloy  is  more  or  less  rich  in  chromium 
(from  87-87-91-7  p.c.).  To  obtain  the  metal  free  from  carbon,  the  alloy  is  broken  into 
large  lumps,  mixed  with  oxide  of  chromium,  put  into  a  crucible  and  covered  with  a 
layer  of  oxide.  This  mixture  is  then  heated  in  the  electric  furnace  and  the  pure  metal 
is  obtained.  This  reduction  can  also  be  carried  on  with  chrome  iron  ore  FeOCr2Os 
which  occurs  in  nature.  In  this  case  a  homogeneous  alloy  of  iron  and  chromium 
is  obtained  If  this  alloy  be  thrown  in  lumps  into  molten  nitre,  it  forms  insoluble 
eesquioxide  of  iron  and  a  soluble  alkaline  chromate.  This  alloy  oj  iron  and 
chromium  dissolved  in  molten  steel  (chrome  steel)  renders  it  hard  and  tough,  so  thai 
Such  eteel  has  many  valuable  applications.  The  alloy,  containing  about  8  p.c.  Cr  and 
about  1-8  p.c.  carbon,  is«even  harder  than  the  ordinary  kinds  of  tempered  steel  and  has  a, 
fine  granular  fracture.  The  usual  mode  of  preparing  the  ferrochromes  for  adding  to 
•teel  is  by  fusing  powdered  chrome  iron  ore  under  fluxes  in  a  graphite  crucible. 


290 


PRINCIPLES  OF  CHEMISTRY 


be  dissolved  in  dilute  nitric  acid,  and  grey  crystalline  chromium  (sp.  gr. 
6'81)  is  left  behind.  Fremy  also  prepared  crystalline  chromium  by  the 
action  of  the  vaptmr  of  sodium  011  anhydrous  chromic  chloride  in  a 
stream  of  hydrogen,  using  the  apparatus  shown  in  the  accompanying 
drawing,  and  placing  the  sodium  and  the  chromic  chloride  in  separate 
porcelain  boats.  The  tube  containing  these  boats  is  only  heated  when 
it  is  quite  full  of  dry  hydrogen.  The  crystals  of  metallic  chromium 
obtained  in  the  tube  are  grey  cubes  having  a  considerable  hardness  and 
withstanding  the  action  of  powerful  acids,  and  even  of  aqua  regia. 
The  chromium  obtained  by  Wbhler  by  the  action  of  a  galvanic  current 
is,  on  the  contrary,  acted  on  under  these  conditions.  The  reason  of 
this  difference  must  be  looked  for  in  the  presence  of  impurities,  and  in 
the  crystalline  structure.  But- in  any  case,  among  the  properties  of 


;^5§:J;^§§§:%l%J%i%P^^ 

•omic  chloride 


FIG   92.—  Apparatus  for  the  preparation  of  metallic  chromium  by  igniting  chr 
and  sodium  iu  a  stream  of  hydrogen. 


metallic  chromium,  the  following  may  be  considered  established  :  it  is 
white  in  colour,  with  a  specific  gravity  of  about  6  -7,  is  extremely  hard 
in  a  crystalline  form,  is  not  oxidised  by  air  at  the  ordinary  temperature, 
and  with  carbon  it  forms  alloys  like  cast  iron  an4  steel. 

The  two  analogues  of  chromium,  molybdenum  and  tungsten  (or  wol- 
fram), are  of  still  rarer  occurrence  in  nature,  and  form  acid  oxides,  RO3, 
which  are  still  less  energetic  than  Cr03.  Tungsten  occurs  in  the  some- 
what rare  minerals,  scheelite,  CaW04,  and  wolfram  ;  the  latter  being  an 
isomorphous  mixture  of  the  normal  tungstates  of  iron  and  manganese, 
(MnFe)WO4.  Molybdenum  is  most  frequently  met  with  as  molybdenite, 
MoS2,  which  presents  a  certain  resemblance  to  graphite  in  its  physical 
properties  and  softness.  It  also  occurs,  but  much  more  rarely,  as  a 
yellow  lead  ore,  PbMoO4.  In  both  these  forms  molybdenum  occurs  in 
the  primary  rocks,  in  granites,  gneiss,  &c.,  and  in  iron  and  copper  ores 


CHROMIUM,  MOLYBDENUM  TUNGSTEN,  URANIUM,  ETC.   291 

in  Saxony,  Sweden,  and  Finland.  Tungsten  ores  are  sometimes  met 
with  in  considerable  masses  in  the  primary  rocks  of  Bohemia  and 
Saxony,  and  also  in  England,  America,  and  the  Urals.  The  pre- 
liminary treatment  of  the  ore  is  very  simple  ;  for  example,  the  sulphide, 
MoS2,  is  roasted,  and  thus  converted  into  sulphurous  anhydride  and 
molybdic  anhydride,  Mo03,  which  is  then  dissolved  in  alkalis,  generally 
in  ammonia.  The  ammonium  molybdate  is  then  treated  with  acids, 
when  the  sparingly  soluble  molybdic  acid  is  precipitated.  Wolfram  is 
treated  in  a  different  manner.  Most  frequently  the  finely -ground  ore  is 
repeatedly  boiled  with  hydrochloric  and  nitric  acids,  and  the  resultant 
solutions  (of  salts  of  manganese  and  iron)  poured  off,  until  the  dark 
brown  mass  of  ore  disappears,  whilst  the  Dungstic  acid  remains,  mixed 
with  silica,  as  an  insoluble  residue  ;  it  is  treated  also  with  ammonia, 
and  is  thus  converted  into  soluble  ammonium  tungstate,  which  passes 
into  solution  and  yields  tungstic  acid  when  treated  with  acids.  This 
hydrate  is  then  ignited,  and  leaves  tungstic  anhydride.  The  general 
character  of  molybdic  and  tungstic  anhydrides  is  analogous  to  that  of 
chromic  anhydride ;  they  are  anhydrides  of  a  feebly  acid  character, 
which  easily  give  polyacid  salts  and  colloid  solutions.8  bis 

s.bti  The  atomic  composition  of  the  tungsten  and  molybdenum  compounds  is  taken  as 
being  identical  with  that  of  the  compounds  of  sulphur  and  chromium,  because  (1)  both 
these  metals  give  two  oxides  in  which  the  amounts  of  oxygen  per  given  amount  of  metal 
etand  in  the  ratio  2:  3;  (2)  the  higher  oxide  is  of  the  latter  kind,  and,  like  chromic 
and  sulphuric  anhydrides, it  has  an  acid  character,  (3)  certain  of  the molybdates  are  iso- 
morphous  with  the  sulphates ;  (4)  the  specific  heat  of  tungsten  is  0'0334,  cgnsequently 
the  product  of  the  atomic  weight  and  specific  heat  is  6' 15,  like  that  of  the  other  elements 
—it  is  the  same  with  molybdenum,  96'0  x  0  0722  —  6'9  ;  (5)  tungsten  forms  with  chlorine 
not  only  compounds  WC16)  WC15,  and  WOC14,  but  also  WO2C12)  a  volatile  substance  the 
analogue  of  chromyl  chloride,  Cr02Cl2,  and  sulphuryl  chloride,  S02C12.  Molybdenum 
gives  the  chlorine  compounds,  MoCl2,  MoCl5(?),  MoCl4  (fuses  at  194°,  boils  at  268° ; 
according  to  Debray  it  contains  MoCl5),  MoOCl4,  Mo02Cl2,  and  Mo02(OH)Cl.  The 
existence  of  tungsten  hexachloride,  WC16,  is  an  excellent  proof  of  the  fact  that  the  type 
6X<j  appears  in  the  analogues  of  suhohur  as  in  S05  ;  (6)  the  vapour  density  accurately 
determined  for  the  chlorine  compounds  MoCl4,  WC1C,  WC15,  WOC14  (Roscoe)  leaves  no 
doubt  as  to  the  molecular  composition  of  the  compounds  of  tungsten  and  molybdenum, 
lor  the  observed  and  calculated  results  entirely  agree. 

Tungsten  is  sometimes  called  scheele  in  honour  of  Scheele,  who  discovered  it  in  1781 
and  molybdenum  in  1778.  Tungsten  is  also  known  as  wolfram ;  the  former  name  was  the 
name  given  to  it  by  Scheele,  because  he  extracted  it  from  the  mineral  then  known  as 
tungsten  and  now  called  scheelite,  CaWO4.  The  researches  of  Roscoe,  Blomstrand  and 
ethers  have  subsequently  thrown  considerable  light  on  the  whole  history  of  the  compounds 
of  molybdenum  and  tungsten. 

The  ammonium  salts  of  tungsten  and  molybdic  acids  when  ignited  leave  the  anhy- 
drides, which  resemble  each  other  in  many  respects.  Tungsten  anhydride,  WOs,  is  a 
yellowish  substance,  which  only  fuses  at  a  strong  heat,  and  has  a  sp  gr.  of  6'8.  It  is 
insoluble  both  in  water  and  acid,  but  solutions  of  the  alkalis,  and  even  of  the  alkali  car"- 
bonates,  dissolve  it,  especially  when  heated,  forming  alkaline  salts.  Molybdic  anhydride, 
MoOs,  is  obtained  by  igniting  the  acid  (hydrate)  or  the  ammonium  salt,  and  forms  a 
white  mass  which  fuses  at  a  red  heat,  and  solidifies  to  a  yellow  crystalline  mass  of  sp.  gr. 


292  PRINCIPLES  OF  CHEMISTRY 

Hydrogen  (which  does  not  directly  form  compounds  with  O,  Mo, 

4-4 ;  whilst  on  further  heating  in  open  vessels  or  in  a  stream  of  air  this  anhydride 
sublimes  in  pearly  scales— this  enables  it  to  be  obtained  in  a  tolerably  pure  state.  Water 
dissolves  it  in  small  quantities — namely,  1  part  requires  600  parts  of  water  for  its  solution. 
The  hydrates  of  molybdic  anhydride  are  soluble  also  in  acids  (a  hydrate,  H.^MoO.,,  is 
obtained  from  the  nitric  acid  solution  of  the  ammonium  salt),  which  forms  one  of  their 
distinctions  from  the  tungstic  acids.  But  after  ignition  molybdic  anhydride  is  insoluble 
in  acids,  like  tungstic  anhydride ;  alkalis  dissolve  this  anhydride,  easily  forming  molybdates. 
Potassium  bitartrate  dissolves  the  anhydride  with  the  aid  of  heat.  None  of  the  acids 
yet  considered  by  us  form  so  many  different  salts  with  one  and  the  same  base  (alkali)  as 
molybdic  and  tungstic  acids.  The  composition  of  these  salts,  and  their  properties  also, 
vary  considerably.  The  most  important  discovery  in  this  respect  was  made  by  Margue- 
rite and  Laurent,  who  showed  that  the  salts  which  contain  a  large  proportion  of  tungstic 
acid  are' easily  soluble  in  water,  and  ascribed  this  property  to  the  fact  that  tungstic  acid 
may  be  obtained  in  several  states.  The  common  tungstates,  obtained  with  an  excess  of 
alkali,  have  an  alkaline  reaction,  and  on  the  addition  of  sulphuric  or  hydrochloric  acid 
first  deposit  an  acid  salt  and  then  a  hydrate  of  tungstic  acid,  which  is  insoluble  both  in 
water  and  acids ;  but  if  instead  of  sulphuric  or  hydrochloric  acids,  we  add  acetic  or  phos- 
phoric acid,  or  if  the  tungstate  be  saturated  with  a  fresh  quantity  of  tungstic  acid,  which, 
may  be  done  by  boiling  the  solution  of  the  alkali  salt  with  the  precipitated  tungstic  acid, 
a  solution  is  obtained  which,  on  the  addition  of  sulphuric  or  a  similar  acid,  does  not 
give  a  precipitate  of  tungstic  acid  at  the  ordinary  or  at  higher  temperatures.  The  solution 
then  contains  peculiar  salts  of  tungstic  acid,  and  if  there  be  an  excess  of  acid  it  also- 
contains  tungstic  acid  itself ;  Laurent,  Riche,  and  others  called  it  metatungslic  acid,  and 
it  is  still  known  by  this  name.  Those  salts  which  with  acids  immediately  give  the  in, 
soluble  tungstic  acid  have  the  composition  R2W04,  RHWO4,  whilst  those  which  give 
the  soluble  metatungstic  acid  contain  a  far  greater  proportion  of  the  acid  elements* 
ficheibler  obtained  the  (soluble)  metatungstic  acid  itself  by  treating  the  soluble  barium 
(meta)  tetratungstate,  BaO,4\VO3,  with  sulphuric  acid.  Subsequent  research  showed  th& 
existence  of  a  similar  phenomenon  for  molybdic  acid.  There  is  no  doubt  that  this  is  a 
case  of  colloidal  modifications. 

Many  chemist^  have  worked  on  the  various  salts  formed  by  molybdic  and  tungstio 
ftcids.  The  tungstates  have  been  investigated  by  Marguerite,  Laurent,  Marignac,. 
Riche,  Scheibler,  Ahthon,  and  others.  The  molybdates  were  partially  studied  by  the 
same  chemists,  but  chiefly  by  Struve  and  Svanberg,  Delafontaine,  and  others.  It  appear* 
that  for  a  given  amount  of  base  the  salts  contain  one  to  eight  equivalents  of  molybdic  or 
tungstic  anhydride  ;  i.e.  if  the  base  have  the  composition  RO,  then  the  highest  properties 
of  base  will  be  contained  by  the  salts  of  the  composition  ROWO3  or  ROMo05 — that  is,  by 
those  salts  which  correspond  with  the  normal  acids  H2WO4  and  H^Mo04,  of  the.  same  nature 
as  sulphuric  acid ;  but  there  also  exist  salts  of  the  composition  RO,2\V05,  KO,3WO3 
RO,8WO3.  The  water  contained  in  the  composition  of  many  of  the  acid  salts  is  often 
not  taken  into  account  in  the  above.  The  properties  of  the  salts  holding  different  pro- 
portions of  acids  vary  considerably,  but  one  salt  may  be  converted  into  another  by  the 
addition  of  acid  or  base  with  great  facility,  and  the  greater  the  proportion  of  the  elements 
of  the  acid  in  a  salt,  the  more  stable,  within  a  certain  limit,  is  its  solution  and  the  salt 
itself. 

The  most  common  ammonium  molybdate  has  the  composition  (NH4HO)6,H2O,7MoOs 
(or,  according  to  Marignac  and  others,  NH4HMoO4),  and  is  prepared  by  evaporating  an 
ammoniacal  solution  of  molybdic  acid.  It  is  used  in  the  laboratory  for  precipitating 
phosphoric  acid,  and  is  purified  for  this  purpose  by  mixing  its  solution  with  a  small 
quantity  of  magnesium  nitrate,  in  order  to  precipitate  any  phosphoric  acid  present,  filter- 
ing, and  then  adding  nitric  acid  and  evaporating  to  dryness.  A  pure  ammonium  molyb- 
date free  from  phosphoric  acid  may  then  be  extracted  from  the  residue. 

Phosphoric  acid  forms  insoluble  compounds  with  the  oxidea  of  uranium  and  iron, 
tin,  bismuth,  etc.,  having  feeble  basic  and  even  acid  properties.  This  perhaps  depends- 


CHROMIUM,   MOLYBDENUM,   TUNGSTEN,  UBANIUM,   ETC.      293 
and  W)  reduces  molybdic.  and  tungstic  anhydride  at  a  red  heat ;  and 

On  the  fact  that  the  atoms  cf  hydrogen   in  phosphoric   acid    are  of   a  very  different 
character,  as  we  saw  above.    Those  atoms  of  hydrogen  which  are  replaced  with  difficulty 
by  ammonium,   sodium,   &c.,  are   probably  easily   replaced  by  feebly  energetic  acid 
groups — that  is,  the  formation  of  particular  complex  substances  may  be  expected  to 
take  place  nt  the  expense  of  these  atoms  of  the  hydrogen  of  phosphoric  acid  and  of 
certain   feeble   metallic  acids;    and  these   substances  will  still  be  acids,  because  the 
hydrogen  of  the  phosphoric  acids  and  metallic  acids,  which  is  easily  replaced  by  metals, 
is  not  removed  by  their  mutual  combination,  but  remains  in  the  resultant  compound. 
Such  a  conclusion  is  verified  in  the  phosphomolybdic  acids  obtained  (1888)  by  Debray 
If  a  solution  of  ammonium  molybdate  be  acidified,  and  a  small  amount  of  a  solution  (it 
Way  be  acid)  containing  orthophosphoric  acid  or  its  salts  be  added  to  it  (so  that  there  are 
ftt  least  40  parts  of  molybdic  acid  present  to  1  part  of  phosphoric  acid),  then  after  a  period 
of  twenty- four  hours  the  whole  of  the  phosphoric  acid  is  separated  as  a  yellow  precipitate, 
containing,  however,   not  more  than  3  to  4  p.c.  of  phosphoric  anhydride,  about  3  p.c.  of 
ammonia,  about  90  p.c.  of  molybdic  anhydride,  and  about  4  p.c.  of  water.     The  formation 
of  tin's  precipitate  is  so  distinct  and  so  complete  that  this  method  is  employed  for  the  dis- 
covery and  separation  of  the  smallest  quantities  of  phosphoric  acid.     Phosphoric  acid  was 
found  to  be  present  in  the  majority  of  rocks  by  this  means.      The  precipitate  is  soluble 
In  ammonia  and  its  salts,  in  alkalis  and  phosphates,  but  is  perfectly  insoluble  in  nitric, 
Sulphuric,  and  hydrochloric  acids  in  the  presence  of  ammonium  molybdate.     The  compo- 
sition of  the  precipitate  appears  to  vary  under  the  conditions  of  its  precipitation,  but  its 
nature  became  clear  when  the  acid  corresponding  with  it  was  obtained.     If  the  above- 
described  yellow  precipitate  be- boiled  in  aqua  regia,   the  ammonia  is  destroyed,  and 
an  acid  is  obtained  in  solution,  which,  when  evaporated  in  the  air,  crystallises  out  in  yellow 
oblique  prisms  of  approximately  the  composition  P2O5,20MoO3,26H2O     Such  an  unusual 
proportion  of  component  parts  is  explained  by  the  above-mentioned  considerations.     We 
saw  above  that  molybdic  acid  easily  gives  salts  R2OnMo03mH20,  which  we  may  imagine 
to  correspond  to  a  hydrate  MoO2(HO)2nMo05mH2O.    And  suppose  that  such  a  hydrate 
reacts   on   orthophosphoric-  acid,   forming  water  and  compounds  of  the  composition 
Mo02(HPO4)?jMoO3??iH2O  or  MoO.^HsPOJ.^MoOsrnHoO  ;  this  is  actually  the  composi- 
tion of  phosphomolybdic  acid.    Probably  it  contains  a  portion  of  the  hydrogen  replaceable 
by  metals  of  both  the  acids  H3PO4  and  of  H2Mo04.    The   crystalline  acid  above  ia 
probably  H.-,MoPO7)9Mo03,12H./).    This  acid   is   really  tribasic,  because  its  aqueous 
solution  precipitates  salts  of  potassium,   ammonium,   rubidium  (but  not  lithium  and 
sodium)   from  acid  solutions,    and  gives  a  yellow    precipitate  of   the    composition 
E3MoPO7,9Mo05,8H20,  where  R  =  NH4.    Besides  these,  salts  of  another  composition 
may  be  obtained,  as  would  be  expected  from  the  preceding.    These  salts  are  only  stable 
in  acid  solutions  (which  is  naturally  due  to  their  containing  an  excess  of  acid  oxides), 
whilst  under  the  action  of  alkalis  they  give  colourless  phosphomolybdates  of  the  compo- 
sition R3MoP05,MoO2,3H20.    The  corresponding  salts  of  potassium,  silver,  ammonium, 
are  easily  soluble  in  water  and  crystalline. 

Phosphomolybdic  acid  is  an  example  of  the  complex  inorganic  acids  first  obtained 
byMarignac  and  afterwards  generalised  and  studied  in  detail  by  Gibbs.  We  shall 
afterwards  meet  with  several  examples  of  such  acids,  and  we  will  now  turn  attention  to 
the  fact  that  they  are  usually  formed  by  weuk  polybasic  acids  (boric,  silicic,  molybdic, 
&c.),  and  in  certain  respects  resemble  the  cobaltic  and  such  similar  complex  compounds, 
with  which  we  shall  become  acquainted  in  the  following  chapter.  As  an  example  we  will 
here  mention  certain  complex  compounds  containing  molybdic  and  tungstic  acids,  aathey 
will  illustrate  the  possibility  of  a  considerable  complexity  in  the  composition  of  salts. 
The  action  of  ammonium  molybdate  upon  a  dilute  solution  of  purpureo-cobaltic  salts  (see 
Chapter  XXII.)  acidulated  with  acetic  acid  gives  a  salt  which  after  drying  at  100°  has  the 
composition  Co.2O310NH37MoO38H2O.  After  ignition  this  salt  leaves  a  residue  having  the 
composition  2CoO7MoO.v  An  analogous  compound  is  also  obtained  for  tungstic  acid,  having 
'the  composition  Co^lONHslOWOjOH.^  In  this  case  after  ignition  there  remains  a  salt 


294  PRINCIPLES  OF  CHEMISTRY 

this  forms  the  means  of  obtaining  metallic  molybdenum  and  tungsten, 

of  the  composition  CoO5WO5  (Carnbt,  1889).  Professor  Kournakoff,  by  treating  a  solution  of 
potassium  and  sodium  molybdates,  containing  a  certain  amount  of  suboxide  of  cobalt,  with 
bromine  obtained  salts  having  the  composition  :  3K2OCo.iO312MoO320HaO  (light  green) 
and  SK^OCojOjlOMoslOHaO  (dark  green) .  Pechard  (1893)  obtained  salts  of  the  four  complex 
phosphotungstic  acids  by  evaporating  equivalent  mixtures  of  solutions  of  phosphoric  acid 
and  metatungstic  acid  (see  further  on) :  phosphotrimetatungstic  acid  P3O512W0548H.iOy 
phosphotetrametatungstic  acid  P20516WO569H3O,  phosphopentametatungstio  acid 
P2O520WO3H2O,  and  phosphohexametatungstic  acid  P2O524WO359H2O.  Kehrmano 
and  Frankel  described  still  more  complex  salts,  such  as :  3Ag2O4BaOP2O;,22W03H2O, 
6Ba02K2OP2O322W0348H2O.  Analogous  double  salts  with  22W03  were  also  obtained 
with  KSr,  KHg,  BaHg,  and  NH4Pb.  Kehrmann  (1892)  considers  the  possibility  of 
obtaining  an  unlimited  number  of  such  salts  to  be  a  general  characteristic  of  such  com- 
pounds. Mahom  and  Friedheim  (1892)  obtained  compounds  of  similar  complexity  for 
naolybdic  and  arsenic  acids. 

For  tungstic  acid  there  are  known :  (1)  Normal  salts— for  example,  KaWO4  ;  (2)  the 
so-called  acid  salts  have  a  composition  like  8K2O,7W03)6H2O  or  K6H8(WO4)7)2H2O ;  (8> 
the  tritungstates  like  Na3O,3WO3,3H3o  =  Na,iH4(WO4)3,H2O.  All  these  three  classes  of 
salts  are  soluble  in  water,  but  are  precipitated  by  barium  chloride,  and  with  acids  in  solu- 
tion give  an  insoluble  hydrate  of  tungstic  acid ;  whilst  those  salts  which  are  enumerated 
below  do  not  give  a  precipitate  either  with  acids  or  with  the  salts  of  the  heavy  metals,  be- 
cause they  form  soluble  salts  even  with  barium  and  lead.  They  are  generally  called  meta- 
tungstates.  They  all  contain  water  and  a  larger  proportion  of  acid  elements  than  the 
preceding  salts  ;  (4)  the  tetratungstates,  like  NaiCMWO^lOILjO  and  BaO,4WO3,9H20  for 
example  ;  (5)  the  octatungstates — for  example,  Na2O,8W03,24H2O.  Since  the  metatung- 
etates  lose  so  much  water  at  100°  that  they  leave  salts  whose  composition  corresponds- 
with  an  acid,  81*20,4 WO3— that  is,  HCW4O,5— whilst  in  the  meta  salts  only  2  hydrogens 
are  replaced  by  metals,  it  is  assumed,  although  without  much  ground,  that  these  salts- 
contain  a  particular  soluble  metatuugstic  acid  of  the  composition  H6W4Oi5. 

^As  an  example  we  will  give  a  short  description  of  the  sodium  salts.  The  normal 
salt,  Na2WO4,is  obtained  by  heating  a  strong  solution  of  sodium  carbonate  with  tungstic 
acid  to  a  temperature  of  80°  ;  if  the  solution  be  filtered  hot,  it  crystallises  in  rhombic 
tabular  crystals,  having  the  composition  Na2WO4,2H2O,  which  remain  unchanged  in  the 
air  and  are  easily  soluble  in  water.  When  this  salt  is  fused  with  a  fresh  quantity  of 
tuugstic  acid,  it  gives  a  ditungstate,  which  is  soluble  in  water  and  separates  from  its 
solution  in  crystals  containing  water.  The  same  salt  is  obtained  by  carefully  adding 
hydrochloric  acid  to  the  solution  of  the  normal  salt  so  long  as  a  precipitate  does  not 
appear,  and  the  liquid  still  has  an  alkaline  reaction.  This  salt,  was  first  supposed  to 
have  the  composition  Na2W2O7,4H2O,  but  it  has  since  been  found  to  contain  (at  100°) 
Na6W7024,16H2O — that  is,  it  corresponds  with  the  similar  salt  of  molybdic  acid. 

(If  this  salt  be  heated  to  a  red  heat  in  a. stream  of  hydrogen,  it  loses  a  portion  of  its 
oxygen,  acquires  a  metallic  lustre,  and  turns  a  golden  yellow  colour,  and,  after  being 
treated  with  water,  alkali,  and  acid,  leaves  golden  yellow  leaflets  and  cubes  which  are 
very  like  gold.  This  very  remarkable  substance,  discovered  by  Wohler,  has,  according 
to  Malaguti's  analysis,  the  composition  No.2W3O9 ;  that,  is*  it,  as  it  were,  contains  a- 
double  tungstate  of  tungsten  oxide,  WO2,  and  of  sodium,  Na2W04;W0.2W03:  '  The 
decomposition  of  the  fused  sodium  salt  is  best  effected  by  finely-divided  tin.  This  sub- 
stance  has  a  sp.  gr.  6'6;  it  conducts  electricity,  like  metals,  and  like  them  has  a  metallic 
lustre.  When  brought  into  contact  with  zinc  and  sulphuric  acid  it  disengages  hydrogen, 
and  it  becomes  covered  with  a  coating  of  copper  in  a  solution  of  copper  sulphate  in  the 
presence  of  zinc — that  is,  notwithstanding  its  complex  composition  it  presents  to  a 
certain  extent  the  appearance  and  reactions  of  the  metals.  It  is  not  acted  on  by  aqua- 
rogia  or  alkaline  solutions,  but  it  is  oxidised  when  ignited  in  air.) 

The  ditungstate  mentioned  above,  deprived  of  water  (having  undergone  a  modifica- 
tion similar  to  that  of  metaphosphoric  acid),  after  being  treated  with  water,  leaves  aor 


CHROMIUM,  MOLYBDENUM,  TUNGSTEN,   URANIUM,   ETC,     295 

Both  metals  are   infusible,  and  both  under  the  action  of  heat  form 

anhydrous,  sparingly  soluble  tetratungstate,  Na-jWO^SWOs,  which,  when  heated  at  120° 
in  a  closed  tube  with  water,  passes  into  an  easily  soluble  metatungstate.  It  may  there- 
fore be  said  that  the  metatungstates  are  hydrated  compounds.  On  boiling  a  solution  of 
the  above-mentioned  salts  of  sodium  with  the  yellow  hydrate  of  tungstic  acid  they  give 
a  solution  of  metatungstate,  which  is  the  hydrated  tetratungstate.  Its  crystals  contain 
Na2W4O13)10H2O.  After  the  hydrafe  of  tungstic  acid  (obtained  from  the  ordinary  tung- 
states  by  precipitation  with  an  acid)  has  stood  a  long  time  in  contact  with  a  solution  (hot 
or  cold)  of  sodium  tungstate,  it  gives  a  solution  which  is  not  precipitated  by  hydrochloric 
acid ;  this  must  be  filtered  and  evaporated  over  sulphuric  acid  in  a  desiccator  (it  is  de- 
composed by  boiling).  It  first  forms  a  very  dense  solution  (aluminium  floats  fh  it)  of 
8p.  gr.  3'0,  and  octahedral  crystals  of  sodium  metatungstate,  Na2"W4013,10H2O,  sp.  gr. 
3'85,  then  separate.  It  effloresces  and  loses  water,  and  at  100°  only  two  out  of  the 
ten  equivalents  of  water  remain,  but  the  properties  of  the  salt  remain  unaltered.  If  the 
salt  be  deprived  of  water  by  further  heating,  it  becomes  insoluble.  At  the  ordinary 
temperature  one  part  of  water  dissolves  ten  parts  of  the  metatungstate.  The  other 
metatungstates  are  easily  obtained  from  this  salt.  Thus  a  strong  and  hot  solution, 
mixed  with  a  like  solution  of  barium  chloride,  gives  on  cooling  crystals  of  barium  meta- 
tuugstate,  BaW4013)9H2O.  These  crystals  are  dissolved  without  change  in  water  con- 
taining hydrochloric  acid,  and  also  in  hot  water,  but  they  are  partially  decomposed  by 
cold  water,  with  the  formation  of  a  solution  of  metatungstic  acid  and  of  the  normal 
bajium  salt  Ba\V04. 

In  order  to  explain  the  difference  in  the  properties  of  the  salts  of  tungstic  acid,  we 
may  add  that  a  mixture  of  a  solution  of  tungstic  acid  with  a.solution  of  silicic  acid  does 
not  coagulate  when  heated,  although  the  silicic  acid  alone  would  do  so ;  this  is  due  to 
the  formation  of  a  silicotungstic  acid,  discovered  by  Marrgnac,  which  presents  a  fresh 
example  of  a  complex  acid.  A  solution  of  a  tungstate  dissolves  gelatinous  silica,  just  as 
it  does  gelatinous  tungstic  acid,  and  when  evaporated  deposits  a  crystalline  salt  of 
silicotungstic  acid.  This  solution  is  not  precipitated  either  by  acids  (a  clear  analogy  to 
the  metatungstates)  or  by  sulphuretted  hydrogen,  and  corresponds  with  a  series  of  salts. 
These  salts  contain  one  equivalent  .of  silica  and  8  equivalents  of  hydrogen  or  metals,  in 
the  same  form  as  in  salts,  to  12  or  10  equivalents  of  tungstic  anhydride  ;  for  example, 
the  crystalline  potassium  salt  has  the  composition  K8W12SiO42,14H.;,O  =  4K2O,12WO3, 
SiOa,14H20.  Acid  salts  are  also  known  in  which  half  of  the  metal  is  replaced  by 
hydrogen.  The  complexity  of  the  composition  of  such  complex  acids  (for  example,  of 
the  phosphomolybdic  acid)  involuntarily  leads  to  .the  idea  of  polymerisation,  which  we 
were  obliged  to  recognise  for  silica,  lead  oxide,  and  other  compounds.  This  polymerisa- 
tion, it  seems  to  me,  may  be  understood  thus  :  a  hydrate  A  (for  example,  tungstic  acid) 
is  capable  of  combining  with  a  hydrate  B  (for  example,  silica  or  phosphoric  acid,  with 
or  without  the  disengagement  of  water),  and  by  reason  of  this  faculty  it  is  capable  of 
polymerisation — that  is,  A  combines  with  A — combines  with  itself — just  as  aldehyde, 
C<jH4O,  or  the  cyanogen  compounds  are  able  to  combine  with  hydrogen,  oxygen,  &c., 
and  are  liable  to  polymerisation.  On  this  view  the  molecule  of  tungstic  acid  is  probably 
much  more  complex  than  we  represent  it ,  this  agrees  with  the  easy  volatility  of  such 
compounds  a^  the  chloranhydrides,  CrO3Cl2,  MoO2Cls,  the  analogues  of  the  volatile 
sulphuryl  chlcride,  SO  C12,  and  with  the  non-volatility,  or  difficult  volatility,  of  chromic 
and  molybdic  annydrides,  the  analogues  of  the  volatile  sulphuric  anhydride.  Such  a 
view  also  finds  a  certain  confirmation  in  the  researches  made  by  Graham  on  the  colloidal 
state  of  tungstic  acid,  because  colloidal  properties  only  appertain  to  compounds  of  a  very 
complex  composition.  The  observations  made  by  Graham  on  the  colloidal  state  of 
tungstic  and  molybdic  acids  introduced  much  new  matter  into  the  history  of  these  sub'- 
stances.  When  sodium  tungstate,  mixed  in  a  dilute  solution  with  an  equivalent  quantity 
of  dilute  hydrochloric  acid,  is  placed  in  a  dialyser,  hydrochloric  acid  and  sodium  chloride 
pass  through  the  membrane,  and  a  solution  of  tungstic  acid  remains  in  the  dialyser. 
Out  of  100  parts  of  tungstic  acid  about  80  ports  remain  in  the  dialyser.  The  solution 
*B 


296  PRINCIPLES  OF  CHEMISTRY 

compounds  with  carbon  and  iron  (the  addition  of  tungsten  to  steel 
renders  the  latter  ductile  and  hard).9  Molybdenum  forms  a  grey  powder,, 
which  scarcely  aggregates  under  a  most  powerful  heat,  and  has  a  specific 
gravity  of  8'5  It  is  not  acted  on  by  the  air  at  the  ordinary  tempera- 
ture, but  when  ignited  it  is  first  converted  into  a  brown,  and  then  into  a 
blue  oxide,  and  lastly  into  molybdic  anhydride.  Acids  do  not  act  on  it 
—that  is,  it  does  not  liberate  hydrogen  from  them,  not  even  from 
hydrochloric  acid — but  strong  sulphuric  acid  disengages  sulphurous 
anhydride,  forming  a  brown  mass,  containing  a  lower  oxide  of  molyb- 
denum. Alkalis  in  solution  do  not  act  on  molybdenum,  but  when  fused 

has  a  bitter,  astringent  taste,  and  does  not  yield  gelatinous  tungstio  acid  (hydrogel) 
either  when  heated  or  on  the  addition  of  acids  or  salts.  It  may  also  be  evaporated  to 
dryness;  it  then  forms  a  vitreous  mass  of  the  hydrosol  of  tungstic  acid,  which  adheres 
strongly  to  the  walls  of  the  vessel  in  which  it  has  bsen  evaporated,  and  is  perfectly 
soluble  in  water.  It  does  not  even  lose  its  solubility  after  having  been  heated  to  200°, 
and  only  becomes  insoluble  when  heated  to  a  red  heat,  when  it  loses  about  2J  p.c.  of 
water.  The  dry  acid,  dissolved  in  a  small  quantity  of  water,  forms  a  gluey  mass,  just 
like  gum  arabic,  which  is  one  of  the  representatives  of  the  hydrosols  of  colloidal 
substances.  The  solution,  containing  5  p.c  of  the  anhydride,  has  a  sp.  gr.  of  1'047  ;  with 
20  p.c.,  of  1-217;  with  50  p.c.,  of  T80 ;  and  with  80  p.c.,  of  8'24.  The  presence  of  a 
polymerised  trioxide  in  the  form  of  hydrate,  H2OW5Oj)  or  HSO4WO5,  must  then  be 
recognised  in  the  solution :  this  is  confirmed  by  Sabaueeff's  cryoscopic  determinations 
(1889).  A  similar  stable  solution  of  molybdic  acid  is  obtained  by  the  dialysis  of  a 
mixture  of  a  strong  solution  of  sodium  molybdate  with  hydrochloric  acid  (the  precipitate 
which  is  formed  is  re-dissolved).  If  MoCl4  be  precipitated  by  ammonia  and  washed  with 
water,  a  point  is  reached  at  which  perfect  solution  takes  place,  and  the  molybdic  acid 
forms  a  colloid  solution  which  is  precipitated  by  the  addition  of  ammonia  (Muthmann). 
The  addition  of  alkali  to  the  solutions  of  the  hydrosols  of  tungstic  and  molybdic  acids 
immediately  results  in  the  re- formation  of  the  ordinary  tungstates  and  molybdates. 
There  appears  to  be  no  doubt  but  that  the  same  transformation  is  accomplished  in  the 
passage  of  the  ordinary  tungstates  into  the  metatungstates  as  takes  place  in  the  passage 
of  tungstic  acid  itself  from  an  insoluble  into  a  soluble  state;  but  this  may  be  even 
actually  proved  to  be  the  case,  because  Scheibler  obtained  a  solution  of  tungstic  acid, 
before  Graham,  by  decomposing  barium  metatungstate  (BaO4WO3)9H2O)  with  sulphuric 
acid.  By  treating  this  salt  with  sulphuric  acid  in  the  amount  required  for  the  precipi- 
tation of  the  baryta,  Scheibler  obtained  a  solution  of  metatungstic  acid  which,  when 
containing  43'75  p.c.  of  acid,  had  a  sp.  gr.  of  T634,  and  with  27'61  p.c.  a  sp.  gr.  of  l'S27— 
that  is,  specific  gravities  corresponding  with  those  found  by  Graham. 

Pechard  found  that  as  much  heat  is  evolved  by  neutralising  metatungstic  acid  as  with 
sulphuric  acid. 

Questions  connected  with  the  metamorphoses  or  modifications  of  tungstic  and 
ttolybdic  acids,  and  the  polymerisation  and  colloidal  state  of  substances,  as  well  as  the 
formation  of  complex  acids,  belong  to  that  class  of  problems  the  solution  of  which 
will  do  much  towards  attaining  a  true  comprehension  of  the  mechanism  of  a  number  of 
chemical  reactions.  I  think,  moreover,  that  questions  of  this  kind  stand  in  intimate  con- 
nection with  the  theory  of  the  formation  of  solutions  and  alloys  and  other  so-called  inde- 
finite compounds. 

9  Moissan  (1893)  studied  the  compounds  of  Mo  and  W  formed  with  carbon  in  the 
electrical  furnace  (they  are  extremely  hard)  from  a  mixture  of  the  anhydrides  and  carbon. 
Poleck  and  Griitzner  obtained  definite  compounds  FeW2  and  FeW2C3  for  tungsten. 
Metallic  W  and  Mo  displace  Ag  from  its  solutions  but  not  Pb.  There  is  reason  for  believing 
that  the  sp.  gr.  of  pure  molybdenum  is  higher  than  that  (8'5)  generally  ascribed  to  it. 


CHROMIUM,  MOLYBDENUM,  TUNGSTEN,  URANIUM,  ETC.  297 

with  it  hydrogen  is  given  off,  which  shows,  as  does  its  whole  character, 
the  acid  properties  of  the  metal.  The  properties  of  tungsten  are  almost 
identical  ;  it  is  infusible,  has  an  iron-grey  colour,  is  exceedingly  hard,  so 
that  it  even  scratches  glass.  Its  specific  gravity  is  19'1  (according  to 
Roscoe),  so  that,  like  uranium,  platinum,  &c.,  it  is  one  of  the  heaviest 
metals.9  bis  Just  as  sulphur  and  chromium  have  their  corresponding 
persulphuric  and  perchromic  acids,  H2S208  and  H2Cr08,  having  the 
properties  of  peroxides,  and  corresponding  to  peroxide  of  hydrogen,  so 
also  molybdenum  and  tungsten  are  known  to  give  permolybdic  and  per- 
tungstic  acids,  H2Md2O8  and  H2W2O8,  which  have  the  properties  of  true 
peroxides,  i.e.  easily  disengage  iodine  from  KI  and  chlorine  from  HC1, 
easily  part  with  their  oxygen,  and  are  formed  by  the  action  of  peroxide 
of  hydrogen,  into  which  they  are  readily  reconverted  (hence  they  rnay 
fee  regarded  as  compounds  of  H202  with  2MoO^  and  2WO3),  <tc.  Their 
formation  (Boerwald  1884,  Kemmerer  1891)  is  at  once  seen  in  the 
coloration  (not  destroyed  by  boiling),  which  is  obtained  on  mixing  a 
solution  of  the  salts  with  peroxide  of  hydrogen,  and  on  treating,  for  in- 
stance, molybdic  acid  with  a  solution  of  peroxide  of  hydrogen  (Pe'chard 
1892).  The  acid  then  forms  an  orange-coloured  solution,  which  after 
evaporation  in  vacuo  leaves  Mo2H2084H2O  as  a  crystalline  powder, 
and  loses  4H2O  at  100°,  beyond  which  it  decomposes  with  the  evolu- 
tion of  oxygen. 9trl 

Uranium,    U  =  240,    has   the    highest  atomic    weight   of  all    the 
analogues  of  chromium,  and  indeed  of  all  the  elements  yet  known.     Its 

9  bis  \Ve  may  conclude  our  description  of  tungsten  and  molybdenum  by  stating  that 
their  sulphur  compounds  have  an  acid  character,  like  carbon  bisulphide  or  stannic  sul- 
phide. If  sulphuretted  hydrogen  be  passed  through,  a  solution  of  a  molybdate  it  doea 
not  give  a  precipitate  unless  sulphuric  acid  be  present,  when  a  dark  brown  precipitate  of 
molybdenum  trisulphide,  MoS3,  is  formed.  When  this  sulphide  is  ignited  without  access 
of  air  it  gives  the  bisulphide  MoSv ;  the  latter  is  not  able  to  combine  with  potassium  sulphide 
like  the  trisulphide  MoS5,  which  forms  a  salt,  KjMoS^  corresponding  with  K2MoO4. 
This  is  soluble  in  water,  and  separates  out  from  its  solution  in  red  crystals,  which  have  a 
metallic  lustre  and  reflect  a  green  light.  It  is  easily  obtained  by  heating  the  native 
bisulphide,  MoS2,  with  potash, ^sulphur,  and  a  small  amount  of  charcoal,  which  serves  for 
deoxidising  the  oxygen  compounds.  Tungsten  gives  similar  compounds,  RjWS4,  where 
R  =  NH4,  K,  Na.  They  are  decomposed  by  acids,  with  the  separation  of  tungsten  trisul- 
phide, WSS)  and  molybdenum  trisulphide,  MoS3.  Rideal  (1892)  obtained  WSN5  by  heating 
WO3  in  NH3.  This  compound  exhibited  the  general  properties  of  metallic  nitrides. 

9iri  When  peroxide  of  hydrogen  acts  upon  a  solution  of  potassium  molybdate  well- 
formed  yellow  crystals  belonging  to  the  triclinic  system  separate  out  in  the  cold.  When 
these  crystals  are  heated  in  vacuo  they  first  lose  water  and  then  decompose,  leaving  a 
residue  composed  of  the  salt  originally  taken.  They  are  soluble  in  water  but  insoluble 
in  alcohol.  Their  composition  Is  represented  by  the  formula  KjMo2082H20.  An  am- 
monium salt  is  obtained  by  evaporating  peroxide  of  hydrogen  with  ammonium  molybdate. 
The  following  salts  ha^e  also  been  obtained  by  the  action  of  peroxide  of  hydrogen  upon  the 
corresponding  molybdates:Na^Ho2066HaO — in  yellow  prismatic  crystals ;  MgMo20810H^O 
—stellar  needles;  BaMo.082HjO — in  microscopic  yellow  octahedra.  A  corresponding 
sodium  pertungstate  has  been  obtained  by  Pe'chard  by  boiling  sodium  tungstate  with  a 


£98  PRINCIPLES  OF 'CHEMISTS 

highest  salt-forming  oxide,  UO3,  shows;  very  feeble  acid  properties. 
Although  it  gives  sparingly-soluble  yellow  compounds  with  alkalis, 
which  fully  correspond  with  the  dichromates — for  example,  Na.2U2Ov 
t=Na20,2U03,10 — yet  it  mo  re  frequently  and  easily  reacts  with  acids,-HX, 

Solution  of  peroxide  of  hydrogen  for  several  minutes.  The  solution  rapidly  turns  yellow, 
and  no  longer  gives  a  precipitate  of  tungstic  anhydride  when  treated  with  nitric  acid. 
When  evaporated  in  vacuo  the  solution  leaves  a  thick  syrupy  liquid  from  which  ray-like 
Crystals  separate  out ;  these  crystals  are  more  soluble  in  water  than  the  salt  originally 
taken.  When  heated  they  also  lose  water  and  oxygen.  Their  composition  answers  to 
the  formula  M2W2O82H.<O,  where  M  =  Na,  NH4,  &c.  The  permolybdates  and  per- 
tungstates  have  similar  properties.  When  treated  with  oxygen  acids  they  give  peroxide 
of  hydrogen,  and  disengage  chlorine  and  iodine  from  hydrochloric  acid  and  potassium 
iodide. 

Piccini  (1891)  showed  that  peroxide  of  hydrogen  not  only  combines  with  the  oxygen 
compounds  of  Mo  and  W,  but  also  with  their  fluo-compounds,  among  which  ammonium 
fluo-molybdate  MoO2F2  2NH4  and  others  have  long  been  known.  (A  few  new  salts  of 
similar  composition  have  been  obtained  by  F.  Moureu  in  1893.)  The  action  of  peroxide 
of  hydrogen  upon  these  compounds  gi-ves  salts  containing  a  larger  amount  of  oxygen  ;  for 
instance,  a  solution  of  Mo02F,/2KFH2O  with  peroxide  of  hydrogen  gives  a  yellow  solu- 
tion which  after  cooling  separates  out  yellow  crystalline  flakes  of  Mo03F22KFELO,  resem- 
bling the  salt  originally  taken  in  their  external  appearance.  By  employing  a  similar  method 
Piccini  also  obtained :  MoO3F.32RbFH20— yellow  monoclinic  crystals ;  MoO5F22CsFHjO, 
—yellow  flakes,  and  the  corresponding  tungstic  compounds.  All  these  salts  re-act  like 
peroxide  of  hydrogen. 

In  speaking  of  these  compounds  I  for  my  part  think  it  may  be  well  to  call  attention 
to  the  fact  that,  in  the  first  place,  the  composition  of  Piccini's  oxy-fluo  compounds  does 
not  correspond  to  that  of  permolybdic  and  pertungstic  acid.  If  the  latter  be  expressed 
by  formulae  with  one  equivalent  of  an  element,  they  will  be  HMo04  and  HW04,  and  the 
oxy-fluo  form  corresponding  to  them  should  have  the  composition  MoO3F  and  W05F 
while  it  contains  MO3F2  and  WO3F2,  i.e.  answers  as  it  were  to  a  higher  degree  of  oxida- 
tion, MoH2O5  and  W  HO5.  But  if  permolybdic  acid  be  regarded  as  2Mo05  +  H2  O2,z'.e. 
as  containing  the  elements  of  peroxide  of  hydrogen,  then  Piccini's  compound  will  also  be 
found  to  contain  the  original  salts  +  H2O  ;  for  example,  from  MoO2F22KFHaO  there  is 
obtained  a  compound  MoOaFal2KFHi02,  i.e.  instead  of  H2O  they  contain  H2O.;.  In  the 
second  place  the  capacity  of  the  salts  of  molybdenum  and  tungsten  to  retain  a  further 
amount  of  oxygen  or  H.^O.^  probably  bears  some  relation  to  their  property  of  giving  com- 
plex acids  and  of  polymerising  which  has  been  considered  in  Note  8  bis.  There  is, 
however,  a  great  chemical  interest  in  the  accumulation  of  data  respecting  these  high 
peroxide  compounds  corresponding  to  molybdic  and  tungstic  acids.  With  regard  to  the 
peroxide  form  of  uranium,  sec  Chapter  XX.,  Note  66. 

10  Uranium  trioxide,  or  uranic  oxide,  shows  its  feeble  basic  and  acid  properties  in  a 
great  number  of  its  reactions.  (1)  Solutions  of  uranic  salts  give  yellow  precipitates  with 
alkalis,  but  these  precipitates  do  not  contain  the  hydrate  of  the  oxide,  but  compounds  of 
it  with  bases ;  for  example,  2UOa(N03)3  +  6KHO  =  4KN03  +  3H2O  +  K2U  ,O7.  There  are 
other  urano-alkali  compounds  of  the  same  constitution  ;  for  example,  (NH4).iU..O7 
(known  commercially  as  uranic  oxide),  MgU.207,  BaU207.  They  are  the  analogues  of  the 
dichromates.  Sodium  uranate  is  the  most  generally  used  under  the  name  of  uranium 
yellow,  Na-jU-jO;.  It  is  used  for  imparting  the  characteristic  yellow -green  tint  to  glass 
and  porcelain.  Neither  heat  nor  water  nor  acids  are  able  to  extract  the  alkali  from 
sodium  uranate,  Na2U2O7,  and  therefore  it  is  a  true  insoluble  salt,  of  a  yellow  colour,  and 
clearly  indicates  the  acid  character  (although  feeble)  of  uranic  oxide.  (2)  The  carbonates 
of  the  alkaline  earths  (for  instance,  barium  carbonate)  precipitate  urarcic  oxide  from  its 
salts,  as  they  do  all  the  salts  of  feeble  bases;  for  example,  R2O3.  (3),  The  alkaline  car- 
bonates, when  added  to  solutions  of  uranic  salts,  give  &  precipitate,  which  is  soluble  in 


CHROMIUM,  MOLYBDENUM,  TUNGSTEN,  URANIUM,  ETC.  £99 

forming  fluorescent  yellowish-green  salts  of  the  composition  UO2X2, 
and  in  this  respect  uranic  trioxide,  UO3,  differs  from  chromic  anhydride, 
Cr03,  although  the  latter  is  able  to  give  the  oxychloride,  Cr02Cl2.  In 
molybdenum  and  tungsten,  however,  we  see  a  clear  transition  from 
chromium  to  uranium.  Thus,  for  example,  chromyl  chloride,  Cr02Cl2, 
is  a  brown  liquid  which  volatilises  without  change,  and  is  completely 
decomposed  by  water ;  molybdenum  oxychloride,  Mo02Cl2,  is  a  crys- 
talline substance  of  a  yellow  colour,  which  is  volatile  and  soluble  in 
water  (Blomstrand),  like  many  salts.  Tungsten  oxychloride,  WO2C12, 
stands  still  nearer  to  uranyl  chloride  in  its  properties  ;  it  forms  yellow- 
scales  on  which  water  and  alkalis  act,  as  they  do  on  many  salts  (zinc 
chloride,  ferric  chloride,  aluminium  chloride,  stannic  chloride,  &c.),  and 
perfectly  corresponds  with  the  difficultly-volatile  salt,  UO2C12  (obtained 
by  Peligot  by  the  action  of  chlorine  on  ignited  uranium  dioxide,  UO2), 
which  is  also  yellow  and  gives  a  yellow  solution  with  water,  like  all  the 

an  excess  of  the  reagent,  and  particularly  so  if  the  acid  carbonates  be  taken.  This  is 
due  to  the  fact  that  (4)  the  uranyl  salts  easily  form  double  salts  with  the  salts  of  the 
alkali  metals,  including  the  salts  of  ammonium.  Uranium,  in  the  form  of  these  double 
salts,  often  gives  salts  of  well-defined  crystalline  form,  although  the  simple  salts  are  little 
prone  to  appear  in  crystals.  Such,  for  example,  are  the  salts  obtained  by  dissolving  potas- 
sium uranate,  KQU2O7,  in  acids,  with  the  addition  of  potassium  salts  of  the  same  acids. 
Thus,  with  hydrochloric  acid  and  potassium  chloride  a  well-formed  crystalline  salt, 
K2(UOS)C14,2H2O,  belonging  to  the  monoclinic  system,  is  produced.  This  salt  decom- 
poses in  dissolving  in  pure  water.  Among  these  double  salts  we  may  mention  the 
double  carbonate  with  the  alkalis,  E4(UO2)(C05)3  (equal  to  2R2C03  +  U02C03) ;  the 
acetates,  E(U02)  (C.;,H3OV)3— for  instance,  the  sodium  salt,  Na(U02)(C2H3O2)3,  and  the 
potassium  salt,  K(U02)(C2H302)3)H2O ;  the  sulphates,  R2(U02)(SO4)3,2H2O,  &c.  In  the 
preceding  formula  R  =  K,  Na,  NH4,  or  Itj  =  Mg,  Ba,  &c.  This  property  of  giving 
comparatively  stable  double  salts  indicates  feebly  developed  basic  properties,  because 
double  salts  are  mainly  formed  by  salts  of  distinctly  basic  metals  (these  form,  as  it  were, 
the  basic  element  of  a  double  salt)  and  salts  of  feebly  energetic  bases  (these  form  the  acid 
element  of  a  double  salt),  just  as  the  former  also  give  acid  salts;  the  acid  of  the  acid 
salts  is  replaced  in  the  double  salts  by  the  salt  of  the  feebly  energetic  base,  which,  like 
water,  belongs  to  the  class  of  intermediate  bases.  For  this  reason  barium  does  not 
give  double  salts  with  alkalis  as  magnesium  does,  and  this  is  why  double  salts  are 
more  easily  formed  by  potassium  than  by  lithium  in  the  series  of  the  alkali  metals. 
(5)  The  most  remarkable  property,  proving  the  feeble  energy  of  uranic  oxide  as  a  base,  is 
Been  in  the  fact  that  when  their  composition  is  compared  with  that  of  other  salts  those  of 
uranic  oxide  always  appear  as  basic  salts.  It  is  well  known  that  a  normal  salt,  R<,>X6, 
corresponds  with  the  oxide  R2O3,  where  X  =  Cl,  N03,  &c.,  or  X3  =  S04,  C05,  &c. ;  but  there 
also  exist  basic  salts  of  the  same  type  where  X  =  HO  or  X3  =  O.  We  saw  salts  of  all 
kinds  among  the  salts  of  aluminium,  chromium,  and  others.  With  uranic  oxide  no  salts 
are  known  of  the  types  UX6  (UC16,  U(S04)3,  alums,  &c.,are  not  known),  nor  even  salts, 
U(HO)2X4  or  UOX4,  but  it \  always  forms  salts  of  the  type  U(HO)4X2  or  UO2X2. 
Judging  from  the  fact  that  nearly  all  the  salts  of  uranic  oxide  retain  water  in  crystallising 
from  their  solutions,  and  that  this  water  is  difficult  to  separate  from  them,  it  may  be 
thought  to  be  water  of  hydration.  This  is  seen  in  part  from  the  fact  that  the  composition 
of  many  of  the  salts  of  uranic  oxide  may  then  be  expressed  without  the  presence  of  water 
of  crystallisation;  for  instance,  U(HO)4KSC14  (and  the  salt  of  NH4)  U(HO)4K2(SOJ  , 
U(HO)4(C2H5O2)2..  Sodium  uranyl  acetate  however  does -not  contain  water. 


800  PRINCIPLES  OF  CHEMISTRY 

salts  U02X2.  The  property  of  uranic  oxide,  UO3,  of  forming  salts 
UO2X2  is  shown  in  the  fact  that  the  hydrated  oxide  of  uranium, 
TJO2(HO).2,  which  is  obtained  from  the  nitrate,  carbonate,  and  other 
salts  by  the  loss  of  the  elements  of  the  acid,  is  easily  soluble  in  acids, 
as  well  as  in  the  fact  that  the  lower  grades  of  oxidation  of  uranium  are 
able,  when  treated  with  nitric  acid,  to  form  an  easily  crystallisable 
uranyl  nitrate,  U02(N03)2,6H20  ;  this  is  the  most  commonly  occurring 
uranium  salt.11 

Uranium,  which  gives  an  oxide,  U03,  and  the  corresponding  salt 
U02X2  and  dioxide  U02,  to  which  the  salts  UX4  correspond,  is  rarely 
Diet  with  in  nature.  Uranite  or  the  double  orthophosphate  of  uranic 

11  Uranyl  nitrate,  or  uranium  nitrate,  UO.j(NO5);,6HjO,  crystallises  from  its  solu- 
tions in  transparent  yellowish-green  prisms  (from  an  acid  solution),  or  in  tabular  crystals 
(from  a  neutral  solution),  which  effloresce  in  the  air  and  are  easily  soluble  in  water, 
alcohol,  and  ether,  have  a  sp.  gr.  of  2'8,  and,  fuse  when  heated,  losing  nitric  acid  and  water 
in  the  process.  If  the  salt  itself  (Berzelius)  or  its  alcoholic  solution  (Malaguti)  be 
heated  up  to  the  temperature  at  which  oxides  of  nitrogen  are  evolved,  there  then  remains 
a  mass  which,  after  being  evaporated  with  water  leaves  uranyl  hydroxide,  UO.,(HO)4 
(sp.  gr.  5-98),  whilst  if  the  salt  be  ignited  there  remains  the  dioxide,  UO2,  as  a  brick-red 
powder,  which  on  further  heating  loses  oxygen  and  forms  the  dark  olive  uranoso-uranio 
oxide,  U3O8.  The  solution  of  the  nitrate  obtained  from  the  ore  is  purified  in  the  following 
manner :  sulphurous  anhydride  is  first  passed  through  it  in  order  to  reduce  the  arsenio 
acid  present  into  arsenious  acid ;  the  solution  is  then  heated  to  60°,  and  sulphuretted 
hydrogen  passed  through  it;  this  precipitates  the  lead,  arsenic,  and  tin,  and  certain 
Other  metals,  as  sulphides,  insoluble  in  water  and  dilute  nitric  acid.  This  liquid  is  then 
filtered  and  evaporated  with  nitric  acid  to  crystallisation,  and  the  crystals  are  dissolved 
in  ether.  Or  else  the  solution  is  first  treated  with  chlorine  in  order  to  convert  the  ferrous 
chloride  (produced  by  the  action  of  the  hydrogen  sulphide)  into  ferric  chloride,  the 
oxides  are  then  precipitated  by  ammonia,  and  the  resultant  precipitate,  containing  the 
oxides  Fe-jOs.UOs,  and  compounds  of  the  latter  with  potash,  lime,  ammonia,  and  other 
bases  present  in  the  solution  (the  latter  being  due  to  the  property  of  uranic  oxide  of 
combining  with  bases),  is  washed  and  dissolved  in  a  strong,  slightly-heated  solution  of 
ammonium  carbonate,,which  dissolves  the  uranic  oxide  but  not  the  ferric  oxide.  The 
solution  is  filtered,  and  on  cooling  deposits  a  well-crystallising  uranyl  ammonium  car- 
bonate, IKX(NH4)4(CO3)3,  in  brilliant  monoclinic  crystals  which  on  exposure  to  air  slowly 
give  off  water,  carbonic  anhydride,  and  ammonia ;  the  same  decomposition  is  readily 
effected  at  300°,  the  residue  then  consisting  of  uranic  oxide.  This  salt  is  not  very  soluble  ' 
in  water,  but  is  readily  so  in  ammonium  carbonate ;  it  is  obvious  that  it  may  readily  be 
converted  into  all  the  other  salts  of  oxides  of  uranium.  Uranium  salts  are  also  purified 
in  the  form  of  acetate,  which  is  very  sparingly  soluble,  and  is  therefore  directly  precipi- 
tated from  a  strong  solution  of  the  nitrate  by  mixing  it  with  acetic  acid. 

We  may  also  mention  the  uranyl  phosphate,  HUPO6,  which  must  be  regarded  as  an 
orthophosphate  in  which  two  hydrogens  are  replaced  by  the  radicle  uranyl,  UO.2,  i.e.  as 
H(UO2)PO4.  This  salt  is  formed  as  a  hydrated  gelatinous  yellow  precipitate,  on  mixing 
a  solution  of  uranyl  nitrate  with  di sodium  phosphate.  The  precipitation  occurs  in  the 
presence  of  acetic  acid,  but  not  in  the-presence  of  hydrocliloric  acid.  If  moreover  an 
excess  of  an  ammonium  salt  be  present,  the  ammonia  enters  into  the  composition  of 
the  bright  yellow  gelatinous  precipitate  formed,  in  the  proportion  U0.2NH4PO4.  This 
precipitate  is  not  soluble  in  water  and  acetic  acid,  and  its  solution  in  inorganic  acids 
when  boiled  entirely  expels  all  the  phosphoric  acid.  This  fact  is  taken  advantage  of  for 
removing  phosphoric  acids  from  solutions — for  instance,  from  those  containing  salts  of 
ofl.lr-.inrp  and  magnesium. 


CHROMIUM,  MOLYBDENUM,  TUNGSTEN,  URANIUM,  ETC.   301 

oxide,  R(U02)H2P2O8,7H2O,  where  R=Cu  or  Ca,  uranium- vitriol 
U(S04)2,H2O,  samarakite,  and  seschynite,  are  very  rarely  found,  and 
then  only  in  small  quantities.  Of  more  frequent  and  abundant 
occurrence  is  the  non-crystalline,  earthy  brown  uranium  ore  known  as 
pitchblende  (sp.  gr.  7'2),  which  is  mainly  composed  of  the  intermediate 
oxide,  U3O8=UO2,2U03.  This  ore  is  found  at  Joachimsthal  in  Bohemia 
and  in  Cornwall.  It  usually  contains  a  number  of  different  impuri- 
ties, chiefly  sulphides  and  arsenides  of  lead  and  iron,  as  well  as  lime 
and  silica  compounds.  In  order  to  expel  the  arsenic  and  sulphur  it  is 
roasted,  ground,  washed  with  dilute  hydrochloric  acid,  which  does  not 
dissolve  the  uranoso-uranic  oxide,  UaO8,  and  the  residue  is  dissolved 
in  nitric  acid,  which  transforms  the  uranium  oxide  into  the  nitrate, 
U02(N03)2. 

It  must  be  observed  that  the  oxide  of  uranium,  first  distinguished 
by  Klaproth  (1789),  was  for  a  long  time  regarded  as  able  to  give 
metallic  uranium  under  the  action  of  charcoal  and  other  reducing  agents 
(with  the  aid  of  heat).  But  the  substance  thus  obtained  was  only  the 
uranium  dioxide,  UO2.  The  compound  nature  of  this  dioxide,12  or  the 
presence  of  oxygen  in  it,  was  demonstrated  by  Peligot  (1841),  by  igniting 
it  with  charcoal  in  a  stream  of  chlorine.  He  thus  obtained  a  volatile 
uranium  tetrqchloride,  UC14,13  which,  when  heated  with  sodium,  gave 

13  Uranium  dioxide,  or  uranyl,  UO  ,  which  "is  contained  in  the  salts  UO2X2,  has 
the  appearance  and  many  of  the  properties  of  a  metal.  Uranic  oxide  may  be  regarded  as 
•uranyl  oxide,  (U02)O,  its  salts  as  salts  of  this  uranyl ;  its  hydroxide,  (UOo)H202,  is  consti- 
tuted like  CaHgOj.  The  green  oxide  of  uranium,  uranoso-uranio  oxide  (easily  formed  from, 
uranic  salts  by  the  loss  of  oxygen),  U5O8  =  UOo,2UO3,  when  ignited  with  charcoal  or 
hydrogen  (dry)  gives  a  brilliant  crystalline  substance  of  sp.  gr.  about  1TO  (Urlaub),  whose 
appearance  resembles  that  of  metals,  and  decomposes  steam  at  a  red  heat  with  the 
evolution  of  hydrogen;  it  does  not,  however,  decompose  hydrochloric  or  sulphuric 
acid,  but  is  oxidised  by  nitric  acid.  The  same  substance  (i.e.  uranium  dioxide  UO^)  is 
also  obtained  by  igniting  the  compound  (UO.2)K2C14  in  a  stream  of  hydrogen,  according 
to  the  equation  UO.K.Cl4  +  Hi  =  UO.2  +  2HCl  +  2KCl.  It  was  at  first  regarded  as  the 
metal.  In  1841  Peligot  found  that  it  contained  oxygen,  because  carbonic  oxide  and 
anhydride  were  evolved  when  it  was  ignited  with  charcoal  in  a  stream  of  chlorine,  and 
from  272  parts  of  the  substance  which  was  considered  to  be  metal  he  obtained  882  parts 
of  a  volatile  product  containing  142  parts  of  chlorine.  Prom  this  it  was  concluded  that 
the  substance  taken  contained  an  equivalent  amount  of  oxygen.  As  142  parts  of  chlorine 
correspond  with  32  parts  of  oxygen,  it  followed  that  272  —  32  =  240  parts  of  maial  were 
combined  in  the  substance  with  82  parts  of  oxygen,  and  also  in  the  chlorine  compound 
obtained  with  142  parts  of  chlorine.  These  calculations  have  been  made  for  the  now 
accepted  atomic  weight  of  uranium  (U  =  240,  see  Note  14).  Peligot  took  another  atomio 
weight,  but  this  does  not  alter  the  principle  of  the  argument. 

15  Uranium  tetrachloride,  uranous  chloride,  UC14,  corresponds  with  uranous  oxide 
M  a  base.  It  was  obtained  by  Peligot  by  igniting  uranic  oxide  mixed  with  charcoal  in  a- 
Stream  of  dry  chlorine.  U03  +  8C  +  2C1>-UC14-+-8CO.  This  green  volatile  compound 
(Note  12)  crystallises  in  regular  octahedra,  is  very  hygroscopic,  easily  soluble  ia  water, 
with  the  development  of  a  considerable  amount  of  heat,  and  no  longer  separates  ou.fr 
from  its  solution  in  an  anhydrous  etate,  but  disengages  hydrochloric  acid  when  evapor 


302  PRINCIPLES  OF   CHEMISTRY 

metallic  uranium  as  a  grey  metal,  having  a  specific  gravity  of  18'7,  and 
liberating  hydrogen  from  acids,  with  the  formation  of  green  urarious 
salts,  UX4,  which  act  as  powerful  reducing  agents.14 

rated.  The  solution  of  uranous  chloride  in  water  is  green.  It  is  also  formed  by  the 
Action  of  zinc  and  copper  (forming  cuprous  chloride)  on  a  solution  of  uranyl  chloride, 
UO.jClj,  especially  in  the  presence  of  hydrochloric  acid  and  sal-ammoniac.  Solutions  of 
uranyl  salts  are  converted  into  uranous  salts  by  the  action  of  various  reducing  agents, 
and  among  others  by  organic  substances  or  by  the  action  of  light,  whilst  the  salts  UX4 
are  converted  into  uranyl  salts,  UO2X^,  by  exposure  to  air  or  by  oxidising  agents.  Solu- 
tions of  the  green  uranyl  salts  act  as  powerful  reducing  agents,  and  give  a  brown  precis 
pitate  of  the  uranous  hydroxide,  UH4O4,  with  potash  and  other  alkalis.  This  hydroxide 
is  easily  soluble  in  acids  but  not  in  alkalis  On  ignition  it  does  not  form  the  oxide  U02, 
because  it  decomposes  water,  but  when  the  higher  oxides  of  uranium  are  ignited  in  a 
stream  of  hydrogen  or  with  charcoal  they  yield  uranous  oxide.  Both  it  and  the  chloride 
UCL,  dissolve  in  strong  sulphuric  acid,  forming  a  green  salt,  U(SO4).;>,2H2O.  The  same 
salt,  together  with  uranyl  sulphate,  UO,(S04),  is  formed  when  the  green  oxide,  U3O3,  is 
dissolved  in  hot  sulphuric  acid.  The  salts  obtained  in  the  latter  instance  may  be 
separated  by  adding  alcohol  to  the  solution,  which  is  left  exposed  to  the  light;  the  alcohol 
reduces  the  uranyl  salt  to  uranous  salt,  an  excess  of  acid  being  required.  An  excess  of 
water  decomposes  this  salt,  forming  a  basic  salt,  which  is  also  easily  produced  under 
.other  circumstances,  and  contains  UO(S04),2H20  (which  corresponds  to  the  uranicsalt). 

14  The  atomic  weight  of  uranium  was  formerly  taken  as  half  the  present  one,  U  =  120, 
.and  the  oxides  U2O3,  suboxide  UO,  and  green  oxide  U504,  were  of  the  same  types  as  the 
•oxides  of  iron.  With  a  certain  resemblance  to  the  elements  of  the  iron  group,  uranium 
presents  many  points  of  distinction  which  do  not  permit  its  being  grouped  with  them. 
Thus  uranium  forms  a  very  stable  oxide,  U2O3(U  =  120),  but  does  not  give  the  corre- 
sponding chloride  U2C16  (Roscoe,  however,  in  1874  obtained  UC15,  like  MoCl3  and  WC15), 
and  under  those  circumstances  (the  ignition  of  oxide  of  uranium  mixed  with  charcoal,  in 
a  stream  of  chlorine),  when  the  formation  of  this, compound  might  be  expected,  it  gives 
.{IT  =  120)  the  chloride  UC12,  which  is  characterised  by  its  volatility;  this  is  not  a  pro- 
perty, to  such  an  extent,  of  any  of  the  bichlorides,  RC12,  of  the  iron  group. 

The  alteration  or  doubling  of  the  atomic  weight  of  uranium — i.e.  the  recognition  of 
U  =  240 — was  made  for  the  first  time  in  the  first  (Russian)  edition  of  this  work  (1871),  and 
in  my  memoir  of  the  same  year  in  Liebig's  Annalen,  on  the  ground  that  with  an  atomic 
weight  1'20,  uranium  could  not  be  placed  in  the  periodic  system.  I  think  it  will  not  be  super- 
fluous to  add  the  following  remarks  on  this  subject :  (1)  In  the  other  groups  (K — Rb — Cs, 
Ca — Sr — Ba,  Cl— Br — I)  the  acid  character  of  the-oxides  decreases  and  their  basic  charac- 
ter increases  with  the  rise  of  atomic  weight,  and  therefore  we  should  expect  to  find  the 
same  in  the  group  Cr — Mo — W — U,  and  if  Cr03,  Mo03,  WO3  be  the  anhydrides  of  acids 
then  we  indeed  find  a  decrease  in  their  acid  character,  and  therefore  uranium  trioxide, 
.U03,  should  be  a  very  feeble  anhydride,  but  its  basic  properties  should  also  be  very 
feeble.  Uranic  oxide  does  indeed  show  these  properties,  as  was  pointed  out  above  (Note 
10).  (2)  Chromium  and  its  analogues,  besides  the  oxides  RO3,  also  form  lower  grades  of 
•oxidation  R02,  R2O3,  and  the  same. is  seen  in  uranium;  it  forms  U03,  U02,  U.^O.-,  and 
their  compounds.  (8)  Molybdenum  and  tungsten,  in  being  reduced  from  RO,-,,  easily  and 
frequently  give  an  intermediate  oxide  of  a  blue  colour,  and  uranium  shows  the  same 
property ;  giving  the  so-called  green  oxide  which,  according  to  present  views,  must 
be  regarded  as  U3Od  =  UO22UO3,  analogous  to  Mo3O8.  (4)  The  higher  chlorides,  RCld, 
possible  for  the  elements  of  this  group,  are  either  unstable  (WC10)  or  do  not  exist  at  all 
{Cr) ;  but  there  is  one  single  lower  volatile  compound,  which  is  decomposed,  by  water, 
and  liable  to  further  reduction  into  a  non-volatile  chlorine  product  and  the  metal.  The 
same  is  observed  in  uranium,  which  forms  an  easily  volatile  chloride,  UC14,  decomposed 
by  water.  (5)  The  high  sp.  gr.  of  uranium  (18'6J  is  explained  by  its  analogy  to  tungsten 
-{sp.  gr.  19'1).  (6)  For  uranium,  as  for  chromium  and  tungsten,  yellow  tints  pre- 


CHROMIUM,  MOLYBDENUM,  TUNGSTEN,  URANIUM,  ETC.   303 

As  the  salts  of  uranic  oxide  are  reduced  in  the  absence  of  organic 
matter  by  the  action  of  light,  and  as  they  impart  a  characteristic 
coloration  to  glass,15  they  find  a  certain  application  in  photography  and 
glass  work. 

If  we  compare  together  the  highly  acid  elements,  sulphur,  selenium, 
and  tellurium,  of  the  uneven  series,  with  chromium,  molybdenum, 
tungsten,  and  uranium  of  the  even  series,  we  find  that  the  resemblance 
of  the  properties  of  the  higher  form  R03  does  not  extend  to  the  lower 
forms,  and  even  entirely  disappears  in  the  elements,  for  there  is 
not  the  smallest  resemblance  between  sulphur  and  chromium  and  their 
analogues  in  a  free  state.  In  other  words,  this  means  that  the  small 
periods,  like  Na,  Mg,  Al,  Si,  P,  S,  Cl,  containing  seven  elements,  do 
not  contain  any  near  analogues  of  chromium,  molybdenum,  <fec.,  and 
therefore  their  true  position  among  the  other  elements  must  be  looked 
for  only  in  those  large  periods  which  contain  two  small  periods,  and 
whose  type  is  seen  in  the  period  containing  :  K,  Ca,  Sc,  Ti,  V,  Cr,  Mn, 
Fe,  Co,  Ni,  Cu,  Zn,  Ga,  Ge,  As,  Se,  Br.  These  large  periods  contain 
Ca  and  Zn,  giving  RO,  Sc,  and  Ga  of  the  third  group,  Ti  and  Ge 
giving  R02,  V  and  As  forming  R205,  Cr  and  Se  of  the  sixth  group, 
Mn  and  Br  of  the  seventh  group,  and  the  remaining  elements,  Fe, 
tlo,  Ni,,  form  connective  members  of  the  intermediate  eighth  group,  to 
the  description  of  the  representatives  of  which  we  shall  turn  in  the 
following  chapters.  We  will  now  proceed  to  describe  manganese, 
Mn  =  55,  as  an  element  of  the  seventh  group  of  the  even  series,  directly 
following  after  Cr  — 52,  which  corresponds  with  Br=80  to  the  same 
degree  that  Cr  does  with  Se  =  79.  For  chromium,  selenium,  and 
bromine  very  close  analogues  are  known,  but  for  manganese  as  yet 
none  have  been  obtained — that  is,  it  is  the  only  representative  of  the 
even  series  in  the  seventh  group.  In  placing  manganese  with  the 

dominate  in  the  form  R05,  whilst  the  lower  form?  are  green  and  blue.  (7)  Zimmermann 
(1881)  determined  the  vapour  densities  of  uranous  bromide,  UBr4,  and  chloride,  UC14 
(19-4  and  13'2),  and'  they  were  found  to  correspond  to  the  formulas  given  above — that  is, 
they  confirmed  the  higher  atomic  weight  U  =  240.  Roscoe,  a  great  authority  on  the 
metals  of  this  group,  was  the  first  to  accept  the  proposed  atomic  weight  of  uranium, 
U  =  240,  which  since  Zimmermann's  work  has  been  generally  recognised. 

15  Uranium  glass,  obtained  by  the  addition  of  the  yellow  salt  K2U207  to  glass,  has  a 
green  yellow  fluorescence,  and  is  sometimes  employed  for  ornaments ;  it  absorbs  the 
violet  rays,  like  the  other  salts  of  uranic  oxide — that  is,  it  possesses  an  absorption  spec- 
trum  in  which  the  violet  rays  are  absent.  The  index  of  refraction  of  the  absorbed  rays 
is  altered,  and  they  are  given  out  again  as  greenish-yellow  rays ;  hence,  compounds  of 
uranic  acid,  when  placed  in  the  violet  portion  of  the  spectrum,  emit  a  greenish-yellow 
light,  and  this  forms  one  of  the  best  examples  (another  is  found  in  a  solution  of  quinine 
sulphate)  of  the  phenomenon  of  fluorescence.  The  rays  of  light  which  pass  through 
uranic  compounds  do  not  contain  the  rays  which  excite  the  phenomena  of  fluorescence 
and  of  chemical  transformation,  as  the  researches  of  Stokes  prove. 


304  PRINCIPLES  OF  CHEMISTRY 

halogens  in  one  group,  the  periodic  system  of  the  elements  only  requires 
that  it  should  bear  an  analogy  to  the  halogens  in  the  higher  type  of 
oxidation — i.e.  in  the  salts  and  acids — whilst  it  requires  that  as  great 
a  difference  should  be  expected  in  the  lower  types  and  elements  as  there 
exists  between  chromium  or  molybdenum  and  sulphur  or  selenium. 
And  this  is  actually  the  case.  The  elements  of  the  seventh  group  form 
a  higher  salt- forming  oxide,  R2O7,  an<*  *ts  corresponding  hydrate, 
HBO4,  and  salts— for  example,  KC104.  Manganese  in  the  form  of 
potassium  permanganate,  KMnO4,  actually  presents  a  great  analogy  in 
many  respects  to  potassium  perchlorate,  KC1O4.  The  analogy  of  the 
crystalline  form  of  both  salts  was  shown  by  Mitscherlich.  The  salts  of 
permanganic  acid  are  also  nearly  all  soluble  in  water,  like  those  of 
perchloric  acid,  and  if  the  silver  salt  of  the  latter,  AgClO4,  be  sparingly 
soluble  in  water,  so  also  is  silver  permanganate,  AgMnO4.  The  specific 
volume  of  potassium  perchlorate  is  equal  to  55,  because  its  speci6o 
gravity=2'54  ;  the  speci6c  volume  of  potassium  permanganate  is  equal 
to  58,  because  its  specific  gravity =2-71.  So  that  the  volumes  of 
equivalent  quantities  are  in  this  instance  approximately  the  same 
whilst  the  atomic  volumes  of  chlorine  (35-5/1-3  =  27)  and  manganese 
(55/7-5)  are  in  the  ratio  4  :  1.  In  a  free  state  the  higher  acids  HC1O4 
and  HMn04  are  both  soluble  in  water  and  volatile,  both  are  powerful 
oxidisers — in  a  word,  their  analogy  is  still  closer  than  that  of  chromic 
and  sulphuric  acids,  and  those  points  of  distinction  which  they  present 
also  appear  among  the  nearest  analogues — for  example,  in  sulphuric  and 
telluric  acids,  in  hydrochloric  and  hydriodic  acids,  <fec.  Besides  Mn2O7 
manganese  gives  a  lower  grade  of  oxidation,  MnO3,  analogous  to 
sulphuric  and  chromic  trioxides,  and  with  it  corresponds  potassium 
manganate,  K2MnO4,  isomorphous  with  potassium  sulphate.16  In  the 
still  lower  grades  of  oxidation,  Mn2O3  and  MnO,  there  is  hardly  any 
similarity  to  chlorine,  whilst  every  point  of  resemblance  disappears 
when  we  come  to  the  elements  themselves — i.e.  to  manganese  and 
chlorine — for  manganese  is  a  metal,  like  iron,  which  combines  directly 
with  chlorine  to  form  a  saline  compound,  MnCl2,  analogous  to  magne- 
sium chloride.17 

Manganese  belongs  to  the  number  of  metals  widely  distributed  in 

16  The  comparison  of  potassium  permanganate  with  potassium  perchlorate,  or  of 
potassium  manganate  with  potassium  sulphate,  shows  directly  that  many  of  the  physical 
and  chemical  properties  of  substances  do  not  depend  on  the  nature  of  the  elements,. 
but  on  the  atomic  types  in  which  they  appear,  on  the  kind  of  movements,  or  on  the  po»i- 
tiona  in  which  the  atoms  forming  the  molecule  occur. 

"  If,  however,  we  compare  the  spectra  (Vol.  I.  p.  665)  of  chlorine,  bromine,  and 
iodine  with  that  of  manganese,  a  certain  resemblance  or  analogy  is  to  be  found  connect- 
ing manganese  both  to  iron  and  to  chlorine,  bromine,  and  iodine. 


CHROMIUM    MOLYBDENUM,   TUNGSTEN,   URANIUM,   ETC       305 

nature,  especially  in  those  localities  where  iron  occurs,  whose  ores 
frequently  contain  compounds  of  manganous  oxide,  MnO,  which  presents 
a  resemblance  to  ferrous  oxide,  FeO,  and  to  magnesia.  In  many  minerals 
magnesia  and  the  oxides  allied  to  it  are  replaced  by  manganous  oxide; 
calcspars  and  magnesites — i.e.  R"CO3  in  general — are  frequently  met 
with  containing  manganous  carbonate,  which  also  occurs  in  a  separate 
state,  although  but  rarely.  The  soil  also  and  the  ash  of  plants  generally 
contain  a  small  quantity  of  manganese.  In  the  analysis  of  minerals 
it  is  generally  found  that  manganese  occurs  together  with  magnesia, 
because,  like  it,  manganous  oxide  remains  in  solution  in  the  presence  of 
ammoniacal  salts,  not  being  precipitated  by  reagents.  The  property  of 
this  manganous  oxido,  MnO,  of  passing  into  the  higher  grades  of  oxida- 
tion under  the  influence' of  heat,  alkalis,  and  air,  gives  an  easy  means 
not  only  of  discovering  the  presence  of  manganese  in  admixture  with 
magnesia,  but  also  of  separating  these  two  analogous  bases.  Magnesia  is 
not  able  to  give  higher  grades  of  oxidation,  whilst  manganese  gives  them 
with  great  facility.  Thus,  for  instance,  an  al/caline  solution  of  sodium 
hypochlorite  produces  a  precipitate  of  manganese  dioxide  in  a  solution  of 
a  manganous  salt  :  MnCl2  +  NaClO  +  2NaHO==Mn02  +  H20  +  3NaCl ; 
whilst  magnesia  is  not  changed  under  these  circumstances,  and  remains 
in  the  form  of  MgCl2.  If  the  magnesia  be  precipitated  owing  to  the 
presence  of.  alkali,  it  may  be  dissolved  in  acetic  acid,  in  which  manganese 
•dioxide  is  insoluble.  The  presence  of  small  quantities  of  manganese 
may  also  be  recognised  by  th6  green  coloration  which  alkalis  acquire 
when  heated  with  manganese  compounds  in  the  air.  This  green  colora- 
tion depends  on  the  property  of  manganese  of  giving  a  green  alkaline 
manganate  :  MnCl2  -f  4KHO  +  02=K2MnO4  +  2KC1  +  2H2O.  Thus 
the  faculty  of  oxidising  in  the  presence  of  alkalis  forms  an  essential 
character  of  manganese.  The  higher  grades  of  oxidation  containing 
Mn2O7  and  Mn03  are  quite  unknown  in  nature,  and  even  Mn02  is  not 
so  widely  spread  in  nature  as  the  ores  composed  of  manganous  com- 
pounds which  are  met  with  nearly  everywhere.  The  most  important 
ore  of  manganese  is  its  dioxide,  or  so-called  peroxide,  Mn02,  which  is 
known  in  mineralogy  as  pyrolusite.  Manganese  also  occurs  as  an 
oxide  corresponding  with  magnetic  iron  ore,  MnO,Mn203=Mn3O4, 
forming  the  mineral  known  as  hausmannite.  The  oxide  .Mn203  also 
occurs  in  nature  as  the  anhydrous  mineral  braunite,  and  in  a  hydrated 
form,  Mn203,H20,  called  manganite.  Both  of  these  often  occur  as  an 
admixture  in  pyrolusite.  Besides  which,  manganese  is  met  with  in 
nature  as  a  rose-coloured  mineral,  rhodonite,  or  silicate,  MnSiOg.  Very 
fine  and  rich  deposits  of  manganese  ores  have  been  found  in.  the 
Caucasus,  the  Urals,  and  along  the  Dnieper.  Those  at  the  Sharapansky 


806  PRINCIPLES  OF  CHEMISTRY 

district  of  the  Government  of  Kutais  and  at  Nicopol  on  the  Dnieper 
are  particularly  rich.  A  large  quantity  of  the  ore  (as  much  as  100,000 
tons  yearly)  is  exported  from  these  localities. 

Thus  manganese  gives  oxides  of  the  following  forms  MnO, 
manganous  oxide,  and  manganous  salts,  MnX2,  corresponding  with  the 
base,  which  resembles  magnesia  and  ferrous  oxide  in  many  respects  ; 
Mn203,  a  very  feeble  base,  giving  salts,  MnX3,  analogous  to  the 
aluminium  and  ferric  salts,  easily  reduced  to  MnX2  ;  MnO2,  dioxide, 
generally  called  peroxide,  an  almost  indifferent  Oxide,  or  feebly  acid  ;18 
Mn03,  manganic  anhydride,  which  forms  salts  resembling  potassium 
sulphate  ; l8  bh  MnaO7l  permanganic  anhydride,  giving  salts  analogous 
to  the  perchlorates. 

All  the  oxides  of  manganese  when  heated  with  acids  give  salts,  MnX2, 
corresponding  with  the  lower  grade  of  oxidation,  manganous  oxide, 
MnO.  Manganic  oxide,  Mn.2O3,  is  a  feebly  energetic  base ;  it  is  true 
that  it  dissolves  in  hydrochloric  acid  and  gives  a  dark  solution  con- 
taining the  salt  MnCU,  but  the  latter  when  heated  evolves  chlorine 
and  gives  a  salt  corresponding  with  manganous  oxide  MnCl2 — i.e.  at 
first:  Mn2O3  +  6HCl=3H2O  +  Mn2Cl6,  and  then  the  Mn2Cl6  decora- 
poses  into  2MnCl2  +  Cl2.  None  of  the  remaining  higher  grades  of 
oxidation  have  a  basic  character,  but  act  as  oxiditsing  agents  in  the 
presence  of  aiids,  disengaging  oxygen  and  passing  into  salts  of  the  lower 
grade  of  oxidation  of  'manganese,  MnO.  Owing  to  this  circumstance, 
the  manganous  salts  are  often  obtained  ;  they  are,  for  instance,  left  in 
the  residue  when  the  dioxide  is  used  for  the  preparation  of  oxygen  and 
chlorine.19 

18  The  name  '  peroxide '  should  only  be  retained  for  those  highest  oxides  (and  MnO2 
stands  between  MnO  and  MnO3)  which  either  by  a  direct  method  of  double  decomposition 
are  able  to  give  hydrogen  peroxide  or  contain  a  larger  proportion  of  oxygen  than  the 
base  or  the  acid,  just  as  hydrogen  peroxide  contains  more  oxygen  than  water.  Their 
type  will  be  H2O2,  and  they  are  exemplified  by  barium  peroxide,  BaO2,  and  sulphur 
peroxide,  82O7,  &c.  Such  a  dioxide  as  MnO2  is,  in  all  probability,  a  salt — that  is,  a 
manganous  manganate,  MnOjMnO,  and  also,  as  a  basic  salt  of  a  feeble  base,  capable  of 
combining  with  alkalis  and  acids.  Hence  the  name  of  manganese  peroxide  should 
be  abandoned,  and  replaced  by  manganese  dioxide.  PbO2  is  better  termed  lead  dioxide 
than  peroxide.  Bisulphide  of  manganese,  MnS2)  corresponding  to  iron  pyrites,  FeS2, 
sometimes  occurs  in  nature  in  fine  octahedra  (and  cube  combinations),  for  instance,  in 
Sicily ;  it  is  called  Hauerite. 

18  bis  On  comparing  the  manganates  with  the  permanganates— for  example,  K2Mn04 
with  KMnO4 — we  find  that  they  differ  in  composition  by  the  abstraction  of  one  equivalent 
of  the  metal.     Snch  a  relation  in  composition  produced  by  oxidation  is  of  frequent 
occurrence— for  instance,  K4Fe(CN)6  in  oxidising  gives  K3Fe(CN)6  ;  H^S04  in  oxidising 
gives  persulphurio  acid,  HSO4,  or  HaS7O8 ;  H^O  forms  HO  or  H.2O2,  &c. 

19  In  the  preparation  of  oxygen  from  the  dioxide  by  means  of  H2SO4,  MnSO4  is 
formed ;  in  the  preparation  of  chlorine  from  HC1  and  MnO2,  MnClj  is  obtained.    These 
two  manganous  salts  may  be  taken   as  examples  of   compounds  MnX?.     Manganous 
sulphate  generally  contains  various  impurities,  and  also  a  large  amount  of  iron  salt 


CHROMIUM,  MOLYBDENUM,  TUNGSTEN,   URANIUM,  ETC.     80? 

As  the  salts  of  manganous  oxide  MnX2  closely  resemble  (and  are 
isomorphous  with)  the  saltflof  magnesia  MgX2  in  many  respects  (with 

(from  the  native  MnO.^),  from  which  it  cannot  be  freed  by  crystallisation.  Their 
removal  may,  however,  be  effected  by  mixing  a  portion  of  the  liquid  with  a  solution  of 
sodium  carbonate ;  a  precipitate  of  manganous  carbonate  is  then  formed.  This  pre- 
cipitate is  collected  and  washed,  and  then  added  to  the  remaining  mass  of  the  impure 
solution  of  manganous  sulphate ;  on  heating  the  solution  with  this  precipitate,  the 
whole  of  the  iron  is  precipitated  as  oxide.  This  is  due  to  the  fact  that  in  the  solution  of 
the  manganese  dioxide  in  sulphuric  acid  the  whole  of  the  iron  is  converted  into  the 
ferric  state  (because  the  dioxide  acts  as  an  oxidising  agent),  which,  as  an  exceedingly 
feeble  base  precipitated  by  calcium  carbonate  and  other  kindred  salts,  is  also  precipitated 
by  manganous  carbonate.  After  being  treated  in  this  manner,  the  solution  of  manganous 
sulphate  is  further  purified  by  crystallisation.  If  it  be  a  bright  red  colour,  it  is  due  to 
the  presence  of  higher  grades  of  oxidation  of  manganese ;  they  may  be  destroyed  by 
boiling  the  solution,  when  the  oxygen  from  the  oxides  of  manganese  is  evolved  and  a 
very  faintly  coloured  solution  of  manganous  sulphate  is  obtained.  This  salt  is  remarkable 
for  the  facility  with  which  it  gives  various  combinations  with  water.  By  evaporating 
the  almost  colourless  solution  of  manganous  sulphate  at  very  low  temperatures,  and  by 
cooling  the  saturated  solution  at  about  0°,  crystals  are  obtained  containing  7  atoms  of 
•water  of  crystallisation,  MnS04,7H.,0,  which  ars  isomorphous  with  cobaltous  and  ferrous 
sulphates.  These  crystals,  even  at  10°,  lose  5  p.c.  of  water,  and  completely  effloresce  at 
15^  losing  about  20  p.c.  of  water.  By  evaporating  a  solution  of  the  salt  at  the  ordinary 
temperature,  but  not  above  20°,  crystals  are  obtained  containing  5  mol.  H2O,  which 
are  isomorphous  with  copper  sulphate  ;  whilst  if  the  crystallisation  be  carried  on  between 
20 '  and  30°,  large  transparent  prismatic  crystals  are  formed  containing  4  mol.  H2O  (see 
Nickel).  A  boiling  solution  also  deposits  these  crystals  together  with  crystals  containing 
3  mol.  H20,  whilst  the  first  salt,  when  fused  and  boiled  with  alcohol,  gives  crystals 
containing  2  mol.  H20.  Graham  obtained  a  monohydrated  salt  by  drying  the  salt  at 
about  200  \  The  last  atom  of  water  is  eliminated  with  difficulty,  as  is  the  case  with  all 
salts  like  MgS04;iH^O.  The  crystals  containing  a  considerable  amount  of  water  are 
rose-coloured,  and  the  anhydrous  crystals  are  colourless.  The  solubility  of  MnSO4,4H2Q 
(Chapter  I.,  Note  24)  per  100  parts  of  water  is  :  at  10°,  127  parts;  at  S7°'5,  149  parts  ;  at 
75°,  145  parts  ;  and  at  101°,  92  parts.  Whence  it  is  seen  that  at  the  boiling-point  this  salt 
is  less  soluble  than  at  lower  temperatures,  and  therefore  a  solution  saturated  at  the 
ordinary  temperature  becomes  turbid  when  boiled.  Manganous  sulphate,  being  analogous 
to  magnesium  sulphate,  is  decomposed,  like  the  latter,  when  ignited,  but  it  does  not  then 
leave  manganous  oxide,  but  the  intermediate  oxide,  Mn504.  It  gives  double  salts  with 
the  alkali  sulphates.  With  aluminium  sulphate  it  forms  fine  radiated  crystals,  whose 
composition  resembles  that  of  the  alums— namely,  MnAl.,(S04)4,  24H20.  This  salt  is 
easily  soluble  in  water,  and  occurs  in  nature. 

Manganous  chloride,  MC12,  crystallises  with  4  mol.  H2O,  like  the  ferrous  salt,  and 
not  with  6  mol.  H20  like  many  kindred  salts — for  example,  those  of  cobalt,  calcium,  and 
magnesium ;  100  parts  of  water  dissolve  38  parts  of  the  anhydrous  salt  at  10°  and  55 
parts  at  62°.  Alcohol  also  dissolves  manganous  chloride,  and  the  alcoholic  solution 
burns  with  a  red  flame.  This  salt,  like  magnesium  chloride,  readily  forms  double  salts. 
A  solution  of  borax  gives  a  dirty  rose-coloured  precipitate  having  the  composition 
MnH4(B05)2H.O,  which  is  used  as  a  drier  in  paint-making.  Potassium  cyanide  pro- 
duces a  yellowish-grey  precipitate,  MnC2N2,  with  raanganons  salts,  soluble  in  »n  excess  of 
the  reagent,  a  double  salt,  K4MnC,.;Nti,  corresponding  with  potassium  ferrocyanide, 
being  formed.  On  evaporation  of  this  solution,  a  portion  of  the  manganese  is  oxidised 
and  precipitated,  whilst  a  salt  corresponding  to  Gmelin's  red  salt,  K^MnCeN6  (see 
Chapter  XXII.),  remains  in  solution.  Sulphuretted  hydrogen  does  not  precipitate 
salts  of  manganese,  not  even  the  acetate,  but  ammonium  sulphide  gives  a  flesh-coloured 
precipitate,  MnS ;  at  820°  this  sulphide  of  manganese  passes  into  a  green  variety  (Antony). 
Oxalic  acid  in  strong  solutions  of  manganous  salts  gives  a  white  precipitate  of  the 


808  PRINCIPLES  OF  CHEMISTRY 

the  exception  of  the  fact  that  MnX2  are  rose  coloured  and  are  easily 
oxidised  in  the  presence  of  alkalis),  we  will  not  dwell  upon  them,  but 

oxalate,  MnC2O4.  This  precipitate  is  insoluble  in  water,  and  is  used  for  the  preparation 
of  manganous  oxide"  itself  because  it  decomposes  like  oxalic  acid  when  ignited  (in  a  tube 
without  access  of  air),  with  the  formation  of  carbonic  anhydride,  carbonic  oxide,  and 
manganous  oxide.  Manganoua  oxide  thus  obtained  is  a  green  powder,  which  however 
oxidises  with  such  facility  that  it  burns  in  air  when  brought  into  contact  with  an 
incandescent  substance,  and  passes  into  the  red  intermediate  oxide  Mn3O4.  In  solutions 
of  manganous  salts,  alkalis  produce  a  precipitate  of  the  hydroxide  MnH2O2,  which 
rapidly  absorbs  oxygen  in  the  presence  of  air  and  gives  the  brown  intermediate  oxide, 
or,  more  correctly  speaking,  its  hydrate. 

Manganous  oxide,  besides  being  obtained  by  the  above-described  method  from  man- 
ganous oxalate,  may  also  be  obtained  by  igniting  the  higher  oxides  jn  a  stream  of 
hydrogen,  and  also  from  manganese  carbonate.  The  manganous  oxide  ignited  in  the 
presence  of  hydrogen  acquires  a  great  density,  and  is  no  longer  so  easily  oxidised.  It 
may  also  be  obtained  in  a  crystalline  form,  if  during  the  ignition  of  the  carbonate  or 
higher  oxide  a  trace  of  dry  hydrochloric  acid  gas  be  passed  into  the  current  of  hydrogen. 
It  Is  thus  obtained  in  the  form  of  transparent  emerald  green  crystals  of  the  regular 
system,  and  in  this  state  is  easily  soluble  in  acids. 

Manganous  oxide  in  oxidising  gives  the  red  oxide  of  manganese,  Mn3O4.  This  is  the 
most  stable  of  all  the  oxides  of  manganese ;  it  is  not  only  stable  at  the  ordinary  but  also 
at  a  high  temperature — that  is,  it  does  not  absorb  or  disengage  oxygen  spontaneously. 
When  ignited,  all  the  higher  oxides  of  manganese  pass  into  it  by  losing  oxygen,  and 
mangauous  oxide  by  absorbing  oxygen.  This  oxide  does  pot  give  any  distinct  salts, 
but  it  dissolves  in  sulphuric  acid,  forming  a  dark  red  solution,  which  contains  both 
manganous  and  manganic  (of  the  oxide,  Mn2O3)  snlphates.  The  latter  with  potassium 
sulphate  gives  a  manganese  alum,  in  which  the  alumina  is  replaced  by  its  isomorphous 
oxide  of  manganese.  But  this  alum,  like  the  solution  of  the  intermediate  oxide  in  sul- 
phuric acid,  evolves  oxygen  and  leaves  a  manganous 'salt  when  slightly  heated. 

Manganese  dioxide  is  still  less  basic  than  the  oxide,  and  disengages  oxygen  or  a 
halogen  in  the  presence  of  acids,  forming  manganous  salts,  like  the  oxide.  However,  if  it 
be  suspended  in  ether,  and  hydrochloric  acid  gas  passed  into  the  mixture,  which  is  kept 
cool,  the  ether  acquires  a  green  colour,  owing  to  the  formation  of  tetra- chloride  of 
manganese,  MnCl4,  corresponding  with  the  dioxide  which  passes  into  solution.  It  i» 
however  very  unstable,  being  exceedingly  easily  decomposed  with  the  evolution  of 
chlorine.  The  corresponding  fluoride,  MnF4)  obtained  by  Nickle's  is  much  more  stable. 
At  all  events,  manganese  dioxide  does  not  exhibit  any  well-defined  basic  character,  but 
has  rather  an  acid  character,  which  is  particularly  shown  in  the  compounds  MnF4  and 
MnCl4  just  mentioned,  and  in  the  property  of  manganese  dioxide  of  combining  with 
alkalis.  If  the  higher  grades  of  oxidation  of  manganese  be  deoxidised  in  the  presence  of 
alkalis,  they  frequently  give  the  dioxide  combined  with  the  alkali— for  example,  in  the 
presence  of  potash  a  compound  is  formed  which  contains  K2O,5MnO2,  which  shows  the 
weak  acid  character  of  this  oxide.  When  ignited  in  the  presence  of  sodium  compounds 
manganese  dioxide  frequently  forms  NaaO.SMnO.,  and  Na2O,12MnO2)  and  lime  when 
heated  with  MnO.j  gives  from  CaO,3Mn02  to  (CaO.)2,Mn02  (Rousseau)  according  to  the 
temperature.  Besides  which,  perhaps,  MnO.>  is  a  saline  compound,  containing 
MnOMnO3  or  (MnO)3Mn2O7,  and  there  are  reactions  which  support  such  a  view  (Spring, 
Richards,  Traube,  and  others) ;  for  instance  it  is  known  that  manganous  chloride  and 
potassium  permanganate  give  the  dioxide  in  the  presence  of  alkalis. 

Manganese  dioxide  may  be  obtained  from  manganous  salts  by  the  action  of  oxidis- 
ing agents.  If  manganous  hydroxide  or  carbonate  be  shaken  up  in  water  through 
which  chlorine  is  passed,  the  hypochlorite  of  the  metal  is  not  formed,  as  is  the  case 
with  certain  other  oxides,  but  manganese  dioxide  is  precipitated  2Mn02H2  +  C19 
c=MnC!2  +  MuO2,H3O  +  H2O.  Owing  to  this  fact,  hypochlorites  in  the  presence  of  alkalis 
and  acetic  acid  when  added  to  a  solution  of  manganous  salts  give  hydrated  manganese 


CHROMIUM,  MOLYBDENUM,  TUNGSTEN  URANIUM,  ETC.  309 

limit  ourselves  to  illustrating  the  chemical  character  of  manganese  by 
describing  the  metal  and  its  corresponding  acids.  The  fact  alone  that 
the  oxides  of  manganese  are  not  reduced  to  the  metal  when  ignited  in 
hydrogen  (whilst  the  oxides  of  iron  give  metallic  iron  under  these 
circumstances),  but  only  to  manganous  oxide,  MnO,  shows  that 
manganese  has  a  considerable  affinity  for  oxygen — that  is,  it  is  difficult 
to  reduce.  This  may  be  effected,  however,  by  means  of  charcoal  or 
sodium  at  a  very  high  temperature.  A  mixture  of  one  of  the  oxides  of 
manganese  with  charcoal  or  organic  matter  gives  fused  metallic  man- 
ganese under  the  powerful  heat  developed  by  coke  with  an  artificial 
draught.  The  metal  was  obtained  for  the  first  time  in  this  manner  by 
Gahn,  after  Pott,  and  more  especially  Scheele,  had  in  the  last  century 
shown  the  difference  between  the  compounds  of  iron  and  manganese 
(they  were  previously  regarded  as  being  the  same).  Manganese  is  pre- 
pared by  mixing  one  of  its  oxides  in  a  finely-divided  state  with  oil  and 
soot ;  the  resultant  mass  is  then  first  ignited  in  order  to  decompose 
the  organic  matter,  and  afterwards  strongly  heated  in  a  charcoal  crucible. 
The  manganese  thus  obtained,  however,  contains,  as  a  rule,  a  consider- 
able amount  of  silicon  and  other  impurities.  Its  specific  gravity  varies 
between  7*2  and  8'0.  It  has  a  light  grey  colour,  a  feebly  metallic 
lustre,  and  although  it  is  very  hard  it  can  be  scratched  by  a  file.  It 
rapidly  oxidises  in  air,  being  converted  into  a  black  oxide  ;  water  acts 
on  it  with  the  evolution  of  hydrogen — this  decomposition  proceeds  very 
rapidly  with  boiling  water,  and  if  the  metal  contain  carbon.20 

dioxide,  as  was  mentioned  above.  Manganous  nitrate  also  leaves  manganese  dioxide 
when  heated  to  200°  It  is  also  obtained  from  manganous  and  manganic  salts  of  tha 
alkalis,  when  they  are  decomposed  in  the  presence  of  a  small  amount  of  acid ;  the  prac- 
tical method  of  converting  the  salts  MnX2  into  the  higher  grades  of  oxidation  is  given  in 
Chapter  II.,  Note  6. 

20  Other  chemists  have  obtained  manganese  by  different  methods,  and  attributed 
different  properties  to  it.  This  difference  probably  depends  on  the  presence  of  carbon 
in  different  proportions.  Deville  obtained  manganese  by  subjecting  the  pure  dioxide, 
mixed  with  pure  charcoal  (from  burnt  sugar),  to  a  strong  heat  in  a  lime  crucible  until  the 
resultant  metal  fused.  The  metal  obtained  had  a  rose  tint,  like  bismuth,  and  like  it 
•was  very  brittle,  although  exceedingly  hard.  It  decomposed  water  at  'the  ordinary 
temperature.  Brunner  obtained  manganese  having  a  specific  gravity  of  about  7'2,  which 
decomposed  water  very  feebly  at  the  ordinary  temperature,  did  not  oxidise  in  air,  and 
was  capable  of  taking  a  bright  polish,  like  steel ;  it  had  the  grey  colour  of  cast  iron,  was 
very  brittle,  and  hard  enough  to  scratch  steel  and  glass,  like  a  diamond.  Brunner'a 
method  was  as  follows  .  He  decomposed  the  manganese  fluoride  (obtained  as  a  soluble 
compound  by  the  action  of  hydrofluoric  acid  on  manganese  carbonate)  with  sodium,  by 
mixing  these  substances  together  in  a  crucible  and  covering  the  mixture  with  a  layer  of 
ealt  and  fluor  spar ,  after  which  the  crucible  was  first  gradually  heated  until  the  reaction 
began,  and  then  strongly  heated  in  order  to  fuse  the  metal  separated.  Glatzel  (1889) 
obtained  25  grms.  of  manganese,  having  a  grey  colour  and  sp.  gr.  7-89,  by  heating  a 
mixture  of  100  grins,  of  MuCl2  with  200  grms.  KC1  and  15  grms.  Mg  to  a  bright  white 
heat.  Moissan  and  others,  by  heating  the  oxides  of  manganese  with  carbon  in  the  electrio 


310  PRINCIPLES  OF  CHEMISTRY 

It  has  been  shown  above  that  if  manganese  dioxide,  or  any 
lower  oxide  of  manganese,  be  heated  with  an  alkali  in  the  presence  of 
air,  the  mixture  absorbs  oxygen,21  and  forms  an  alkaline  manganate  of  a 
green  colour:  2KHO  +  Mn02  +  O  =  K2MnO4  +  H2O.  Steam  is  disen- 
gaged during  the  ignition  of  the  mixture,  and  if  this  does  not  take 
place  there  is  no  absorption  of  oxygen.  The  oxidation  proceeds  much 
more  rapidly  if,  before  igniting  in  air,  potassium  chlorate  or  nitre  be 
added  to  the  mixture,  and  this  is  the  method  of  preparing  potassium 
manganate,  K2MnOj.  The  resultant  mass  dissolved  in  a  small  quantity 
of  water  gives  a  dark  green  solution,  which,  when  evaporated  under  the 
receiver  of  an  air  pump  over  sulphuric  acid,  deposits  green  crystals  of 
exactly  the  same  form  as  potassium  sulphate —namely,  six-sided  prisms 
and  pyramids.  The  composition  of  the  product  is  not  changed  by  being 
redissolved,  if  perfectly  pure  water  free  from  air  and  carbonic  acid  bo 
taken.  But  in  the  presence  of  even  very  feeble  acids  the  solution  of 
this  salt  changes  its  colour  and  becomes  red,  and  deposits  manganese 
dioxide.  The  same  decomposition  takes  place  when  the  salt  is  heated 
with  water,  but  when  diluted  with  a  large  quantity  of  unboiled  water 
manganese  dioxide  does  not  separate,  although  the  solution  turns  red. 
This  change  of  colour  depends  on  the  fact  that  potassium  manganate, 
K2MnO4,  whose  solution  is  green,  is  transformed  into  potassium  per- 
manganate, KMnO4,  whose  solution,  is  of  a  red  colour.  The  reaction 
proceeding  under  the  influence  of  acids  and  a  large  quantity  of  water 

furnace,  obtained  carbides  of  manganese — for  example,  Mn5C — and  remarked  that  the  metal 
volatilised  in  the  heat  of  the  voltaic  arc.  Metallic  manganese  is,  however,  not  prepared  on' 
a  large  scale,  but  only  its  alloys  with  carbon  (they  readily  and  rapidly  oxidise)  and/erro- 
manganese  or  a  coarsely  crystalline  alloy  of  iron,  manganese  and  carbon,  which  is 
emelted  in  blast-furnaces  like  pig-iron  (see  Chapter  XXII.)  This  ferro-manganese  is 
employed  in  the  manufacture  of  steel  by  Bessemer's  and  other  processes  (see  Chapter 
XXII.)  and  for  the  manufacture  of  manganese  bronze.  However,  in  America,  Green  and 
Wahl  (1895)  obtained  almost  pure  metallic  manganese  on  a  large  scale.  They  first  treat 
the  ore  of  MnO2  with  80  p.c.  sulphuric  acid  (which  extracts  all  the  oxides  of  iron 
present  in  the  ore),  and  then  heat  it  in  a  reducing  flame  to  convert  it  into  MnO,  which 
they  mix  with  a  powder  of  Al,  lime  and  CaF^  (as  a  flux),  and  heat  the  mixture  in  a 
crucible  lined  with  magnesia  ;  a  reaction  immediately  takes  place  at  a  certain  temperature, 
and  a  metal  of  specific  gravity  7'3  is  obtained,  which  only  contains  a  small  trace  of  iron. 

Manganese  gives  two  compounds  with  nitrogen,  Mn5N2  and  Mn^No.  They  were 
obtained  by  Prelinger  (1894)  from  the  amalgam  of  manganese  Mn^Hg^  (obtained  on  a 
mercury  anode  by  the  action  of  an  electric  current  upon  a  solution  of  MnCl  >) ;  the 
mercury  may  be  removed  from  this  amalgam  by  heating  it  in  an  atmosphere  of  hydrogen, 
and  then  metallic  manganese  is  obtained  as  a  grey  porous  mass  of  specific  gravity  7'42. 
If  this  amalgam  be  heated  in  dry  nitrogen  it  gives  Mn5N2  (grey  powder,  sp.  gr.  6'58),  but 
if  heated  in  an  atmosphere  of  NH3  it  gives  (as  also  does  MnaN2)  Mn5N2,  (a  dark  mass 
with  a  metallic  lustre,  sp.  gr.  G'21),  which,  when  heated  in  nitrogen  is  converted  into 
MiiiN-i,  and  if  heated  in  hydrogen  evolves  NH3  and  disengages  hydrogen  from  a  solution 
of  NH4C1.  At  all  events,  manganese  is  a  metal  which  decomposes  water  more  easily 
than  iron,  nickel,  and  cobalt. 

»  Volume  I.  D.  157»  Note  7. 


CHROMIUM,  MOLYBDENUM,  TUNGSTEN,  URANIUM,  ETC.  811 

is  expressed  in  the  following  manner  :  3K2MnO4  4  2H20=2KMn04 
+  Mn02  +  4KHO.  If  there  is  a  large  proportion  of  acid  and  the  de- 
composition is  aided  by  heat,  the  manganese  dioxide  and  potassium 
permanganate  are  also  decomposed,  with  formation  of  raanganous  salt. 
Exactly  the  same  decomposition  as  takes  place  under  the  action  of  acids 
is  also  accomplished  by  magnesium  sulphate,  which  reacts  in  many  cases 
like  an  acid.  When  water  holding  atmospheric  oxygen  in  solution  acts 
on  a  solution  of  potassium  manganate,  the  oxygen  combines  directly 
with  the  manganate  and  forms  potassium  permanganate,  without 
precipitating  manganese  dioxide,  2K2MnO4  4-  O  +  H2O  =  2KMnO4 
+  2KHO.  Thus  a  solution  of  potassium  manganate  undergoes  a  very 
characteristic  change  in  colour  and  passes  from  green  to  red  ;  hence  this 
salt  received  the  name  of  chameleon  mineral.2'2 

Potassium  permanganate,  KMnO4,  crystallises  in  well-formed,  long 
red  prisms  with  a  bright  green  metallic  lustre.  In  the  arts  the  potash 
is  frequently  replaced  by  soda,  and  by  other  alkaline  bases,  but  no  salt 
of  permanganic  acid  crystallises  so  well  as  the  potassium  salt,  and 
therefore  this  salt  is  exclusively  used  in  chemical  laboratories.  On& 
part  of  the  crystalline  salt  dissolves  in  15  parts  of  water  at  the  ordinary 
temperature.  The  solution  is  of  a  very  deep  red  colour,  which  is  so 
intense  that  it  is  still  clearly  observable  after  being  highly  diluted  with 
writer.  In  a  solid  state  it  is  decomposed  by  heat,  with  evolution  of- 

22  It  was  known -to  the  alchemists  by  this  name,  but  the  true  explanation  of  tits- 
change  in  colour  is  due  to  the  researches  of  Chevillot,  Edwards,  Mitscherlich,  and. 
Forchhammer.  The  change  in  colour  of  potassium  manganate  is  due  to  its  insta- 
bility and  to  its  splitting  up  into  two  other  manganese  compounds,  a  higher  and  a/ 
lower  8MnO3  =  Mn^O;  +  MnO.2.  Manganese  trioxide  is  really  decomposed  in  this  manner 
by  the  action  of  water  (see  later)  :  8MnO3  +  H2O  =  2MnH04  +  MnO2  (Franke,  Thorpe, 
and  Humbly).  The  instability  of  the  salt  is  proved  by  the  fact  of  its  being  deoxidised  by 
organic  matter,  with  the  formation  of  manganese  dioxide  .and  alkali,  so  that,  for  instance, 
a  solution  of  this  salt  cannot  be  filtered  through  paper.  The  presence  of  an  excess  of 
alkali  increases  the  stability  of  the  salt ;  when  heated  it  breaks  up  in  the  presence  of 
water,  with  the  evolution  of  oxygen. 

The  method  of  preparing  potassium  permanganate  will  be  understood  from  the  above. 
There  are  many  recipes  for  preparing  this  substance,  as  it  is  now  used  in  considerable 
quantities  both  for  technical  and  laboratory  purposes.  But  in  all  cases  the  essence  of 
the  methods  is  one  and  the  same :  a  mixture  of  alkali  with  any  oxide  of  manganese 
(even  manganons  hydroxide,  which  may  be  obtained  from  manganous  chloride)  is  first 
heated  in  the  presence  of  air  or  of  an  oxidising  substance  (for  the  sake  of  rapidity,  with 
potassium  chlorate) ,  the  resultant  mass  is  then  treated  with  water  and  keated,  when 
manganese  dioxide  is  precipitated  and  potassium  permanganate  remains  in  solution. 
This  solution  may  be  boiled,  as  the  liquid  will  contain  free  alkali ;  but  the  solution 
cannot  be  evaporated  to  dryness,  because  a  strcnj  solution,  as  well  as  the  solid  salt,  is 
decomposed  by  heat. 

By  adding  a  dilute  solution  of  manganons  sulphate  to  a  boiling  mixture  oi  lead 
dioxide  and  dilute  nitric  acid,  the  whole  of  the  manganese  may  be  converted  into  per* 
manganic  acid  (Crum) 


312  PRINCIPLES  OF  CHEMISTRY 

oxygen,  a  residue  consisting  of  the  lower  oxides  of  manganese  and 
potassium  oxide  being  left.22  bi3  A  mixture  of  permanganate  of  potas- 
sium, phosphorous  and  sulphur  takes  fire  when  struck  or  rubbed,  a 
mixture  of  the  permanganate  with  carbon  only  takes  fire  when  heated, 
not  when  struck.  The  instability  of  the  salt  is  also  seen  in  the  fact 
that  its  solution  is  decomposed  by  peroxide  of  hydrogen,  which  at  the 
same  time  it  decomposes.  A  number  of  substances  reduce  potassium 
permanganate  to  manganese  dioxide  (in  which  case  the  red  solution 
becomes  colourless).23  Many  organic  substances  (although  far  from 
all,  even  when  boiled  in  a  solution  of  permanganate)  act  in  this  manner, 
being  oxidised  at  the  expense  of  a  portion  of  its  oxygen.  Thus,  a 
solution  of  sugar  decomposes  a  cold  solution  of  potassium  permanganate. 
In  the  presence  of  an  excess  of  alkali,  with  a  small  quantity  of  sugar, 
the  reduction  leads  to  the  formation  of  potassium  manganate,  because 
2KMn04  +  2KHO=O  +  2K2Mn04  +  H2O.  With  a  considerable  amount 
of  sugar  and  a  more  prolonged  action,  the  solution  turns  brown  and 
precipitates  manganese  dioxide  or  even  oxide.  In  the  oxidation  of 
many  organic  bodies  by  an  alkaline  solution  of  KMnO4  generally  three- 
fcighths  of  the  oxygen  in  the  salt  are  utilised  for  oxidation  :  2KMn04 
=K2O  -f  2MnO2  +  O3.  A  portion  of  the  alkali  liberated  is  retained  by 
the  manganese  dioxide,  and  the  other  portion  generally  combines  with 
the  substance  Oxidised,  because  the  latter  most  frequently  gives  an  acid 
with  an  excess  of  alkali.  A  solution  of  potassium  iodide  acts  in  a 
similar  manner,  being  converted  into  potassium  iodate  at  the  expense  of 
the  three  atoms  of  oxygen  disengaged  by  two  molecules  of  potassium 
permangan'ate. 

In  the  presence  of  acids,  potassium  permanganate  acts  as  an  oxidising 
agent  with  still  greater  energy  than  in  the  presence  of  alkalis.  At  any 
rate,  a  greater  proportion  of  oxygen  is  then  available  for  oxidation, 
namely,  not  §,  as  in  the  presence  of  alkalis,  but  $,  because  in  the  first 
instance  manganese  dioxide  is  formed,  and  in  the  second  case  mangan- 
ous  oxide,  or  rather  the  salt,  MnX2,  corresponding  with  it.  Thus,  for 

82  bii  The  solution  of  this  salt  with  an.  excess  of  impure  commercial  alkali  generally 
acquires  a  green  tint. 

23  A  solution  of  potassium  permanganate  gives  a  beautiful  absorption  spectrum 
(Chapter  XIII.)  If  the  light  in  passing  through  this  solution  loses  a  portion  of  its  raya 
in  it  (if  one  may  so  account  for  it),  this  is  partially  explained  by  the  increased  oxidising 
power  which  the  solution  then  acquires.  We  may  here  also  remark  that  a  dilute  solution 
of  permanganate  of  potassium  forms  a  colourless  solution  with  nickel  salts,  because 
the  green  colour  of  the  solution  of  nickel  salts  is  complementary  to  the  red.  Such  a 
decolorised  solution,  containing  a  large  proportion  of  nickel  and  a  small  proportion  of 
manganese,  decomposes  after  a  time,  throws  down  a  precipitate,  and  re-acquires  the 
green  colour  proper  to  the  nickel  salts.  The  addition  of  a  solution  of  a  cobalt  salt  (rose- 
red)  to  the  nickel  salt  also  destroys  the  colour  of  both  salts. 


CHROMIUM,  MOLYBDENUM,  TUNGSTEN,   URANIUM,   ETC.     313 

instance,  in  the  presence  of  an  excess  of  sulphuric  acid,  the  decom- 
position is  accomplished  in  the  following  manner  :  iiKMnC^  +  SH^SO,! 
=K2SO4-f2MnS04  + 3H2O  +  5O.  This  decomposition,  however,  does 
not  proceed  directly  on  mixing  a  solution  of  the  salt  with  sulphuric 
acid,  and  crystals  of  the  salt  even  dissolve  in  oil  of  vitriol  without  the 
evolution  of  oxygen,  and  this  solution  only  decomposes  by  degrees  after 
a  certain  time.  This  is  due  to  the  fact  that  sulphuric  acid  liberates 
free  permanganic  acid  from  the  permanganate,24  which  acid  is  stable 
in  solution.  But  if,  in  the  presence  of  acids  and  a  permanganate,  there 

24  If  sulphuric  acid  is  allowed  to  act  on  potassium  permanganate  without  any  special 
precautions,  a  large  amount  of  oxygen  is  evolved  (it  may  even  explode  and  inflame),  and 
ft  violet  spray  of  the  decomposing  permanganic  acid  is  given  off.  But  if  the  pure  salt 
(i.e.  free  from  chlorine)  be  dissolved  in  pure  well-cooled  sulphuric  acid,  without 
any  rise  in  temperature,  a  green-coloured  liquid  settles  at  the  bottom  of  the  vessel. 
This  liquid  does  not  contain  any  sulphuric  acid,  and  consists  of  permanganic  anhydride, 
Mn207  (Aschoff,  Terreil).  It  is  impossible  to  prepare  any  considerable  quantity  of  the 
anhydride  by  this  method,  as  it  decomposes  with  an  explosion  as  it  collects,  evolving 
oxygen  and  leaving  red  oxide  of  manganese.  Permanganic  anhydride,  Mn.jO7,  in 
dissolving  in  sulphuric  acid,  gives  a  green  solution,  which  (according  to  Franke,  1887)  con- 
tains a  compound  Mn2S010  =  (MnO3)iSO4 — that  is,  sulphuric  acid  in  which  both  hydro- 
gens are  replaced  by  the  group  Mn03,  which  is  combined  with  OK  in  permanganate  of 
potassium.  This  mixture  with  a  small  quantity  of  water  gives  Mn.^O7,  according  to  the 
equation:  (MnO3)iS044-HiO  =  H^SC^  +  Mn.2O7,  and  when  heated  to  80°  it  gives  man- 
ganese trioxide,  (Mn05)2SO4-f  H.jO  =  2MnOi  +  H2S04  +  O.  Pure  manganese  trioxide  is 
(obtained  if  the  solution  of  (MnO3)2SO4  be  poured  in  drops  on  to  sodium  carbonate.  Then, 
together  with  carbonic  anhydride,  a  spray  of  manganese  trioxide  passes  over,  which 
may  be  collected  in  a  well-cooled  receiver,  and  this  shows  that  the  reaction  proceeds 
according  to  the  equation  .  (Mn03)2SO4  +  Na,CO3  =  Na.;S04  +  2MnO3  +  CO2  +  0  (Thorpe). 
The  trioxide  is  decomposed  by  water,  forming  manganese  dioxide  and  a  solution  of 
permanganic  acid:  3MnO5  +  H4O  =  MnOj  +  2HMn04.  The  same  acid  is  obtained  by 
dissolving  permanganic  anhydride  in  water. 

Barium  permanganate  when  treated  with  sulphuric  acid  gives  the  same  acid.  This 
barium  salt  may  be  prepared  by  the  action  of  barium  chloride  on  the  difficultly  soluble 
silver  permanganate,  AgMn04,  which  is  precipitated  on  mixing  a  strong  solution  of  the 
potassium  salt  with  silver  nitrate.  The  solution  of  permanganic  acid  forms  a  bright  red 
liquid  which  reflects  a  dark  violet  tint.  A  dilute  solution  has  exactly  the  same  colour 
as  that  of  the  potassium  salt.  It  deposits  manganese  dioxide  when  exposed  to  the  action 
of  light,  and  also  when  heated  above  60°,  and  this  proceeds  the  more  rapidly  the  more 
dilute  the  solution.  It  shows  its  oxidising  properties  in  many  cases,  as  already 
mentioned.  Even  hydrogen  gas  is  absorbed  by  a  solution  of  permanganic  acid ;  and 
charcoal  and  sulphur  are  also  oxidised  by  it,  as  they  are  by  potassium  permanganate. 
This  may  be  taken  advantage  of  in  analysing  gunpowder,  because  when  it  is  treated 
with  a  solution  of  potassium  permanganate,  all  the  sulphur  is  converted  into  sulphuric 
acid  and  all  the  charcoal  into  carbonic  anhydride.  Finely-divided  platinum  immediately 
decomposes  permanganic  acid.  With  potassium  iodide  it  liberates  iodine  (which  may 
afterwards  be  oxidised  into  iodic  acid)  (Mitscherlich,  Fromherz,  Aschoff,  and  others). 
Ammonia  does  not  form  a  corresponding  salt  with  free  permanganic  acid,  because  it  ia 
oxidised  with  evolution  of  nitrogen.  The  oxidising  action  of  permanganic  acid  in  a 
strong  solution  may  be  accompanied  by  flame  and  the  formation  of  violet  fumes  of 
permanganic  acid  ;  thus  a  strong  solution  of  it  takes  fire  when  brought  into  contact  with 
paper,  alcohol,  alkaline  sulphides,  fats,  &c. 

We  may  add  that,  according  to  Franke,  1  part  of  potassium  permanganate  with  18 


814  PRINCIPLES  OF  CHEM1STKY 

is  a  substance  capable  of  absorbing  oxygen— for  instance,  capable  o£ 
passing  into  a  higher  grade  of  oxidation  — then  the  reduction  of  the 
permanganic  acid  into  inanganous  oxides  sometimes  proceeds  directly 
at  the  ordinary  temperature.  This  reduction  is  very  clearly  seen, 
because  the  solutions  of  potassium  permanganate  are  red  whilst  the 
manganous  salts  are  almost  colourless.  Thus,  for  instance,  nitrous  acid 
and  its  salts  are  converted  into  nitric  acid  and  decolorise  the  acid  solution 
of  the  permanganate.  Sulphurous  anhydride  and  its  salts  immediately 
decolorise  potassium  permanganate,  forming  sulphuric  acid.  Ferrous 
salts,  and  in  general  salts  of  lower  grades  of  oxidation  capable  of  being 
oxidised  in  solution,  act  in  exactly  the  same  manner.  Sulphuretted 
hydrogen  is  also  oxidised  to  sulphuric  acid  ;  even  mercury  is  oxidised 
at  the  expense  of  permanganic  acid,  and  decolorises  its  solution,  being 
converted  into  mercuric  oxide.  Moreover,  the  end  point  of  these  reactions 
may  easily  be  seen,  and  therefore,  having  first  determined  the  amount 
of  active  oxygen  in  one  volume  of  a  solution  of  potassium  permanganate, 
and  knowing  how  many  volumes  are  required  to  effect  a  given  oxidation, 
it  is  easy  to  determine  the  amount  of  an  oxidisable  substance  in  a 
solution  from  the  amount  of  permanganate  expended  (Marguerite's 
method). 

The  oxidising  action  of  KMnO4,  like  all  other  chemical  reactions, 
is  not  accomplished  instantaneously,  but  only  gradually.  And,  as  the 
course  of  the  reaction  is  here  easily  followed  by  determining  the  amount 
of  salt  unchanged  in  a  sample  taken  at  a  given  moment,25  the  oxidising 
reaction  of  potassium  permanganate,  in  an  acid  liquid,  was  employed  by 
Harcourt  and  Esson  (1865)  as  one  of  the  first  cases  for  the  investigation 
of  the  laws  of  the  rate  of  chemical  change26  as  a  subject  of  great  import- 
ance in  chemical  mechanics.  In  their  experiments  they  took  oxalic  acid,- 

parts  of  sulphuric  acid  at  100°  gives  brown  crystals  of  the  salt  Mn.2(SO4)3,H2SO4)4H,0, 
which  gives  a  precipitate  of  hydrated  -manganese  dioxide,  H2MnO3  =  MuO.jH.jO,  when 
treated  with  water. 

Spring,  by  precipitating  potassium  permanganate  with  sodium  sulphite  and  washing 
the  precipitate  by  decantation,  obtained  a  soluble  colloidal  manganese  oxide,  whose 
composition  was  the  mean  between  Mn-Os  and  Mn0.2— namely,  MnaO5,4(MnO.jH,iO). 

25  For  rapid  and  accurate  determinations  of  this  kind,  advantage  is  taken  of  those 
methods  of  chemical  analysis  which  are  known  as  '  titrations'  (volumetric  analysis),  and 
consist  in  measuring  the  volume  of  solutions  of  known  strength  required  for  the  complete 
conversion  of  a  given  substance.  Details  respecting  the  theory  and  practice  of  titvation, 
iu  which  potassium  permanganate  is  very  frequently  employed,  must  be  looked  for  in 
works  on  analytical  chemistry. 

20  The  measurements  of  velocity  and  acceleration  serve  for  determining  the  measure 
of  forces  in  mechanics,  but  in  that  case  the  velocities  are  magnitudes  of  length  or  paths 
passed  over  in  a  unit  of  time.  The  velocity  of  chemical  change  embodies  a  conception  of 
quite  another  kind.  In  the  first  place,  the  velocities  of  reactions  are  magnitudes  of  the 
masses  which  have  entered  into  chemical  transformations ;  in  the  second  place,  these 
velocities  can  only  be  relative  Quantities.  Hence  the  concention  of '  velocitv  '  hoaauite  a. 


CHROMIUM,  MOLYBDENUM,  TUNGSTEN,  URANIUM,  ETC.   315 

C2H2O4,  which  in  oxidising  gives  carbonic  anhydride,  whilst,  with 
an  excess  of  sulphuric  acid,  the  potassium  permanganate  is  converted 
into  manganous  sulphate,  MnS04,  so  that  the  ultimate  oxidation 
will  be  expressed  by  the  equation:  5C2H2O4  +  2MnK04 +  3H2SO4 
==10C02  +  K2S04  +  2MnSO4  +  8H2O.  The  influence  of  the  relative 
amount  of  sulphuric  acid  is  seen  from  the  annexed  table,  which  gives 
the  measure  of  reaction  p  per  100  parts  of  potassium  permanganate, 
taken  four  minutes  after  mixing,  using  n  molecules  of  sulphuric  acid, 
H2S04,  per  2KMn04  +  5C2H204 

n  =  2  4  6  8         12         16         22 

p  =22  36        51         63        77        86        92. 

showing  that  in  a  given  time  (4  minutes)  the  oxidation  is  the  more 
perfect  the  greater  the  amount  of  sulphuric  acid  taken  for  given  amounts 
of  KMn04  and  C2H2O4.  It  is  obvious  also  that  the  temperature  and 
relative  amount  of  every  one  of  the  acting  and  resulting  substances 
should  show  its  influence  on  the  relative  velocity  of  reaction  ;  thus,  for 
instance,  direct  experiment  showed  the  influence  of  the  admixture 
of  manganous  sulphate.  When  a  large  proportion  of  oxalic  acid  (108 
molecules)  was  taken  to  a  large  mass  of  water  and  to  2  molecules  of 
permanganate  14  molecules  of  manganous  sulphate  were  added,  the 
quantity  x  of  the  potassium  permanganate  acted  on  (in  percentages 
of  the  potassium  permanganate  taken)  in  t  minutes  (at  16°}  was  as 
follows  : 

«=2        5        8        11         14        44        47        53        61         68 
a=  5-2    12-1    18-7      25-1      31-3      68-4      71-7     "75-8      79-8     83-0 

These  figures  show  that  the  rate  of  reaction — that  is,  the  quantity  of 
permanganate  changed  in  one  minute — decreases  proportionally  to  the 
decrease  in  the  amount  of  unchanged  potassium  permanganate.  At  the 

different  meaning  in  chemistry  from  what  it  has  in  mechanics.  Their  only  common  factor 
is  time.  If  dt  be  the  increment  of  time  and  dx  the  quantity  of  a  substance  changed  in 
this  space  of  time,  then  the  fraction  (or  quotient)  dx.'dt  will  express  the  rate  of  the 
reaction.  The  natural  conclusion,  come  to  both  by  Havcourt  and  Esson,  and  previously  to 
them  (1850)  by  Wilhelmj  (who  investigated  the  rate  of  conversion,  or  inversion,  of  sugar 
in  its  passage  into  glucose),  consists  in  establishing  that  this  velocity  is  proportional  to 
the  quantity  of  substances  still  unchanged — i.e.  that  dx'dt  =  C(A.-x),  where  C  is  a 
constant  coefficient  of  proportionality,  and  where  A  is  the  quantity  of  a  substance  taken 
for  reaction  at  the  moment  when  t=Q  and  a*  =  0 — that  is,  at  the  beginning  of  the 
experiment,  from  which  the  time  t  and  quantity  x  of  substance  changed  is  counted. 
On  integrating  the  preceding  equation  we  obtain  log(A/A  —  x)~Jct,  where  k  is  a  new 
constant,  if  we  take  ordinary  (and  not  natural)  logarithms.  Hence,  knowing  A,  x,  and  £, 
for  each  reaction,  we  find  k,  and  it  proves  to  be  a  constant  quantity.  Thus  from  the 
figures  cited  in  the  text  for  the  reaction  2KMnO4  +  108C2H.2O4  +  14MnS0.1,  it  may  be 
calculated  that  &  =  0'0114;  for  example,  *  =  44,  z  =  C8'4  (A  =  100),  whence  ft*  =  0'5004  and 
k  =  0-0114,  (see  also  Chapter  XIV.,  Note  3,  and  Chapter  XXVII.,  Note  25  bis). 


316  PRINCIPLES  OF  CHEMISTRY 

commencement,  about  2'6  per  cent,  of  the  salt  taken  was  decomposed  in 
the  course  of  one  minute,  whilst  after  an  hour  the  rate  was  about 
0'5  per  cent.  The  same  phenomena  are  observed  in  every  case  which 
has  been  investigated,  and  this  branch  of  theoretical  or  physical 
chemistry,  now  studied  by  many,27  promises  to  explain  the  course  of 
chemical  transformations  from  a  fresh  point  of  view,  which  is  closely 
allied  to  the  doctrine  of  affinity,  because  the  rate  of  reaction,  without 
doubt,  is  connected  with  the  magnitude  of  the  affinities  acting  between 
the  reacting  substances. 

Si  The  researches  made  by  Hood,  Van't  Hoff,  Ostwald,  Warder,  Menschutkin,  Kono- 
valoff,  and  others  have  a  particular  significance  in  this  direction.  Owing  to  the  com- 
parative novelty  of  this  subject,  and  the  absence  of  applicable  as  well  as  indubitable 
deductions,  I  consider  it  impossible  to  enter  into  this  province  of  theoretical  chemistry, 
although  I  am  quite  confident  that  its  development  should  lead  to  very  important  results, 
especially  in  respect  to  chemical  equilibria,  for  Van't  Hoff  has  already  shown  that 
the  limit  of  reaction  in  reversible  reactions  is  determined  by  the  attainment  of  equal 
velocities  for  tho  opposite  reactions. 


317 


CHAPTER  XXII 

IRON,    COBALT,    AND   NICKEL 

JUDGING  from  the' atomic  weights,  and  the  forms  of  the  higher  oxides 
of  the  elements  already  considered,  it  is  easy  to  form  an  idea  of 
the  seven  groups  of  the  periodic  system.  Such  are,  for  instance,  the 
typical  series  Li,  Be,  B,  C,  N,  O,  F,  or  the  third  series,  Na,  Mg,  Al,  Si, 
P,  S,  Cl.  The  seven  usual  types  of  oxides  from  R20  to  R2O7  correspond 
with  them  (Chapter  XV.)  The  position  of  the  eighth  group  is  quite 
separate,  and  is  determined  by  the  fact  that,  as  we  have  already  seen, 
in  each  group  of  metals  having  a  greater  atomic  weight  .than  potassium 
a  distinction  ought  to  be  .made  between  the  elements  of  the  even  and 
uneven  series*  The  series  of  even  elements,  commencing  with  a 
strikingly  alkaline  element  (potassium,  rubidium,  caesium),  together  with 
the  uneven  series  following  it,  and  concluding  with  a  haloid  (chlorine, 
bromine,  iodine),  forms  a  large  period,  the  properties  of  whose  members 
repeat  themselves  in  other  similar  periods.  The  elements  of  the  eighth 
group  are  situated  between  the  elements  of  the  even  series  and  the  ele- 
ments of  the  uneven  series  following  them.  And  for  this  reason  elements 
of  the  eighth  group  are  found  in  the  middle  of  each  large  period.  The 
properties  of  the  elements  belonging  to  it,  in  many  respects  independent 
and  striking,  are  shown  with  typical  clearness  in  the  case  of  iron,  the 
well-known  representative  of  this  group. 

Iron  is  one  of  those  elements  which  are  not  only  widely  diffused  in 
the  crust  of  the  earth,  but  also  throughout  the  entire  universe.  Its 
oxides  and  their  various  compounds  are  found  in  the  most  diverse 
portions  of  the  earth's  crust ;  but  here  iron  is  always  found  combined 
with  some  other  element.  Iron  is  not  found  on  the  earth's  surface  in 
a  free  state,  because  it  easily  oxidises  under  the  action  of  air.  It  is 
occasionally  found  in  the  native  state  in  meteorites,  or  aerolites,  which 
fall  upon  the  earth. 

Meteoric  iron  is  formed  outside  the  earth. l  Meteorites  are  fragments 
\vhich  are  carried  round  the  sun  in  orbits,  and  fall  upon  the  earth 

The  composition  of  meteoric  iron  is  variable.  It  generally  contains  nickel,  phos« 
phorus,  carbon,  &c.  The  schreibersite  of  meteoric  stones  contains  Fe4Ni2P. 


318  PRINCIPLES  OF  CHEMISTRY 

when  coming  into  proximity  with  it  during  their  motion  in  space.  The 
meteoric  dust,  on  passing  through  the  upper  parts  of  the  atmosphere, 
and  becoming  incandescent  from  friction  with  the  gases,  produces  that 
phenomenon  which  is  familiar  under  the  name  of  falling  stars.2  Such  is 

8  Comets  and  the  rings  of  Saturn  ought  now  to  be  considered  as  consisting  of  an 
accumulation  of  such  meteoric  cosmic  particles.  Perhaps  the  part  played  by  these 
minute  bodies  scattered  throughout  space  is  much  more  important  in  the  formation 
of  the  largest  celestial  bodies  than  has  hitherto  bean  imagined.  The  investigation  of 
this  branch  of  astronomy,  due  to  Schiaparelli,  has  a  bearing  on  the  whole  of  natural 
Science. 

The  question  arises  as  to  why  the  iron  in  meteorites  is  in  a  free  state,  whilst  on  earth 
it  is  in  a  state  of  combination.  Does  not  this  tend  to  show  that  the  condition  of  our 
globe  is  very  different  from  that  of  the  rest  ?  My  answer  to  this  question  has  been 
already  given  in  Volume  I.  p.  877,  Note  57.  It  is  my  opinion  that  inside  the  earth  there 
is  a  mass  similar  in  composition  to  meteorites — that  is,  containing  rocky  matter  and 
metallic  iron,  partly  carburetted.  In  conclusion,  I  consider  it  will  not  be  out  of  place 
to  add  the  following  explanations.  According  to  the  theory  of  the  distribution  of  pres- 
«ures  (sae  my  treatise,  On  Barometrical  Levelling,  1876,  pages  48  et  scg.)  in  an  atmo- 
sphere of  mixed  gases,  it  follows  that  two  gases,  whose  densities  are  d  and  dlt  and  whose 
relative  quantities  or  partial  pressures  at  a  certain  distance  from  the  centre  of  gravity 
are  h  and  hi,  will,  when  at  a  greater  distance  from  the  centre  of  attraction,  present  a 
different  ratio  of  their  masses  x  :  art — that  is,  of  their  partial  pressures— which  may  be 
found  by  the  equation  ^(log  h  —  log  x)  =  d(\og  hi  —  log  xj.  If,  for  instance,  d  :  (Z,  =  2 : 1, 
and  h  =  hi  (that  is  to  say,  the  masses  are  equal  at  the  lower  height)  =  1000,  then  when 
#  =  10  the  magnitude  of  Xi  will  not  be  10  (i.e.  the  mass  of  a  gas  at  a  higher  level  whose 
density  =1  will  not  be  equal  to  the  mass  of  a  gas  whose  density  =2,  as  was  the  case  at 
a  lower  level),  but  much  greater— namely,  a;^  100— that  is,  the  lighter  gas  will  pre- 
•dominate  over  a  heavier  one  at  a  higher  level.  Therefore,  when  the  whole  mass  of  the 
earth  was  in  a  state  of  vapour,  the  substances  having  a  greater  vapour  density  accumu- 
lated about  the  centre  and  those  with  a  lesser  vapour  density  at  the  surface.  And  as 
•the  vapour  densities  depend  on  the  atomic  and  molecular  weights,  those  substances  which 
"have  small  atomic  and  molecular  weights  ought  to  have  accumulated  at  the  surface,  and 
•those  with  high  atomic  and  molecular  weights,  which  are  the  least  volatile  and  the  easiest 
to  condense,  at  the  centre.  Thus  it  becomes  apparent  why  such  light  elements  as 
'hydrogen,  carbon,  nitrogen,  oxygen,  sodium,  magnesium,  aluminium,  silicon,  phosphorus, 
•sulphur,  chlorine,  potassium,  calcium,  and  their  compounds  predominate  at  the  surface 
And  largely  form  the  earth's  crust.  There  is  also  now  much  iron  in  the  sun,  as  spectrum 
analysis  shows,  and  therefore  it  must  have  entered  into  the  composition  of  the  earth 
And  other  planets,  but  would  have  accumulated  at  the  centre,  because  the  density  of 
its  vapour  is  certainly  large  and  it  easily  condenses.  There  was  also  oxygen  near  the 
centre  of  the  earth,  but  not  sufficient  to  combine  with  the  iron.  The  former,  as  a  much 
lighter  element,  principally  accumulated  at  the  surface,  where  we  at  the  present  time 
find  all  oxidised  compounds  and  even  a  remnant  of  free  oxygen.  This  gives  the 
possibility  not  only  of  explaining  in  accordance  with  cosmogonic  theories  the  pre- 
dominance of  oxygen  compounds  on  the  surface  of  the  earth,  with  the  occurrence  of 
unoxidised  iron  in  the  interior  of  the  earth  and  in  meteorites,  but  also  of  understanding 
•why  the  density  of  the  whole  earth  (over  5)  is  far  greater  than  that  of  the  rocks  (1  to  3) 
composing  its  crust.  And  if  all  the  preceding  arguments  and  theories  (for  instance 
the  supposition  that  the  sun,  earth,  and  all  the  planets  were  formed  of  an  elementary 
homogeneous  mass,  formerly  composed  of  vapours  and  gases)  be  -true,  it  must  be  ad- 
mitted  that  the  interior  of  the  earth  and  other  planets  contains  metallic  (unoxidised)  iron, 
which,  however,  is  only  found  on  the  surface  as  aerolites.  And  then  assuming  that 
aerolites  are  the  fragments  of  planets  which  have  crumbled  to  pieces  so  to  say 
.during  cooling  (this  has  been  held  to  be  the  case  by  astronomers,  judging  from  the  path* 


IRON,   COBALT    AND   NICKEL  319 

the  doctrine  concerning  meteorites,  and  therefore  the  fact  of  their 
containing  rocky  (siliceous)  matter  and  metallic  iron  shows  that  outside 
the  earth  the  elements  and  their  aggregation  are  in  some  degree  the 
same  as  upon  the  earth  itself. 

The  most  widely  diffused  terrestrial  compound  of  iron  is  iron 
bisulphide,  FeS2,  or  iron  pyrites,  tt  occurs  in  formations  of  both 
aqueous  and  igneous  origin,  and  sometimes  in  enormous  masses.  It  is 
a  substance  having  a  greyish-yellow  colour,  with  a  metallic  lustre,  and  a 
specific  gravity  of  5'0  ;  it  crystallises  in  the  regulau  system.2  bi8 

The  oxides  are  the  principal  ores  used  for  producing  metallic  iron. 
The  majority  of  the  ores  contain  ferric  oxide,  Fe.2O3,  either  in  a 
free  state  or  combined  with  water,  or  else  in  combination  with  ferrous 
oxide,  FeO  The  species  and  varieties  of  iron  ores  are  numerous  and 
diverse.  Ferric  oxide  in  a  separate  form  appears  sometimes  as  crystals 
of  the  rhombohedric  system,  having  a  metallic  lustre  and  greyish  steel 
colour  ;  they  are  brittle,  and  form  a  red  powder,  specific  gravity  about 
5-25.  Ferric  oxide  in  type  of  oxidation  and  properties  resembles 
alumina  ;  it  is,  however,  although  with  difficulty,  soluble  in  acids  even 
when  anhydrous.  The  crystalline  oxide  bears  the  name  of  specular 
iron  ore,  but  ferric  oxide  most  often  occurs  in  a  non-crystalline  form, 
in  masses  having  a  red  fracture,  and  is  then  known  as  red  hcematite. 
In  this  form,  however,  it  is  rather  a  rare  ore,  and  is  principally  found 
in  veins.  The  hydrates  of  ferric  oxide,  ferric  hydroxides,3  are  most, 

of  aerolites),  it  is  readily  understood  why  they  should  be  composed  of  metallic 
iron,  and  this  would  explain  its  occurrence  in  the  depths  of  the  earth,  which  we 
assumed  as  the  basis  of  our  theory  of  the  formation  of  naphtha  (Chapter  VIII.,  Notes 
67-60). 

2bU  Immense  deposits  of  iron  pyrites  are  known  in  various  parts  of  Russia.  On  the 
river  Msta,  near  Borovitsi,  thousands  of  tons  are  yearly  collected  from  the  detritus  of 
the  neighbouring  rocks.  In  the  Go vernments  of  TouJa,  Riazan,  and  in  the  Donets  district 
continuous  layers  of  pyrites  occur  among  the  coal  seam<».  Very  thick  beds  of  pyrites 
are  also  known  in  many  parts  of  the  Caucasus.  But  the  deposits  of  the  Urals  are  par- 
ticularly vast,  and  have  been  worked  for  a  long  time.  Amongst  these  I  will  only  indicate 
the  deposits  on  the  Soymensky  estate  near  the  Kiohteimsky  works;  the  Kaletinsky 
deposits  near  the  Virhny-lsetsky  works  (containing  1-2  p.c.  Cu) ;  on  the  banks  of  the 
river  Koushaivi  near  Koushvi  (3-5  p.c.  Cu),  and  the  deposits  near  the  Bogoslovsky 
works  (3-5  p.c.  Cu).  Iron  pyrites  (especially  that  containing  copper  which  is  extracted 
ifter  roasting)  is  now  chiefly  employed  for  roasting,  as  a  source  of  S03  for  the  manufac- 
ture of  chamber  sulphuric  acid  (Vol.1,  p.  291),  but  the  remaining  oxide  of  iron  is  per- 
fectly suitable  for  smelting  into  pig  iron,  although  it  gives  a  sulphurous  pig  iron  (the 
sulphur  may  be  easily  removed  by  subsequent  treatment,  especially  with  the  aid  of 
ferro-manganese  in  Bessemer's  process).  The  great  technical  importance  of  iron  pyrites 
leads  to  its  sometimes  being  imported  from  great  distances;  for  instance,  into  England 
from  Spain.  Besides  which,  when  heated  in  closed  retorts  FeS^  gives  sulphur,  and  if 
allowed  to  oxidise  in  damp  air,  green  vitriol,  FeSO^. 

3  The  hydrated  ferric  oxide  is  found  in  nature  in  a  dual  form  It  is  somewhat  rarely 
met  with  in  the  form  of  a  crystalline  mineral  called  gitthite,  whose  specific  gravity  is  4*4 

*c 


820  PRINCIPLES  OF  CHEMISTRY 

often  found  in  aqueous  or  stratified  formations,  and  are  known  as 
brown  haematites  ;  they  generally  have  a  brown  colour,  form  a  yellowish- 
brown  powder,  and  have  no  metallic  lustre  but  an  earthy  appearance. 
They  easily  dissolve  in  acids  and  diffuse  through  other  formations,  espe- 
cially clays  (for  instance,  ochre) ;  they  sometimes  occur  in  reniform  and 
similar  masses,,  evidently  of  aqueous  origin.  Such  are,  for  instance, 
the  so-called  bog  or  lake  and  peat  ores  found  at  the  bottom  of  marshes 
and  lakes,  and  also  under  and  in  peat  beds.  This  ore  is  formed  from 
water' containing  ferrous  carbonate  in  solution,  which,  after  absorbing 
oxygen,  deposits  ferric  hydroxide.  In  rivers  and  springs,  iron  is  found 
in  solution  as  ferrous  carbonate  through  the  agency  of  carbonic 
acid  :  hence  the  existence  of  chalybeate  springs  containing  FeCO3. 
This  ferrous  carbonate,  or  siderite,  is  either  found  as  a  non-crystalline 
product  of  evidently  aqueous  origin,  or  as  a  crystalline  spar  called 
spathic  iron  ore.  The  reniform  deposits  of  the  former  are  most  re- 
markable ;  they  are  called  spherosiderites,  and  sometimes  form  whole 
strata  in  the  Jurassic  and  carboniferous  formations.  Magnetic 
iron  ore,  Fe3O4  =  FeO,Fe2O3,  in  virtue  of  its  purity  and  practical 
uses,  is  a  very  important  ore ;  it  is  a  compound  of  the  ferrous  and 
ferric  oxides,  is  naturally  magnetic,  has  a  specific  gravity  of  5'lr 
crystallises  in  well-formed  crystals  of  the  regular  system,  is  "with  diffi- 
culty soluble  in  acids,  and  sometimes  forms  enormous  masses,  as,  for 
instance,  Mount  Blagodat  in  the  Ural..  However,  in  most  cases — for 
instance,  at  Korsak-Mogila  (to  the  north  of  Berdiansk  and  Nogaiska, 
near  the  Sea  of  Azov),  or  at  Krivoi  Rog  (to  the  west  of  Ekaterinoslav)— . 
the  magnetic  iron  ore  is  mixed  with  other  iron  ores.  In  the  Urals,  the 
Caucasus  (without  mentioning  Siberia),  and  in  the  districts  adjoining  the 
basin  of  the  Don,  Russia  possesses  the  richest  iron  ores  in  the  world. 
To  the  south  of  Moscow,  in  the  Governments  of  Toula  and  Nijni- 
novgorod,  in  the  Olonetz  district,  and  in  the  Government  of  Orloffsky 
(near  Zinovieff  in  the  district  of  Kromsky),  and  in  many  other  places, 
there  are  likewise  abundant  supplies  of  iron  ores  amongst  the  deposited 
aqueous  formations  ;  the  siderite  of  Orloffsky,  for  instance,  is  dis- 
tinguished by  its  great  purity.4 

end  composition  Fe^H2O4,  or  FeHO^ — that  is,  one  of  oxide  of  iron  to  one  of  water, 
Fe.jOj.IijO;  frequently  found  as  brown  ironstone,  forming  a  dense  mass  of  fibrous, 
reniform  deposits  containing  2FeiO5,3HvjO— that  is,  having  a  composition  Fe4H6O9.  In 
bog  ore  and  other  similar  ores  we  most  often  find  a  mixture  of  this  hydrated  ferric  oxide 
with  clay  and  other  impurities.  The  specific  gravity  of  such  formations  is  rarely  as  high 
as  4-0. 

4  The  ores  of  iron,  similarly  to  all  substances  extracted  from  veins  and  deposits,  are 
worked  according  to  mining  practice  by  means  of  vertical,  horizontal,  or  inclined 
shafts  which  reach  and  penetrate  the  veins  and  strata  containing  the  ore  deposits. 
The  mass  of  ore  excavated  is  raised  to  the  surface,  then  sorted  either  by  hand  or  else  in 


IKON.   COBALT,   AND  NICKEL  S21 

Iron  is  also  found  in  the  form  of  various  other  compounds — for 
instance,  in  certain  silicates,  and  also  in  some  phosphates  ;  but  these 
forms  are  comparatively  rare  in  nature  in  a  pure  state,  and  have  not 
the  industrial  importance  of  those  natural  compounds  of  iron  pre- 
viously mentioned.  In  small  quantities  iron  enters  into  the  composi- 
tion of  every  kind  of  soil  and  all  rocky  formations.  As  ferrous  oxide, 
FeO,  is  isornorphous  with  magnesia,  and  ferric  oxide,  Fe2O3,  with 
alumina,  isomorphous  substitution  is  possible  here,  and  hence  minerals 
are  not  unfrequently  found  in  which  the  quantity  of  iron  varies  con- 
siderably ,  such,  for  instance,  are  pyroxene,  amphibole,  certain  varieties 
of  mica,  &c.  Although  much  iron  oxide  is  deleterious  to  the  growth  of 
vegetation,  still  plants  do  not  flourish  without  iron  ;  it  enters  as  an 
indispensable  component  into  the  composition  of  all  higher  organisms  ; 
in  the  ash  of  plants  we  always  find  more  or  less  of  its  compounds.  It 
also  occurs  in  blood,  and  forms  one  of  the  colouring  matters  in  it ; 
100  parts  of  the  blood  of  the  highest  organisms  contain  about  0'05  of  iron. 

The  reduction  of  the  ores  of  iron  into  metallic  iron  is  in  prin- 
ciple very  simple,  because  when  the  oxides  of  iron  are  strongly  heated 
with  charcoal,  hydrogen,  carbonic  oxide,  and  other  reducing  agents,* 
they  easily  give  metallic  iron.  But  the  matter  is  rendered  more 

special  sorting  apparatus  (generally  acting  with  water  to  wash  the  ore),  and  is  subjected 
to  roasting  and  other  treatment.  In  every  case  the  ore  contains  foreign  matter.  In  the 
extraction  of  iron,  which  is  one  of  the  cheapest  metals,  the  dressing  of  an  ore  is  in  most 
cases  unprofitable,  and  only  ores  rich  in  metal  are  worked — namely,  those  containing  at 
least  20  p.c.  It  is  often  profitable  to  transport  very  rich  and  pure  ores  (with  as  much  as 
70  p.c.  of  iron)  from  long  distances.  The  details  concerning  the  working  and  extraction 
of  metals  will  be  found  in  special  treatises  on  metallurgy  and  mining. 

5  The  reduction  of  iron  oxides  by  hydrogen  belongs  to  the  order  of  reversible  re- 
actions (Chapter  II.),  and  is  therefore  determined  by  a  limit  which  ie  here  expressed 
by  the  attainment  of  the  same  pressure  as  in  the  case  where  hydrogen  acts  on  iron 
oxides,  and  as  in  the  case  where  (at  the  same  temperature)  water  is  decomposed  by 
metallic  iron.  The  calculations  referring  to  this  matter  were  made  by  Henri  Sainte-Claire 
Deville  (1870).  Spongy  iron  was  placed  in  a  tube  having  a  temperature  t,  one  end  of 
which  was  connected  with  a  vessel  containing  water  at  0°  (vapour  tension  =  4'6  mm.) 
and  the  other  end  with  a  mercury  pump  and  pressure  gauge  which  determined  the 
limiting  tension  attained  by  the  dry  hydrogen  p  (subtracting  the  tension  of  the  water 
vapour  from  the  tension  observed).  A  tube  was  then  taken  containing  an  excess  of  iron 
oxide.  It  was  filled  with  hydrogen,  and  the  tension  pl  observed  of  the  residual  hydrogen 
when  the  water  was  condensed  at  0°. 

t  =  200°  440°  860°  1040° 

p  =  95'  9  25'8  12-8  9'2  inm. 

2>i=   —  12-8  9'4  mm. 

The  equality  of  the  pressure  (tension)  of  the  hydrogen  in  the  two  cases  is  evident.  The 
hydrogen  here  behaves  like  the  vapour  of  iron  or  of  its  oxide. 

By  taking  ferric  oxide,  Fe2O3,  Moissan  observed  that  at  850'  it  passed  into 
magnetic  oxide,  Fe5O4,  at  500°  into  ferrous  oxide,  FeO,  and  at  600°  into  metallic  iron. 
"Wright  and  Luff  (1878),  whilst  investigating  the  reduction  of  oxides,  found  that  (a)  the 
temperature  of  reaction  depends  on  the  condition  of  the  oxide  taken — for  instance 


'822  PRINCIPLES  OF  CHEMISTBY 

difficult  by  the  fact  that  the  iron  does  not  melt  at  the  heat  developed 
by  the  combustion  of  the  charcoal,  and  therefore  it  does  not  separate 
from  those  mechanically  mixed  impurities  which  are  found  in  the  iron 
ore.  This  is  obviated  by  the  following  very  remarkable  property  of 
iron  :  at  a  high  temperature  it  is  capable  of  combining  with  a  small 
quantity  (from  2  to  5  p.c  )  of  carbon,  and  then  forms  cast  iron,  which 
easily  melts  in  the  heat  developed  by  the  combustion  of  charcoal  in  air. 
For  this  reason  metallic  iron  is  not  obtained  directly  from  the  ore,  but 
is  only  formed  after  the  further  treatment  of  the  cast  iron  ,-  the  first 
product  extracted  from  the  ore  being  cast  iron.  The  fused  mass  dis- 
poses itself  in  the  furnace  below  the  slag — that  is,  the  impurities  of  the 
ore  fused  by  the  heat  of  the  furnace.  If  these  impurities  did  not  fuse 
they  would  block  up  the  furnace  in  which  the  ore  was  being  smelted, 
and  the  continuous  smelting  of  the  cast  iron  would  not  be  possible  ; 6 
it  would  be  necessary  periodically  to  cool  the  furnace  and  heat  it  up 
again,  which  means  a  wasteful  expenditure  of  fuel,  and  hence  in  the 
production  of  cast  iron,  the  object  in  view  is  to  obtain  all  the  earthy 
impurities  of  the  ore  in  the  shape  of  a  fused  mass  or  slag.  Only 
in  rare  cases  does  the  ore  itself  form  a  mass  which  fuses  at  the 
temperature  employed,  and  these  cases  are  objectionable  if  much  iron 
oxide  is  carried  away  in  the  slag.  The  impurities  of  the  ores  most 
often  consist  of  certain  mixtures — for  instance,  a  mixture  of  clay  and 
sand,  or  a  mixture  of  limestone  and  clay,  or  quartz,  &c.  These 

precipitated  ferric  oxide  is  reduced  by  hydrogen  at  85°,  that  obtained  by  oxidising  the 
metal  or  from  its  nitrate  at  175° ;  (b)  when  other  conditions  are  the  same  the  reduction 
by  carbonic  oxide  commences  earlier  than  that  by  hydrogen,  and  the  reduction  by 
hydrogen  still  earlier  than  that  by  charcoal ;  (c)  the  reduction  is  effected  with  greater 
facility  when  a  greater  quantity  of  heat  is  evolved  during  the  reaction.  Ferric  oxide 
obtained  by  heating  ferrous  sulphate  to  a  red  heat  begins  to  be  reduced  by  carbonic 
oxide  at  202°,  by  hydrogen  at  260°,  by  charcoal  at  480°,  whilst  for  magnetic  oxide,  FesO*, 
the  temperatures  are  200°,  290°,  and  450°  respectively. 

c  The  primitive  methods  of  iron  manufacture  were  conducted  by  intermittent  pro- 
cesses in  hearths  resembling  smiths'  fires.  As  evidenced  by  the  uninterrupted  action 
of  the  steam  boiler,  or  the  process  of  lime  burning,  and  the  continuous  preparation  and 
condensation  of  sulphuric  acid  or  the  uninterrupted  smelting  of  iron,  every  industrial 
process  becomes  increasingly  profitable  and  complete  under  the  condition  of  the  con- 
tinuous action,  as  far  as  possible,  of  all  agencies  concerned  in  the  production.  This 
continuous  method  of  production  is  the  first  condition  for  the  profitable  production 
on  the  large  scale  of  nearly  all  industrial  products.  This  method  lessens  the  cost  of 
labour,  simplifies  the  supervision  of  the  work,  renders  the  product  uniform,  and  fre- 
quently introduces  a  very  great  economy  in  the  expenditure  of  fuel  and  at  the  same  time 
presents  the  simplicity  and  perfection  of  an  equilibrated  system.  Hence  every  manu- 
facturing operation  should  be  a  continuous  one,  and  the  manufacture  of  pig  iron  and 
sulphuric  acid,  which  have  long  since  become  so,  may  be  taken  as  examples  in  many 
tespects.  A  study  of  these  two  manufactures  should  form  the  commencement  of  an 
acquaintance  with  all  the  contemporary  methods  of  manufacturing  both  from  a  tech- 
nical and  economical  point  of  view. 


IRON,  COBALT,  AND  NICKEL  328 

impurities  do  not  separate  of  themselves,  or  do  not  fuse.  The  difficulty 
of  the  industry  lies  in  forming  an  easily-fusible  slag,  into  which  the 
whole  of  the  foreign  matter  of  the  ore  would  pass  and  flow  down  to  the 
bottom  of  the  furnace  above  the  heavietr  cast  iron.  This  is  effected  by 
mixing  certain  fluxes  with  the  ore  and  charcoal,  A  flux  is  a  substance 
which,  when  mixed  with  the  foreign  matter  of  the  ore,  forms  a  fusible 
vitreous  mass  or  slag.  The  flux  used  for  silica  is  limestone  with  clay  ; 
for  limestone  a  definite  quantity  of  silica  is  used,  the  best  procedure 
having  been  arrived  at  by  experiment  and  by  long  practice  in  iron 
smelting  and  other  metallurgical  processes.7 

Thus  the  following  materials  have  to  be  introduced  into  the  furnace 
where  the  smelting  of  the  iron  ore  is  carried  on  :  (1)  the  iron  ore, 
composed  of  oxide  of  iron  and  foreign  matter  ;  (2)  the  flux  required  to 
form  a  fusible  slag  with  the  foreign  matter ;  (3)  the  carbon  which  is 
necessary  (a)  for  reducing,  (6)  for  combining  with  the  reduced  iron 
to  form  cast  iron,  (c)  principally  -for  the  purpose  of  combustion  and 
the  heat  generated  thereby,  necessary  not  only  for  reducing  the  iron 
and  transforming  it  into  cast  iron,  but  also  for  melting  the  slag,  as  well 
as  the  cast  iron — and  (4)  the  air  necessary  for  the  combustion  of  the 
charcoal.  The  air  is  introduced  after  a  preparatory  heating  in  order  to 
economise  fuel  and  to  obtain  the  highest  temperature.  The  air  is 
forced  in  under  pressure  by  means  of  a  special  blast  arrangement. 
This  permits  of  an  exact  regulation  of  the  heat  and  rate  of  smelting. 
All  these  component  parts  necessary  for  the  smelting  of  iron  must  be 
contained  in  a  vertical,  that  is)  shaft  furnace,  which  at  the  base  must 
have  a  receptacle  for  the  accumulation  of  the  slag  and  cast  iron  formed, 
in  order  that  the  operation  may  proceed  without  interruption.  The 
walls  of  such  a  furnace  ought  to  be  built  of  fireproof  materials  if  it  be 

7  The  composition  of  slag  suitable  for  iron  smelting  most  often  approaches  the 
following :  50  to  60  p.c.  Si02,  5  to  20  ALjO3,  the  rest  of  the  mass  consisting  of  MgO, 
CaO,  MnO,  FeO.  Thus  the  most  fusible  slag  (according  to  the  observations  of 
Bodeman)  contains  the  alloy  ALjOj^CaO^SiOj.  On  altering  the  quantity  of  magnesia 
and  lime,  and  especially  of  the  alkalis  (which  increases  the  fusibility)  and  of  silica 
(which  decreases  it),  the  temperature  of  fusion  changes  with  the  relation  between  the  total 
quantity  of  oxygen  and  that  in  the  silica.  Slags  of  the  composition)  BO,SiO2  are  easily 
fusible,  have  a  vitreous  appearance,  and  are  very  common.  Basic  slags  approach  the 
composition  2EO,Si02.  Hence,  knowing  the  composition  and  quantity  of  the  foreign 
matter  in  the  ore,  it  is  at  once  e&sy  to  find  the  quantity  and  quality  of  the  flux  which 
must  be  added  to  form  a  suitable  slag.  The  smelting  of  iron  is  rendered  more  complex 
by  the  fact  that  the  silica,  SiO4,  which  enters  into  the  slag  and  fluxes  is  capable  of  form- 
ing a  slag  with  the  iron  oxides.  In  order  that  the  least  quantity  of  iron  may  pass  into 
the  slag,  it  is  necessary  for  it  to  be  reduced  before  the  temperature  is  attained  at  which 
the  slags  are  formed  (about  1000°),  which  is  effected  by  reducing  the  iron,  not  with  char- 
coal itself,  but  with  carbonic  oxide.  From  this  it  will  be  understood  how  the  progress  of 
the  whole  treatment  may  be  judged  by  the  properties  of  the  slags.  Details  of  this 
complicated  and  well-studied  subject  will  be  found  in  works  on  metallurgy. 


824  PRINCIPLES  OF  CHEMISTRY 

designed  to  serve  for  the  continuous  production  of  cast  iron  by  charging 
the  ore,  fuel,  and  flux  into  the  mouth  of  the  furnace,  forcing  a  blast  of 
air  into  the  lower  part,  and  running  out  the  molten  iron  and  slag  from 
below.  The  whole  operation  is  conducted  in  furnaces  known  as  blast 
furnaces.  The  annexed  illustration,  fig.  93  (which  is  taken  by  kind 
permission  from  Thorpe's  Dictionary  of  Applied  Chemistry),  represents 
the,  vertical  section  of  such  a  furnace.  These  furnaces  are  generally 
of  large  dimensions — varying  from  50  to  90  feet  in  height.  They  are 
sometimes  built  against  rising  ground  in  order  to  afford  easy  access  to 
the  top  where  the  ore,  flux,  and  charcoal  or  coke  are  charged.8 

8  The  section  of  a  blast  furnace  is  represented  by  two  truncated  cones  joined  at  their 
bases,  the  upper  cone  being  longer  than  the  lower  one ;  the  lower  cone  is  terminated  by 
the  hearth,  or  almost  cylindrical  cavity  in  which:  the  cast  iron  and  slag  collect,  one 
Bide  being  provided  with  apertures  for  drawing  off  the-  iron  and  slag.  The  air  is  blown 
into  the  blast  furnace  through  special  pipes,  situated  over  the  hearth,  as  shown  in  the 
section.  The  air  previously  passes  through  a  series  .of  cast-iron  pipes,  heated  by  the 
combustion  of  the  carbonic  oxide  obtained  from  Ihe  upper  parts  of  the  furnace,  where 
it  is  formed  as  in  a  '  gas-producer.'  The  blast  furnace  acts  continuously  until  it  is  worn 
out ;  the  iron  is  tapped  off  twice- a  day,  and  the  furnace  is  allowed  to  cool  a  little  from 
lime  to  time  so  as  not  to  be  spoilt  by  the  increasing  heat,  and  to  enable  it  to  withstand 
long  usage. 

Blast  furnaces  worked",  with  charcoal  fuel  are  not  so  high,  and  in  general  give  a 
smaller  yield  than  those  using  coke,  because  the  latter  are  worked  with  heavier  charges 
than  those  in  which  charcoal  is  employed.  Coke  furnaces  yield  20,000  tons  and  over  oj  pig 
iron  a  year.  In  the  United  States  there  are  blast  furnaces  80  metres  high,  and  upwards 
of  600  cubic  metres  capacity,  yielding  as  much  as  180,000  tons  of  pig  iron,  requiring  a  blast 
of  about  750  cubic  metres  of  air  per  minute,  heated  to  600°,  and  consuming  about  0-85 
part  of  coke  per  1  part  of  pig  iron  produced.  At  the  present  time  the  world  produces  as 
much  as  80  million  tons  of  pig  iron  a  year,  about  -^  of  which  is  converted  into  wrought 
iron  and  steel.  The  chief  producers  are  the  United  States  (about  10  million  tons  a  year) 
and  England  (about  9  million  tons  a  year) ;  Russia  yields  about  1$  million  tons  a  year. 
The  world's  production  has  doubled  during  the  last  20  years^  and  in  this  respect  the 
United  States  have  outrun  all  other  countries.  The  reason  of  this  increase  of  production 
must  be  looked  for  in  the  increased  demand  for  iron  and  steel  for  "railway  purposes,  for 
structures  (especially  ship-building),  and  in  the  fact  that :  (a)'  the  cost  of  pig  iron  has 
fallen,  thanks  to  the  erection  of  large  furnaces  and  a  fuller  study  of  the  processes  taking 
place  in  them,  and  (b)  that  every  kind  of  iron  ore  (even  sulphurous  and  phosphoritic)  can 
now  be  converted  into  a  homogeneous  steel. 

In  order  to  more  thoroughly  grasp  the  chemical  process  which  takes  place  in  blast 
furnaces,  it  is  necessary  to  follow  the  course  of  the  material  charged  in  at  the  top  and  of 
'the  air  passing  through  the  furnace.  From  60  to  200  parts  of  carbon  are  expended  on  100 
parts  of  iron.  The  ore,  flux,  and  coke  are  charged  into  the  top  of  the  furnace,  in 
layers,  as  the  cast  iron  is  formed  in  the  lower  parts  and  flowing  down  to  the  bottom 
causes  the  whole  contents  of  the  furnace  to  subside,  thus  forming  an  empty  space  at 
the  top,  which  is  again  filled  up  with  the  afore-mentioned  mixture.  During  its  down* 
ward  course  this  mixture  is  subjected  to  increasing  heat.  This  rise  of  temperature 
first  drives  off  the  moisture  of  the  ore  mixture,  and  then  leads  to  the  formation  of 
the  products  of  the  dry  distillation  of  coal  or  charcoal.  Little  by  little  the  subsiding 
mass  attains  a  temperature  at  which  the  heated  carbon  reacts  with  the  carbonic  anhydride 
passing  upwards  through  the  furnace  and  transforms  it  into  carbonic  oxide.  This  is 
the  reason  why  carbonic  anhydride  is  not  evolved  from  the  furnace,  but  only  carbonic 
oxide.  As  regards  the  ore  itself,  on  being  heated  to  about  600°  to  800°  it  is  reduced  at 
the  expense  of  the  carbonic  oxide  ascending  the  furnace,  and  formed  by  the  contact  of 


IKON,  COBALT,  AND  NICKEL 


325 


The  cast  iron  formed  in  blast  furnaces  is  not  always  of  the  same 
quality.     When  slowly  cooled  it  is  soft,  has  a  grey  colour,  and  is  not. 

the  carbonic  anhydride  with  the  incandescent  charcoal,  so  that  the  reduction  in  the  blast 
furnace  is  without  doubt  brought  about  by  the  formation  and  decompDsition  of  carbonic 
oxide  and  not  by  carbon  itself— thus,  Fe2O3  +  3CO  =  Fe2  +  3C02.  The  reduced  iron,  on 
further  subsidence  and  contact  with  carbon,  forms  cast  iron,  which  flows  to  the  bottom 
of  the  furnace.  In  these  lower  layers,  where  the  temperature  is  highest  (about  1,800°), 


PlO.  93.— Vertical  section  of  a  modern  Cleveland  blast  furnace  capable  of  producing  300  to  1,000  tons 
of  pig  iron  weekly.  The  outer  casing  is  of  riveted  iron  plates,  the  furnace  being  lined  with  re- 
fractory fire-brick.  It  is  closed  at  the  top  by  a  'cap  and  cone '  arrangement,  by  means  of  which 
the  charge  can  be  fed  into  the  furnace  at  suitable  intervals  by  lowering  the  moveable  cone. 

the  foreign  matter  of  the  ore  finally  forms  slag,  which  also  is  fusible,  with  the  aid  of 
fluxes.  The  air  blown  in  from  below,  through  the  so-called  tuyeres,  encounters  carbon 
in  the  lower  layers  of  the  furnace,  and  burns  it,  converting  it  into  carbonic  anhydride. 
It  is  evident  that  this  develops  the  highest  temperature  in  these  lower  layers  of  the 
furnace,  because  here  the  combustion  of  the  carbon  is  effected  by  heated  and  compressed 
air.  This  is  very  essential,  for  it  is  by  virtue  of  this  high  temperature  that  the 
process  of  forming  the  slag  and  of  forming  and  fusing  the  cast  iron  are  effected 


826  PRINCIPLES  OF  CHEMISTRY 

completely  soluble  in  acids.  When  treated  with  acids  a  residue  of 
graphite  remains  ;  it  is  known  as  grey  or  soft  cast  iron.  This  is  the 
general  form  of  the  ordinary  cast  iron  used  for  casting  various  objects, 
because  in  this  state  it  is  not  so  brittle  as  in  the  shape  of  white  cast 
iron,  which  does  not  leave  particles  of  graphite  when  dissolved,  but 
yields  its  carbon  in  the  form  of  hydrocarbons.  This  white  cast  iron 
is  characterised  by  its  whitisn-grey  colour,  dull  lustre,  the  crystalline 
structure  of  its  fracture  (more  homogeneous  than  that  of  grey  iron),  and 
such  hardness  that  a  file  will  hardly  cut  it.  When  white  cast  iron  is 
produced  (from  manganese  ore)  at  high  temperatures  (and  with  an  ex- 
cess of  lime),  and  containing  little  sulphur  and  silica  but  a  considerable 
amount  of  carbon  (as  much  as  5  p.c.),  it  acquires  a  coarse  crystalline 
structure  which  increases  in  proportion  to  the  amount  of  manganese, 
and  it  is  then  known  under  the  name  of  '  spiegeleisen '  (and  'ferro- 
manganese  ').9 

simultaneously  in  these  lower  portions  of  the  furnace.  The  carbonic  acid  formed  in 
these  parts  rises  higher,  encounters  incandescent  carbon,  and  forms  with  it  carbonic 
oxide.  This  heated  carbonic  oxide  acts  as  a  reducing  agent  on  the  iron  ore,  and  is  re- 
converted by  it  into  carbonic  anhydride ;  this  gas  meets  with  more  carbon,  and  again 
forms  carbonic  oxide,  which  again  acts  as  a  reducing  agent.  The  final  transformation 
of  the  carbonic  anhydride  into  carbonic  oxide  is  effected  in  those  parts  of  the  furnace 
where  the  reduction  of  the  oxides  of  iron  does  not  take  place,  but  where  the  temperature- 
is  still  high  enough  to  reduce  the  carbonic  anhydride.  The  ascending  mixture  of 
carbonic  oxide  and  nitrogen,  CO2,  &c.,  is  then  withdrawn  through  special  lateral 
apertures  formed  in  the  upper  cold  parts  of  the  furnace  walls,  and  is  conducted  through 
pipes  to  those  stoves  which  are  used  for  heating  the  air,  and  also  sometimes  into  other 
furnaces  used  for  the  further  processes  of  iron  manufacture.  The  fuel  of  blast  furnaces 
consists  of  wood  charcoal  (this  is  the  most  expensive  material,  but  the  pig  iron  pro- 
duced in  the  purest,  because  charcoal  does  not  contain  any  sulphur,  while  coke  does), 
anthracite  (for  instance,  in  Pennsylvania,  and  in  Russia  at  Pastouhoff's  works  in  the 
Don  district),  coke,  coal,  and  even  wood  and  peat.  It  must  be  borne  in  mind  that  the 
utilisation  of  naphtha  and  naphtha  refuse  would  probably  give  very  profitable  results 
in  metallurgical  processes. 

The  process  just  described  is  accompanied  by  a  series  of  other  processes.  Thus,  for 
instance,  in  the  blast  furnace  a  considerable  quantity  of  cyanogen  compounds  are  formed. 
This  takes  place  because  the  nitrogen  of  the  air  blast  comes  into  contact  with  incan- 
descent carbon  and  various  alkaline  matters  contained  in  the  foreign  matter  of  the  ores. 
A  considerable  quantity  of  potassium  cyanide  is  formed  when  wood  charcoal  is  employed 
for  iron  smelting,  as  its  ash  is  rich  in  potash. 

9  The  specific  gravity  of  white  cast  iron  is  about  7'5.  Grey  cast  iron  has  a  much  lower 
specific  gravity,  namely,  7'0.  Grey  cast  iron  generally  contains  less  manganese  and 
more  silica  than  white;  but  both  contain  from  2  to  3  p.c.  of  carbon.  The  difference 
between  the  varieties  of  cast  iron  depends  on  the  condition  of  the  carbon  which 
enters  into  the  composition  of  the  iron.  In  white  cast  iron  the  carbon  is  in  combination 
with  the  iron — in  all  probability,  as  the  compound  CFe4  (Abel  and  Osmond  and  others 
extracted  this  compound,  which  is  sometimes  called  'carbide,'  from  tempered  steel, 
which  stands  to  unannealed  steel  as  white  cast  iron  does  to  grey),  but  perhaps  in  the  state 
of  an  indefinite  chemical  compound  resembling  a  solution.  In  any  case  the  compound  of 
the  iron  and  carbon  in  white  cast  iron  is  chemically  very  unstable,  because  when  slowly 
cooled  it  decomposes,  with  separation  of  graphite,  just  as  a  solution  when  slowly  cooled 


IRON,   COBALT,  AND  NICKEL  827 

Cast  iron  is  a  material  which  is  either  suitable  for  direct  application 
for  casting  in  moulds  or  else  for  working  up  into  wrought  iron  and 
eteel.  The  latter  principally  differ  from  cast  iron  in  their  containing 
less  carbon— thus,  steel  contains  from  1  p.c.  to  0-5  p.c.  of  carbon  and 
far  less  ^silicon  and  manganese  than  cast  iron  ;  wrought  iron  does 
not  generally  contain  more  than  0'25  p.c.  of  carbon  and  not  more  than 
0-25  p.c.  of  the  other  impurities.  Thus  the  essence  of  the  working  up 
of  cast  iron  into  steel  and  wrought  iron  consists  in  the  removal  of  the 
greater  part  of  the  carbon  and  other  elements,  S,  P,  Mn,  Si,  &c.  This 
is  effected  by  means  of  oxidation,  because  the  oxygen  of  the  atmosphere, 
oxidising  the  iron  at  a  high  temperature,  forms  solid  oxides  with  it ; 
and  the  latter,  coming  into  contact  with  the  carbon  contained  in  the 
cast  iron,  are  deoxidised,  forming  wrought  iron  and  carbonic  oxide, 
which  is  evolved  from  the  mass  in  a  gaseous  form.  It  is  evident  that 
the  oxidation  must  be  carried  on  with  a  molten  mass  in  a  state  of 
agitation,  so  that  the  oxygen  of  the  air  may  be  brought  into  contact 
with  the  whole  mass  of  carbon  contained  in  the  cast  iron,  or  else  the 
operation  is  effected  by  means  of  the  addition  of  oxygen  compounds 
of  iron  (oxides,  ores,  as  in  Martin's  process).  Cast  iron  melts  much 
more  easily. than  wrought  iron  and  steel,  and,  therefore,  as  the  carbon 
separates,  the  mass  in  the  furnace  (in  puddling)  or  hearth  (in  the 
bloomery  process)  becomes  more  and  more  solid  ;  .moreover  the  degree  of 
hardness  forms,  to  a  certain  extent,  a  measure  of  the  amount  of  carbon 
separated,  and  the  operation  may  terminate  either  in  the  formation  of 
steel  or  wrought  iron.10  In  any  case,  the  iron  used  for  industrial  pur- 
yields  a  portion  of  the  substance  dissolved.  The  separation  of  carbon  in  the  form  of 
graphite  on  the  conversion  of  white  cast  iron  into  grey  is  never  complete,  however  slowly 
the  separation  be  carried  on;  part  of  the  carbon  remains  in  combination  with  the 
iron  in  the  same  state  in  which  it  exists  in  white  cast  iron.  Hence  when  grey  cast  iron  is 
treated  with  acids,  the  whole  of  the  carbon  does  not  remain  in  the  form  of  graphite,  but  a 
part  of  it  is  separated  as  hydrocarbons,  which  proves  the  existence  of  chemically-combined 
carbon  in  grey  cast  iron.  It  is  sufficient  to  re-melt  grey  cast  iron  and  to  cool  it  quickly  to 
transform  it  into  white  cast  iron.  It  is  not  carbon  alone  that  influences  the  properties  of 
oast  iron ;  when  it  contains  a  considerable  amount  of  sulphur,  cast  iron  remains  white 
even  after  having  been  slowly  cooled.  The  same  is  observed  in  cast  iron  very  rich  in 
manganese  (5  to  7  p.c.),  and  in  this  latter  case  the  fracture  is  very  distinctly  crystalline 
tod  brilliant.  When  cast  iron  contains  a  large  amount  of  manganese,  the  quantity  of 
carbon  may  also  be  increased.  Crystalline  varieties  of  cast  iron  rich  in  manganese  are  in 
practice  called  ferro-manganese  (p.  310),  and  are  prepared  for  the  Bessemer  process. 
Grey  cast  iron  not  having  an  uniform  structure  is  much  more  liable  to  various  changes 
than  dense  and  thoroughly  uniform  white  cast  iron,  and  the  latter  oxidises  much  more 
elowly  in  air  than  the  former.  White  cast  iron  is  not  only  used  for  conversion  into  wrought 
iron  and  steel,  but  also  in  those  cases  where  great  hardness  is  required,  although  it  be  ac- 
companied by  a  certain  brittleness  ,  for  instance,  for  making  rollers,  plough- shares,  &c. 

10  This  direct  process  of  separating  the  carbon  from  cast  iron  is  termed  puddling.  It 
is  conducted  in  reverberatory  furnaces.  The  cast  iron  is  placed  on  the  bed  of  the 
furnace  and  melted  ;  through  a  special  aperture,  the  puddler  stirs  up  the  oxidising  raasa 


328  PRINCIPLES  OF  CHEMISTRY 

poses  contains  impurities.     Chemically  pure  iron  may  He  obtained  by 
precipitating  iron  from  a  solution  (a  mixture  of  ferrous  sulphate  with 

of  cast  iron,  pressing  the  oxides  into  the  molten  iron.    This  resembles  kneading  dough, 
and  the  process  introduced  in  England  became  known  as  puddling.    It  is  evident  that 
the  puddled  mass,  or  bloom,  is  a  heterogeneous  substance  obtained  by  mixing,  and 
hence  one  part  of  the  mass  will  still  be  rich  in  carbon,  another  will  be  poor,  some  parts 
will  contain  oxide  not  reduced,  &c.    The  further  treatment  of  the  puddled  mass  consists 
in  hammering  and  drawing  it  out  into  flat  pieces,  which  on  being  hammered  become 
more  homogeneous,  and  when  several  pieces  are  welded  together  and  again  hammered 
out  a  still  more  homogeneous  mass  is  obtained.    The  quality  of  the  steel  and  iron  thua 
formed  depends  principally  on  their  uniformity.    The  want  of  uniformity  depends  on 
the  oxides  remaining  inside  the  mass,  and  on  the  variable  distribution  of  the  carbon 
throughout  the  mass.      In  order  to  obtain  a  more  homogeneous  metal  for  manufac- 
turing articles  out  of  steel,  it  is  drawn   into    thin  rods,  which    are  tied  together  in 
bundles  and  then  again  hammered  out.     As  an  example  of  what  may  be  attained  in  this 
direction,   imitation  Damascus  steel  may  be  cited ;  it  consists  of  twisted  and  plaited 
wire,  which   is  then  hammered  into  a  dense  mass.      (Real   damascened  wootz   steel 
may  be  made  by  melting  a  mixture  of  the  best  iron  with  graphite  (fV)  and  iron  rust; 
the  article  is  then  corroded  with  acid,  and  the  carbon  remains  in  the  form  of  a  pattern.) 
Steel  and  wrought  iron  are  manufactured  from  cast  iron  by  puddling.    They  are,  how- 
ever, obtained  not  only  by  this  method  but  also  by  the  bloomery  process,  which  is  carried 
out  in  a  fire  similar  to  a  blacksmith's  forge,  fed  with  charcoal  and  provided  with  a  blast , 
a  pig  of  cast  iron  is  gradually  pushed  into  the  fire,  and  portions  of  it  melt  and  fall  to  the 
bottom  of  the  hearth,  coming  into  contact  with  an  air  blast,  and  are  thus  oxidised.     Tho 
bloom  thus  formed  is  then  squeezed  and  hammered.    It  is  evident  that  this  process  ia 
only  available  when  the  charcoal  used  in  the  fire  does  not  contain  any  foreign  matter 
which  might  injure  the  quality  of  the  iron  or  steel — for  instance,  sulphur  or  phosphorus 
— and  therefore  only  wood  charcoal  may  be  used  with  impunity,  from  which  it  follows 
that  this  process  can  only  be  carried  on  where  the  manufacture  of  iron  can  be  conducted 
with  this  fuel.     Coal  and  coke  contain  the  above-mentioned  impurities,  and  would 
therefore  produce  iron  of  a  brittle  nature,  and  thus  it  would  be  necessary  to  have 
recourse  to  puddling,  where  the  fuel  is  burnt  on  a  special  hearth,  separate  from  the 
cast  iron,  whereby  the  impurities  of  the  fuel  do  not  come  into  contact  with  it.    The 
manufacture  of  steel  from  cast  iron  may  also  be  conducted  in  fires;  but,  in,  addition  to 
this,  it  is  also  now  prepared  by  many  other  methods.     One  of  the  long-known  processes 
is  called  cementation,  by  which  steel  is  prepared  from  wrought  iron  but  not  from  cast 
iron.    For  this  process  strips  of  iron  are  heated  red-hot  for  a  considerable  time  whilst 
immersed  in  powdered  charcoal ;  during  this  operation  the  iron  at  the  surface  combines 
with  the  charcoal,  which  however  does  not  penetrate ;  after  this  the  iron  strips  are 
re-forged,  drawn  out  again,  and  cemented  anew,  repeating  this  process  until  a  steel  of  the 
desired  quality  is  formed — that  is,  containing  the  requisite  proportion  of  carbon.    The 
Bessemer  process  occupies  the  front  rank  among  the  newer  methods  (since  1856)  ;  it 
is  so  called  from  the  name  of  its  inventor.     This  process  consists  in  running  melted 
cast  iron   into   converters  (holding  about  6  tons  of  cast  iron) — that  is,  egg-shaped 
receivers,  fig.    94,  capable  of   revolving  on    trunnions  (in  order    to    charge  in    the 
cast  iron  and  discharge  the  steel),  and  forcing  a  stream  of  air  through  small  apertures 
at  a  considerable  pressure.     Combustion  of  the  iron  and  carbon  at  an  elevated  tempera- 
ture then  taken  place,  resulting  from  the  bubbles  of  oxygen  thus  penetrating  the  mass 
of  the  cast  iron.     The  carbon,  however,  burns  to  &  greater  extent  than  the  iron,  and 
therefore  a  mass  is  obtained  which  is  much  poorer  in  carbon  than  cast  iron.    As  the 
combustion  proceeds  very  rapidly  in  the  mass  of  metal,  the  temperature  rises  to  such  an 
extent  that  even  the  wrought  iron  which  may  be  formed  remains  in  a  molten  condition, 
whilst  the  steel,  being  more  fusible  than  the  wrought  iron,  remains  very  liquid.      In 
half  an  hour  the  mass  is  ready.     The  purest  possible  cast  iron  is  used  in  the  Bessemer 


IRON,   COBALT,  AND  NICKEL  329 

magnesium  sulphate  or  ammonium  chloride)  by  the  prolonged  action  of 
&  feeble  galvanic  current ;  the  iron  may  be  then  obtained  as  a  dense 

process,  because  sulphur  and  phosphorus  do  not  burn  out  like  carbon,  silicon,  and 
manganese. 

The  presence  of  manganese  enables  the  sulphur  to  be  removed  with  the  slag,  and  the 
presence  of  lime  or  magnesia,  which  are  introduced  into  the  lining  of  the  converter, 


Wi  TBi 


PJO.  94.— Bessemer  converter,  constructed  of  iron  plate  and  lined  with  ganister.  The  air  is  carried 
by  the  tubes,  L,  0,  D  to  the  bottom,  M,  from  which  it  passes  by  a.  number  of  holes  into  the  con- 
verter. The  converter  is  rotated  on  the  trunnion  d  by  means  of  the  rack  and  pinion  H,  when  it 
is  required  either  to  receive  molten  cast  iron  from  the  melting  furnaces  or  to  pour  out  the  steel. 

facilitates  the  removal  of  the  phosphorus.  This  basic  Bessemer  process,  or  Thomas 
Oilchrist  process,  introduced  about  1880,  enables  ores  containing  a  considerable  amount 
•>f  phosphorus,  which  had  hitherto  only  been  used  for  cast  iron,  to  be  used  for  making 
vrought  iron  and  steel.  Naturally  the  greatest  uniformity  will  be  obtained  .by  re-melting 
the  metal.  Steel  is  re-melted  in  small  wind  furnaces,  in  masses  not  exceeding  80  kilos ; 
a  liquid  metal  is  formed,  which  may  be  cast  in  moulds.  Admixture  of  wrought  and  cast 
iron  is  often  used  for  making  cast  steel  (the  addition  ot  a  small  amount  of  metallic  Al 
improves  the  homogeneity  of  the  castings,  by  facilitating  the  passage  of  the  impurities 
into  slag).  Large  steel  castings  are  made  by  simultaneous  fusion  in  several  furnaces  and 
•crucibles ;  in  this  way,  castings  up  to  80  tons  or  more,  such  as  large  ordnance,  may  be 
made.  This  molten,  and  therefore  homogeneous,  steel  is  called  cast  steel.  Of  late  years 
the  Martin's  process  for  the  manufacture  of  steel  has  come  largely  into  use ;  it  was 
invented  in  France  about  1860,  and  with  the  use  of  regenerative  furnaces  it  enables  large 
quantities  of  cast  steel  to  be  made  at  a  time.  It  is  based  on  the  melting  of  cast  iron  with 
iron  oxides  and  iron  itself — for  instance,  pure  ores,  scrap,  &c.  There  the  carbon  of  the 
cast  iron  and  the  oxygen  of  the'  oxide  form  carbonic  oxide,  and  the  carbon  therefore 
burns  out,  and  thus  cast  steel  is  obtained  from  cast  iron,  providing,  naturally, 'that  there 
is  a  requisite  proportion  and  corresponding  degree  of  heat.  The  advantage  of  this 


330  PRINCIPLES  OF  CHEMISTRY 

mass.  This  method,  proposed  by  Bottcher  and  applied  by  Klein,  gives, 
as  R.  Lenz  showed,  iron  containing  occluded  hydrogen,  which  is  dis- 

process  is  that  not  only  do.  the  carbon,  silicon,  and  manganese,  but  also  a  great  part  of 
the  sulphur  and  phosphorus  of  the  cast  iron  burn  out  at  the  expense  of  the  oxygen  of  the 
iron  oxides.  During  the  last  decade  the  manufacture  of  steel  and  its  application  for 
rails,  armour  plate,  guns,  boilers,  &c.,  has  developed  to  an  enormous  extent,  thanks  to 
the  invention  of  cheap  processes  for  the  manufacture  of  large  masses  of  homogeneous 
cast  steel.  Wrought  iron  may  also  be  melted,  but  the  heat  of  a  blast  furnace  is  insufficient 
for  this.  It  easily  melts  in  the  oxyhydrogen  flame.  It  may  be  obtained  in  a  molten 
state  directly  from  cast  iron,  if  the  latter  be  melted  with  nitre  and  sufficiently  stirred  up. 
Considerable  oxidation  then  takes  place  inside  the  mass  of  cast  iron,  and  the  temperature 
rises  to  such  an  extent  that  the  wrought  iron  formed  remains  liquid.  A  method  is  also 
known  for  obtaining  wrought  iron  directly  from  rich  iron  ores  by  the  action  of  carbonic 
oxide :  the  wrought  iron  is  then  formed  as  a  spongy  mass  (which  forms  an  excellent 
filter  for  purifying  water),  and  may  be  worked  up  into  wrought  iron  or  steel  either  by 
forging  or  by  dissolving  in  molten  cast  iron.  / 

Everybody  is  more  or  less  familiar  with  the  difference  in  the  properties  of  steel  and 
wrought  iron.  Iron  is  remarkable  for  its  softness,  pliability,  and  small  elasticity,  whilst 
steel  may  be  characterised  by  its  capability  of  attaining  elasticity  and  hardness  if  it  be 
cooled  suddenly  after  having  been  heated  to  a  definite  temperature,  or,  as  it  is  termed, 
tempered.  But  if  tempered  steel  be  re-heated  and  slowly  cooled,  it  becomes  as  soft  as 
wrought  iron,  and  can  then  be  cut  with  the  file  and  forged,  and  in  general  can  be  made 
to  assume  any  shape,  like  wrought  iron.  In  this  soft  condition  it  is  called  annealed  steel. 
The  transition  from  tempered  to  annealed  steel  thus  takes  place  in  a  similar  way  to  the 
transition  from  white  to  grey  cast  iron.  Steel,  when  homogeneous,  has  considerable 
lustre,  and  such  a  fine  granular  structure  that  it  takes  a  very  high  polish.  Its  fracture 
clearly  shows  the  granular  nature  of  its  structure.  The  possibility  of  tempering  steel 
enables  it  to  be  used  for  making  all  kinds  of  cutting  instruments,  because  annealed  steel 
can  be  forged,  turned,  drawn  (under  rollers,  for  instance,  for  making  rails,  bars,  &c.),  filed, 
&c.,  and  it  may  then  be  tempered,  ground  and  polished.  The  method  and  temperature 
of  tempering  and  annealing  steel  determine  its  hardness  and  other  qualities.  Steel  is 
generally  tempered  to  the  required  degree  of  hardness  in  the  following  manner  :  It  is 
first  strongly  heated  (for  instance,  up  to  600°),  and  then  plunged  into  water — that  is, 
hardened  by  rapid  cooling  (it  then  becomes  as  brittle  as  glass).  It  is  then  heated  until 
the  surface  assumes  a  definite  colour,  and  finally  cooled  either  quickly  or  slowly. 
When  steel  is  heated  up  to  220°,  its  surface  acquires  a  yellow  colour  (surgical  instru- 
ments) ;  it  first  of  all  becomes  straw-coloured  (razors,  &c.),  and  then  gold-coloured ;  then 
at  a  temperature  of  250°  it  becomes  brown  (scissors),  then  red,  then  light  blue  at  285° 
(springs),  then  indigo  at  300°  (files),  and  finally  sea-green  at  about  840°.  These  colours 
are  only  the  tints  of  thin  films,  like  the  hues  of  soap  bubbles,  and  appear  on  the  steel 
because  a  thin  layer  of  oxides  is  formed  over  its  surface.  Steel  rusts  more  slowly  than 
wrought  iron,  and  is  more  soluble  in  acids  than  cast  iron,  but  less  so  than  wrought  iron. 
Its  specific  gravity  is  about  7'6  to  7'9. 

As  regards  the  formation  of  steel,  it  was  a  long  time  before  the  process  of  cementation 
was  thoroughly  understood,  because  in  this  case  infusible  charcoal  permeates  unfused 
wrought  iron.  Caron  showed  that  this  permeation  depends  on  the  fact  that  the  charcoal 
nsed  in  the  process  contains  alkalis,  which,  in  the  presence  of  the  nitrogen  of  the 
air,. form  metallic  cyanides;  these  being  volatile  and  fusible,  permeate  the  iron,  and, 
giving  up  their  carbon  to  it,  serve  as  the  material  for  the  formation  of  steel.  This 
explanation  is  confirmed  by  the  fact  that  charcoal  without  alkalis  or  without  nitrogen 
will  not  cement  iron.  The  charcoal  used  for  cementation  acts  badly  when  used  over 
again,  as  it  has  lost  alkali.  The  very  volatile  ammonium  cyanide  easily  conduces  to  the 
formation  of  steel.  Although  steel  is  also  formed  by  the  action  of  cyanogen  compounds, 
nevertheless  it  does  not  contain  more  nitrogen  than  cast  or  wrought  iron  (O'Ol  p.c.),  and 


IRON,  COBALT,  AND  NICKEL  391 

engaged  on  heating.  This  galvanic  deposition  of  iron  is  used  for 
making  galvanoplastic  cliches,  which  are  distinguished  for  their  great 

these  latter  contain  it  because  their  ores  contain  titanium,  which  combines  directly  with 
nitrogen.  Hence  the  part  played'  by  nitrogen  in  steel  is  but  an  insignificant  one.  16 
may  be  useful  here  to  add  some  information  taken  from  Caron's  treatise  concerning  the 
influence  of  foreign  matter  on  the  quality  of  steel.  The  principal  properties  of  steel  ar* 
those  of  tempering  and  annealing.  The  compounds  of  iron  with  silicon  and  boron  have 
not  these  properties.  They  are  more  stable  than  the  carbon  compound,  and  this  latter1 
is  capable  of  changing  its  properties;  because  the  carbon  in  it  either  enters  into 
combination  or  else  is  disengaged,  which  determines  the  condition  of  hardness  or  softness 
of  steel,  as  in  white  and  grey  cast  iron.  When  slowly  cooled,  steel  splits  up  into  & 
mixture  of  soft  and  carburetted  iron ;  but,  nevertheless,  the  carbon  does  not  separate 
from  the  iron.  If  such  steel  be  again  heated,  it  forms  a  uniform  compound,  and  hardens 
when  rapidly  cooled.  If  the  same  steel  as  before  be  taken  and  heated  a  long  time,  then, 
after  being  slowly  cooled,  it  becomes  much  more  soluble  in  acid,  and  leaves  a  residue  of 
pure  carbon.  This  shows  that  the  combination  between  the  carbon  and  iron  in  steel 
becomes  destroyed  when  subjected  to  heat,  and  the  steel  becomes  iron  mixed  with 
carbon.  Such  burnt  steel  cannot  be  tempered,  but  may  be  corrected  by  continued 
forging  in  a  heated  condition,  which  has  the  effect  of  redistributing  the  carbon  equally 
throughout  the  whole  mass.  After  the  forging,  if  the  iron  is  pure  and  the  carbon  has 
not  been  burnt  out,  steel  is  again  formed,  which  may  be  tempered.  If  steel  be  re- 
peatedly or  strongly  heated,  it  becomes  burnt  through  and  cannot  be  tempered  or 
annealed ;  the  carbon  separates  from  the  iron,  and  this  is  effected  more  easily  if  the 
steel  contains  other  impurities  which  are  capable  of  forming  stable  combinations  with 
iron,  such  as  silicon,  sulphur,  or  phosphorus.  If  there  be  much  silicon,  it  occupies  the 
place  of  the  carbon,  and  then  continued  forging  will  not  induce  the  carbon  once 
separated  to  re-enter  into  combination.  Such  steel  is  easily  burnt  through  and  cannot 
be  corrected;  when  burnt  through,  it  is  hard  and  cannot  be  annealed — this  is  tough 
steel,  an  inferior  kind.  Iron  which  contains  sulphur  and  phosphorus  cements  badly, 
combines  but  little  with  carbon,  and  steel  of  this  kind  is  brittle,  both  hot  and  cold. 
Iron  in  combination  with  the  above-mentioned  substances  cannot  be  annealed  by  slow 
cooling,  showing  that  these  compounds  are  more  stable  than  those  of  carbon  and  iron^ 
and  therefore  they  prevent  the  formation  of  the  latter.  Such  metals  as  tin  and  zino 
combine  with  iron,  but  not  with  carbon,  and  form  a  brittle  mass  which  cannot  he 
annealed  and  is  deleterious  to  steel.  Manganese  and  tungsten,  on  the  contrary,  are 
capable  of  combining  with  charcoal ;  they  do  not  hinder  the  formation  of  steel,  but  even 
remove  the  injurious  effects  of  other  admixtures  (by  transforming  these  admixed  sub- 
stances into  new  compounds  and  slags),  and  are  therefore  ranked  with  the  substances 
which  act  beneficially  on  steel ;  but,  nevertheless,  the  best  steel,  which  is  capable  of 
renewing  most  often  its  primitive  qualities  after  burning  or  hot  forging,  is  the  purest. 
The  addition  of  Ni,  Cr,  W,  and  certain,  other  metals  to  steel  renders  it  very  suitable  fo? 
certain  special  purposes,  and  is  therefore  frequently  made  use  of. 

It  is  worthy  of  attention  that  steel,  besides  temper,  possesses  many  variable 
properties,  a  review  of  which  may  be  made  in  the  classification  of  the  sorts  of  steel 
(1878,  Cockerell).  (1)  Very  mild  steel  contains  from  0'05  to  0'20  p.c.  of  carbon,  breaks 
with  a  weight  of  40  to  50  kilosjper  square  millimetre,  and  has  an  extension  of  20  to 
80  p.c.;  it  may  be  welded, like  wrought  iron,  but  cannot  be  tempered;  is  used  in  sheets 
for  boilers,  armour  plate  and  bridges,  nails,  rivets,  &c.,  as  a  substitute  for  wrought  iron  ; 
(2)  mild  steel,  from  0'20  to  0'85  p.c.  of  carbon,  resistance  to  tension  50  to  60  kitoa, 
extension  15  to  20  p.c.,  not  easily  welded,  and  tempers  badly,  used  for  axles,  rails,  and 
railway  tyres,  for  cannons  and  guns,  and  for  parts  of  machines  destined  to  resist  bending 
and  torsion ;  (3  hard  steel,  carbon  0'85  to  0'50  p.c.,  breaking  weight  60  to  70  kilos  per 
square  millimetre,  extension  10  to  15  p.c.,  cannot  be  welded,  takes  a  temper;  used  for 
jails,  all  kinds  of  springs,  swords,  parts  of  machinery  in  motion  subjected  to  friction, 


832  PRINCIPLES  OF  CHEMISTRY 

hardness.  Electro- deposited  iron  is  brittle,  but  if  heated  (after  the 
separation  of  the  hydrogen)  it  becomes  soft.  If  pure  ferric  hydroxide, 
which  is  easily  prepared  by  the  precipitation  of  solutions  of  ferric 
salts  by  means  of  ammonia,  be  heated  in  a  stream  of  hydrogen,  it 
forms,  first  of  all,  a  dull  black  powder  which  ignites  spontaneously  in 
air  (pyrophoric  iron),  and  then  a  grey  powder  of  pure  iron.  The 
powdery  substance  first  obtained  is  an  iron  suboxide  ;  when  thrown 
into  the  air  it  ignites,  forming  the  oxide  Fe3O4.  If  the  heating  in 
hydrogen  be  continued,  more  water  and  pure  iron,  which  does  not 
ignite  spontaneously,  will  be  obtained.  If  a  small  quantity  of  iron  be 
[fused  in  the  oxyhydrogen  flame  (with  an  excess  of  oxygen)  in  a  piece 
'of  lime  and  mixed  with  powdered  glass,  pure  molten  iron  will  be 
formed,  because  in  the  oxyhydrogen  flame  iron  melts  and  burns,  but 
the  substances  mixed  with  the  iron  oxidise  first.  The  oxidised  im- 
purities, here  either  disappear  (carbonic  anhydride)  in  a  gaseous  form, 
or  turn  into  slag  (silica,  manganese,  oxide,  and  others)— that  is,  fuse 
with  the  glass.  Pure  iron  has  a  silvery  white  colour  and  a  specific 
gravity  of  7 -84 ;  it  melts  at  a  temperature  higher  than  the  melting- 
points  of  silver,  gold,  nickel,  and  steel,  i.e.  about  1400°-!  500°  and 


spindles  of  looms,  hammers,  spades,  hoes,  &c. ;  (4)  very  hard  steel,  carbon  0*5  to  0*65 
p.c.,  tensile  breaking  weight  70  to  80  kilos,  extension  5  to  10  p.c.,  does  not  weld,  but 
tempers  easily ;  used  for  small  springs,  saws,  files,  knives  and  similar  instruments. 

The  properties  of  ordinary  wrought  iron  are  well  known1.  The  best  iron  is  the  most 
tenacious — that  is  to  say,  that  which  does  not  break  up  when  struck  with  the  hammer 
or  bent,  and  yet  at  the  same  time  is  sufficiently  hard.  There  is,  however,  a  distinction 
between  hard  and  soft  iron.  Generally  the  softest  iron  is  the  most  tenacious,  and  can 
best  be  welded,  drawn  into  wire,  sheets,  &c.  Hard,  especially  tough,  iron  is  often 
characterised  by  its  breaking  when  bent,  and  is  therefore  very  difficult  to  work,  and 
objects  made  from  it  are  less  serviceable  in  many  respects.  Soft  iron  is  most  adapted 
for  making  wire  and  sheet  iron  and  such  small  objects  as  nails.  Soft  iron  is  characterised 
by  its  attaining  a  fibrous  fracture  after  forging,  whilst  tough  iron  preserves  its  granular 
structure  after  this  operation.  Certain  sorts  of  iron,  although  fairly  soft  at  the  ordinary 
temperature,  become  brittle  when  heated  and  are  difficult  to  weld.  These  sorts  are 
less  suitable  for  being  worked  up  into  small  objects.  The  variety  of  the  properties  of 
iron  depends  on  the  impurities  which  it  contains.  In  general,  the  iron  used  in  the  arts 
still  contains  carbon  and  always  a  certain  quantity  of  silicon,  manganese,  sulphur, 
phosphorus,  &c.  A  variety  in  the  proportion  of  these  component  parts  changes  th« 
quality  of  the  iron.  In  addition  to  this  the  change  which  soft  wrought  iron,  having  a 
fibrous  structure,  undergoes  when  subjected  to  repeated  blows  and  vibrations  is  con- 
siderable ;  it  then  becomes  granular  and  brittle.  This  to  a  certain  degree  explains  the 
want  of  stability  of  some  iron  objects — such  as  truck  axles,  which  must  be  renewed  after 
a  certain  term  of  service,  otherwise  they  become  brittle.  It  is  evident  that  there  are 
innumerable  intermediate  transitions  from  wrought  iron  to  steel  and  cast  iron. 

At  the  present  day  the  greater  part  of  the  cast  iron  manufactured  is  converted  into 
steel,  generally  cast  steel  (Bessemer's  and  Martin's).  I  may  add  the  Urals,  Donetz 
district,  and  other  parts  of  Russia  offer  the  greatest  advantages  for  the  development  of 
an  iron  industry,  because  these  localities  not  only  contain  vast  supplies  of  excellent  iron 
ore,  but  also  coal,  which  is  necessary  for  smelting  it. 


IRON,  COBALT,  AND  NICKEL  333 

below  the  melting  point  of  platinum  (17500).11  But  pure  iron  becomes 
soft  at  a  temperature  considerably  below  that  at  which  it  melts,  and 
may  then  be  easily  forged,  welded,  and  rolled  or  drawn  into  sheets  and 
wireiibis  Pure  iron  may  be  rolled  into  an  exceedingly  thin  sheet, 
weighing  less  than  a  sheet  of  ordinary  paper  of  the  same  size.  This 
ductility  is  the  most  important  property  of  iron  in  all  its  forms,  and  is 
most  marked  with  sheet  iron,  and  least  so  with  cast  iron,  whoso 
ductility,  compared  with  wrought  iron,  is  small,  but  it  is  still  very 
considerable  when  compared  with  other  substances — such,  for  instance, 
as  rocks.12 

The  chemical  properties  of  iron  have  been,  already  repeatedly 
mentioned  in  preceding  chapters.  Iron  rusts  in  air  at  the  ordinary 
temperature — that  is  to  say,  it  becomes  covered  with  a  layer  of  iroa 
oxides.  Here,  without  doubt,  the  moisture  of  the  air  plays  a  part, 
because  in  dry  air  iron  does  not  oxidise  at  all,  and  also  because,  more 

11  According  to  information  supplied  by  A.  T.  Skinder's  experiments  at  the  Oboukoft 
Steel  Works,  140  volumes  of  liquid  molten  steel  give  128  volumes  of  solid  metal.    By 
means  of  a  galvanic  current  of  great  intensity  and  dense  charcoal  as  one  electrode,  aru$ 
iron  as  the  other,  Bernadoss  welded  iron  and  fused  holes  through  sheet  iron.    Soft 
wrought  iron,  like  steel  and  soft  malleable  cast  iron,  may  be  melted  in  Siemens* 
regenerative  furnaces,  and  in  furnaces  heated  with  naphtha. 

u  bis  (}ore  (1869),  Tait,  Barret,  Tchernoff,  Osmond,  and  others  observed  that  at  a. 
temperature  approaching  600° — that  is,  between  dark  and  bright  red  heat — all  kinda  of 
wrought  iron  undergo  a  peculiar  change  called  recalescence,  i.e.  a  spontaneous  rise  of 
temperature.  If  iron  be  considerably  heated  and  allowed  to  cool,  it  may  be  observed 
that  at  this  temperature  the  cooling  stops— that  is,  latent  heat  is  disengaged,  corre- 
sponding with  a  change  in  condition.  The  specific  heat,  electrical  conductivity,  magnetic* 
and  other  properties  then  also  change.  In  tempering,  the  temperature  of  recalescence 
must  not  be  reached,  and  so  also  in  annealing,  &c.  It  is  evident  that  a  change  of  the- 
internal  condition  is  here  encountered,  exactly  similar  to  the  transition  from  a  solid  to  a, 
liquid,  although  there  is  no  evident  physical  change.  It  is  probable  that  attentive  study 
would  lead  to  the  discovery  of  a  similar  change  in  other  substances. 

12  The  particles  of  steel  are  linked  together  or  connected  more  closely  than  those  of 
the  other  metals ;  this  is  shown  by  the  fact  that  it  only  breaks  with  a  tensile  strain  of 
60-80  kilos  per  sq.  mm.,  whilst  wrought  iron  only  withstands  about  80  kilos,  cast  iron, 
10,  copper  85,  silver  28,  platinum  30,  wood  8.     The  elasticity  of  iron,  steel,  and  other 
metals  i#  expressed  by  the  so-called  coefficient  of  elasticity.    Let  a  rod  be  taken  whose 
length  is  L ;  if  a  weight,  P,  be  hung  from  the  extremity  of  it,  it  will  lengthen  to  I, 
The  less  it  lengthens  under  other  equal  conditions,  the  more  elastic  the  material,  if  ifc 
resumes  its  original  length  when  the  weight  is  removed.  It  has  been  shown  by  experiment* 
that  the  increase  in  length  I,  due  to  elasticity,  i«  directly  proportional  to  the  length  L 
and  the  weight  P,  and  inversely  proportional  to  the  section,  but  changes  with  tha 
material.    The  coefficient  of  elasticity  expresses  that  weight   (in  kilos  per  sq.  mm.) 
under  which  a  rod  having  a  square  section  taken  as  1  (we  take  1  sq.  mm.)  acquires 
double  the  length  by  tension.     Naturally  in  practice  materials  -do  not  withstand  such  a. 
lengthening,  under  a  certain  weight  they  attain  a  limit  of  elasticity,  i.e.  they  stretch, 
permanently  (undergo  deformation).    Neglecting  fractions  (as  the  elasticity  of  metal$ 
varies  not  only  with  the  temperature,  but  also  with  forging,  purity,  &c.),  the  coefficient 
of  elasticity  of  steel  and  iron  is  20,000,  copper  and  brass  10,000,  silver  7,000,  glass  6,000, 
lead  2,000,  and  wood  1,200. 


834  PBINCIPLES  OF  CHEMISTRY 

particularly,  ammonia  is  always  found  in  iron  rust ;  the  ammonia  must 
arise  from  the  action  of  the  hydrogen  of  the  water,  at  the  moment  of  its. 
separation,  on  the  nitrogen  of  the  air.  Highly-polished  steel  does  not 
rust  nearly  so  readily,  but  if  moistened  with  water,  it  easily  becomes 
coated  with  rust.  As  rust  depends  on  the  access  of  moisture,  iron  may 
be  preserved  from  rust  by  coating  it  with  substances  which  prevent 
the  moisture  having  access  to  it.  Thus  arises  the  practice  of  covering 
iron  objects  with  paraffin,13  varnish,  oil,  paints,  or  enamelling  it 
with  a  glassy-looking  flux  possessing  the  same  coefficient  of  expansion  as 
iron,  or  with  a  dense  scoria  (formed  by  the  heat  of  superheated  steam), 
or  with  a  compact  coating  of  various  metals.  Wrought  iron  (both  as 
eheet  iron  and  in  other  forms),  cast  iron,  and  steel  are  often  coated  with 
tin,  copper,  lead,  nickel,  and  similar  metals,  which  prevent  contact  with 
the  air.  These  metals  preserve  iron  very  effectually  from  rust  if  they 
form  a  completely  compact  surface,  but  in  those  places  where  the  iron 
becomes  exposed,  either  accidentally  or  from  wear,  rust  appears  much 
more  quickly  than  on  a  uniform  iron  surface,  because,  towards  these 
metals  (and  also  towards  the  rust),  the  iron  will  then  behave  as  an 
electro-positive  pole  in  a  galvanic  couple,  and  hence  will  attract 
•oxygen.  A  coating  of  zinc  does  not  produce  this  inconvenience,  because 
iron  is  electro-negative  with  reference  to  zinc,  in  consequence  of  which 
galvanised  iron  does  not  easily  rust,  and  even  an  iron  boiler  containing 
some  lumps  of  zinc  rusts  less  than  one  without  zinc.14  Iron  oxidises 
at  a  high  temperature,  forming  iron  scale,  Fe3O4,  composed  of  ferrous 
•and  ferric  oxides,  and,  as  has  been  seen,  decomposes  water  and  acids 
with  the  evolution  of  hydrogen.  It  is  also  capable  of  decomposing 
salts  and  oxides  of  other  metals,  which  property  is  applied  in  the  arts 
for  the  extraction  of  copper,  silver,  lead,  tin,  &c.  For  this  reason 
iron  is  soluble  in  the  solutions  of  many  salts — for  instance,  in  cupric 
sulphate,  with  precipitation  of  copper  and  formation  of  ferrous  sul- 
phate.15 When  iron  dels  on  acids  it  always  forms  compounds  FeX2 — 

18  Paraffin  is  one  of  the  best  preservatives  for  iron  against  oxidation  in  the  air.  I 
found  this  by  experiments  about  1860,  and  immediately  published  the  fact.  This  method 
is  now  very  generally  applied. 

14  See  Chapter  XVIII.,  Note  84  bis.  Based  on  the  rapid  oxidation  of  iron  and  its 
increase  in  volume  in  the  presence  of  water  and  salts  of  ammonium,  a  packing  is  used 
for  water  mains  and  steam  pipes  which  is  tightly  hammered  into  the  socket  joints. 
This  packing  consists  of  a  mixture  of  iron  filings  and  a  small  quantity  of  sal-ammoniac 
(and  sulphur)  moistened  with  water ;  after  a  certain  lapse  of  time,  especially  after  the 
pipes  have  been  used,  this  mass  swells  to  such  an  extent  that  it  hermetically  seals  the 
joints  of  the  pipes. 

16  Here,  however,  a  ferric  salt  may  also  be  formed  (when  all  the  iron  has  dissolved 
And  the  cupric  salt  is  still  in  excess),  because  the  cupric  salts  are  reduced  by  ferrous 
••alts.  Cast  iron  is  also  dissolved. 


IRON,   COBALT,  AND  NICKEL  335 

that  is,  corresponding  to  the  suboxide  FeO  -  and  answering  to  magnesium 
compounds — and  hence  two  atoms  of  hydrogen  are  replaced  by  one 
atom  of  iron.  Strongly  oxidising  acids  like  nitric  acid  may  transform 
the  ferrous  salt  which  is  forming  into  the  higher  degree  of  oxidation  or 
ferric  salt  (corresponding  with  the  sesquioxide,  Fe2O3),  but  this  is  a 
secondary  reaction.  Iron,  although  easily  soluble  in  dilute  nitric  acid, 
loses  this  property  when  plunged  into  strong  fuming  nitric  acid  ;  after 
this  operation  it  even  loses  the  property  of  solubility  in  other  acids 
until  the  external  coating  formed  by  the  action  of  the  strong  nitric 
acid  is  mechanically  removed.  This  condition  of  iron  is  termed  the 
passive  state.  The  passive  condition  of  iron  depends  on  the  formation, 
on  its  surface,  of  a  coating  of  oxide  due  to  the  iron  being  acted  on  by 
the  lower  oxides  of  nitrogen  contained  in  the  fuming  nitric  acid.16 
Strong  nitric  acid  which  does  not  contain  these  lower  oxides,  does  not 
render  iron  passive,  but  it  is  only  necessary  to  add  some  alcohol  or 
other  reducing  agent  which  forms  these  lower  oxides  in  the  nitric  acid, 
and  the  iron  will  assume  the  passive  state. 

Iron  readily  combines  with  non-metals — for  instance,  with  chlorine, 
iodine,  bromine,  sulphur,  and  even  with  phosphorus  and  carbon ;  but 
on  the  other  hand  the  property  of  combining  with  metals  is  but  little 
developed  in  it —that  is  to  say,  it  does  not  easily  form  alloys.  Mercury, 
which  acts  on  most  metals,  does  not  act  directly  on  iron,  and  the  iron 
amalgam,  or  solution  of  iron  in  mercury,  which  is  used  for  electrical 
machines,  is  only  obtained  in  a  particular  way — namely,  with  the 
co-operation  of  a  sodium  amalgam,  in  which  the  iron  dissolves  and  by 
means  of  which  it  is  reduced  from  solutions  of  its  salts. 

When  iron  acts  on  acids  it  forms  ferrous  salts  of  the  type  FeX2, 
and  in  the  presence  of  air  and  oxidising  agents  they  change  by  degrees 
into  ferric  salts  of  the  type  Fe"X3.  This  faculty  of  passing  from  the 
ferrous  to  the  ferric  state  is  still  further  developed  in  ferrous  hydroxide. 
If  sodium  hydroxide  be  added  to  a  solution  of  ferrous  sulphate  or 
green  vitriol,  FeSO4,17a  white  precipitate  of  ferrous  hydroxide,  FeH2O2, 

16  Powdery  reduced  iron  is  passive  with  regard  to  nitric  acid  of  a  specific  gravity  of 
1*87,  bnt  when  heated  the  acid  acts  on  it.    This  passiveness  disappears  in  the  magnetic 
field.    Saint-Edme  attributes  the  passiveness  of  iron  (and  nickel)  to  the  formation  of 
nitride  of  iron  on  the  surface  of  the  metal,  because  he  observed  that  when  heated  in  dry 
hydrogen  ammonia  is  evolved  by  passive  iron. 

Remsen  observed  that  if  a  strip  of  iron  be.  immersed  in  acid  and  placed  in  the  mag- 
netic field,  it  is  principally  dissolved  at  its  middle  part— that  is,  the  acid  acts  more  feebly 
at  the  poles.  According  to  Etard  (1891)  strong  nitric  acid  dissolves  iron  in  making  it 
passive,  although  the  action  is  a  very  slow  one. 

17  Iron  vitriol  or  green  vitriol,  sulphate  of  iron  or  ferrous  sulphate,  generally  crys- 
tallises from   solutions,  like  magnesium    sulphate,  with  seven  molecules    of   water, 
FeS04,7H20.    This  salt  is  not  only  formed  by  the  action  of  iron  on  sulphuric  acid,  but 


836  PRINCIPLES  OF  CHEMISTRY 

is  obtained  ;  but  on  exposure  to  the  air,  even  under  water,  it  turns 
green,  becomes  grey,  and  finally  turns  brown,  which  is  due  to  the 
oxidation  that  it  undergoes.  Ferrous  hydroxide  is  very  sparingly 
soluble  in  water  ;  the  solution  has,  however,  a  distinct  alkaline  reaction, 
which  is  due  to  its  being  a  fairly  energetic  basic  oxide.  In  any  case, 
ferrous  oxide  is  far  more  energetic  than  ferric  oxide,  so  that  if  ammonia 
be  added  to  a  solution  containing  a  mixture  of  a  ferrous  and  ferric 
salt,  at  first  ferric  hydroxide  only  will  be  precipitated.  If  barium 
carbonate,  BaCO3,  be  shaken  up  in  the  cold  with  ferrous  salts,  it 
does  not  precipitate  them — that  is,  does  not  change  them  into  ferrous 
carbonate  ;  but  it  completely  separates  all  the  iron  from  the  ferric 
salts  in  the  cold,  according  to  the  equation  Fe2Cl6  +  3BaCO3  -t-  3H20 
e?  Fe2O3,3H2O  +  3BaCl2  +  3C02.  If  ferrous  hydroxide  be  boiled  with, 
a  solution  of  potash,  the  water  is  decomposed,  hydrogen  is  evolved,  and 
the  ferrous  hydroxide  is  oxidised.  The  ferrous  salts  are  in  all. respects 
similar  to  the  salts  of  magnesium  and  zinc  ;  they  are  isomorphous 
with  them,  but  differ  from  them  in  that  the  ferrous  hydroxide  is  not 
soluble  either  in  aqueous  potash  or  ammonia.  In  the  presence  of  an 
excess  of  ammonium  salts,  however,  a  certain  proportion  of  the  iron 

also  by  the  action  of  moisture  and  air  on  iron  pyrites,  especially  when  previously  roasted 
(FeS2  +  Oa  =  FeS  +  S02),  and  in  this  condition  it  easily  absorbs  the  oxygen  of  damp  air 
(FeS  •+•  O4  =  FeS04).  Green  vitriol  is  obtained  in  many  processes  as  a  bye-product. 
Ferrous  sulphate,  like  all  the  ferrous  salts,  has  a  pale  greenish  colour  hardly  perceptible 
in  solution.  If  it  be  desired  to  preserve  it  without  change — that.is,  so  as  not  to  contain 
ferric  compounds — it  is  necessary  to  keep  it  hermetically  sealed.  This  is  best  done  by. 
expelling  the  air  by  means  of  sulphurous  anhydride  or  ether ,  sulphurous  anhydride, 
6O2,  removes  oxygen  from  ferric  compounds,  which  might  be  formed,  and  is  itself 
changed  into  sulphuric  acid,  and  hence  the  oxidation  of  the  ferrous  compound  does  aot 
take  place  in  its  presence.  Unless  these  precautions  are  taken,  green  vitriol  turnft 
brown,  partly  changing  into  the  ferric  salt.  When  turned  brown,  it  is  not  completely 
Soluble  in  water,  because  during  its  oxidation  a  certain  amount  of  free  insoluble  ferric 
oxide  is  formed:  6FeSO4  +  O3  =  2Fe2(SO4)3+Fe2O3.  In  order  to  cleanse  such  mixed 
green  vitriol  from  the  oxide,  it  is  necessary  to  add  some  sulphuric  acid  and  iron  and  boil 
the  mixture;  the  ferric  salt  is  then  transformed  into  the  ferrous  state:  Fes(S04)3+Fe 
e=3FeSO4. 

Green  vitriol  is  used  for  the  manufacture  of  Nordhausen  sulphuric  acid  (Chapter 
XX.),  for  preparing  ferric  .oxfde,  in  many  dye  works  (for  preparing  the  indigo  vats  and 
reducing  blue  indigo  to  white),  and  in  many  other  processes,  it  is  also  a  very  good 
disinfectant,  and  is  the  cheapest  salt  from  which  other  compounds  of  iron  may  be 
obtained. 

The  other  ferrous  salts  (excepting  the  yellow  prussiate,  which  will  be  mentioned  later 
ore  but  little  used,  and  it  is  therefore  unnecessary  to  dwell  upon  them.  We  will  only 
mention  ferrous  chloride,  which,  in  the  crystalline  state,  has  the  composition 
FeCl2,4H2O.  It  is  easily  prepared  .  for  instance,  by  the  action  of  hydrochloric  acid  on 
iron,  and  in  the  anhydrous  state  by  the  action  of  hydrochloric  acid  gas  on  metallic  iron 
at  a  red  heat.  The  anhydrous  ferrous  chloride  then  volatilises  in  the  form  of  colourless 
cubic  crystals.  Ferrous  oxalate  (or  the  double  potassium  salt)  acts  as  a  powerful 
reducing  agent,  and  is  frequently  employed  in  photography  (as  a  developer). 


IRON,  COBALT,   AND  NICKEL 


337 


'is  not  precipitated  by  alkalis  and  alkali  carbonates,  which  fact  points 
to  the  formation  of  double  ammonium  salts.18  The  ferrous  salts  have 
a  dull  greenish  colour,  and  .form  solutions  also  of  a  pale  green  colour, 
whilst  the  ferric  salts  have  a  brown  or  reddish- brown  colour.  Tho 
ferrous  salts,  being  capable  of  oxidation,  form  very  active  reducing 
agents— for  instance,  under  their  action  gold  chloride,  AuCl3,  deposits 
metallic  gold,  nitric  acid  is  transformed  into  lower  oxides,  and  the 
xrighest  oxides  of  manganese  also  pass  into  the  lower  forms  of  oxidation. 
All  these  reactions  take  place  with  especial  ease  in  the  presence  of  an 
excess  of  acid.  This  depends  on  the  fact  that  the  ferrous  oxide,  FeO 
(or  salt),  acting  as  a  reducing  agent,  turns  into  ferric  oxide,  Fe2O3  (or 
salt),  and  in  the  ferric  state  it  requires  more  acid  for  the  formation 
of  a  normal  salt  than  in  the  ferrous  condition.  Thus  in  the  normal 
ferrous  sulphate,  FeSO4,  there  is  one  equivalent  of  iron  to  one 
equivalent  of  sulphur  (in  the  sulphuric  radicle),  but  in  the  neutral 
;  ferric  salt,  Fe2(SO4)3,  there  is  one  equivalent  of  iron  to  one  and  'a 
half  of  sulphur  in  the  form  of  the  elements  of  sulphuric  acid.19 

The  most  simple  oxidising  agent  for  transforming  ferrous  into  ferric 
salts  is  chlorine  in  the  presence  of  water— for  instance,  2Fe012  -f  012 

18  Ferrous  sulphate,  like  magnesium  sulphate,  easily  forms  double  salts — for  instance, 
(NHJjSO^FeSO^elLjO.    This  salt  does  not  oxidise  in  air  so  readily  as  green  vitriol,  and 
is  therefore  used  for  standardising  K'MnO^ 

19  The  transformation  of  ferrous  oxide  into  ferric  oxide  is  not  completely  effected  in 
air,  as  then  only  a  part  of  the  suboxide  is  converted  into  ferric  oxide.    Under  these 
circumstances  the  so-called  magnetic  oxide  of  iron  is  generally  produced,  which  contains 
atomic  quantities  of  the  suboxide  and  oxide — namely,  FeO.FeaOs  =  FesO^     This  sub- 
stance, as  already  mentioned,  is  found  in  nature  and  in  iron  scale.    It  is  also  formed 
when  most  ferrous  and  ferric  salts  are  heated  in  air ;  thus,  for  instance,  when  ferrous 
carbonate,  FeCO3  (native  or  the  precipitate  given  by  soda  in  a  solution  of  FX2),  is 
heated  it  loses  the  elements  of  carbonic  anhydride,  and  magnetic  oxide  remains.    This 
oxide  of  iron  is  attracted  by  the  magnet,  and  is  on  this  -account  called  magnetic  oxide, 
although  it  does  not  always  show  magnetic  properties.    If  magnetic  oxide  be  dissolved  in 
any  acid — for  instance,  hydrochloric — which  does  not  act  as  an  oxidising  agent,  a  ferrous 
salt  is  first  formed  and  ferric  oxide  remains,  which  is  also  capable  of  passing  into 
solution.    The  best  way  of  preparing  the  hydrate  of  the  magnetic  oxide  is  by  decomposing 
a  mixture  of  ferrous  and  ferric  salts  with  ammonia ;  it  is,  however,  indispensable  to  pour 
this  mixture  into  the  ammonia,  and  not  vice  versa,  as  in  that  case  the  ferrous  oxide  would 
at  first  be  precipitated  alone,  and  then  the  ferric  oxide.    The  compound  thus  formed  has  a 
bright  green  colour,  and  when  dried  forms  a  black  powder.    Other  combinations  of 
ferrous  with  ferric  oxide  are  known,  as  are  also  compounds  of  ferric  oxide  with  other 
bases.    Thus,  for  instance,  compounds  are  known  containing  4  molecules  of  ferrous  oxide 
to  1  of  ferric  oxide,  and  also  6  of  ferrous  to  1  of  ferric  oxide.    These  are  also  magnetic, 
and  are  formed  by  heating  iron  in   air.      The  magnesium  compound  MgO,Fe203  is 
prepared  by  passing  gaseous  hydrochloric  acid  over  a  heated  mixture  of  magnesia  and 
ferric  oxide.    Crystalline  magnesium  oxide  is  then  formed,  and  black,  shiny,  octahedral 
crystals  of  the  above-mentioned  composition.     This  compound  is  analogous'  to  the 
oluminates— for  instance,  to  spinel.     Bernheim  (1888)  and  Eousseau  (1891)  obtained 
many  similar  compounds  of  ferric  oxide,  and  their  composition  apparently  corresponds 
to  the  hydrates  (Note  22)  known  for  the  oxide. 


838  PRINCIPLES  OF  CHEMISTRY 

=  Fe2Cl6,  or,  generally  speaking,  2FeO  +  Cl,  -f  H2O=Fe2O3  +  2HC) 
When  such  a  transformation  is  required  it  is  best  to  add  potassium 
chlorate  and  hydrochloric  acid  to  the  ferrous  solution  ;  chlorine  is 
formed  by  their  mutual  reaction  and  acts  as  an  oxidising  agent. 
Nitric  acid  produces  a  similar  effect,  although  more  slowly  Ferrous 
salts  may  be  completely  and  rapidly  oxidised  into  ferric  salts  by  means 
Of  chromic  acid  or  permanganic  acid,  HMnO4,  in  the  presence  of  acids — 
for  example,  10FeSO4  +  2KMnO4  +  8H2SO4  =  5Fe2(SO4)3  +  2MnS04 
M-  K2SO4  +  8H20.  This  reaction  is  easily  observed  by  the  change  of 
colour,  and  its  termination  is  easily  seen,  because  potassium  perman- 
ganate forms  solutions  of  a  bright  red  colour,  and  when  added  to  a 
ablution  of  a  ferrous  salt  the  above  reaction  immediately  takes  place  in 
the  presence  of  arid,  and  the  solution  then  becomes  colourless,  because  all 
'the  substances  formed  are  only  faintly  coloured  in  solution.  Directly  all 
the  ferrous  compound  lias  passed  into  the  ferric  state,  any  excess  of 
permanganate  which  is  added  communicates  a  red  colour  to  the  liquid 
(see  Chapter  XXI.) 

Thus  when  ferrous  salts  are  acted  on  by  oxidising  agents,  they  pass 
into  the  ferric  form,  and  under  the  action  of  reducing  agents  the 
reverse  reaction  occurs.  Sulphuretted  hydrogen  may,  for  instance,  be 
used  for  this  complete  transformation,  for  under  its  influence  ferric  salts 
are  reduced  with  separation  of  sulphur— for  example,  Fe2Cl6  +  H2S 
=  2FeCl2  +  2HC1  +  S.  Sodium  thionulphate  acts  in  a  similar  way  : 
Fe2Cl6  +  Na2S2O3  +  H2O  =  2FeCl2  -f  Na2SO4  +  2HC1  +  S,  Me- 
tallic  iron  or  zinc,20  in  the  presence  of  acids,  or  sodium  amalgam,  <fec., 
acts  like  hydrogen,  and  has  also  a  similar  reducing  action,  and  this 
furnishes  the  best  method  for  reducing  ferric  salts  to  ferrous  salts— 
for  instance,  Fe2Cl6  +  Zn  =  2FeCl8  +  ZnCl2.  Thus  the  transition  from 
ferrous  salts  to  ferric  satis  and  vice  versd  is  always  possible.*1 

20  Copper  and  cuprous  salts  also  reduce  ferric  oxide  to  ferrous  oxide,  and  are  them* 
selves  turned  into  cupric  salts.  The  essence  of  the  reactions  is  expressed  by  the  following 
equations .  Fe2O,  +  Cu..,O+  2FeO  +  2CuO ;  Fe2O3  •+•  Cu = 2FeO  +  CuO.  This  fact  is  made 
use  of  in  analysing  copper  compounds,  the  quantity  of  copper  being  ascertained  by  the 
amount  of  ferrous  salt  obtained.  An  excess  of  ferric  salt  is  required  to  complete  the 
reaction.  Here  we  have  an  example  of  reverse  reaction ;  the  ferrous  oxide  or  its  salt  in 
the  presence  of  alkali  transforms  the  cupric  oxide  into  cuprous  oxide  and  metallic  copper, 
aa  observed  by  Lovel,  Knopp,  and  others. 

Jl  We  will  here  mention  the  reactions  by  means  of  which  it  may  be  ascertained 
whether  the  ferrous  compound  has  been  entirely  converted  into  a  ferric  compound  or 
vice  versd.  There  are  two  substances  which  are  best  employed  for  this  purpose: 
potassium  ferricyanide.  FeK5C6N6)  and  potassium  thiocyanate,  KCNS.  The  first  salt 
gives  with  ferrous  salts  a  blue  precipitate  of  an  insoluble  salt,  having  a  composition 
Fe5Cj2Nl9 ,  but  with  ferric  salts  it  does  not  form  any  precipitate,  and  only  gives  a  brown 
colour,  and  therefore  when  transforming  a  ferrous  salt  into  a  ferric  salt,  the  completion 
of  the  transformation  may  be  detected  by  taking  a  drop  of  the  liquid  on  paper  or  on  a 
porcelain  plate  and  adding  a  drop  of  the  ferricyanide  solution.  If  a  blue  precipitate  be 


IRON,  COBALT,  AND  NICKEL  839 

Ferric  oxide,  or  sesquioxide  of  iron,  Fe203,  is  found  in  nature, 
and  is  artificially  prepared  in  the  form  of  a  red' powder  by  many 
methods.  Thus  after  heating  green  vitriol  a  red  oxide  of  iron  remains, 
called  colcothar,  which  is  used  as  an  oil  paint,  principally  for  painting 
wood.  The  same  substance  in  the  form  of  a  very  tine  powder  (rouge) 
is  used  for  polishing  glass,  steel,  and  other  objects.  If  a  mixture  of 
ferrous  sulphate  with  an  excess  of  common  salt  be  strongly  heated, 
crystalline  ferric  oxide  will  be  formed,  having  a  dark  violet  colour,  and 
resembling  some  natural  varieties  of  this  substance.  When  iron  pyrites 
is  heated  for  preparing  sulphurous  anhydride,  ferric  oxide  also  remains 
behind  ;  it  is  used  as  a  pigment.  On  the  addition  of  alkalis  to  a 
solution  of  ferric  salts,  a  brown  precipitate  of  ferric  hydroxide  is  formed, 
•which  when  heated  (even  when  boiled  in  water,  that  is,  at  about  100°, 
according  to  Tomassi)  easily  parts  with  the  water,  and  leaves  red 
anhydrous  ferric  oxide.  Pure  ferric  oxide  does  not  show  any  mag- 
netic properties,  but  when  heated  to  a  white  heat  it  loses  oxygen  and  is 
converted  into  the  magnetic  oxide.  Anhydrous  ferric  oxide  which  has 
been  heated  to  a  high  temperature  is  with  difficulty  soluble  in  acids 
(but  it  is  soluble  when  heated  in  strong  acids,  and  also  when  fused  with 
potassium  hydrogen  sulphate),  whilst  ferric  hydroxide,  at  all  events 
that  which  is  precipitated  from  salts  by  means  of  alkalis,  is  very  readily 
soluble  in  acids.  The  precipitated  ferric  hydroxide  has  the  composition 
2Fe2033H2O,  or  Fo4H609.  If  this  ordinary  hydroxide  be  rendered 
anhydrous  (at  100°),  at  a  certain  moment  it  becomes  incandescent 
—that  is,  loses  a  certain  quantity  of  heat.  This  self-incandescence 
depends  on  internal  displacement  produced  by  the  transition  of  the 
easily- soluble  (in  acids)  variety  into  the  difficultly-soluble  variety, 
and  does  not  depend  on  the  loss  of  water,  since  the  anhydrous  oxide 
.undergoes  the  same  change.  In  addition  to  this  there  exists  a  ferric 
hydroxide,  or  hydrated  oxide  of  iron,  which,  like  the  strongly-heated 
anhydrous  iron  oxide,  is  difficultly  soluble  in  acids.  This  hydroxide  on 
losing  water,  or  after  the  loss  of  water,  does  not  undergo  such  self- 
incandescence,  because  no  such  state  of  internal  displacement  occurs 
(loss  of  energy  or  heat)  with  it  as  that  which  is  peculiar  to  the  ordinary 
oxide  of  iron.  The  ferric  hydroxide  which  is  difficultly  soluble  in  acids 
has  the  composition  Fe203,H2O.  This  hydroxide  is  obtained  by  a  pro-, 

formed,  then  part  of  the  ferrous  salt  still  remains ;  if  there  is  none,  the  transformation  is 
Complete.  The  thiocyanate  does  not  give  any  marked  coloration  with  ferrous  salts  ;  but 
.with  ferric  salts  in  the  most  diluted  state  it  forms  a  bright  red  soluble  compound,  and 
therefore  when  transforming  a  ferric  salt  into  a  ferrous  salt  we  must  proceed  as  before, 
testing  a  drop  of  the  solution  with  thiocyanate,  when  the  absence  of  a  red  colour  will 
prove  the  total  transformation  of  the  ferric  salt  into  the  ferrous  state,  and  if  a  red  colour 
{9  apparent  it  shows  that,  the  transformation  ia  not  yet  complete. 


840  PRINCIPLES  OF  CHEMISTRY' 

longed  ebullition  of  water  in  which  fert-ic  hydroxide  prepared  by  tbe 
oxidation  of  ferrous  oxide  is  suspended,  and  also  sometimes  by  similar 
treatment  of  the  ordinary  hydroxide  after  it  has  been  for  a  long  time 
in  contact  with  water.  The  transition  of  one  hydroxide  to  another  is 
apparent  by  a  change  of  colour  ;  the  easily-soluble  hydroxide  is  redder, 
and  the  sparingly-soluble  hydroxide  more  yellow  in  colour.22 

The  normal  salts  of  the  composition  Fe2X6  or  FeX3  correspond 
with  ferric  oxide — for  example,  the  exceedingly  volatile  ferric  chloride, 
Fe2Cl6,  which  is  easily  prepared  in  the  anhydrous  state  by  the  action 
of  chlorine  on  heated  iron.23  Such  also  is  the  normal  ferric  nitrate, 

22  The  two  ferric  hydroxides  are  not  only  characterised  by  the  above-mentioned 
properties,  but  also  by  the  fact  that  the  first  hydroxide  forms  immediately  with  potassium 
ferrocyanide,  K4FeCeNti,  a  blue  colour  depending  on  the  formation -of  Prussian  blue, 
whilst  the  second  hydroxide  does  not  give  any  reaction  whatever  with  this  salt.  The 
first  hydroxide  is  entirely  soluble  in  nitric, hydrochloric,  and  all  other  acids;  whilst  the 
second  sometimes  (not  always)  forms  a  brick-coloured  liquid,  which  appears  turbid 
and  does  not  give  the  reactions  peculiar^  to  the  ferric  salts  (Pe"an  de  Saint-GUles, 
Scheurer-Kestner).  In  addition  to  this,  when  the  smallest  quantity  of  an  alkaline  salt 
is  added  to  this  liquid,  ferric  oxide  is  precipitated.  Thus  a  colloidal  solution  is  formed 
(hydrosol),  which  is  exactly  similar  to  silica  hydrosol  (Chapter  XVII.),  according  to 
which  example  the  hydrosol  of  ferric  oxide  may  be  obtained. 

If  ordinary  ferric  hydroxide  be  dissolved  in  acetic  acid,  a  solution  of  the  colour  of  red 
wine  is  obtained,  which  has  all  the  reactions  characteristic  of  ferric  salts.  But  if  this 
solution  (formed  in  the  cold)  be  heated  to  the  boiling-point,  its  colour  is  very  rapidly 
intensified,  a  smell  of  acetic  acid  becomes  apparent,  and  the  solution  then  contains  a 
new  variety  of  ferric  oxide.  If  the  boiling  of  the  solution  be  continued,  acetic  acid  is 
evolved,  and  the  modified  ferric  oxide  is  precipitated.  If  the  evaporation  of  the  acetic 
acid  be  prevented  (in  a  closed  or  sealed  vessel),  and  the  liquid  be  heated  for  some  time, 
the  whole  of  the  ferric  hydroxide  then  passes  into  the  insoluble  form,  and  if  some  alkaline 
salt  be  added  (to  the  hydrosol  formed),  the  whole  of  the  ferric  oxide  is  then  precipitated, 
in  its  insoluble  form.  This  method  may  be  applied  for  separating  ferric  oxide  from 
solutions  of  its  salts. 

All  phenomena  observed  respecting  ferric  oxide  (colloidal  properties,  various  forms, 
formation  of  double  basic  salts)  demonstrate  that  this  substance,  like  silica,  alumina, 
lead  hydroxide,  &c.,  is  polymerised,  that  the  composition  is  represented  by  (Fe^Qj),,. 

*»  The  ferric  compound  which  is  most  used  in  practice  (for  instance,  in  medicine,  for 
cauterising,  stopping  bleeding,  &c. — Oleum  Martis)  is  ferric  chloride,  Fe^Cle,  easily 
obtained  by  dissolving  the  ordinary  hydrated  oxide  of  iron  in  hydrochloric  acid.  It"  is 
obtained  in  the  anhydrous  state  by  the  action  of  chlorine  on  heated  iron.  The  experi- 
ment is  carried  on  in  a  porcelain  tube,  and  a  solid  volatile  substance  is  then  formed  in 
the  shape  of  brilliant  violet  scales  which  very  readily  absorb  moisture  from  the  air,  and 
when  heated  with  water  decompose  into  crystalline  ferric  oxide  and  hydrochloric  acid  : 
Fe2Cl6  +  3H2O  =  6HCl  +  Fe2O.-,.  Ferric  chloride  is  so  volatile  that  the  density  of  its 
vapour  may  be  determined.  At  440°  it  .is  equal  to  1G4'0  referred  to  hydrogen  ;  the 
formula  Fe.jClc  corresponds  with  a  density  of  162'5.  An  aqueous  solution  of  this  salt 
has  a  brown  colour.  On  evaporating  and  cooling  this  solution,  crystals  separate  con- 
taining 6  or  12  molecules  of  H.,O.  Ferric  chloride  is  not  only  soluble  in  water,  but  also 
in  alcohol  (similarly  to  magnesium  chloride,  &c.)  and  in  ether.  If  the  latter  solutions 
are  exposed  to  the  rays  of  the  sun  they  become  colourless,  and  deposit  ferrous  chloride, 
Fed,,  chlorine  being  disengaged.  After  a  certain  lapse  of  time,  the  aqueous  solutions 
of  ferric  chloride  decompose  with  precipitation  of  a  basic  salt,  thus  demonstrating  the 
instability  of  ferric  chloride,  like  the  other  salts  of  ferric  oxide  (Note  22).  This  salt  is 


IRON,   COBALT,  AND  NICKEL 


341 


;  it  is  obtained  by  dissolving  iron  in  an  excess  of  nitric  acidL 

much  more  stable  in  the  form  of  double  salts,  like  all  tlie  ferric  salts  and  also  the 
salts  of  many  other  feeble  bases.  Potassium  or  ammonium  chloride  forms  with  it  very 
beautiful  red  crystals  of  a  double  salt,  having  the  composition  Fe3Cl6,4KCl,2H2O. 
When  a  solution  of  this  salt  is  evaporated  it  decomposes,  with  separation  of  potassium 
chloride. 

B.  Boozeboom  (1892)  studied  in  detail  (as  for  CaCl2,  Chapter  XIV.,  Note  50)  the 
eeparation  of  different  hydrates  from  saturated  solutions  of  Fe2ClG  at  various  concen- 
trations and  temperatures ;  he  found  that  there  are  4  crystallohydrates  with  12,  7,  5,  and 
4  molecules  of  water.  An  orange  yellow  only  slightly  hygroscopic  hydrate,  Fe2Cl6,12H20, 
is  most  easily  and  usually  obtained,  which  melts  at  87° ;  its  solubility  at  different  tempera- 
tures is  represented  by  the  curve  BCD  in  the  accompanying  figure,  where  the  point  B 


-so' 


0'  50' 

FIG.  95.— Diagram  of  the  solubility  of  Fe2CU 


100' 


Corresponds  to  the  formation,  at  —  55°,  of  a  cryohydrate  containing  about  Fe2Cl«  +  36H2O, 
the  point  C  corresponds  to  the  melting-point  (  +  3T*)  of  the  hydrate  Fe2Cl6,12H2O,  and 
the  curve  CD  to  the  fall  in  the  temperature  of  crystallisation  with  an  increase  in  the 
amount  of  salt,  or  decrease  in  the  amount  of  water  (in  the  figure  the  temperatures  are 
taken  along  the  axis  of  abscissae,  and  the  amount  of  n  in  the  formula  nEe.^Clg  -i- 100 H  O 
along  the  axis  of  ordinates).  When  anhydrous  FeaCLj  is  added  to  the  above  hydrate 
(12H20),  or  some  of  the  water  is  evaporated  from  the  latter,  very  hygroscopic 
crystals  of  Fe,Cl6,5H2O  (Fritsche)  are  formed  ;  they  melt  at  56°,  their  solubility  is 
expressed  by  the  curve  HJ,  which  also  presents  a  small  branch  at  the  end  J  This 
again  gives  the  fall  in  the  temperature  of  crystallisation  with  an  increase  in  the  amount 
of  Fe2Cl6.  Besides  these  curves  and  the  solubility  of  the  anhydrous  salt  expressed  by 
the  lino  KL  (up  to  100°,  beyond  which  chlorine  is  liberated),  Roozeboom  also  gives  the 
two  curves,  EFG  and  JK,  corresponding  to  the  crystallohydrates,  Fe2Cl6,7H2O  (melts  at 
+  820>5,  that  is  lower  than  any  of  the  others)  and  FejCle.4H.jO  (melts  at  73°P5),  which 
be  discovered  by  a  systematic  research  on  the  solutions  of  ferric  chloride.  The  curve 
AB  represents  the  separation  of  ice  from  dilute  solutions  of  the  salt. 

The  researches  of  the  same  Dutch  chemist  upon  the  conditions  of  the  formation  of 
Crystals  from  the  double  salt  (NH4Cl)4FeoClcJ2H2O  are  even  more  perfect.  This  salt 
was  obtained  in  1839  by  Fritsche,  and  is  easily  formed  from  a  strong  solution  of  Fe2Cl« 
by  adding  sal-ammoniac,  when  it  separates  in  crimson  rhombic  crystals,  which,  after 
dissolving  in  water,  only  deposit  again  on  evaporation,  together  with  the  sal-ammoniac. 

Boozeboom  (1892)  found  that  when  the  solution  contains  b  molecules  of  FewClc,  and 


842 


PRINCIPLES  OF  CHEMISTRY 


taking  care  as  far  as  possible  to  prevent  any  rise  of  temperature.11  The 
normal  salt  separates  from  the  brown  solution  when  it  is  concentrated 

a  molecules  of  NH4C1,  per  100  molecules  H2O,  then  at  15°  one  of  the  following  separa- 
tions takes  place :  (1)  crystals,  Fe2Cl6,12H2O,  when  a  varies  between  0  and  11,  and  & 
between  4*65  and  4*8,  or  (2)  a  mixture  of  these  crystals  and  the  double  salt,  when  a 
^1-86,  and  &  =  4'47,  or  (8)  the  double  salt,  Fe^Cl^NI^Cl^HjO,  when  a  varies 
between  2  and  ll'S,  and  6  between  8'1  and  4'56,  or  (4)  a  mixture  of  sal-ammoniac  with 
the  iron  salt  (it  crystallises  in  separate  cubes,  Retgers,  Lehmann),  when  a  varies 
between  7'7  and  10'9,  and  6  is  less  than  8'88,  or  (5)  sal-ammoniac,  when  a=ll'88.  And 
as  in  the  double  salt,  a  '  &"4  : 1  it  is  evident  that  the  double  salt  only  separates  out 
when  the  ratio  a-b  is  less  than  4  . 1  (i.e.  when  F^Cle  predominates).  The  above  is 

seen  more  clearly  in  the  accom- 
panying figure,  where  a,  or  the 
number  of  molecules  of  NH4C1 
per  100H2O,  is  taken  along  the 
axis  of  abscisses,  and  b,  or  the 
number  of  molecules  of  Fe2Cl6, 
along  the  ordinates.  The  curves 
ABCD  correspond  to  saturation 
and  present  an  iso-therm  of  16°. 
The  portion  AB  corresponds  to 
the  separation  of  chloride  of  iron 
(the  ascending  nature  of  this 
curve  shows  that  the  solubility  of 
Fe2Cle  is  increased  by  the  pre- 
sence of  NH4C1,  while  that,  of 
NH4C1  decreases  in  the  presence 
of  FejCle),  the  portion  EC  to  the 
double  salt,  and  the  portion  CD 
to  a  mixture  of  sal-ammoniac-  and 
ferric  chloride,  while  the  straight 

line  OF  corresponds  to  the  ratio  FeaCl6,4NH4Cl,  or  a  :  6-4  : 1.  The  portion  CE  shows 
that  more  double  salt  may  be  introduced  into  the  solution  without  decomposition,  but- 
then  the  solution  deposits  a  mixture  of  sal-ammoniac  and  ferric  chloride  (see  Chapter 
XXIV.  Note  Obta).  If  there  were  more  such  well-investigated  cases  of  solutions,  our 
knowledge  of  double  salts,  solutions,  the  influence  of  water,  equilibria,  isomorphous 
mixtures,  and  such-like  provinces  of  chemical  relations  might  be  considerably  advanced. 
M  The  normal  ferric  salts  are  decomposed  by  heat  and  even  by  water,  forming  basic 
salts,  which  may  be  prepared  in  various  ways.  Generally  ferric  hydroxide  is  dissolved 
in  solutions  of  ferric  nitrate ;  if  it  contains  a  double  quantity  of  iron  the  basic  salt  is 
formed  which  contains  Fe2O3  (in  the  form  of  hydroxide)  +2Fe2(NO3)6=8Fe2O(NO3)4, 
ft  salt  of  the  type  Fe2OX4.  Probably  water  enters  into  its  composition.  With  con- 
siderable quantities  of  ferric  oxide,  insoluble  basic  salts  are  obtained  containing  various 
amounts  of  ferric  hydroxide.  Thus  when  a  solution  of  the  above-mentioned  basic  acid 
is  boiled,  a  precipitate  is  formed  containing  4(Fe2O5)8,2(N2O5),8H2O,  which  probably 
contains  2Fe2Os(NOz)2-f  2FesO3,3H2O  If  a  solution  of  basic  nitrate  be  sealed  in  a 
tube  and  then  immersed  in  boiling  water,  the  colour  of  the  solution  changes  just  in 
the  same  way  as  if  a  solution  of  ferric  acetate  had  been  employed  (Note  22).  The 
solution  obtained  smells  strongly  of  nitric  acid,  and  on  adding  a  drop  of  sulphuric  or 
hydrochloric  acid  the  insoluble  variety  of  hydrated  ferric  oxide  is  precipitated. 

Normal  ferric  orthophosphaf  is  soluble  in  sulphuric,  hydrochloric,  and  nitric  acids, 
but  insoluble  in  others,  such  as,  for  instance,  acetic  acid.  The  composition  of  this  salt 
in  the  anhydrous  state  is  FePO4,  because  in  orthophosphoric  acid  there  are  three  atoms 
of  hydrogen,  and  iron,  in  the  ferric  state,  replaces  the  three  atoms  of  hydrogen.  This 
.Bait  is  obtained  from  ferric  acetate,  which,  with  disodium  phosphate,  forms  a  white  pre- 


J    4     5    6     7     8    &    10    II    12 


FlO.  86.— Diagram  of  the  formation,  at  15°,  of  the  double 
salt  FeaC1.4NH4C12HaO  or  Fe(NH4)3Cl.H,0,  (After 
Roozeboom.) 


IRON,   COBALT,  AND  NICKEL 


343 


ler  a  bell  jar  over  sulphuric  acid.  This  salt,  Fe2(N03)6,9H20,  then 
crystallises  in  well-formed  and  perfectly  colourless  crystals,25  which 
deliquesce  in  air,  melt  at  35°,  and  are  soluble  in  and  decomposed  by 
water.  The  decomposition  may  be  seen  from  the  fact  that  the  solution 
is  brown  and  does  not  yield  the  whole  of  the  salt  again,  but  gives 
partly  basic  salt.  The  normal  salt  (only  stable  in  the  presence  of  an 
excess  of  HNO3)  is  completely  decomposed  with  great  facility  by  heat- 
ing with  water,  even  at  130°,  and  this  is  made  use  of  for  removing  iron 
(and  also  certain  other  oxides  of  the  form  R2O3)  from  many  other 
bases  (oi  the  form  RO)  whose  nitrates  are  far  more  stable.  The  ferric 
salts,  FeX3,  in  passing  into  ferrous  salts,  act  as  oxidising  agents,  as  is 
seen  from  the  fact  that  they  not  only  liberate  S  from  SH2,  but  also 
iodine  from  KI  like  many  oxidising  agents.25  bis 

cipitute  of  FePO4,  containing  water.  If  a  solution  of  ferric  chloride  (yellowish-red 
colour)  be  mixed  with  a  solution  of  sodium  acetate  in  excess,  the  liquid  assumes  an 
intense  brown  colour  which  demonstrates  the  formation  of  a  certain  quantity  of  ferric 
acetate ;  then  the  disodium  phosphate  directly  forms  a  white  gelatinous  precipitate  of  ferrio 
phosphate.  By  this  means  the  whole  of  the  iron  may  be  precipitated,  and  the  liquid  which 
was  brown  then  becomes  colourless.  If  this  normal  salt  be  dissolved  in  orthophosphorio 
acid,  the  crystalline  acid  salt  FeH3(PO4).2  is  formed.  If  there  be  an  excess  of  ferric  oxide 
in  the  solution,  the  precipitate  will  consist  of  the  basic  salt.  If  ferric  phosphate  be 
dissolved  in  hydrochloric  acid,  and  ammonia  be  added,  a  salt  is  precipitated  on  heating 
which,  after  continued  washing  in  water  and  heating  (to  remove  the  water),  has  the 
composition  Fe4P2Ou — that  is,  aFe^Oj^Oj.  In  an  aqueous  condition  this  salt  may  be 
considered  as  ferric  hydroxide,  Eea(OH)6,  in  which  (OH)3  is  replaced  by  the  equivalent 
group  P04.  Whenever  ammonia  is  added  to  a  solution  containing  an  excess  of  ferric 
salt  and  a  certain  amount  of  phosphoric  acid,  a  precipitate  is  formed  containing  the 
whole  of  the  phosphoric  acid  in  the  mass  of  the  ferric  oxide". 

Ferric  oxide  is  characterised  as  a  feeble  base,  and  also  by  the  fact  of  its  forming  double 
salts — for  instance,  potassium  iron  alum,  which  has  a  composition  Few(SO4)5,K2SO4, 
2411,0  or  FeK(SO4)2,12H2O.  It  is  obtained  in  the  form  of  almost  colourless  or  light 
rose-coloured  large  octahedra  of  the  regular  system  by  simply  mixing  solutions  of 
potassium  sulphate  and  the  ferric  sulphate  obtained  by  dissolving  ferric  oxide  in  sul- 
phuric acid. 

25  It  would  seem  that  all  normal  ferric  salts  are  colourless,  and  that  the  brown  colour 
which  is  peculiar  to  the  solutions  is  really  due  to  basic  ferric  salts.  "A  remarkable 
example  of  the  apparent  change  of  colour  of  salts  is  represented  by  the  ferrous  and  ferric 
oxalates.  The  former  in  a  dry  state  has  a  yellow  colour,  although  as  a  rule  the  ferrous 
salts  are  green,  and  the  latter  is  colourless  or  pale  green.  When  the  normal  ferric  salt  is 
dissolved  in  water  it  is,  like  many  salts,  probably  decomposed  by  the  water  into  acid 
and  basic  salts,  and  the  latter  communicates  a  brown  colour  to  the  solution.  Iron  alum 
is  almost  colourless,  is  easily  decomposed  by  water,  and  is  the  best  proof  of  our  asser- 
tion. The  study  of  the  phenomena  peculiar  to  ferric  nitrate  might,  in  ray  opinion,  give 
a  very  useful  addition  to  our  knowledge  of  the  aqueous  solutions  of  salts  in  general. 

25  bis  The  reaction  FeX5  +  KI  =  FeXa  -f  KX  + 1  proceeds  comparatively  slowly  in  solu- 
tions, is  not  complete  (depends  upon  the  mass),  and  is  reversible.  In  this  connection  we 
may  cite  the  following  data  from  Seubert  and  Rohrer's  (1894)  comprehensive  researches. 
The  investigations  were  conducted  with  solutions  containing  T\j  gram — equivalent 
weights  of  Fe.j(SO4)3  (i.e.'  containing  20  grams  of  salt  per  litre),  and  a  corresponding 
solution  of  KI ;  the  amount  of  iodine  liberated  being  determined  (after  the  addition  of 
Btarch)  by  a  solution  (also  T\j  normal)  of  Na^Os  (see  Chapter  XX.,  Note  42).  The  pro- 

*D 


344  PRINCIPLES  OF  CHEMISTRY 

Iron  forms  one  other  oxide  besides  the  ferric  and  ferrous  oxides  ; 
this  contains  twice  as  much  oxygen  as  the  former,  but  is  so  very 
unstable  that  it  can  neither  be  obtained  in  the  free  state  nor  as  a 
hydrate.  Whenever  sjuch  conditions  of  double  decomposition  occur  as 
should  allow  of  its  separation  in  the  free  state,  it  decomposes  into 
oxygen  and  ferric  oxide.  It  is  known  in  the  state  of  salts,  and  is  only 
stable  in  the  presence  of  alkalis,  and  forms  salts  with  them  which  have 
a  decidedly  alkaline  reaction  ;  it  is  therefore  a  feebly  acid  oxide.  Thus 
|  when  small  pieces  of  iron  are  heated  with  nitre  or  potassium  chlorate 
a  potassium  salt  of  the  composition  K2Fe04  is  formed,  and  therefore 
the  hydrate  corresponding  with  this  salt  should  have  the  composition 
H2FeO4.  It  is  called  ferric  acid.  Its  anhydride  ought  to  contain 
Fe03  or  Fe2O0— twice  as  much  oxygen  as  ferric  oxide.  If  .a  solution 
of  potassium  ferrate  be  mixed  with  acid,  the  free  hydrate  ought 
to  be  formed,  but  it  immediately  decomposes  (2K2FeO4  +  5HaS04 
=  2K2SO4  +  Fe2(SO4)3  +  5H2O  +  O3),  oxygen  being  evolved.  If  a 
small  quantity  of  acid  be  taken,  or  if  a  solution  of  potassium  ferrate 
be  heated  with  solutions  of  other  metallic  salts,  ferric  oxide  is  sepa- 
rated— for  instance  : 

2CuSO4  +  2K2FeO4  =  2K2S04  +  03  +  Fe?03  +  2CuO. 

Both  these  oxides  are  of  course  deposited  in  the  form  of  hydrates. 
This  shows  that  not  only  the  hydrate  H2FeO4,  but  also  the  suits  of  the 
heavy  metals  corresponding  with  this  higher  oxide  of  iron,  are  not 
formed  by  reactions  of  double  decomposition.  The  solution  of  potas- 
sium ferrate  naturally  acts  as  a  powerful  oxidising  agent ;  for  instance, 
it  transforms  manganous  oxide  into  the  dioxide,  sulphurous  into 
sulphuric  acid,  oxalic  acid  into  carbonic  anhydride  and  water,  <fec.2fi 
Iron  thus  combines  with  oxygen  in  three  proportions  :  RO,  R203, 

grass  of  the  reaction  was  expressed  by  the  amount  of  liberated  iodine  in  percentages 
of  the  theoretical  amount.  For  instance,  the  following  amount  of  iodide  of  potassium 
was  decomposed  when  Fej(S04)3  +  2HKI  was  taken : 

n-                1                 28                0               10  20 

After  16'               11 '4            26'8           40'6           78'5           91'6  96'0 

„      80'                14-0            85'8            47-8            78'5            94'3  97'4 

„       Ihour        19-0            42-7            66-0            84'0            95'7  97'6 

„      10    „           82-6            56'0            75-7            98'2            96'5  97'6 

„      48.  „           89-4            07'7            82-6            98'4            96'6  97'6 

Similar  results  were  obtained  for  FeCl5,  but  then  the  amount  of  iodine  liberated  was 
somewhat  greater.  Similar  results  were  also  obtained  by  increasing  the  mass  of  FeXj 
per  KI,  and  by  replacing  it  by  HI  (see  Chapter  XXL,  Note  26). 

i  M  If  chlorine  be  passed  through  a  strong  solution  of  potassium  hydroxulo  in  which 
hydrated  ferric  oxide  is  suspended,  the  turbid  liquid  acquires  a  dark  pomegranato-rcd 
colour  and  contains  potassium  ferrate :  10KHO  +  Fe3O3  +  8C13  -  2KaFeO4  +  6KC1  +  5H,O. 
The  chlorine  must  not  be  in  excess,  otherwise  the  salt  is  again  decomposed,  although  tho 
mode  of  decomposition  is  unkhown  ;  however,  ferric  chloride  and  potassium  chlora  .e 
are  probably  formed.  Another  way  in  which  the  above- described  salt  is  formed  is  also 


IRON,   COBALT,  AND  NICKEL  845 

and  RO3.  It  might  have  been  expected  that  there  would  be  inter- 
mediate stages  RO2  (corresponding  to  pyrites  FeS2)  and  R2O5,  but  for 
iron  these  are  unknown.20  bis  The  lower  oxide  has  a  distinctly  basic 
character,  the  higher  is  feebly  acid.  The  only  one  which  is  stable,  in 
the  free  state  is  ferric  oxide,  Fe2O3  ;  the  suboxide,  FeO,  absorbs 
oxygen,  and  ferric  anhydride,  FeO3,  evolves  it.  It  is  also  the  same 
for  other  elements  ;  the  character  of  each  is  determined  by  the  relative 
degree  of  stability  of  the  known  oxides.  The  salts  FeX2  correspond 
with  the  suboxide,  the  salts  FeX3  or  Fe2X6  with  the  sesquioxide,  and 
Fe\X6  represents  those  of  ferric  acid,  as  its  potassium  salt  is  FeO2(OK).,, 
corresponding  with  K2SO4,  K2MnO4,  K2CrO4,  *fec.  Iron  therefore 
forms  compounds  of  the  types  FeX2,  FeX3,  and  FeXG,  but  this  latter, 
like  the  type  NX.,,  does  not  appear  separately,  but  only  when  X  re- 
presents heterogeneous  elements  or  groups  ;  for  instance,  for  nitrogen 
in  the  form  of  NO2(OH),  NH4C1,  &c.,  for  iron  in  the  form  of 
FeO2(OK)2.  But  still  the  type  FeX6  exists,  and  therefore  FeX2  and 
FeX3are  compounds  which,  like  ammonia,  NH3,  are  capable  of  further 
combinations  up  to  FeX6 ;  this  is  also  seen  in  the  property  of 
ferrous  and  ferric  salts  of  forming  compounds  with  water  of  crystallisa- 
tion, besides  double  and  basic  salts,  whose  stability  is  determined  by 
the  quality  of  the  elements  included  in  the  types  FeX2  and  FeX3.26  trt 
It  is  therefore  to  be  expected  that  there  should  be  complex  compounds 

remarkable;  a  galvanic  current  (from  6  Grove  elements)  is  passed  through  cast  iron 
and  platinum  electrodes  into  -a  strong  solution  of  potassium  hydroxide.  The  cast- 
iron  electrode  is  connected  with  the  positive  pole,  and  the  platinum  electrode  is  sur- 
rounded by  a  porous  earthenware  cylinder.  Oxygen  would  bo  evolved  at  the  cast- 
iron  electrode,  but  it  is  used  up  in  oxidation,  and  a  dark  solution  of  potassium  ferrate  is 
therefore  formed  about  it.  It  is  remarkable  that  the  cast  iron  cannot  be  replaced  by 
wrought  iron. 

»6bu  When  Moiul  and  his  assistants  obtained  the  remarkable  volatile  compound 
Ni(CO).,  (described  later,  Chapter  XXII.),  it  was  shown  subsequently  by  Mond  and 
Quincke  (1891),  and  also  by  Berthelot,  that  iron,  under  certain  conditions,  in  a  stream  of 
carbonic  oxide,  also  volatilises  and  forms  a  compound  like  that  given  by  nickel.  Roscoe 
and  Scudder  then  showed  that  when  water  gas  is  passed  through  and  kept  under 
pressure  (8  atmospheres)  in  iron  vessels  a  portion  of  the  iron  volatilises  from  the 
sides  of  the  vessel,  and  that  when  the  gas  is  burnt  it  deposits  a  certain  amount  of  oxides 
of  iron  (the  same  result  is  obtained  with  ordinary  coal  gas  which  contains  a  small  amount 
of  CO).  To  obtain  the  volatile  compound  of  iron  with  carbonic  oxide,  Mond  prepared 
a  finely  divided  iron  by  heating  the  oxalate  in  a  stream  of  hydrogen,  and  after  cooling  it 
to  80° — 45°  he  passed  CO  over  the  powder.  The  iron  then  formed  (although  very  slowly) 
a  volatile  compound  containing  Fe(CO)5  (as  though  it  answered  to  a  very  high  type, 
FeXIO),  which  when  cooled  condenses  into  a  liquid  (slightly  coloured,  probably  owing  to 
Incipient  decomposition),  sp.  gr.  T47,  which  solidifies  at  —21°,  boils  at  about  108°,  and 
has  a  vapour  density  (about  6'5  with  respect  to  air)  corresponding  to  the  above  formula; 
it  decomposes  at  180°.  Wator  and  dilute  acids  do  not  act  upon  it,  but  it  decomposes 
under  the  action  of  light  and  forms  a  hard,  non-volatile  crystalline  yellow  compound 
Fea(CO)7  which  decomposes  at  80°  and  again  forms  Fe(CO)5. 

so  trt  When  the  molecular  FeaCl6  is  produced  instead  of  FeCl3  this  complication  of 
the  type  also  occurs. 


846  PRINCIPLES  OF  CHEMISTRY 

derived  from  ferrous  and  ferric  oxides.  Amongst  these  the  series, 
of  cyanogen  compounds  is  particularly  interesting  j  their  formation 
and  character  is  not  only  determined  by  the  property  which  iron 
possesses  of  forming  complex  types,  but  also  by  the  similar  faculty  of 
the  cyanogen  compounds,  -"which,  like  nitriles  (Chapter  IX.),  have 
clearly  developed  properties  of  polymerisation  and  in  general  of  forming 
complex  compounds.27 

In  the  cyanogen  compounds  of  iron,  two  degrees  might  be  expected  : 
Fe(CN)2,  cor  respond  ing 'with  ferrous  oxide,  and  Fe(CN)3,  correspond- 
ing with  ferric  oxide.  There  are  actually,  however,  many  other  known 
compounds,  intermediate  and  far  more  complex.  They  correspond 
with  the  double  salts  so  easily  formed  by  metallic  cyanides.  The  two 
following  double  salts  are  particularly  well  known,  very  stable,  often 
used,  and  easily  prepared.  Potassium  ferrocyanide  or  yellow  prussiate 
of  potash,  a  double  salt  of  cyanide  of  potassium  and  ferrous  cyanide, 
has  the  composition  FeC2N2,4KCN  ;  its  crystals  contain  3  mol.  of  water  : 
K4FeC6N6,3H2O.  The  other  is  potassium  ferricyanide  or  red prussiate 
of  potash.  It  is  also  known  as  Gmelin's  salt,  and  contains  cyanide  of 
potassium  with  ferric  cyanide ;  its  composition  is  Fe(CN)3,3KCN  or 
K3FeC6N6.  Its  crystals  do  not  contain  water.  It  is  obtained  from 
the  first  by  the  action  of  chlorine,  which  removes  one  atom  of  the 
potassium.  A  whole  series  of  other  ferrocyanic  -compounds  correspond 
with  these  ordinary  salts. 

Before  treating  of  the  preparation  and  properties  of  these  two 
remarkable  and  very  stable  salts,  it  must  be  observed  that  with  ordi- 
nary reagents  neither  of  them  gives  the  same  double  decompositions 
as  the  other  ferrous  and  ferric  salts,  and  they  both  present  a  series  of 
remarkable  properties.  Thus  these  salts  have  a  neutral  reaction,  are 
unchanged  by  air,  dilute  acids,  or  water,  unlike  potassium  cyanide  and 
even  some  of  its  double  salts.  When  solutions  of  these  salts  are  treated 
with  caustic  alkalis,  they  do  not  give  a  precipitate  of  ferrous  or  ferric 
hydroxides,  neither  are  they  precipitated  by  sodium  carbonate.  This 
led  the  earlier  investigators  to  recognise  special  independent  groupings 
in  them.  The  yellow  prussiate  was  considered  to  contain  the  complex 
radicle  FeC6N6  combined  with  potassium,  namely  with  K4,  and  K3 
was  attributed  to  the  red  prussiate.  This  was  confirmed  by  the  fact 
that  whilst  in  both  salts  any  other  metal,  even  hydrogen,  might  be 
substituted  for  potassium,  the  iron  remained  unchangeable,  just  as 
nitrogen  in  cyanogen,  ammonium,  and  nitrates  does  not  enter  into 
27  Some  light  may  be  thrown  upon  the  faculty  of  Fe  of  forming  various  compounds  with 
CN,  by  the  fact  that  Fe  not  only  combines  with  carbon  but  also  with  nitrogen.  Nitride 
of  iron  Fe2N  was  obtained  by  Fowler  by  heating  finely  powdered  iron  in  a  stream  of 
NH3  at  the  temperature  of  melting  lead. 


IRON,  COBALT,  AND.  NICKEL  84?' 

double  decomposition,  being  in  the  state  of  the  complex  radicles  CN, 
NH4,  NO2.  Such  a  representation  is,  however,  completely  superfluous 
for  the  explanation  of  the  peculiarities  in  the  reactions  of  such  com- 
pounds as  double  salts.  If  a  magnesium  salt  which  can  be  precipitated 
by  potassium  hydroxide  does  not  form  a  precipitate  in  the  presence  of 
ammonium  chloride,  it  is  very  clear  that  it  is  owing  to  the  formation 
of  a  soluble  double  salt  which  is  not  decomposed  by  alkalis.  And 
there  is  no  necessity  to  account  for  the  peculiarity  of  reaction  of  a 
double  salt  by  the  formation  of  a  new  complex  radicle.  In  the  same 
way  also,  in  the  presence  of  an  excess  of  tartaric  acid,  cupric  salts  do 
not  form  a  precipitate  with  potassium  hydroxide,  because  a  double  salt 
is  formed.  These  peculiarities  are  more  easily  understood  in  the  case 
of  cyanogen  compounds  than  in  all  others,  because  all  cyanogen  com- 
pounds, as  unsaturated  compounds,  show  a  marked  tendency  to. 
complexity.  This  tendency  is  satisfied  in  double  salts.  The  appear^ 
ance  of  a  peculiar  character  in  double  cyanides  is  the  more  easily 
understood  since  in  the  case  of  potassium  cyanide  itself,  and  also  in 
hydrocyanic  acid,  a  great  many  peculiarities  have  been  observed 
which  are  not  encountered  in  those  haloid  compounds,  potassium 
chloride  and  hydrochloric  acid,  with  which  it  was  usual  to  compafe 
cyanogen  compounds.  These  peculiarities  become  more  comprehensible 
on  comparing  cyanogen  compounds  with  ammonium  compounds.  Thus 
in  the  presence  of  ammonia  the  reactions  of  many  compounds  change 
considerably.  If  in  addition  to  this  it  is  remembered"  that  the 
presence  of  many  carbon  (organic)  compounds  frequently  completely 
disturbs  the  reaction  of  salts,  the  peculiarities  of  certain  double  cyanides 
will  appear  still  less  strange,  because  they  contain  carbon.  The  fact 
that  the  presence  of  carbon  or  another  element  in  the  compound  pro- 
duces a  change  in  the  reactions,  may  be  compared  to  the  action  of 
oxygen,  which,  when  entering  into  a  combination,  also  very  materially 
changes  the  nature  of  reactions.  Chlorine  is  not  detected  by  silver 
nitrate  when  it  is  in,  the  form  of  potassium  chlorate,  KC1O3,  as  it  is 
detected  in  potassium  chloride,  KC1.  The  iron  in  ferrous  and  ferric 
compounds  varies  in  its,  reactions.  In  addition-to  the  above-mentioned 
facts,  consideration  ought  to  be  given  to  the  circumstance  that  the 
easy  mutability  of  nitric  acid  undergoes  modification  in  its  alkali 
salts.,  and  in  general  the  properties  of  a  salt  often  differ  much  from 
those  of  the  acid.  Every  double  salt  ought  to  be  regarded  as  a  pecu- 
liar kind  of  saline  compound  :  potassium  cyanide  is,  as  it  were,  a  basic, 
and  ferrous  cyanide  an  acid,  element.  They  may  be  unstable  in  the 
separate  state,  but  form  a  stable  double  compound  when  combined 
together ;  the  act  of  combination  disengages  the  energy  of  the  elements, 


848  PKINCIPLES  OF  CHEMISTRY 

and  they,  so  to  speak,  saturate  each  other.  Of  course,  all  this  is  not  a 
definite  explanation,  but  then  the  supposition  of  a  special  complex  radicte 
can  even  less  be  regarded  as  such. 

Potassium  ferrocyanide,  K4FeC6N6,  is  very  easily  formed  by  mixing 
solutions  of  ferrous  sulphate  and  potassium  cyanide.  First,  a  white 
precipitate  of  ferrous  cyanide,  FeC2N2,  is  formed,  which  becomes  blue 
on  exposure  to  air,  but  is  soluble  in  an  excess  of  potassium  cyamde} 
forming  the  ferrocyanide.  The  same  yellow  prussiate  is  obtained  on 
heating  animal  nitrogenous  charcoal  or  animal  matters— such  as 
horn,  leather  cuttings,  <fec. — with  potassium  carbonate  in  iron 
vessels,27  bis  the  mass  formed  being  afterwards  boiled  with  water  with 
exposure  to  air,  potassium  cyanide  first  appearing,  which  gives  yellow 
prussiate.  The  animal  charcoal  may  be  exchanged  for  wood  charcoal, 
.permeated  with  potassium  carbonate  and  heated  in  nitrogen  or 
ammonia ;  the  mass  thus  produced  is  then  boiled  in  water  with  ferric 
Oxide.28  In  this  manner  it  is  manufactured  on  the  large  scale,  and  is 
.called  'yellow  prussiate'  ('prussiate  de  potasse,'  Blutlaugensalz). 

It  is  easy  to  substitute  other  metals  for  the  potassium  in  the  yellow 
prussiate.  The  hydrogen  salt  or  hydroferrocyanic  acid,  H4FeC6NG,  is 
obtained  by  mixing  strong  solutions  of  yellow  prussiate  and  hydro- 
chloric acid.  If  ether  be  added  and  the  air  excluded,  the  acid  is 
obtained  directly  in  the  form  of  a  white  scarcely  crystalline  precipitate 
which  becomes  blue  on  exposure  to  air  (as  ferrous  cyanide  does  from  the 
formation*  of  blue  compounds  of  ferrous  and  ferric  cyanides,  and  it  is 
On  this  account  used  in  cotton  printing).  It  is  soluble  in  water  and 
alcohol,  but  not  in  ether,  has  marked  acid  properties,  and  decomposes 
carbonates,  which  renders  it  Easily  possible  to  prepare  ferrocyanides  of 

** bl*  The  sulphur  of  the  animal  refuse  here  forms  the  compound  FeKS2,  which 
by  the  action  of  potassirim  cyanide  yields  potassium  sulphide,  thiocyanate,  and  ferro- 
cyanide. 

M  Potassium  ferrocyanide  may  also  be  obtained  from  Prussian  blue  by  boiling  with  a 
solution  of  potassium  hydroxide,  and  from  the  ferricyanide  by  the  action  of  alkalis  and 
reducing  substances  (because  the  red  prussiate  is  a  product  of  oxidation  produced  by 
the  action  of  chlorine :  a  ferric  salt  is  reduced  to  a  ferrous  salt),  &c.  In  many  works 
(especially  in  Germany  and  France)  yellow  prussiate  is  prepared  from  the  mass,  con- 
taining oxide  of  iron,  and  employed  for  purifying  coal  gas  (Vol.  L,  p.  361),  which 
generally  contains  cyanogen  compounds.  About  2  p.c.  of  the  nitrogen  contained  in  coal  is 
Converted  into  cyanogen,  which  forms  Prussian  blue  and  thiocyanates  in  the  mass  used 
for  purifying  the  gas.  OH  evaporation  the  solution  yields  large  yellow  crystals  containing 
8  molecules  of  water,  which  is  easily  expelled  by  heating  above  100°.  100  parts  of  water 
at  the  ordinary  temperature  are  capable  of  dissolving  25  parts  of  this  salt ;  its  sp.  gr.  is 
1-83.  Wnen  ignited  it  forms  potassium  cyanide  and  iron  carbide,  FeC2  (Chapter  XIII., 
Note  12).  Oxidising  substances  change  it  into  potassium  ferricyanide.  With  strong 
sulphuric  acid  it  gives  carbonic  oxide,  and  with  dilute  sulphuric  acid,  when  heated, 
prussic  acid  is  evolved  according  to  the  equation:  2K4FeC6N6  +  8H2S04  =  K2FeiC6N9 
H-SKjSO,,*  6HCN;  hence  in  the  yellow  prussiate  Ko  replaces  Fe. 


IRON,   COBALT,   AND  NICKEL  849 

^he  metals  of  the  alkalis  and  alkaline  earths  ;  these  are  readily  soluble, 
have  a  neutral  reaction,  and  resemble  the  yellow  prussiate.  Solutions 
of  these  salts  form  precipitates  with  the  salts  of  other  metals,  because  the 
ferrocyanides  of  the  heavy  metals  are  insoluble.  Here  either  the  whole 
pf  the  potassium  of  the  yellow  prussiate,  or  only  a  part  of  it,  is  exchanged 
for  an  equivalent  quantity  of  the  heavy  metal.  Thus,  when  a  cupric 
salt  is  added  to  a  solution  of  yellow  prussiate,  a  red  precipitate  is  obtained 
which  still  contains  half  the  potassium  of  the  yellow  prussiate  : 

K4FeC6N6  +  CuS04  =  K2CuFeC6N6  +  K2S04. 

But  if  the  process  be  reversed  (the  salt  of  copper  being  then  in  excess) 
the  whole  of  the  potassium  will  be  exchanged  for  copper,  forming  a 
reddish-brown  precipitate,  Cu2FeC6N6,9H2O.  This  reaction  and 
those  similar  to  it  are  very  sensitive  and  may  be  used  for  detecting 
metals  in  solution,  more  especially  as  the  colour  of  the  precipitate 
very  often  shows  a  marked  difference  when  one  metal  is  exchanged 
for  another.  Zinc,  cadmium,  lead,  antimony,  tin,  silver,  cuprous  and 
aurous  (  salts  form  white  precipitates ;  cupric,  uranium,  titanium 
and  molybdenum  salts  reddish-brown ;  those  of  nickel,  cobalt, 
and  chromium,  green  precipitates  ;  with  ferrous  salts,  ferrocyanide 
forms,  as  has  been  already  mentioned,  a  white  precipitate — namely, 
kFe2FeC6N6,  or  FeC2N2 — which  turns  blue  on  exposure  to  air,  and 
with  ferric  salts  a  blue  precipitate  called  Prussian  blue.  Here  the 
potassium  is  replaced  by  iron,  the  reaction  being  expressed  thus  : 
2Fe2Clc  +  3K4FeC6N6  =  12KCl  +  Fe4Fe3C,8N18,  the  latter  formula 
expressing  the  composition  of  Prussian  blue.  It  is  therefore  the 
compound  4Fe(CN)3-f  3Fe(CN)2.  The  yellow  prussiate  is  prepared  in 
chemical  works  on  a  large  scale  especially  for  the  manufacture  of  this 
blue  pigment,  which  is  used  for  dyeing  cloth  and  other  fabrics  and 
also  as  one  of  the  ordinary  blue  paints.  It  is  insoluble  in  water,  and 
the  stuffs  are  therefore  dyed  by  first  soaking  them  in  a  solution  of  a 
ferric  salt  and  then  in  a  solution  of  yellow  prussiate.  If  however 
an  excess  of  yellow  prussiate  be  present  complete  substitution  between 
potassium  and  iron  does  not  occur,  and  soluble  Prussian  blue  is 
formed  j  KFe2(CN)6=  KCN,Fe(CN)2,Fe(CN)3.  This  blue  salt  is 
colloidal,  is  soluble  in  pure  water,  but  insoluble  and  precipitated 
when  other  salts — for  instance,  potassium  or  sodium  chloride — ate 
present  even  in  small  quantities,  and  is  therefore  first  obtained  as  a 
precipitate.29 

29  Skraup  obtained  this  salt  both  from  potassium  ferrocyanide  with  ferric  chloride 
end  from  ferricyanide  with  ferrous  chloride,  which  evidently  shows  that  it  contains  iron 


$50  PRINCIPLES  OF  CHEMISTRY 

Potassium  ferricyanide,  or  red  prussiate  of  potash,  K3FeC6N6,  is 
called  'Gmelin's  salt,'  because  this  savant  obtained  it  by  the  action 
of  chlorine  on  a  solution  of  the  .yellow  prussiate  :  K4FeC6N6  +  Cl 
t=  K3FeC6Nc  +  KCl.  The  reaction  is  due  to  the  ferrous  salt  being 
changed  by  the  action  of  the  chlorine  into  a  ferric  salt.  It  separates 
from  solutions  in  anhydrous,  well-formed  prisms  of  a  red  colour,  but 
the  solution  has  an  oliye  colour  ;  100  parts  of  water,  at  10°,  dis- 
solve 37  parts  of  the  salt,  and  at  100°,  78  parts.30  The  red  prus- 
siate gives  a  blue  precipitate  with  ferrous  salts,  called  TurnbulVs  blue, 
very  much  like  Prussian  blue  (and  the  soluble  variety),  because  it  also 
contains  ferrous  cyanide  and  ferric  cyanide,  although  in  another  propor- 

in  both  the  ferric  and  ferrous  states.  With  ferrous  chloride  it  forms  Prussian  blue,  and 
with  ferric  chloride  TurnbuH's  bine. 

Prussian  blue  was  discovered  in  the  beginning  of  the  last  century  by  a  Berlin 
manufacturer,  Diesbach.  It  was  then  prepared,  as  it  sometimes  is  also  at  present, 
directly  from  potassium  cyanide  obtained  by  heating  animal  charcoal  with  potassium 
carbonate.  The  mass  thus  obtained  Is  dissolved  in  water,  alum  is  added  to  the 
solution  in  order  to  saturate  the  free  alkali,  and  then  a  solution  of  green  vitriol  is  added 
which  has  previously  been  sufficiently  exposed  to  the  air  to  contain  both  ferric  and 
ferrous  salts.  If  the  solution  of  potassium  cyanide  be  mixed  with  a  solution  containing 
both  salts,  Prussfan  blue  will  be  formed,  because  it  is  a-compound  of  ferrous  cyanide, 
FeC2N2,  and  ferric  cyanide,  Fe2C6N6.  A  ferric  salfr  with  potassium  ferTocyanide  forms 
a  blue  colour,  because  ferrous  cyanide  is  obtained  from  the  first  salt  and  ferric  cyanide 
from  the  second.  During  the  preparation  of  this  compound  alkali  must  be  avoided,  aa 
otherwise  the  precipitate  would  contain  oxides  of  iron.  Prussian  blue  has  not  a  crystal- 
line structure  ;  it  forms  a  blue  mass  with  a  copper-red  metallic  lustre.  Both  acids  and 
alkalis  act  on  it.  The  action  is  at  first  confined  to  the  ferric  suit  it  contains.  Thus 
alkalis  form  ferric  oxide  and  ferrocyanide  in  solution:  2Fe2C6N6,8FeC2N2-f'12KHO 
»  2(Fe2O-,,8H.jO)  +  SK^FeCgNfl.  Various  ferrocyanides  may  thus  be  prepared.  Prussian 
bine  is  soluble  in  an  aqueous  solution  of  oxalic  acid,  forming  blue  ink.  In  air,  when 
exposed  to  the  action  of  light,  it  fades;  but  in  the  dark  again  absorbs  oxygen  and 
becomes  blue,  which  fact  is  also  sometimes  noticed  in  blue  cloth.  An  excess  of  potassium 
ferrocyanide  renders  Prussian  blue  soluble  in  water,  although  insoluble  in  various  saline 
solutions— that  is,  it  converts  it  into  the  soluble  variety.  Strong  hydrochloric  acid  also 
dissolves  Prussian  blue. 

80  An  excess  >f  chlorine  must  not  be  employed  in  preparing  this  compound,  otherwise 
the  reaction  goes  further.  It  is  easy  to  find  out  when  the  action  of  the  chlorine  on  potassium 
ferrocyanide  must  cease  ;  it  is  only  necessary  to  take  a  sample  of  the  liquid  and  add  a 
solution  of  a  ferric  salt  to  it:  If  a  precipitate  of  Prussian  blue  is  formed,  more  chlorine 
must  be  added,  as  there  is  stilrsome  undecomposed  ferrocyanide,  for  the  ferricyanide 
does  not  give  a  precipitate  with  ferric  salts.  Potassium  ferricyanLde,  like  the  ferro- 
cyanide, easily  exchanges  its  potassium  for  hydrogen  and  various  metals  by  double 
decomposition.  With  the  salts  of  tin,  silver,  and  mercury  it  forms  yellow  precipitates, 
and  with  those  of  uranium,  nickel,  cobalt,  copper,  and  bismuth  brown  precipitates.  The 
lead  salt  under  the  action  of  sulphuretted  hydrogen  fonns  lead  sulphide  and  a  hydrogen 
salt  or  acid,  H3FeCbN6,  corresponding  with  potassium  ferricyanide,  which  is  soluble, 
crystallises  in  red  needles,  and  resembles  hydroferrocyanic  acid,  H4FeC6NG.  Under  the 
action  of  reducing  agents — for  instance,  sulphuretted  hydrogen,  copper — potassium  ferri- 
cyanide is  changed  into  ferrocyanide,  especially  in  the  presence  of  alkalis,  and  thus  forms 
a  rather  energetic  oxidising  agent— capable,  for  instance,  of  changing  manganous  oxido 
into  dioxide,  bleaching  tissues,  &c. 


IRON.   COBALT.  AND  NICKEL  851 


'tion,  being  formed  accordirig  to  the  equation: 

•=6KCl  +  Fe3Fe2Oj2N12,  or  3FeC2N2,Fe206N6  ;   in  Prussian  blue  we 

haveFe7Cy18,  and  here  Fe5Cy,2.     A  ferric  salt  ought  to  form  ferric 

^cyanide  Fe2C6N6,  with  red  prussiate,  but  ferric  cyanide  is  soluble, 

and  therefore  no  precipitate  is  obtained,  and  the  liquid  only  becomes 

•brown.31 

If  chlorine  and  sodium  are  representatives  of  independent  groups 
of  elements,  the  same  may  also  be  said  of  iron.  Its  nearest  ana- 
logues show,  besides  a  similarity  in  character,  a  likeness  as  regards 
physical  properties  and  a  proximity  in  atomic  weight.  Iron  occupies  a. 
medium  position  amongst  its  nearest  analogues,  both  with  respect  to 
properties  and  faculty  of  forming  saline  oxides,  and  also  as  regards 
•atomic  weight.  On  the  one  hand,  cobalt,  58,  and  nickel,  59,  approach. 

5i  It  is  important  to  mention  a  series  of  readily  crystallisable  salts  formed  by  the- 
action  of  nitric  acid  on  potassium  and  other  ferrocyanides  and  ferricyanides.  These- 
salt  contain  the  elements  of  nitric  oxide,  and  are  therefore  called  nitro-(nitroso} 
ferricyanides  (nitroprussides).  Generally  a  crystalline  sodium  salt  is  obtained,. 
Na2FeC5N6O,2HjO.  In  its  composition  this  salt  differs  from  the  red  sodium  salt, 
Na3FeC6N6,  by  the  fact  that  in  it  one  molecule  of  sodium  cyanide,  NaCN,  is  replaced  by 
nitric  oxide,  NO.  In  oi-dcr  to  prepare  it,  potassium  ferrocyanide  in  powder  is  mixed. 
with  five-sevenths  of  its  weight  of  nitric  acid  diluted  with  an  equal  volume  of  water. 
The  mixture  is  at  first  left  at  the  ordinary  temperature,  and  then  heated  on  a 
water-bath.  Here  ferricyanide  is  first  of  all  formed  (as  shown  by  the  liquid  giving  a. 
precipitate  with  ferrous  chloride),  which  then  disappears  (no  precipitate  with  ferrous, 
chloride),  and  forms  a  green  precipitate.  The  liquid,  when  cooled,  deposits  crystals 
'of  nitre.  The  liquid  is  then  strained  off  and  mixed  with  sodium  carbonate,  boiled,. 
filtered,  and  evaporated  ;  sodium  nitrate  and  the  salt  described  are  deposited  in  crystals. 
It  separates  in  prisms  of  a  red  colom-.  Alkalis  and  salts  of  the  alkaline  earths  do  not 
give  precipitates:  they  are  soluble,  but  the  salts  of  iron,  zinc,  copper,  and  silver  form 
precipitates  where  sodium  is  exchanged  with  these  metals.  It  is  remarkable  that  the- 
sulphides  of  the  alkali  metals  give  with  this  salt  an  intense  bright  purple  coloration. 
This  series  of  compounds  was  discovered  by  Gmelin  and  studied  by  Playfair  and  others. 
(1849). 

This  series  to  a  certain  extent  resembles  the  nitro-sulphide  series  described  by 
Roussin.  Here  the  primary  compound  consists  of  black  crystals,  which  are  obtained  as 
follows  :—  Solutions  of  potassium  hydrosulphide  and  nitrate  are  mixed,  and  the  mixture 
is  agitated  whilst  ferric  chloride  is  added,  then  boiled  and  filtered  ;  on  cooling,  black 
Crystals  aro  deposited,  having  the  composition  Fe6S3  (NO)10,H3O  (Rosenberg),  or,  accord- 
ing to  Demel,  FeNO^NH^S.  They  have  a  slightly  metallic,  lustre,  and  are  soluble  in 
water,  alcohol,  and  ether.  They  absorb  the  latter  as  easily  as  calcium  chloride  absorbs 
water.  In  the  presence  of  alkalis  these  crystals  remain  unchanged,  but  with  acids  tney 
evolve  nitric  oxides.  There  are  several  compounds  which  are  capable  of  interchanging,. 
and  correspond  with  Roussin's  salt.  Here  we  enter  into  the  series  of  the  nitrogen 
compounds  which  have  been  as  yet  but  little  investigated,  and  will  most  probably  in 
time  form  most  instructive  material  for  studying  the  nature  of  that  element.  These- 
series  of  compounds  are  as  unlike  the  usual  saline  compounds  of  inorganic  chemistry  as. 
are  organic  hydrocarbons.  There  is  no  necessity  to  describe  these  series  in  detail,  because- 
their  connection  with  other  compounds  is  not  yet  clear,  and  they  have  not  yet  any 
Application. 


852  PRINCIPLES  OF  CHEMISTKY 

iron,  56  ;  they  are  metals  of  a  more  basic  character,  they  do  not  form 
stable  acids  or  higher  degrees  of  oxidation,  and  are  a  transition  to 
copper,  63,  and  zinc,  65.  On  the  other  hand,  manganese,  55,  and 
chromium,  52,  are  the  nearest  to  iron  ;  they  fo'rm  both  basic  and  acid 
oxides,  and-  are  a  transition  to  the  metals  possessing  acid  properties. 
In  addition  to  having  atomic  weights  approximately  alike,  chromium, 
manganese,  iron,  cobalt,  nickel,  and  copper  have  also  nearly  the  same 
specific  gravity,  so  that  the  atomic  volumes  and  the  molecules  of  their 
analogous  compounds  are  also  near  to  one  another  (see  table  at  the 
beginning'  of  this  volume).  Besides  this,  the  likeness  between  the 
Above-mentioned  elements  is  also  seen  from  the  following  : 

They  form  suboxides,  RO,  fairly  energetic  bases,  isomorphous  with 
magnesia— for  instance,  the  salt  RSO4,7H2O,  akin  to  MgSO4,7H2O, 
And  FeSO4,7H20,  or  to  sulphates  containing  less  water ;  with  alkali 
sulphates  all  form  double  salts  crystallising  with  6H20  ;  all  are  capable 
of  forming  ammonium  salts,  <fec.  The  lower  oxides,  in  the  cases  of 
nickel  and  cobalt,  .are  tolerably  stable,  are  not  easily  oxidised  (the 
nickel  compound  with  more  difficulty  than  cobalt,  a  transition  to 
copper);  with  manganese,  and  .especially  with  chromium,  they  are 
more  Qasily  oxidised  than  with  iron  and  pass  into  higher  oxides. 
They  also  form  oxides  of  the  form  R2O3,  and  with  nickel,  cobalt, 
•and  manganese  this  oxide  is  very  unstable,  and  is  more  easily  reduced 
than  ferric  oxide  ;  but,  in  the  case  of  chromium,  it,  is  very  stable,  and 
forms  the  ordinary  kind  of  salts.  It  is. isomorphous  with  ferric  oxide, 
forms  alums,  is  a  feeble  base,  &c.  Chromium,  manganese,  and  iron  are 
oxidised  by  alkali  and  oxidising  agents,  forming  salts  like  Na2S04  ; 
but  cobalt  and  nickel  are  difficult  to  oxidise  ;  their  acids  are  not  known 
with  any  certainty,  and  are,  in  all  probability,  still  less  stable  than  the 
ferrates.  Cr,Mn  and  Fe  form  compounds  R2C1C  which  are  like  Fe2Cl3 
in  many  respects ;  in  Co  this  faculty  is  weaker  and  in  Ni  it  has  almost 
disappeared.  The  cyanogen  compounds,  especially  of  manganese  and 
cobalt,  are  very  near  akin  to  the  corresponding  ferrocyanides.  Thd 
oxides  of  nickel  and  cobalt  are  more  easily  reduced  to  metal  than  those 
of  iron,  but  those  of  manganese  and  chromium  are  not  reduced  so 
easily  as  iron,  and  the  metals  themselves  are  not  easily  obtained  in  a 
pure  state  ;  they  are  capable  of  forming  varieties  resembling  cast  iron. 
The  metals  Cr,Mn,Fe,Co,  and  Ni  have  a  grey  iron  colour  and  are  very 
difficult  to  melt,  but  nickel  and  cobalt  can  be  melted  in  the  reverbera- 
tory  furnace  and  are  more  fusible  than  iron,  whilst  chromium  is  more 
.difficult  to  melt  than  platinum  (Deville).  -These  metals  decompose 
water,  but  with  greater  difficulty  as  the  atomic  weight  rises,  forming  a 
•transition  to  copper,  which  does  not  decompose  water.  All  the  com- 


IRON,   COBALT,  AND  NICKEL  353 

pounds  of  these  metals  have  various  colours,  which  are  sometimes  very 
bright,  especially,  in  the  higher  stages  of  oxidation. 

These  metals  of  the  iron  group  are  often  met  with  together  in 
nature.  Manganese  nearly  everywhere  accompanies  iron,  and  iron  is 
always  an  ingredient  in  the  ores  of  manganese.  Chromium  is  found 
principally  as  chrome  ironstone — that  is,  a  peculiar  kind  of  magnetic 
oxide  in  which  Fe.2O3  is  replaced  by  Cr2O3. 

Nickel  and  cobalt  are  as  inseparable  companions  as  iron  and 
manganese.  The  similarity  between  them  even  extends  to  such 
remote  properties  as  magnetic  qualities.  In  this  series  of  metals  we 
6nd  those  which  are  the  most  magnetic  :  iron,  cobalt,  and  nickel. 
There  is  even  a  magnetic  oxide  among  the  chromium  compounds,  such 
being  unknowh  in  the  other  series.  Nickel  easily  becomes  passive  in 
strong  nitric  acid.  It  absorbs  hydrogen  in  just  the  same  way  as  iron. 
Tn  short,  in  the  series  Cr,  Mn,  Fe,  Co,  and  Ni,  there  are  many  points 
in  common  although  there  are  many  differences,  as .  will  be  seen  still 
more  clearly  on  becoming  acquainted  with  cobalt  and  nickel. 

In  nature  cobalt  is  principally  found  in  combination  with  arsenic 
and  sulphur.  Cobalt  arsenide,  or  cobalt  speiss,  CoAs2,  is  found  in 
brilliant  crystals  of  the  regular  system,  principally  in  Saxony.  Cobalt 
glance,  CoAs2CoS2,  resembles  it  very  much,  and  'also  belongs  to  the 
regular  system  ;  it  is  found  in  Sweden,  Norway,  and  the  Caucasus. 
Kupfernickel  is  a-  nickel  ore  in  combination  with  arsenic,  but  of  a 
different  composition  from  cobalt  arsenide,  having  the  formula  NiAs  ; 
it  is  found  in  Bohemia  and  Saxony.  It  has  a  copper-red  colour  and  is 
rarely  crystalline ;  it  is  so  called  because  the  miners  of  Saxony  first 
mistook  it  for  an  ore  of  copper  (Kupfer),  but  were  unable  to  extract 
copper  from  it.  Nickel  glance^  NiS2,NiAs2,  corresponding  with  cobalt 
glance,  is  also  known.  Nickel  accompanies  the  ores  of  cobalt  and. 
cobalt  those  of  nickel,  so  that  both  metals  are  found  together.  The 
ores  of  cobalt  are  worked  in  the  Caucasus  in  the  Government  of 
Elizavetopolsk.  Nickel  ores  containing  aqueous  hydrated  nickel  silicate 
are  found  in  the  Ural  (Revdansk).  Large  quantities  of  a  similar  ore 
are  exported  into  Europe  from  New  Caledonia.  Both  ores  contain 
about  12  per  cent.  Ni.  Garnierite,  (RO)5(SiO2)4l|H2O,  where  R=Ni 
and  Mg,  predominates  in  the  New  Caledonian  ore.  Large  deposits  of 
nickel  have  been  discovered  in  Canada,  where  the  ore  (as  nickelous 
pyrites)  is  free  from  arsenic.  Cobalt  is  principally  worked  up  into 
cobalt  compounds,  but  nickel  is  generally  reduced  to  the  metallic  state,  in 
which  it  is  now  often  used  for  alloys — for  instance,  for  coinage  in  many 
European  States,  and  for  plating  other  metals,  because  it  does  not 
oxidise.  Cobalt  arsenide  and  cobalt  glance  are  principally  used,  for  the 


854  PRINCIPLES  OF  CHEMISTRY 

preparation  of  cobalt  compounds  ;  they  are  first  sorted  by  discarding 
the  rocky  matter,  and  then  roasted.  During  this  process  most  of  the 
sulphur  and  arsenic  disappears  ;  the  arsenious  anhydride  volatilises 
with  the  sulphurous  anhydride  and  the  metal  also  oxidises.3'2  It  is  a 
simple  matter  to  obtain  nickel  and  cobalt  from  their  oxides.  In  order 
to  obtain  the  latter,  solutions  of  their  salts  are  treated  with  sodium 

32  The  residue  from  the  roasting  of  cobalt  ores  is  called  zajftor,  and  is  often  met  with 
in  commerce.  From  this  the  purer  compounds  of  cobalt  may  be  prepared.  The  ores  of 
nickel  are  also  first  roaste'd,  and  the  oxides  dissolved  in  acid,  nickelous  salts  being  then 
obtained. 

The  further  treatment  of  cobalt  and  nickel  ores  is  facilitated  if  the  arsenic  can  be 
almost  entirely  removed,  which  may  be  effected  by  roasting  the  ore  a  second  time  with  a 
small  addition  of  nitre  and  sodium  carbonate  ;  the  nitre  combines  with  the  arsenic, 
forming  an  arsenious  salt,  which  may  be  extracted  with  water.  The  remaining  mass  is 
dissolved  in  hydrochloric  acid,  mixed  with  a  small  quantity  of  nitric  acid.  Copper,  iron, 
manganese,  nickel,  cobalt,  &c.,  pass  into  solution.  By  passing  hydrogen  sulphide 
through  the  solution,  copper,  bismuth,  lead,  and  arsenic  are  dene-sited  as  metallic  sul- 
phides ;  but  iron,  cobalt,  nickel,  and  manganese  remain  in  solution.  If  an  alkaline  solu- 
tion of  bleaching  powder  be  then  added  to  the  remaining  solution,  the  whole  of  the 
manganese  will  first  be  deposited  in  the  form  of  dioxide,  then  the  cobalt  as  hydrated 
cobaltic  oxide,  and  finally  the  nickel  also.  It  is,  however,  impossible  to  rely  on  this 
method  for  effecting  a  complete  separation,  the  more  so  since  the  higher  oxides  of  the 
three  above-mentioned  metals  have  all  a  black  colour ;  but,  after  a  few  trials,  it  will  be 
easy  to  find  how  much  bleaching  powder  is  required  to  precipitate  the  manganese,  and 
the  amount  which  will  precipitate  all  the  cobalt.  The  manganese  may  also  be  separated 
'  from  cobalt  by  precipitation  .from  a  mixi  are  of  the  solutions  of  both  metals  (in  the  form  of 
the  '  ous '  salts)  with  ammonium  sulphide,  and  then  treating  the  precipitate  with  acetic 
acid  or  dilute  hydrochloric  acid,  in  which  manganese  sulphide  is  easily  soluble  and  cobalt 
sulphide  almost  insoluble.  Further  particulars  relating  to  the  separation  of  cobalt  from 
nickel  may  be  found  in  treatises  on  analytical  chemistry.  In  practice  it  is  usual  to  rely  on 
the  rough  method  of  separation  founded  oirthe  fact  that  nickel  is  more  easily  reduced  and 
more  difficult  to  oxidise  than  cobalt.  The  New  Caledonian  ore  is  smelted  with  CaS04 
and  CaC03  on  coke,  and  a  metallic  regulus  is  obtained  containing  all  the  Ni,  Fe,  and  S. 
This  is  roasted  with  Si02,  which  converts  all  the  iron  into  slag,  whilst  the  Ni  remains 
combined  with  the  S;  this  residue  on  further  roasting  gives  NiO,  which  is  reduced  by  the 
carbon  to  metallic  Ni.  The  Canadian  ore  (a  pyrites  containing  11  p.  c.  Ni)  is  frequently 
treated  in  America  (after  a  preliminary  dressing)  by  smelting  it  with  Na^SC^  and 
charcoal ;  the  resultant  fusible  Na2S  then  dissolves  the  CuS  and  FeS?,  while  the  NiS  ia 
obtained  in  a  bottom  layer  (Bartlett  and  Thomson's  process)  from  which  Ni  is  obtained 
in  the  manner  described  above. 

For  manufacturing  purposes  somewhat  impure  cobalt  compounds  are  frequently  used, 
which  are  converted  into  smalt.  This  is  glass  containing  a  certain  amount  of  cobalt 
oxide  ;  the  glass  acquires  a  bright  blue  colour  from  this  addition,  so  that  when  powdered 
it  may  be  used  as  a  blue  pigment;  it  is  also  unaltered  at  high  temperatures,  BO 
that  it  used  to  take  the  place  now  occupied  by  Prussian  blue,  ultramarine,  &c.  At 
present  smalt  is  almost  exclusively  used  for  colouring  glass  and  china.  To  prepare 
smalt,  ordinary  impure  cobalt  ore  (zaffre)  is  fused  in  a  crucible  with  quartz  and  potassium 
carbonate.  A  fused  mass  of  cobalt  glass  is  thus  formed,  containing  silica,  cobalt  oxide, 
and  potassium  oxide,  and  a  metallic  mass  remains  at  the  bottom  of  the  crucible,  con- 
taining almost  all  the  other  metals, .arsenic,  nickel,  copper,  silver,  &c.  This  metallic 
mass  is  called  speiss,  and  is  used  as  nickel  ore  for  the  extraction  of  nickel.  Smalt  usually 
contains  70  p.c.  of  silica,  20  p.c.  of  potash  and  soda,  and  about  5  to  6  p.c.  of  cobaltous 
oxide;  the  remainder  consisting  of  other  metallic  oxides. 


IRON,  COBALT,  .AND  NICKEL  355 

carbonate  and  the  precipitated  carbonates  are  heated;  the  suboxides 
are  thus  obtained,  and  these  latter  are  reduced'  in  a  stream  of 
hydrogen,  or  even  by  heating  with  ammonium  chloride.  They  easily 
oxidise  when  in  the  state  of  powder.  When  the  chlorides  of  nickel 
and  cobalt  are  heated  in  a  stream  of  hydrogen,  the  metal  is  deposited 
in  brilliant  scales.  Nickel  is-  always  much  more  easily  and  quickly 
reduced  than  cobalt.  Nickel  melts  more  easily  than  cobalt,  and  this 
even  furnishes  a  means  of  testing  the  heating  powers  of  a  reverberatory 
furnace.  Cobalt  fuses  at  a  temperature  only  a  little  lower  than  that 
•at  which  iron  does.  In  general,  cobalt  is  nearer  to  iron  than  nickel, 
nickel  being  nearer  to  copper. 32tli3  Both  nickel  and  cobalt  have  mag- 
netic properties  like  iron,  but  Co  is  less  magnetic  than  Fe,  and  Ni  still 
less  so.  The  specific  gravity  of  nickel  reduced  by  hydrogen  is  9-1  and' 
that  of  cobalt  8'9.  Fused  .cobalt  has  -a  specific  gravity  of  8-5,  the 
density  of  ordinary  nickel  being  almost  the  same.  Nickel  has., a-  greyish 
silvery- white  colour  ;  it  is  brilliant  and  very  ductile,  so  that  the  finest 
wire  may  be  easily  drawn  from  it.  This  wire  has  a  resistance  to 
tension  equal  to  iron  wire.  The  beautiful  colour  of  nickel,  and  the 
high  polish  which  it  is  capable  of  receiving  and  retaining,  as  it  does 
not  oxidise,  render  it  a  useful  metal  for  many  purposes,  arid  in 
many  ways  it  resembles  silver.32  tri  It  is  now  very  common  to  cover 

32bi«  All  we  know  respecting  the  relations  of  Co  and  Ni  to  Fe  and  Cu  confirms  the 
fact  that  Co  is  more  closely  related  to  Fe  and  Ni  to  Cu  ;  and  as  the  atomic  weight  of 
Fe  =>  56  and  of  Cu  =  63,  then  according  to  the  principles  of  the  periodic  system  it  would 
be  expected  that  the  atomic  weight  of  Co  would  be  about  59.-60,  whilst  that  of  Ni  should 
be  greater  than  that  of  Co  but  less  than  that  of  Cu,  i.e.  about  50:5  -60'5.  However,  as 
yet  the  majority  of  the  determinations  of  the  atomic  -weights  ;of  Co  and  Ni  give  a 
different  result  and  show  that  a  lower  atomic  weight  is  obtained  for  Ni  than  for  Co. 
Thus  K.  Winkler  (1894)  obtained  (employing  metals  deposited  electrolytically  and  deter- 
mining the  amount  of  iodine  which  combined  with  them)  Ni  =  58'72  and  Co  =  59-S7  (if 
H  =  1  and  I  =  126'53).  In  my  opinion  this  should  not  be  regarded  as  proving  that  the 
principles  of  the  periodic  system  cannot  be  applied  in  this  instance,  nor  as  a  reason  for 
altering  the  position  of  these  elements  in  the  system  (i.e.  by  placing  Ni  after  Fe,  and  Co 
next  to  Cu),  because  in  the  first  place  '.the  figures  given  by  different  chemists  (for  instance, 
Zimmermann,  Kriiss,  and  others)  are  somewhat  divergent,  and  in  the  second  place  the 
majority  of  the  latest  modes  of  determining  the  atomic  weights  of  Co  and  Ni  aim  at 
finding  what  weights  of  these  metals  react  with  known  weights  of  other  elements  without 
ta"king  into  account  the  faculty  they  have  of  absorbing  hydrogen;  since  this  faculty  ia 
more  developed  in  Ni  than  in  Co  the  hydrogen  (occluded  in  Ni)  should  lower  the  atomic 
weight  of  Ni  more  than  that  of  Co.  On  the  whole,  the  question  of  the  atomic 
weights  of  Co  and  Ni  cannot  yet  be  considered  as.  .decided,  notwithstanding  the 
•  numerous  researches  which  have  been  made ;  still  there  can  be  no  doubt  that  the  atomic 
weights  of  these  two  metals  are  very  nearly  equal,  and  greater  than  that  of  Fe,  but  less 
than  that  of  Cu.  This  question  is  of  great  interest,  not  only  for  completing  our  know- 
ledge of  these  metals,  but  also  for  perfecting  our  knowledge  of  the  periodic  system'of  the 
elements. 

32  tri  por  instance,  the  alkalis  may  be  fused  in  nickel  vessels  as  well  as  in  silver^ 
because  they  have  no  action  upon  either  metal.  Nickel,  like  silver,  is  not  acted  upon  bjf 


356  PRINCIPLES  OF  CHEMISTRY 

.Other  metals  with  a  layer  of  nickel  (nickel  plating).  This  is  done  by  a 
process  of  electro-plating,  using  a  solution  oi  a  nickel  salt.  The 
colour  of  cobalt  is  dark  and  redder  ;  it  is  also  ductile,  and  has  a 
greater  tensile  resistance  than  iron.  Dilute  acids  act  very  slowly  on 
nickel  and  cobalt  ;  nitric  acid  may  be  considered  as  the  best  solvent 
for  them.  The  solutions  in  every  case  contain  salts  corresponding  with 
the  ferrous  salts — that  is,  the  salts  CoX2,  NiX2,  correspond  with  the 
suboxides  of  these  metals.  These  salts  in  their  types  are  similar  to  the 
magnesium  salts.  The  salts  of  nickel  when  crystallising  with  water 
have  a  green  colour,  and  form  bright  green  solutions,  but  in  the  anhy- 
drous state  they  most  frequently  have  a  yellow  colour.  The  salts  of 
cobalt  are  generally  rose-coloured,  and  generally  blue  when  in  the 
anhydrous  state.  Their  aqueous  solutions  are  rose-coloured.  Cobaltous 
:chloride  is  easily  soluble  in  alcohol,  and  forms  a  solution  of  an  intense 
blue  colour.33 

dilute  acids.  Only  nitric  acid  dissolves  both  metals  well.  Nickel  is  harder,  and  fuses  at 
a  higher  temperature  than  silver.  For  castings,  a  small  quantity  of  magnesium  (O'OOl 
part  hy  weight)  is  added  to  nickel  to  render  it  more  homogeneous  (just  as  aluminium  is 
added  to  steel).  Nickel  forms  many  valuable  alloys.  Steel  containing  8  p.c.  Ni  is  par- 
ticularly valuable,  its  limit  of  elasticity  is  higher  and  its  hardness  is  greater;  it  is  used, 
for  armour  plate  and  other  large  pieces.  The  alloys  of  nickel,  especially  with  copper  and 
zinc  (melchior,  see  later),  aluminium  and  silver,  although  used  in  certain  cases,  are  now 
replaced  by  nickel-plated  or  nickel-deposited  goods  (deposited  by  electricity  from  a 
solution  of  the  ammonium  salts). 

35  The  change  of  colour  re  dependent  in  all  probability  on  the  combination  with 
.water,  or  according  to  others  on  polymeric  transformation.  It  enables  a  solution  of 
cobalt  chloride  to  be  used  as  sympathetic  ink.  If  something  be  written  with  cobalt 
chloride  on  white  paper,  it  will  be  invisible  on  account  of  the  feeble  colour  of  the  solution, 
and  when  dry  nothing  can  be  distinguished.  If,  however,  the  paper  be  heated  before  the 
fire,  the  rose-coloured  salt  will  be  changed  into  a  less  hydrous  blue  salt,  and  the  writing 
will  become  quite  visible,  but  fade  away  when  cool. 

The  change  of  colour  which  takes  place  in  solutions  of  CoCl2  under  the  influence  not 
Only  of  solution  in  water  or  alcohol,  but  also  of  a  change  of  temperature,  is  a  character- 
istic of  all  the  halogen  salts  of  cobalt.  Crystalline  iodide  of  cobalt,  CoI26H20,  gives  a 
•  dark  red  solution  between  —  22°'and  +20°;  above  +20°  the  solution  turns  brown  and 
passes  from  olive  to  green,  from  +  85°  to  820°  the  solution  remains  green.  According  to 
Etard  the  change  of  colour  is  due  to  the  fact  that  at  first  the  solution  contains  the 
hydrate  CoI2H2O,  and  that  above  85°  it  contains  CoI24H2O.  These  hydrates  can  be 
crystallised  from  the  solutions;  the  former  at  ordinary  temperature  and  the  latter  on 
heating  the  solution.  The  intermediate  olive  colour  of  the  solutions  corresponds  to  the 
incipient  decomposition  of  the  hexahydrated  salt  and  its  passage  into  CoI24H2O.  A 
solution  of  the  hexahydrated  chloride  of  cobalt,  CoCl2GH2O,  is  rose-coloured  between 
-  22°  and  +  25° ;  but  the  colour  changes  starting  from  +  25°,  and  passes  through  all 
the  tints  between  red  and  blue  right  up  to  50°  ;  a  true  blue  solution  is  only  obtained 
a.t  65°  and  remains  up  'to  800°.  This  true  blue  solution  contains  another  hydrate. 
CoCl22H2O. 

The  dependence  between  the  solubility  of  the  iodide  and  chloride  of  cobalt  and 
he  temperature  is  expressed  by  two  almost  straight  lines  corresponding  to  the  hexa- 
and  di-hydrates ;  the  passage  of  the  one  into  the  other  hydrate  being  expressed  by  a 
icurve.  The  same  character  of  phenomena  is  seen  also  in  the  variation  of  the  vapour 


IRON,   COBALT,   AND  NICKEL  357 

If  a  solution  of  potassium  hydroxide  be  added  to  a  solution  of  a 
«obalt  salt,  a  blue  precipitate  of  the  basic  salt  will  be  formed.  If  a 

tension  of  solutions  of  chloride  of  cobalt  with  the  temperature.  We  have  repeatedly 
Been  that  aqueous  solutions  (for  instance,  Chapter  XXII.,  Note  28  for  Fe2Cl6)  deposit 
different  crystallo-hydrates  at  different  temperatures,  and  that  the  amount  of  water 
in  the  hydrate  decreases  as  the  temperature  t  rises,  so  that  it  is  not  surprising  that 
CoCl22H20  (or  according  to  Potilitzin  CoCl2H20)  should  sepa»ate  out  above  55°  and 
CoCl26H2O  at  25°  and  below.  Nor  is.  it  exceptional  that  the  colour  of  a  salt  variea 
according  as  it  contains  different  amounts  of  H2O.  But  in  this  instance  it  is  character- 
istic that  the  change  of  colour  takes  place  in  solution  in  the  presence  of  an  excess  of 
water  This  apparently  shows  that  the  actual  solution  may  contain  either  CoClQGH2O  or 
CoCl22H2O  And  as  we  know  that  a  solution  may  contain  both  metaphosphoric  PHO3 
and  orthophosphoric  acid  H3PO4  =  HP03+H2O,  as  well  as  certain  other  anhydrides, 
the  question  of  the  state  of  substances  in  solutions  becomes  still  more  complicated. 

Nickel  sulphate  crystallises  from  neutral  solutions  at  a  temperature  of  from  15°  to  20° 
in  rhombic  crystals  containing  7H20.  Its  form  approaches  very  closely  to  that  of  the 
Salts  of  zinc  and  magnesium.  The  planes  of  a  vertical  prism  for  magnesium  salts  are 
inclined  at  an  angle  of  90°  80',  for  zinc  salts  at  an  angle  of  91°  7',  and  for  nickel  salts  at 
an,  angle  of  91°  10'.  Such  is  also  the  form  of  the  zinc  and  magnesium  selenates  and 
chromates.  Cobalt  sulphate  containing  7  molecules  of  water  is  deposited  in  crystals 
of  the  monoclinic  system,  like  the  corresponding  salts  of  iron  and  manganese.  The  angle 
of  a  vertical  prism  for  the  iron  salt  =  82°  20',  for  cobalt  =  82°  22',  and  the  inclination  of 
the  horizontal  pinacoid  to  the  vertical  prism  for  the  iron  salt  =  99°  2',  and  for  the  cobalt 
salt  99°  36'  All  the  isomorphous  mixtures  of  the  salts  of  magnesium,  iron,  cobalt, 
nickel  and  manganese  have  the  same  form  if  they  contain  7  mol.  H2O  and  iron  or  cobalt 
predominate,  whilst  if  there  is  a  preponderance  of  magnesium,  zinc,  or  nitkel,  the 
crystals  have  a  rhombic  form  like  magnesium  sulphate.  Hence  these  sulphates  are 
dimorphous,  but  for  some  the  one  form  is  more  stable  and  for  others  the  other.  Brooke, 
Moss,  Mitscherlich,  Rammelsberg,  and  Marignac  have  explained  these  relations.  Brooke 
and  Mitscherlich  also  supposed  that  NiS04,7H2O  is  not  only  capable  of  assuming  these 
forms,  but  also  that  of  the  tetragonal  system,  because  it  is  deposite'd  in  this  form  from 
acid,  imd  especially  from  slightly-heated  solutions  (80°  to  40°).  But  Marignac  demon- 
strated that  the  tetragonal  crystals  do  not  contain  7,  but  6,  molecules  of  water,  NiSO4,6H2O. 
He  also  observed  that  a  solution  evaporated  at  50°  to  70°  deposits  monoclinic  crystals, 
but  of  a  different  form  from  ferrous  sulphate,  FeSO4)7H20— namely,  the  angle  of  the 
prism  is  71°  52',  that  of  the  pinacoid  95°  6'.  This  salt  appears  to  be  the  same  with  6 
molecules  of  water  as  the  tetragonal.  Marignac  also  obtained  magnesium  and  zino 
salts  with  6  molecules  of  water  by  evaporating  their  solutions  at  a  higher  tem- 
perature, and  these  salts  were  found  to  be  isomorphoua  with  the  monoclinic  "nickel  salt. 
In  addition  to  this  it  must  be  observed  that  the  rhombic  crystals  of  nickel  sulphate  with 
7H2O  become  turbid  under  the  influence  of  heat  and  light,  lose  water,  and  change  into 
the  tetragonal  salt.  The  monoclinic  crystals  in  time  also  become  turbid,  and  change 
their  structure,  so  that  the  tetragonal  form  df  this  salt  is  the  most  stable.  Let  us  also 
add  that  nickel  sulphate  in  all  its  shapes  forms  very  beautiful  emerald  green  crystals, 
which,  when  heated  to  280°,  assume  a  dirty  greenish-yellow  hue  and  then,  contain  one 
molecule  of  water. 

Klobb  (1891)  and  Langlot  and  Lenoir  obtained  anhydrous  CoSO4  and  NiSO4  by 
igniting  the  hydrated  salt  with  (NH4)2SO4^  until  the  ammonium  fsalt  had  completely 
volatilised  and  decomposed. 

We  may  add  that  when  equivalent  aqueous  solutions  of  NiX2  (green)  and  CoX2  (red) 
are  mixed  together  they  give  an  almost  colourless  (grey)  solution,  in  which  the  green  and 
ired  colour  of  the  component  parts  disappears  owing  to  the  combination  of  the  comple- 
mentary colours. 

A  double  salt  NiKF3  is  obtained  by  heating  NiCl2  withKFHPin  a  platinum' crucible  j 
KCoF3  is  formed  in  a  similar  manner.  The  nickel  salt  occurs  in  fine  green  plates,  easily 


658  PRINCIPLES  OF  CHEMISTEY 

solution  of  a  cobalt  salt  be  heated  almost  to  the  boiling-point,  and  the 
Solution  be  then  mixed  with  a  boiling  solution  of  an  alkali  hydroxide, 
a,  pink  precipitate  of  cobaltous  hydroxide,  CoH2O2,  will  be  formed.  If 
air  be  not  completely  excluded  during  the  precipitation  by  boiling,  the 
precipitate  will  also  contain  brown  cobaltic  hydroxide  formed  by  the 
further  oxidation  of  the  cobaltous  oxide.34  Under  similar  circumstances 
nickel  salts  form  a  green  precipitate  of  nickelous  hydroxide,  the  forma- 
tion of  which  is  not  hindered  by  the  presence  of  ammonium  salts,  but 
in  that  case  only  requires  more  alkali  to  completely  separate  the 
nickel.  The  nickelous  oxide  obtained  by  heating  the  hydroxide,  or 
from  the  carbonate  or  nitrate,  is  a  grey  powder,  easily  soluble  in  acids 
and  easily  reduced,  but  the  same  substance  may  be  obtained  in  the 
crystalline  form  as  an  ordinary  product  from  the  ores  ;  it  crystallises 
in  regular  octahedra,  with  a  metallic  lustre,  and  is  of  a  grey  colour. 
In  this  state  the  nickelous  oxide  almost  resists  the  action  of  acids.34  bl3 

soluble  in  water  but  scarcely  soluble  in  ethyl  and  methyl  alcohol.  They  decompose  into 
.green  oxide  of  nickel  and  potassium  fluoride  when  heated  in  a  current  of  air.  The 
analogous  salt  of  cobalt  crystallises  in  crimson  flakes. 

If  instead  of  potassium  fluoride,  CoCl2  or  NiCl2  be  fused  with  ammonium  fluoride, 
they  also  form  double  salts  with  the  latter.  This  gives  the  possibility  of  obtaining 
anhydrous  fluorides  NiF2  and  CoF2.  Crystalline  fluoride  of  nickel,  obtained  by 
beating  the  amorphous  powder  formed  by  decomposing  the  double  ammonium  salt  in 
a  stream  of  hydrofluoric  acid,  occurs  in  beautiful  green  prisms,  sp.  gr.  4'68,  which  are 
insoluble  in  water,  alcohol,  and  ether ;  sulphuric,  hydrochloric,  and  nitric  acids  also  have 
no  action  upon  them,  even  when  heated ;  NiF2  is  decomposed  by  steam,  with  the  forma- 
tion of  black  oxide,  which  retains  the  crystalline  structure  of  the  salt.  Fluoride  of 
cobalt,  obtained  as  a  rose-coloured  powder  by  decomposing  the  double  ammonium  salt 
with  the  aid  of  heat  in  a  stream  of  hydrofluoric  acid,  fuses  into  a  ruby-coloured  masa 
which  bears  distinct  signs  of  a  crystalline  structure;  sp.  gr.  4'4S.  The  molten  salt 
only  Volatilises  at  about  1400°,  which  forms  a  clear  distinction  between  CoF2  and  the 
volatile  NiF2.  Hydrochloric,  sulphuric,  and  nitric  acids  act  upon  CoF2  even  in  the  cold, 
although  slowly,  while  when  heated  the  reaction  proceeds  rapidly  (Poulenc,  1892). 

w  Hydrated  suboxide  of  cobalt  (de  Schulten,  1889)  is  obtained  in  the  following 
manner.  A  solution  of  10  grams  of  CoCl26H2O  in  60  c.c.  of  water  is  heated  in  a  flask 
with  250  grams  of  caustic  potash  and  a  stream  of  coal  gas  is  passed  through  the  solution. 
When  heated  the  hydrate  of  the  suboxide  of  cobalt  which  separates  out,  dissolves  in  the 
caustic  potash  and  forms  a  dark  blue  solution.  This  solution  is  allowed  to  stand  for  24 
hours  in  an  atmosphere  of  coal  gas  (in  order  to  prevent  oxidation).  The  crystalline  masa 
which  separates  out  has  a  composition  Co(OH)2,  and  to  the  naked  eye  appears  as  a  violet 
powder,  which  is  seen  to  be  crystalline  under  the  microscope.  The  specific  gravity  of 
this  hydrate  is  8'597  at  15°.  It  does  not  undergo  change  in  the  air ;  warm  acetic  acid 
dissolves  it,  but  it  is  insoluble  in  warm  and  cold  solutions  of  ammonia  and  sal- 
ammoniac. 

54  bis  The  following  reaction  may  be  added  to  those  of  the  cobaltous  and  nickelous 
salts:  potassium  cyanide  forms  a  precipitate  with  cobalt  salts  which  is  soluble  ify  an 
excess  of  the  reagent  and  forms  a  green  solution.  On  heating  this  and  adding  a  certain 
quantity  of  acid,  a  double  cobalt  cyanide  is  formed  which  corresponds  with  potassium 
.ferricyanide.-  Its  formation  is  accompanied  with  the  evolution  of  hydrogen,  and  is 
founded  upon  the  property  which  cobalt  has  of  oxidising  in  an  alkaline  solution,  the  de- 
velopment of  which  has  been-observed  in  such  a  considerable  measure  in  the  cobaltamine 
Baits.  The  process  which  goes  on  here  may  be  expressed  by  the  following  equation  ; 


IRON,  COBALT,  AND  NICKEL  359 

It  is  interesting  to  note  the  relation  of  the  cobaltous  and  nickelous 
hydroxides  to  ammonia  ;  aqueous  ammonia  dissolves  the  precipitate  of 
cobaltous  and  nickelous  hydroxide.  The  blue  ammoniacal  solution  of 
nickel  resembles  the  same  solution  of  cupric  oxide,  but  has  a  somewhat 
reddish  tint.  It  is  characterised  by  the  fact  that  it  dissolves  silk  in 
the  same  way  as  the  ammoniacal  cupric  oxide  dissolves  cellulose.  Am- 
monia likewise  dissolves  the  precipitate  of  cobaltous  hydroxide,  forming 
a  brownish  liquid,  which  becomes  darker  in  air  and  finally  assumes  a 
bright  red  hue,  absorbing  oxygen.  The  admixture  of  ammonium  chloride 
prevents  the  precipitation  of  cobalt  salts  by  ammonia  ;  when  the  am- 
monia is  added,  a  brown  solution  is  obtained  from  which,  as  in  the 
case  of  the  preceding  solution,  potassium  hydroxide  does  not  separate 
the  cobaltous  oxide.  Peculiar  compounds  are  produced  in  this  solution. ; 
they  are  comparatively  stable,  containing  ammonia  ancl  an  excess  of 
oxygen  ;  they  bear  the  name  cobaltoamine  and  cobaltiamine  salts.  They 
have  been  principally  investigated  by  Oenth,  Fremy,  Jorgenson  and 
others.  Genth  found  that  when  a  cobalt  salt,  mixed  with  an  excess  of 
ammonium  chloride,  is  treated  with  ammonia  and  exposed  to  the  air, 
after  a  certain  lapse  of  time,  on  adding  hydrochloric  acid  and  boiling, 
a  red  powder  is  precipitated  and  the  remaining  solution  contains  an 
orange  salt.  The  study  of  these  compounds  led  to  the  discovery  of  a 
whole  series  of  similar  salts,  some  of  which  correspond  with  particular 
higher  degrees  of  oxidation  of  cobalt,  which  are  described  later.35 

CoC2N2H-4KCN  first  forms  CoK4C6N6,  which  salt  with  water,  H2O,  forms  potassium 
hydroxide,  KHO,  hydrogen,  H,  and  the  salt,  K3CoC6N6.  Here  naturally  the  presence  of 
the  acid  is  indispensable  in  consequence  of  its  being  required  to  combine  with  the  alkali. 
From  aqueous  solutions  this  salt  crystallises  in  transparent,  hexagonal  prisms  of  a  yellow 
colour,  easily  soluble  in  water.  The  reactions  of  double  decomposition,  and  even  the 
formation  of  the  corresponding  acid,  are  here  completely  the  same  as  in  the  case  of  the 
ferricyanide.  If  a  nickelous  salt  be  treated  in  precisely  the  same  manner  as  that  just 
described  for  a  salt  of  cobalt,  decomposition  will  occur. 

35  The  cobalt  salts  may  be  divided  into  at  least  the  following  classes,  which  repeat 
themselves  for  Cr,  Ir,  Rh  (we  shall  not  stop  to  consider  the  latter,  particularly  as  they 
closely  resemble  the  cobalt  salts) :  — 

(a)  Ammonium  cobalt  salts,  which  are  simply  direct  compounds  of  the  cobaitoue 
<palts  CoX2  with  ammonia,  similar  to  various  other  compounds  of  the  salts  of  silver, 
copper,  and  even  calcium  and  magnesium,  with  ammonia.    They  are  easily  crystallised 
from  an  ammoniacal  solution,  and  have  a  pink  colour.    Thus,  for  instance,  when 
cobaltous  chloride  in   solution  is  mixed  with  sufficient  ammonia  to  redissolve    the 
precipitate  first  formed,  octahedral  crystals  are  deposited  which  have  a  composition 
CoCljjH^O.GNHj.     These  salts  are  nothing  else  but  combinations  with  ammonia  of 
crystallisation — if  it  may  be  so  termed — likening  them  in  this  way  to  combinations  with 
water  of  crystallisation.    This  similarity  is  evident  both  from  their  composition  and  from 
their  capability  of  giving  off  ammonia  at  various  temperatures.    The  most  important 
point  to  observe  is  that  all  these  salts  contain  6  molecules  of  ammonia  to  1  atom  of  cobalt, 
and  this  ammonia  isheld  in  fairly  stable  connection.  Water  decomposes  these  salts.  (Nickel 
behaves  similarly  without  forming  other  compounds  corresponding  to  the  true  cobaltic.) 

(b)  The  solutions  of  the  above-mentioned  salts  are  rendered  turbid  by  the  action  of 


860  PRINCIPLES  OF  CHEMISTRY 

Nickel  does  not  possess  this  property  of  absorbing  the  oxygen  of  the  air 
when  in  an  ammoniacal  solution.  In  order  to  understand  this  distinc- 

the  art ;  they  absorb  oxygen  and  become  covered  with  a  crust  of  oxycobaltamirie  salts. 
The  latter  are  sparingly  soluble  in  aqueous  ammonia,  have  a  brown  colour,  and  are 
characterised  by  the  fact  that  with  warm  water  they  evolve  oxygen,  forming  salts  of  the 
following  category :  The  nitrate  may  be  taken  as  an  example  of  this  kind  of  salt ;  its 
composition  is  CoN2O7,5NH3,H20.  It  differs  from  cobaltous  nitrate,  Co(NO3)2,  in  con- 
taining an  extra  atom  of  oxygen— that  is,  it  corresponds  with  cobalt  dioxide,  CoO2,  in 
the  same  way  that  the  first  salts  correspond  with  cobaltous  oxide ;  they  contain  5,  and 
oot  6,  molecules  of  ammonia,  as  if  NH3  had  been  replaced  by  O,  but  we  shall  afterwards 
meet  compounds  containing  either  5NH3  or  6NH3  to  each  atom  of  cobalt. 

(c)  The  luteocobaltic  salts  are  thus  called  because  they  have  a  yellow  (luteus) 
colour.    They  are  obtained  from  the  salts  of  the  first  kind  by  submitting  them  in  dilute 
solution  to  the  action  of  the  air  ;  in  this  case  salts  of  the  second  kind  are  not  formed, 
be'cause  they  are  decomposed  by  an  excess  of  water,  with  the  evolution  of  oxygen  and 
the  formation  of  luteocobaltic  salts.     By  the  action  of  ammonia  the  salts  of  the  fifth 
kind  (roseocobaltic)  are  also  converted  into  luteocobaltic  salts.    These  last-named  salts 
generally  crystallise  readily,  and  have  a  yellow  colour ;  they  are  comparatively  much 
more  stable  than  the  preceding  ones,  and  even  for  a  certain  time  resist  the  action  of 
boiling  water.     Boiling  aqueous  potash  liberates  ammonia  and  precipitates  hydrated 
cobaltic  oxide,  Co2O3,3H2O,  from  them.    This  shows  that  the  luteocobaltic  salts  corre- 
spond  with  cobaltic  oxide,  Co2O5,  and  those  of   the   second  kind  with  the  dioxide. 
When   a  solution   of  luteocobaltic   sulphate,  Co2(SO4)5,12NH3,4H2O,  is  treated  with 
baryta,  barium   sulphate    is    precipitated,    and    the    solution    contains    luteocobaltic 
hydroxide,  Co(OH)3,6NH3,  which  is  soluble  in  water,  is  powerfully  alkaline,  absorbs 
the  oxygen  of  the  air,  and  when   heated  is  decomposed   with   the  evolution  of  am- 
monia.    This  compound  therefore  corresponds  to  a  solution  of   cobaltic  hydroxide  in 
ammonia.    The  luteocobaltic   salts  contain   2   atoms  of  cobalt  and  12  molecules  of 
ammonia — that  is,  CNH3  to  each  atom  of  cobalt,  like  the  salts  of  the  first  kind.     The 
C«X2  salts  have  a  metallic  taste,  whilst  those  of  luteocobalt  and  others  have  a  purely 
saline  taste,  like  the  salts  of  the  alkali  metals.     In  the  luteo-salts  all  the  X's  react  (are 
ionised,  as  some  chemists  say)  as  in  ordinary  salts — for  instance,  all  the  C12  is  pre- 
cipitated by  a  solution  of  AgNO3 ;   all  the  (SO4)3  gives  a  precipitate  with  BaX2,  &c. 
The  double  salt  formed  with  PtCl4  is  composed  in  the  same  manner  as  the  potassium 
salt,  K2PtCl4  =  2KCl  +  PtCl4,that  is,  contains  (CoCl3)6NH3)2)8PtCl4,  or  the  amount  of 
chlorine  in  the  PtCl4  is  double  that  in  the  alkaline  salt.    In  the  rosepentamino  (c),  and 
rosetetramine  (/),  salts,  also  all  the  X's  react  or  are  ionised,  but  in  the  (g)  and  (h)  salts 
only  a  portion  of  the  X's  react,  and  they  are  equal  to  the  (e)  and  (/)  salts  minus  water ; 
this  means  that  although  the  water  dissolves  them  it  is  not  combined  with  them,  as 
PHO5  differs  from  PH3O5;   phenomena  of  this  class  correspond  exactly  to  what  has 
been  already  (Chapter  XXI.,  Note  7)  mentioned  respecting  the  green  and  violet  salts  of 
oxide  of  chromium. 

(d)  The  fuscocobaltic  salts.    An  ammoniacal  solution  of  cobalt  salts  acquires  a  brown 
colou   in  the  air,  due  to  the  formation  of  these  salts.    They  are  also  produced  by  the 
decomposition  of  salts  of  the  second  kind ;  they  crystallise  badly,  and  are  separated  from 
their  solutions  by  addition  of  alcohol  or  an  excess  of  ammonia.    When  boiled  they  give 
op  the  ammonia  and  cobaltic  oxide  which  they  contain.    Hydrochloric  and  nitric  acids 
give  a  yellow  precipitate  with  these  salts,  which  turns  red  when  boiled,  forming  salts  of 
the  next  category.    The  following  is  an  example  of  the  composition  of  two  of  the  fusco-' 
cobaltic  salts,  Co2O(S(X,)7,8NH3,4H2O  and  Co2OCl4,8NH5,8H2O.    It  is  evident  that  the 
fuscocobaltic  salts  are  ammoniacal  compounds  of  basic  cobaltic  salts.     The  normal  co- 
baltio  sulphate  ought  to  have  the  composition  Co2(SO4)3  =  Co2O3,8SO3 ;    the  simplest 
basic  salts  will  be  Co2O(SO4)2  =  Co2O3r2SO3,  and  Co2O2(SO4)  =  Co2O3,SO3.    The  fusco- 
cobaltic salts  correspond  with  the  first  type  of  basic  salts.    They  are  changed  (in  con- 
centrated  solutions)  into  oxycobaltamine  salts  by  absorption  of  one  atom  of  oxygen, 


IEON,   COBALT,  AND  NICKEL  361 

tion,  and  in  general  the  relation  of  nickel,  it  is  important  to  observe 
that  cobalt  more  easily  forms  a  higher  degree  of  oxidation— namely, 

Co2O2(S04)2.  The  whole  process  of  oxidation  will  be  as  follows :  first  of  all  Co2X4,  a 
cobaltous  salt,  is  in  the  solution  (X  a  univalent  haloid,  2  molecules  of  the  salt  being 
taken),  then  Co2OX4,  the  basic  cobaltic  salt  (4th  series),  then  Co2O2X4,  the  salt  of  the 
dioxide  (2nd  series);  The  series  of  basic  salts  with  an  acid,  2HX,  forms  water  and  a 
normal  salt,  Co2X6  (in^S,  5,  6  series).  These  salts  are  combined  with  various  amounts  o! 
•water  and  ammonia.  XJnder  many  conditions  the  salts  of  fuscocobalt  are  easily  trans- 
formed into  salts  oi  the  next  series.  The  salts  of  the  series  that  has  just  been  described 
contain  4  molecules  of  ammonia  to  1  atom  of  cobalt, 

(e)  'The  roseocobaltic  (or  rosepentamine),  CoX2H20,5NH3,  salts,  like  the  luteo- 
cobaltic,  correspond  with  the  normal  cobaltic  salts,  but  contain  less  ammonia,  and  an 
extra  molecule  of  water.  Thus  the.  sulphate  is  obtained  from  cobaltous  sulphate 
dissolved  in  ammonia  and  left  exposed  to  the  air  until  transformed  into  a  brown  solution 
of  the  fuscocobaltic  salt ;  when  this  is  treated  with  sulphuric  acid  a  crystalline  powder 
of  the  roseocobaltic  salt,  Co2(S04)3,10NH3,5H20,  separates.  The  formation  of  this  salt 
is  easily  understood  :  cobaltous  sulphate  in  the  presence  of  ammonia  absorbs  oxygen,  and 
the  solution  of  the  fuscocobaltic  salt  will  therefore  contai.n,  like  cobaltous  sulphate,  one 
part  of  sulphuric  acid  to  every  part  of  cobalt,  so  that  the  whole  process  of  formation  may 
be  expressed  by  the  equation:  10NH3  +  2CoSO4  +  H2S04  +  4H2O  +  O  =  Co2(S04)3,10NH3, 
5H2O.  This  salt  forms  tetragonal  crystals  of  a  red  colour,  slightly  soluble  in  cold,  but 
readily  soluble  in  warm  water.  When  the  sulphate  is  treated  with  baryta,  roseocobaltic 
hydroxide  is  formed  in  the  solution,  which  absorbs  the  carbonic  anhydride  of  the  air. 
It  is  obtained  from  the  next  series  by  the  action  of  alkalis. 

(/)  The  rosetetramine  cobaltic  salts  CoCl2,2H20,4NH3  were  obtained  by  Jbrgenson, 
and  belong  to  the  type  of  the  luteo-salts,  only  with  the  substitution  of  2NH3  for  H2O. 
Like  the  luteo-  and  roseo-salts  they  give  double  salts  with  PtCl4,  similar  to  the  alkaline 
double  salts,  for  instance  (Co2H2O,4NH3)2(SO4)2Cl2PtCl4.  They  are  darker  in  colour 
than  the  preceding,  but  also  crystallise  well.  They  are  formed  by  dissolving  CoCO5  in 
sulphuric  acid  (of  a  given  strength),  and  after  NH3  and  carbonate  of  ammonium  have 
been  added,  air  is  passed  through  the  solution  (for  oxidation)  until  the  latter  turns  red. 
It  is  then  evaporated  with  lumps  of  carbonate  of  ammonium,  filtered  from  the  precipi- 
tate and  crystallised.  A  salt  of  the  composition  Co2(C03)2(S04),  (2H2O,4NH3)2  is  thus 
obtained,  from  which  the  other  salts  may  be  easily  prepared. 

(g)  The  purpureocobaltic  salts,  CoX3,5NH3,  are  also  products  of  the  direct  oxidation 
of  ammoniacal  solutions  of  cobalt  salts.  They  are  easily  obtained  by  heating  the  roseo- 
cobaltic and  luteo-salts  with  strong  acids.  They  are  to  all  effects  the  same  as  the 
roseocobaltic  salts,  only  anhydrous.  Thus,  for  instance,  the  purpureocobaltic  chloride, 
Co2Cl6,10NH3,  or  CoCl3,5NH3,  is  obtained  by  boiling  the  oxycobaltamine  salts  with 
ammonia.  There  is  the  same  distinction  between  these  salts  and  the  preceding  ones  as 
between  the  various  compounds  of  cobaltous  chloride  with  water.  In  the  purpureo- 
cobaltic only  X2  out  of  the  X3  react  (are  ionised)  To  the  rosetetramine  salts  (/)  there 
correspond  the  purpureotetramine  salts,  CoX3H2O,4NH3.  The  corresponding  chromium 
purpureopentamine  salt,  CrCl3,6NH3  is  obtained  with  particular  ease  (Christensen,  1893). 
Dry  anhydrous  chromium  chloride  is  treated  with  anhydrous  liquid  ammonia  in  a 
freezing  mixture  composed  of  liquid  CO2  and  chlorine,  and  after  some  time  the  mixture 
is  taken  out  of  the  freezing  mixture,  so  that  the  excess  of  'NH3  boils  away ;  the  violet 
crystals  then  immediately  acquire  the  red  colour  of 'the  salt,  CrCl3,5NH3,  which  is  formed. 
The  product  is  washed  with  water  (to  extract  the  luteo-salt,  CrCl3,6NH5),  which  does  not 
dissolve  the  salt,  and  it  is  then  recrystallised  from  a  hot  solution  of  hydrochloric  acid. 

(h)  The  prazeocobaltic  salts,  CoX3,4NH3,  are  green,  and  form,  with  respect  to  the 
tosetetramine  salts  (/),  the  products  of  ultimate  dehydration  (for  example,  like  meta- 
phosphoric  acid  with  respect  to  orthophosphoric  acid,  but  in  dissolving  in  water  they  give 
neither  rosetetramine  nor  tetramine  salts.  (In  my  opinion  one  should  expect  salts  with 
a  still  smaller  amount  of  NH3,  of  the  blue  colour  proper  to  the  low  hydrated  compounds 


362  PRINCIPLES  OF  CHEMISTRY 

nesquioxide  of  cobalt,  cobaltic  oxide,  Co2O3 — than  nickel,  especially  in 
the  presence  of  hypochlorous  acid.  If  a  solution  of  a  cobalt  salt  be 

of  cobalt ;  the  green  colour  of  the  prazeo-salts  already  forma  a  step  towards  the  blue.) 
Jb'rgenson  obtained  salts  for  ethylene-diamine,  N2H4C2H4  which  replaces  2NH5.  After 
being  kept  a  long  time  in  aqueous  solution  they  give  rosetetramine  salts,  just  as  meta- 
phosphoric  acid  gives  orthophosphoric  acid,  while  the  rosetetramine  salts  are  converted 
into  prazeo-salts  by  Ag2O  and  NaHO.  Here  only  one  X  is  ionised  out  of  the  X5.  There 
are  also  basic  salts  of  the  same  type;  but  the  best  known  is  the  chromium  salt  called  the 
rhodozochromic  salt,  Cr2(OH)3Cl3,6NH3,2H2O,  which  is  formed  by  the  prolonged  action 
of  water  upon  the  corresponding  roseo-salt.- 

The  cobaltamine  compounds  differ  essentially  but  little  from  the  ammoniacal  com- 
pounds of  other  metals.  The  only  difference  is  that  here  the  cobaltic  oxide  is  obtained 
from  the  cobaltous  oxide  in  the  presence  of  ammonia.  In  any  case  it  is  a  simpler  question 
than  that  of  the  double  cyanides.  Those  forces  in  virtue  of  which  such  a  considerable 
number  of  ammonia  molecules  are  united  with  a  molecule  of  a  cobalt  compound,  apper- 
tain naturally  to  the  series  of  those  slightly  investigated  forces  which  exist  even  in  the 
highest  degrees  of  combination  of  the  majority  of  elements.  They  are  the  same  forces' 
which  lead  to  the  formation  of  compounds  containing  water  of  crystallisation,  double 
salts,  isomorphous  mixtures  and  complex  acids '  (Chapter  XXI.,  Note  8  bis).  The 
simplest  conception,  according  to  my  opinion,  of  cobalt  compounds  (much  more  so  than 
by  assuming  special  complex  radicles,  with  Schiff,  Weltzien,  Glaus,  and  others),  may  be 
formed  by  comparing  them  with  other  ammoniacal  products.  Ammonia,  like  water,  com- 
bines in  vanous  proportions  with  a  multitude  of  molecules.  Silver  chloride  and  calcium 
chloride,  just  like  cobalt  chloride,  absorb  ammonia,  forming  compounds  which  are  some- 
times slightly  stable,  and  easily  dissociated,  sometimes  more  stable,  in  exactly  the  same 
way  as  water  combines  with  certain  substances,  forming  fairly  stable  compounds  called 
hydroxides  or  hydrates,  or  less  stable  compounds  which  are  called  compounds  with  water 
of  crystallisation.  Naturally  the  difference  in  the  properties  in  both  cases  depends  on 
the  properties  of  those  elements  which  enter  into  the  composition  of  the  given  substance, 
and  on  those  kinds  of  affinity  towards  which  chemists  have  not  as  yet  turned  their 
attention.  If  boron  fluoride,  silicon  fluoride,  &c.,  combine  with  hydrofluoric  acid,  if 
platinic  chloride,  and  even  cadmium  chloride,  combine  with  hydrochloric  acid,  these 
compounds  may  be  regarded  as  double  salts,  because  acids  are  salts  of  hydrogen.  But 
evidently  water  and  ammonia  have  the  same  saline  faculty,  more  especially  an  they,  like 
haloid  acids,  contain  hydrogen,  and  are  both  capable  of  further  combination — for  instance, 
ammonia  with  hydrochloric  acid.  Hence  it  is  simpler  to  compare  complex  ammoniacal 
with  double  salts,  hydrates,  and  similar  compounds,  but  the  ammonia-metallic  salts 
present  a  most  complete  qualitative  and  quantitative  resemblance  to  the  hydrated  salts 
of  metals.  The  composition  of  the  latter  is  MXnwH2O,  where  M  =  metal,  X  =  the 
haloid,  simple  or  complex,  and  n  and  m  the  quantities  of  the  haloid  and  so-called  water 
of  crystallisation  respectively.  The  composition  of  the  ammoniacal  salts  of  metals  is 
MXnmNH3.  The  water  of  crystallisation  is  held  by  the  salt  with  more  or  less  stability,  and 
some  salts  even  do  not  retain  it  at  all ;  some  part  with  water  easily  when  exposed  to  the  air, 
others  when  heated,  and  then  with  difficulty.  In  the  case  of  some  metals  all  the  salts  com- 
bine with  water,  whilst  with  others  only  a  few,  and  the  water  so  combined  may  then  be 
easily  disengaged.  All  this  applies  equally  well  to  the  ammoniacal  salts,  and  therefore  the 
combination  of  ammonia  may  be  termed  the  ammonia  of  crystallisation.  Just  as  the 
water  which  is  combined  with  a  salt  is  held  by  it  with  different  degrees  of  force,  so  it  is  with 
ammonia.  In  combining  with  2NH5,  PtCl2  evolves  81,000  cals. ;  while  CaCl2  only  evolves 
14,000  cals. ;  and  the  former  compound  parts  with  its  NHS  (together  with  HC1  in  this 
case)  with  more  difficulty,  only  above  2003,  while  the  latter  disengages  ammonia  at  180°. 
ZnCl2,2NHs  in  forming  ZnCl2,  4NH3  evolves  only  11,000  cals.,  and  splits  up  again  into 
its  components  at  80°.  The  amount  of  combined  ammonia  is  as  variable  as  the  amount 
of  water  of  crystallisation— for  instance,  Snl^NHs.CrCljSNHj.CrClseNHs.CrClsKNHs, 
PtCl34NHs,  &c.  are  known.  Very  often  NH*  is  replaceable  by  OH2  and  conversely.  A. 


IRON,  COBALT,  AND  NICKEL  363 

mixed  with  barium  carbonate  and  an  excess  of  hypochlorous  acid  be 
added,  or  chlorine  gas  be  passed  through  it,  then  at  the  ordinary 

colourless,  anhydrous  cupric  salt — for  instance,  cupric  sulphate — when  combined  with 
water  forms  blue  and  green  salts,  and  violet  when  combined  with  ammonia.  If  steam  b© 
passed  through  anhydrous  copper  sulphate  the  salt  absorbs  water  and  becomes  heated ;  if 
ammonia  be  substituted  for  the  water  the  heating  becomes  much  more  intense,  and  the 
salt  breaks  up  into  a  fine  violet  powder.  With  water  CuS04,5H2O  is  formed,  and  with 
ammonia  CuS04,5NH3)  the  number  of  water  and  ammonia  molecules  retained  by  the 
Bait  being  the  same  in  each  case,  and  as  a  proof  of  this,  and  that  it  is  not  an  isolated 
coincidence,  the  remarkable  fact  must  be  borne  in  mind  that  water  and  ammonia  con- 
secutively, molecule  for  molecule,  are  capable  of  supplanting  each  other,  and  forming  the 
compounds  CuS04,5H2O,  CuS04)4H2O,NH3;  CuS04,3H2O,2NH3 ;  CuS04,2H20,3NH3 ; 
CuSO4,H2O,4NH3,  and  CuSO4,5NH3.  The  last  of  these  compounds  was  obtained  by 
Henry  Rose,  and  my  experim'ents  have  shown  that  more  ammonia  than  this  cannot  be 
retained.  By  adding  to  a  strong  solution  of  cupric  sulphate  sufficient  ammonia  to 
dissolve  the  whole  of  the  oxide  precipitated,  and  then  adding  alcohol,  Berzelius  obtained 
the  compound  CuS04,H20,4NH3,  &c.  The  law  of  substitution  also  assists  in  rendering 
these  phenomena  clearer,  because  a  compound  of  ammonia  with  water  forms  ammonium 
hydroxide,  NH4HO,  and  therefore  these  molecules  combining  with  one  another  may  also 
interchange,  as  being  of  equal  value.  In  general,  those  salts  form  stable  ammoniacal 
compounds  which  are  capable  of  forming  stable  compounds  with  water  of  crystallisation ; 
and  as  ammonia  is  capable  of  combining  with  acids,  and  as  some  of  the  salts  formed  by 
slightly  energetic  bases  in  their  properties  more  closely  resemble  acids  (that  is,  salts  of 
hydrogen)  than  those  salts  containing  more  energetic  bases,  we  might  expect  to  find 
more  stable  and  more  easily-formed  ammonio-metallic  salts  with  metals  and  their 
oxides  having  weaker  basic  properties  than  with  those  which  form  energetic  bases.  Thi.s 
explains  why  the  salts  of  potassium,  barium,  &c.,  do  not  form  ammonio-metallic  salts, 
whilst  the  salts  of  silver,  copper,  zinc,  &c.,  easily  form  them,  and  the  salts  RX3  still 
more  easily  and  with  greater  stability.  This  consideration  also  accounts  for  the  great 
stability  of  the  ammoniacal  compounds  of  cupric  oxide  compared  with  those  of  silver 
oxide,  since  the  former  is  displaced  by  the  latter.  It  also  enables  us  to  see  clearly  the 
distinction  which  exists  in  the  stability  of  the  cobaltamine  salts  containing  salts  corre- 
ponding  with  cobaltous  oxide,  and  those  corresponding  with  higher  oxides  of  cobalt, 
for  the  latter  are  weaker  bases  than  cobaltous  oxides.  The  nature  of  the  forces 
and  quality  oj  the  phenomena  occurring  during  the  formation  of  the  most  stable  sub- 
stances, and  of  such  compounds  as  crystallisable  compounds,  are  one  and  the  same, 
although  perhaps  exhibited  in  a  different  degree.  This,  in  my  opinion,  may  be  best 
cdnfirm,  d  by  examining  the  compounds  of  carbon,  because  for  'this  element  the  nature 
of  the  forces  acting  during  the  formation  of  its  compounds  is  well  known.  Let  us  take 
as  an  example  two  unstable  compounds  of  carbon.  Acetic  acid,  C2H4O2  (specific  gravity 
1-06),  with  water  forms  the  hydrate,  C2H402,H20,  denser  (1'07)  than  either  of  the  com- 
ponents, but  unstable  and  easily  decomposed,  generally  simply  referred  to  as  & 
solution.  Such  also  is  the  crystalline  compound  of  ojcalic  acid,  C2H2O4,  with  water, 
C2H2O4)2H20.  Their  formation  might  be  predicted  as  starting  from  the  hydrocarbon 
CjH6,  in  which,  as  in  any  other,  the  hydrogen  may  be  exchanged  for  chlorine,  the 
water  residue  (hydroxyl),  &c.  The  first  substitution  product  with  hydroxyl,  C2H5(HO), 
is  stable ;  it  can  be  distilled  without  alteration,  resists  a  temperature  higher  than  100°, 
and  then  does  not  give  off  water.  This  is  ordinary  alcohol.  The  second,  C2H4(HO)2, 
Can  also  be  distilled  without  change,  but  can  be  decomposed  into  water  and  C2HaO 
(ethylene  oxide  or  aldehyde) ;  it  boils  at  about  197°,  whilst  the  first  hydrate  boils  at  78°, 
a  difference  of  about  100°  The  compound  C2H3(HO)3  will  be  the  third  product  of  such 
substitution  ;  it  ought  to  boil  at  about  300°,  but  does  not  resist  this  temperature — it  de- 
Composes  into  H20  and  CiH402,  where  only  one  hydroxyl  group  remains,  and  the  other 
atom  of  oxygen  is  left  in  the  same  condition  as  in  ethyler.e  oxide,  C2H40.  There  is  a  proof 
of  this.  Glycol,  C2H4(HO)2,  boils  at  197°,  and  forms  water  and  ethylene  oxide,  which 


364  PRINCIPLES  OF  CHEMISTRY 

temperature  on  shaking,  the  whole  of  the  cobalt  will  be  separated 
in  the  form  of  black  cobaltic  oxide  :  2CoSO4  -f  C1HO  +  2BaCO3 

boils  at  13°  (aldehyde,  its  isomeride,  boils  at  21°) ;  therefore  the  product  disengaged  by 
the  splitting  up  of  the  hydrate  boils  at  184°  lower  than  the  hydrate  C2H4(HO)2.  Thus 
the  hydrate  C2H3(HO)3,  which  ought  to  boil  at  about  800°,  splits  up  in  exactly  the  same 
way  into  water  and  the  product  C2HjO2,  which  boils  at  117° — that  is,  nearly  183°  lower 
than  the  hydrate,  C2H3(HO)3.  But  this  hydrate  splits  up  before  distillation.  The 
above-mentioned  hydrate  of  acetic  acid  is  such  a  decomposable  hydrate — that  is  to 
Bay,  what  is  called  a  solution.  Still  less  stability  may  be  expected  from  the  following 
hydrates.  C2H2(HO)4  also  splits  up  into  water  and  a  hydrate  (it  contains  two  hydroxyl 
groups)  called  glycollic  acid,  C2H20(HO)2=C2H4O3.  The  next  product  of  substitution 
will  be  O2H(HO)5;  it  splits  up  into  water,  H2O,  and  glyoxylic  acid,  C2H404  (three 
hydroxyl  groups).  The  last  hydrate  which  ought  to  be  obtained  from  C2H6,  and  ought 
to  contain  C2(HO)6,  is  the  crystalline  compound  of  oxalic  acid,  C2H.2O4  (two  hydroxyl 
groups),  and  water,  2H2O,  which  has  been  already  mentioned.  The  hydrate  C2(HO)a 
=  C2Ha04,2H?O,  ought,  according  to  the  foregoing  reasoning,  to  boil  at  about  600° 
(because  the  hydrate,  C2H4(HO)2,  boils  at  about  200°,  and  the  substitution  of  4  hydroxyl 
groups  for  4  atoms  of  hydrogen  will  raise  the  boiling-point  400°).  It  does  not  resist  this 
temperature,  but  at  a  much  lower  point  splits  up  into  water,  2H20,  and  the  hydrate 
C202(HO)2,  which  is  also  capable  of  yielding  water.  Without  going  into  further  dis- 
cussion of  this  subject,  it  may  be  observed  that  the  formation  of  the  hydrates,  or  com- 
pounds with  water  of  crystallisation,  of  acetic  and  oxalic  acids  has  thus  received  an 
accurate  explanation,  illustrating  the  point  we  desired  to  prove  in  affirming  that  com- 
pounds with  water  of  crystallisation  are  held  together  by  the  same  forces  as  those  which 
act  in  the  formation  of  other  complex  substances,  and  that  the  easy  displaceability 
of  the  water  of  crystallisation  is  only  a  peculiarity  of  a  local  character,  and  nok 
a  radical  point  of  distinction.  All  the  above-mentioned  hydrates,  C2X6,  or  pro- 
ducts of  their  destruction,  are  actually  obtained  by  the  oxidation  of  the  first  hydrate, 
C2H5(HO),  or  common  alcohol,  by  nitric  acid  (Sokoloff  and  others).  Hence  the  forces 
which  induce  salts  to  combine  with  nH2O  or  with  NH3  are  undoubtedly  of  the  same 
order  as  the  forces  which  govern  the  formation  of  ordinary  '  atomic '  and  saline  com- 
pounds. (A  great  impediment  in  the  study  of  the  former  was  caused  by  the  conviction 
which  reigned  in  the  sixties  and  seventies,  that  'atomic'  were  essentially  different 
from  'molecular'  compounds  like  crystallohydrates,  in  which  it  was  assumed  that 
there  was  a  combination  of  entire  molecules,  as  though  without  the  participation  of  the 
atomic  forces.)  If  the  bond  between  chlorine  and  different  metals  is  not  equally  strong, 
so  also  the  bond  uniting  nH2O  and  »iNH3  is  exceeding  variable;  there  is  nothing  very 
surprising  in  this.  And  in  the  fact  that  the  combination  of  different  amounts  of  NH5 
and  H2O  alters  the  capacity  of  the  haloids  X  of  the  salts  RX2  for  reaction  (for  instance, 
in  the  luteo-salts  all  the  X3,  while  in  the  purpureo,  only  2  out  of  the  8,  and  in  the  prazeo- 
«alts  only  1  of  the  8  X's  reacts),  we  should  see  in  the  first  place  a  phenomenon  similar 
to  what  we  met  with  in  Cr2Cl<j  (Chapter  XXI.,  Note  7  bis),  for  in  both  instances  the  essence 
of  the  difference  lies  in  the  removal  of  water;  a  molecule  RC13,6H20  or  RC15,6NH3 
oontains  the  halogen  in  a  perfectly  mobile  (ionised)  state,  while  in  the  molecule 
HC13)5H2O  or  RC13,5NH3  a  portion  of  the  halogen  has  almost  lost  its  faculty  for  reacting 
•with  AgNO5)  just  as  metalepsical  chlorine  has  lost  this  faculty  which  is  fully  developed  in 
the  chloranhydride.  tJntil  the  reason  of  this  difference  be  clear,  we  cannot  expect  that 
ordinary  points  of  view  and  generalisation  can  give  a  clear  answer.  However,  we  may 
assume  that  here  the  explanation  lies  in  the  nature  and  kind  of  motion  of  the'atoms  in  the 
molecules,  although  as  yet  it  is  not  clear  how.  Nevertheless,  I  think  it  well  to  call 
attention  again  (Chapter  I.)  to  the  fact  that  the  combination  of  water,  and  hence,  also, 
of  any  other  element,  leads  to  most  diverse  consequences ;  the  water  in  the  gelatinous 
tydrate  of  alumina  or  in  the  decahydrated  Glauber  salt  is  very  mobile,  and  easily  reacts 
like  water  in  a  free  state  ;  but  the  same  water  combined  with  oxide  of  calcium,  or  C2H4 
(for  instance,  in  C2HeO  and  in  C4H10O),or  with  P2O5,has  become  quite  different,  and  no 


IR01S,   COBALT,   AND  NICKEL  865 

=Co203  -f  2BaS04  4-  HC1  +  2CO2.  Under  these  circumstances  nickelous 
oxide  does  not  immediately  form  black  sesquioxide,  but  after  a  consider- 
able space  of  time  it  also  separates  in  the  form  of  sesquioxide,  Ni2O3, 
but  always  later  than  cobalt.  This  is  due  to  the  relative  difficulty  of 
further  oxidation  of  the  nickelous  oxide.  It  is,  however,  possible  to 
oxidise  it ;  if,  for  instance,  the  hydroxide  NiH202  be  shaken  in  water 
and  chlorine  gas  be  passed  through  it,  then  nickel  chloride  will  be 
formed,  which  is  soluble  in  water,  and  insoluble  nickelic  oxide  in  the 
form  of  a  black  precipitate:  3NiH202  +  Cl2=NiCl2-r-Ni2O3,3H20. 
Nickelic  oxide  may  also  be  obtained  by  adding  sodium  hypochlorite 
mixed  with  alkali  to  a  solution  of  a  nickel  £alt.  Nickelic  and  cobaltio 
hydrates  are  black.  Nickelic  oxide  evolves  oxygen  with  all  acids,  and 
in  consequence  of  this  it  is  not  separated  as  a  precipitate  in  the  presence 
of  acids  ;  thus  it  evolves  chlorine  with  hydrochloric  acid,  exactly  like 
manganese  dioxide.  When  nickelic  oxide  is  dissolved  in  aqueous 
ammonia  it  liberates  nitrogen,  and  an  ammoniacal  solution  of  nickelous 
oxide  is  formed.  When  heated,  nickelic  oxide  loses  oxygen,  forming 

longer  acts  like  water  in  a  free  state.  We  see  the  same  phenomenon  in  many  other 
cases— for  example,  the  chlorine  in  chlorates  no  longer  gives  a  precipitate  of  chloride  of 
silver  with  AgNO3.  Thus,  although  the  'instance  which  is  found  in  the  difference 
between  the  roseo-  and  purpureo-salts  deserves  to  be  fully  studied  on  account  of  its  sim- 
plicity, still  it  is  far  from  being  exceptional,  and  we  cannot  expect  it  to  be  thoroughly 
explained  unless  a  mass  of  similar  instances,  which  are  exceedingly  common  among 
chemical  compounds,  be  conjointly  explained.  (Among  the  researches  which  add  to 
our  knowledge  respecting  the  complex  ammoniacal  compounds,  I  think  it  indispensable 
to  call  the  reader's  attention  to  Prof.  Kournakoff's  dissertation  '  On  complex  metallic 
bases,'  1893.) 

Kournakoff  (1894)  showed  that  the  solubility  of  the  luteo-salt,  CoCl3,6NH3,  at  0° 
=  4-30  (per  100  of  water),  at  20°  =  7'7,  that  in  passing  into  the  roseo-salt,CoCl5H205NH3, 
the  solubility  rises  considerably,  and  at  0°  =  16'4,  and.  at  20°= about  27,  whilst  the 
passage  into  the  purpureo-salt,  CoCl3,5NH3,  is  accompanied  by  a  great  fall  in  the 
solubility,  namely,  at  0°  =  0'28,  and  at  20°  =  about  0'5.  And  as  crystallohydrates  with  a 
smaller  amount  of  water  are  usually  more  soluble  than  the  higher  crystallohydrates  (Le 
Chatelier),- whilst  here  we  find  that  the  solubility  falls  (in  the  purpureo-salt)  with  a  loss 
of  water,  that  water  which  is  contained  in  the  roseo-salt  cannot  be  compared  with  the 
water  of  crystallisation.  Kournakoff,  therefore,  connects  the  fall  in  solubility  (in  the 
passage  of  the  roseo-  into  the  purpureo-salts)  with  the  accompanying  loss  in  the  reactive 
capacity  of  the  chlorine. 

In  conclusion,  it  may  be  observed  that  the  elements  of  the  eighth  group — that  is,  the 
analogues  of  iron  and  platinum — according  to  my  opinion,  will  yield  most  fruitful  results 
when  studied  as  to  combinations  with  whole  molecules,  as  already  shown'  by  the  examples 
of  complex  ammoniacal,  cyanogen,  nitro-,  and  other  compounds,  which  are  easily  formed 
in  this  eighth  group,  and  are  remarkable  for  their  stability.  This  faculty  of  the  elements 
of  the  eighth  group  for  forming  the  complex  compounds  alluded  to,  is  in  all  probability 
connected  with  the  position  which  the  eighth  group  occupies  with  regard  to  the  others. 
Following  the  seventh,  whi'ch  forms  the  type  RX7,  it  might  be  expected  to  contain  \he 
most  complex  type,  BX8.  This  is  met  with  in  Os04.  The  other  elements  of  the  eighth 
group,  however,  only  form  the  lower  types  RX2,  RX3,  RX4  ....  and  these  accordingly 
should  be  expected  to  aggregate  themselves  into  the  higher  types,  which  is  accom- 
plished in  the  formation  of  the  above-mentioned  complex  compounds. 


366  PRINCIPLES  OF  CHEMISTRY 

nickelous  oxide.  Cobaltic  oxide,  Co2O3,  exhibits  more  stability  than 
nickelic  oxide,  and  shows  feeble  basic  properties  ;,  thus  it  is  dissolved 
in  acetic  acid  without  the  evolution  of  oxygen.35  bis  But  ordinary  acids, 
especially  on  heating,  evolve  oxygen,  forming  a'  solution  of  a  cobaltous 
salt.  The  presence  of  a  cobaltic  salt  in  a  solution  of  a  cobaltous  salt 
may  be  detected  by  the  brown  colour  of  the  solution  and  the  black 
precipitate  formed  by  the  addition  of  alkali,  and  also  from  the  fact  that 
such  solutions  evolve  chlorine  when  heated  with  hydrochloric  acid, 
Cobaltic  oxide  may  not  only  be  prepared  by  the  above-mentioned 
methods,  but  also  by  heating  cobalt  nitrate,  after  which  a  steel-coloured 
mass  remains  which  retains  traces  of  nitric  acid,  but  when  heated 
further  to  incandescence  evolves  oxygen,  leaving  a  compound  of 
cobaltic  and  cobaltous  oxides,  similar  to  magnetic  ironstone.  Cobalt 
(but  not  nickel)  undoubtedly  forms  besides  Co2O3  a  dioxide.  Co02. 
This  is  obtained  36  when  the  cobaltous  oxide  is  oxidised  by  iodine  or 
peroxide  of  barium.37 

35  bis  Marshall  (1891)  obtained  cobaltic  sulphate,  Co2(S04)3,18H3O,  by  the  action  of  an 
electric  current  upon  a  strong  solution  of  CoSO4. 

36  The  action  of  an  alkaline  hypochlorite  or  hypobromite  upon  a  boiling  solution  of 
cobaltous  salts,  according  to  Schroederer  (1889),  produces  oxides,  whose  composition 
varies  between  Co3Os  (Hose's  compound)  and  Co2O3,  and  also  between  Co5O8  and 
Co12O19.      If  caustic  potash  and  then  bromine  be  added  to  the  liquid,  only  Co^Os  is 
formed.    The  action  of  alkaline  hypochlorites  or  hypo-bromites,  or  of  iodine,  upon 
cobaltic  salts,  gives  a  highly-coloured  precipitate  which  has  a  different  colour  to  the 
hydrate  of  the  oxide  Co2(OH)6.    According  to  Carnot  the  precipitate  produced  by  the 
hypochlorites  has  a  composition  Co10Oi6,  whilst  that  given  by  iodine  in  the  presence  of 
an  alkali  contains  a  larger  amount  of  oxygen.    Fortmann  (1891)  reinvestigated  the 
composition  of  the  higher  oxygen  oxide  obtained  by  iodine  in  the  presence  of  alkali,  and 
found  that  the  greenish  precipitate  (which  disengages  oxygen  when  heated  to  100°) 
corresponds  to  the  formula  CoO9.    The  reaction  must  be  expressed  by  the  equation: 
CoX8 + 12  +  4KHO  =  Co02 + 2KX + 2KI  +  2H2O. 

37  Prior  to  Fortmann,  Bousseau  (1889)  endeavoured  to  solve  the  question  as  to 
•whether  CoO2  was  able  to  combine  with  bases.    He  succeeded  in  obtaining  a  barium 
compound  corresponding  to  this  oxide.    Fifteen  grams  of  BaCl2  or  BaBr2  are  triturated 
with  5-6  grams  of  oxide  of  barium,  and  the  mixture  heated  to  redness  in  a  closed 
platinum  crucible ;  1  gram  of  oxide  of  cobalt  is  then  gradually  added  to  the  fused  mass. 
Each  addition  of  oxide  is  accompanied  by  a  violent  disengagement  of  oxygen.    After  a 
short  time,  however,  the  mass  fuses  quietly,  and  a  salt  settles  at  the  bottom  of  the 
crucible,  which,  when  freed  from  the  residue,  appears  as  black  hexagonal,  very  brilliant 
crystals.    In  dissolving  in  water  this  substance  evolves  chlorine ;  its  composition  corre- 
sponds to  the  formula  2(CoO2)BaO.    If  the  original  mass  be  neated  for  a  long  time 
(40  hours),  the  amount  of  dioxide  in  the   resultant  mass  decreases.    The  author  ob- 
tained a  neutral  salt  having  the  composition  CoO^BaO   (this  compound  =  BaO2CoO) 
by  breaking  up  the  mass  as  it  agglomerates  together,  and  bringing  the  pieces  into 
contact  with  the  more  heated  surface  of  the  crucible.    This  salt  is  formed  between  the 
somewhat  narrow  limits  of  temperature  1,000°-1,100° ;  above  and  below  these  limits 
compounds  richer  or  poorer  in  CoO2  are  formed.     The  formation  of  CoO2  by  the  action 
of  BaO2,  and  the  easy  decomposition  of  CoO2  with  the  evolution  of  oxygen,  give  reason 
for  thinking  that  it  belongs  tQ  the  class  of  peroxides  (like  Cr2O7,  CaO2,  &c.) ;  it  is  not  yet 
tnown  whether  they  give  peroxide  of  hydrogen  like  the  true  peroxides.    The  fact  that 


IRON,   COBALT,   AND  NICKEL  867 

Nickel  alloys  possess  qualities  which  render  them  valuable  for 
technical  purposes,  the  alloy  of  nickel  with  iron  being  particularly 
remarkable.  This  alloy  is  met  with  in  nature  as  meteoric  iron.  The 
Pallasoffsky  mass  of  meteoric  iron,  preserved  in  the  St.  Petersburg 
Academy,  fell  in  Siberia  in  the  last  century  j  it  weighs  about  15  cwt. 
and  contains  88  p.c.  of  iron  and  about  10  p.c.  of  nickel,  with  a 
small  admixture  of  other  metals.  In  the  arts  German  silver  is  most 
extensively  used  ;  it  is  an  alloy  containing  nickel,  copper,  and  zinc  in 
various  proportions.  It  generally  consists  of  about  50  parts  of  copper, 
25  parts  of  zinc,  and  25  parts  of  nickel.  This  alloy  is  characterised  by 
its  white  colour  resembling  that  of  silver,  and,  like  this  latter  metal,  it 
does  not  rust,  and  therefore  furnishes  an  excellent  substitute  for  silver 
in  the  majority  of  cases  where  it  is  used.  Alloys  which  contain  silver 
in  addition  to  nickel  show  the  properties  of  silver  to  a  still  greater 
extent.  Alloys  of  nickel  are  used  for  currency,  and  if  rich  deposits  of 
nickel  are  discovered  a  wide  field  of  application  lies  before  it,  not  only 
in  a  pure  state  (because  it  is  a  beautiful  metal  and  does  not  rust)  but 
also  for  use  in  alloys.  Steel  vessels  (pressed  or  forged  out  of  sheet 
steel)  covered  with  nickel  have  such  practical  merits  that  their  manu- 
facture, which  has  not  long  commenced,  will  most  probably  be  rapidly 
developed,  whilst  nickel  steel,  which  exceeds  ordinary  steel  in  its 
tenacity,  has  already  proved  its  excellent  qualities  for  many  purposes 
(for  instance,  for  armour  plate). 

Until  1890  no  compound  of  cobalt  or  nickel  was  known  of  sufficient 
volatility  to  determine  the  molecular  weights  of  the  compounds  of  these 
metals  ;  but  in  1890  Mr.  L.  Mond,  in  conducting  (together  with  Langer 
and  Quiucke)  his  researches  on  the  action  of  nickel  upon  carbonic  oxide 
(Chapter  IX.,  Note  24-  bis),  observed  that  nickel  gradually  volatilises  in 
a  stream  of  carbonic  oxide  ;  this  only  takes  place  at  low  temperatures, 
and  is  seen  by  the  coloration  of  the  flame  of  the  carbonic  oxide.  This 
observation  led  to  the  discovery  of  a  remarkable  volatile  compound  of 
nickel  and  carbonic  'oxide,  having  as  molecular  composition  Ni(CO)4,38 

it  is  obtained  by  means  of  iodine  (probably  through  H1O),  and  its  great  resemblance 
to  Mn02,  leads  rather,  to  the  supposition  that  CoO2  is  a  very  feeble  saline  oxide.  The 
form  Co02  is  repeated  in  the  cobaltic  compounds  (Note  85),  and  the  existence  of  CoO3 
should  have  long  ago  been  recognised  upon  this  basis. 

38  This  compound  is  known  as  nickel  tetra-carbonyl.  It  appears  to  me  yet  premature 
to  judge  of  the  structure  of  such  an  extraordinary  compound  as  Ni(CO)4.  It  has  long 
been  known  that  potassium  combines  with  CO  forming  Kn(CO)n  (Chapter  IX.,  Note  81), 
but  this  substance  is  apparently  saline  and  non-volatile,  and  has  as  little  in  common 
with  Ni(CO)4  as  Na^H  has  with  SbH3.  However,  Berthelot  observed  that  when  NiC4O4 
is  kept  in  air,  it  oxidises  and  gives  a  colourless  compound,  Ni3C2O3)10H2O,  having 
apparently  saline  properties.  We  may  add  that  Schutzenberger,  on  reducing  NiCl2  by 
heating  it  in  a  current  of  hydrogen,  observed  that  a  nickel  compound  partially  volatilises 
with  the  HC1  and  gives  metallic  nickel  when  heated  again.  The  platinum  compound, 

*E 


368  PRINCIPLES  OF  CHEMISTRY 

as  determined  by  the  vapour  density  and  depression  of  the  freezing 
point.  Cobalt  and  many  other  metals  do  not  form  volatile  compounds 
under  these  conditions,  but  iron  gives  a  similar  product  (Note  26  bis). 
Ni(CO)4  is  prepared  by  taking  finely  divided  Ni  (obtained  by  reducing 
NiO  by  heating  it  in  a  stream  of  hydrogen,  or  by  igniting  the  oxalate 
NiC2O4)  39  and  passing  (at  a  temperature  below  50°,  for  even  at  60° 
decomposition  may  take  place  and  an  explosion)  a  stream  of  CO  over 
it  >  the  latter  carries  over  the  vapour  of  the  compound,  which  condenses 
(in  a  well-cooled  receiver)  into  a  perfectly  colourless  extremely  mobile 
liquid,  boiling  without  decomposition  at  43°,  and  crystallising  in  needles 
at  -25°  (Mond  and  Nasini,  1891).  Liquid  Ni(CO)4  has  a  sp.  gr.  1-356 
at  0°,  is  insoluble  in  water,  dissolves  in  alcohol  and  benzene,  and  burns 
with  a  very  smoky  flame  due  to  the  liberation  of  Ni.  The  vapour  when 
passed  through  a  tube  heated  to  180°  and  above  deposits  a  brilliant 
coating  of  metal,  and  disengages  CO.  If  the  tube  be  strongly  heated 
the  decomposition  is  accompanied  by  an  explosion.  If  Ni(CO)4  as 
vapour  be  passed  through  a  solution  of  CuCl2,  it  reduces  the  latter  to 
metal  ;  it  has  the  same  action  upon  an  ammoniacal  solution  of  AgCl,  strong 
nitric  acid  oxidises  Ni(CO)4,  dilute  solutions  of  acids  have  no  action  j 
if  the  vapour  be  passed  through  strong  sulphuric  acid,  CO  is  liberated, 
chlorine  gives  NiCl  and  COC12  ;  no  simple  reactions  of  double  decom- 
position are  yet  known  for  Ni(CO)4,  however,  so  that  its  connection 
with  other  carbon  compounds  is  not  clear.  Probably  the  formation  of 
this  compound  could  be  applied  for  extracting  nickel  from  it  ores.40 

PtCl2(CO)3  (Chapter  XXIII.,  Note  11),  offers  the  greatest  analogy  to  Ni(CO)4.  This 
compound'  was  obtained  as  a  volatile  substance  by  Schutzenberger  by  moderately 
heating  (to  235°)  metallic  platinum  in  a  mixture  of  chlorine  and  carbonic  oxide.  If  we 
designate  CO  by  Y,  and  an  atom  of  chlorine  by  X,  then  taking  into  account  that, 
according  to  the  periodic  system,  Ni  is  an  analogue  of  Pt,  a  certain  degree  of  corre- 
spondence is  seen  in  the  composition  NiY4  and  PtX2Y2.  It  would  be  interesting  to 
compare  the  reactions  of  the  two  compounds. 

59  According  to  its  empirical  formula  oxalate  of  nickel  also  contains  nickel  and 
Carbonic  oxide. 

40  The  following  are  the  thenno-chemical  data  (according  to  Thomsen,  and  referred 
to  gram  weights  expressed  by  the  formula,  in  large  calories  or  thousand  units  of  heat) 
lor  the  formation  of  corresponding  compounds  of  Mn,  Fe,  Co,  Ni,  and  Cu  (  +  Aq  signifies 
that  the  reaction  proceeds  in  an  excess  of  water)  : 

R  +  Clo  +  Aq 


R  =  Mn 
128 
106 
76 
95 
193 
+  16 

Fe 
100 
78 
48 
68 
169 
18 

Co 
95 
73 
43 
63 
168 
18 

Ni 
94 
72 
41 
61 
163 
19 

Cu 
68 
41 
82 
88 
180 
11 

R  +  O  +  H,O 
R  +  0.,  +  S 
RCl2  +  Aq 

These  examples  show  that  for  analogous  reactions  the  amount  of  heat  evolved  In 
passing  from  Mn  to  Fe,  Co,  Ni,  and  Cu  varies  in  regular  sequences  as  the  atomic  weight 
increases.  A  similar  difference  is  to  be  found  in  other  groups  and  series,  and  proves 
that  thermo-chemical  phenomena  are  subject  to  the  periodic  law 


369 


CHAPTER  XXIII 

THE     PLATINUM     METALS 

THE  six  metals  :  ruthenium,  Ru,  rhodium,  Rh,  palladium,  Pd,  osmium, 
Os,  iridium,  Ir,  and  platinum,  Pt,  are  met  with  associated  together  in 
nature.  Platinum  always  predominates  over  the  others,  and  hence 
they  are  known  as  the  platinum  metals.  By  their  chemical  character 
their  position  in  the  periodic  system  is  in  the  eighth  group,  correspond- 
ing with  iron,  cobalt,  and  nickel. 

The  natural  transition  from  titanium  and  vanadium,  to  copper  and 
zinc  by  means  of  the  elements  of  the  iron  group  is  demonstrated  by  all 
the  properties  of  these  elements,  and  in  exactly  the  same  manner  a 
transition  from  zirconium,  niobium,  and  molybdenum  to  silver,  cadmium, 
and  indium,  through  ruthenium,  rhodium,  and  palladium,  is  in  perfect 
accordance  with  fact  and  with  the  magnitude  of  the  atomic  weights,  as 
also  is  the  position  of  osmium,  iridium,  and  platinum  between  tantalum 
and  tungsten  on  the  one  side,  and  gold  and  mercury  on  the  other.  In 
all  these  three  cases  the  elements  of  smaller  atomic  weight  (chromium, 
molybdenum,  and  tungsten)  are  able,  in  their  higher  grades  of 
oxidation,  to  give  acid  oxides  having  the  properties  of  distinct  but 
feebly  energetic  acids  (in  the  lower  oxides  they  give  bases),  whilst  the 
elements  of  greater  atomic  weight  (zinc,  cadmium,  mercury),  even  in 
their  higher  grades  of  oxidation,  only  give  bases,  although  with  feebly 
developed  basic  properties.  The  platinum  metals  present  the  same 
intermediate  properties  such  as  we  have  already  seen  in  iron  and  the 
elements  of  the  eighth  group. 

In  the  platinum  metals  the  intermediate  properties  of  feebly  acid 
and  feebly  basic  metals  are  developed  with  great  clearness,  so  that 
there  is  not  one  sharply-defined  acid  anhydride  among  their  oxides, 
although  there  is  a  great  diversity  in  the  grades  of  oxidation  from  the 
type  R04  to  R20.  The  feebleness  of  the  chemical  forces  observed  in 
the  platinum  metals  is  connected  with  the  ready  decomposability  of 
their  compounds,  with  the  small  atomic  volume  of  the  metals  themr 


870  PRINCIPLES  OF  CHEMISTKY 

selves,  and  with  their  large  atomic  weight.  The  oxides  of  platinum, 
indium,  and  osmium  can  scarcely  be  termed  either  basic  or  acid  ;  they 
are  capable  of  combinations  of  both  kinds,  each  of  which  is  feeble. 
They  are  all  intermediate  oxides. 

The  atomic  weights  of  platinum,  iridium,  and  osmium  are  nearly 
1*91  to  196,  and  of  palladium,  rhodium,  and  ruthenium,  104  to  106. 
Thus,  strictly  speaking,  we  have  here  two  series  of  metals,  which 
are,  moreover,  perfectly  parallel  to  each  other  ;  three  members  in 
the  first  series,  and  three  members  in  the  second — namely,  platinum 
presents  an  analogy  to  palladium,  iridium  to  rhodium,  and  osmium 
to  ruthenium.  As  a  matter  of  fact,  however,  the  whole  group  of  the 
platinum  metals  is  characterised  by  a  number  of  common  properties, 
both  physical  and  chemical,  and,  moreover,  there  are  Several  points  of 
resemblance  between  the  members  of  this  group  and  those  pf  the  iron 
group  (Chapter  XXII.)  The  atomic  volumes  (Table  III.,  column  18) 
of  the  elements  of  this  group  are  nearly  equal  and  very  small.  The  iron 
metals  have  atomic  volumes  of  nearly  7,  whilst  that  of  the  metals  allied 
to  palladium  is  nearly  9,  and  of  those  adjacent  to  platinum  (Pt,  Ir,  Os,) 
nearly  9 '4.  This  comparatively  small  atomic  volume  corresponds  with 
the  great  infusibility  and  tenacity  proper  to  all  the  iron  and  platinum 
metals,  and  to  their  small  chemical  energy,  which  stands  out  very 
clearly  in  the  heavy  platinum  metals.  All  the  platinum  metals  are 
very  easily  reduced  by  ignition  and  by  the  action  -of  various  reducing 
agents,  in  which  process  oxygen,  or  a  haloid  group,  is  disengaged  from 
their  compounds  and  the  metal  left  behind.  This  is  a  property  of  the 
platinum  metals  which  determines  many  of  their  reactions,  and  the 
circumstance  of  their  always  being  found  in  nature  in  a  native  state. 
In  Russia  in  the  Urals  (discovered  in  1819)  and  in  Brazil  (1735) 
platinum  is  obtained  from  alluvial  deposits,  but  in  1892  Professor 
Inostrantseff  discovered  a  vein  deposit  of  platinum  in  serpentine  near 
Tagil  in  the  Urals.1  The  facility  with  which  they  are  reduced  is  so 
great  that  their  chlorides  are  even  decomposed  by  gaseous  hydrogen, 
especially  when  shaken  up  and  heated  under  a  certain  pressure.  Hence 
it  will  be  readily  understood  that  such  metals  as  zinc,  iron,  <fcc,,  separate 
them  from  solutions  with  great  ease,  which  fact  is  taken  advantage  of 
in  practice  and  in  the  chemical  treatment  of  the  platinum  metals.1  bls 

1  Wells  and  Penfield  (1888)  have  described  a  mineral  sperryllite  found  in  the  Canadian 
gold-bearing  quartz  and  consisting  of  platinum  diarsenide,  PtAs2.  It  is  a  noticeable  fact 
that  this  mineral  clearly  confirms  the  position  of  platinum  in  the  same  group  as  iron, 
because  it  corresponds  in  crystalline  form  (regular  octahedron)  and  chemical  composition 
with  iron  pyrites,  FeS2. 

i  t>u  Some  light  is  thrown  upon  the  facility  with  which  the  platinum  compounds 
decompose  by  Thomson's  data,  showing  that  in  an  excess  of  water  ( +  Aq)  the  formation 


THE  PLATINUM  METALS  371 

All  the  platinum  metals,  like  those  of  the  iron  group,  are  grey,  with 
a  comparatively  feeble  metallic  lustre,  and  are  very  infusible.  In  this 
respect  they  stand  in  the  same  order  as  the  metals  of  the  iron  series  ; 
nickel  is  more  fusible  and  whiter  than  cobalt  and  iron,  so  also  palla- 
dium is  whiter  and  more  fusible  than  rhodium  and  ruthenium,  and 
platinum  is  comparatively  more  fusible  and  whiter  than  iridium  or 
osmium.  The  saline  compounds  of  these  metals  are  red  or  yellow,  like 
those  of  the  majority  of  the  metals  of  the  iron  series,  and  like  the 
latter,  the  different  forms  of  oxidation  present  different  colours.  More- 
P.ver,  certain  complex  compounds  of  the  platinum  metals,  like  certain 
complex  compounds  of  the  iron  series,  either  have  particular  character- 
istic tints  or  else  are  colourless. 

The  platinum  metals  are  found  in  nature  associated  together  in 
alluvial  deposits  in  a  few  localities,  from  which  they  are  washed, 
owing  to  their  very  considerable  density,  which  enables  a  stream  of 
water  to  wash  away  the  sand  and  clay  with  which  they  are  mixed. 
Platinum  deposits  are  chiefly  known  in  the  Urals,  and  also  in  Brazil 
and  a  few  other  localities.  The  platinum  ore  washed  from  these 
alluvial  deposits  presents  the  appearance  of  more  or  less  coarse  grains, 
and  sometimes,  as  it  were,  of  semi-fused  nuggets.2 

All  the  platinum  metals  give  compounds  with  the  halogens,  and  the 
highest  haloid  type  of  combination  for  all  is  KX4.  For  the  majority 
of  the  platinum  metals  this  type  is  exceedingly  unstable  ;  the  lower 
compounds  corresponding  to  the  type  RX2,  which  are  formed  by  the 
separation  of  X2,  are  more  stable.  In  the  type  RX2  the  platinum 
metals  form  more  stable  salts,  which  offer  no  little  resemblance  to 

from  platinum,  of  such  a  double  salt  as  PtCl2,2KCl,  is  accompanied  by  a  comparatively 
small  evolution  of  heat  (see  Chapter  XXL,  Note  40),  for  instance,  Pt  +  Cl2  +  2KCl  + Aq 
only  evolves  about  33,000  calories  (hence  the  reaction,  Pt  +  Cl2  +  Aq,.  will  evidently 
disengage  still  less,  because  PtCl2  +  2KC1  evolves  a  certain  amount  of  heat),  whilst  oni 
the  other  hand,  Fe  +  Cl2  +  Aq  gives  100,000  calories,  and  even  the  reaction  with  copper 
(for  the  formation  of  the  double  salt)  evolves  63,000  calories. 

2  The  largest  amount  of  platinum  is  extracted  in  the  Urals,  about  five  tons  annually. 
A  certain  amount  of  gold  is  extracted  from  the  washed  platinum  by  means  of  mercury, 
which  does  not  dissolve  the  platinum  metals  but  dissolves  the  gold  accompanying  the 
platinum  in  its  ores.  Moreover,  the  ores  of  platinum  always  contain  metals  of  the  iron 
series  associated  with  them.  The  washed  and  mechanically  sorted  ore  in  the  majority 
of  cases  contains  about  70  to  80  p.c.  of  platinum,  about  5  to  8  p.c.  of  iridium,  and  a  some- 
what smaller  quantity  of  osmium.  The  other  platinum  metals — palladium,  rhodium,  and 
ruthenium — occur  in  smaller  proportions  than  the  three  above  named.  Sometimes  grains 
of  almost  pure  osmium-iridium,  containing  only  a  small  quantity  of  other  metals,  are 
found  in  platinum,  ores.  This  osmium-iridium  may  be  easily  separated  from  the  other 
platinum  metals,  owing  to  its  being  nearly  insoluble  in  aqua  regia,  by  which  the  latter 
are  easily  dissolved.  There  are  grains  of  platinum  which  are  magnetic.  The  grains  of 
osmium-iridium  are  very  hard  and  malleable,  and  are  therefore  used  for  certain  pur- 
poses, for  instance,  for  the  tips  of  gold  pens. 


87'2  PRINCIPLES  OF  CHEMISTRY 

the  kindred  compounds  of  the  iron  series — for  example,  to  nickelou? 
chloride,  NiCl2,  cobaltous  chloride,  CoCl2,  &c.  This  even  expresses 
itself  in  a  similarity  of  volume  (platinous  chloride,  PtCl2,  volume,  46  ; 
aickelous  chloride,  NiCl2  =  50),  although  in  the  type  RX2  the  true  iron 
metals  give  very  stable  compounds,  whilst  the  platinum  metals  fre- 
quently react  after  the  manner  of  suboxides,  decomposing  into  the 
metal  and  higher  types,  2RX2  =  R  +  RX4.  This  probably  depends  on 
the  facility  with  which  RX2  decomposes  into  R  and  X2,  when  X2 
combines  with  the  remaining  portion  of  RX2 

As  in  the  series  iron,  cobalt,  nickel,  nickel  gives  NiO  and  Ni203, 
whilst  cobalt  and  iron  give  higher  and  varied  forms  of  oxidation,  so 
also  among  the  platinum  metals,  platinum  and  palladium  only  give  the 
forms  RX2  and  RX4,  whilst  rhodium  and  indium  torm  another  and 
intermediate  type,  RX3,  also  met  .with  in  cobalt,  corresponding  with 
the  oxide,  having  the  composition  R2O3,  besides  which  they  form 
an  acid  oxide,  like  ferric  acid,  which  is  also  known  in  the  form  of 
salts,  but  is  in  every  respect  unstable.  Osmium  and  ruthenium^  like 
manganese,  form  still  higher  oxides,  and  in  this  respect  exhibit  the 
greatest  diversity.  They  not  only  give  RX2,  RX3,  RX4,  and  RX6> 
but  also  a  still  higher  form  of  oxidation,  R04,  which  is  not  met  with  in 
any  other  series.  This  form  is  exceedingly  characteristic,  owing  to  the 
Cact  that  the  oxides,  OsO4  and  Ru04,  are  volatile  and  have  feebly  acid 
properties.  In  this  respect  they  most  resemble  permanganic  anhydride, 
which  is  also  somewhat  volatile.3 

When  dissolved  in  aqua  regia  (PtCl4  is  formed)  and  liberated  from. 
the  solution  by  sal-ammoniac  ( (NH4)2  PtCl6  is  formed)  and  reduced  by 
ignition  (which  may  be  done  by  Zn  and  other  reducing  agents,  direct 
from  a  solution  of  PtCl4)  platinum 3  bis  forms  a  powdery  mass,  known 

5  In  characterising  the  platinum  metals  according  to  their  relation  to  the  iron  metals, 
it  is  very  important  to  add  two  more  very  remarkable  points.  The  platinum  metals  are 
capable  of  forming  a  sort  of  unstable  compound  with  hydrogen  ;  they  absorb  it  and  only 
part  with  it  when  somewhat  strongly  heated.  This  faculty  is  especially  developed  in 
platinum  and  palladium,  and  it  is  very  characteristic  that  nickel,  which  exactly  corresponds 
with  platinum  and  palladium  in  the  periodic  system,  should  exhibit  the  same  faculty  for 
tetaining  a  considerable  quantity  of  hydrogen  (Graham's  and  Raoult's  experiments). 
Another  characteristic  property  of  the  platinum  metals  consists  in  their  easily  giving 
(like  cobalt  which  forms  the  cobaltic  salts)  stable  and  characteristic  saline  compounds 
with  ammonia,  and  like  Fe  and  Co,  double  salts  with  the  cyanides  of  the  alkali  metals, 
especially  in  their  lower  forms  of  combination.  All  the  above  so  clearly  brings  the 
elements  of  the  iron  series  in  close  relation  to  the  platinum  metals,  that  the  eighth  group 
acquires  as  natural  a  character  as  can  be  required,  with  a  certain  originality  or  indivi- 
duality for  each  element. 

s  bts  Platinum  was  first  obtained  in  the  last  century  from  Brazil,  where  it  was  called 
silver  (platinus).  Watson  in  1750  characterised  platinum  as  a  separate  independent 
metal.  In  1803  Wollaston  discovered  alladiura  and  rhodium  in  crude  platinum,  and  at 


THE  PLATINUM  METALS  373 

as  spongy  platinum  or  platinum  black.  If  this  powder  of  platinum  be 
heated  and  pressed,  or  hammered  in  a  cylinder,  the  grains  aggregate  or 
forge  together,  and  form  a  continuous,  though  of  course  not  entirely 
homogeneous,  mass.  Platinum  was  formerly,  and  is  even  now,  worked 
up  in  this  manner.  The  platinum  money  formerly  used  in  Russia  was 
made  in  this  way.  Sainte-Claire  Deville,  in  the  fifties,  for  the  first 
time  melted  platinum  in  considerable  quantities  by  employing  a  special 
furnace  made  in  the  form  of  a  small  reverberatory  furnace,  and  com- 
posed of  two  pieces  of  lime,  on  which  the  heat  of  ihe  oxyhydrogen  flame 
has  no  action.  Into  this  furnace  (shown  in  fig.  34,  Vol.  I.  p.  175)— or, 
more  strictly  speaking,  into  the  cavity  made  in  the  pieces  of  lime — the 
platinum  is  introduced,  and  two  orifices  are  made  in  the  lime  j  through 
one,  the  upper,  or  side  orifice,  is  introduced  an  oxyhydrogen  gas  burner, 
in  which  either  detonating  gas  or  a  mixture  of  oxygen  and  coal-gas  is 
burnt,  whilst  the  other  orifice  serves  for  the  escape  of  the  products  of 
combustion  and  certain  impurities  which  are  more  volatile  than  the 
platinum,  and  especially  the  oxidised  compounds  of  osmium,  ruthenium, 
and  palladium,  which  are  comparatively  easily  volatilised  by  heat.  In 
this  manner  the  platinum  is  converted  into  a  continuous  metallic  form 
by  means  of  fusion,  and  this  method  is  now  used  for  melting  consider- 
able masses  of  platinum  4  and  its  alloys  with  iridiura. 

about  the  same  time  Tennant  distinguished  indium  and  osmium  in  it.  Professor  Claua, 
of  Kazan,  in  his  researches  on  the  platinum  metals  (about  1840)  discovered  ruthenium 
in  them,  and  to  him  are  due  many  important  discoveries  with  regard  to  these  elements, 
such  as  the  indication  of  the  remarkable  analogy  between  the  series  Pd— Rh— Ru  and 
pt— Ir— Os. 

The  treatment  of  platinum  ore  is  chiefly  carried  on  for  the  extraction  of  the  platinum 
itself  and  its  alloys  with  iridium,  because  these  metals  offer  a  greater  resistance  to  the 
action  of  chemical  reagents  and  high  temperatures  than  any  of  the  other  malleable  and 
ductile  metals,  and  therefore  the  wire  so  often  used  in  the  laboratory  and  for  technical 
purposes  is  made~from  them,  as  also  are  various  vessels  used  for  chemical  purposes  in 
the  laboratory  and  in  works.  Thus  sulphuric  acid  is  distilled  in  platinum  retorts,  and 
many  substances  are  fused,  ignited,  and  evaporated  in  the  laboratory  in  platinum 
crucibles  and  on  platinum  foil.  Gold  and  many  other  substances  are  dissolved  in  dishes 
made  of  iridium-platinum,  because  the  alloys  of  platinum  and  iridium  are  but  slightly 
attacked  when  subjected  to  the  action  of  aqua  regia. 

The  comparatively  high  density  (about  21 -5),  hardness,  ductility,  and  infusibility  (it 
does  not  melt  at  a  furnace  heat,  but  only  in  the  oxyhydrogen  flame  or  electric  furnace), 
as  well  as  the  fact  of  its  resisting  the  action  of  water,  air,  and  other  reagents,  renders  an 
alloy  of  90  parts  of  platinum  and  10  parts  of  iridium  (Deville's  platinum-iridium  alloy)  a 
most  valuable  material  for  making  standard  weights  and  measures,  such  as  the  metre, 
kilogram,  and  pound,  and  therefore  all  the  newest  standards  of  most  countries  are  made 
of  this  alloy. 

1  This  process  has  altered  the  technical  treatment  of  platinum  to  a  considerable 
extent.  It  has  in  particular  facilitated  the  manufacture  of  alloys  of  platinum  with 
tridium  and  rhodium  from  the  pure  platinum  ores,  since  it  is  sufficient  to  fuse  the 
Ore  in  order  for  the  greater  amount  of  the  osmium  to  burn  off,  and  for  the  mass  to  fuse 
into  a  homogeneous,  malleable  alloy,  which  can  be  directly  made  use  of.  There  is  very 
little  ruthenium  in  the  ores  of  platinum.  If  during  fusion  lead  be  added,  it  dissolves 


874  PRINCIPLES  OF  CHEMISTRY 

To  obtain  pure  platinum,  the  ore  is  treated  with  aqua  regia  in  which 
only  the  osmium  and  indium  are  insoluble.  The  solution  contains  the 
platinum  metals  in  the  form  RC14,  and  in  the  lower  forms  of  chlorina- 
tion,  RC13  and  RC12,  because  some  of  these  metals — for  instance, 
palladium  and  rhodium — form  such  unstable  chlorides  of  the  type  RX4 
that  they  partially  decompose  even  when  diluted  with  water,  and  pass 
into  the  stable  lower  type  of  combination  ;  in  addition  to  which  the 
chlorine  is  very  easily  disengaged  if  it  comes  in  contact  with  substances 
on  which  it  can  act.  In  this  respect  platinum  resists  the  action  of 
heat  and  reducing  agents  better  than  any  of  its  companions — that  is, 
it  passes  with  greater  difficulty  from  PtCl4  to  the  lower  compound 
PtCl2.  On  this  is  based  the  method  of  preparation  of  more  or  less 
pure  platinum.  Lime  or  sodium  hydroxide  is  added  to  the  solution  in 
aqua  regia  until  neutralised,  or  only  containing  a  very  slight  excess  of 
alkali.  It  is  best  to  first  evaporate  and  slightly  ignite  the  solution,  in 
order  to  remove  the  excess  of  acid,  and  by  heating  it  to  partially  con- 
vert the  higher  chlorides  of  the  palladium,  &c.,  into  the  lower.  The 
addition  of  alkalis  completes  the  reduction,  because  the  chlorine  held 
in  the  compounds  RX4  acts  on  the  alkali  like  free  chlorine,  converting 
it  into  a  hypochlorite.  Thus  palladium  chloride,  PdCl4,  for  example, 
is  converted  into  palladious  chloride,  PdCl2,  by  this  means,  according 
to  the  equation  PdCl4  +  2NaHO==PdCl2  +  NaCl  +  NaClO  +  H2O.  In 
a  similar  manner  iridic  chloride,  IrCl4,  is  converted  into  the  trichloride, 
IrCl3,  by  this  method.  When  this  conversion  takes  place  the  platinum 
still  remains  in  the  form  of  platinic  chloride,  PtCl4.  It  is  then  possible 
to  take  advantage  of  a  certain  difference  in  the  properties  of  the  higher 
and  lower  chlorides  of  the  platinum  metals.  Thus  lime  precipitates  the 
lower  chlorides  of  the  members  of  the  platinum  metals  occurring  in 
solution  without  acting  on  the  platinic  chloride,  PtCl4,  and  hence  the 
addition  of  a  large  proportion  of  lime  immediately  precipitates  the 
associated  metals,  leaving  the  platinum  itself  in  solution  in  the  form 
of  a  soluble  double  salt,  PtCl4,CaCl2.  A  far  better  and  more  perfect 

the  platinum  (and  other  platinum  metals)  owing  to  ita  being  able  to  form  a  very  charac- 
teristic alloy  containing  PtPb.  If  an  alloy  of  the  two  metals  be  left  exposed  to  moist 
air,  the  excess  of  lead  is  converted  into  carbonate  (white  lead)  in  the  presence  of  the 
water  and  carbonic  acid  of  the  air,  whilst  the  above  platinum  alloy  remains  unchanged. 
The  white  lead  may  be  extracted  by  dilute  acid,  and  the  alloy  PtPb  remains  unaltered. 
The  other  platinum  metals  also  give  similar  alloys  with  lead.  The  fusibility  of  these, 
alloys  enables  the  platinum  metals  to  be  separated  from  the  gangue  of  the  ore,  and  they 
may  afterwards  be  separated  from  the  lead  by  subjecting  the  alloy  to  oxidation  in 
furnaces  furnished  with  a  bone  ash  bed,  because  the  lead  is  then  oxidised  and  absorbed 
by  the  bone  ash,  leaving  the  platinum  metals  untouched.  This  method  of  treatment 
was  proposed  by  H.  Sainte-Claire  Deville  in  the  sixties,  and  is  also  used  in  the  analysis  ofc 
these  metals  (see  further  on). 


THE  PLATINUM  METALS  875 

Separation  is  effected  by  means  of  ammonium  chloride^  which  gives,  with 
platinic  chloride,  an  insoluble  yellow  precipitate,  PtCl4,2NH4Cl,  whilst 
it  forms  soluble  double  salts  with  the  lower  chlorides  RC12  and  RC13, 
eo  that  ammonium  chloride  precipitates  the  platinum  only  from  the 
solution  obtained  by  the  preceding  method.  These  methods  are 
employed  for  preparing  the  platinum  which  is  used  for  the  manufacture 
of  platinum  articles,  because,  having  platinum  in  solution  as  calcium 
platinochloride,  PtCaCl6,  or  as  the  insoluble  ammonium  platinochloride, 
Pt(NH4)2Cl6,  the  platinum  compound  in  every  case,  after  drying  or 
ignition,  loses  all  the  chlorine  from  the  platinic  chloride  and  leaves  finely- 
divided  metallic  platinum,  which  may  be  converted  into  homogeneous 
metal  by  compression  and  forging,  or  by  fusion.5 

5  For  the  ultimate  purification  of  platinum  from  palladium  and  iridium  the  metals 
must  be  re-dissolved  in  aqua  regia,  -and  the  solution  evaporated  until  the  residue  begins 
to  evolve  chlorine.  The  residue  is  then  re-precipitated  with  ammonium  or  potassium 
chloride.  The  precipitate  may  still  contain  a  certain  amount  of  iridium,  which  passes 
with  greater  difficulty  from  the  tetrachloride,  IrCLj,  into  the  trichloride,  IrCl3>  but  it  will 
be  quite  free  from  palladium,  because  the  latter  easily  loses  its  chlorine  and  passes  into 
palladious  chloride,  PdCl2,  which  gives  an  easily-soluble  salt  with  potassium  chloride. 
The  precipitate,  containing  a  small  quantity  of  iiydium,  is  then  heated  with  sodium 
carbonate  in  a  crucible,  when  the  mass  decomposes,  giving  metallic  platinum  and 
iridium  oxide.  If  potassium  chloride  has  been  employed,  the  residue  after  ignition  is 
washed  with  water  and  treated  with  aqua  regia.  The  iridium  oxide  remains  undissolved, 
and  the  platinum  easily  passes  into  solution.  Only  cold  and  dilute  aqua  regia  must  be 
used.  The  solution  will  then  contain  pure  platinic  chloride,  which  forms  the  starting- 
point  for  the  preparation  of  all  platinum  compounds.  Pure  platinum  for  accurate 
researches  (for  instance,  for  the  unit  of  light,  according  to  Violle's  method)  may  be 
obtained  (Mylius  and  Foerster,  1892)  by  Finkener's  method,  by  dissolving  the  impure 
metal  in  aqua  regia  (it  should  be  evaporated  to  drive  off  the  nitrogen  compounds),  and 
adding  NaCl  so  as  to  form  a  double  sodium  salt,  which  is  purified  by  crystallising  with  a 
small  amount  of  caustic  soda,  washing  the  crystals  with  a  strong  solution  of  NaCl,  and 
then  dissolving  them  in  a  hot  1  p.c.  solution  of  soda,  repeating  the  above  and  ultimately 
igniting  the  double  salt,  previously  dried  at  120°,  in  a  stream  of  hydrogen ;  platinum 
black  and  NaCl  are  then  formed.  The  three  following  are  very  sensitive  tests  (to 
thousandths  of  a  per  cent.)  for  the  presence  of  Ir,  Eu,  Rh,  Pd  (osmium  is  not  usually 
present  in  platinum  which  has  once  been  purified,  since  it  easily  volatilises  with  C12 
and  CO2,  and  in  the  first  treatment  of  the  crude  platinum  either  passes  off  as  Os04 
or  remains  undissolved),  Fe,  Cu,  Ag,  and  Pb  :  (1)  the  assay  is  alloyed  with  10  parts  of 
pure  lead,  the  alloy  treated  with  dilute  nitric  acid  (to  remove  the  greater  part  of  the 
Pb),  and  dissolved  in  aqua  regia;  the  residue  will  consist  pf  Ir  and  Ru;  the  Pb  is 
precipitated  from  the  nitric  acid  solution  by  sulphuric  acid,  whilst  the  remaining 
platinum  metals  are  reduced  from  the  evaporated  solution  by  formic  acid,  and  the 
resultant  precipitate  fused  with  KHSO4 ;  the  Pd  and  Rh  are  thus  converted  into  soluble 
salts,  and  the  former  is  then  precipitated  by  HgC2N2.  (2)  Iron  may  be  detected  by  the 
Usual  reagents,  if  the  crude  platinum  be  dissolved  in  aqua  regia,  and  the  platinum 
metals  precipitated  from  the  solution  by  formic  acid.  (8)  If  crude  platinum  (as  foil  or 
sponge)  be  heated  in  a  mixture  of  chlorine  and  carbonic  oxide  it  volatilises  (with  a 
Certain  amount  of  Ir,  Pd,  Fe,  &c.)  as  PtCl2,2CO  (Note  11),  whilst  the  whole  of  the  Rh, 
Ag,  and  Cu  it  may  contain  remains  behind.  Among  other  characteristic  reactions  for 
the  platinum  metals,  we  may  mention :  (1)  that  rhodium  is  precipitated  from  the  solution 
obtained  after  fusion  with  KHS04  (in  which  Pt  does  not  dissolve)  by  NH3)  acetic  and 
formic  acids  ;  (2)  that  dilute  aqua  regia  dissolves  precipitated  Pt,  but  not  Rh ;  (8)  that 


376  PRINCIPLES  OF  CHEMISTKY 

Metallic  platinum  in  a  fused  state  has  a  specific  gravity  of  21  ;  ft 
is  grey;  softer  than  iron  but  harder  than  copper,  exceedingly  ductile, 
and  therefore  easily  drawn  into  wire  and  rolled  into  thin  sheets,  and 
may  be  hammered  into  crucibles  and  drawn  into  thin  tubes,  &c.  In 
the  state  in  which  it  is  obtained  by  the  ignition  of  its  compounds, 
it  fonns  a  spongy  mass,  known  as  spongy  platinum,  or  else  as  powder 
(platinum  black).6  In  either  case  it  is  dull  grey,  and  is  characterised, 
as  we  already  know,  by  the  faculty  of  absorbing  hydrogen  and  other 
gases.  Platinum  is  not  acted  on  by  hydrochloric,  hydriodic,  nitric,  and 
sulphuric  acids,  or  a  mixture  of  hydrofluoric  and  nitric  acids.  Aqua 
regia,  and  any  liquid  containing  chlorine  or  able  to  evolve  chlorine  or 
bromine,  dissolves  platinum.  Alkalis  are  decomposed  by  platinum  at 
a  red  heat,  owing  to  the  faculty  of  the  platinum  oxide,  Pt02,  formed  to 
combine  with  alkaline  bases,  inasmuch  as  it  has  a  feebly-developed  acid 
character  (see  Note  8).  Sulphur,  phosphorus  (the  phosphide,  PtP2, 

if  the  insoluble  residue  of  the  platinum  metals  (Ir,  Ru,  Os)  obtained,  after  treating  with 
aqua  regia,  be  fused  with  a  mixture  of  1  part  of  KNO3  and  3  parts  of  K2CO3  (in  a  gold 
crucible),  and  then  treated  with  water,  it  gives  a  solution  containing  the  Ru  (and  a 
portion  of  the  Ir),  but  which  throws  it  all  down  when- saturated  with  chlorine  and 
boiled ;  (4)  that  if  iridium  be  fused  with  a  mixture  of  KHO  and  KNO3,  it  gives  a  soluble 
potassium  salt,  IrK8O4  (the  solution  is  blue),  which,  when  saturated  with  chlorine,  gives 
IrCI4,  which  is  precipitated  by  NH4C1  (the  precipitate  iff  black),  forming  a  double  salt, 
leaving  metallic  Ir  after  ignition ;  (5)  that  rhodium  mixed  with  NaCl  and  ignited  in  a 
current  of  chlorine  gives  a  soluble  double  salt  (from  which  sal-ammoniac  separates  Pt 
and  Ir>,  which  gives  (according  to  Jb'rgensen)  a  difficultly  soluble  purpureo-salt  (Chapter 
XXH.,  Note  85),  Rh2Cl3,5NH3,  when  treated  with  NH3 ;  in  this  form  the  Rh  may  be 
easily  purified  and  obtained  in  a  metallic  form  by  igniting  in  hydrogen ;  and  (6)  that 
palladium,  dissolved  in  aqua  regia  and  dried  (NH4C1  throws  down  any  Pt),  gives  soluble 
PdCl2,  which  forms  an  easily  crystallisable  yellow  salt,  PdCl2NH3,  with  ammonia ;  this, 
salt  (Wilm)  may  be  easily  purified  by  crystallisation,  and  gives  metallic  Pd  when 
ignited.  These  reactions  illustrate  the  method  of  separating  the  platinum  metals  from 
each  other. 

«  We  have  already  become  acquainted  with  the  effect  of  finely-divided  platinum  on 
many  gaseous  substances.  It  is  best  seen  in  the  so-called  platinum  black,  which  is  a 
coal-black  powder  left  by  the  action  of  sulphuric  acid,  on  the  alloy  of  zinc  and  platinum, 
or  which  is  precipitated  by  metallic  zinc  from  a  dilute  solution  of  platinum.  In  any 
case,  finely-divided  platinum  absorbs  gases  more  powerfully  and  rapidly  the  more 
finely  divided  and  porous  it  is.  Sulphurous  anhydride,  hydrogen,  alcohol,  and  many 
organic  substances  in  the  presence  of  such  platinum  are  easily  oxidised  by  the  oxygen  of 
the  air,  although  they  do  not  combine  with  it  directly.  The  absorption  of  oxygen  is  as 
much  as  several  hundred  volumes  per  one  volume  of  platinum,  and  the  oxidising  power 
of  such  absorbed  oxygen  is  taken  advantage  of  not  only  in  the  laboratory  but  even  in 
manufacturing  processes.  Asbestos  or  charcoal,  soaked  in  a  solution  of  platinic  chloride 
and  ignited,  is  very  useful  for  this  purpose,  because  by  this  means  it  becomes  coated  with 
platinum  black.  If  50  grams  of  PtCl4  be  dissolved  in  60  c.c.  of  water,  and  70  c.c.jof  a 
strong  (40  p.c.)  solution  of  formic  aldehyde  added,  the  mixture  cooled,  and  then  a 
solution  of  60  grams  of  NaHO  in  50  grams  of  water  added,  the  platinum  is  pre- 
cipitated. After  washing  with  water  the  precipitate  passes  into  solution  and  forms  a 
black  liquid  containing  soluble  colloidal  platinum  (Loew,  1890).  If  the  precipitated 
platinum  be  allowed  to  absorb  oxygen  on  the  filter,  the  temperature  rises  40°,  and  a 
very  porous  platinum  black  is  obtained  which  vigorously  facilitates  oxidation. 


THE  PLATINUM  JOJTALS  377 

is  formed),  arsenic  and  silicon  all  act  more  or  less  rapidly  on  platinum, 
under  the  influence  of  heat.  Many  of  the  metals  form  alloys  with  it. 
Even  charcoal  combines  with  platinum  when  it  is  ignited  with  it,  and 
therefore  carbonaceous  matter  cannot  be  subjected  to  prolonged  and 
powerful  ignition  in  platinum  vessels.  Hence  a  platinum  crucible  soon 
becomes  dull  on  the  surface  in  a. smoky  flame.  Platinum  also  forms 
alloys  with  zinc,  lead,  tin,  copper,  gold,  and  silver.7  "Although  mercury 
does  not  directly  dissolve-  platinum,  still  it  forms  a  solution  or  amalgam 
with  spongy  platinum  in  the  presence  of  sodium  amalgam  ;  a  similar 
amalgam  is  also  formed  by  the  action  of  sodium  amalgam  on  a  solution 
of  platinum  chloride,  and  is  used  for  physical  experiments. 

There  are  two  kinds  of  platinum  compounds,  PtX4  and  PtX2. 
The  former  are  produced  by  an  excess  of  halogen  in  the  cold,  and  the 
latter  by  the  aid  of  heat  or  by  the  splitting  up  of  the  former.  The 
starting-point  for  the  platinum  compounds  is,  platinum  tetrachloride, 
platinic  chloride,  PtCl4,  obtained  by  dissolving  platinum  in  aqua 
regia.7  bis  The  solution  crystallises  in  the  cold,  in  a  desiccator,  in  the 
form  of  reddish-brown  deliquescent  crystals  which  contain  hydrochloric 
acid,  PtCl4,2HCl,6HoO,  and  behave  like  a  true  acid  whose  salts  cor- 
respond  to  the  formula  R2PtCl6— ammonium  platinochloride,  for 
example.7  trl  The  hydrochloric  acid  is  liberated  from  these  crystals  by 
gently  heating  or  evaporating  the  solution  to  dryness  ;  or,  better  still, 
after  treatment  with  silver  nitrate  a  reddish-brown  mass  remains 
behind,  which  dissolves  in  water,  and  forms  a  yellowish-red  solution 
which  on  cooling  deposits  crystals  of  the  composition  PtCl4,8H20. 
The  tendency  of  PtCl4  to  combine  with  hydrochloric  acid  and  water — 
that  is,  to  form  higher  crystalline  compounds — is  evident  in  the 
platinum  compounds,  and  must  be  taken  into  account  in  explaining 
the  properties  of  platinum  and  the  formation  of  many  other  of  its 
complex  compounds.  Dilute  solutions  of  platinic  chloride  are  yellow, 
and  are  completely  reduced  by  hydrogen,  sulphurous  anhydride,  and 
many  reducing  agents,  which  first  convert  the  platinic  chloride  into 

7  It  is  necessary  to  remark  that  platinum  when  alloyed  with  silver,  or  as  amalgam, 
is  soluble  in  nitric  acid,  and  in  this  respect  it  differs  from  gold,  so  that  it  is  possible, 
by  alloying  gold  with  silver,  and  acting  on  the  alloy  with  nitric  acid,  to  recognise 
the  presence  of  platinum  in  the  gold,  because  nitric  acid  does  not  act  on  gold  alloyed 
with  silver. 

7  bts  ptC!4  is  also  formed  by  the  action  of  a  mixture  of  HC1  vapour  and  air,  and  by 
the  action  of  gaseous  chlorine  upon  platinum. 

7  lr<  Pigeon  (1891)  obtained  fine  yellow  crystals  of  PtH2Cl6,4H20  by  adding  strong  sul- 
phuric  acid  to  a  strong  solution  of  PtH2Cl6,6H2O.  If  crystals  of  H2PtCl6,6H2O  be 
melted  in  vacuo  (60°)  in  the  presence  of  anhydrous  potash,  a  red-brown  solid  hydrate  is 
obtained  containing  less  water  and  HC1,  which  parts  with  the  remainder  at  200°,  leaving 
anhydrous  PtCl4.  The  latter  does  not  disengage  chlorine  before  220°,  and  is  perfectly 
soluble  in  water. 


378  PRINCIPLES  OF  CHEMISTRY 

the  lower  compound  platinous.  chloride,  PtCl2.  That  faculty  which 
reveals  itself  in  platinum  tetrachloride  of  combining  with  water  o£ 
crystallisation  and  hydrochloric  acid  is  distinctly  marked  in  its  pro- 
perty, with  which  we  are  already  acquainted,  of  giving  precipitates 
with  the  salts  of  potassium,  ammonium,  rubidium,  &c.  In  general  it 
readily  forms  double  salts,  R2PtCl6=PtCl4  +  2RCl,  where  R  is  a 
univalent  metal  such  as  potassium  or  NH4.  Hence  the  addition  of  a 
solution  of  potassium  or  ammonium  chloride  to  a  solution  of  platinio 
chloride  is  followed  by  the  formation  of  a  yellow  precipitate,  which  is 
sparingly  soluble  in  water  and  almost  entirely  insoluble  in  alcohol  and 
ether  (platinic  chloride  is  soluble  in  alcohol,  potassium  iridiochloride, 
IrK3Cl6,  i.e.  a  compound  of  IrCl3,  is  soluble  in  water  but  not  in  alcohol). 
It  is  especially  remarkable  in  this  case,  that  the  potassium  compounds 
here,  as  in  a  number  of  other  instances,  separate  in  an  anhydrous  form, 
whilst  the  sodium  compounds,  which  are  soluble  in  water  and  alcohol, 
form  red  crystals  containing  water.  The  composition  Na2PtCl6,6H2O 
exactly  corresponds  with  the  above-mentioned  hydrochloric  compound. 
The  compounds  with  barium,  BaPtCl6,4H2O,  strontium,  SrPtCl6,8H2O, 
calcium,  magnesium,  iron,  manganese,  and  many  other  metals  are  all 
soluble  in  watW.8 

8  Nilson  (1877),  who  investigated  the  platinochlorides  of  various  metals  subsequently 
to  Bonsdorff,  Topsb'e,  Clove,  Marignac,  an£  others,  found  that  univalent  and  bivalent 
metals— such  as  hydrogen,  potassium,  ammonium  .  .  .  beryllium,  calcium,  barium- 
give  compounds  of  such  a  composition  that  there  is  always  twice  as  much  chlorine  in 
the  platinic  chloride  as  in  the  combined  metallic  chloride ;  for  example,  K2Cl2,PtCl4 } 
BeCl2,PtCl4,8H2O,  &c.  Such  trivalent  metals  as  aluminium,  iron  (ferric),  chromium,  di- 
dymium,  cerium  (cerous)'  f arm  compounds  of  the  type  RCl3PtCl4,  in  which  the  amounts  of 
chlorine  are  in  the  ratio  3  :  4.  Only  indium  and  yttrium-give  salts  of  a  different  composi- 
tion—namely, 2lnCl3,5PtCl4,86H2O  and  4YCl3,6PtCl4)5lH2O.  Such  quadrivalent  metala 
as  thorium,  tin,  zirconium  give  compounds  of  the  type  RCL^PtCLj,  in  which  the  ratio  of 
the  chlorine  is  1:1.  In  this  manner  the  valency  of  a  metal  may,  to  a  certain  extent,  be 
judged  from  the  composition  of  the  double  salts  formed  with  platinic  chloride. 

Platinic  bromide,  PtBr4)  and  iodide,  Ptl^,  are  analogous  to  the  tetrachloride,  but  the 
iodide  is  decomposed  still  more  easily  than  the  chloride.  If  sulphuric  acid  be  added  to 
platinic  chloride,  and  the  solution  evaporated,  it  forms  a  black  porous  mass  like  char- 
coal, which  deliquesces  in  the  air,  and  has  the  composition  Pt(S04)2.  But  this,  the 
only  oxygen  salt  of  the  type  PtX4,  is  exceedingly  unstable.  This  is  due  to  the  fact  that 
platinum  oxide,  the  oxide  of  the  type  Pt02)  has  a  feeble  acid  character.  This  is  shown 
in  a  number  of  instances.  Thus  if  a  strong  solution  of  platinic  chloride  treated  with 
sodium  carbonate  be  exposed  to  the  action  of  light  or  evaporated  to  dryness  and  then 
washed  with  water,  a  sodium  platinate,  Pt3Na2O7)6H2O,  remains..  The  composition  of 
this  salt,  if  wo  regard  it  in  the  same  sense  as  we  did  the  salts  of  silicic,  titanic,  molybdic 
and  other  acids,  will  be  PtO(ONa)2,2PtO2,6H2O— that  is,  the  same  type  is  repeated  as 
we  saw  in  the  crystalline  compounds  of  platinum  tetrachloride  with  sodium  chloride,  or 
with  hydrochloric  acid— namely,  the  type  PtX48Y,  where  Y  is  the  molecule  H2O,HC1,  &c. 
Similar  compounds  are  also  obtained  with  other  alkalis.  They  will  be  platinates  of  the 
alkalis  in  which  the  platinic  oxide,  Pt02,  plays  the  part  of  an  acid  oxide.  Rousseau 
(1889)  obtained  different  grades  of  combination  BaOPtO2,  8(BaO)2PtO2,  &c.,  by  igniting 
a  mixture  of  PtCl4  and  caustic  baryta.  If  such  an  alkaline  compound  of  platinum  be 


THE  PLATINUM  METALS  379 

Platinous  chloride,  PtCl2,  is  formed  when  hydrogen  platino<?hloride, 
PtH2Cl6,  is  ignited  at  300°,  or  when  potassium  is  heated  at  230°  in  a 
stream  of  chlorine.  The  undecomposed  tetrachloride  is  extracted  from 
the  residue  by  washing  it  with  water,  and  a  greenish-grey  or  brown 
insoluble  mass  of  the  dichloride  (sp.  gr.  5'9)  is  then  obtained.  It  is 
soluble  in  hydrochloric  acid,  giving  an  acid  solution  of  the  composition 
PtCl2,2HCl,  corresponding  with  the  type  of  double  salts  PtR2Cl4. 
Although  platinous  chloride  decomposes  below  500°,  still  it  is  formed  to 
a  small  extent  at  higher  temperatures.  Troost  and  Hautefeuille,  and 
Seelheim  observed  that  when  platinum  was  strongly  ignited  in  a  stream  of 
chlorine,  the  metal,  as  it  were,  slowly  volatilised  and  was  deposited  in 
crystals  ;  a  volatile  chloride,  probably  platinous  chloride,  was  evidently 
formed  in  this  case,  and  decomposed  subsequently  to  its  formation, 
depositing  crystals  of  platinum. 

The  properties  of  platinum  above- described  are  repeated  more  or  less 
distinctly,  or  sometimes  with  certain  modifications,  in  the  above-men- 
tioned associates  and  analogues  of  this  metal.  Thus  although  palladium 
fortns  PdCl4,  this  form  passes  into  PdCl2  with  extreme  ease.9  Whilst 

treated  with  acetic  acid,  the  alkali  combines  with  the  latter,  and  a  platinic  hydroxide^ 
Pt(OH)4,  remains  as  a  brown  mass,  which  loses  water  and  oxygen  when  ignited,  and  in 
BO  doing  decomposes  with  a  slight  explosion.  When  slightly  ignited  this  hydroxide  first 
loses  water  and  gives  the  very  unstable  oxide  Pt02.  Piatinic  sulphide,  PtS2)  belongs  to 
the  same  type;  it  is  precipitated  by  the  action  of  sulphuretted  hydrogen  on  a  solution 
of  platinum  letrachloride.  The  moist  precipitate  is  capable  of  attracting  oxygen,  and  19 
then  converted  into  the  sulphate  above  mentioned,  which  is  soluble  in  water.  This 
absorption  of  oxygen  and  conversion  into  sulphate  is  another  illustration  of  the  basic 
nature  of  Pt02,  so  that  it  clearly  exhibits  both  basic  and  acid  properties.  The  latter 
appear,  for  instance,  in  the  fact  that  platinic  sulphide,  PtS2,  gives  crystalline  compounds 
with  the  alkali  sulphides. 

9  In  comparing  the  characteristics  of  the  platinum  metals,  it  must  be  observed  thafe 
palladium  in  its  form  of  combination  PdX2  gives  saline  compounds  of  considerable 
stability.  Amongst  them  palladous  chloride  is  formed  by  the  direct  action  of  chlorine 
or  aqua  regia  (not  in  excess  or  in  dilute  solutions)  on  palladium.  It  forms  a  brown 
solution,  which  gives  a  black  insoluble  precipitate  of  palladous  iodide,  PdI2,  with 
solutions  of  iodides  (in  this  respect,  as  in  many  others,  palladium  resembles  mercury  in 
the  mercuric  compounds  HgX2).  With  a  solution  of  mercuric  cyanide  it  gives  a  yellowish 
white  precipitate,  palladous  cyanide,  PdC8N2,  which  is  soluble  in  potassium  cyanide,  and 
gives  other  double  salts,  M2PdC4N4. 

That  portion  of  the  platinum  ore  which  dissolves  in  aqua  regia  and  is  precipitated 
by  ammonium  or  potassium  chloride  does  not  contain  palladium.  It  remains  in  solu- 
tion, because  the  palladic  chloride,  PdCl4,  is  decomposed  and  the  palladous  chloride 
formed  is  not  precipitated  by  ammonium  chloride ;  the  same  holds  good  for  all  the  other 
lower  chlorides  of  the  platinum  metals.  Zinc  (and  iron)  separates  out  all  the  unprecipi- 
tated  platinum  metals  (and  also  copper,  &c.)  from  the  solution.  The  palladium  is  found 
ih  these  platinum  residues  precipitated  by  zinc.  If  this  mixture  of  metals  be  treated 
with  aqua  regia,  all  the  palladium  will  pass  into  solution  as  palladous  chloride  with 
some  platinic  chloride.  By  this  treatment  the  main  portion  of  the  iridiura,  rhodium,  &o. 
remains  almost  undissolved,  the  platinum  is  separated  from  the  mixture  of  pallado,u» 
and  platinic  chlorides  by  a  solution  of  ammonium  chloride,  and  the  solution  of  palladiuln 


880  PRINCIPLES  OF  CHEMISTRY 

rhodium  and  iridium  in  dissolving  in  aqua  regia  also  form  RhCl4  and 

is  precipitated  by  potassium  iodide  or  mercuric  cyanide.  Wilm  (1881)  showed  that 
palladium  may  be  separated  from  an  impure  solution  by  saturating  it  with  ammonia ;  all 
the  iron  present  is  thus  precipitated,  and,  after  filtering,  the  addition  of  hydrochloric 
acid  to  the  filtrate  gives  a  yellow  precipitate  of  an  ammonio-palladium  compound, 
PdCI2,2NH3,  whilst  nearly  all  the  other  metals  remain  in  solution.  Metallic  palladium 
is  obtained  by  igniting  the  ammonio-compound  or  the  cyanide,  PdC2N2.  It  occurs 
native,  although  rarely,  and  is  a  metal  of  a  whiter  colour  than  platinum,  sp.  gr.  1T4, 
melts  at  about  1,500°  ;  it  is  much  more  volatile  than  platinum,  partially  oxidises  on  the 
surface  when  heated  (Wilm  obtained  spongy  palladium  by  igniting  PdCl2,2NH3,  and 
observed  that  it  gives  P.dO  when  ignited  in  oxygen,  and  that  on  further  ignition  this 
oxide  forms  a  mixture  of  Pd2O  and  Pd),  and  loses  its  absorbed  oxygen  on  a  further  rise 
of  temperature.  It  does  not  blacken  or  tarnish  (does  not  absorb  sulphur)  in  the  air  at 
the  ordinary  temperature,  and  is  therefore  better  suited  than  silver  for  astronomical  and 
Other  instruments  in  which  fine  divisions  have  to  be  engraved  on  a  white  metal,  in  order 
that  the  fine  lines  should  be  clearly  visible.  The  most  remarkable  property  of  palladium, 
discovered  by  Graham,  consists  in  its  capacity  for  absorbing  a  large  amount  of  hydrogen. 
Ignited  palladium  absorbs  as  much  as  940  volumes  of  hydrogen,  or  about  0'7  p.c.  of  its 
Own  weight,  which  closely  approaches  to  the  formation  of  the  compound  Pd3H2).  and 
probably  indicates  the  formation  of  palladium  hydride,  Pd2H.  This  absorption 
also  takes  place  at  the  ordinary  temperature — for  example,  when  palladium  serves  as 
an  electrode  at  which  hydrogen  is  evolved.  In  absorbing  the  hydrogen,  the  palladium 
does  not  change  in  appearance,  and  retains  all  its  metallic  properties,  only  its  volume 
increases  by  about  10  p.c. — 'that  is,  the  hydrogen  pushes  out  and  separates  the  atoms  of 
the  palladium  from  each  other,  and  is  itself  compressed  to  ^jj  of  its  volume.  This  com- 
pression indicates  a  great  force  of  chemical  attraction,  and  is  accompanied  by  the  evolu- 
tion of  heat  (Chapter  II.,  Note  88).  The  absorption  of  1  grm.  of  hydrogen  by  metallic 
palladium  (Favre)  is  accompanied  by  the  evolution  of  4'2  thousand  calories  (for  Pt  20, 
for  Na  13,  for  K  10  thousand  junits  of  heat).  Troost  showed  that  the  dissociation 
pressure  of  palladium  hydride  is  inconsiderable  at  the  ordinary  temperature,  but  reaches 
the  atmospheric  pressure  at  about  140°.  This  subject  was  subsequently  investigated  by 
A.  A.  Cracow  of  St.  Petersburg  -(1894),  who  slewed  that  at  first  the  absorption  of 
hydrogen  by  the  palladium  proceeds  like  solution,  according  to  the  law  of  Dalton  and 
Henry,  but  that  towards  the  end.it  proceeds  like  a  dissociation  phenomenon  in  definite 
compounds;  this  forms  another  link  between  the  phenomenon  of  solution  and  of  the 
formation  of  definite  atomic  compounds.  Cracow's  observations  for  a  temperature  18°, 
showed  that  the  electro-conductivity  and  tension  vary  until  a  compound  Pd2H  is  reached, 
and  namely,  that  the  tension  ^  rises  with  the  volume  v  of  hydrogen  absorbed,  according 
fb  the  law  of  Dalton  and  Henry — for  instance,  for 

j»  =  2'l  8'2  6'5  7'7  mm. 

v=  14  20  84  47 

The  maximum  tension  at  18°  is  9  mm.  At  a  temperature  of  about  140°  (in  the  vapour  of 
xylene)  the  maximum  tension  is  about  760mm.,  and  when  v  =  10  — 60  vols.  the  tension 
(according  to  Cracow's  experiments)  stands  at  90-450  mm.— that  is,  increases  in  pro- 
portion to  the  volume  of  hydrogen  absorbed.  But  from  the  point  of  view  of  chemical 
mechanics  it  is  especially  important  to  remark  that  Moutier  clearly  showed,  through 
palladium  hydride,  the  similarity  of  the  phenomena  which  proceed  in  evaporation  and 
dissociation,  which  fact  Henri  Sainte-Claire  DeviDe  placed  as  a  fundamental  proposition 
in  the  theory  of  dissociation.  It  is  possible  upon  the  basis  of  the  second  law  of  the 
theory  of  heat,  according  to  the  law  of  the  variation  of  the  tension  p  of  evaporation  with 
the  temperature  T  (counted  from  -273°),  to  calculate  the  latent  heat  of  evaporation 
L  (see  works  on  physics)  because  424  L  =  T  (1/d-l/D)  dpldt,  where  d  and  D  are  the 
weights  of  cubic  measures  of  the  gas  (vapour)  and  liquid.  (Thus,  for  instance,  fof 
,-jrater,  when  <-100°,  T-873,  d  =  0605,  D  =  960,  dpldt~ 0'027  m.,  18,596  =  867,  L-630, 


THE  PLATINUM  METALS  381 

IrCl4,  but  they  pass  into  RhCl3  and  IrCl3  9  bis  very  easily  when  heated 

whence  424  L  =  227,264,  and  the  second  portion  of  the  equation  226,144,  which  is 
sufficiently  near,  within  the  limits  of  experimental  error,  see  Chapter  I.,  Note  11.) 
The  same  equation  is  applicable  to  the  dissociation  of  Na^H  and  K2H— (Chapter  XII., 
Note  42) — but  it  has  only  been  verified  in  this  respect  for  Pd2H,  since  Moutier,  by 
calculating  the  amount  of  heat  L  evolved,  for  £  =  20,  according  to  the  variation  of 
the  tension  (dptdt)  obtained  4'1  thousand  calories,  which  is  very  near  the  figure 
obtained  experimentally  by  Favre  (see  Chapter  XII.,  Note  44).  The  absorbed  hydrogen 
is  easily  disengaged  by  ignition  or  decreased  pressure.  The  resultant  compound 
does  not  decompose  at  the  ordinary  temperature,  but  when  exposed  to  air  the  metal 
sometimes  glows  spontaneously,  owing  to  the  hydrogen  burning  at  the  expense  of 
the  atmospheric  oxygen.  The  hydrogen  absorbed  by  palladium  acts  towards  many 
solutions  as  a  reducing  agent ;  in  a  word,  everything  here  points  to  the  formation  of  a 
definite  compound  and  at  the  same  time  of  a  physically-compressed  gas,  and  forms  one/ 
of  the  best  examples  of  the  bond  existing  between  chemical  and  physical  processes,  td 
which  we  have  many  times  drawn  attention.  It  must  be  again  remembered  that  the 
other  metals  of  the  eighth  group,  even  copper,  are,  like  palladium  and  platinum,  able  to- 
combine  with  hydrogen.  The  permeability  of  iron  and  platinum  tubes  to  hydrogen  is 
naturally  due  to  the  formation  of  similar  compounds,  but  palladium  is  the  most 
permeable. 

9  bts  Rhodium  is  generally  separated,  together  with  iridiura,  from  the  residues  left 
after  the  treatment  of  native  platinum,  because  the  palladium  is  entirely  separated  from 
them,  and  the  ruthenium  is  present  in  them  in  very  small  traces,  whilst  the  osmium  at 
any  rate  is  easily  separated,  as  we  shall  soon  see.  The  mixture  of  rhodium  and  iridium 
which  is  left  undissolved  in  dilute  aqua  regia  is  dissolved  in  chlorine  water,  or  by  the 
action  of  chlorine  on  a  mixture  of  the  metals  with  sodium  chloride.  In  either  case  both 
metals  pass  into  solution.  They  may  be  separated  by  many  methods.  In  either  case 
(if  the  action  be  aided  by  heat)  the  rhodium  is  obtained  in  the  form  of  the  chloride 
RhCl3,  and  the  iridium  as  iridious  chloride,  IrCl5.  They  both  form  double  salts  with 
sodium  chloride  which  are  soluble  in  water,  but  the  iridium  salt  is  also  partially  soluble 
in  alcohol,  whilst  the  rhodium  salt  is  not.  A  mixture  of  the  chlorides,  when  treated  with 
dilute  aqua  regia,  gives  iridic  chloride,  IrClj,  whilst  the  rhodium  chloride,  RhCl3,  re- 
mains  unaltered  ;  ammonium  chloride  then  precipitates  the  iridium  as  ammonium  iridio- 
chloride,  Ir(NH4)2Cl6,  and  on  evaporating  -the  rose-coloured  filtrate  the  rhodium  gives 
a  crystalline  salt,  Rh(NH4)3Cl6.  Rhodium  and  its  various  oxides  are  dissolved  when 
fused  with  potassium  hydrogen  sulphate,  and  give  a  soluble  double  sulphate  (whilst 
iridium  remains  unacted  on) ;  this  fact  is  very  characteristic  for  this  metal,  which  offers 
in  its  properties  many  points  of  resemblance  with  the  iron  metals.  When  fused  with 
potassium  hydroxide  and  chlorate  it  is  oxidised  like  iridium,  but  it  is  not  afterwards 
soluble  in  water,  iu  which  respect  it  differs  from  ruthenium.  This  is  taken  advantage  of  for 
separating  rhodium,  ruthenium,  and  iridiura.  In  any  case,  rhodium  under  ordinary 
conditions  always  gives  salts  of  the  type  RX3,  and  not  of  any  other  type  ;  and  not  only 
halogen  salts,  but  also  oxygen  salts,  are  known  in  this  type,  which  is  rare  among 
the  platinum  metals.  Rhodium  chloride,  RhCl3,  is  known  in  an  insoluble  anhydrous 
and  also  in  a  soluble  form  (like  CrX3  or  salts  of  chromic  oxides),  in  which  it  easily  gives 
double  salts,  compounds  with  water  of  crystallisation,  and  forms  rose-coloured  solutions. 
In  this  form  rhodium  easily  gives  double  salts  of  the  two  types  RhM5Cl6  and  RhM2Cl5— 
for  example,  K3RhCl6,3H20  and  K2RhCl5)H2O.  Solutions  of  the  salts  (at  least,  the 
ammonium  salt)  of  the  first  kind  give  salts  of  the  second  kind  when  they  are  boiled.  If 
a  strong  solution  of  potash  be  added  to  a  red  solution  of  rhodium  chloride  and  boiled,  a 
black  precipitate  of  the  hydroxide  Rh(OH)3  is  formed ;  but  if  the  solution  of  potash  is 
added  little  by  little,  it  gives  a  yellow  precipitate  containing  more  water.  This  yellow 
hydrate  of  rhodium  oxide  gives  a  yellow  solution  when  it  is  dissolved  in  acids,  which 
only  becomes  rose-coloured  after  being  boiled.  It  is  obvious  a  change  here  takes  place, 
like  the  transmutations  of  the  salts  of  chromic  oxide.  It  is  also  a  remarkable  fact  that 


882  PRINCIPLES  OF  CHEMISTRY 

or  when  acted  upon  by  substances  capable  of  taking  up  chlorine  (evea 
alkalis,  which  form  bleaching  salts).  Among  the  platinum  metals, 
ruthenium  and  osmium  have  the  most  acid  character,  and  although  they 
give  EuCl4  and  OsCl4  they  are  easily  oxidised  to  RuO4  and  Os04  by 
the  action  of  chlorine  in  the  presence  of  water  ;  the  latter  are  volatile 
and  may  be  distilled  with  the  water  and  hydrochloric  acid,  from  a 
solution  containing  other  platinum  metals.9  tri  Thus  with  respect  to 

the  black  hydroxide,  like  many  other  oxidised  compounds  of  the  platinoid  metals,  does 
not  dissolve  in  the  ordinary-  oxygen  acids,  whilst  the  yellow  hydroxide  is  easily  soluble 
and  gives  yellow  solutions,  which  deposit  imperfectly  crystallised  salts.  Metallic, 
rhodium  is  easily  obtained  by  igniting  its  oxygen  and  other  compounds  in  hydrogen,  or 
by  precipitation  with  zinc.  It  resembles  platinum,  and  has  a  sp.  gr.  of  12'1.  At  the 
ordinary  temperature  it  decomposes  formic  acid  into  hydrogen  and  carbonic  anhydride, 
with  development  of  heat  (Deville).  With  the  alkali  sulphites,  the  salts  of  rhodium  and 
indium  of  the  type  RX3  give  sparingly-soluble  precipitates  of  double  sulphites  of  the 
comp"osition  R(S03Na)3)H2O,  by  means  of  which  these  metals  may  be  separated  from 
solution,  and  also  may  be  separated  from  each  other,  for  a  mixture  of  these  salts  when 
treated  with  strong  sulphuric  acid  gives  a  soluble  iridium  sulphate  and  leaves  a  red  in- 
soluble double  salt  of  rhodium  and  sodium.  It  may  be  remarked  that  the  oxides  Ir4O3  and 
Rh2O3  are  comparatively  stable  and  are  easily  formed,  and  that  they  also  form  different 
double  salts  (for  instance,  IrCl3,3KCl3H2O,  RhCl3,2NH4Cl4H20,  RhCl3,8NH1ClHH2O) 
and  compounds  like  the  cobaltia  compounds  (for  instance,  luteo-salts  RhX3,6NH3,  roseo- 
salts,  RhX3H205NH3,  and  purpureo-salts  IrX3,5NH3.  &c.)  Iridious  oxide,  Ir205)  is 
obtained  by  fusing  iridious  chloride  and  its  compounds  with  sodium  carbonate,  and 
treating  the  mass  with  water..  The  oxide  is  then  left  as  a  black  powder,  which,  when 
strongly  heated,  is  decomposed  into  iridium  and  oxygen;  it  is  easily  reduced,  and  is 
insoluble  in  acids,  which  indicates  the  feeble  basic  character  of  this  oxide,  in  many 
respects  resembling  such  oxides  as  cobaltic  oxide,  eerie  or  lead  dioxide,  &c.  It  does  not 
dissolve  when  fused  with  potassium  hydrogen  sulphate.  Rhodium  oxide,  Rh2O3,  is  a 
far  more  energetic  base.  It  dissolves  when  fused  with  potassium  hydrogen  sulphate. 

From  what  has  been  said  respecting  the  separation  of  platinum  and  rhodium  it  will 
be  understood  how  the  compounds  of  iridium,  which  is  the  main  associate  of  platinum, 
are  obtained.  In  describing  the  treatment  of  osmiridium  we  shall  again  have  an 
opportunity  of  learning  the  method  of  extraction  of  the  compounds  of  this  metal,  which 
has  in  recent  times  found  a  technical  application  in  the  form  of  its  oxide,  Ir2O3; 
this  is  obtained  from  many  of  the  compounds  of  iridium  by  ignition  with  water,  ia 
easily  reduced  by  hydrogen,  and  is  insoluble  in  acids.  It  is  used  in  painting  on  china, 
for  giving  a  black  colour.  Iridium  itself  is  more  difficultly  fusible  than  platinum,  and 
when  fused  it  does  not  decompose  acids  or  even  aqua  regia ;  it  is  extremely  hard,  and  ia 
not  malleable  ;  its  sp.  gr.  is  22'4.  In  the  form  of  powder  it  dissolves  in  aqua  regia,  and 
is  even  partially  oxidised  when  heated  in  air,  sets  fire  to  hydrogen,  and,  in  a  word,  closely 
resembles  platinum.  Heated  in  an  excess  of  chlorine  it  gives  iridic  chloride,  IrCl4)  but 
this  loses  chlorine  at  60° ;  it  is,  however,  more  stable  in  the  form  of  double  salts,  which 
have  a  characteristic  black  colour — for  instance,  Ir(NH4)3Cle — but  they  give  iridious 
chloride,  IrCl3,  when  treated  with  sulphuric  aeid. 

9  tri  \ve  have  yet  to  become  acquainted  with  the  two  remaining  associates  of  platinum 
—ruthenium  and  osmium — whose  most  important  property  is  that  they  are  oxidised 
even  when  heated  in  air,  and  that  they  are  able  to  give  volatile  oxides  of  the  form  Ru04 
and  OsO4 ;  these  have  a  powerful  odour  (like  iodine  and  nitrous  anhydride).  Both  these 
higher  oxides  are  solids ;  they  volatilise  with  great  ease  at  100° ;  the  former  is  yellow 
and  the  latter  white.  They  are  known  as  ruthenic  and  osmic  anhydrides,  although  their 
aqueous  solutions  (they  both  slowly  dissolve  in  water)  do  not  show  an  acid  reaction,  and 
although  they  do  not  even  expel  carbonic  anhydride  from  potassium  carbonate,  do  not 


THE  PLATINUM  METALS  383 

the  types  of  combination,  all  the  platinum  metals,  under  certain  circum- 
stances, give  compounds  of  the  type  BX4 — for  instance,  RQ2,  RC14,  &o. 

give  crystalline  salts  with  bases,  and  their  alkaline  solutions  partially  deposit  them 
again  when  boiled  (an  excess  of  water  decomposes  the  salts).  The  formulae  OsO4  and 
RuO4  correspond  with  the  vapour  density  of  these  oxides.  Thus  Deville  found  the 
vapour  density  of  osmic  anhydride  to  be  128  (by  the  formula  127'5)  referred  to  hydrogen. 
Tennant  and  Vauquelin  discovered  this,  compound,  and  Berzelius,  Wb'hler,  Fritzsche, 
Strove",  Deville,  Claus,  Joly,  .and  others,  helped  in  its  investigation ;  nevertheless  there 
are  still  many  questions  concerning  it  which  remain  unsolved.  It  should  be  observed 
that  RO4  is  the  highest  known  form  for  an  oxygen  compound,  and  RH4  is  the  highest 
known  form  for  a  compound  of  hydrogen;  whilst  the  highest  forms  of  acid  hydrates 
contain  SiH4O4,  PH304,  SH2O4,  C1HO4— all  with  four  atoms  of  oxygen,  and  therefore  in 
this  number  there  is  apparently  the  limit  for  the  simple  forms  of  combination  of  hydrogen 
and  oxygen.  In  combination  with  several  atoms  of  an  element,  or  several  elements, 
there  may  be  more  than  O4  or  H4,  but  a  molecule  never  contains  more  than  four  atoms 
of  either  O  or  H  to  one  atom  of  another  element.  Thus  the  simplest  forms  of  combina- 
tion of  hydrogen  and  oxygen  are  exhausted  by  the  list  BH4,  RH3,  RH2,  RH,  RO,  RO2, 
R03,  R04.  The  extreme  members  are  RH4  and  R04,  and  are'  only  met  with  for  such 
elements  as  carbon,  silicon,  osmium,  ruthenium,  which  also  give  RCl4  with  chlorine. 
In  these  extreme  forms,  RH4  and  RO^  the  compounds  are  the  least  stable  (com- 
pare SiH4,  PH3,  SH2,  C1H,  or  RuO^  Mo03,  Zr02,  SrO),  and  easily  give  up  part,  or  even 
all,  their  oxygen  of  hydrogen. 

The  primary  source  from  which  the  compounds  of  ruthenium  and  osmium  are 
obtained  is  either  osmiridium  (the  osmium  predominates,  from  IrOs  to  IrOs4,  sp.  gr. 
from  16  to  2*1),  which  Occurs  in  platinum  ores  (it  is  distinguished  from  the  grains  of 
platinum  by  its  crystalline  structure,  hardness,  and  insolubility  in  aqua  regia),  or  else 
those  insoluble  residues  which  are  obtained,  as  we  saw  above,  after  treating  platinum 
with  aqua  regia.  Osmium  predominates  in  these  materials,  which  sometimes  contain 
from  80  p.c.  to  40  p.c.  of  it,  and  rarely  more  than  4  p.c.  to  5  p.c.  of  ruthenium.  The 
process  for  their  treatment  is  as  follows :  they  are  first  fused  with  6  parts  of  zinc>  and 
the  zinc  is  then  extracted  with  dilute  hydrochloric  acid.  The  osmiridium  thus  treated 
is,  according  to  Fritzsche  and  Struve"'s  method,  then  added  to  a  fused  mixture  of 
potassium  hydroxide  and  chlorate  in  an  iron  crucible ;  the  mass  as  it  begins  to  evolve 
oxygen  acts  on  the  metal,  and  the  reaction  afterwards  proceeds  spontaneously.  The 
dark  product  is  treated  with  water,  and  gives  a  solution  of  osmium  and  ruthenium  in 
the  form  of  soluble  salts,  R2OsO4  and  RaRuC^,  whilst  the  insoluble  residue  contains  a 
mixture  of  oxides  of  iridium  (and  some  osmium,  rhodium,  and  ruthenium),  and  grains 
of  metallic  iridium  still  unacted  on.  According  to  Fre*my's  method  the  lumps  of 
osmiridium  are  straightway  heated  to  whiteness  in  a  porcelain  tube  in  a  stream  of  air  or 
oxygen,  when  the  very  volatile  osmic  anhydride  is  obtained  directly,  and  is  collected  in 
a  well-cooled  receiver,  whilst  the  ruthenium  gives  a  crystalline  sublimate  of  the  dioxide, 
RuO2,  which  is,  however,  very  difficultly  volatile  (it  volatilises  together  with  osmic 
anhydride),  and  therefore  remains  in  the  cooler  portions  of  the  tube ;  this  method  does 
not  give  volatile  ruthenic  anhydride,  and  thedridium  and  other  metals  are  not  oxidised 
or  give  non-volatile  products.  This  method  is  simple,  and  at  once  gives  dry,  pure  osmic 
anhydride  in  the  receiver,  and  ruthenium  dioxide  in  the  sublimate.  The  air  which 
passes  through  the  tube  should  be  previously  passed  through  sulphuric  acid,  not  only  in 
order  to  dry  it,  but  also  to  remove  the  organic  and  reducing  dust.  The  vapour  of  osmic 
anhydride  must  be  powerfully  cooled,  and  ultimately  passed  over  caustic  potash.  A 
third  mode  of  treatment,  which  is  most  frequently  employed,  was  proposed  by  Wb'hler, 
and  consists  in  slightly  heating  (in  order  that  the  sodium  chloride  should  not  melt)  an 
intimate  mixture  of  osmiridium  and  common  salt  in  a  stream  of  moist  chlorine.  The 
metals  then  form  compounds  with  chlorine  and  sodium  chloride,  whilst  the  osmium 
forms  the  chloride,  OsCl4,  which  reacts  with  the  moisture,  and  gives  osmic  anhydride, 
which  is  condensed.  The  ruthenium  in  this,  as  in  the  pther  processes,  does  not  directly 


884  PRINCIPLES  OF  CHEMISTRY" 

'But  this  is  the  highest  form  for  only  platinum  and  palladium1/ 
The  remaining  platinum  metals  further,  like  iron,  give  acids  of  the  type 

give  ruthenic  anhydride,  but  is  always  extracted  as  the  soluble  ruthenium  salt,  K2Ru04, 
obtained  by  fusion  with  potassium  hydroxide  and  chlorate  or  nitrate.  When  the  orange- 
coloured  ruthenate,  K2Ru04,  is  mixed  with  acids,  the  liberated  ruthenic  acid  immediately 
decomposes  into  the  volatile  ruthenic  anhydride  and  the  insoluble  ruthenic  .oxide : 
2K2RuO4  +  4HNO3=Ru04+Ru02,2H20  +  4KNO3.  When  once  one  of  the  above  com' 
.pounds  of  ruthenium  or  osmium  is  procured  it  is  easy  to  obtain  all  the  remaining 
compounds,  and  by  reduction  (by  metals,  hydrogen,  formic  acid,  &c.)  the  metals 
themselves. 

Osmic  anhydride,  Os04,  is  'very  easily  deoxidised  by  many  methods.  It'  blackens 
Organic  substances,  owing  to  reduction,  and  is  therefore  used  in  investigating  vegetable 
and  animal,  and  especially  nerve,  preparations  under  the  microscope.  Although  osmio 
anhydride  may  be  distilled  in  hydrogen,  still  complete  reduction  is  accomplished  when  a 
mixture  of  hydrogen  and  osmic  anhydride  is  slightly  ignited  (jnst  before  it  inflames).  If 
osmium  be  placed  in  the  flame  it  is  oxidised,  and  gives  vapours  of  osmic  anhydride,  which 
become  reduced,  and  the  flame  gives  a  brilliant  light.  Osmic  anhydride  deflagrates 
like  nitre  on  red-hot  charcoal ;  zinc,  and  even  mercury  and  silver,  reduce  osmic  anhydride 
from  its  aqueous  solutions  into  the  lower  oxides  or  metal;  such  reducing  agents  as 
hydrogen  sulphide,  ferrous  sulphate,  or  sulphurous  anhydride,  alcohol,  &c.,  act  in  the 
Bame  manner  with  great  ease. 

The  lower  oxides  of  osmium,  ruthenium,  and  of  the  other  elements  of  the  platinum 
series  are  not  volatile,  and  it  is  noteworthy  that  the  other  elements  behave  differently. 
On  comparing  S02,  SO3;  As2O3,  As2O5 ;  P203,  P20?;  CO,  CO2,  &c.,  we  observe  a 
converse  phenomenon  ;  the  higher  oxides  are  less  volatile  than  the  lower.  In  the  case 
of  osmium  all  the  oxides,  w'ith  the  exception  of  the  highest,  are  non-volatile,  and  it  may 
therefore  be  thought  that  this  higher  form  is  more  simply  constituted  than  the  lower. 
It  is  possible  that  osmic  oxide,  OsO2,  stands  in  the  same  relation  to  the  anhydride  as 
C2H4  to  CH4— i.e.  the  lower  oxide  is  perhaps  Os2O4,  or  is  still  more  polymerised,  which 
would  explain  why  the  lower  oxides,  having  a  greater  molecular  weight,  are  less  volatile 
than  the  higher  oxides,  just  as  we  saw  in  the  case  of  the  nitrogen  oxides,  N2O  and  NO. 

Ruthenium  and  osmium,  obtained  by  the  ignition  or  reduction  of  their  compounds 
in  the  form  of  powder,  have  a  density  considerably  less  than  in  the  fused  form,  and  differ 
in  this  condition  in  their  capacity  for  reaction  ;  they  are  much  more  difficultly  fused  than 
platinum  and  iridium,  although  ruthenium  is  more  fusible  than  osmium.  Ruthenium 
in  powder  has  a  specific  gravity  of  8'5,  the  fused  metal  of  12'2 ;  osmium  in  powder  has  a 
specific  gravity  of  20'0,  and  when  semi-fused — or,  more  strictly  speaking,  agglomerated— 
in  the  oxy-hydrogen  flame,  of  21'4,  and  fused  22'5.  The  powder  of  slightly-heated  osmium 
oxidises  very  easily  in  the  air,  and  when  ignited  burns  like  tinder,  directly  forming  the 
odoriferous  osmic  anhydride  (hence  its  name,  from  the  Greek  word  signifying  odour) ; 
ruthenium  also  oxidises  when  heated  in  air,  but  with  more  difficulty,  forming  the  oxide 
EuO2.  The  oxides  of  the  types  RO,  R*O3,  and  RO2  (and  their  hydrates)  obtained  by 
reduction  from  the  higher  oxides,  and  also  from  the  chlorides,  are  analogous  to  those 
given  by  the  other  platinum  metals,  in  which  respect  osmium  and  ruthenium  closely 
resemble  them.  We  may  also  remark  that  ruthenium  has  been  found  rn  the  platinum 
deposits  of  Borneo  in  the  form  of  laurite,  Ru^Ss,  in  grey  octahedra  of  sp.  gr.  7-0. 

For  osmium,  Moraht  and  Wischin  (1893)  obtained  free  osmic  acid,  HoOsOj,  by 
decomposing  K2OsO4  with  water,  and  precipitating  with  alcohol  in  a  current  of  hydrogen 
(because  in  air  volatile  OsO4  is  formed) ;  with  H2S,  osmic  acid  gives  Os05(HS)2  at  the 
ordinary  temperature. 

Debray  and  Joly  showed  that  ruthenic  anhydride,  RuO4,  fuses  at  25°,  boils  at  100°, 
and  evolves  oxygen  when  dissolved  in  potash,  forming  the  salt  KRuO4  (not  isomorphous 
with  potassium  permanganate). 

Joly  (1891),  who  studied  the  ruthenium  compounds  in  greater  detail,  showed  that  the 
easily-formed  KRuO4  gives  RuKO^RuOu  when  ignited,  but  it  resembles  KMnO4  in  many 


THE  PLATINUM  METALS  885 

R03or  hydrates,  H2R04t=R02(HO)2  (the  type  of  sulphuric  acid) ;  but 
they,  like  ferric  and  manganic  acids,  are  chiefly  known  in  the  form  of 
salts  of  the  composition  K2RO4  or  K2R207  (like  the  dichromate).  These 
salts  are  obtained,  like  the  manganates  and  ferrates,  by  fusing  the  oxides, 
or  even  the  metals  themselves,  with  nitric,  or,  better  still,  with  potassium 
peroxide.  They  are  soluble  in  water,  are  easily  deoxidised  and  do  not 
yield  the  acid  anhydrides  under  the  action  of  acids,  but  break  up,  either 
(like  the  ferrate)  forming  oxygen  and  a  basic  oxide  (iridium  and  rhodium 
react  in  this  manner,  as  they  do  not  give  higher  forms  of  oxidation),  or 
passing  into  a  lower  and  higher  form  of  oxidation — that  is,  reacting 
like  a  manganate  (or  partly  like  nitrite  or  phosphite).  Osmium  and 
ruthenium  react  according  to  the  latter  form,  as  they  are  capable  of 
giving  higher  forms  of  oxidation,  OsO4  and  Ru04,  and  therefore  their 
reactions  of  decomposition  may  be  essentially  represented  by  the  equa- 
tion •  20s03=Os02  +  OsO4.10 

respects,  In  general,  Ru  has  much  in  common  with  Mn,  Joly  (1889)  also  showed  that 
if  KN03  be  added  to  a  solution  of  RuCl3  containing  HC1,  ihe  solution  becomes  hot,  and 
a  salt,  RuCl3N02KCl,  is  formed,  which  enters  into  double  decomposition  and  is  very 
stable.  Moreover,  if  RuCl3  be  treated  with  an  excess  of  nitric  acid,  it  forms  a  salt, 
BuCljNOHiO,  after  being  heated  (to  boiling)  and  the  addition  of  HC1.  The  vapour 
density  of  Ru04,  determined  by  Debray  and  Joly,  corresponds  to  that  formula. 

10  Although  palladium  gives  the  same  types  of  combination  (with  chlorine)  as 
platinum,  its  reduction  to  RX2  is  incomparably  easier  than  that  of  platinic- chloride,  and 
in  the  case  of  iridium  itj  is  also  very  easy.  Iridic  chloride,  IrCl4,  acts  as  an  oxidising 
agent,  readily  parts  with  a  fourth  of  its  •  chlorine  to  a  number  of  substances,  readily 
evolves  chlorine  when  heated,  and  it  is  only  at  low  temperatures  that  chlorine  and  aqua 
regia  convert  iridium  into  iridic  chloride.  In  disengaging  chlorine  iridium  more  often 
and  easily  gives  the  very  stable  iridious  chloride,  IrClj  (perhaps  this  substance  J8 
Ir2Cl6=IrCl2,IrCl4,  insoluble  in  water,  but  soluble  in  potassium  chloride,  because  it 
forms  the  double  salt  K3IrCl6>,  than  the  dichloride,  IrCl2.  This  compound,  corresponding 
to  IrX2,  is  very  stable,  and  corresponds  with  the  basic  oxide,  Ir2O3,  resembling  the 
oxides  Fe2O3,  Co2O3.  To  this  form  there  correspond  amm«oniacal  compounds  similar  to 
those  given  by  cobaltic  oxide.  Although  iridium  also  gives  an  acid  in  the  form  of  the 
salt  K2Ir207,  ii  does  not,  like  iron  (and  chromium),  form  the  corresponding  chloride, 
IrCl6.  In  general,  in  this  as  in  the  other  elements,  it  is  impossible  to  predict  the  chlorine 
compounds  from  those  of  oxygen.  Just  as  there  is  no  chloride  SC16,  but  only  SC12,  so 
also,  although  IrO3  exists,  IrCle  is  wanting,  the  only  chloride  being  IrCl4,  and  thia 
is  unstable,  like  SC12,  and  easily  parts  >with  its  chlorine.  In  this  respect  rhodium  is 
very  much  like  iridium  (as  platinum  is  like  palladium).  For  RhCl4  decomposes  with 
extreme  ease,  whilst  rhodium  chloride,  RhCl3,  is  very  -stable,  like  many  of  the  salts  of 
the  type  RhX3,  although  like  the  platinum  elements  these  salts  are  easily  reduced  to 
metal  by  the  action  of  heat  and  powerful  reagents.  There  is  as  close  a  resemblance 
between  osmium  and  ruthenium.  Osmium  when  submitted  to  the  action  of  dry  chlorine 
gives  osmic  chloride,  OsCl4,  but  the' latter  is  converted  by  water  (as  is  osmium  by  moist 
chlorine)  into  osmic  anhydride,  although  the  greater  portion  is  then  decomposed  >into 
Os(HO)4  and  4HC1,  like  a  chloranhydride  of  an  acid.  In  general  this  acid  character  ia 
more  developed  in  osmium  than  in  platinum  and  iridium.  Having  parted  with  chlorine, 
osmic  chloride,  OsCl4,  gives  the  unstable  trichloride,  OsCl3,  and  the  stable  soluble 
dichloride,  OsClj,  which  corresponds  with  platinous  chloride  in  its  properties  and 
reactions.  'The  relation  of  ruthenium  to  the  halogens  is  of  the  same  nature. 


886  PRINCIPLES  OF  CHEMISTRY 

Platinum  and  its  analogues,  like  iron  and  its  analogues,  are  able  t6 
form  complex  and  comparatively  stable  cyanogen  and  ammonia  com- 
pounds, corresponding  with  the  ferrocyanides  and  the  ammoniacal  com- 
pounds of  pobalt,  which  we  have  already  considered  in  the  preceding 
chapter. 

If  platinous  chloride,  PtCl2  (insoluble  in  water),  be  added  by  degrees 
to  a  solution  of  potassium  cyanide,  it  is  completely  dissolved  (like 
silver  chloride),  and  on  evaporating  the  solution  deposits  rhombic 
prisms  of  potassium  platinocyanidet  PtK2(CN)4,3H20.  This  salt,  like 
all  those  corresponding  with  it,  has  a  remarkable  play  of  colours,  due  to 
the  phenomena  of  dichromism,  and  even  polychromism,  natural  to  all 
the  platinocyanides.  Thus  it  is  yellow  and  reflects  a  bright  blue 
light.  It  is  easily  soluble  in  water,  effloresces  in  air,  then  turns  red, 
and  at  100°  orange,  when  it  loses  all  its  water.  The  loss  of  water 
does  not  destroy  its  stability — that  is,  it  still  remains  unchanged,  and 
its  stability  is  further  shown  by  the  fact  that  it  is  formed  when 
potassium  ferrocyanide,  K4Fe(CN)6,  is  heated  with  platinum  black. 
This  salt,  first  obtained  by  Gmelin,  shows  a  neutral  reaction  with 
litmus  ;  it  is  exceedingly  stable  under  the  action  of  air,  like  potassium 
ferrocyanide,  which  it  resembles  in  many  respects.  Thus  the  platinum 
in  it  cannot  be  detected  by  reagents  such  as  sulphuretted  hydrogen  j 
the  potassium  may  be  replaced  by  other  metals  by  the  action  of  their 
salts,  so  that  it  corresponds  with  a  whole  series  of  compounds,  R2Pt(CN)4, 
and  it  is  stable,  although  the  potassium  cyanide  and  platinous  salts,  of 
which  it  is  composed,  individually  easily  undergo  change.  When 
treated  with  oxidising  agents  it  passes,  like  the  ferrocyanide,  into  a 
higher  form  of  combination  of  platinum.  If  salts  of  silver  be  added 
to  its  solution,  it  gives  a  heavy  white  precipitate  of  silver  platino- 
cyanide,  PtAg2(CN)4,  which  when  suspended  in  water  and  treated 
with  sulphuretted  hydrogen,  enters  into  double  decomposition  with  the 
latter  and  forms  insoluble  silver  sulphide,  Ag2S,  and  soluble  hydro- 
platinocyanic  acid,  H2Pt(CN)4.  If  potassium  platinocyanide  be  mixed 
with  an  equivalent  quantity  of  sulphuric  acid,  the  hydroplatino- 
cyanic  acid  liberated  may  be  extracted  by  a  mixture  of  alcohol  and 
ether.  The  ethereal  solution,  when  evaporated  in  a  desiccator,  deposits 
bright  red  crystals  of  the  composition  PtH2(CN)4,5H2O.  This  acid 
colours  litmus  paper,  liberates  carbonic  anhydride  from  sodium  car- 
bonate, and  saturates  alkalis,  so  that  it  presents  an  analogy  to  hydro- 
ferrocyanic  acid.11 

11  This  acid  character  is  explained  by  the  influence  of  the  platinum  on  the  hydrogen, 
and  by  the  attachment  of  the  cyanogen  groups.  Thus  cyanuric  acid,  H3(CN)3O3,  is  an 
energetic  acid  compared  with  cyanic  acid,  HCNO.  And  the  formation  of  a  compound 


THE  PLATINUM  METALS  387 

Ammonia,  like  potassium  cyanide,  has  the  faculty  of  easily  reacting 
with  platinum  dichloride,  forming  compounds  similar  to  the  platino 

with  five  molecules  of  water  of  crystallisation,  (PtH2(CN)4,5H2O),  confirms  the  opinion 
that  platinum  is  able  to  form  compounds  of  still  higher  types  than  that  expressed 
in  its  saline  compounds,  and,  moreover,  the  combination  of  hydroplatinocyanic  acid 
with  water  does  not  reach  the  limit  of  the  compounds  which  appears  in  PtCl4,2HCl, 
6H2O. 

A  whole  series  of  platinocyanides  of  the  common  type  PtR2(CN)4nH20  is  obtained 
by  means  of  double  decomposition  with  the  potassium  or  hydrogen  or  silver  salts.  For 
example,  the  salts  of  sodium  and  lithium  contain,  like  the  potassium  salt,  three  molecules 
of  water;  The  sodium  salt  is  soluble  in  water  and  alcohol.  The  ammonium  salt  has  the 
composition  Pt(NH4)2(CN)4,2H20  and  gives  crystals  which  reflect  blue  and  rose-coloured 
light.  This  ammonium  salt  decomposes  at  800°,  with  evplution  of  water  and  ammonium 
cyanide,  leaving  a  greenish  platinum  dicyanide,  Pt(CN)o,  which  is  insoluble  in  water 
and  acid  but  dissolves  in  potassium  cyanide,  hydrocyanic  acid,  and  other  cyanides.  The 
game  platinous  cyanide  is  obtained  by  the  action  of  sulphuric  acid  on  the  potassium 
salts  in  the  form  of  a  reddish-brown  amorphous  precipitate.  The  most  characteristic  of  the 
platinocyanides  are  those  of  the  alkaline  earths.  The  magnesium  salt  PtMg(CN)4,7H2O 
crystallises  in  regular  prisms,  whose  side  faces  are  of  a  metallic  green  colour  and  terminal 
planes  dark  blue.  It  shows  a  carmine-red  colour  along  the  main  axis,  and  dark  red 
along  the  lateral  axes ;  it  easily  loses  water,  (2H2O),  at  40°,  and  then  turns  blue  (it  then 
contains  5H2O,  which  is  frequently  the  case  with  the  platinocyanides).  Its  aqueous 
solution  is  colourless,  and  an'  alcoholic  solution  deposits  yellow  crystals.  The  remainder 
Of  the  water  is  given  off  at  230°.  It  is  obtained  by  saturating  platinocyanic  acid  with 
magnesia,  or  else  by  double  decomposition  between  the  barium  salt  and  magnesium  sul- 
phate. The  strontium  salt,  SrPt(CN)4,4H2O  crystallises  in  milk-white  plates  having 
a  violet  and  green  iridescence.  When  it  effloresces  in  a  desiccator,  its  surfaces  have 
a  violet  and  metallic  green  iridescence.  A  colourless  solution  of  the  barium  salt 
PtBa(CN)4,4H2O  is  obtained  by  saturating  a  solution  of  hydroplatinocyanic  acid  with 
baryta,  or  by  boiling  the  insoluble  copper  platinocyanide  in  baryta  water  It  crystallises 
Cn  monoclinic  prisms  of  a  yellow  colour,  with  blue  and  green  iridescence ;  it  loses  half  its 
water  at  100°,  and  the  whole  at  150°.  The  ethyl  salt,  Pt(C2H5)2(CN)4,2HoO,  is  also 
very  characteristic ;  its  crystals  are  isomorphous  with  those  of  the  potassium  salt,  and 
are  obtained  by  passing  hydrochloric  acid  into  an  alcoholic  solution  of  hydroplatino- 
cyanic acid.  The, facility  with  which  they  crystallise,  the  regularity  of  their  forma, 
and  their  remarkable  play  of  colours,  renders  the  preparation  of  the  platinocyanides  one 
Of  the  most  attractive  lessons  of  the  laboratory. 

By  the  action  of  chlorine  or  dilute  nitric  acid,  the  platinocyanides  are  converted  into 
Baits  of  the  composition  PtM2(CN)5,  which  corresponds  with  Pt(CN)3,2KCN— that  is, 
they  express  the  type  of  a  non-existent  form  of  oxidation  of  platinum,  PtX3  (i.e.  oxide 
Pt2O3),  just  as  potassium  ferricyanide  (FeCy3,3KCy)  corresponds  with  ferric  oxide,  and 
the  ferrocyanide  corresponds  with  the  ferrous  oxide.  The  potassium  salt  of  this  series 
contains  PtK2(CN)5,3H2O,  and  forms  brown  regular  prisms  with  a  metallic  lustre,  and  is 
soluble  in  water  but  insoluble  in  alcohol.  Alkalis  re-convert  this  compound  into  the 
ordinary  platinocyanide  K2Pt(CN)4,  taking  tip  the  excess  of  cyanogen.  It  is  remarkable 
that  the  salts  of  the  type  PtM2Cy5  contain  the  same  amount  of  water  of  crystallisation 
as  those  of  the  type  PtM2Cy4.  Thus  the  salts  of  potassium  and  lithium  contain  three, 
and  the  salt  of  magnesium  seven,  molecules  of  water,  like  the  corresponding  salts  of  the 
type  of  platinous  oxide.  Moreover,  neither  platinum  nor  any  of  its  associates  gives  any 
cyanogen  compound  corresponding  with  the  oxide,  i.e.  having  the  composition  PtK2Cy6, 
Just  as  there  are  no  compounds  higher  than  those  which  correspond  to  KCy3nMCy  foe 
cobalt  or  iron.  This  would  appear  to  indicate  the  absence  of  any  such  cyanides,  and 
indeed,  for  no  element  are  there  yet  known,  any  poly-cyanides  containing  more  than  three 
equivalents  of  cyanogen  for  one  equivalent  of  the  element.  The  pheupmenoa  is  perhaps 


888  PRINCIPLES  OF  CHEMISTRY 

cyanide  and  cobaltia  compounds,  which  are  comparatively  stable.-  But 
as  ammonia  does  not  contain  any  hydrogen  easily  replaceable  by 

connected  with  the  faculty  of  cyanogen  of  giving  tricyanogen  polymerides,  such  as  cyanurio 
acid,  solid  cyanogen  chloride,  &c.  Under  the  action  of  an  excess  of  chlorine,  a  solution 
of  PtK2(CN)4  gives  (besides  PtK2Cy5)  a  product  PtK2Cy4Cl2,  which  evidently  contains 
the  form  PtX4,  but  at  first  the  action  of  the  chlorine  (or  the  electrolysis  of,  or  addition 
of  dilute  peroxide  of  hydrogen  to,  a  solution  of  PtK2Cy4,  acidulated  with  hydrochloric 
acid)  produces  an  easily  soluble  intermediate  salt  which  crystallises  in  thin  copper-red 
needles  (Wilm,  Hadow,  1889).  It  only  contains  a  small  amount  of  chlorine,  and 
apparently  corresponds  to  a  compound  5PtK2Cy4  +  PtK2Cy4Cl2  +  24H2O.  Under  the 
action  of  an  excess  of  ammonia  both  these  chlorine  products  are  converted  either  com- 
pletely or  in  part  (according  to  Wilm  ammonia  does  not  act  upon  PtK2Cy4)  into 
PtCy2,2NH5,  i.e.  a  platino-ammonia  compound  (see  further  on).  It  is  also  necessary  to 
pay  attention  to  the  fact  that  ruthenium  and  osmium — .which,  as  we  know,  give  higher 
forms  of  oxidation  than  platinum — are  also  able  to  combine  with  a  larger  proportion  of 
potassium  cyanide  (but  not  of  cyanogen)  than  platinum.  Thus  ruthenium  forms  a 
crystalline  hydroruthenocyanic  acid,  RuH4(CN)6,  which  is  soluble  in  water  and  alcohol, 
and  corresponds  with  the  salts  M4Ru(CN)6.  There  are  exactly  similar  osmic  com- 
pounds— for  example,  K4Os(CN)6,8H20.  The  latter  is  obtained  in  the  form  of  colourless, 
sparingly-soluble  regular  tablets  on  evaporating  the  solution  obtained  from  a  fused 
mixture  of  potassium  osmiochloride,  K2OsCl6).and  potassium  cyanide.  These  osmic  and 
ruthenic  compounds  fully  correspond  with  potassium  ferrocyanide,  K4Fe(CN)6,8H2O,  not 
Only  in  their  composition  but  also  in  their  crystalline  form  and  reactions,  which  again 
demonstrates  the  close  analogy  between  iron,  ruthenium,  and  osmium,  which  we  have 
shown  by  giving  these  three  elements  a  similar  position  (in  the  eightli  group)  in  the 
periodic  system.  For  rhodium  and  iridium  only  salts  of  the  same  type  as  the  ferricyanides, 
M3RCy6,  are  known,  and  for  palladium  only  of  the  type  M2PdCy4,  which  are  analogous 
to  the  platinum  salts.  In  all  these  examples  a  constancy  of  the  types  of  the  double 
cyanides  is  apparent.  In  the  eighth  group  we  have  iron,  cobalt,  nickel,  copper,  and  their 
analogues  ruthenium,  rhodium,  palladium,  silver,  and  also  osmium,  iridium,  platinum, 
gold.  The  double  cyanides  of  iron,  ruthenium,  osmium  have  the  type  K4R(CN)6 ;  of 
cobalt,  rhodium,  iridium,  the  type  K5R(CN)6 ;  of  nickel,  palladium,  platinum  the  type 
K2R(CN)4  and  K2R(CN)5;  and  for  copper,  silver,  gold  there  are  known  KR(CN)2,  so 
that  the  presence  of  4,  8,  2,  and  1  atoms  of  potassium  corresponds  with  the 
order  of  the  elements  in  the  periodic  system.  Those  types  which  we  have  seen 
in  the  ferrocyanidea  and  ferricyanides  of  iron  repeat  themselves  in  all  the  platinoid 
metals,  and  this  naturally  leads  to  the  conclusion  that  the  formation  of  similar 
so-called  double  salts  is  of  exactly  the  same  ^nature  as  that  of  the  ordinary  salts.  If,  in 
expressing  the  union  of  the  elements  in  the  oxygen  salts,  the  existence  of  an 
aqueous  residue  (hydroxyl  group)  be  admitted,  in  which  the  hydrogen  is  replaced  by  a 
metal,  we  have  then  only  to  apply  this  mode  of  expression  to  the  double  salts  and  the 
analogy  will  be  obvious,  if  only  we  remember  that  C12,  (CN)2,  SO4)  &c.,  are  equivalent  to 
O,  as  we  see  in  RO,  RC12,  RSO4,  &c.  They  all  =  X2)  and,  therefore,  in  point  of  fact, 
wherever  X  (  =  C1  or  OH,  &c.)  can  be  placed,  there  (C12H),  (SO4H),  &c.,  can  also  stand. 
And  as  C12H  =  Cl  +  HC1  and  SO4H  =  OH  +  SO3,  &c.,  it  follows  that  molecules  HC1  or  SO3, 
or,  in  general,  whole  molecules — for  instance,  NH3)  H2O,  salts,  &c.,  can  annex  themselves 
to  a  compound  containing  X.  (This  is  an  indirect  consequence  of  the  law  of  substitution 
which  explains  the  origin  of  double  salts,  ammonia  compounds,  compounds  with  water 
of  crystallisation,  &c.,  by  one  general  method.)  Thus  the  double  salt  MgSO4,K2SO4, 
according  to  this  reasoning,  may  be  considered  as  a  substance  of  the  same  type  as 
MgCl2,  namely,  as  =  Mg(S04K)2,  and  the  alums  as  derived  from  A1(OH)(SO4),  namely,  as 
A1(SO4K)(SO4).  Without  stopping  to  pursue  this  digression  further,  we  will  apply 
these  considerations  to  the  type  of  the  ferrocyanides  and  ferricyanides  and  thei» 
platinum  analogues.  Such  a  salt  as  K2PtCy4  may  accordingly  be  regarded  as  Pt(Cy2K)j> 


THE  PLATINUM  METALS  889 

metals,  and  As  ammonia  itself  is  able  to  combine  with  acids,  the 
PtX2  plays,  as  it  were,  the  part  of  an  acid  with  reference  to  the 

like  Pt(OH)2 ;  and  such  a  salt  as  PtK2Cy5  as  .PtCy(Cy2K)2,  the  analogue  of  PtX(OH)2, 
or  A1X(OH)2,  and  other  compounds  of  the  type  RX3.  "Potassium  ferricyanide  and  the 
analogous  compounds  of  cobalt,  iridium,  and  rhodium,  belong  to  the  same-type,  with  the 
same  difference  as  there  is  between  RX(OH)2  and  R(OH)3,  since  FeK3Cy<j=Fe(Cy2K)s. 
Limiting  myself  to  these  considerations,  which  may  partially  elucidate  the  nature 
of  double  salts,  I  will  now  pass  'again  to  the  complex,  saline  compounds  known  fo.r 
platinum. 

(A)  On  mixing  a  solution  of  potassium  thiocyanate  with  a  solution  of  potassium 
platinosochloride,  K2PtCl4,  they  form  a  double  thiocyanate,  PtK2(CNS)f4,  which  is  easily 
soluble  in  water  and  alcohol,  crystallises  in  red  prisms,  and  gives  an  orange-coloured, 
solution,  which  precipitates  salts  of  the  heavy  metals.    The  action  of  sulphuric  acid 
on  the  lead  salt  of  the  same  type  gives  the  acid  jtself,  PtH2(SCN)4,  which  corre- 
sponds with  these  salts.    The  type  of  these  compounds  is  evidently  the  same  as  that  of 
the  cyanides. 

(B)  Platinous  chloride,  PtCl2,  which  is  insoluble  in  water,  forms  double  salts  .with 
the  metallic  chlorides.    These  double  chlorides  are  soluble  in  water,  and  capable  of" 
crystallising.    Hence  when  a  hydrochloric  acid  solution  of  platinous  chloride  is  mixed 
with  solutions  of  metallic  salts  and  evaporated  it  forms  crystalline  salts  of  a  red  or 
yellow  colour.    Thus,  for   example,  the  potassium,  salt,  PtK2Cl4,  is  red,  and  easily 
soluble    in    water;    the    sodium  salt  is  also  soluble  in    alcohol;1  the    barium  salt, 
PtBaCl4,3H2O,  is  soluble  in  water,  but  the  silver  salt,  PtAg2Cl4,  is  insoluble  in  water, 
and  may  be  used  for  obtaining  the  remaining  salts  by  means  of  double  decomposition, 
with  their  chlorides. 

(C)  A  remarkable  example  of  the  complex  compounds  of  platinum  was  observed  by 
Schiitzenberger  (1868).     He  showed  that  finely-divided  platinum  in  the  presence  of 
chlorine  and  carbonic  oxide  at  250°-800°  gives  phosgene  and  a  volatile  compound  con- 
taining platinum.    The  same  substance  is  formed  by  the  action  of  carbonic  oxide  on 
platinous  chloride.    It  decomposes  with  an  explosion  in  contact  with  water.    Carbon 
tetrachloride  dissolves  a  portion  of  this  substance,  and  on  evaporation  gives  crystals  of 
2PtCl2,3CO,  whilst  the  compound  PtCl2,2CO  remains  undissolved.    When  fused  and 
sublimed  it  gives  yellow  needles  of  PtCl2,CO,  and  in  the  presence  of  an  excess  of 
carbonic  oxide  PtCl2,2CO  is  formed.    These  compounds  are  fusible  (the  first  at  250°,  the 
second  at  142°,  and  the  third  at  195°).    In  this  case  (as  in  the  double  cyanides)  com- 
bination takes  place,  because  both  carbonic  oxide  and  platinous  chloride  are  unsaturated 
compounds  capable  of  further  combination.    The  carbon  tetrachloride  solution  absorbs 
NH3  and  gives  PtCl2,CO,2NH3,  and  PtCl2,2CO,2NH3,  and  these  substances  are  analo- 
gous (Foerster,  Zeisel,  Jb'rgensen)  to  similar  compounds  containing  complex  amines  (for 
instance,  pyridine,  C5H5N),  instead  of  NH3,  and  ethylene,  &c.,  instead  of  CO,  so  that  here 
we  have  a  whole  series  of   complex,  platino-compounds.     The  compound  PtCl2CO 
dissolves  in  hydrochloric  acid  without  change,  and  the  solution  disengages  all  the 
carbonic  oxide  when  KCN  is  added  to  it,  which  shows  that  those  forces  which  bind 
2  molecules  of  KCN  to  PtCl2  can  also  bind  the  molecule  CO,  or  2  molecules  of  CO. 
When  the  hydrochloric  acid  solution  of  PtCl2CO  is  mixed  with  a  solution  of  sodium 
acetate  or  acetic  acid,  it  gives  a  precipitate  of  PtOCO,  i.e.  the  C12  is  replaced  by  oxygen 
(probably  because  the  acetate  is  decomposed  by  water).    This  oxide,  PtOCO,  splits  up 
into  Pt-f  CO2  at  350°,    PtSCO  is  obtained  by  the  action  of  sulphuretted  hydrogen  upon 
PtCl2CO.    All  this  leads  to  the  conclusion  that  the  group  PtCO  is  able  to  assimilate 
X2=C12,  S,  O,  &c.  (Mylius,  Foerster,   1891).     Pullinger  (1891),  by  igniting  spongy 
platinum  at  250°,  first  in  a  stream  of  chlorine,  and  then  in  a  stream  of  carbonic  oxide, 
obtained  (besides  volatile  products)  a  non-volatile  yellow  substance  which  remained 
unchanged  in  air  and  disengaged  chlorine  and  phosgene  gas  when  ignited ;  its  compo- 
sition was  PtCl6(CO)3,  which  apparently  proves  it  to  be  a  compound  of  PtCl.2  and 


890  PRINCIPLES  OF  CHEMISTRY 

ammonia.     Owing  to  the  influence   of  the  ammonia,  the   X2  in  the 
resultant  compound   will  represent  the   same  character  as  it  has  in 

2COC12,  as  PtCl2  is  able  to  combine  with  oxychlorides,  and  forms  somewhat  stable 
compounds. 

(D)  The  faculty  of  platinous  chloride  for  forming  stable  compounds  with  divers  sub- 
Btances  shows  itself  in  the  formation  of  the  compound  PtCl2,PCl3  by  the  action  of  phos- 
phorus pentachloride  at  250°   on  platinum  powder  (Pd  reacts  in   a  similar  manner, 
according  to  Fink,  1892).      The  product  contains  both  phosphorus   pentachloride  and 
platinum,  whilst  the  presence  of  PtCl2  is  shown  in  the  fact  that  the  action  of  water 
produces  chlorplatino-phosphorous  acid,  PtCl2P(OH)3. 

(E)  After  the.  cyanides,  the  double  salts  of  platinum  formed  by  sulphurous  acid  are 
most  distinguished  for  their  stability  and  characteristic  properties.    This  is  all  the  more 
instructive,  as  sulphurous  acid  is  only  feebly  energetic,  and,  moreover,  in  these,  as  in  all 
its  compounds,  it  exhibits  a  dual  reaction.    The  salts  of  sulphurous  acid,  R2SO3,  either 
react  as  salts  of  a  feeble  bibasic  acid,  where  the  group  SO3  presents  itself  as  bivalent, 
and  consequently  equal  to  X2,  or  else  they  react  after  the  manner  of  salts  of  a  monobasic 
acid  containing  the  same  residue,  RSO3,  as  occurs  in  the  salts  of  sulphuric  acid.    In  sul- 
phurous acid  this  residue  is  combined  with  hydrogen,  H(SO3H),  whilst  in  sulphuric  acid 
it  is  united  with  the  aqueous  residue  (hydroxyl),  OH(SO3H).    These  two  forms  of  action 
of  the  sulphites  appear  in  their  reactions  with  the  platinum  salts— that  is  to  say,  salts  of 
both  kinds  are  formed,  and  they  both  correspond  with  the  type  PtH2X4.     The  one 
series  of  salts  contain  PtH2(SO3)2,  and  their  reactions  are  due  to  the  bivalent  residue 
of  sulphurous  acid,  which  replaces  X2.      The  others,  which  have   the  composition 
PtR2(SO3H)4,  contain  sulphoxyl.    The  latter  salts  will  evidently  react  like,  acids ;  they 
are  formed  simultaneously  with  the  salts  of  the  first  kind,  and  pass  into  them.    These 
salts  are  obtained  either  by  directly  dissolving  platinous  oxide  in  water  containing  sul- 
phurous acid,  or  by  passing  sulphurous  anhydride  into  a  solution  of  platinous  chloride 
in  hydrochloric  acid.    If  a  solution  of  platinous  chloride  or  platinous  oxide  in  sulphurous 
acid  be  saturated  with  sodium  carbonate,  it  forms  a  white,  sparingly  soluble  precipitate 
containing  PtNa2(SO3Na)4,7H2O.    If  this  precipitate  be  dissolved  in  a  small  quantity  of 
hydrochloric  acid  and  left  to  evaporate  at  the  ordinary  temperature,  it  deposits  a  salt  of 
tie  other  type,  PtNa2(SO3)2,H2O,  in  the  form  of  a  yellow  powder,  which  is  sparingly 
soluble  in  water.    The  potassium  salt  analogous  to  the  first  salt,  PtK2(S03K)4,2H2O,  ia 
precipitated  by  passing  sulphurous  anhydride  into  a  solution  of  potassium  sulphite  in 
which  platinous  oxide  is  suspended.    A  similar  salt  is  known  for  ammonium,  and  with 
hydrochloric  acid  it  gives  a  salt  of  the  second  kind,  Pt(NH4)2(SO3)2)H2O.    If  ammonio- 
chloride  of  platinum  be  added  to  an  aqueous  solution  of  sulphurous  anhydride,  it  is  first 
deoxidised,  and  chlorine  is  evolved,  forming  a  salt  of  the  type  PtX2  ;  a  double  decompo- 
sition then  takes  place  with  the  ammonium  sulphite,  and  a  salt  of  the  cotnposition 
Pt(NH4)2Cl3(S03H)  is  formed  (in  a  desiccator).    The  acid  character  of  this  substance  is 
explained  by  the  fact  that  it  contains  the  elements  SO3H— sulphoxyl,  with  the  hydrogen 
not  yet  displaced  by  a  metal.    On  saturating  a  solution  of  this  acid  with  potassium 
carbonate  it  gives  orange-coloured  crystals  of  a  potassium   salt  of  th«  composition 
Pt(NH4)2Cl3(SO3K).    Here  it  is  evident  that  an  equivalent  of  chlorine  in  Pt(NH4)2Cl4  is 
replaced  by  the  univalent  residue  of  sulphurous  acid.    Among  these  salts,  that  of  the 
composition  Pt(NH4)2Cl2(S03H)2,H2O  is  very  readily  formed,  and  crystallises  in  well- 
formed  colourless  crystals;   it  is  obtained  by  dissolving  ammonium  platinosochloride, 
Pt(NH4)2014,  in  an  aqueous  solution  of  sulphurous  acid.     The  difficulty  with  which  sul- 
phurous anhydride  and  platinum  are  separated  from  these  salts  indicates  the  same  basic 
character  in  these  compounds  as  is  seen  in  the  double  cyanides  of  platinum.     In  their 
passage   into  a  complex  salt,   the  metal  platinum   and  the  group  SO2   modify  their 
relations  (compared  with  those  of  PtX2  or  SO2X2),  just  as  the  chlorine  in  the  salts  KC1O, 
KC1O3,  and  KC1O4  is  modified  in  its  relations  as  compared  with  hydrochloric  acid  or 
potassium  chloride. 


THE  PLATINUM  METALS  891 

ammoniacal  salts  ;  consequently,  the  ammoniacal  compounds  produced 
from  PtX2  will  be  salts  in  which  X  will  be  replaceable  by  various 
other  haloids,  just  as  the  metal  is  replaced  in  the  cyanogen  salts  ;  such 
is  the  nature  of  the  platino- ammonium  compounds.  PtX2  forms  com- 
pounds with  2NH3  and  with  4NH3,  and  so  also  PtX4  gives  (not 
directly  from  PtX4  and  ammonia,  but  from  the  compounds  of  PtX2  by 
the  action  of  chlorine,  &c.)  similar  compounds  with  2NHS  and  with 
4NH,.12 

(F)  No  less  characteristic  are  the  platinonitrites  formed  by  platinous  oxide.  They 
correspond  with  nitrous  acid,  whose  salts,  ENO2,  contain  the  univalent  radicle,  NO2> 
which  is  capable  of  replacing  chlorine,  and  therefore  the  salts  of  this  kind  should  form  a 
common  type  PtR2(NO2)4,  and  such  a  salt  of  potassium  has  actually  been  obtained  by 
mixing  a  solution  of  potassium  platinosochloride  with  a  solution  of  potassium  nitrite, 
when  the  liquid  becomes  colourless,  especially  if  it  be  heated,  which  indicates  the  change 
in  the  chemical  distribution  of  the  elements.  As  the  liquid  decolorises  it  gradually 
deposits  sparingly  soluble,  colourless  prisms  of  the  potassium  salt  K2Pt(N02)4,  which 
does  not  contain  any  water.  With  silver  nitrate  a  solution  of  this  salt  gives  a  precipitate 
of  silver  platinonitrite,  PtAg2(t?O2)4.  The  silver  of  this  salt  may  be  replaced  by  other 
metals  by  means  of  double  decomposition  with  metallic  chlorides.  The  sparingly  soluble 
barium  salt,  when  treated  with  an  equivalent  quantity  of  sulphuric  acid,  gives  a  soluble 
acid,  which  separates,  under  the  receiver  of  an  air-pump,  in  red  crystals ;  this  acid  has 
the  composition  PtH2(NO2)4.  -To  the  potassium  salt,  K2Pt(NO2)4,  there  correspond 
(Vezes,  1892)  K2Pt(NO2)4Br2  and  K2Pt(N02)4Cl2  and  other  compounds  of  the  same  type 
K2PtX6,  where  X  is  partly  replaced  by  Cl  or  Br  and  partly  by  (N02),  shpwing  a 
transition  towards  the  type  of  the  double  salts  like  the  platino-ammoniacal  salts.  (The 
corresponding  double  sodium  nitrite  salt  of  cobalt  is  soluble  in  water,  while  he  K,NH4 
and  many  other  salts  are  insoluble  in  water,  as  I  was  informed  by  Prof.  K.  Winkler 
in  1894). 

In  all  the  preceding  complex  compounds  of  Pt  we  see  a  common  type  PtX2,2MX 
(i.e.  of  double  salts  corresponding  to  PtO)  or  PtM2X4  =  Pt(MX2)2,  corresponding  to 
Pt(HO)2  with  the  replacement  of  O  by  its  equivalent  X2.  Two  other  facts  must  also  be 
noted.  In  the  first  place  these  X's  generally  correspond  to  elements  (like  chlorine)  or 
groups  (like  CN,  NO2,  S03,  &c.),  which  are  capable  of  further  combination.  In  the 
Second  place  all  the  compounds  of  the  type  PtM2X4  are  capable  of  combining  with 
chlorine  or  similar  elements,  and  thus  passing  into  compounds  of  the  types  PtX3  or  PtX4. 

12  The  platinum  salt  and  ammonia,  when  once  combined  together,  are  no  longer 
subject  to  their  ordinary  reactions  but  form  compounds  which  are  comparatively  very 
stable.  The  question  at  once  suggests  itself  to  all  who  are  acquainted  with  these  pheno- 
jnena,  as  to  what  is  the  relation  of  the  .elements  contained  in  these  compounds.  The  first 
explanation  is  that  these  compounds  are  salts  of  ammonium  in  which  the  hydrogen  is 
partially  replaced  by  platinum.  This  is  the  view,  with  certain  shades  of  difference,  held 
by  many  respecting  the  platino-atnmonium  compounds.  They  were  regarded  in  this 
light  by  Gerhardt,  Schiff,  Kolbe,  Weltzien,  and  many  others.  If  we  suppose  the  hydro- 
gen in  2NH4X  to  be  replaced  by  bivalent  platinum  (as  in  the  salts  PtX2),  we  shall 

obtain  ^§3Pt^— that  is,  the  compound  PtX2,2NH3.    The  compound  with  4NH3  will 

then  be  represented  by  a  'further  substitution  of  the  hydrogen  in  ammonia  by  ammo- 
nium itself— i.e.  as  NH2(NH4X)2Pt  or  PtX2,4NH3.  A  modification  of  this  view  is  found 
in  that  representation  of  compounds  of  this  kind  which  is  based  on  atomicity.  As 
platinum  in  PtX2  is  bivalent,  has  two  affinities,  and  ammonia,  NH3,  is  also  bivalent, 
because  nitrogen  is  quinquivalent  and  is  here  only  combined  with  H3,  it  is  evident  what 
bonds  should  be  represented  in  PtX2,2NH3  and  in  PtX2,4NH3.  %  In  the  former,  Pt(NH3Cl)a, 
the  nitrogen  of  each  atom  of  ammonia  is  united  by  three  affinities  with  H3,  by  one  with 


892  PRINCIPLES  OF  CHEMISTRY 

If  ammonia  acts  on  a  boiling  solution  of  platinous  chloride  in 
hydrochloric    acid,    it    produces   the  green  salt  of  Magnus  (1829), 

platinum,  and  by  the  fifth  with  chlorine.  The  other  compound  is  Pt(NH3'NH3Cl)2— that 
is,  the  N  is  united  by  one  affinity  with  the  other  N,  whilst  the  remaining  bonds  are  the 
same  as  in  the  first  salt.  It  is  evident  that  this  union  or  chain  of  ammonias  has  no 
obvious  limit,  and  the  most  essential  fault  of  such  a  mode  of  representation  is  that  it  does 
not  indicate  at  all  what  number  of  ammonias  are  capable  of  being  retained  by  platinum. 
Moreover,  it  is  hardly  possible  to  admit  the  bond  between  nitrogen  and  platinum  in  such 
stable  compounds,  for  these  kinds  of  affinities  are,  at.all  events,  feeble,  and  cannot  lead 
to  stability,  but  would'  rather  indicate  explosive  and  easily-decomposed  compounds. 
Moreover,  it  is  not  clear  why  this  platinum,  which  is  capable  of  giving  PtX^  does  not  act 
with  its  remaining  affinities  when  the  addition  of  ammonia  to  PtX3  takes  place.  These, 
and  certain  other  considerations  which  indicate  the  imperfection  of  this  representation  of 
the  structure  of  the  platino-ammonium  salts,  cause  many  chemists  to  incline  more  to  the 
representations  of  Berzelius,  Glaus,  Gibbs,  and  others,  who  suppose  that  NH^is  able  to 
combine  with  substances,  to  adjoin  itself  or  pair  itself  with  them  (this  kind  of  combination 
is  called  '  Paarung ')  without  altering  the  fundamental  capacity  of  a  substance  for  further- 
combinations.  Thus,  in  PtX2,2NH3,  the  ammonia  is  the  associate  of  PtX2,  as  is 
expressed  by  the  formula  NgHgPtXg.  Without  enlarging  on  the  exposition  of  the  details 
of  this  doctrine,  we  will  only  mention  that  it,  like  the  first,  does  not  render  it  possible  to 
foresee  a  limit  to  the  compounds  with  ammonia ;  it  isolates  compounds  of  this  kind 
into  a  special  and  artificial  class ;  does  not  show  the  connection  between  compounds  of 
this  and  of  other  kinds,  and  there'fore  it  essentially  only  expresses  the  fact  of  the  com- 
bination with  ammonia  and  the  modification  in  its  ordinary  reactions.  For  these 
reasons  we  do  not  hold  to  either  of  these  proposed  representations  of  the  ammonio- 
platinum  compounds,  but  regard  them  from  the  point  of  view  cited  above  with  reference 
to  double  salts  and  water  of  crystallisation — that  is,  we  embrace  all  these  compounds 
under  the  representation  of  compounds  of  complex  types.  The  type  of  the  compound 
PtX2,2NH3  is  far  more  probably  the  same  as  that  of  PtX2,2Z— i.e.  as  PtX4,  or,  still  more 
•accurately  and  truly,  it  is  a  compound  of  the  same  type  as  PtX2,2KX  or  PtX2,2H2O,  &c. 
Although  the  platinum  first  entered  into  PtK2X4  as  the  type  PtX2,  yet  its  character  has 
changed  in  the  same  manner  as  the  character  of  sulphur  changes  when  from  SO2  the  com- 
pound SOo(OH)3  is  obtained,or  when  KCIO^,  the  higher  form,  is  obtained  from  KC1.  For  us 
as  yet  there  is  no  question  as  to  what  affinities  hold  X2  and  what  hold  2NH3,  because  this 
is  a  question  which  arises  from  the  supposition  of  the  existence  of  different  affinities  in  the 
atoms,  which  there  is  no  reason  for  taking  as  a  common  phenomenon.  It  seems  to  me 
that  it  is  most  important  as  a  commencement  to  render  clear  the  analogy  in  the  formation 
of  various  complex  compounds,  and  it  is  this  analogy  of  the  ammonia  compounds  with 
those  of  water  of  crystallisation  and  double  salts  that  forms  the  main  object  of  the 
primary  generalisation.  We  recognise  in  platinum,  at  all  events,  not  only  the  four, 
affinities  expressed  _in  the  compound  PtCl4,  but  a  much  larger  number  of  them,  if  only 
the  summation  of  affinities  is  actually  possible.  Thus,  in  sulphur  we  recognise  not  two 
but  a  much  greater  number  of  affinities;  it  is  clear  that  at  least  six  affinities  can  act. 
So  also  among  the  analogues  of  platinum :  osmic  anhydride,  Os04,  Ni(CO)4)  PtH2Cl6l  &c. 
indicate  the  existence  of  at  least  eight  affinities ;  whilst,  in  chlorine,  judging  from  the 
compound  KC1O4  =  C1O3(OK)  =  C1X7,  we  must  recognise  at  least  seven  affinities, 
instead  of  the  one  which  is  accepted.  The  latter  mode  of  calculating  affinities  is  a  tribute 
to  that  period  of  the  development  of  science  when  only  the  simplest  hydrogen  compounds 
were  considered,  and  when  all  complex  compounds  were  entirely  neglected  (they  were 
placed  under  the  class  of  molecular  compounds).  This  is  insufficient  for  the  present 
state  of  knowledge,  because  we  find  that,  in  complex  compounds  as  in  the  most  simple,  the 
came  constant  types  or  modes  of  equilibrium  are  repeated,  and  the  character  of  certaim 
elements  is  greatly  modified  in  the  passage  from  the  most  simple  into  very  complex 
compounds. 


THE  PLATINUM  METALS  393 

PtCl2,2NH3,  insoluble  in  water  and  hydrochloric  acid.  But,  judging 
by  its  reactions,  this  salt  has  twice  this  formula.  Thus,  Gros  (1837), 
on  boiling  Magnus's  salt  with  nitric  acid,  observed  that  half  the  chlorine 
was  replaced  by  the  residue  of  nitric  acid  and  half  the  platinum  was 
disengaged  >  2PtCl2(NH3)2  +  2HNO3  =  PtCl2(NO3)2(NH3)4  +  2PtCl2. 
The  Gros's  salt  thus  obtained,  PtCl2(NO3)24NH3  (if  Magnus's  salt 

Judging  from  the  most  complex  platino-ammonium  compounds  PtCl4,4NH3,  we 
should  admit  the  possibility  of  the  formation  of  compounds  of  the  type  PtX4Y4,  where 
Y4  =  4X2  =  4NH3,'  and  this  shows  that  those  forces  which  form  such  a  characteristic 
Beries  of  double  platinocyanides  PtK2(CN)4,3H20,  probably  also  determine  the  formation 
of  the  higher  ammonia  derivatives,  as  is  seen  on  comparipg — 

PtCl2       NH3     C12      8NH5 

Pt(CN),  KCN  KCN  8H,0. 

Moreover,  it  is  obviously  much  more  natural  to  ascribe  the  faculty  for  combination 
with  wY  to  the  whole  of  the  acting  elements— that  is;  to  PtX2  or  PtX4)  and  not  to 
platinum  alone.  Naturally  such  compounds  are  not  produced  with  any  Y.  With 
certain  X's  there  only  combine  certain  Y's.  The  best  known  and  most  frequently- 
formed  compounds  of  this  kind  are  those  with-  water— that  is,  compounds  with  water  of 
crystallisation.  Compounds  with  salts  are  double  salts;  also  we  know  that  similar 
compounds  are  also  frequently  formed  by  means  of  ammonia.  Salts  of  zinc,  ZnXj, 
copper,  CuX2,  silver,  AgX,  and  many  others  give  similar  compounds,  but  these  and  many 
other  ammonia-metallic  saline  compounds  are  unstable,  and  readily  part  with  their 
combined  ammonia,  and  it  is  only  in  the  elements  of  the  platinum  'group  and  in  the 
group  of  the  analogues  of  iron,  that  we  observe  the  faculty  to  form  stable  ammonio- 
metallic  compounds.  It  must  be  remembered  that  the  metals  of  the  platinum  and  iron 
groups  are  able  to  form  several  high  grades  of  oxidation  which  have  an  acid  character, 
and  consequently  in  the  lower  degrees  of  combination  there  yet  remain  affinities  capable 
of  retaining  other  •  elements,  and  they  probably  retain  ammonia,  and  hold  it  the  more 
stably,  because  all  the  properties  of  the  platinum  compounds  are  rather  acid  than  basio 
—that  is,  PtXn  recalls  rather  HX  or  SnXrt  or  CXn  than  KX,  CaX2,  BaX2,  &c.,  and 
ammonia  naturally  will  rather  combine  with  an  acid  than  with  a  basic  substance. 
Further,  a  dependence,  or  certain  connection  of  the  forms  of  oxidation  with  the  ammonia 
compounds,  is  seen  on  comparing  the  following  compounds : 

PdCL,2NH5)H20  PdCl2,4NH3,H20 

PtCl2,2NH3  PtCl4)4NH3 

RhCl3,5NH3  EuCl2,4NH3,8H20 

IrCl3,5NH3  OsCl2,4NH3,2H2O 

We  know  that  platinum  and  palladium  give  compounds  of  lower  types  than  iridium 
and  rhodium,  whilst  ruthenium  and  osmium  give  the  highest  forms  of  oxidation ;  this 
shows  itself  in  this  case  also.  We  have  purposely  cited  the  same  compounds  with  4NH5 
for^osmium  and  ruthenium  as  we  have  for  platinum  and  palladium,  and  it  is  then  seen 
that  Ru  and  Os  are  capable  of  retaining  2H2O  and  8H20,  besides  C12  and  NH3,  which 
.the  compounds  of  platjjmm  and  palladium  are  unable  to  do.  The  same  ideas 
which  were  developed  in  Note  85,  Chapter  XXII.  respecting  the  cobaltia  compounds  are 
perfectly  applicable  to  the  present  case,  i.e.  to  the  platinia  compounds  or  ammonia 
compounds  of  the  platinum  metals,  among  which  Rh  and  Ir  give  compounds  which  are 
perfectly  analogous  to  the  cobaltia  compounds. 

Iridium  and  rhodium,  which  easily  give  compounds  of  the  type  RX3,  give  compound* 
(Glaus)  of  the  type  IrX3,5NH3,  of  a  rose  colour,  and  RhX3,5NH3,  of  a  yellow  colour. 
Jbrgensen,  in  his  researches  on  these  compounds,  showed  their  entire  analogy  with  the 
.cobalt  compounds,  as  was  to  be  expected  from  the  periodic  system. 


894  PRINCIPLES  OF  CHEMISTRY 

belongs  to  the  type  PtX2,  then  Gros's  salt  belongs  to  the  type  PtX4),  is 
soluble  in  water,  and  the  elements  of  nitric  acid,  but  not  the  chlorine, 
contained  in  it  are  capable  of  easily  submitting  themselves  to  double 
saline  decomposition.  Thus  silver  nitrate  does  not  enter  into  double 
decomposition  with  the  chlorine  of  Gros's  salt.  Most  instructive  was 
the  circumstance  that  Gros,  by  acting  on  his  salt  with  hydrochloric 
acid,  succeeded  in  substituting  the  residue  of  nitric  acid  in  it  by 
chlorine,  and  the  chlorine  thus  introduced,  easily  reacted  with  silver 
nitrate.  Thus  it  appeared  that  Gros's  salt  contained  two  varieties  of 
chlorine  —  one  which  reacts  readily,  and  the  other  which  reacts  with 
difficulty.  The  composition  of  Gros's  first  salt  is  PtCl2(NH3)4(ISr03)2  ; 
it  may  be  converted  into  PtCl2(NH3)4(S04),  and  in  general  into 


The  salt  of  Magnus  when  boiled  with  a  solution  of  ammonia  gives 
the  salt  (of  Reiset's  first  base)  PtCl2(NH3)4,  and  this,  when  treated 
With  bromine,  forms  the  salt  PtCl2Br2(NH3)4,  which  has  the  same 
composition  and  reactions  as  Gros's  salt.  To  Reiset's  salts  there 
corresponds  a  soluble,  colourless,  crystalline  hydroxide,  Pt(OH)2(NH3)4, 
having  the  properties  of  a  powerful  and  very  energetic  alkali  ;  it 
attracts  carbonic  anhydride  from  the  atmosphere,  precipitates  metallic 
Salts  like  potash,  saturates  active  acids,  even  sulphuric,  forming 
colourless  (with  nitric,  carbonic,  and  hydrochloric  acids),  or  yellow 
(with  sulphuric  acid),  salts  of  the  type  PtX2(NH3)4J4  The  com- 

13  Subsequently,  a  whole  series  of  such  compounds  was  obtained  with  various 
elements  in  the  place  of  the  (non-reacting)  chlorine,  and  nevertheless  they,  like  the 
chlorine,  reacted  with  difficulty,  whilst  the  second  portion  of  the  X's  introduced  into 
such  salts  easily  underwent  reaction.    This  formed  the  most  important  reason  for  the 
interest  which  the  study  of  the  composition  and  structure  of  the  platino-ammonium 
salts  subsequently  presented  to  many  chemists,  such  as  Eeiset,  Bloinstrand,  Peyrone, 
Kaeffski,  Gerhardt,  Buckton,  Cleve,  Thomsen,  Jb'rgensen,  Kournakoff,  Verner,  and 
others.    The  salts  PtX4,2NH3,  discovered  by  Gerhardt,  also  exhibited  several  different 
properties  in  the  two  pairs  of  X's.    In  the  remaining  platino-ammonium  salts  all  the 
X's  appear  to  react  alike. 

The  quality  of  the  X's,  retainable  in  the  platino-ammonium  salts,  may  be  considerably 
modified,  and  they  may  frequently  be  wholly  or  partially  replaced  by  hydroxyl.  For 
example,  the  action  of  ammonia  on  the  nitrate  .of  Gerhardt's  base,  Pt(NO5)4,2NH3,  in  a 
boiling  solution,  gradually  produces  a  yellow  crystalline  precipitate  which  is  nothing 
else  than  a  basic  hydrate  or  alkali,  Pt(OH)4,2NH5.  It  is  sparingly  soluble  in  water,  but 
gives  directly  soluble  salts  PtX.j,2NH3  with  acids.  The  stability  of  this  hydroxide  is 
such  that  potash  does  not  expel  ammonia  from  it,  even  on  boiling,  and  it  does  not  change 
below  130°.  Similar  properties  are  shown  by  the  hydroxide  Pt(OH)2,2NH5  and  the 
oxide  PtO,2NH3  of  Reiset's  second  base.  But  the  hydroxides  of  the  compounds  con- 
taining 4NH3  are  particularly  remarkable.  The  presence  of  ammonia  renders  them 
soluble  and  energetic.  The  brevity  of  this  work  does  not  permit  us,  however,  to 
mention  many  interesting  particulars  in  connection  with  this  subject. 

14  Hydroxides  are  known  corresponding  with  Gros's  salts,  which  contain  one  hydroxyl 
group  in  the  place  of  that  chlorine  or  haloid  which  in  Gros's  salts  reacts  with  difficulty, 


THE  PLATINUM  METALS  895 

parative  stability  (for  instance,  as  compared  with  AgCl  and  NH3)  of 
such  compounds,  and  the  existence  of  many  other  compounds  analogous 

and  these  hydroxides  do  not  at  once  show  the  properties  of  alkalis,  just  as  the  chlorine 
which  stands  in  the  same  place  does  not  react  distinctly ;  but  still,  after  the  prolonged 
action  of  acids,  this  hydroxyl  group  is  also  replaced  by  acids.  Thus,  for  example,  the  action 
of  nitric  acid  on  Pt(N03)2Cl2,4NH3  causes  the  non-active  chlorine  to  react,  but  in  the 
product  all  the  chlorine  is  not  replaced  by  NO3,  but  only  half,  and  the  other  half  is  replaced 
by  the  hydroxyl  group :  Pt(NO3)2Cl2,4NH3  +  HNO3  +  H2O  =  Pt(NO3)3(OH),4NH5  +  2HC1 ; 
and  this  is  particularly  characteristic,  because  here  the  hydroxyl  group  has  not  reacted 
with  the  acid — an  evident  sign1  of  the  non-alkaline  character  of  this  residue.  I  think  it 
may  be  well  to  call  attention  to  the  fact  that  the  composition  of  the  ammonio-metalla- 
salts  very  often  exhibits  a  correspondence  between  the  amount  of  X's  and  the  amount 
of  NH3,  of  such  a  nature  that  we  find  they  contain  either  XNH3  or  the  grouping 
X2NH3;  for  example,  Pt(XNH3)2  and  Pt(X2NH3)2,  Co(X2NH3)3,  Pt(XNH3)4,  &o. 
Judging  from  this,  the  view  of  the  constitution  of  the  double  cyanides  of  platinum 
given  hi  Note  11  finds  some  confirmation  here,  but,  in  my  opinion,  all  questions 
respecting  the  composition  (and  structure)  of  the  ammoniacal,  double,  complex,  and 
crystallisation  compounds  stand  connected  with  the  solution  of  questions  respecting  the 
formation  of  compounds  of  various  degrees  of  stability,  among  which  a  theory  of 
solutions  must  be  included,  and  therefore  I  think  that  the  time  has  not  yet  come  for  a 
complete  generalisation  of  the  data  which  exist  for  these  compounds ;  and  here  I  again 
refer  the  reader  to  Prof.  Kournakoff's  work  cited  in  Chapter  XXII.,  Note  85.  However, 
we  may  add  a  few  individual  remarks  concerning  the  platinia  compounds. 

To.  the  common  properties  of  the  platino-ammonium  salts,  we  must  add  not  only  their 
Stability  (feeble  acids  and  alkalis  do  not  decompose  them,  the  ammonia  is  not  evolved 
by  heating,  &c.),  but  also  the  fact  that  the  ordinary  reactions  of  platinum  are  concealed 
in  them  to  as  great  ah  extent  as  those  of  iron  in  the  ferricyanides.  Thus  neither  alkalis 
nor  hydrogen  sulphide  will  separate  the  platinum  from  them.  For  example,  sulphuretted 
hydrogen  in  acting  on  Gros's  salts  gives  sulphur,  removes  half  the  chlorine  by'  means  of 
its  hydrogen,  and  forms  salts  of  Reiset's  first  base.  This  may  be  understood  or  explained 
by  considering  the  platinum  in  the  molecule  as  covered,  walled  up  by  the  ammonia,  or 
situated  in  the  centre  of  the  molecule,  and  therefore  inaccessible  to  reagents.  On  this 
assumption,  however,  we  should  expect  to  find  clearly-expressed  ammoniacal  properties, 
and  this  is  not  the  case.  Thus  ammonia  is  easily  decomposed  by  chlorine,  whilst  iu 
acting  on  the  platino-ammonium  salts  containing  PtX2  and  2NH3  or  4NH3,  chlorine 
combines  and  does  not  destroy  the  ammonia ;  it  converts  Eeiset's  salts  into  those  of. 
Gros  and  Gerhardt.  Thus  from  PtX2,2NH3  there  is  formed  PtX2Cl2,2NH3,  and  from 
PtX2,4NH5  the  salt  of  Gros's  base  PtX2Cl2,4NH3.  This  shows  that  the  amount  .'of 
chlorine  which  combines  is  not  dependent  on  the  amount  of  ammonia  present,  but  is  due 
to  the  basic  properties  of  platinum.  Owing  to  this  some  chemists  suppose  the  ammonia 
to  be  inactive  or  passive  in  certain  compounds.  It  appears  to  me  that  these  relations, 
these  modifications,  in  the  usual  properties  of  ammonia  and  platinum  are  explained 
directly  by  their  mutual  combination.  Sulphur,  in  sulphurous  anhydride,  SO2,  and 
hydrogen  sulphide,  SH2,  is  naturally  one  and  the  same,  but  if  we,  only  knew  of  it  in  the 
form  of  hydrogen  sulphide,  then,  having  obtained  it  in  the  form  of  sulphurous  anhydride, 
we  should  consider  its  properties  as  hidden.  The  oxygen  in  magnesia,  MgO,  and  in 
nitric  peroxide,  NO2,  is  so  different  that  there  is  no  resemblance.  Arsenic  no  longer 
reacts  in  its  compounds  with  hydrogen  as  it  reacts  in  its  compounds  with  chlorine,  and 
In  their  compounds  with  nitrogen  all  metals  modify  both  their  reactions  and  their  physical 
properties.  We  are  accustomed  to  judge  the  metals  by  their  saline  compounds  with 
haloid  groups,  and  ammonia  by  its  compounds  with  acid  substances,  and  here,  in  the 
platino-compounds,  if  we  assume  the  platinum  to  be  bound  to  the  entire  mass  of  the 
ammonia — to  its  hydrogen  and  nitrogen — we  shall  understand  that  both  the  platinum 
and  ammonia  modify  their  characters.  Far  more  complicated  is  the  question  why  a  por« 


896  PRINCIPLES  OF  CHEMISTRY 

'to  them,  endows  them  with  a  particular  chemical  interest.  Thus 
Kournakoff  (1889)  obtained  a  series  of  corresponding  compounds  contain- 

tion  of  the  chlorine  (and  other  haloid  simple  and  complex  groups)  in  Gros's  salts  acts  Ui 
a  different  manner  from  the  other  portion,  and  why  only  half  of  it  acts  in  the  usual  way. 
But  this  also  is  not  an  exclusive  case.  The  chlorine  in  potassium  chlorate  or  in  carbon 
tetrachloride  does  not  react  with  the  same  ease  with  metals  as  the  chlorine  in  the  salts 
corresponding  with  hydrochloric  acid.  In  this  case  it  is  united  to  oxygen  and  carbon, 
whilst  in  the  platino-ammonium  compounds  it  is  united  partly  to  platinum  and  partly  to 
the  platino-ammonium  group.  Many  chemists,  moreover,  suppose  that  a  part  of  the 
chlorine  is  united  directly  to  the  platinum  and  the  other  part  to  the  nitrogen  of  the 
ammonia,  and  thus  explain  the  difference  of  the  reactions;  but  chlorine  united  to 
platinum  reacts  as  well  with  a  silver  salt  as  the  chlorine  of  ammonium  chloride,  NH4C1, 
or  nitrosyl  chloride,  NOC1,  although  there  is  no  doubt  that  in  this  case  there  is  a 
union  between  the  chlorine  and  nitrogen.  Hence  it  is  necessary  to  explain  the  absence 
of  a  facile  reactive  capacity  in  a  portion  of  the  chlorine  by  the  conjoint  influence 
Of  the  platinum  and  ammonia  on  it,  whilst  the  other  portion  may  be  admitted  as 
being  under  the  influence  of  the  platinum  only,  and  therefore  as  reacting  as  in  other 
Baits.  By  admitting  a  certain  kind  of  stable  union  in  the  platino-ammonium  grouping, 
it  is  possible  to  imagine  that  the  chlorine  does  not  react  with  its  customary  facility, 
because  access  to  a  portion  of  the  atoms  of  chlorine  in  this  complex  grouping  is  difficult, 
and  the  chlorine  union  is  not  the  same  as  we  usually  meet  in  the  saline  compounds  of 
chlorine.  These  are  the  grounds  on  which  we,  in  refuting  the  now  accepted  explanations 
of  the  reactions  and  formation  of  the  platino-compounds,  pronounce  the  following  opinion 
as  to  their  structure. 

In  characterising  the  platino-ammomum  compounds,  it  is  necessary  to  bear  in  mind 
that  compounds  which  already  contain  PtX4  do  not  combine  directly  with  NH3,  and  that 
such  compounds  as  PtX4,4NH3  only  proceed  from  PtX2,  and  therefore  it  is  natural  to 
conclude  that  thbse  affinities  and  forces  which  cause  PtX2  to  combine  with  X2  also  cause 
it  to  combine  with  2NH3.  And  having  the  compound  PtX^NHj,  and  supposing  that  in 
subsequently  combining  with  C12  it  reacts  with  those  affinities  which  produce  the  com- 
pounds of  platinic  chloride,  PtCl4,  with  water,  potassium  chloride,  potassium  cyanide, 
hydrocliloric  acid,  and  the  like,  we  explain  not  only  the  fact  of  combination,  but  also 
many  of  the  reactions  occurring  in  the  transition  of  one  kind  of  platino-ammonium  salts 
into  another.  Thus  by  this  means  we  explain  the  fact  that  (1)  PtX2,2NH3  combines 
with  2NH3,  forming  salts  of  Beiset's  first  base ;  (2)  and  the  fact  that  this  compound 
•(represented  as  follows  for  'distinctness),  PtX2,2NH3,2NH3,  when  heated,  or  even  when 
boiled  in  solution,  again  passes  into  PtX2)2NH3  (which  resembles  the  easy  disengage- 
ment of  water  of  crystallisation,  &c.) ;  (8)  the  fact  that  PtX2,2NH3  is  capable  of  absorbing, 
under  the  action  of  the  same  forces,  a  molecule  of  chlorine,  PtX2,2NH3,Cl2,  which  it 
then  retains  with  energy,  because  it  is  attracted^  not  only  by  the  platinum,  but  also  by  the 
hydrogen  of  the  ammonia;  (4)  the  fact  that  this  chlorine  held  in  this  compound  (of 
Gerhardt)  will  have  a  position  unusual  in  salts,  which  will  explain  a  certain  (although 
very  feebly-marked)  difficulty  of  reaction;  (5)  the  fact  that  this  does  not  exhaust  the 
faculty  of  platinum  for  further  combination  (we  need  only  recall  the  compound 
PtCl4,2HCl,16H2O), and  that  therefore  both  PtX2,2NH3,Cl2  and  PtX2,2NH3,2NH3  are  still 
capable  of  combination,  whence  the  latter,  with  chlorine,  gives  PtXo,2NH3,2NH3,Cl2, 
after  the  type  of  PtX4Y4  (and  perhaps  higher) ;  (6)  the  fact  that  Gros's  compounds 
thus  formed  are  readily  re-converted  into  the  salts  of  Reiset's  first  base  when  acted  on 
by  reducing  agents ;  (7)  the  fact  that  in  Gros's  salts,  PtX2,2NH5(NH3X)2,  the  newly- 
attached  chlorine  or  haloid  will  react  with  difficulty  with  salts  of  silver,  &c.,  because  it  is 
attached  both  to  the  platinum  and  to  the  ammonia,  for  both  of  which  it  has  an  attraction ; 
(8)  the  fact  that  the  faculty  for  further  combination  is  not  even  yet  exhausted  in  tho 
type  of  Gros's  salts,  and  that  we  actually  have  a  compound  of  Gros's  chlorine  salt  with 
platinous  chloride  and  with  platinic  chloride;  the  salt  PtS04,2NH3,2NH3,SO4  com- 
bines further  also  with  HjO  j  (9)  the  fact  that  such  a  faculty  of  combination  with  now 


THE  PLATINUM  METALS  £97 

!ing  thiocarbamide,  CSN2H4,  in  the  place  of  ammonia,  PtCl2,4CSN8H4, 
and  others  corresponding  with  Reiset's  salts.  Hydroxylaraine,  and 
other  substances  corresponding  with  ammonia,  also  give  similar  com- 
pounds. The  common  properties  and  composition  of  such  compounds 
show  their  entire  analogy  to  the  cobaltia  compounds  (especially  for 
ruthenium  and  iridium)  and  correspond  to  the  fact  that  both  the 
platinum  metals  and  cobalt  occur  in  the  same,  eighth,  group. 

molecules  is  naturally  more  developed  in  the  lower  forms  of  combination  than  in  the 
higher.  Hence  the  salts  of  Reiset's  first  base— for  example,  PtClo,2NH3,2NH3— both 
Combine  with  water  and  give  precipitates  (soluble  in  water  but  not  in  hydrochloric  acid) 
of  double  salts  with  many  salts  of  the  heavy  metals — for  example,  with  lead  chloride, 
cupric  chloride,  and  also  with  platinic  and  platinous  chlorides' (Buckton's  salts).  The 
latter  compounds-  will  have  the  composition  PtCl2,2NH3,2NH3,PtCl3 — that  is,  the  same 
composition  as  the  salts  of  ReiBet's  second  base,  but  it  cannot  be  identical  with  it. 
Such  an  interesting  case  does  actually  exist.  The  first  salt,  PtCl2,4NH3,PtCl2,  is  green, 
insoluble  in  water  and  in  hydrochloric  acid,  and  is  known  as  Magnus's  salt,  and  the 
second,  PtCl2,2NH3,  is  Reiset's  yellow,  sparingly  soluble  (in  water).  They  are  polymeric, 
tiamely,  the  first  contains  twice  the  number  of  elements  held  in  the  second,  and  at  the 
same  time  they  easily  pass  into  each  other.  If  ammonia  be  added  to  a  hot  hydrochloric 
acid  solution  of  platinous  chloride,  it  forms  the  salt  PtCl2,4NH3,  but  in  the  presence  of 
an  excess  of  platinous  chloride  it  gives  Magnus's  salt.  On  boiling  the  latter  in  ammonia  it 
gives  a  colourless  soluble  salt  of  Reiset's  first  base,  PtCl2,4NH3,  and  if  this  be  boiled  with 
ttrater,  ammonia  is  disengaged,  and  a  salt  of  Reiset's  second  T>ase,  PtCl2,2NH3,  is  obtained. 
A  class  of  platino-ammonium  isomerides  (obtained  by  Millon  and  Thomson)  are  also 
known.  Buckton's  salts — for  example,  the  copper  salt — were  obtained  by  them  from  the 
salts  of  Reiset's  first  base,  PtCl2,4NH3,  by  treatment  with  a  solution  of  cupric  chloride, 
&c.,  and  therefore,  according  to  our  method  of  expression,  Buckton's  copper  salt  will  be 
PtCl2,4NH3,CuCl2.  This  salt  is  soluble  in  water,  but  not  in  hydrochloric  acid.  In  ic 
the  ammonia  must  be  considered  as  united  to  the  platinum.  But  if  cupric  chloride  be 
dissolved  in  ammonia,  and  a  solution  of  platinous  chloride  in  ammonium  chloride  is 
added  to  it,  a  violet  precipitate  is  obtained  of  the  same  composition  as  Buckton's 
salt,  which,  however,  is  insoluble  in  water,  but  soluble  in  hydrochloric  acid.  In  this  a 
portion,  if  not  all,  of  the  ammonia  must  be  regarded  asN  united  to  the  copper,  and  it  must 
therefore  be  represented  as  CuCl2,4NH3,PtCl2.  This  form  is  identical  in  composition 
but  different  in  properties  (is  isomeric)  with  the  preceding  salt  (Buckton's).  The  salt  of 
Magnus  is  intermediate  between  them,  PtCl2,4NH3,PtCl.i ;  it  is  insoluble  in  water. and 
hydrochloric  acid.  These  and  certain  other  instances  of  isomeric  compounds  in  the 
series  of  the  platino-ammQnium  salts  throw  a  light  on  the  nature  of  the  compounds  in 
.question,  just  as  the  study  of  the  isomerides  of  the  carbon  compounds  has  served  and' 
still  serves  as  the  chief  cause  of  the  rapid  progress  of  organic  chemistry.  In  conclusion, 
we  may  add  that  (according  to  the  law  of  substitution)  we  must  necessarily  expect  all 
kinds  of  intermediate  compounds  between  the  platino  and  analogous  ammonia  deriva- 
tives on  the  one  hand,  and  the  complex  compounds  of  nitrous  acid  on  the  other, 
Perhaps  the  instance  of  the  reaction  of  ammonia  upon  osmic  anhydride,  Os04,  observed 
by  Fritsche,  Fre"my,  and  others,  and  more  fully  studied  by  Joly  (1891),  belongs  to  this 
class.  The  latter  showed  that  when  ammonia  acts  upon  an  alkaline  solution  of  OsO4 
the  reaction  proceeds  according  to  the  equation:  Os04  +  KHO  +  NH3  =  OsNK03  +  2H2O. 
It  might  be  imagined  that  in  this  case  the  ammonia  is  oxidised,  probably  forming  the 
residue  of  nitrous  acid  (NO),  while  the  type  Os04  is  deoxidised  into  Os02,  and  a  salt, 
OsO(NO)(KO),  of  the  type  ©8X4  is  formed.  This  salt  crystallises  well  in  light  yellow 
octahedra.  It  corresponds  to  osmiamic  acid,  OsO(ON)(HO),  whose  anhydride, 
[OsO(NO)]2,  has  the  composition  Os2N2O5,  which  equals  2Os  +  N205  to  the  same  extent 
&s  the  above-mentioned  compound  PtC02  equals  Pt  +  C0«  (see  Note  11). 


398  PRINCIPLES  OF  CHEMISTRY 


CHAPTER  XXIV 

COPPER,   SILVER,   AND  GOLD 

THAT  degree  of  analogy  and  difference  which  exists  between  iron, 
cobalt,  and  nickel  repeats  itself  in  the  corresponding  triad  ruthenium, 
rhodium,  and  palladium,  and  also  in  the  heavy  platinum  metals, 
osmium,  iridium,  and  platinum.  These  nine  metals  form  Group  VIII. 
pf  the  elements  in  the  periodic  system,  being  ther  intermediate  group 
between  the  even  elements  of  the  large  periods  and  the  uneven,  among 
which  we  know  zinc,  cadmium,  and  mercury  in  Group  II.  Copper, 
silver,  and  gold  complete  l  this  transition,  because  their  properties 
place  them  in  proximity  to  nickel,  palladium,  and  platinum  on  the  one 
hand,  and  to  zinc,  cadmium,  and  mercury  on  the  other.  Just  as  Zn, 
Cd,  and  Hg  ;  Fe,  Ru,  and  Os  ;  Co,  Rh,  and  Ir  ;  Ni,  Pd,  and  Pt, 
resemble  each  other  in  many  respects,  so  also  do  Cu,  Ag,  and  Au. 
Thus,  for  example,  the 'atomic  weight  of  copper  Cu  =  63,  and  in  all  its 
properties  it  stands  between  Ni  =  59  and  Zn  =  65.  But  as  the  tran- 
sition from  Group  VIII.  to  Group  II.,  where  zinc  is  situated,  cannot  be 
otherwise  than  through  Group  L,  so  in  copper  there  are  certain  pro- 
perties of  the  elements  of  Group  I.  Thus  it  gives  a  suboxide,  Cu20, 
and  salts,  CuX,  like  the  elements'  of  Group  I.,  although  at  the  same 
time  it  forms  an  oxide,  CuO,  and  salts  CuX2,  like  nickel  and  zinc.  In 
the  state  of  the  oxide,  CuO,  and  the  salts,  CuX2,  copper  is  analogous  to 
zinc,  judging  from  the  insolubility  of  the  carbonates,  phosphates,  and 
similar  salts,  and  by  the  isomorphism,  and  other  characters.2  In  the 
cuprous  salts  there  is  undoubtedly  a  great  resemblance  to  the  silver 

1  The  perfectly  unique  position  held  by  copper,  silver,  and  gold  in  the  periodic  system 
of  the  elements,  and  the  degree  of  affinity  which  is  found  between  them,  is  all  the  more 
remarkable,  as  nature  and  practice  have  loug  isolated  these  metals  from  all  others  by 
having    employed    them — for    example,    for    coinage — and    determined  their  relative 
importance  and  value  in  conformity  with  the  order  (silver  between  copper  and  gold)  of 
their  atomic  weights,  &c. 

2  Cupric  sulphate  contains  5  molecules  of  water,  CuS04,5H2O,  and  the  isomorphous 
mixtures  with  ZnSO4,7H2O  contain  either  5  or  7  equivalents,  according  to  whether  copper 
or  zinc  predominates  (Vol.  II.  p.  G).     If  there  be  a  large  proportion  of  copper,  and  if  the 
mixture  contain  5H2O,  the  form  of  the  isomorphous  mixture  (triclinic)  will  be  isomorphous 
with  cupric  sulphate,  CuS04,5H2O,  but  if  a  large  amount  of  zinc  (or  magnesium,  iron, 
nickel,  or  cobalt)  be  present  the  form  (rhombic  or  monoclinic)  will  be  nearly  the 


COPPER,   SILVER,  AND  GOLD  399 

salts — thus,  for  example,  silver  chloride,  AgCl,  is  characterised  by  its 
insolubility  and  capacity  of  combining  with  ammonia,  and  in  this  respect 
cuprous  chloride  closely  resembles  it,  for  it  is  also  insoluble  in  water, 
and  combines  with  ammonia  and  dissolves  in  it,  &c.  Its  composition  is 
also  RC1,  the  same  as  AgCl,  NaCl,  KC1,  &c.,  and  silver  in  many  com- 
pounds resembles,  and  is  even  isomorphous  with,  sodium,  so  that  this 
again  justifies  their  being  brought  together.  Silver  chloride,  cuprous 
chloride,  and  sodium  chloride  crystallise  in  the  regular  system. 
Besides  which,  the  specific  heats  of  copper  and  silver  require  that  they 
should  have  the  atomic  weights  ascribed  to  them.  To  the  oxides  Cu2O 
and  Ag2O  there  are  corresponding  sulphides  Ag2S  and  Cu2S.  They 
both  occur  in  nature  in  crystals  of  the  rhombic  system,  and,  what  is 
most  important,  copper  glance  contains  an  isomorphous  mixture  of 
them  both,  and  retains  the  form  of  copper  glance  with  various  pro- 
portions of  copper  and  silver,  and  therefore  has  the  composition  R2S 
where  E  =  Cu,  Ag. 

Notwithstanding  the  resemblance  in  the  atomic  composition  of  the 
tuprous  compounds,  CuX,  and  silver  compounds,  AgX,  with  the  com- 
.pouhds  of  the  alkali  metals  KX,  NaX,  there  is  a  considerable  degree 
of  difference  between  these  two  series  of  elements.  This  difference  is 
clearly  seen  in  the  fact  that  the  alkali  metals  belong  to  those  elements 
which  combine  with  extreme  facility  with  oxygen,  decompose  water, 
and  form  the  most  alkaline  bases  ;  whilst  silver  and  copper  are 
oxidised  with  difficulty,  form  less  energetic  oxides,  and  do  not  decom- 
pose water,  even  at  a  rather  high  temperature.  Moreover,  they  only 
displace  hydrogen  from  very  few  acids.  The  difference  between  them 
is  also  see/i  in  the  dissimilarity  of  the'  properties  of  many  of  the 
corresponding  compounds.  Thus  cuprous  oxide,  Cu2O,  and  silver  oxide, 
Ag2O,  are  insoluble  in  water :  the  cuprous  and  silver  carbonates, 
chlorides,  and  sulphates  are  also  sparingly  soluble  in  water.  The 
oxides  of  silver  and  copper  are  also  easily  reduced  to  metal.  This 
difference  in  properties  is  in  intimate  relation  with  that  difference  in 
the  density  of  the  metals  which  exists  in  this  case.  The  alkali  metals 
belong  to  the  lightest,  and  copper  and  silver  to  the  heaviest,  and  there- 
fore the  distance  between  the  molecules  in  these  metals  is  very  dis- 
similar— it  is  greater  for  the  former  than  the  latter  (tables  in  Chapter 
XV.).  From  the  point  of  view  of  the  periodic  law,  this  difference 
between  copper  and  silver  and  such  elements  of  Group  I.  as  potassium 
and  rubidium,  is  clearly  seen  from  the  fact  that  copper  and  silver 

as  that  of  zinc  sulphate,  ZnSO4,7H20.  Supersaturated  solutions  of  each  of  these  salts 
crystallise  in  that  form  and  with  that  amount  of  water  which  is  contained  in  a  crystal 
of  one  or  other  of  the  salts  brought  in  contact  with  the  solution  (Chapter  XIV.,  Note  27). 


400  PRINCIPLES  OF  CHEMISTRY- 

stand  in  the  middle  of  those  large  periods  (for  example,  R,  Oa,  Sc,  Ti, 
V,  Or,  Mn,  Fe,  Co,  Ni,  Cu,  Zn,  Ga,  Ge,  As,  Se,  Br)  which  start  with 
the  true  metals  of  the  alkalis — that  is  to  say,  the  analogy  and  difference 
"between  potassium  and  copper  are  of  the  same  nature  as  that  between 
chromium  and  selenium,  or  vanadium  and  arsenic. 

Copper  is  one  of  the  few  metals  which  have  long  been  known  in  a 
metallic  form.  The  Greeks  and  Romans  imported  copper  chiefly  from 
the  island  of  Cyprus — whence  its  Latin  name,  cuprum.  It  was  known 
to  the  ancients  before  iron,  and  was  used,  especially  when  alloyed  with 
other  metals,  for  arms  and  domestic  utensils.  That  copper  was  known 
to  the  ancients  will  be  understood  from  the  fact  that  it  occurs,  although 
rarely,  in  a  native  state,  and  is  easily  extracted  from  its  other  natural 
compounds.  Among  the  latter  are  the  oxygen  compounds  of  copper. 
When  ignited  with  charcoal,  they  easily  give  up  their  oxygen  to 
it,  and  yield  metallic  copper  j  hydrogen  ateo  easily  takes  up  the 
oxygen  from  copper  oxide  when  heated.  Copper  occurs  in  a  native 
state,  sometimes  .in  association  with  other  ores,  in  many  parts  of  the 
Urals  and  in  Sweden,  and  in  considerable  masses  in  America,  espe-; 
cially  in  the  neighbourhood  of  the  great  American  lakes  ;  and  also  in 
Chili,  Japan,  and  China.  The  oxygen  compounds  of  copper  are  also  of 
somewhat  common  occurrence  in  certain  localities  ;  in  this  respect 
certain  deposits  of  the  Urals  are  especially  famous.  The  geological 
period  of  the  Urals  (Permian)  is  characterised  by  a  considerable  dis- 
tribution of  copper  ores.  Copper  is  met  with  in  the  form  of  cuprous 
oxide,  or  suboxide  of  copper,  Cu2O,  and  is  then  known  as  red  copper 
ore,  because  it  forms  red  masses  which  not  unf requently  are  crystallised 
in  the  Tegular  system.  It  is  found  much  more  rarely  in  the  estate  of 
cupric  oxide,  CuO,  and  is  then  called  black  copper  ore.  The  mosf 
common  of  the  oxygenised  compounds  of  copper  are  the  basic  carbonates 
corresponding  with  the  oxides.  That  these  compounds  are  undoubtedly 
of  aqueous  origin,  is  apparent,  not  only  from  the  fact  that  specimens 
are  frequently  found  of  a  gradual  transition  from  the  metallic,  sul- 
phuretted, and  oxidised  copper  into  its  various  carbonates,  but  also  from 
the  presenc^  of  water  in  their  composition,  and  from  the  laminar, 
reniform  structure  which  many  of  them  present.  In  this  respect  mala* 
chite  is  particularly  well  known  ;  it  is  used  as  a  green  paint  and  also 
for  ornaments,  owing  to  the  diversity  of  the  shades  of  colour  presented 
by  tho  different  layers  of  deposited  malachite.  The  composition  of 
malachite  corresponds  with  "the  basic  carbonate  containing  one  molecule 
of  cupric  carbonate  to  one  of  hydroxide  :  •  CuCO^CuH^Oj.-  In  this 
•form  the  copper  frequently  occurs  in  admixture  with  various  sedi- 
.mentary  rocks,  forming  large  strata,  which  confirms  the  abuebus  origin 


COPPER,  SILVER,  AND  GOLD  401 

of  these  compounds.  There  are  many  such  localities  in  the  Perm  and 
other  Governments  bounding  the  Urals.  Blue  carbonate  of  copper,  or 
azurite,  is  also  often  met  with  in  the  same  localities  ;  it  contains  the 
same  ingredients  as  malachite,  but  in  a  different  proportion,  its  com- 
position being  CuH2O2,2CuC03.  Both  these  substances  may  be  ob 
tained  artificially  by  the  action  of  the  alkali  carbonates  on  solutions 
of  cupric  salts  at  various  temperatures.  These  native  carbonates  are 
often  used  for  the  extraction  of  copper,  all  the  more  as  they  very 
readily  give  metallic  copper,  evolving  water  and  carbonic  anhydride 
when  ignited,  and  leaving  the  easily-reducible  cupric  oxide.  Copper 
is,  however,  still  more  often  met  with  in  the  form  of  the  sulphides. 
The  sulphides  of  copper  generally  occur  in  chemical  combination 
with  the  sulphides  of  iron.3  These  copper-sulphur  compounds  (copper 
pyrites  CuFeS2,  variegated  copper  ore  Cu3FeS3,  &c.)  generally  occur  in 
veins  in  a  rock  gangue. 

The  extraction  of  copper  from  its  oxide  ores  does  not  present  any 
difficulty,  because  the  copper,  when  ignited  with  charcoal  and  melted, 
is  reduced  from  the  impurities  which  accompany  it.  This  mode  of 
smelting  copper  ores  is  carried  on  in  cupola-  or  cylindrical  furnaces, 
fluxes  forming  a  slag  being  added  to  the  mixture  of  ore  and  charcoal. 

5  Iron  pyrites,  FeS2,  very  often  contain  a  small  quantity  of  copper  sulphide  (see 
Chapter  XXII.,  Note  2  bis),  and  on  burning  the  iron  pyrites  for  sulphurous  anhydride  the 
copper  oxide  remains  in  the  residue,  from  which  the  copper  is  often  extracted  with  profit. 
For  this  purpose  the  whole  of  the  sulphur  is  not  burnt  off  from  the  iron  pyrites,  but  a 
portion  is  left  behind  in  the  ore,  which  is  then  slowly  ignited  (roasted)  with  access  of  air. 
Cupric  sulphate  is  then  formed,  and  is  extracted  by  water ;  or  what  is  better  and  more 
frequently  done,  the  residue  from  the  roasting  of  the  pyrites  is  roasted  with  common 
salt,  and  the  solution  of  cupric  chloride  obtained  by  lixiviating  is  precipitated  with  iron. 
A  far  greater  amount  of  copper  is  obtained  from  other  sulphuretted  ores.  Among  these 
copper  glance,  Cu2S,  is  more  rarely  met  with.  It  has  a  metallic  lustre,  is  grey,  generally 
crystalline,  and  is  obtained  in  admixture  with  organic  matter  ;  so  that  there  is  no  doubt 
that  its  origin  is  due  to  the  reducing  action  of  the  latter  on  solutions  of  cupric  sulphate. 
Variegated  copper  ore,  which  crystallises  in  octahedra,  not  infrequently  forms  an 
admixture  in  copper  glance;  it  has  a  metallic  lustre,  and  is  reddish-brown;  it  has  a 
superficial  play  of  colours,  due  to  oxidation  proceeding  on  its  surface.  Its  composition  is 
Cn3FeS3.  But  the  most  common  and  widely-distributed  copper  ore  is  copper  pyrites, 
which  crystallises  in  regular  octahedra ;  it  has  a  metallic  lustre,  a  sp.  gr.  of  4'0,  and 
yellow  colour.  Its  composition  is  CdFeS2.  It  must  be  remarked  that  the  sulphurous 
ores  of  copper  are  oxidised  in  the  presence,  of  water  containing  oxygen  in  solution, 
and  form  cupric  sulphate,  blue  vitriol,  which  is  easily  soluble  in  water.  If  this  water 
contains  calcium  carbonate,  gypsum  and  cupric  carbonate  are  formed  by  double 
decomposition:  CuS04  +  CaCO3  =  CuC03+CaSO4.  Hence  copper  sulphide  in  the  form 
of  different  ores  must  be  considered  as  the  primary  product,  and  the  many  other  copper 
ores  as  secondary  products,  formed  by  water.  This  is  confirmed  by  the  fact  that  at  the 
present  time  the  water  extracted  from  many  copper  mines  contains  cupric  sulphate  in 
solution.  From  this  liquid  it  is  easy  to  extract  cupric  oxide  bjv  the  action  of  organic 
matter  and  various  impurities  of  water.  Hence  metallic  copper  is  sometimes  found  in 
natural  products  of  the  modification  of  copper  sulphide  and  is  probably  deposited  by 
the  action  of  organic  matter  present  in  the  water. 


402  PRINCIPLES  OF  CHEMISTRY 

The  smelted  copper  still  contains  sulphur,  iron,  and  other  metallic 
impurities,  from  which  it  is  freed  by  fusion  in  reverberatory  furnaces, 
with  access  df  air  to  the  surface  of  the  molten  metal,  as  the  iron  and 
sulphur  are  more  easily  oxidised  than  the  copper.  The  iron  then 
separates  as  oxides,  which  collect  in  the  slag.4 

4  Copper  ores  rich  in  oxygen  are  very  rare" ;  the  sulphur  ores  are  of  more  common 
occurrence,  but  the  extraction  of  tho  copper  from'  them  is  much  more  difficult.  The 
problem  here  not  only  consists  in  the  removal  of  the  sulphur,  but  also  in  the  removal  of 
the  iron  combined  with  the  sulphur  and  copper.  This  ifTattained  by  a  whole  series  of 
operations,  after  which 'there  still  sometimes  remains  the  extraction  of  the  metallic  silver 
Which  generally  accompanies  the  copper,  although  in  but  small  quantity.  These 
jprocesses  commence  with  the  roasting— i.e.  calcination — of  thejore  with  access  of  air,  by 
which  means  the  sulphur  is  converted  into  sulphurous  anhydride.  It  should  here  be; 
Remarked  that  iron  sulphide  is  more  easily  oxidised  than  copper  sulphide,  and  therefore 
the  greater  part  of  the  iron  in  the  residue  from  roasting  is  no  longer  in  the  form  of 
sulphide  but  of  oxide  of  iron.  The.  roasted  ore  is  mixed  with  charcoal,  and  siliceous  fluxes, 
and  smelted  in  a  cupola  furnace.  The  iron  then  passes  into  tho  slag,  because  its  oxide- 
gives  an  easily-fusible  mass  with  the  silica,  whilst  the  copper,  in  the  form  of  sulphide,, 
fuses  and  collects  under  the  slag.  The  greater  part  of  the  iron  is  removed  from,  the 
mass  by  this  smelting.  The  resultant  coarse  metal  is  again  roasted  in  order  to 
remove  the  greater  part  of  the"  sulphur  from  the  copper  sulphide,  and  to  convert  the 
metal  into  oxide,  after  which  the  mass  is  again  smelted.  These  processes  are  repeated 
several  times,  according  to  the  richness  of  the  ore.  During  these  smeltings  a  portion  of 
the  copper  is  already  obtained  in  a  metallic  form,  because  copper  sulphide  gives 
metallic  copper  with  the  oxide  (CuS  +  2CuO  =  8Cu+SO2).  We  will  not  here  describe 
the  furnaces  used.or  the  details  of  this  process,  but  the  above  remarks  include  the  ex- 
planation of  those  chemical  processes  which  are  accomplished  u>  the  various  tech- 
nical operations  which  are  made  use  of  in  the  process  (for  details  see  works  on. 
metallurgy). 

Besides  the  smelting  of  copper  there  also  exist  methods  for  its  extraction  from 
solutions  in  the  wet  way,  as  it  is  called.  Recourse  is  generally  had  .to  these  methods  for 
poor  copper  ores.  The  copper  is  brought  into  solution,  from  which  it  is  separated  by 
means  of  metallic  iron  or  by  other  methods  (by  the  action'of  an  electric  current).  The 
sulphides  are  roasted  in  such  a  manner  that  the  greater  part  of  the  copper  is  oxidised 
into  cupric  sulphate,  whilst  at  the  same  'time  the  corresponding  iron  salts  are  as  far  as 
possible  decomposed.  This  process  is  based  on  the  fact  that  the  copper  sulphides  absorb 
oxygen  when  they  are  calcined  .in  the  presence  of  air,  forming  cupric  sulphate.  The 
roasted  ore  is  treated  with  water,  to  which  acid  is  sometimes  added,  and  after  lixivia- 
tion  the  resultant  solution  containing  copper  is  treated  either  with  metallic  iron  or  with 
milk  of  lime,  which  precipitates  cupric  hydroxide  from  the  solution.  Copper  oxide 
ores  poor  in  metal  may  be  treated  with  dilute  acids  'in  order  to  obtain  the  copper 
oxides  in  solution,  from  which  the  copper  is  then  easily  precipitated  either  by  iron  or 
as  hydroxide  by  lime.  According  to  Hunt  and  Douglas's  method,  the  copper  in  the  ore 
is  converted  by  calcination  into  the  cupric  oxide,  which  is  brought  into  solution  by 
the  action  of  a  mixture  of  solutions  of  ferrous  sulphate  and  sodium  chloride; 
the  oxide  converts  the  ferrous"  chloride  into  ferric  oxide,  forming  copper  chlorides, 
according  to  the  equation  3CuO  +  2FeCl2  =  CuClo  +  2CuCl  +  Fe2Os.  The  cupric  chloride 
is  soluble  in  water,  whilst  the  cuprous  chloride  is  dissolved  in  the  solution  of  sodium 
chloride,  and  therefore  all  the  copper  passes  into  solution,  from  which  it  is  precipitated 
by  iron. 

The  same  American  metallurgists  give  the  following  wet  method  for  extracting  the 
AgandAu  occurring  in  many  copper  ores,  especially  in  sulphurous  ores  :  (1)  The  Cu2S  is 
first  converted  into  oxide  by  roasting  in  a  calciner ;  (2)  the  CuO  is  extracted  by  the 
dilute  sulphuric  acid  obtained  in  the  fourth  pfocess,  the  Cu  then  passes  into  solution, 


COPPER,  SILVER,  AND  GOLD  .  403 

Copper  is  characterised  by  its  -red  colour,  which  distinguishes  it 
from  all  other  metals.  Pure. copper  is  soft,  and  may  be  beaten  out  by 
a  hammer  at  the  ordinary  temperature,  and  when  hot  may  be  rolled 
into  very  thin  sheets.  Extremely  thin  leave's  of  copper  transmit  a 
green  light.  The  tenacity  of  copper  is  also  considerable,  and  next'  to 
iron  it  is  one  of  the  most  durable  metals  in  this  respect.  Copper  wire 
of  1  sq.  millimetre  in  section  only  breaks  under  a  weight  of  45  kilograms. 
The  specific  gravity  of  copper  is  8'8,  unless  it  contains  cavities  due  to  the 
fact  that  molten  copper  absorbs  oxygen  from  the  air,  which  is  disen- 
gaged on  cooling, -and  therefore  gives  a  porous  mass  whose  density  is 
much  less.  Rolled  copper,  and  also  that  which  is  deposited  by  the  electric 
current,  has  a  comparatively  high  density.  Copper  melts  at  a  bright 
red  heat,  about  1050°,  although  below  the  temperature  at  which  many 
kinds  of  cast  iron  melt.  At  a  high  temperature  it  is  converted  into 
vapour,  which  communicates  a  green  colour  to  the  flame.  Both  native 
copper  and  that  cooled  from  a  molten  state  crystallise  in  regular 
octahedra.  Copper  is  not  oxidised  in  dry  air  at  the  ordinary  tempera- 
ture, but  when  calcined  it  becomes  coated  with  a  layer  of  oxide,  and  it 
does  not  burn  even  at  the  highest  temperature.  Copper,  when  calcined 
in  air,  forms  either  the  red  cuprous  oxide  or  the  black  cupric  oxide, 

while  the  Ag,  Au  and  oxides  of  iron  remain  behind  in  the  residue  (from  which  the  noble 
metals  may  be  extracted) ;  (3)  a  portion  of  the  copper  in  solution  is  converted  into  CuClj 
(and.  CaSO4  precipitated)  by  means  of  the  CaCl3  obtained  in  the  fifth  process ;  (4)  the 
mixture  of  solutions  of  CuSC^  and  CuCl2  is  converted  into  the  insoluble  CuCl  (salt  of  the 
suboxide)  by  the  action  of  the  SOj  obtained  by  roasting  the  ore  (in  the  first  operation), 
sulphuric  acid  is  then  formed  in  the  solution,  according  to  the  equation :  CuSC>+  CuCl2 
.•  +  SO2  +  2H2O  =  2H2S04  +  2CuCl;  (5)  the  precipitated  CuCl  is  treated  with  lime  and 
water,  and  gives  CuCl2  in  solution  and  CuO  in  the  residue ;  and  lastly  (6)  the  Cu20  is 
reduced  to  metallic  Cu  by  carbon  in  a  furnace.  According  to  Crboke's  method  the  impure 
copper  regulus  obtained  by  roasting  and  smelting  the  ore,  is  broken  up  and  immersed 
jrepeatedly  in  molten  lead,  which  extracts  the  Ag  and  Au  occurring  in  the  regulus.  The 
regulus  is  then  heated  in  a  reverberatory  furnace  to  run  off  the  lead,  and,  is  then  smelted 
for  Cu. 

The  copper  brought  into  the  market  often  contains  small  quantities  of  various  impuri- 
ties, Among  these  there  are  generally  present  iron,  lead,  silver,  arsenic,  and  sometimes 
small  quantities  of  oxides  of  copper.  As  copper,  when  mixed  with  ,a  small  amount  of 
foreign  substances,  loses  its  tenacity  to  a  certain  degree,  the  manufacture  of  very  thin 
sheet  copper  requires  the  use  of  Chili  copper,  which  is  distinguished  for  its  great  softness, 
and  therefore  when  it  is  desired  to  have  pure  copper,  it  is  best  to  take  thin  sheet  copper, 
like  that  which  is  used  in  the  manufacture  of  cartridges.  But  the  purest  copper ,is  electro* 
lytic  copper— that  is,  that  which  is  deposited  from  a  solution  by  the  action  of  an 
electric  current. 

If  the  copper  contains  silver,  as  is  often  the  case,  it  is  used  in  gold  refineries  for  the 
precipitation  of  silver  from  its  solutions  in  sulphuric  acid.  Iron  and  zinc  reduce  copper 
salts,  but  copper  reduces  mercury  and  silver  salts.  "  The  precipitate  contains  not  only  the 
silver  which %was  previously  in  solution,  but  also  all  that  which  was  in  the*  copper.-  The 
silver  solutions  in  sulphuric  acid  are  obtained  in  the  separation  of  silver  from  goffl  by 
treating  their  alloys  with  sulphuric  acid,  which  only  dissolves  the  silver. 


404  PRINCIPLES  OF  CHEMISTRY 

according  to  the  temperature  and  quantity  of  air  supplied.  Tn  ai» 
at  the  ordinary  temperature,  copper — as  everyone  knows— becomes 
coated  with  a  brown  layer  of  oxides  or  a  green  coating  of  basic  salts, 
•due  to  the  action  of  the  damp  air  containing  carbonic  acid.  If  this 
action  continue  for  a  prolonged  time,  the  copper  is  covered  with  a  thick 
coating  of  basic  carbonate,  or  the  so-called  verdigris  (the  cerugo  nobilia 
of  ancient  statues).  This  is  due  to  the  fact  that  copper,  although 
scarcely  capable  of  oxidising  by  itself,5  in  the  presence  of  water  and 
acids — even  very  feeble  acids,  like  carbonic  acid— absorbs  oxygen  from 
the  air  and  forms  salts,  which  is  a  very  characteristic  property  of  it  (and 
of  lead).6  Copper  does  not  decompose  water y  and  therefore  does  not  cKsen- 

5  Sciiiitzenberger  showed  that  when  the  basic  carb&nate  of  copper  is  decomposed  by 
an  electric  current  it  gives,  besides  the  ordinary  copper,  an  allotropic  form  which  grows 
on  the  negative  platinum  electrode,  if  its  surface  be  smaller  than  that  of  the  positive 
copper  electrode,  in  the  form  of  brittle  crystalline  growths  of  sp.  gr.  8'1.    It  differs  from 
ordinary  copper  by  giving  not  nitric  oxide  but  nitrous  oxide  when  treated  with  nitric 
acid,  and  in  being  very  easily  oxidised  in  air,  and  coated  with  red  shades  of  colour.    It 
is  possible  that  this  is  copper  hydride,  or  copper  which  has  occluded  hydrogen.    Spring 
(1892)  observed  that  copper  reduced  from  the  oxide  by  hydrogen  at  the  lowest  possible 
temperature  was  pulverulent,  while  that  reduced  from  CuCl2  at  a  somewhat  high  tem- 
perature appeared  in  bright  crystals.    The  same  difference  occurs  with  many  other 
metals,  and  is  probably  partly  due  to  the  volatility  of  the  metallic  chlorides. 

6  This  is  taken  advantage  of  in  practice ;  for  instance,  by  pouring  dilute  acids  ovei 
copper  turnings  on  revolving  tables  in  the  preparation  of  copper  salts,  such  as  verdigris, 
oar  the  basic  acetate  2C4H6CuO4,CuH2O2,5HoO,  which  is  so  much  used  as  an  oil  paint  (i.e. 
with  boiled  oil).    The  capacity  of  copper  for  absorbing  oxygen  in  the  presence  of  acids 
is  so  great  that  it  is  possible  by  this  means  (by  taking,  for  example,  thin  copper  shavings 
moistened  with  sulphuric  acid)  to  take  up  all  the  oxygen  from  a  given  volume  of  air,  and 
.this  is  even  employed  for  the  analysis  of  air. 

The  combination  of  copper  with  oxygen  is  not  only  aided  by  acids  but  also  by  alkalis, 
although  cupric  oxide  does  not  appear  to  have  an  acid  character.  Alkalis  do  not  act  on 
copper  except  in  the  presence  of  air,  when  they  produce  cupric  oxide,  which  does  not 
appear  to  combine  with  such  alkalis  as  caustic  potash  or  soda.  But  the  action  of 
ammonia  is  particularly  distinct  (Chapter  V.,  Note  2).  In  the  action  of  a  solution  of 
ammonia  not  only  is  oxygen  absorbed  by  the  copper,  but  it  also  acts  on  the  ammonia, 
and  a  definite  quantity  of  ammonia  is  always  acted  on  simultaneously  with  the  passage 
of  the  copper  into  solution.  The  ammonia  is  then  converted  into  nitrous  acid,  according 
to  the  reaction  :  NH3  +  O3 = NHO2  +  H2O,  and  the  nitrous  acid  thus  formed  passes  into  the 
state  of  ammonium  nitrite,  NH4NO2.  In  this  manner  three  equivalents  of  oxygen  are 
expended  on  the  oxidation  of  the  ammonia,  and  six  equivalents  of  oxygen  pass  over  to 
the  copper,  forming  six  atoms  of  cupric  oxide.  The  latter  does  not  remain  in  the  state 
of  oxide,  but  combines  with  the  ammonia. 

A  strong  solution  of  common  salt  does  not  act  on  copper,  but  a  dilute  solution  of  tne 
Bait  corrodes  copper,  converting  it  into  oxychloride — that  is,  in  the  presence  of  air. 
This  action  of  salt  water  is  evident  in  those  cases  where  the  bottoms  of  ships  are  coated 
,with  sheet  copper.  From  what  has  been  said  above  it  will  be  evident  that  copper  vessels 
ehould'not  be  employed  in  the  preparation  of  food,  because  this  contains  salts  and  acids 
which  act  on  copper  in  the  presence  of  air,  and  give  copper  salts,  which  are  poisonoue, 
and  therefore  the  food  prepared  in  untinned  copper  vessels  may  be  poisonous.  Hence 
tinned  vessels  are  employed  for  this  purpose— that  is,  copper  vessels  coated  with  a  thin 
layer  of  tin,  on  which  acid  and  saline  solutions  do  not  act. 


COPPER,   SILVER,  AND  GOLD  405 

gage  hydrogen  from  it  either  at  the  ordinary  or  at  high  temperatures. 
Nor  does  copper  liberate  hydrogen  from  the  oxygen  acids  ,  these  act  on 
it  in  two  ways  :  they  either  give  up  a  portion  of  their  oxygen,  form- 
ing lower  grades  of  oxidation,  or  else  only  react  in  the  presence  of 
air.  Thus,  when  nitric  acid  acts  on  copper  it  evolves  nitric  oxide,  the 
copper  being  oxidised  at  the  expense  of  the  nitric  acid.  In  the  same 
way  copper  converts  sulphuric  acid  into  the  lower  grade  of  oxidation— 
into  sulphurous  anhydride,  SO2.  In  these  cases  the  copper  is  oxidised 
to  copper  oxide,  which  combines  with  the  excess  of  acid  taken,  and 
therefore  forms  a  cupric  salt,  CuX2.  Dilute  nitric  acid  does  not  act 
on  copper  at  the  ordinary  temperature,  but  when  heated  it  reacts 
with  great  ease  ;  dilute  sulphuric  acid  does  not  act  on  copper  except 
in  presence  of  air. 

Both  the  oxides  of  copper,  Cu2O  and  CuO,  are  unacted  on  by 
air,  and,  as  already  mentioned,  they  both  occur  in  nature.6  bis  How- 
ever, in  the  majority  of  cases  copper  is  obtained  in  the  form  oi 
'cupric  oxide  and  its  salts — and  the  copper  compounds  used  indus- 
trially generally  belong  to  this  type.  This  is  due  to  the  fact  that  the 
cuprous  compounds  absorb  oxygen  from  the  air  and  pass  into  cupric 
compounds.  The  cupric  compounds  may  serve  as  the  source  for  the 
preparation  of  cuprous  oxide,  because  many  reducing  agents  are 
capable  of  deoxidising  the  oxide  into  the  suboxide.  Organic  sub- 
stances are  most  generally  employed  for  this  purpose,  and  especially 
saccharine  substances,  which  are  able,  in  the  presence  of  alkalis,  to 
undergo  oxidation  at  the  expense  of  the  oxygen  of  the  cupric  oxide, 
and  to  give  acids  which  combine  with  the  alkali :  2CuO  —  O  =  Cu2O, 
In  this  case  the  deoxidation  of  the  copper  may  be  carried  further  and 
metallic  copper  obtained,  if  only  the  reaction  be  aided  by  heat.  Thus, 
for  example,  a  tine  powder  of  metallic  copper  may  be  obtained  by  heat- 
ing an  ammoniacal  solution  of  cupric  oxide  with  caustic  potash 
and  grape  sugar.  But  if  the  reducing  action  of  the  saccharine 
substance  proceed  in  the  presence  of  a  sufficient  quantity  of  alkali  in 

8  bu  Copper,  besides  the  cuprous  oxide,  Cu2O,  and  cupric  oxide,  CuO,  gives  two  khown 
higher  forms  of  oxidation,  but  they  have  scarcely  been  investigated,  and  even  their 
composition  is  not  well  known.  Copper  dioxide  (Ca02,  or  CuO>,H20,  perhaps  CuOH202) 
is  obtained  by  the  action  of  hydrogen  peroxide  on  cupric  hydroxide,  when  the  green 
colour  of  the  latter  is  changed  to  yellow.  It  is  very  unstable,  and  is  decomposed  even 
by  boiling  water,  with  the  evolution  of  oxygen,  whilst  the  action  of  acids  gives  cuprio 
salts,  oxygen  being  also  disengaged.  A  still  higher  copper  peroxide  is  formed  by  heating 
a  mixture  of  caustic  potash,  nitre,  and  metallic  copper  to  a  red  heat,  and  by  dissolving 
cupric  hydroxide  in  solutions  of  the  hypochlorites  of  the  alkali  metals.  A  slight  heating 
of  the  soluble  salt  formed  is  enough  for  it  to  be  decomposed  into  oxygen  and  copper 
dioxide,  which  is  precipitated.  Judging  from  Fre'my's  researches,  the  composition  of  the 
copper-potassic  compound  should  be  KjCuO4.  Perhaps  this  is  a  compound  of  th* 
peroxides  of  potassium,  K^Oi,  and  of  copper,  CuOa. 


406  PKINCIfLES  OF  CHEMISTRY 

solution,  and  at  not  too  high  a  temperature,  cuprous  oxide  is  ob- 
tained. To  see  this  reaction  clearly,  it  is  not  sufficient  to  take  any 
cupric  salt,  because  the  alkali  necessary  for  the  reaction  might  pre- 
cipitate cupric  oxide — it  is  necessary  to  add  previously  some  substance 
which  will  prevent  this  precipitation.  Among  such  substances, 
tartaric  acid,  C4H6O6,  is  one  of  the  best.  In  the  presence  of  a  suffi- 
cient quantity  of  tartaric  acid,  any  amount  of  alkali  may  be  added  to  a 
solution  of  cupric  salt  without  producing  a  precipitate,  because  a  soluble 
double  salt  of  cupric  oxide  and  alkali  is  then  formed.  If  glucose  (for 
instance,  honey  or  molasses)  be  added  to  such  an  alkaline  tartario 
solution,  and  the  temperature  be  slightly  raised,  it" first  gives  a  yellow 
precipitate  (this  is  cuprous  hydroxide,  CuHO),  and  then,  on  boiling, 
a  red  precipitate  of  (anhydrous)  cuprous  oxide.  If  such  a  mixture 
be  left  for  a  long  time  at  the  ordinary  temperature,  it  deposits  well- 
formed  crystals  of  anhydrous  cuprous  oxide  belonging  to  the  regular 
system.7 

7  Colourless  solutions  of  cuprous  salts  may  also  be  obtained  by  the  action  of  sul- 
phurous or  phosphorous  acid  and  similar  lower  grades  of  oxidation  on  the  blue  solutions  of 
the  cupric  salts.  This  is  very  clearly  and  easily  effected  by  means  of  sodium  thio- 
eulphate,  Na^Oj,  which  is  oxidised  in  the  process.  Cuprous  oxide  can  not  only  be 
obtained  by  the  deoxidation  of  cupric  oxide,  but  also  directly  from  metallic  copper  itself, 
because  the  latter,  in  oxidising  at  a  red  heat  in  air,  first  gives  cuprous  oxide.  It  is  pre- 
pared in  this  manner  on  a  large  scale  by  heating  sheet  copper  rolled  into  spirals  in 
reverberatory  furnaces.  Care  must  be  taken  that  the  air  is  not  in  great  excess,  and  that 
the  coating  of  red  cuprous  oxide  formed  does  not  begin  to  pass  into  the  black  cupric  oxide. 
If  the  oxidised  spiral  sheet  is  then  unbent,  the  brittle  cuprous  oxide  falls  away  from 
the  soft  metal  The  suboxide  obtained  in  this  manner  fuses,  with  ease.  It  is  necessary 
to  prevent  the  access  of  air  during  the  fusion,  and  if  the  mass  contains  cupric  oxide  it 
must  be  mixed  with  charcoal,  which  reduces  the  latter.  Cuprous  chloride,  CuCl,  corre- 
sponding with  cuprous  oxide  (as  sodium  chloride  corresponds  with  sodium  oxide),  when 
calcined  with  sodium  carbonate,  gives  sodium  chloride  and  cuprous  oxide,  carbonic 
anhydride  being  evolved,  because  it  doe's  not  combine  with  the  cuprous  oxide  under  these 
conditions.  The  reaction  can  bs  expressed,  by  the  follo'wing  equation :  2CuCl  +  Na^COj 
«=Cu2<!)'+3NaCl  +  CO2.  The  cupric  oxide  itself,  when  calcined  with  finely-divided  copper 
this  copper  powder  may  be  obtained  by  many  methods — for  instance,  by  immersing  zinc 
in  a  solution  of  a  copper  salt,  or  by  igniting  cupric  oxide  in  hydrogen),  gives  the  fusible 
cuprous  oxide:  Cu  +  CuO  =  CuaO.  Both  the  native  and  artificial  cuprous  oxide  have'a 
ep.  gr.  of  5'6.  It  is  insoluble  in  water;  and  is  not  acted  on  by  (dry)  air.  When  heated 
with  acids  the  suboxide  forms  a  solution  of  a  cupric  salt  and  metallic  copper — for  example, 
Cu2O  +  H2SO4  =  Cu  +  CuSO4  +  H2O.  However,  strong  hydrochloric  acid  does  not  separate 
metallic  copper  on  dissolving  cuprous  oxide,  which  is  due  to  the  fact  that  the  cuprou.8 
cnloride  formed  is  soluble  in  strong  -hydrochloric  acid.  Cuprous  oxide  also,  dissolves  in 
a  solution  of  ammonia,  and  in  the  absence  of  air  gives  a  colourless  solution,  which  turna 
blue  in  the  airr  absorbing  oxygen,  owing  to  the  conversion  of  the  cuprous  oxide  into 
cupric  oxide.  The  blue,  solution  thus  formed  may  be  again  reconverted  into  a  colourless 
cuprous  solution  by  immersing  a  copper  strip  in  it,  because  the  metallic  copper  then 
deoxidises  the  cupric  oxide  in  the  solution  into  cuprous  oxide.  •  Cuprous  oxide  is  charac- 
terised by  the  fact  that  it  gives  red  glasses  when  fused  with  glass  or  with  salts  forming " 
vitreous  alloys.  Glass  tinted  with  cuprous  oxide  is  used  for  ornaments.  The  access  of 
trtr  must  be  avoided  during  its  preparation,  because  the  colour  then  becomes  green,  owing 


COPPEB,  SILVEB,  AND  GOLD  407 

Cupric  chloride,  CuCl2,  when  ignited,  gives  cuprous  chloride,  CuCl 
— t.e.  the  salt  corresponding  with  sub.oxide  of  copper — and  therefore 
cuprous  chloride  is  always  formed  when  copper  enters  into  reaction 
•with  chlorine  at  a  high  temperature.  Thus,  for  example,  when  copper 
is  calcined  with  mercuric  chloride,  jt  forms  cuprous  chloride  and  vapoura 
of  mercury.  The  same  substance  is  obtained  on  heating  metallic 
copper  in  hydrochloric  acid,  hydrogen  being  disengaged  ;  but  this  reac- 
tion only  proceeds  with  finely-divided  copper,  as  hydrochloric  acid  acts 
very  feebly  on  compact  masses  of  copper,  and,  in  the  presence  of  air, 
gives  cupric  chloride.  The  green  solution  of  cupric  chloride  is  decolo- 
rised by  metallic  copper,  cuprous  chloride  being  formed  ;  but  this 
reaction  is  only  accomplished  with  ease  when  the  sojutionis  very  con- 
centrated and  in  the  presence  of  an  excess  of  hydrochloric  acid  to 
dissolve  the  cuprous  chloride.  The  addition  of  water  to  the  solu- 
tion precipitates,  the  cuprous  chloride,  because  it  is  less  soluble  in 
dilute  than  in  strong  hydrochloric  acid.  Many  reducing  agents  which 
are  able  to  take  up  half  the  oxygen  from  cupric  oxide  are  able,  in  the 
presence  of  hydrochloric  acid,  to  form  cuprous  chloride.  Stannous 
salts,  sulphurous  anhydride,  alkali  sulphites,  phosphorous-  and  hypo- 
phosphorous  acids,  and  many  similar  reducing  agents,  act  in  this 
manner.  The  usual  method  of  preparing  cuprous  chloride  consists  in 
passing  sulphurous  anhydride  into  a  very  strong  solution  o£  cupric 
chloride  :  2CuCl2  +  SO2  +  2H20  =  2CuCl  +  2HC1  +  ELjSCV  Cuprous 
chloride  forms  colourless  cubic  crystals  which  are  insoluble  in  water. 
It  is  easily  fusible,  and  even  volatile.  Under  the  action  of  oxidising 
agents,  it  passes  into  the  cupric  salt,  and  it  absorbs  oxygen  from  moist 
air,  forming  cupric  oxy chloride,  Cu^Cl^O.  Aqueous  ammonia  easily 
dissolves,  cuprous  chloride  as  well  as  cuprous  oxide  ;  the  solution  also 
turns  blue  on  exposure  to  the  air.  Thus  an  ammoniacal  solution  of 
cuprous  chloride  serves  as  an  excellent  absorbent  for  oxygen  ;  but  this 
solution  absorbs  not  only  oxygen,  but  also  certain  other  gases — for 
example,  carbonic  oxide  and  acetylene.8 

to  the  formation  of  cupric  oxide,  which  colours  glass  blue.  This  may  even  be  taken 
Advantage  of  in  testing  for  copper  under  the  blow-pipe  by  heating  the  copper  compound 
with  borax  in  the  flame  of  a  blow-pipe ;  a  red  glass  is  obtained  in  the  reducing  flame, 
and  a  blue  glass  in  the  oxidising  flame,  owing  to  the  conversion  of  the  cuprous  into  cuprio 
oxide. 

Etard  (1882),  by  passing  sulphurous  anhydride  into  a  .solution  of  cupric  acetate,  ob« 
tained  a  white  precipitate  of  cuprous  sulphite,  Cu2S05,H2O,  whilst  he  obtained  the  same 
Bait,  of  a  red  colour,  from  the  double  salt  of  sodium  and  copper ;  but  there  are  not  any 
convincing  proofs  of  isomerisin  in  this  case. 

8  The  solubility  of  cuprous  chloride  in  ammonia  is  due  to  the  formation  of  compounds 
between  the  ammonia  and  the  chloride.  In  a  warm  solution  the  compound  NHs^CuCl. 
is  formed,  and  at  the  ordinary- temperature  CuCl,NH3.  This  salt  is  soluble  in  hydro, 
chloric  acid,  and  then  forms  a  corresponding  double  salt  of  cuprous  chloride  and  ammpt 


L408  PRINCIPLES  OF  CHEMISTRY 

When  copper  is  oxidised  with  a  considerable  quantity  of  oxygen  at 
a  high  temperature,  or  at  the  ordinary  temperature  in  the  presence  of 
acids,  and  also  when  it  decomposes  acids,  converting  them  into  lower 
grades  of  oxidation  (for  example,  when  submitted  to  the  action  of 
nitric  and  sulphuric  acids),  it  forms  cupric  oxide,  CuO,  or,  in  the 
presence  of  acids,  cupric  salts.  Copper  rust,  or  that  black  mass  which 
forms  on  the  surface  of  copper  when  it  is  calcined,  consists  of  cupric 
oxide.  The  coating  of  the  oxidised  copper  is  very  easily  separated 
from  the  metallic  copper,  because  it  is  brittle  and  very  easily  peels  off, 
when  it  is  struck  or  immersed  in  water.  Many  copper  salts  (for 

nium  chloride.  By  the  action  of  a  certain  excess  of  ammonia  on  a  hydrochloric  acid 
solution  of  cuprous  chloride,  very  well  formed  crystals,  having  the  composition 
CuCl,NH3,HjO,  are  obtained.  Cuprous  chloride  is  not  only  soluble  in  ammonia  and 
hydrochloric  acid,  but  it  also  dissolves  in  solutions  of  certain  other  salts  —  for  example, 
in  sodium  chloride,  potassium  chloride,  sodium  thiosulphate,  and  certain  others.  All 
the  solutions  of  cuprous  chloride  act  in  many  cases  as  very  powerful  deoxidising 
substances;  for  example,  it  is  easy,  by  means  of  these  solutions,  to  precipitate 
gold  from  its  solutions  in  a  metallic  form,  according  to  the  equation  Au.Cl3  +  8CuCl 


Among  the  other  compounds  corresponding  with  cuprous  oxide,  cuprous  iodide,  Cul, 
is  worthy  of  remark.  It  is  a  colourless  substance  which  is  insoluble  in  water  and 
sparingly  soluble  in  ammonia  (like  silver  iodide),  but  capable  of  absorbing  it,  and  in  thia 
respect  it  resembles  cuprous  chloride.  It  is  remarkable  from  the  fact  that  it  is  exceed- 
ingly easily  formed  from  the  corresponding  cupric  compound  CuI2.  A  solution  of  cupric 
iodide  easily  decomposes  into  iodine  and  cuprous  iodide,  even  at  the  ordinary  tempera- 
ture, whilst  cupric  chloride  only  suffers  a  similar  change  on  ignition.  If  a  solution  of  a 
cupric  salt  be  mixed  with  a  solution  of  potassium  iodide  the  cupric  iodide  formed  imrne- 
fliately  decomposes  into  free  iodine  and  cuprous  iodide,  which  separates  out  as  a  precipi- 
tate. In  this  case  the  cupric  salt  acts  in  an  oxidising  manner,  like,  for  example,  nitrous 
acid,  ozone,  and  other  substances  which  liberate  iodine  from  iodides,  but  with  this  differ- 
ence, that  it  only  liberates  half,  whilst  they  aet  free  the  whole  of  the  iodine  from  potas- 
sium Iodide  :  2KI  +  CuCl2  =  2KC1  +  Cul  +  1. 

It  must  also  be  remarked  that  cuprous  oxide,  when  treated  with  hydrofluoric  acid, 
gives  an  insoluble  cuprous  fluoride,  CuF.  Cuprous  cyanide  is  also  insoluble  in  water, 
and  is  obtained  by  the  addition  of  hydrocyanic  acid  to  a  solution  of  cupric  chloride 
Saturated  with  sulphurous  anhydride.  This  cuprous  cyanide,  like  silver  cyanide,  gives 
a  double  soluble  salt  with  potassium  cyanide.  The  double  cyanide  of  copper  and 
potassium  is  tolerably  stable  in  the  air,  and  enters  into  double  decompositions  with 
various  other  salts,  like  those  double  cyanides  of  iron  with  which  we  are  already 
acquainted. 

Copper  hydride,  CuH,  also  belongs  to  the  number  of  the  cuprous  compounds.  It 
was  obtained  by  Wiirtz  by  mixing  a  hot  (70°)  solution  of  cupric  sulphate  with  a  solution 
of  hypophosphorous  acid,  HjPO.^.  The  addition  of  the  reducing  hypophosphorous  acid 
must  be  stopped  when  a  brown  precipitate  makes  its  appearance,  and  when  gas  begins 
to  be  evolved.  The  brown  precipitate  is  the  hydrated  cuprous  hydride.  When  gently 
heated  it  disengages  hydrogen  ;  it  gives  cuprous  oxide  when  exposed  to  the  air,  burns 
in  a  stream  of  chlorine,  and  liberates  hydrogen  with  hydrochloric  acid:  CuH  +  HCl 
=  CuCl  +  H2.  Zinc,  silver,  mercury,  lead,  and  many  other  heavy  metals  do  not  form 
such  a  compound  with  hydrogen,  neither  under  these  circumstances  nor  under  the  action 
of  hydrogen  at  the  moment  of  the  decomposition  of  salts  by  a  galvanic  current.  The 
greatest  resembance  is  seen  between  cuprous  hydride  and  the  hydrogen  compounds  of 
potassium,  sodium,  Fd,  Ca,  and  Ba. 


COPPER,  SILVEH,  AND  GOtD  409 

Instance,  the  nitrite  and  carbonate)  leave  oxide  of  copper8"3  in  the 
form  of  friable  black  powder,  after  being  ignited.  If  the  ignition  be 
carried  further,  'Cu2O  may  be  formed  from  the  CuO.8  tri  Anhydrous 
cupric  oxide  is  very  easily  dissolved  in  acids,  forming  Cupric  salts,  CuX2. 
They  are  analogous  to  the  salts  MgX2,  ZnX7,  NiX2,  FeX2,  in  many 
respects.  On  adding  potassium  or  ammonium  hydroxide  to  a  solution 
of  a  cupric  salt,  it  forms  a  gelatinous  blue  precipitate  of  the  hydrated 
oxide  of  copper,  CuH2O2,  insoluble  in  water.  The  resultant  precipitate 
i$  redissolved  by  an  excess  of  ammonia,  and  gives  a  very  beautiful 
^fizure  blue  solution,  of  so  intense  a  colour  that  the  presence  of  small 
traces  of  cupric  salts  may  be  discovered  by  this  means.9  An  excess  of 

8  bl»  The  oxide  of  copper  obtained  by  igniting  the  nitrate  is  frequently  used  for. 
Organic  analyses.  It  is  hygroscopic  and  retains  nitrogen  (1'5  c.c.  per  gram)  when  the 
nitrate  is  heated  in  vacuo  (Richards  and  Rogers,  1893); 

8  trl  Oxide  of  copper  is  also  capable  of  dissociating  when  heated.    Debray  and 
Joannis  showed  that  it.  then  disengages  oxygen,  whose  maximum  tension  is  constant 
for  a  given  temperature,  providing  that  fusion  does  not  take  place  (the  CuO  then 
dissolves  in  the  molten  Cu2O) ;  that  this  loss  of  oxygen  is  followed  by  the  formation  of 
euboxide,  and  that  on  cooling,  the  oxygen  is  again  absorbed,  forming  CuO. 

9  Cupric  oxide  and  many  of  its  salts  are  able  to  give  definite,  although  unstable, 
compounds  with  ammonia.    This  faculty  already  shows  itself  in  the  fact  that  cupric 
cxide,  as  well  as  the.  salts  of  copper,  dissolves  in  aqueous  ammonia,  and  also  in  the  fact 
that  salts  of  copper  absorb  ammonia  gas.    If  ammonia  be  added  to  a  solution  of  any 
cupric  salt,  it  first  forms  a  precipitate  of  cupric  hydroxide,  which  then  dissolves  in  an 
excess  of  ammonia.    The  solution  thus  formed,  when  evaporated  or  on  the  addition  of 
alcohol,  frequently  deposits  crystals  of  salts  containing  both  the  elements  of  the  salt  of 
copper  taken  and  of  ammonia.     Several  such  compounds  are  generally  formed.    Thus 
cupric  chloride,  CuCL,  according  to  Deherain,  forms  four  compounds  with  ammonia — 
namely,  with  one,  two,  four,  and  six  molecules  of  ammonia.      Thus,  for  example, 
if   ammonia  gas  be  passed  into  a  boiling  saturated   solution   of   cupric   chloride,  on 
cooling,    small    octahedral    crystals    of    a    blue    colour    separate    out,    containing 
CuCl2,2NH3,HoO.    At  150°  this  substance  loses  half  the  ammonia  and -all  the  water 
contained  in  it,  leaving  the  compound  CuCl2,NH3.    Nitrate  of  copper  form's  the  com- 
pound Cu(N03).2,2NH5.      This  compound  remains  unchanged  on  evaporation.      Dry 
cupric  sulphate  absorbs  ammonia  gas,  and  gives-a  compound  containing  five  molecules  of 
ammonia  to  one  of  sulphate  (Vol.  I.,  p.  257,  and  Chapter  XXII.,  Note  35).    If  this  com- 
pound is  dissolved  in  aqueous  ammonia,  on  evaporation  it  deposits  a  crystalline  substance 
containing  CuS04,4NH3)H20.    At  150°  this  substance  loses  the  molecule  of  water  and 
one-fourth  of  its  ammonia.    On  ignition  all  these  compounds  part  with  the  remaining 
ammonia  in  the  form  of  an  ammoniacal  salt,  so  that  the  residue  consists  of  cupric  oxide. 
Both  the  hydrated  and  anhydrous  cupric  oxide  are  soluble  in  aqueous  ammonia. 

The  solution  obtained  by  the  action  of  aqueous  ammonia  and. air  on  copper  turnings 
(Note  6)  is  remarkable  for  its  faculty  of  dissolving  cellulose,  which  is  insoluble  in  water, 
dilute  acids,  and  alkalis.  Paper  soaked  in  such  a  solution  acquires  the  property  of  not 
rotting,  of  being  difficultly  combustible,  and  ^aterproof,  &c.  It  has  therefore  been 
Applied,  especially  in  England,,  to  many  practical  purposes — for  example,  to  the  con- 
struction of  temporary  buildings,  for  covering  roofs,  &c.  The  composition  of  the 
substance  held  in  solution  is  Cu(HO)2,4NH3. 

If  dry  ammonia  gas  be  passed  over  cupric  oxide  heated  to  265°,  a  portion  of  the  oxide 
of  copper  remains  unaltered,  whilst  the  other  portion  gives  copper  nitride,  the  oxygen  of 
the  eopper  oxide  combining  with  the  hydrogen  and  forming  water.  The  oxide  of  copper 
which"  remains  unchanged  is  easily  removed  by  washing  the  resultant  product  with 


410  PRINCIPLES  OF  CHEMISTRY 

potassium  or  sodium  hydroxide  does  not  dissolve  cupric  hydroxide, 
A  hot  solution  gives  a  black  precipitate  of  the  anhydrous  oxide 
instead  of  the  blue  precipitate,  and  the  precipitate  of  the  hydroxide 
of  copper  becomes  granular,  and  turns  black  when  the  solution 
is  heated.  This  is  due  to  the  fact  that  the  blue  hydroxide  is 
exceedingly  unstable,  and  when  slightly  heated  it  loses  the  elements 
of  water  and  gives  the  black  anhydrous  cupric  oxide  :  CuH2O2 
=  CuO  +  H2O. 

Cupric  oxide  fuses  at  a  strqng  heat,  and  on  cooling  forms  a  heavy 
crystalline  mass,  which  is  black,  opaque,  and  somewhat  tenacious.  It 
is  a  feebly  energetic  base,  so  that  not  only  do  the  oxides  of  the  metals 
of  the  alkalis  and  alkaline  earths  displace  it  from  its  compounds,  but 
even  such  oxides  as  those  of  lead  and  silver  precipitate  it  from  solutions, 
which  is  partially  due  to  these  oxides  being  soluble,  although  but  slightly 
so,  in  water.  However,  cupric  oxide,  and  especially  the  hydroxide, 
easily  combines  with  even  the  least  energetic  acids,  and  does  not  give 
any  compounds  with  bases  ;  but,  on  the  other  hand,  it  easily  forms 
basic  salts, 9  bis  and  in  this  respect  outstrips  magnesium  and  recalls  the 

aqueous  ammonia.  Copper  nitride  is  very  stable,  and  js  insoluble ;  it  has  the  composi- 
tion Cu3N  (i.e.  the  copper  is  monatomic  here  as  in  Cu20),  and  is  an  amorphous  green 
powder,  which  is  decomposed  when  strongly  ignited,  and  gives  cuprous  chloride  and 
ammonium  chloride  when  treated  with  hydrochloric  acid.  Like  the  other  nitrides,  copper' 
nitride,  Cu3N,  has  scarcely  been  investigated..  Granger  (1892),  by  heating  copper  in  the 
vapour  of  phosphorus,  obtained  hexagonal  prisms  of  'Cu5P,  which  passed  into_Cu6P 
(previously  obtained  by  Abety  when  heated  in  nitrogen.  Arsenic  is  easily  absorbed  by 
copper,  and  its  presence  (like  P),  even  in  small  quantities,  has  a  great  influence  upon 
the  properties  of  copper — for  instance,  pure  copper  wire  1  sq.  mm.  in  section  breaks 
under  a  load  of  85  kilos,  while,  the  presence  of  0'22  p.c.  of  arsenic  raises  the  breaking 
load  to  42  kilos. 

9  bu  As  a  comparatively  feeble  base,  oxide  of  copper  easily  forms  both  basic  and 
double  salts.  As  an  instance  we  may  mention  the  double  sails  composed  of  tha 
dichloride  CuCl2,2H2O  and  potassium  chloride.  The,  double  salt  CuK2Cl4,2H2O 
crystallises  from  solutions  in  blue  plates,  but  when  heated  alone  or  with  substancea 
taking  up  water  easily  gives  brown  needles  CuKCl3  and  at  the  same  time  KC1,  and  thia 
reaction  is  reversible  at  92 '  as  Meyerhoffer  (1889)  showed  (i.e.  above  92°  the  simpler 
double  salt  is  formed  and  below  92°  the  more  complex  salt).  With  an  excess  of  the 
copper  salt,  KC1  gives  another  double  salt,  Cu2KCl5)4H2O,  the_transition  temperature  of 
which  is  65p.  The  instances  of  equilibria  which  are  encountered  in  such  complex 
relations  (see  Chapter  XIV-.,  Note  25,  ,astrakhanite,  and  Chapter  XXII.,  Note  23)^  are 
embraced  by  the,  law  of  phases  given  by  Gibbs  (Transactions  of  the  Connecticut 
Academy  of  Sciences,  1875-1878,  in  J.  Willard  Gibbs'  memoir  '  On  the  equilibrium  of 
heterogeneous  substances : '  and  in"  a  clearer  and  more  accessible  form  in  H.  W, 
Bakhuis  Roozeboom's  papers,  Rec.  trav.  chim.,  Vol.  VI.,  and  in  W.  Meyerhoffer's  memoir 
Die  Phasenregel  und  ihre  Anwendungen,  1893,  to  which  sources  we  refer  those  desiring 
fuller  information  respecting  this  law).  Gibbs  calls  '  bodies  '  substances  (simple  or  com- 
pound) capable  of  forming  homogeneous  complexes  (for  instance,  solutions  or  inter- 
combinations)  of  a  varied  composition ;  &phase — a  mechanically  separable  portion  of  such 
,  bodies  or  of  their  homogeneous  complexes  (for  instance,  a  vapour,  liquid  or  precipitated 
!«olid),  perfect  egut7i&riwm— euoh  a  state  •  of  bodies  and  of  their  complexes  as  Is 


COPPER,  SILVER,  AND  GOLD  411 

Oxides  of  leacf  or  mercury.  Hence  the  hydroxide  of  copper  dissolves  in 
solutions  of  neutral  cupric  salts.  The  cupric  salts  are  generally  blue  or 
green,  because  cupric  hydroxide  itself  is  coloured.  But  some  of  the 
salts  in  the  anhydrous  state  are  colourless.10 

Characterised  by  a  constant  pressure  at  a  constant  temperature  even  under  a  change  in 
the  amount  of  one  of  the  component  parts  (for  instance,  of  a  salt  in  a  saturated  solution), 
while  an  imperfect  equilibrium  is  such  a  one  for  which  such  a  change  corresponds  with 
a  change  of  pressure  (for  instance,  an  unsaturated  solution).  The  law  of  phases  consists 
in  the  fact  that :  n  bodies  only  give  a  perfect  equilibrium  when  w+  1  phases  participate 
in  that  equilibrium — for  example,  in  the  equilibrium  of  a  salt  in  its  saturated  solution 
in  water  there  are  two  bodies  (the  salt  and  water)  and  three  phases,  namely,  the  salt, 
solution,  and  vapour,- which  can  be  mechanically  separated  from  each  other,  and  to  this 
equilibrium  there  corresponds  a  definite  tension.  At  the  same  time,  n  bodies  may 
occur  inn  +  2  phases,  but  only  at  one  definite  temperature  and  one  pressure  ;  a  change 
of  one  of  these  may  bring  about  another  state  (perfect  or  not — equilibrium  stable  or 
unstable).  Thus  water  when  liquid  at  the  ordinary  temperature  offers  two  phases 
(liquid  and  vapour)  and  is  in  perfect  equilibrium  (as  also  is  ice  below  0°),  but  water,  ice, 
and  vapour  (three  phases  and  only  one  body)  can  only  be  in  equilibrium  at  0°,  and  at  the 
ordinary  pressure ;  with  a  change  of  t  there  will  remain  either  only  ice  and  vapour  or 
only  liquid  water  and  vapour ;  whilst  with  a  rise  of  pressure  not  only  will  the  vapour 
pass  into  the  liquid  (there  again  only  remain  two  phases)  but  also  the  temperature  of 
the  formation  of  ice  will  fall  (by  about  7°  per  1000  atmospheres).  The  same  laws  of 
phases  are  applicable  to  the  consideration  of  the  formation  of  simple  or  double  salts 
from  saturated  Solutions  and  to  a  number  of  other  purely  chemical  relations.  Thus,  for 
example,  in  the  above-mentioned  instance,  when  the  bodies  are  KC1,  CuCl2)  and  H20, 
perfect  equilibrium  (which  here  has  reference  to  the  solubility)  consisting  of  four  phases, 
corresponds  to  the  following  seven  cases,  considering  only  the  phases  (above  0°) 
A  =  CuCl.,,2KCl,2H2O;  B  =  CuCl2KCl ;  C  =  CuCl2,2H20,KCl,  solution  and  vapour: 
(1)  A +  B  + solution +tvapour;  (2)  A  +  C  +  solution  f  vapour ;  (3)  A  +  KC1  +  solution. 
+ vapour;  (4)  A  +  B  +  C  + vapour  (it  follows  that  B  +  KC1  +  solution  gives  A);  (5) 
A + C  +  KC1  +  vapour ;  (6)  B  +  C  +  solution  +  vapour ;  and  (7)  B  +  KC1  -f  solution  +  vapour. 
Thus  above  92°  A  gives  B  +  KC1.  The  law  of  phases  by  bringing  complex  instances  of 
chemical  reaction  under  simple  physical  schemes,  facilitates  their  study  in  detail  and 
gives  the  means'  of  seeking  the  simplest  chemical  relations  dealing  with  solutions,  dis- 
sociation, double  decompositions  and  similar  cases,  and  therefore  deserves  consideration, 
but  a  detailed  exposition  of  this  subject  must  be  looked  for  in  works  on  physical 
chemistry. 

10  The  normal  cupric  nitrate,  CuN.2Od,3H20,  is  obtained  as  a  deliquescent  salt  of  a  blu& 
colour  (soluble  in  water  and  in  alcohol)  by  dissolving  copper  or  cupric  oxide  in  nitric  acid. 
It  is  so  easily  decomposed  by  the  action  of  heat  that  it  is  impossible  to  drive  off  the  water 
of  crystallisation  from  it  before  it  begins  to  decompose.  During  the  ignition  of  the  normal 
salt  the  cupric  oxide  formed  enters  into  combination  with  the  remaining  undecomposed 
normal  salt,  and  gives  a  basic  salt,  CuN266,2CuH2O2.  The  same  basic  salt  is  obtained 
if  a  certain  quantity  of  alkali  or  cupric  hydroxide  or  carbonate  be  added  to  the  solution 
of  the  normal  salt,  which  is  even  decomposed  when  boiled  with  metallic  copper,  and  forms, 
the  basic  salt  as  a  green  powder,  which  easily  decomposes  under  the  action  of  heat  and 
leaves  a  residue  of  cupric  oxide.  The  basic  salt,  having  the  composition  CuN2O6,3CuH203, 
is  nearly  insoluble  in  water. 

The  normal  carbonate  of  copper,  CuCOj,  occurs  in  nature,  although  extremely  rarely. 
If  solutions  of  cupric  salts  be  mixed  with  solutions  of  alkali  carbonates,  then,  as  in  the 
case  of  magnesium,  carbonic  anhydride  is  evolved  and  basic  salts  are  formed,  which  vary 
in  composition  according  to  the  temperature  and  conditions  of  the  reaction.  By  mixing1 
cold  solutions,  a  voluminous  blue  precipitate  is.  formed,  containing  an  equivalent  pro- 
portion of  cupric  hydroxide  and  carbonate  (after  standing  or  heating,  its  composition 


412  PEINCIPLES  OF  CHEMISTRY 

The  commonest  normal  salt  is  blue  vitriol  —  i.e.  the  normal  cupric 
sulphate.  It  generally  contains  five  molecules  of  water  of  crystallisa- 
tion, CuSO4,5H2O.  It  forms  the  product  of  the  action  of  strong  sul- 
phuric acid  on  copper,  sulphurous  anhydride  being  evolved.  The  same 
salt  is  obtained  in  practice  by  carefully  roasting  sulphuretted  ores  of 
copper,  and  also  by  the  action  of  water  holding  oxygen  in  solution  on 
them  :  CuS-f-O4  =  CuSO4.  This  salt  forms  a  by-product,  obtained  in 
gold  refineries,  when  the  silver  is  precipitated  from  the  sulphuric  acid 
solution  by  means  of  copper.  It  is  also  obtained  by  pouring  dilute 
sulphuric  acid  over  sheet  copper  in  the  presence  of  air,  or  by  heating 
cupric  oxide  or  carbonate  in  sulphuric  acid.  The  crystals  of  this  salt 
belong  to  the  triclinic  system,  have  a  specific  gravity  of  2-19,  are  of  a 
beautiful  blue  colour,  and  give  a  solution  of  the  same  colour.  100 
parts  of  water  at  0°  dissolve  15,  at  25°  23,  and  at  100°  about  45  parts 
of  cupric  sulphate,  CuS04.10  bi8  At  100°  this  salt  loses  a  portion  of  its 


is  the  same  as  malachite,  sp.  gr.  8'5)  :  2CuS04 
-f  2Na8SO4  +  CO2.  If  the  resultant  blue  precipitate  be  heated  in  the  liquid,  it  loses  water 
and  is  transformed  into  a  granular  green  mass  of  the  composition  Cu2C04  —  i.e.  into  a 
compound  of  the  normal  salt  with  anhydrous  cupric  oxide.  This  salt  of  the  oxide  corre- 
eponds  with  orjhocarbonic  acid,  C(OH)4  =  CH4O4,  where  4H  is  replaced  by  2Cu.  On 
further  boiling  this  salt  loses  a  portion  of  the  carbonic  acid,  forming  black  cupric  oxide, 
eo  unstable  is  the  compound  of  copper  with  carbonic  anhydride.  Another  basic  salt  which 
occurs  in  nature,  2CuC03,CuH2O2,  is  known  as  azurite,  or  blue  carbonate  of  copper  ;  it 
also  loses  carbonic  acid  when  boiled  with  water.  On  mixing  a  solution  of  cupric  sulphate 
with  sodium  sesquicarbonate  no  precipitate  is  at  first  obtained,  but  after  boiling  a  pre- 
cipitate is  formed  having  the  composition  of  malachite.  Debray  obtained  artificial 
azurite  by  heating  cupric  nitrate  with  chalk. 

10  bl»  Although  sulphate  of  copper  usually  crystallises  with  5H2O,  that  is,  differently 
to  the  sulphates  of  Mg,  Fe,  and  Mn,  it  is  nevertheless  perfectly  isomorphous  with  them, 
as  is  seen  not  only  in  the  fact  that  it  gives  isomorphous  mixtures  with  them,  containing 
a  similar  amount  of  water  of  crystallisation,  but  also  in  the  ease  with  which  it  forms, 
like  all  bases  analogous  to  MgO,  double  Baits,  R-jCXSO^.^eH.^O,  where  R  =  K,  Rb,  Cs, 
of  the  monoclinic  system. 

Salts  of  this  kind,  like  CuCl2,2KCl,2H20,PtK2Cy4,  &c.,  present  a  composition 
CuX2  if  the  representation  of  double  salts  given  in  Chapter  XXIII.,  Note  11,  be 
admitted,  because  they,  like  Cu(HO)2,  contain  Cu(X2K)2,  where  X2  =  SO4,  i.e.  the 
resiGjue  of  sulphuric  acid,  which  combines  with  H2,  and  is  therefore  able  to  replace 
the  H2  by1  X2  or  O.  A  detailed  study  of  the  crystalline  forms  of  these  salts,  made  by 
Tutton  (1893)  (see  Chapter  XIII.,  Note  1),  showed  :  (1)  that  22  investigated  salts  of  the 
composition  R2M(SO4),6H2O,  where  R  =  K,  Rb,  Cs,  and  M  =  Mg,  Zn,  Cd,  Mn,  Fe,  Co,  Ni, 
Cu,  present  a  complete  crystallographic  resemblance  ;  (2)  that  in  all  respects  the  Rb 
salts  present  a  transition  between  the  K  and  Cs  salts;  (3)  that  the  Cs  salts  form 
crystals  most  easily,  and  the  K  salts  the  most  difficultly,  and  that  for  the  K  salts  of  Cd 
and  Mn  it  was  even  impossible  to  obtain  well-formed  crystals  ;  (4)  that  notwithstanding 
the  closeness  of  their  angles,  the  general  appearance  (habit)  of  the  potassium  compound 
differs  very  clearly  from  the  Cs  salts,  while  the  Rb  salts  present  a  distinct  transition  in 
this  respect  ;  (5)  that  the  angle  of  the  inclination  of  one  of  the  axes  to  the  plane  of  the  two 
Other  axes  showed  that  in  the  K  salts  (angle  from  75°  to  75°  38')  the  inclination  is  least, 
in  the  Cs  salts  (from  72°  52'  to  73°  50')  greatest,  and  in  the  Rb  salts  (from  73°  57'  to 
74°  42')  intermediate  between  the  two  ;  the  replacement  of  Mg.  .  .  Cu  produces  but  a 


COPPER,  SILVER,  AND  GOLD  413 

water  of  crystallisation,  which  it  only  parts  with  entirely  at  a  high 
temperature  (220°)  and  then  gives  a  white  powder  of  the  anhydrous 
sulphate  ;  and  the  latter,  on  further  calcination,  loses  the  elements  of 
sulphuric  anhydride,  leaving  cupric  oxide,  like  all  the  cupric  salts.  The 
anhydrous  (colourless)  cupric  sulphate  is  sometimes  used  for  absorbing 
water  ;  it  turns  blue  in  the  process.  It  offers  the  advantage  that  it 
retains  both  hydrochloric  acid  and  water,  but  not  carbonic  anhydride.11 
Cupric  sulphate  is  used  for  steeping  seed  corn  ;  this  is  said  to  prevent 
the  growth  of  certain  parasites  on  the  plants.  In  the  arts  a  consider- 
able quantity  of  cupric  sulphate  is  also  used  in  the  preparation  of  other 
copper  salts — for  instance,  of  certain  pigments  n  bis — and  a  particularly 

very  small  change  in  this  angle;  (6)  that  the  other  angles  and  the  ratio  of  the 
axes  oi  the  crystals  exhibit  a  similar  variation ;  and  (7)  that  thus  the  variation  of  the 
form  is  chiefly  determined  by  the  atomic  weight  of  the  alkaline  metal.  As  an  example 
we  cite  the  magnitude  of  the  inclination  of  the  axes  of  R2M(S04)2)GHoO. 

R=  K  Rb  Cs 

M  =  Mg  75°  12'  74°  1'  72°  54' 

Zn  75°  12'  74°  7'  72°  59' 

Cd  74°  7'  72°  49' 

Mn  73°  8'  72°  53' 

Fe  75°  28'  74°  16'  73°  8' 

Co  75°  5'  73°  59'  72°  52' 

Ni  75°  0'  73°  57'  72°  58' 

Ca  75°  82'  74°  42'  73°  50' 

Thia  shows  clearly  (within  the  limits  of  possible  error,  which  may  be  as  much  as 
80')  the  almost  perfect  identity  of  the  independent  crystalline  forms  notwithstanding  the 
difference  of  the  atomic  weights  of  the  diatomic  elements,  M  =  Mg  :  .  .  Cu. 

11  In  addition  to  what  has  been  said  (Chapter  I.,  Note  65,  and  Chapter  XXII., 
Note  85)  respecting  the  combination  of  CuS04  with  water  and  ammonia,  we  may  add 
that  Lachinoff  (1893)  showed  that  CuS04,5H20  loses  4£H20  at  180°,  that  CuSO4,5NH3 
also  loses  4f  NH3  at  320°,  and  that  only  £H20  and  £NH3  remain  in  combination  with 
the  CuSO4.  The  last  £H2O  can  only  be  driven  off  by  heating  to  200°,  and  the  last 
$NH3  by  heating  to  860°  Ammonia  displaces  water  from  CuS04,5'H20,  but  water 
cannot  displace  the  ammonia  from  CuSO^SNHs-  If  hydrochloric  acid  gas  be  passed 
over  CuSC^SliaO  at  the  ordinary  temperature,  it  first  forms  CuSQ4,5H20,3liCl,  and 
then  CuSO4)2H20,2HCl.  When  air  is  passed  over  the  latter  compound  it  passes  into 
CuSO4H2O  with  a  small  amount  of  HC1  (about  $HC1).  At  100°  CuS04,5H2O'  in  a 
stream  of  hydrochloric  acid  gas  gives  CuS04,£H20,2HCl,  and  then  CuS04,$H2OHCl, 
whilst  after  prolonged  heating- CuS04  remains,  which  rapidly  passes  into  CuS04,5H30 
when  placed  under  a  bell  jar  over  water.  Over  sulphuric  acid,  however,  CuSO4,5H2O 
only  parts  with  8H2O,  and  if  CuS04,2H20  be  -  placed  over  water  it  again  forms 
CuS04)5H20,  and  so  on. 

11  bis  Commercial  blue  vitriol  generally  contains  ferrous  sulphate.  The  salt  is  purified 
by  converting  the  ferrous  salt  into  a  ferric  salt  by  heating  the  solution  with  chlorine  or 
nitric  acid.  The  solution  is  then  evaporated  to  dryness,  and  the  unchanged  cupric  sul- 
phate extracted  from  the  residue,  which  will 'contain  the  larger  portion  of  the  ferric 
oxide.  The  remainder  will  be  separated  if  cupric  hydroxide  is  added  to  the  solution  andj 
boiled ;  the  cupric.  oxide,  CuO,  then  precipitates  the  ferric  oxide,  Fe203,  just  as  it  is  itself 
precipitated  by  silver  oxide.  But  the  solution  will  contain  a  small  proportion-  of  a  basic 
salt  of  copper,  and  therefore  sulphuric  acid  must  be  added  to  the  filtered  solution,  and  the 
salt  allowed  to  crystallise.  Acid  salts  are  not  formed,  and  cupric  sulphate  itself  has  an 
acid  reaction  on  litmus  paper. 


414  PRINCIPLES  OP  CHEMISTRY 

large  quantity  18  used  in  the  galvanoplastie  process,  Which  consists  hi 
the  deposition  of  copper  from  a  solution  of  eupric  sulphate  by  the 
action  of  a  galvanic  current,  when  the  metallic  copper  is  deposited 
on  the  negative  pole  and  takes  the  shape  of  the  latter.  The  d&- 
scription  of  the  processes  of  galvanoplastic  art  introduced  by  Jacobi 
in  St.  Petersburg  forms  a  part  of  applied  physics,  and  will  not  be 
touched  on  here,  and  we  will  only  mention  that,  although  first  intro- 
duced for  small  articles,  it  is  now  used  for  such  articles  as  type  moulds 
(cliches],  for  maps,  prints,  &c.,  and  also  for  large  statues,  and  for  the 
•deposition  of  iron,  zinc,  nickel,  gold,  silver,  &c.  on  other  metals  and 
materials.  The  beginning  of  the  application  of  the  galvanic  current  to 
the  practical  extraction  of  metals  from  solutions  has  also  been  estab- 
lished, especially  since  the  dynamo- electric  machines  of  Gramme, 
Siemens,  and  others  have  rendered  it  possible  to  cheaply  convert  the 
mechanical  motion  of  the  steam  engine  into  an  electric  current.  It  is 
to  be  expected  that  the  application  of  the  electric  current,  which  has 
long  since  given  such  important  results  in  chemistry,  will,  in  the  near 
future,  play  an  important  part  in  technical  processes,  the  example  being 
shown  by  electric  lighting. 

The  alloys  of  copper  with  certain  metals,  and  especially  with  zinc 
>and  tin,  are  easily  formed  by  directly  melting  the  metals  together. 
They  are  easily  cast  into  moulds,  forged,  and  worked  like  copper, 
whilst  they  are  much  more  durable  in  the  air,  and  are  therefore  fre- 
quently used  in  the  arts.  Even  the  ancients  used  exclusively  alloys 
of  copper,  and  not  pure  copper,  but  its  alloys  with  tin  or  different 
kinds  of  bronze  (Chapter  XVIII.,  Note  35).  The  alloys  of  copper  with 
.zinc  are  called  brass  or  'yellow  metal.'  Brass  contains  about  32  p.c.  of 
zinc  ;  generally,  however,  it  does  not  contain  more  than  65  p.c.  of 
copper.  The  remainder  is  composed  of  lead  and  tin,  which  usually 
occur,  although  in  small  quantities,  in  brass.  Yellow  metal  contains 
about  40  p.c.  of  zinc.12  The  addition  of  zinc  to  copper  changes  the 

IS  Among  the  alloys  of  copper  resembling  brass,  delta  metal,  invented  by  A.  Dick 
(London)  is  largely  used  (since  1888).  It  contains  55  p.c.  Cu,  and  41  p.c.  Zn,  the 
'•remaining  4  p.c.  being  composed  of  iron  (as  much  as  8J  p.c.,  which  is  first  alloyed  with 
jrinc),  or  of  cobalt,  and  manganese,  afrd  certain  other  metals.  The  sp.  gr.  of  delta  metal 
is  8-4.  It  melts  at  950°,  and  then  becomes  so  fluid  that  it  fills  up  all  the  cavities  in  a 
•mould  and  forms  excellent  castings.  It  has  a  tensile  strength  of  70  kilos  per  sq.  mm. 
fgnn  metal  about  20,  phosphor  bronze  about  80).  It  is  very  soft,  especially  when  heated 
to  600°,  but  after  forging  and  rolling  it  becomes  very  hard ;  it  is  more  difficultly  acted 
•upon  by  air  and  water  than  other  kinds  of  brass,  and  preserves  its  golden  yellow  colour 
lor  any  length  of  time,  especially  if  well  polished.  It  is  used  for  making  bearings,  screw 
•propellers,  valves,  and  many  other  articles.  In  general  the  alloys  of  Cu  and  Zn  con- 
taining about  5  p.c.  by  weight  of  copper  were  for  a  long  time  almost  exclusively  made  in 
Sweden  and  England  (Bristol,  Birmingham).  Thesfe  alloys  for  the  most  part  are  cheaper, 
Jiarder,  and  more  fusible  than  copper  alone,  and  form  good  castings.  The  alloys  con- 


COPPER,  SILVER,  AND   GOLD  415 

colour  of  the  latter  to  a  considerable  degree  /  with  a  certain  amount  of 
zinc  the  colour  of  the  copper  becomes  yellow,  and  with  a  still  larger 
proportion  of  zinc  an  alloy  is  formed  which  has  a  greenish  tint  In  those 
alloys  of  zinc  and  copper  which  contain  a  larger'umount  of  zinc  than 
of  copper,  the  yellow  colour  disappears  and  is  replaced  by  a  greyish 
colour.  But  when  the  amount  of  zinc  is  diminished  to  about  20  p.c.j. 
the  alloy  is  red  and  hard,  and  is  called  '  tombac.'  A  contraction  takes 
place  in  alloying  copper  with  zinc,  so  that  the  volume  of  the  alloy  is 
less  than  that  of  either  metal  individually.  The  zinc  volatilises  on 
prolonged  heating  at  a  high  temperature  and  the  excess  of  metallic 
copper  remains  behind.  When  heated  in  the  air,  the  zinc  oxidises 
before  the  copper,  so  that  all  the  zinc  alloyed  with  copper  may  be 
removed  from  the  copper  by  this  means.  An  important  property  of 
brass  containing  about  30  p.c.  of  zinc  is  that  it  is  soft  and  malleable  in; 
the  cold,  but  becomes  somewhat  brittle  when  heated.  We  may  also 
mention  that  ordinary  copper  coins  contain,  in  order  to  render  them 
hard,  tin,  zinc,  and  iron  (Cu  =  95  p.c.) ;  that  it  is  now  customary  to  add 
a  small  amount  of  phosphorus  to  copper  and  bronze,  for  the  same  pur- 
pose ;  and  also  that  copper  is  added  to  silver  and  gold  in  coining,  &c. 
to  render  it  hard  ;  moreover,  in  Germany,  Switzerland,  and  Belgium, 
and  other  countries,  a  silver- white  alloy  (melchior,  German  silver,  &c.), 
for  base  coinage  and  other  purposes,  is  prepared  from  brass  and  nickel 
(from  10  to  20  p.c.  of  nickel  ;  20  to  30  p.c.  zinc  :  50  to  70  p.c.  copper), 
or  directly  from  copper  and  nickel,  or,  more  rarely,  from  an  alloy  con- 
taining silver,  nickel,  and  copper.12  bia 

Copper,  in  its  cuprous  compounds,  is  so  analogous  to  silver,  that 

taining  45-80  p.c.  Cu  crystallise  in  cubes  if  slowly  cooled  (Bi  also  gives  crystals).  By 
washing  the  surface  of  brass  with  dilute  sulphuric  acid,  Zn  is  removed  and  the  article 
acquires  the  colour  of  copper.  The  alloys  approaching  Zn^Cu-  in  their  composition 
exhibit  the  greatest  resistance  (under  other  equal  conditions  ;  of  purity,  forging,  rolling, 
&c.)  The  addition  of  3  p.c.  Al,  or  5  p.c.  Sn,  improves  the  quality  of  brass.  Respecting 
aluminium  bronze  see  Chapter  XVII.  p.  88. 

12  bu  Ball  (also  Kamensky),  1888,  by  investigating  the  electrical  conductivity  of  the 
alloys  of  antimony  and  copper  with  lead,  came  to  the  conclusion  that  only  two  definite 
compounds  of  antimony  and  copper  exist,  whilst  the  other  alloys  are  either  alloys  of  these 
two  together  or  with  antimony  or  with  copper.  These  compounds  are  Cu.2Sb  and 
Cu4Sb — one  corresponds  with  the  maximum,  and  the  other  with  the  minimum,  electrical 
resistance.  In  general,  the  resistance  offered  to  an  electrical  current  forms  one  of  the 
methods  by  which  the  composition  of  definite  alloys  (for  example,  Pb^Zn-,)  is  often 
established,  whilst  the  electromotive  force  of  alloys  affords  (Laurie,  1888)  a  still  more 
accurate  method — for  instance,  several  definite  compounds  were  discovered  by  this 
method  among  the  alloys  of  copper  with  zinc  and  tin ;  but  we  will  not  enter  into  any 
details  of  this  subject,  because  we  avoid  all  references  to  electricity,  although  the  reader 
is  recommended  to  make  himself  acquainted  with  this  branch  of  science,  which  has  many 
points  in  common  with  chemistry.  The  study  of  alloys  regarded  as  solid  solutions  should, 
in  my  opinion,  throw  much  light  upon  the  question  of  solutions,  which  is  still  obscure f 
in  many  aspects  and  in  many  branches  of  chemistry. 

*G 


416  PRINCIPLES  OF  CHEMISTRY 

were  thefT?  no  cupric  compounds,  or  if  silver  gave  stable  compounds 
of  the  higher  oxide,  AgO,  the  resemblance  would  be  as  close  as  that 
between  chlorine  and  bromine  or  zinc  and  cadmium  ;  but  silver 
compounds  corresponding  to  AgO  are  quite  unknown.  Although 
silver  peroxide — which  was  regarded  as  AgO,  but  which  Berthelot 
(1880)  recognised  as  the  sesquioxide  Ag2O3 — is  known,  still  it  does 
not  form  any  true  salts,  and  consequently  cannot  be  placed  along 
with  cupric  oxide.  In  distinction  to  copper,  silver  as  a  metal  does 
not  oxidise  under  the  influence  of  heat ;  and  its  oxides,  Ag2O  and 
Ag2O3,  easily  lose  oxygen  (see  Note  8  tri).  Silver  does  not  oxidise  in 
air  at  the  ordinary  pressure,  and  is  therefore  classed  among  the 
so-called  noble  metals.  It  has  a  white  colour,  which  is  much  purer 
than  that  of  any  other  known  metal,  especially  when  the  metal  is  chemi- 
cally pure.  In  the  arts  silver  is  always  used  alloyed,  because  chemi- 
cally-pure silver  is  so  soft  that  it  wears  exceedingly  easily,  whilst  when 
fused  with  a  small  amount  of  copper,  it  becomes  very  hard,  without 
losing  its  colour.13 

15  There  are  not  many  soft  metals ;  lead,  tin,  copper,  silver,  iron,  and  gold  are  some- 
what soft,  and  potassium  and  sodium  very  soft.  The  metals  of  the  alkaline  earths 
are  sonorous  and  hard,  and  many  other  metals  are  even  brittle,  especially,  bismuth 
and  antimony.  But  the  very  slight  significance  which  these  properties  have  in 
determining  the  fundamental  chemical  properties  of  substances  (although,  however,  of 
immense  importance  in  the  practical  applications  of  metals)  is  seen  from  the  example 
Shown  by  zinc,  which  is  hard  at  the  ordinary  temperature,  soft  at  100°,  and  brittle 
at  200°. 

As  the  value  of  silver  depends  exclusively  on  its  purity,  and  as  there  is  no  possibility. 
of  telling  the  amount  of  impurities  alloyed  with  it  from  its  external  appearance,  it  is 
customary  in  most  countries  to  mark  an  article  with  the  amount  of  pure  silver  ib  contains 
after  an  accurately-made  analysis  known  as  the  assay  of  the  silver.  In  France  the 


FJO.  95.— Cupel  for  silver  assaying.  FIG.  96.— Clay  muffle. 

assay  of  silver  shows  the  amount  of  pure  silver  in  1,000  parts  by  weight ;  in  Russia  the 
amount  of  pure  silver  in  96  parts — that  is,  the  assay  shows  the  number  of  zolotniks 
(4'26  grams)  of  pure  silver  in  one  pound  (410  grams)  of  alloyed  silver.  Russian  silver  is 
generally  84  assay— that  is,  contains  84  parts  by  weight  of  pure  silver  and  12  parts  of 
dopper  and  other  metals.  French  money  contains  90  p.c.  (in  the  Russian  system  this 
Will  be  86-4  assay)  by  weight  of  silver  [English  coins  and  jewellery  contain  92'5  p.c.  of 
silver]  ;  the  silver  rouble  is  of  83£  assay— that  is,  it  contains  86'8  p.c.  of  silver— and  the 
smaller  Russian  silver  coinage  is  of  48  assay,  and  therefore  contains  50  p.c.  of  silver. 
Silver  ornaments  and  articles  are  usually  made  in  Russia  of  84  and  72  assay.  As 
the  alloys  of  silver  and  copper,  especially  after  being  subjected  to  the  action  of  heat,  are 
Dot  so  white  as  pure  silver,  they  generally  undergo  a  process  known  as  '  blanching  '  (or 


COPPEE,   SILVER,  AND  GOLD 


417 


Silver  occurs  in  nature,  both  in  a  native  state  and  in  certain  com* 
poundsT    Native  silver,  however,  is  of  rather  rare  occurrence.     A  far 

1  pickling ')  after  being  worked  up.  This  consists  in  removing  the  copper  from  the  surface) 
of  the  article  by  subjecting  it  to  a  dark-red  heat  and  then  immersing  it  in  dilute  acid. 
During  the  calcination  the  copper  on  the  surface  is  oxidised,  whilst  the  silver  remains 
unchanged  ;  the  dilute  acid  then  dissolves  the  copper  oxides  formed,  and  pure  silver  13 
left  on  the  surface.  The  surface  is  dull  after  this  treatment,  owing  to  the  removal  of  a 
portion  of  the  metal  by  the  acid.  After  being  polished  the  article  acquires  the  desired 
lustre  and  colour,  so  as  to  be  indistinguishable  from  a  pure  silver  object.  In  order  to 
test  a  silver  article,  a  portion  of  its  mass  must  be  taken,  not  from  the  surface,  but  to  a 
certain  depth.  The  methods  of  assay  used  in  practice  are  very  varied.  The  commonest 
and  most  often  used  is  that  known  as  cupellation.  It  is  based  on  the  difference  in  the 
oxidisability  of  copper,  lead,  and  silver.  The  cupel  is  a  porous  cup  with  thick  aides,. 


FIG.  97.— Portable  muffle  furnace 

made  by  compressing  bone  ash.  The  porous  mass  of  bone  ash  absorbs  the  fused  oxides, 
especially  the  lead  oxide,  which  is  easily  fusible,  but  it  does  not  absorb  the  unoxidised 
metal.  The  latter  collects  into  a  globule  under  the  action  of  a  strong  heat  in  the  cupel, 
and  on  cooling  solidifies  into  a  button,  which  may  then  be  weighed.  Several  cupels  are 
placed  in  a  muffle.  A  muffle  is  a  semi-cylindrical  clay  vessel,  shown  in  the  accompanying 
drawing.  The  sides  of  the  muffle  are  pierced  with  several  orifices,  which  allow  the  access 
of  air  into  it.  The  muffle  is  placed  in  a  furnace,  where  it  is  strongly  heated.  Under  the 
action  of  the  air  entering  the  muffle  the  copper  of  the  silver  alloy  is  oxidised,  but  as  the 
oxide  of  copper  is  infusible,  or,  more  strictly  speaking,  difficultly  fusible,  a  certain  quan- 
tity of  lead  is  added  to  the  alloy ;  the  lead  is  also  oxidised  by  the  air  at  the  high  tern- 
perature  of  the  muffle,  and  gives  the  very  fusible  lead  oxide.  The  copper  oxide  then 
fuses  with  the  lead  oxide,  and  is  absorbed  by  the  cupel,  whilst  the  silver  remains  as  a 


418  PRINCIPLES  OF  CHEMISTRY 

greater  quantity  of  silver  occurs  in  combination  with  sulphur,  and 
especially  in  the  form  of  silver  sulphide,  Ag2S,  with  lead  sulphide 
or  copper  sulphide,  or  the  ores  of  various  other  metals.  The  largest 
amount  of  silver  is  extracted  from  the  lead  in  which  it  occurs.  If  this 
lead  be  calcined  in  the  presence  of  air,  it  oxidises,  and  the  resultant 
lead  oxide,  PbO  ('  litharge '  or  '  silberglatte,'  as  it  is  called),  melts  into 
a  mobile  liquid,  which  is  easily  removed.  The  silver  remains  in  an 
unoxidised  metallic  state.14  This  process  is  called  cupellation. 

bright  white  globule.  If  the  weight  of  the  alloy  taken  and  of  the  silver  left  on  the  cupel 
be  determined,  it  is  possible  to  calculate  the  composition  of  the  alloy.  Thus  the  essence 
of  cupellation  consists  in  the  separation  of  the  oxidisable  metals  from  silver,  which  does 
not  oxidise  under  the  action  of  heat.  A  more  accurate  method,  based  on  the  precipitation 
of  silver  from  its  solutions  in  the  form  of  silver  chloride,  is  described  in  detail  in  works 
on  analytical  chemistry. 

14  In  America,  whence  the  largest  amount  of  silver  is  now  obtained,  ores  are  worked 
containing  not  more  than  £  p.c.  of  silver,  whilst  at  i  p.c.  its  extraction  is  very  profitable. 
Moreover,  the  extraction  of  silver  from  ores  containing  not  more  than  O'Ol  p.c.  of  this 
tnetal  is  sometimes  profitable.  The  majority  of  the  lead  smelted  from  galena  contains 
silver,  which  is  extracted  from  it.-  Thus  near  Arras,  in  Prance,  an  ore  is  worked 
which  contains  about  65  parts  of  lead  and  0'088  part  of  silver  in  100  parts  of  ore,  which 
corresponds  with  186  parts  of  silver  in  100,000  parts  of  lead.  At  Freiberg,  in  Saxony,  the 
ore  used  (enriched  by  mechanical  dressing)  contains  about  0*9  of  silver,  160  of  lead,  and 
2  of  copper  in  10,000  parts.  In  every  case  the  lead  is  first  extracted  in  the"  manner 
described  in  Chapter  XVHL,  and  this  lead  will  contain  all  the  silver.  Not  unfrequently 
other  ores  of  silver  are  mixed  with  lead  ores,  in  order  to  obtain  an  argentiferous  lead  as 
the  product.  The  extraction  of  small  quantities  of  silver  from  lead  is  facilitated  by  the 
fact  (Pattinson's  process)  that  molten  argentiferous-  lead  in  cooling  first  deposits 
Crystals  of  pure  lead,  which  fall  to  the  bottom  of  the  cooling  vessel,  whilst  the  proper^, 
tion  of  silver  in  the  unsolidified  mass  increases  owing  to  the  removal  of  the  crystals 
of  lead.  The  lead  is  enriched  in  this  manner  until  it  contains  7^  part  of  'silver,  and 
is  then  subjected  to  cupellation  on  a  larger  scale.  According  to  Park's  process,  zinc  is- 
added  to  the  molten  argentiferous  lead,  and  the  alloy  of  Pb  and  Zn,  which  first  separates 
out  on  cooling,  is  collected.  This  alloy  is  found  to  contain  all  the  silver  previously  con- 
tained in  the  lead.  The  addition  of  0'6  p.c.  of  aluminium  to  the  zinc  (Bossier  and  Edelman) 
facilitates  the  extraction  of  the  Ag  from  the  resultant  alloy  besides  preventing  oxida- 
tion ;  for,  after  re-melting,  nearly  all  the  lead  easily  runs  'off  (remains  fluid),  and 
leaves  an  alloy  containing  about  80  p.c.  Ag  and  about  70  p.c.  Zn.  This  alloy  may  be  used 
as  an  anode  in  a  solution  of  ZnCla,  when  the  Zn  is  deposited  on  the  cathode,  leaving  the 
silver  with  a  small  amount  of  Pb,  &c.  behind.  The  silver  can  be  easily  obtained  pure  by 
treating  it  with  dilute  acids  and  cupelling. 

The  ores  of  silver  which  contain  a  larger  amount  of  it  are  :  silver  glance,  Ag2S  (sp. 
gt.  7'2) ;  argentiferous-copper  glance,  CuAgS  ;  horn  silver  or  chloride  of  silver,  AgCl ; 
argentiferous  grey  copper  ore;  polybasite,  M9ES6  (where  M  =  Ag,  CUj  and  R  =  Sb,  As), 
And  argentiferous  gold.  The  latter  is  the  usual  form  in  which  gold  is  found  in  alluvial 
deposits  and  ores.  The  crystals  of  gold  from  the  Berezoffsky  mines  in  the  Urals  contain 
90  to  95  of  gold  and  5  to  9  of  silver,  and  the  Altai  gold  contains  50  to  65  of  gold  and  86  to 
68  of  silver.  The  proportion  of  silver  in  native  gold  varies  between  these  limits  in  other 
localities.  Silver  ores,  which  generally  occur  in  veins,  usually  contain  native  silver  and 
Various  sulphur  compounds.  The  most  famous  mines  in  Europe  are  in  Saxony  (Frei- 
berg), which  has  a  yearly  output  of  as  much  as  26  tons  of  silver,  Hungary,  and  Bohemia 
(41  tons).  In  Russia,  silver  is  extracted  in  the  Altai  and  at  Nerchinsk  (17  tons).  The 
richest  silver  mines  known  are  in  America,  especially  in  Chili  (as  much  as  70  tons), 
Mexico  (200  tons),  and  more  particularly  in  the  Western  States  of  North  America.  The 


COPPER,  SILVER,  AND  GOLD  419 

Commercial  silver  generally  contains  copper,  and,  more  rarely,  other 
metallic  impurities  also.  Chemically  pure  silver  is  obtained  either  by 
cupellation  or  by  subjecting  ordinary  silver  to  the  following  treatment. 
The  silver  is  first  dissolved  in  nitric  acid,  which  converts  it  and  the 
copper  into  nitrates,  Cu(N03)2  and  AgNO3  ;  hydrochloric  acid  is  then 
added  to  the  resultant  solution  (green,  owing  to  the  presence  of  the 
cupric  salt),  which  is  considerably  diluted  with  water  in  order  to  retain 
the  lead  chloride  in  solution  if  the  silver  contained  lead.  The  copper 
and  many  other  metals  remain  in  solution,  whilst  the  silver  is  precipi- 
tated as  silver  chloride.  The  precipitate  is  allowed  to  settle,  and  the 
liquid  is  decanted  off ;  the  precipitate  is  then  washed  and  fused  with 
sodium  carbonate.  A  double  decomposition  then  takes  place,  sodium 
chloride  and  silver  carbonate  being  formed  ;  but  the  latter  decomposes 
into  metallic  silver,  because  the  silver  oxide  is  decomposed  by  heat  : 
Ag2CO3  =  Ag2  +  0  +  CO2.  The  silver  chloride  may  also  be  mixed, 
with  metallic  zinc,  sulphuric  acid,  and  water,  and  left  for  some  time, 
when  the  zinc  removes  the  chlorine  from  the  silver  chloride  and  pre- 
cipitates the  silver  as  a  powder.  This  finely-divided  silver  is  called. 
'  molecular  silver.' 15 

richness  of  these  mines  may  be  judged  from  the  fact  that  one  mine  in  the  State  of 
Nevada  (Comstock,  near  Washoe  and  the  cities  of  Gold  Hill  and  Virginia),  which  was  dis- 
covered in  1859,  gave  an  output  of  400  tons  in  1866.  In  place  of  cupellation,  chlpri- 
nation  may  also  be  employed  for  extracting  silver  from  its  ores.  The  method  of 
chlorination  consists  in  converting  the  silver  in  an  ore  into  silver  chloride.  This  is 
either  done  by  a  wet  or  by  a  dry  method,  roasting  the  ore  with  NaCl.  When  the  silver 
chloride  is  formed,  the  extraction  of  the  metal  is  also  done  by  two  methods.  The  first 
consists  in  the  silver  chloride  being  reduced  to  metal  by  means  of  iron  in  rotating 
barrels,  with  the  subsequent  addition  of  mercury  which  dissolves  the  silver, .  but 
does  not  act  on  the  other  ^metals.  The  mercury  holding  the  silver  in  solution  is  distilled, 
when  the  silver  remains  behind.  This  method  is  called  amalgamation.  The  other 
method  is  less  frequently  used,  and  consists  in  dissolving  the  silver  chloride  in  sodium 
chloride  or  in  sodium  thiosulphate,  and  then  precipitating  the  silver  from  the  solution. 
s  The  amalgamation  is  then  carried  on  in  rotating  barrels  containing  the  roasted  ore  mixed 
with  water,  iron,  and  mercury.  The  iron  reduces  the  silver  chloride  by  taking  up  the 
chlorine  from  it.  The  technical  .details  of  these  processes  are  described  in  works  on 
metallurgy.  The  extraction  of  AgCl  by  the  wet  method  is  carried  on  (Patera's  process) 
by  means  of  a  solution  of  hyposulphite  of  sodium  which  dissolves  AgCl  (see  Note  23),  or 
by  lixiviating  with  a  2  p.c.  solution  of  a  double  hyposulphite  of  Na  and  Cu  (obtained  by 
adding  CuSO4  to  NajS-jO,).  The  resultant  solution  of  AgCl  is  first  treated  with  soda 
to  precipitate  PbCO3,  and  then  with  Na-jS,  which  precipitates  the  Ag  and  Au.  The 
process  should  be  carried  on  rapidly  to  prevent  the  precipitation  of  C^S  from  the  solu- 
tion of  CuSO4  and  Na^Os- 

15  There  is  another  practical  method  which  is  also  suitable  for  separating  the  sjlver 
from  the  solutions  obtained  in  photography,  and  consists  in  precipitating  the  silver  by 
oxalic  acid.  In  this  case  the  amount  of  silver  in  the  solution  must  be  known,  and  23 
grams  of  oxalic  acid  dissolved  in  400  grams  of  water  must  be  added  for  every  60  grams 
of  silver  in  solution  in  a  litre  of  water.  A  precipitate  of  silver  oxalate,  Ag2C204,  is  then 
obtained,  which  is  insoluble  in  water  but  soluble  in  acids.  Hence,  if  the  liquid  contain 
any  free  acid  it  must  be  previously  freed  from  it  by  the  addition  ol  sodium  carbonate. 


420  PRINCIPLES  OF  CHEMISTRY 

Chemically-pure  silver  has  an  exceeding  pure  white  colour,  and  a 
^Specific  gravity  of  10'5.  Solid  silver  is  lighter  than  the  molten  metal, 
and  therefore  a  piece  of  silver  floats  on  the  latter.  The  fusing- 
point  of  silver  is  about  950°  C.,  and  at  the  high  temperature  attained 
by  the  combustion  of  detonating  gas  it  volatilises.16  By  employing 
eilver  reduced  from  silver  chloride  by  milk  sugar  and  caustic  potash, 
and  distilling  it,  Stas  obtained  silver  purer  than  that  obtained  by  any 
other  means  ;  in  fact,  this  was  perfectly  pure  silver.  The  vapour  of 
silver  has  a  very,  beautiful  green  colour,  which  is  seen  when  a  silver 
wire  is  placed  in  an  oxyhydrogen  flame.17 

It  has  long  been  knowr\  (Wohler)  that  when  nitrate  of  silver, 
AgNO3,  reacts  as  an  oxidising  agent  upon  citrates  and  tartrates,  it  is 
able  under  certain  conditions  to  give  either  a  salt  of  suboxide  of  silver 
(see  Note  19)  or  a  red  solution,  or  to  give  a  precipitate  of  metallic 
silver  reduced  at  the  expense  of  the  organic  substances.  In  1889  Carey 
"Lea,  in  his  researches  on  this  class  of  reactions,  showed  that  soluble 

The  resultant  precipitate  of  silver  oialate  is  dried,  mixed  with  an  equal  weight  of  dry 
eodium  carbonate,  and  thrown  into  a  gently-heated  crucible.  The  separation  of  the 
eilver  then  proceeds  without  an  explosion,  whilst  the  silver  oxalate  if  heated  alone 
decomposes  with  explosion. 

According  to  Stas,  the  best  method  for  obtaining  silver  from  its  solutions  is  by  the 
reduction  of  silver  chloride  dissolved  in  ammonia  by  means  of  anammoniacal  solution  of 
cuprous  thiosulphate ;  the  silver  is  then  precipitated  in  a  crystalline  form.  A  solution  of 
ammonium  sulphite  may  be  used  instead  of  the  cuprous  salt. 

J6  Silver  is  very  malleable  and  ductile ;  it  may  be  beaten  into  leaves  0'002  mm.  in 
thickness.  Silver  wire  may  be  made  so  fine  that  I  gram  is  drawn  into  a  wire  2J  kilo-, 
metres  long.  In  this  respect  silver  is  second  only  to  gold.  A  wire  of  2  mm.  diameter 
breaks  under  a  strain  of  20  kilograms. 

17  In  melting,  silver  absorbs  a  considerable  amount  of  oxygen,  which  is  disengaged  on 
solidifying.  One  volume  of  molten  silver  absorbs  as  much  as  22  volumes  of  oxygen.  In 
solidifying,  the  silver  forms  cavities  like  the  craters  of  a  volcano,  and  throws  off  metal, 
owing  to  the  evolution  of  the  gas ',  all  these  phenomena  recall  a  volcano  on  a  miniature 
scale  (Dumas).  Silver  which  contains  a  small  quantity  of  copper  or  gold,  &c.,  does  not 
.show  this  property  of  dissolving  oxygen. 

The  absorption  of  oxygen  by  molten  silver  ;s,  however,  an  oxidation,  but  it  is  at  the 
same  time  a  phenomenon  of  solution.  One  cubic  centimetre  of  molten  silver  can 
dissolve  twenty-two  cubic  centimetres  of  oxygen,  which,  even  at  0°,  only  weighs  0'08 
gram,  whilst  1  cubic  centimetre  of  silver  weighs  at  least  10  grams,  and  therefore  it  is 
impossible  to  suppose  that  the  absorption  of  the  oxygen  is  attended  by  the  formation  of 
any  definite  compound  (rich  in  oxygen)  of  silver  and  oxygen  (about  45  atoms  of  silver-  to 
1  of  oxygen)  in  any  other  but  a  dissociated  form,  and  this  is  the  state  in  which  sub- 
stances  in  solution  must  be  regarded  (Chapter  I.) 

Le  Chatelier  showed  that  at  "800°  and  15  atmospheres  pressure  silver  absorbs  so 
much  oxygen  that  it  maybe  regarded  as  having  formed  the  compound  Ag40,  or  a 
mixture  of  Ag2  and  Ag2O.  Moreover,  silver  oxide,  Ag20,  only  decomposes  at  800°  under, 
low  pressures,  whilst  at  pressures  above  10  atmospheres  there  is  no  decomposition  at 
800°  but  only  at  400°. 

Stas  showed  that  silver  is  oxidised  by  air  in  the  presence  of  acids.  V.  d.  Pfordten 
.confirmed  this,  and  showed  that  an  acidified  solution  of  potassium  permanganate  rapidly 
dissolves  silver  in  the  presence  of  air. 


COPPER,  SILVER,  AND  GOLD  421, 

eilver  is  here  formed,  which  he  called  allotropic  silver.  It  may  be 
obtained  by  taking  200  c.c.  of  a  10  per  cent,  solution  of  AgNO3  and 
quickly  adding  a  mixture  -(neutralised  with  NaHO)  of  200  c.c.  of  a 
80  per  cent,  solution  of  FeSO4  and  200  c.c.  of  a  40  per  cent,  solution 
of  sodium  citrate.  A  lilac  precipitate  is  obtained,  which  is  collected 
on  a  filter  (the  precipitate  becomes  blue)  and  washed  with  a  solution  of 
NH4N03.  It  then  becomes  soluble  in  pure  water,  forming  a  red 
perfectly  transparent  solution  from  which  the  dissolved  silver  is  preci- 
pitated on  the  addition  of  many  soluble  foreign  bodies.  Some  of  the 
latter — for  instance,  NH4NO3,  alkaline  sulphates,  nitrates,  and  citrates 
— give  a  precipitate  which  redissolves  in  pure  water,  whilst  others — for 
instance,  MgSO4,  FeS04,  K2Cr2O7,  AgN63,Ba(NO3)2  and  many  others- 
convert  the  precipitated  silver  into  a  new  variety,  which,  although  no 
longer  soluble  in  water,  regains  its  solubility  in  a  solution  of  borax 
and  is  soluble  in  ammonia.  Both  the  soluble  and  insoluble  silver  are 
rapidly  converted  into  the  ordinary  grey-metallic  variety  by  sulphuric 
acid,  although  nothing  is  given  off  in  the  reaction ;  the  same  changd 
takes  place  on  ignition,  but  in  this  case  CO2  is.  disengaged  ;  the  latterv 
is  formed  from  the  organic  substances  which  remain  (to  the  amount  of 
3  per  cent.)  in  the  modified  silver  (they  are  not  removed  by  soaking  in 
alcohol  or  water).  If  the  precipitated  silver  be  slightly  washed  and 
laid  in  a  smooth  thin  layer  on  paper  or  glass,  it  is  seen  that  the  soluble 
variety  is  red  when  moist  and  a  fine  blue  colour  when  dry,  whilst  the  in- 
soluble variety  has  a  blue  reflex.  Besides  these,  under  special  conditions 18 

18  When- solutions  of  AgNO'3)  FeSO4)  sodium  citrate,  and  NaHO  are  mixed  together 
in  the  manner  described  above,  they  throw  down  a  precipitate  of  a  beautiful  lilao 
colour;  when  transferred  to  a  filter  paper  the  precipitate  soon  changes  colour,  and 
becomes  dark  blue.  To  obtain  the,  substance  as  pure  as  possible  it  is  washed  with  a 
6-10  p.c.  solution  of  ammonium  nitrate;  the  liquid  is  decanted,  and  150  c.c.  of  water 
poured  over  the  precipitate.  It  then  dissolves  entirely  in  the  water.  A  small  quantity  of 
a  saturated  solution  of  ammonium  nitrate  is  added  to  the  solution,  and  the  silver 
in  solution  again  separates  out  as  a  precipitate.  These  alternate  solutions  and 
precipitations  are  repeated  seven  or  eight  times,  after  which  the  precipitate  is  trans- 
ferred to  a  filter  and  .washed  with  95  p.c.  alcohol  until  the  filtrate  gives  no  residue  on 
evaporation.  An  analysis  of  the  substance  so  obtained  showed  that  it  contained  from 
97'IB  p.c.  to  97'Sl  p.c.  of  metallic  silver.  It  remained  to  discover  what  the  remaining 
2-3  p.c.  were  composed  of.  Are  they  merely  impurities,  or  is  the  substance  some  com- 
pound of  silver  with  oxygen  or  hydrogen,  or  does  it  contain  citric  acid  in  combination 
which  might  account  for  its  solubility?  The  first  suppositign  is  set  aside  by  the  fact 
that  no  gases  are  disengaged  by  the  precipitate  of  silver,  either  under  the  action  of  gases 
or  whe.n  heated.  The  second  supposition  is  shown  to  be  impossible  by  the  fact  that 
there  is  no  definite  relation  between  the  silver  and  citric  acid.  A  determination  of  the 
amount  of  silver  in  solution  showed  that  the  amount  of  citric  acid  varies  greatly  for  one 
and  the  same  amount  of  silver,  and  there  is  no  simple  ratio  between  them.  Among 
other  methods  of  preparing  soluble  silver  given  by  Carey  Lea,  we  may  mention  the 
method  published  by  him  in  1891.  AgNO3  is  added  to  a  solution  of  dextrine  in  caustic 
coda,  or  potash ;  at  first  a  precipitate  of  brown  oxide  of  silver  is  thrown  down,  but  the 


422  PRINCIPLES  OF  CHEMISTRY 

a  golden  yellow  variety  may  be  obtained,  which  gives  a  brilliant  golden^ 
yellow  coating  on  glass  ;  but  it  is  easily  converted  into  the  ordinary 
grey-metallic  state  by  friction  or  trituration.  There  is  no  doubt  18  bi» 
that  there  is  the  same  relation  between  ordinary  silver  which  is  per- 
fectly insoluble  in  water  and  the  varieties  of  silver  obtained  by  Carey 
Lea  18  trl  as  there  is  between  quartz  and  soluble  silica  or  between 

brown  colour  then  changes  into  a  reddish  chocolate,  owing  to  the  reduction  of  the  silver 
by  the  dextrine,  and  the  solution  turns  a  deep  red.  A  few  drops  of  this  solution  turn 
water  bright  red,  and  give  a  perfectly  transparent  liquid.  The  dextrine  solution  is  pre- 
pared by  dissolving  40  grams  of  caustic  soda  and  the  same  amount  of  ordinary  brown 
dextrine  in  two  litres  of  water.  To  this  solution  is  gradually  added  28  grams  of  AgNOs 
dissolved  in  a  small  quantity  of  water. 

The  insoluble  allotropic  silver  is  obtained,  as  was  mentioned  above,  from  a  solution 
of  silver  prepared  in  the  manner  described,  by  the  addition  of  sulphate  of  copper, 
iron,  barium,  magnesium,  &c.  In  one  experiment  Lea  succeeded  in  obtaining  the 
insoluble  allotropic  Ag  in  a  crystalline  form.  The  red  solution,  described  above,  after 
standing  several  weeks,  deposits  crystals  spontaneously  in  the  form  of  short  black 
needles  and  thin  prisms,  the  liquid  becoming  colourless.  This  insoluble  variety,  when 
rubbed  upon  paper,  has  the  appearance  of  bright  shining  green  flakes,  which  polarise 
light. 

The  gold  variety  is  obtained  in  a  different  manner  to  the  two  other  varieties.  A 
solution  is  prepared  containing  200  c.c.  of  a  10  p.c.  solution  of  nitrate  of  silver,  200  c.c. 
of  a  20  p.c.  solution  of  Rochelle  salt,  and  800  c.c.  of  water.  Just  as  in  the  previous'  case 
the  reaction  consisted  in  the  reduction  of  the  citrate  of  silver,  so  in  this  case  it  consists 
in  the  reduction  of  the  tartrate,  which  here  first  forms  a  red,  and  then  a  black  precipitate 
of  allotropic  Ag,  which,  when  transferred  to  the,  filter,  appears  of  a  beautiful  bronze 
colour.  After  washing  and  drying,  this  precipitate  acquires  the  lustre  and  colour 
peculiar  to  polished  gold,  and  this  is  especially  remarked  where  the  precipitate  comes 
into  contact  with  glass  or  china.  An  analysis  of  the  golden  variety  gave  a  percentage 
composition  of  98-750  to  98'749  Ag.  Both  the  insoluble  varieties  (the  blue  and  gold) 
have  a  different  specific  gravity  from  ordinary  silver.  Whilst  that  of  fused  silver  is  10'50, 
and  of  finely-divided  silver  10-62,  the  specific  gravity  of  the  blue  insoluble  variety  is  9'58, 
and  of  the  gold  variety  8'51r  The  gold  variety  passes  into  ordinary  Ag  with  great  ease. 
This  transition  may  even  be  remarked  on  the  filter  in  those  places  which  have  acciden- 
tally not  been  moistened  with  water.  A  simple  shock,  and  therefore  friction  of  one 
particle  upon  another,  is  enough  to  convert  the  gold  variety  into  normal  white  silver. 
Carey  Lea  sent  sample's  of  the  gold  variety  for  a  long  distance  by  rail  packed  in  three 
;tubes,  in  which  the  silver  occupied  about  the  quarter  of  their  volume ;  in  one  tube  only 
he  filled  up  this  space  with  cotton-wool.  It  was  afterwards  found  that  the  shaking  of 
the  particles  of  Ag  had  completely  converted  it  into  ordinary  white  silver,  and  that  only 
the  tube  containing  the  cotton-wool  had  preserved  the  golden  variety  intact. 

The  soluble  variety  of  Ag  also  passes  into  the  ordinary  state  with  great  ease,  the 
heat  of  conversion  being,  .as  Prange  showed  in  1890,  about  +60  calories. 

18  t>b  The  opinion  of  the  nature  of  soluble  silver  given  below  was  first  enunciated  in 
the  Journal  of  the  Eussian  Chemical  Society,  February  1>  1890,  Vol.  XXII.,  Note  78. 
This  view  is,  at  the  present  time,  generally  accepted,  and  this  silver  is  frequently  known 
M  the  '  colloid '  variety.  I  may  add  that  Carey  Lea  observed  the  solution  of  ordinary 
molecular  silver  in  ammonia  without  the  access  of  air. 

18  trt  It  is,  however,  noteworthy  that  ordinary  metallic  lead  has  long  been  considered 
soluble  in  water,  that  boron  has  been  repeatedly  obtained  in  a -brown  solution,  and  thai 
observations  upon  the  development  of,  certain  bacteria  have  shown  that  the  latter  die  in 
water  which  has  been  for  some  time  in  contact  with  metals.  This  seems  to  indicate  the 
passage  of  small  quantities  of  metals  into  water  (however,  the  formation  of  peroxide  of 
hydrogen  may  be  supposed  to  have  some  influence  in  these  oases) 


COPPER,  SILVER,  AND  GOLD  42$ 

CuS  and  As2S.2  in  their  ordinary  insoluble  forms  and  in  the  state  of 
the  colloid  solution  of  their  hydrosols  (see  Chapter  I.,  Note  57,  and 
Chapter  XVII.,.  Note  25  bis).  Here,  however,  an  important  step  in 
advance  has  been  made  in  this  respect,  that  we  are  dealing  with  the 
solution  of  a  simple  body,  and  moreover  of  a  metal — i.e.  of  a  particu- 
larly characteristic  state  of  matter.  And  as  boron,  gold,  and  certain 
other  simple  bodies  have  already  been  obtained  in  a  soluble  (colloid) 
form,  and  as  numerous  organic  compounds  (albuminous  substances, 
gum,  cellulose,  starch,  &c.)  and  inorganic  substances  are  also  known  in 
this  form,  it  might  be  said  that  the  colloid  state  (of  hydrogels  and  hydro- 
Sols)  can  be  acquired,  if  not  by  every  substance,  at  all  .events  by  sub- 
stances  of  most  varied  chemical  character  under  particular  condition^ 
of  formation  from  solutions.  And  this  being  the  case,  we  may  hope 
that  a  further  study  of  soluble  colloid  compounds,  which  apparently 
present  various  transitions  towards  emulsions,  may  throw  a  new  light 
upon  the  complex  question  of  solutions,  which  forms  one  of  the  problems 
of  the  present  epoch  of  chemical  science.  Moreover,  we  may  remark  that 
Spring  (1890)  clearly  proved  the  colloid  state  of  soluble  silver  by  means 
of  dialysis  as  it  did  not  pass  through  the  membrane. 

As  regards  the  capacity  of  silver  for  chemical  reactions,  it  is 
remarkable  for  its  small  capacity  for  combination  with  oxygen,  and  for 
its  considerable  energy  of  combination  with  sulphur,  iodine,  and  cer- 
tain kindred  non-metals.  Silver  does  not  oxidise  at  any  temperature, 
and  its  oxide,  Ag2O,  is  decomposed  by  heat.  It  is  also  a  very  impor- 
tant fact  that  silver  is  not  oxidised  by  oxygen  either  in  the  presence  of 
alkalis,  even  at  exceedingly  high  temperatures,  or  in  the  presence  of 
acids — at  least,  of  dilute  acids — which  properties  render  it  a  very 
important  metal  in  chemical  industry  for  the  fusion  of  alkalis,  and  also 
for  many  purposes  in  everyday  life ;  for  instance,  for  making  spoons, 
salt-cellars,  <fec.  Ozone,  however,  oxidises  it.  Of  all  acids  nitric  acid 
has  the  greatest  action  on  silver.  The  reaction  is  accompanied  by  the 
formation  of  oxides  of  nitrogen  and  server  nitrate,  AgNO3,  which 
dissolves  in  water  and  does  not,  therefore,  hinder  the  further  action  of 
the  acid  on  the  metal.  The  halogen  acids,  especially  hydriodic  acid, 
act  on  silver,  hydrogen  being  evolved  ;  but  this  action  soon  stops, 
owing  to  the  halogen  compounds  of  silver  being  insoluble  in  water  and 
•'only  very  slightly  soluble  in  acids ;  they  therefore  preserve  the  remaining 
mass  of  metal  from  the  further  action  of  the  acid  ;  in  consequence  of 
this  the  action  of  the  halogen  acids  is  only  distinctly  seen  with  finely- 
divided  silver.  Sulphuric  acid  acts  on  silver  in  the  same  manner  that 
it  does  on  copper,  only  it  must  be  concentrated  and  at  a  higher 
temperature.  Sulphurous  anhydride,  and  not  hydrogen,  is  then  evolved, 


424  PRINCIPLES  OF  CHEMISTRY 

but  there  is  no  action  at  the  ordinary  temperature,  even  in  the  presence 
of  air.  Among  the  various  salts,  sodium  chloride  (in  the  presence  of 
moisture,  air,  and  carbonic  acid)  and  potassium  cyanide  (in  the  presence 
of  air)  act  on  silver  more  decidedly  than  any  others,  converting  it  respec- 
tively into  silver  chloride  and  a  double  cyanide. 

Although  silver  does  not  directly  combine  with  oxygen,  still  three 
different  grades  of  combination  with  oxygen  may  be  obtained  indi- 
rectly  from  the  salts  of  silver.  They  are  all,  however,  unstable,  and 
decompose  into  oxygen  and  metallic  silver  when  ignited.  These  three 
oxides  of  silver  have  the  following  composition  :  silver  suboxide, 
Ag4O,19  corresponding  with  the  (little  investigated)  suboxides  of  the 
alkali  metals  ;  silver  oxide,  Ag2O,  corresponding  with  the  oxides  of  the 
alkali  metals  and  the  ordinary  salts  of  silver,  AgX  ;  and  silver  peroxide, 
AgO,19  bl8  or,  judging  from  Berthelot's  researches,  Ag2O3.  Silver  oxide 
is  obtained  as  a  brown  precipitate  (which  when  dried  does  not  contain 
water)  by  adding  potassium  hydroxide  to  a  solution  of  a  silver  salt — 
for  example,  of  silver  nitrate.  The  precipitate  formed  seems,  however, 

19  Silver  suboxide  (Ag4O)  or  argentous  oxide  is  obtained  from  argentic  citrate  by 
heating  it  to  100°  in  a  stream  of  hydrogen.  Water  and  argentous  citrate  are  then 
formed,  and  the  latter,  although  but  slightly  soluble  in  water, '  gives  a  reddish* 
brown  solution  of  colloid  silver  (Note  18),  and  when  boiled  this  solution  becomes 
colourless  and  deposits  metallic  silver,  the  argentic  salt  being  again  formed.  Wohler, 
who  discovered  this  oxide,  obtained  it  as  a  black  precipitate  by  adding  potassium 
hydroxide  to  the  above  solution  of  argentous  citrate.  With  hydrochloric  acid  the 
euboxide  gives  a  brown  compound,  Ag3Cl.  Since  the  discovery  of  soluble  silver  the 
above  data -cannot  be  regarded  as  perfectly  trustworthy ;  it  is  probable  that  a  mixture  of 
Ag3  and  Ag2O  was.  being  dealt  with,  so  that  the  actual  existence  of  Ag4O  is  now 
doubtful,  but  there  can  be  no  doubt,  as  to.  the  formation  of  a  subchloride,  Ag2Cl  (see 
Note  25),  corresponding  to  the  suboxide.  The  same  compound  is  obtained  by  the  action 
of  light  on  the  higher  chloride.  Other,  acids  do  not  combine  with  silver  suboxide,  but 
convert  it  into  an  argentic  salt  and  metallic  silver.  In  this  respect  cuprous  oxide 
presents  a  certain  resemblance  to  these  suboxides.  But  copper  forms  a  suboxide  of 
the  composition  Cu4O,  which  is  obtained  by  the  action  of  an  alkaline  solution  of 
stannous  oxide  on  cupric  hydroxide,  and  is  decomposed  by  acids  into  cupric  salts  and 
metallic  copper.  The  problems  offered  by  the  suboxides,  as  well  as  by  the  peroxides, 
cannot  be  considered  as  fully  solved. 

19  bb  Silver  peroxide,  AgO  or  AgaOs,  is  obtained  by  the  decomposition  of  a  dilute 
(10  p.c.)  solution  of  silver  nitrate  by  the  action  of  a  galvanic  current  (Hitter).  On  the 
positive  pole,  where  oxygen  is  usually  evolved  in  the  decomposition  of  salts,  brittle  grey 
needles  with  a  metallic  lustre,  which  occasionally  attain  a  somewhat  considerable  size, 
are  then  formed.  They  are  insoluble  in  water,  and  decompose  with  the  evolution  of 
oxygen  when  they  are  dried.. and  heated,  especially  up  to  150°,  and,  like  lead  dioxide, 
tarium  peroxide,  &c.,  their  action  is  strongly  oxidising.  When  treated  with  acids,  oxygen 
fa  evolved  and  a  salt  of  the  oxide  formed.  Silver  peroxide  absorbs  sulphurous  anhydride 
and  forme  silver  sulphate.  Hydrochloria  acid  evolves  chlorine  ;  ammonia  reduces  the 
silver,  and  is  itself  oxidised,  forming  water  and  gaseous  nitrogen.  Analyses  of  the  above- 
mentioned  crystals  show  that  they  contain  silver  nitrate,  peroxide,  and  water.  According 
to  Fisher,  they  have  the  composition  4AgO,AgNO5,HaO,  and,  according  to  Bertbeloti 


COPPER,  SILVEK,  AND  GOLD  425 

to  be  an  hydroxide,  AgHO,  i.e.  AgN03  +  KHO  =  KN03  *  AgHO, 
and  the  formation  of  the  anhydrous  oxide,  2AgHO  =  Ag2O  -f  H2O, 
may  be  compared  with  the  formation  of  the  anhydrous  cupric  oxide  by 
the  action  of  potassium  hydroxide  on  hot  cupric  solutions.  Silver 
hydroxide  decomposes  into  water  and  silver  oxide,  even  at  low 
.temperatures  j  at  least,  the  hydroxide  no  longer  exists  at  60°,  but 
'forms  the  anhydrous  oxide,  Ag20.19tri  Silver  oxide  is  almost 
.insoluble  in  water ;  but,  nevertheless,  it  is  undoubtedly  a  rather 
powerful  basic  oxide,  because  it  displaces  the  oxides  of  many  metals 
from  their  soluble  salts,  and  saturates  such  acids  as  nitric  acid, 
forming  with  them  neutral  salts,  which  do  not  act  on  litmus  paper.20 
Undoubtedly  water  dissolves  a  small  quantity  of  silver  oxide, 
which  explains  the  possibility  of  its  action  on  solutions  of  salts — for 
example,  on  solutions  of  cupric  salts.  Water  in  which  silver  oxide 
is  shaken  up  has  a  distinctly  alkaline  reaction.  The  oxide  is  dis- 
tinguished by  its  great  instability  when  heated,  so  that  it  loses  all  its 
oxygen  when  slightly  heated.  Hydrogen  reduces  it  at  about  80°. 20  bis 
The  feebleness  of  the  affinity  of  silver  for  oxygen  is  shown  by  the  fact 
that  silver  oxide  decomposes  under  the  action  of  light,  so  that  it  must  be 
kept  in  opaque  vessels.  The  silver  salts  are  colourless  and  decompose 
when  heated,  leaving  metallic  silver  if  the  elements  of  the  acid  are 
volatile.20*"  They  have  a  peculiar  metallic  taste,  and  are  exceedingly 
poisonous ;  the  majority  of  them  are  acted  on  by  light,  especially  in 
the  presence  of  organic  substances,  which  are  then  oxidised.  The 
alkaline  carbonates  give  a  white  precipitate  of  silver  carbonate, 
Ag2CO3,  which  is  insoluble  in  water,  but  soluble  in  ammonia  and, 
ammonium  carbonate.  Aqueous  ammonia,  added  to  a  solution  of  a 
normal  silver  salt,  first  acts  like  potassium  hydroxide,  but  the  precipitate 
dissolves  in  an  excess  of  the  reagent,  like  the  precipitate  of  cupric 

19  tri  According  to  Carey  Lea,  however,  oxide  of  silver  still  retains  water  even  at  100°, 
and  only  parts  with  it  together  with  the  oxygen.    Oxide  of  silver  is  used  for  colouring 
glass  yellow. 

20  The  reaction  of  Pb(OH)2  upon  AgHO  in  the  presence  of  NaHO  leads  to  the 
formation  of  a  compound  of  both  oxides,  PbOnAg2O,  from  which  the  oxide  of  lead 
cannot  be  removed  by  alkalies  (Wohler,  Leton).    Wb'hler,  Welch,  and  others  obtained 
crystalline  double  salts,  RsAgX3,  by  the  action  of  strong  solutions  of  RX  of  the  halogen 
salts  of  the  alkaline  metals  upon  AgX,  where  R  =  Cs,  Rb,  K. 

20  bis  According  to  Muller,  ferric  oxide  is  reduced  by  hydrogen"  (see  Chapter  XXII., 
Note  5)  at  295°  (into  what  ?),  cupric  oxide  at  140°,  NiQO3  at  150°  ;  nickelous  oxide,  NiO, 
is  reduced  to  the  suboxide,  Ni2O,  at  195°,  and  to  nickel  at  270° ;  zinc  oxide  requires  so 
high  a  temperature  for  its  reduction  that  the  glass  tube  in  which  Muller  conducted  the 
experiment  did  not  stand  the  heat ;  antimony  oxide  requires  a  temperature  of  215°  for 
its  reduction ;  yellow  mercuric  oxide  is  reduced  at  130°  and  the  red  oxide  at  280°  ;  silver 
oxide  at  85°,  and.  platinum  oxide  even  at  the  ordinary  temperature. 

30 M  A  silica  compound,  Ag2OSiO2  is  obtained  by  fusing  AgNO3  with  silica;  this  salt 
is  able  to  decompose  with  the  evolution  of  oxygen,  leaving  Ag  +  SiOa. 


426  PRINCIPLES  OF  CHEMISTRY 

hydroxide.21  Silver  oxalate  and  the  halogen  compounds  of  silver  are 
insoluble  in  water ;  hydrochloric  acid  and  soluble  chlorides  give, 
as  already  repeatedly  observed,  a  white  precipitate  of  silver  chloride 
jn  solutions  of  silver  salts.  Potassium  iodide  gives  a  yellowish 
precipitate  of  silver  iodide.  Zinc  separates  all  the  silver  in  a  metallic 
form  from  solutions  of  silver  salts.  Many  other  metals  and  reducing 
agents — for  example,  organic  substances — also  reduce  silver  from  the 
solutions  of  its  salts. 

Silver  nitrate,  AgNO3,  is  known  by  the  name  of  lunar  caustic 
(or  lapis  infernalis) ;  it  is  obtained  by  dissolving  metallic  silver 
in  nitric  acid.  If  the  silver  be  impure,  the  resultant  solution  will 
contain  a  mixture  of  the  nitrates  of  copper  and  silver.  If  this  mixture, 
be  evaporated  to  dryness  and  the  residue  carefully  fused  at  an\ 
incipient  red  heat,  all  the  cupric  nitrate  is  decomposed,  whilst  the 
greater  part  of  the  silver  nitrate  remains  unchanged.  On  treating 
the  fused  mass  with  water  the  latter  is  dissolved,  whilst  the  cupric 
oxide  remains  insoluble.  If  a  certain  amount  of  silver  oxide  be  added 
to  the  solution  containing  the  nitrates  of  silver  and  copper,  it  displaces 
all  the  cupric  oxide.  In  this  case  it  is  of  course  not  necessary  to  take 
pure  silver  oxide,  but  only  tp-pour  off  some  of  the  solution  and  to  add 
potassium  hydroxide  to  one  portion,  and  to  mix  the  resultant  pre- 
cipitate of  the  hydroxides,  Cu(OH)2  and  AgOH,  with  the  remaining 
portion.22  By  these  methods  all  the  copper  can  be  easily  removed  and 

81  If  a  solution  of  a  silver  salt  be  precipitated  by  sodium  hydroxide,  and  aqueous 
ammonia  is  added  drop  by  drop  until  the  precipitate  is  completely  dissolved,  the 
liquid  when  evaporated  deposits  a  violet  mass  of  crystalline  silver  oxide.  If  moist  silver 
oxide  be  left  in  a  strong  solution  of  ammbnia  it  gives  a  black  mass,  which  easily  decom- 
poses with  a  loud  explosion,  especially  when  struck.  This  black  substance  is  called 
fulminating  silver.  Probably  this  is  a  compound"  like  the  other  compounds  of  oxides 
with  ammonia,  and  in  exploding  the  oxygen  of  the  silver  oxide  forms  water  with  the 
hydrogen  of  the  ammonia,  which  is  naturally  accompanied  by  the  evolution  of  heat  and 
formation  of  gfcseous  nitrogen,  or,  as  Raschig  states,  fulminating  silver  contains  NAg3  or 
ope  of  the  amides  (for  instance,  NHAg3  =  NH3  +  Ag2O  -  H20).  Fulminating  silver  is  also 
formed  when  potassium  hydroxide  is  added  to  a  solution  of  silver  nitrate  in  ammonia. 
The  dangerous  explosions  which  are  produced  by,  this  Compound  render  it  needful  thatfl 
great  care  be  taken  when  salts  of  silver  come  into  contact  with  ammonia  and  alkalis' 
(*ee  Chapter  XVI.,  Note  26). 

•'  **  So  that  we  here  encounter  the  following  phenomena  :  Copper  displaces  silver  from 
fche  solutions  of ,  its  salts,  and  silver  oxide  displaces  copper  oxide  from  cupric  salts. 
Guided  by  the  conceptions  enunciated  in  Chapter  XV.,  we  can  account  for  this  in  the 
following  manner:  The  atomic  volume  of  silver =  10'8,  and  of  copper =7'2,  of  silver 
oxide  =H2,  and  of  copper  oxide  =  18.  A  greater  contraction  has  taken  place  in  the  for- 
mation of  cupric  oxide,  CuO,  than  in  the  formation  of  silver  oxide,  Ag2O,  since  in  the 
former  (18—7  =  6)  the  volume  after  combination  with  the  oxygen  has  increased  by  very 
IJttle,  whilst  the  volume  of  silver  oxide  is  considerably  greater  than  that  of  the  metal  it 
contains  [82-(2x  10-8)  =  11'4].  Hence  silver  oxide  is  less  compact  than  cupric  oxide, 
and  is  therefore  less  stable;  but,  on  the  other  hand,  there  are  greater  intervals 
betweeo.tho  atoms  in  silver  oxide  than  in  cupric  oxide,  and  therefore  silver  oxide  is  able  to 


COPPER,  SILVER,  AND  GOLD  427 

pure  silver  nitrate  obtained  (its  solution  is  colourless,  while  the  presence 
of  Cu  renders it  blue),  which  may  be  ultimately  purified  by  crystallisa- 
tion. It  crystallises  in  colourless  transparent  prismatic  plates,  which 
are  not  acted  on  by  air.  They  are  anhydrous.  Its  sp.  gr.  is  4 '34  ;  it 
dissolves  in  half  its  weight  of  water  at  the  ordinary  temperature. 22bi9 
The  pure  salt  is  not  acted  on  by  light,  but  it  easily  acts  in  an  oxidising 
manner  on  the  majority  of  organic  substances,  which  it  generally 
blackens.  This  is  due  to  the  fact  that  the  organic  substance  is  oxidised 
by  the  silver  nitrate,  which  is  reduced  to  metallic  silver  ]  the  latter  is 
thus  obtained  in  a  finely-divided  state,  which  causes  the  black  stain. 
This  peculiarity  is  taken  advantage  of  for  marking  linen.  Silver  nitrate 
is  for  the  same  reason  used  for  cauterising  wounds  and  various  growths 
;on  the  body.  Here  again  it  acts  by  virtue  of  its  oxidising  capacity  in 
destroying  the  organic  matter,  which  it  oxidises,  as  is  seen  from  the 
separation  of  a  coating  of  black  metallic  powdery  silver  from  the  part 
^cauterised.22  tri  From  the  description  of  the  preparation  of  silver  nitrate 
it  will  have  been  seen  that  this  salt,  which  fuses  at  218°,  does  not 

give  more  stable  compounds  than  those  of  copper  oxide.  This  is  verified  by  the  figures 
and  data  of  their  reactions.  It  is  impossible  to  calculate  for  cupric  nitrate,  because  this 
salt  has  not  yet  been  obtained  in.  an  anhydrous  state ;  but  the  sulphates  of  both  oxides 
are  known.  The  specific  gravity  of  copper  sulphate  in  an  anhydrous  state  is  8'58,  and  of 
silver  sulphate  5'86;  the  molecular  volume  of  the  former  is  45,  and  of  the  latter  58. 
The  group  SO3  in  the"  copper  occupies,  as  it  were,  a  volume  45—18  =  82,  and  in  the  silver 
salt  a  volume  58— 82  =  26 ;  hence  a  smaller  contraction  has  taken  place  in  the  formation 
of  the  copper  salt  from  the  oxide  than  in  the  formation  of  the  silver  salt,  and  conse- 
quently the  latter  should  be  more  stable  than  the  former.  Hence  silver  oxide  ia 
able  to  decompose  the  salt  of  copper  oxide,  whilst  with  respect  to  the  metals  both  salts 
have  been  formed  with  an  almost  identical  contraction,  since  58  volumes  of  the  silver 
salt  contain  21  volumes  of  metal  (difference  =  87),  and  45  volumes  of  the  copper  salt 
contain  7  volumes  of  copper  (difference  =  38).  Besides  which,  it  must  be  observed  that 
copper  o'xide  displaces  iron  oxide/ just  .as  silver,  oxide  displaces  copper  oxide.  Silver, 
copper,  and  iron,  in  the  form  of  oxides,  displace  each  other  in  the  above  order,  but  in  the 
form  of  metals  in  a  reverse  order  (iron,  copper,  silver).  The  cause  of  this  order  of  the 
displacement  of  the  oxides  lies,  amongst  other  things,  in  their  composition.  They  have 
the  composition  Ag2O,  Cu3O2,  Fe2O3 ;  the  oxide  containing  a  less  proportion  of  oxygen 
.displaces  that  containing  a  larger  proportion,  because  the  basic  character  diminishes 
,  with  the  increase  of  contained  oxygen. 

Copper  also  displaces  mercury  from  its  salts.  It  may  here  be  remarked  that  Spring 
(1888),  on  leaving  a  mixture  of  dry  mercurous  chloride  and  copper  for  two  hours, 
observed  a  distinct  reduction,  which  belongs  to  the  category  of  those  phenomena  which 
demonstrate  the  existence  of  a  mobility  of  parts  (i.e.  atoms  and  molecules)  in  solid-sub, 
stances. 

ti  bis  The  reaction  of  1  part'  by  weight  of  AgN03  requires  (according  to  Kremers)  the 
following  amounts  of  water:  at  0°,  0'82  part,  at  19°'5,  0'41  part,  at  54°,  0!20  part, 
at  110",  0'09  part,  and,  according  to  Tilden,  at  125°,  O'OCIT  part,  and  at  183°,  0'051S 
part. 

Mtrt  It  may  be  remarked  that  the  black  stain  produced  by  the  redaction  of  metallic 
silver  disappears  under  the  action  of  a  solution  of,  mercuric  chloride  or  of  potassium 
cyanide,  because  these  salts  act  on  finely-divided  silver, 


428  PRINCIPLES  OF  CHEMISTRY 

decompose  at  an  incipient  red  heat ;  when  cast  into  sticks  it  is  usually 
employed  for  cauterising.  On  further  heatingr the  fused  salt  undergoes 
decomposition,  first  forming  silver  nitrite  and  then  metallic  silver. 
With  ammonia,  silver  nitrate  forms,  on  evaporation  of  the  solution, 
colourless  crystals  containing  AgN08,2HN3  (Marignac).  In  general 
the  salts  of  silver,  like  cuprous,  cupric,  zinc,  &c.  salts,  are  able  to  give 
Several  compounds  with  ammonia  ;  for  example,  silver  nitrate  in  a  dry 
state  absorbs  three  molecules  (Rose).  The  ammonia  is  generally  easily 
expelled  from  these  compounds  by  the  action  of  heat. 

Nitrate  of  silver  easily  forms  dc/uble  salts  like  AgN032NaN03  and 
AgNO8KNO3.  Silver  nitrate  under  the  action  of  water  and  a  halogen 
gives  nitric  acid  (see  Vol.  I.  p»  280,  formation  of  N2O5),  a  halogen  salt  of 
silver,  and  a  silver  salt  of  an  oxygen  acid  of  the  halogen.  Thus,  for 
example,  a  solution  of  chlorine  in  water^  when  mixed  with  a  solution  of 
silver  nitrate,  gives  silver  chloride  and  chlorate.  It  is  here  evident  that 
the  reaction  of  the  silver  nitrate  is  identical  with  the  reaction  of  the 
caustic  alkalis,  as  the  nitric  acid  is  all  set  free  and  the  silver  oxide  only 
reacts  in  exactly  the  same  way  in  which  aqueous  potash  acts  on  free 
chlorine.  Hence  the  reaction  may  be  expressed  in  the  following 
manner  :  6AgNO3  +  3C12  +  3H2O  =  SAgCl  +  AgC103  +  6NH03. 

Silver  nitrate,  like  the  nitrates  of  the  alkalis,  does  not  contain  any 
water  of  crystallisation.  Moreover  the  other  salts  of  silver  almost 
always  separate  out  without  any  water  of  crystallisation.  The  silver 
salts  are  further  characterised  by  the  fact  that  they  give  neither 
basic  nor  acid  salts,  owing  to  which  the  formation  of  silver  salts 
generally  forms  the  means  of  determining  the  true  composition  of 
acids— thus,  to  any  acid  HnX  there  corresponds  a  salt  AgnX— for 
instance,  Ag3P04  (Chapter  XIX.,  Note  15). 

Silver  gives  insoluble  and  exceedingly  stable  compounds  with  the 
halogens.  They  are  obtained  by  double  decomposition  with  great 
facility  whenever  a  silver  salt  comes  in  contact  with  halogen  salts. 
Solutions  of  nitrate,  sulphate,  and  all  other  kindred  salts  of  silver  give 
a  precipitate  of  silver  chloride  or  iodide  in  solutions  of  chlorides  and 
iodides  and  of  the  halogen  acids,  because  the  halogen  salts  of  silver  are 
insoluble  both  in  water 23  and  in  other  acids.  Silver  chloride,  AgOl,  ia 

23  Silver  chloride  is  almost  perfectly  insoluble  in  water,  but  is  somewhat  soluble  ift 
water  containing  sodium  chloride  or  hydrochloric  acid,  or  other  chlorides,  and  many  salts, 
in  solution.  Thus  at  100°,  100  parts  of  water  saturated  with  sodium  chloride  dissolve 
0'4  part  of  silver  chloride.  Bromide  and  iodide  of  silver  are  less  soluble  in  this  respect, 
as  also  in  regard  to  other  solvents.  It  should  be  remarked  that  silver  chloride  dissolve* 
in  solutions  of  ammonia,  potassium  cyanide,  and  of  sodium  thiosulphate,  NaaS-jOj. 
Silver  bromide  is  almost  perfectly  analogous  to  the  chloride,  but  silver  iodide  is  nearly 
insoluble  in  a  solution  of  ammonia.  Silver  chloride  even  absorbs  dry  ammonia  ga% 


COPPER,  SILVER,  AND  GOLD  429 

then  obtained  as  a  white  flocculent  precipitate,  silver  bromide  forms  a 
yellowish  precipitate,  and  silver  iodide  has  a  very  distinct  yellow 
colour.  These  halogen  compounds  sometimes  occur  in  nature ;  they 
are  formed  by  a  dry  method — by  the  action  of  halogen  compounds  on 
silver  compounds,  especially  under  the  influence  of  heat.  Silver  chlo- 
ride easily  fuses  at  451°  on  cooling  from  a  molten  state  ;  it  forms 
a  somewhat  soft  horn-like  mass  which  can  be  cut  with  a  knife 
and  is  known  as  horn  silver.  It  volatilises  at  a  higher  tempera- 
ture. Its  ammoniacal  solution,  on  the  evaporation  of  the  ammonia, 
deposits  crystalline  chloride  of  silver,  in  octahedra.  Bromide  and 
iodide  of  silver  also  appear  in  forms  of  the  regular  system,  so  that  in 
this  respect  the  halogen  salts  of  silver  resemble  the  halogen  salts  of  the 
alkali  metals;24 

forming  very  unstable  ammoniacal  compounds.  When  heated,  these  compounds  (Vol.  I. 
p.  250,  Note  8)  evolve  the  ammonia,  as  they  also  do  under  the  action  of  all  acids.  Silver 
chloride  enters  into  double  decomposition  with  potassium  cyanide,  forming  a  soluble 
double  cyanide,  which  we  shall  presently  describe  ;  it  also  forms  a  soluble  double  salt, 
NaAgS2Os,  with  sodium  thiosulphate. 

Silver  chloride  offers  different  modifications  in  the  structure  of  Us  molecule,  as  is  seen 
in  the  variations  in  the  consistency  of  the  precipitate,  and  in  the  differences  in  the  action 
of  light  which  partially  decomposes  AgCl  (see  Note  25),  Stas  and  Carey  Lea  investigated 
this  subject,  which  has  a  particular  importance  in  photography,  because  silver  bromide 
also  gives  photo-salts.  There  is  still  much  to  be  discovered  in  this  respect,  since  Abney 
showed  that  perfectly  dry  AgCl  placed  in  a  vacuum  in  the  dark  is  not  in  the  least  acted 
upoii  when  subsequently  exposed  to  light. 

24  Silver  bromide  and  iodide  (which  occur  as  the  minerals  bromito  and  iodite) 
resemble  the  chloride  in  many  respects,  but  the  degree  of  affinity  of  silver  for  iodine  id 
greater  than  that  for  chlorine  and  bromine,  although  less  heat  is  evolved  (see  Note  28  Bis). 
Deville  deduced  this  fact  from  a  number  of  experiments.  Thus  silver  chloride,  when 
treated  with  hydriodic  acid,  evolves  hydrochloric  acid,  and  forms  silver  iodide.  Finely, 
divided  silver  easily  liberates-  hydrogen  when  treated  with  hydriodic  acid ;  it  produces 
the  same  decomposition  with  hydrochloric  acid,  but  in  a  considerably  less  degree  and 
only  on  the  surface.  The  difference  between  silver  chloride  and  iodide  is  especially 
remarkable,  sinco  the  formation  of  the  former  is  attended  with  a  greater  contraction 
than  that  of  the  latter.  The  volume  of  AgCl  =  26 ;  of  chlorine  27,  of  silver  10,  the  sum 
=  37,  hence  a  contraction  has  ensued;  and  in  the  formation  of  silver  iodide  an  expansion 
takes  place,  for  the  volume  of  Ag  is  10,  of  I  26,  and  of  Agl  89  instead  of  86  (density, 
AgCl,  6'69 ;  Agl,  6'67).  The  atoms  of  chlorine  have  united  with  the  atoms  of  silver 
without  moving  asunder,  whilst  the  atoms  of  iodine  must  have  moved  apart  in 
combining  with  the  silver.  It  is  otherwise  with  respect  to  the  metal ;  the  distance 
between  its  atoms  in  the  metal  =  2'2,  in  silver  chloride = 8'0,  and  in  silver  iodide 
=  8'5;  hence  its  atoms  have  moved  asunder  considerably  in  both  cases.  It  is  also  very 
remarkable,  as  Fizeau  observed,  that  the  density  of  silver  iodide  increases  with  a  rise  0% 
temperature — that  is,  a  contraction  takes  place  when  it  is  heated  and  an  expansion  wheq|- 
it  is  cooled. 

In  order  to  explain  the  fact  that  in  silver  compounds  the  iodide  is  more  stable  than? 
the  chloride  and  oxide,  Professor  N.  N.  Beketoff,  in  his  '  Researches  on  the  Phenomena 
of  Substitutions  '  (Kharkoff,  1865),  proposed  the  following  original  hypothesis,  which  wd 
will  give  in  almost  the  words  of  the  author : — In  the  case  of  aluminium,  the  oxide,  AljOs, 
is  more  stable  than  the  chloride,  A12C16,  and  the  iodide,  A12I6.  In  the  oxide  the  amount 
of  the  metal  is  to  the  amount  of  the  element  .Combined  with  it  ae  54*8  (A1-  27'8)  is  to  48, 


430  PRINCIPLES  QE  CHEMISTRY 

Silver  chloride  may  be  decomposed,  with  the  separation  of  silver 
oxide,   by  heating  it  with  a  solution  of  aa  alkali,  and  if  an  organic 

or  in  the  ratio  112  :  100  ;  for  the  chloride  the  ratio  is=S5  :  100  ;  for  the  iodide  it  =  7  :  100 
In  the  case  of  silver  the  oxide  (ratio  =  1350:  100)  is  less  stable  than  the  chloride  (ratio 
e=S04  :  100),  and  the  iodide  (ratio  of  the  weight  of  metal  to  the  weight  of  the  halogen 
=  85  :  100)  is  the  most  stable.  From  these  and  similar  examples  it  follows  that  the  most 
stable  compounds  are  those  in  which  the  weights  of  the  combined  substances  are  equal. 
This  may  be  partly  explained  by  the  attraction  of  similar  molecules  even  after  their 
having  passed  into  combination  with  others.  This  attraction  is  proportional  to  the 
product  of  the  acting  masses.  In  silver  oxide  the  attraction  of  Ag,  for  Ag,  =  216x  216 
=  46,656,  and  the  attraction  of  Ag2  for  O  =  216  x  16  =  8,456.  The  attraction  of  like  mole- 
cules thus  counteracts  the  attraction  of  the  unlike  molecules.  The  former  naturally 
does  not  overcome  the  latter,  otherwise  there  would  be  a. disruption,  but  it  nevertheless 
diminishes  the  stability.  In  the  case  of  an  equality  or  proximity  of  the  magnitude  of 
the  combining  masses,  the  attraction  of  the  like  parts  will  counteract  the  stability  of  the 
compound  to  the  least  extent — in  other  words,  with  an  inequality  of  the  combined  masses, 
the  molecules  have  an  inclination  to  return  to  an  elementary  state,  to  decompose,  which 
does  riot  exist  to  such  an  extent  where  the  combined  masses  are  equal.  There  is,  there- 
fore, a  tendency  for  large  masses  to  combine  with  large,  and  for  small  masses  to  combine 
with  small.  Hence' Ag2O  +  2KI 'gives  K3O  +  2AgI.  The  influence  of  an  equality  of 
masses  on  the  stability  is  seen  particularly  clearly  in  the  effect  of  a  rise  of  temperature. 
Argentic,  mercuric,  auric  and  other  oxides  composed  of  unequal  masses,  are  somewhat 
readily  decomposed  by  heat,  whilst  the  oxides  of  the  lighter  metals  (like  water)  are  not  so 
easily  decomposed  by  heat.  Silver  chloride  and  iodide  approach  the  condition  of 
equality,  and  are  not  decomposed  by  heat.  The  most  stable  oxides  under  the  action  of 
heat  are  those  of  magnesium,  calcium,  silicon,  and  aluminium,  since  they  also  approach 
the  condition  of  equality.  For  the  same  reason  hydriodic  acid  decomposesrwith  greater 
facility  than  hydrochloric  acid.  Chlorine  does  not  act  on  magnesia  or  alumina,  but  it 
acts  on  lime  and  silver  oxide,  &c.  This  is  partially  explained  by  the  fact  that  by  con- 
sidering heat  as  a  mode  of  motion,  and  knowing  that  the  atomic  heats  of  the  free  elemepts 
are  equal,  it  must  be  supposed  that  the  amount  of  the  motion  of  atoms  (their  vis  viva)  is 
equal,  and  as  it  is  equal  to  the  product  of  the  mass  (atomic  weight)  into  the  square  of  the 
velocity,  it  follows  that  the  greater  the  combining  weight  the  smaller  will  be  the  square 
of  the  velocity,  and  if  the  combining  weights  be  nearly  equal,  then  the  velocities  also  will 
be  nearly  equal.  Hence  the  greater  the  difference  between  the  weights  of  the  combined 
atoms  the  greater  will  be  the  difference  between  their  velocities.  The  difference  between 
the  velocities  will  increase  with  the  temperature,  and  therefore  the  temperature  of  de- 
composition will  be  the  sooner  attained  the  greater  be.  the  original  difference — that  is, 
the  greater  the  difference  of  the  weights  of  the  combined  substances.  The  nearer  these 
weights  are  to  each  other,  the  more  analogous  the  motion  of  the  unlike  atoms,  and  con- 
sequently, the  more  stable  the  resultant  compound. 

The  instability  of  cupric  chloride  and  nitric  oxide,  the  'absence  of  compounds  of  fluorine 
with  oxygen,  whilst  there  are  compounds  of  oxygen  with  chlorine,  the  greater  stability  of 
the  oxygen  compounds  of  iodine  than  those  of  chlorine,  the  stability  of  boron  nitride,  and 
the  instability  of  cyanogen,  and  a  number  of  similar  instances,  v  here,  j  udging  from  the  above 
argument,  one  would  expect  (owing  to  the  closeness  of  the  atomic  weights)  a  stability, 
show  that  Beketoff's  addition  to  the  mechanical  theory  of  chemical  phenomena  is  still 
far  from  sufficient  for  explaining  the  true  relations  of  affinities.  Nevertheless,  in  his 
mode  of  explaining  the  relative  stabilities  of  compounds^  we  find  an  exceedingly  interest- 
ing treatment  of  questions  of  primary  importance.  Without  such  efforts  it  would  be 
Impossible  to  generalise  the  complex  data  of  experimental  knowledge. 

Fluoride  of  silver,  AgF,  is  obtained  by  dissolving  Ag.2O  or  Ag2CO3  in  hydrofluoric  acid. 
It  differs  from  the  other  halogen  salts  of  silver  in  being  soluble  in  water  (1  part  of  salt  in 
.0'55of  water).  It  crystallises  from  its  solution  in  prisms,  AgFH2O  (Marignac),  or  AgF2H2O 
(Pfaundler),  which  lose  their  water  in  vacuo.  Giintz  (1891),  by  electrolising  a  saturated 


COPPEB,  SILVER,  AKD  GOLD  431 

substance  be  added  to  the  alkali  the  chloride  can  easily  be  reduced  to 
metallic  silver,  the  silver  oxide  being  reduced  in  the  oxidation  ot  the 
organic  substance.  Iron,  zinc,  and  many  other  metals  reduce  silver 
chloride  in  the  presence  of  water.  Cuprous  and  mercurous  chlorides 
and  many  organic  substances  are  also  able  to  reduce  the  silver  from 
chloride  of  silver.  This  shows  the  rather  easy  decomposability  of  the^ 
halogen  compounds  of  silver.'  Silver  iodide  is  much  more  stable  in  this 
respect  than  the  chloride.  The  same  is  also  observed  with  respect  to 
the  action  of  light  upon  moist  AgCL  White  silver  chloride  soon  acquires 
a  violet  -colour  when  exposed  to  the  action  of  light,  and  especially 
under  the  direct  action  of  the  sun's  rays.  After  being  acted  upon 
by  light  it  is  no  longer  entirely  soluble  in  ammonia,  ,but  leaves 
metallic  silver  undissolved,  from  which  it  might  be  assumed  that  the 
action  of  light  consisted  in  the  decomposition  of  the  silver  chloride 
into  chlorine  and  metallic  silver  and  in  fact  the  silver  chloride  be- 
comes in  time  darker  and  darker.  Silver  bromide  and  iodide  are  much 
more  slowly  acted  on  by  light,  and,  according  to  certain  observations, 
when  pure  they  are  even  quite  unacted  on  ;  at  least  they  do  not  change 
in  weight,24  bis  so  that  if  they  are  acted  on  by  light,  the  change  they 
undergo  must  be  one  of  a  change  in  the  structure  of  their  parts  and  not 
of  decomposition,  as  it  is  in  silver  chloride.  The  silver  chloride  under 
the  action  of  light  changes  in  weight,  which  indicates  the  formation  of 
a  volatile  product,  and  the  deposition  of  metallic  silver  on  dissolving 
in  ammonia  shows  the  loss  of  chlorine.  The  change  does  actually 
occur  under  the  action  of  light,  but  the  decomposition  does  not  go 
as  far  as  into  chlorine  and  silver,  but  only  to  the  formation  of  a  sub- 
chloride  of  silver,  Ag2Cl,  which  is  of  a  brown  colour  and  is  easily  de- 
composed into  metallic  silver  and  silver  chloride,  Ag2Cl  =  AgCl  +  Ag. 
This  change  of  the  chemical  composition  and  structure  of  the  halogen 
salts  of  silver  under  the  action  of  light  forms  the  basis  of  photography, 
because  the  halogen  compounds  of  silver,  after  having  been  exposed  to 
light,  give  a  precipitate  of  finely-divided  silver,  of  a -black  colour, 
when  treated  with  reducing  agents.25 

solution  of  Ag2F,  obtained  poly  fluoride  of  silver,  Ag2F,  which  is  decomposed  by  water 
into  AgF  +  Ag.  It  is  also  formed  by  the  action  of  a  strong  solution  of  AgF  upon  finely- 
divided  (precipitated)  silver. 

24  bu  The  changes  brought  about  by  the  action  of  light  necessitate  distinguishing  the 
photo-salts  of  silver. 

85  In  photography  these  are  called  '  developers.'  The  most  common  developers  are : 
solutions  of  ferrous  sulphate,  pyrogallol,  ferrous  oxalate,  bydroxylamine,  potassium  sul- 
phite, hydroquinone  (the  last  acts  particularly  well  and  is  very  convenient  to  use),  &c.  The 
chemical  processes  of  photography  are  of  great  practical  and  theoretical  interest ;  but  it 
would  be  impossible  in  this  work  to  enter  into  this  special  branch  of  chemistry,  which  has  as 
yet  been  very  little  worked  out  from  a  theoretical  point  of  view.  Nevertheless,  we  .will  pause 


482  PRINCIPLES  OF  CHEMISTRY 

The  insolubility  of  the  halogen  compounds  of  silver  forms  tnd 
basis  of  many  methods  used  in  practical  chemistry.  Thus  by  means  of 
this  reaction  it  is  possible  to  obtain  salts  of  other  acids  from  a  halogen 
salt  of  a  given  metal,  for  instance,  RC12  +  2AgNO3=R(Nr03)2  +  2AgCL 
The  formation  of  the  halogen  compounds  of  silver  is  very  frequently 
used  in  the.  investigation  of  organic  substances  ;  for  example,  if  any 
product  of  metalepsis  containing  iodine  or  chlorine  be  heated  with  a 
silver  salt  or  silver  oxide,  the  silver  combines  with  the  halogen  and 
gives  a  halogen  salt,  whilst  the  elements  previously  combined  with  the 
silver  replace  the,  halogen.  For  instance,  ethylene  dibromide,  C2H4Bra, 
is  transformed  into  ethylene  diacetate,  C2H4(C2H302)2,  and  silver 

to  consider  certain'  aspects  of  this  subject  which  are  of  a  purely  chemical  interest,  and 
especially  the  facts  concerning  subchloride  of  silver,  Ag3Cl  (see  -Note  19),  and  the  photo- 
salts  (Note  28).  There  is  no  doubt  that  under  the  action  of  light,  AgCl  becomes  darker 
in,colour,  decreases  hi  weight,  and  probably  forms  a  mixture  of  AgCl,  Ag2Cl,  and  Ag. 
But  the  isolation  of  the  subchloride  has'  only  been  recently  accomplished  by  Giintz  by 
means  of  the  Ag2P,  discovered  by  him  (see  Note  24).  Many  chemists  (and  among  them 
Hodgkinson)  assumed  that  an  oxychloride  of  silver  was  formed  by  the  decomposition  of 
AgCl  under  the  action  of  light.  Carey  Lea's  (1889)  and  A.  Richardson's  (1891)  experiments 
showed  that  the  product  formed  does  not,  however,  contain  any  oxygen  a*  all,  and  the 
change  in  colour  produc«d  by  th&  action  of  light  upon  AgCl  is  most  probably  due  to  the 
formation  of  Ag2Cl.  This  substance  was  isolated  by  Giintz  (1891)  by  passing  HC1  over 
crystals  of  Ag2F.  He  also  obtained  Ag2I  in  a  similar  manner  by  passing  HI,  and  Ag2S 
by  passing  H2S  over  AgoF.  Ag2Cl  is  best  prepared  by  the  action  of  phosphorus  tri- 
chloride upon  Ag2F.  At  the  temperature  of  its  formation  Ag2Cl  has  an  easily  changeable 
tint,  with  shades  of  violet  red  to  violet  black.  Under  the  action  of  light  a  similar 
(isomeric)  substance  is  obtained,  which  splits  up  into  AgCl  +  Ag  when  heated.  With 
potassium  cyanide  Ag2Cl  gives  Ag  +  AgCN  +  KC1,  whence  it  is  possible  to  calculate  the 
heat  of  formation  of  Ag._>Cl ;  it  =  29:7,  whilst  the  heat  of  formation  of  AgCl  =  29'2— i.e.  the 
reaction  2AgCl  =  Ag2Cl  +  Cl  corresponds  to  an  absorption  .of  28'7  major  calories.  If  we 
admit  the  formation  of  such  a  compound  by  the  action  of  light,  it  is  evident  that  the  energy 
of  the  light  is  consumed  in  the  above  reaction.  Carey  Lea  (1892)  subjected  AgCl,  AgBr,  and 
Agl  to  a  pressure  (of  course  in  the  dark)  of  8,000  atmospheres,  and  to  trituration  with 
water  in  a  mortar,  and  observed  a  change  of  colour  indicating  incipient  decomposition, 
which  is  facilitated  under  the  action  of  light  by  the  molecular  currents  set  up  (Lermontoff, 
Egoroff).  The  change  of  colour;  of  the  halogen  salts  of  silver  under  the  action  of  light, 
and  their  faculty  of  subsequently  giving  a  visible  photographic  image  under  the  action  of 
4  developers,'  must  now  be  regarded  as  connected  with  the  decomposition  of  AgX,  leading 
to  the  formation  of  Ag.>X,  and  the  different  tinted  photo- salts  must  be  considered  as 
systems  containing  such  Ag2X*B.  Carey  Lea  obtained  photo-salts  of  this  kind  not  only  by 
the  action  of  light  but  also  in  many  other  ways,  which  we  will  enumerate  to  prove  that 
they  contain  the  products  of  an  incomplete  combination  of  Ag  with  the  halogens,  (for  the 
salts  Ag2X  must  be  regarded  as  such).  The  photo-salts  have  been  obtained  (1)  by  the 
imperfect  chlorination  of  silver  ;  (2)  by  the  incomplete  decomposition  of  Ag2O  or  Ag2COs 
by  alternately  heating  and  treating  with  a  halogen  acid  ;  (3)  by  the  action  of  nitric  acid 
or  Na-zS-jOj  upon  Ag.>Cl;  (4)  by  mixing  a  solution  of  AgNO5  with  the  hydrates  of  FeO, 
MnO  and  CrO,  and  precipitating  by  HC1 ;  (5)  by  the  action  of  HC1  upon  the  product 
obtained  by  the  reduction  of  citrate  of  silver  in  hydrogen  (Note  19),  and  (6)  by  the  action 
of  milk  sugar  upon  AgN05  together  with  soda  and  afterwards  acidulating  with  HC1.  All 
these  reactions  should  lead  to  the  formation  of  products  of  imperfect  combination  with 
the  halogens  and  give  photo-salts  of  a  similar  diversity  of  colour  to  those  produced 
by  the  action  of  developers  upon  the  halogen  salts  of  silver  after  exposure  to  light. 


COPPEE,  SILVER,  AND  GOLD  433 

.bromide  by  heating  it  with  silver  acetate,  2C2H3O2  Ag.  The  insolubility 
of  th'e  halogen  compounds  of  silver  is  still  more  frequently  taken  ad- 
vantage  of  in  determining  the  amount  of  silver  and  halogen  in  a  given 
solution.  If  it  is  required,  for  instance,  to  determine  the  quantity  of 
chlorine  present  in  the  form  of  a  metallic  chloride  in  a  given  solution, 
a  solution  of  silver  nitrate  is  added  to  it  so  long  as  it  gives  a  pre- 
cipitate. On  shaking  or  stirring  the  liquid,  the  silver  chloride  easily 
settles  in  the  form  of  heavy  flakes.  It  is  possible  in  this  way  to 
precipitate  the  whole  of  the  chlorine  from  a  solution,  without  adding 
an  excess  of  silver  nitrate,  since  it  can  be  easily  seen  whether  the 
addition  of  a  fresh  quantity  of  silver  nitrate  produces  a  precipitate  in 
the  clear  liquid.  In  this  manner  it  is  possible  to  add  to  a  solution 
containing  chlorine,  as  much  silver  as  is  required  for  its  entire  precipi- 
tation, and  to  calculate  the  amount  of  chlorine  previously  in  solution 
from  the  amount  of  the  solution  of  silver  nitrate  consumed,  if  the 
quantity  of  silver  nitrate  in  this  solution  has  been  previously  deter- 
mined.25 bis  The  atomic  proportions  and  preliminary  experiments  with 
a  pure  salt — for  example,  with  sodium  chloride — will  give  the  amount 
of  chlorine  from  the  quantity  of  silver  nitrate.  Details  of  these 
methods  will  be  found  in  works  on  analytical  chemistry.25  trl 

25  t>i«  jn  order  to  determine  when  the  reaction  is  at  an  end,  a  few  drops  of  a  solution 
of  K2CrO4  are  added  to  the  solution  of  the  chloride.  Before  all  the  chlorine  is  precipitated 
as  AgCl,  the  precipitate  (after  shaking)  is  white  fsince  Ag2Cr04  with  2RC1  gives  2AgCl) ; 
but  when  all  the  chlorine  is  thrown  down  Ag2CrO4  is  formed,  which  colours  tie  precipi- 
tate reddish-brown.  In  order  to  obtain  accurate  results  the  liquid  should  be  neutral 
to  litmus. 

«5  trl  Silver  cyanide,  AgCN,  is  -closely  analogous  to  the  haloid  salts  of  silver.  It  is 
obtained,  in  similar  manner  to  silver  chloride,  by  the  addition  of  potassium  cyanide  to 
silver  nitrate.  A  white  precipitate  is  then  formed,  which  is  almost  insoluble  in  boiling 
water.  It  is  also,  like  silver  chloride,  insoluble  in  dilute  acids.  However,  it  is  dissolved 
when  heated  with  nitric  acid,  and  both  hydriodic  and  hydrochloric  acids  act  on  it,  con- 
verting it  into  silver  chloride  and  iodide.  Alkalis,  however,  do  not  act  on  silver  cyanide, 
although  they  act  on  the  other  haloid  salts  of  silver.  Ammonia  and  solutions  of  the 
cyanides  of  the  alkali  metals  dissolve  silver  cyanide,  as  they  do  the  chloride.  In  the 
latter  case  double  cyanides  are  formed — for  example,  KAgC2N2.  This  salt  is  obtained  in 
a  crystalline  state  on  evaporating  a  solution  of  silver  cyanide  in  potassium  cyanide.  It 
is  much  more  stable  than  silver  cyanide  itself.  It  has  a  neutral  reaction,  does  not 
change  in  the  air,  and  does  not  smell  of  hydrocyanic  acid.  Many  acids,  in  acting  on  a 
solution  of  this  double  salt,  precipitate  the  insoluble  silver  cyanide.  Metallic  silver  dis- 
solves in  a  solution  of  potassium  cyanide  in  the  presence  of  air,  with  formation  of 
the  same  double  salt  and  potassium  hydroxide,  and  when  silver  chloride  dissolves  in 
potassium  cyanide  it  forms  potassium  chloride,  besides  the  salt  KAgC2N2.  This  double 
salt  of  silver  is  used  in  silver  plating.  For  this  purpose  potassium  cyanide  is  added  to 
its  solution,  as  otherwise  silver  cyanide,  and  not  metallic  silver,  is  deposited  by  the 
electric  current.  If  two  electrodes — one  positive  (silver)  and  the  other  negative  (copper)— 
be  immersed  in  such  a  solution,  silver  will  be  deposited  upon  the  latter,  and  the 
silver  of  the  positive  electrode  will  be  dissolved  by  the  liquid,  which  will  thus  preserve 
the  same  amount  of  metal  in  solution  as  it  originally  contained.  If  instead  of  the 
negative  electrode  a  copper  object  be  taken,  well  cleaned  from  all  dirt,  the  silver 


,434  PRINCIPLES  OF  CHEMISTRY 

Accurate  experiments,  and  more  especially  the  researches  of 
Stas  at  Brussels,  show  the  proportion  in  which  silver  reacts  with 
metallic  chlorides.  These  researches  have  led  -to  the  determina- 
tion of  the  combining  weights  of  silver,  sodium,  potassium,  chlorine, 
bromine,  iodine,  and  other  elements,  and  are  distinguished  for  their 
model  exactitude,  and  we  will  therefore  describe  them  in  some  detail. 
As  sodium  chloride  is  the  chloride  most  generally  used  for  the  pre- 
cipitation of  silver,  since  it  can  most  easily  be  obtained  in  a  pure  state, 
we  will  here  cite  the  quantitative  observations  made  by  Stas  for  show- 
ing the  co-relation  between  the  quantities  of  chloride  .of  sodium  and 
silver  which  react  together.  In  order  to  obtain  perfectly  pure  sodium 

will  be  deposited  in  an  even  coating;  this,  indeed,  forms  the  mode  of  silver  plating  by 
the  wet  method,  'which  is  most  often  used  in  practice.  A  solution  of  one  part  of  silver 
jiitrate  in  80  to  50  parts  of  water,  and  mixed  with  a  sufficient  quantity  of  a  solution  ot 
potassium  cyanide  to  redissolve  the  precipitate  of  silver  cyanide  formed,  gives  a  dull 
coating  of  silver,  but  if  twice  as  much  water  be  used  the  same  mixture  gives  a  bright 
coating. 

.Silver  plating  in  the  wet  way  has  now  replaced  to  a  considerable  extent  the  old 
process  of  dry  silvering,  because  this  process,  which  consists  in  dissolving  .silver  in 
mercury  and  applying  the  amalgam  to  the  surface  of  the  objects,  and  then  vaporising 
the  mercury,  offers  the  great  disadvantage  of  the  poisonous  mercury  fumes.  Besides 
these,  there  is  another  method  of  silver  plating,  based  on.  the  direct  displacement  of 
silver  from  its  salts  by  other  metals — for  example,  by  copper.  The  copper  reduces  the 
silver  from  its  compounds,  and  the  silver  separated  is  deposited  upon  the  copper.  Thus 
a  solution  of  silver  chloride  in  sodium  thiosulphate  deposits  a  coating  of  silver  upon  a 
atrip  of  copper  immersed  in  it.  It  is  best  for  this  purpose  to  take  pure  silver  sulphite. 
This  is  prepared  by  mixing  a  solution  of  silver  nitrate  with  an  excess  of  ammonia,  and 
adding  a  saturated  solution-  of  sodium  sulphite  and  then  alcohol,  which  precipitates 
silver  sulphite  from  the  solution.  The  latter  and  its  solutions  are  very  easily  decomposed 
by  copper.  Metallic  iron  produces  the  same  decomposition,  and  iron  and  steel  articles 
may  be  very  readily  silver-plated  by  means  of  the  thiosulphate  solution  of  silver  chloride. 
Indeed,  copper  and  similar  metals  may  even  be  silver-plated  by  means  of  silver  chloride ; 
if  the  chloride  of  silver,  with  a  small  amount  of  acid,  be  rubbed  upon  the  surface  of  the 
copper,  the  latter  becomes  covered  with  a  coating  of  silver,  which  it  has  reduced. 

Silver  plating  is  not  only  applicable  to  metallic  objects,  but  also  to  glass,  china,  &c. 
Glass  is  silvered  for  various  purposes — for  example,  glass  globes  silvered  internally  are 
used  for  ornamentation,  and  have  a  mirrored  surface.  Common  looking-glass  silvered 
upon  one  side  forms  a  mirror  which  is  better  than  the  ordinary  mercury  mirrors,  owing 
to  the  truer  colours  of  the  image  due  to  the  whiteness  of  the  silver.  For  optical  in- 
struments— for  example,  telescopes — concave  mirrors  are  now  made  of  silvered  glass, 
which  has  first  been  ground  and  polished  into  the  required  form.  The  silvering  of  glass 
is  based  on  the  fact  that  silver  which  is  reduced  from  certain  solutions  deposits  itself  uni- 
formly in  a  perfectly  homogeneous  and  continuous  but  very  thin  layer,  forming  a  bright 
reflecting  surface.  Certain  organic  substances  have  the  property  of  reducing  pilver  in  this 
form.  The  best  known  among  these  are  certain  aldehydes—for  instance,  ordinary- 
acetaldehyde,  C2H4O,  which  easily  oxidises  in  the  air  and  forms  acetic  acid,  CSH4O2. 
This  oxidation  also  easily  takes  place  at  the  expense  of  silver  oxide,  when  a  certain  amount 
of  ammonia  is  added  to  the  mixture.  The  oxide  of  silver  gives  up  its  oxygen  to  the 
aldehyde,  and  the  silver  reduced  from  it  is  deposited  in  a  metallic  state  in  a  uniform 
bright  coating.  The  same  action  is  produced  by  certain  saccharine  substances  and 
certain  organic  acids,  such  as  tartaric  acid,  &c. 


COPPER,  SILVEE,  AND  GOLD  435 

chloride,  he  took  pure  rock  salt,  containing  only  a  small  quantity  of 
magnesium  and  calcium  compounds  and  a  small  amount  of  potassium 
salts.  This  salt  was  dissolved  in  water,  and  the  'Saturated  solution 
evaporated  by  boiling.  The  sodium  chloride  separated  out  during  the 
boiling,  and  the  mother  liquor  containing  the  impurities  was  poured 
off.  Alcohol  of  65  p.c.  strength  and  platinic  chloride  were  added 
to  the  resultant  salt,  in  order  to  precipitate  all  the  potassium  and 
a  certain  part  of  the  sodium  salts.  The  resultant  alcoholic  solution, 
containing  the  sodium  and,  platinum  chlorides,  was  then  mixed  with  a 
solution  of  pure  ammonium  chloride  in  order  to  remove  the  platinic 
chloride.  After  this  precipitation,  the  solution  was  evaporated  in  a 
platinum  retort,  and  then  separate  portions  of  this  purified  sodium 
chloride  were  collected  as  they  crystallised.  The  same  salt  was  pre- 
pared from  sodium  sulphate,  tartrate,  nitrate,  and  from  the  platino- 
ohloride,  in  order  to  have  sodium  chloride  prepared  by  different  methods 
and  from  different  sources,  and  in  this  manner  ten  samples  of  sodium 
chloride  thus  prepared  were  purified  and  investigated  in  their  relation 
to  silver.  After  being  dried,  weighed  quantities  of  all  ten  samples 
of  sodium  chloride  were  dissolved  in  water  and  mixed  with  a  solution 
in  nitric  acid  of  a  weighed  quantity  of  perfectly  pure  silver.  A 
slightly  greater  quantity  of  silver  was  taken  than  would  be  required 
for  the  decomposition  of  the  sodium  chloride,  and  when,  after  pour- 
ing in  all  the  silver  solution,  the  silver  chloride  had  settled,  the 
amount  of  silver  remaining  in  excess  was  determined  by  means  of  a 
solution  of  sodium  chloride  of  known  strength.  This  solution  of 
sodium  chloride  was  added  so  long  as  it  formed  a  precipitate.  In  this 
manner  Stas  determined  how  many  parts  of  sodium  chloride  corre- 
spond to  100  parts  by  weight  of  silver.  The  result  of  ten  determina- 
tions was  that  for  the  entire  precipitation  of  100  parts  of  silver, 
from  54-2060  to  54-2093  parts  of  sodium  chloride  were  required.  The 
difference  is  so  inconsiderable  that  it  has  no  perceptible  influence 
on  the  subsequent  calculations.  The  mean  of  ten  experiments  was 
that  100  parts  of  silver  react  with  54-2078  parts  of  sodium  chloride. 
In  order  to  learn  from  this  the  relation  between  the  chlorine  and 
silver,  it  was  necessary  to  determine  the  quantity  of  chlorine  contained  in 
54-2078  parts  of  sodium  chloride,  or,  what  is  the  same-  thing,  the  quantity 
of  chlorine  which  combines  with  100  parts  of  silver.  For  this  purpose 
Stas  made  a  series  of  observations  on  the  quantity  of  silver  chloride 
obtained  from  100  parts  of  silver.  Four  syntheses  were  made  by  him 
for  this  purpose.  The  first  synthesis  consisted  in  the  formation  of 
silver  chloride  by  the  action  of  chlorine  on  silver  at  a  red  heat.  This 
experiment  showed  that  100  parts  of  silver  give  132-841,  132-843  and 


436  PEINCIPLES  OF  CHEMISTRY 

132-843  of  silver  chloride.  The  second  method  consisted  in  dissolving 
a  given  quantity  of  silver  in  nitric  acid  and  precipitating  it  by  means* 
of  gaseous  hydrochloric  acid  passed  over  the  surface  of  the  liquid  ;  the 
resultant  mass  was  evaporated  in  the  dark  to  drive  off  the  nitric  acid 
and  excess  of  hydrochloric  acid,  and  the  remaining  silver  chloride  was 
fused  first  in  an  atmosphere  of  hydrochloric  acid  gas  and  then  in  aif. 
In  this  process  the  silver  chloride  was  not  washed,  and  therefore  there 
could  be  no  loss  from  solution.  Two  experiments  made  by  this 
method  showed  that  100  parts  of  silver  give  132-849  and  132-846 
parts  of  silver  chloride.  A  third  series  of  determinations  was  also 
made  by  precipitating  a  solution  of  silver  nitrate  with  a  certain 
excess  of  gaseous  hydrochloric  acid.  The  amount  of  silver  chloride 
obtained  was  altogether  132-848.  Lastly,  a  fourth  determination  was 
made  by  precipitating  dissolved  silver  with  a  solution  of  ammonium 
chloride,  when  it  was  found  that  a  considerable  amount  of  silver 
(0-3175)  had  passed  into  solution  in  the  washing ;  for  100  parts 
of  silver  there  was  obtained  altogether  132-8417  of  silver  chloride. 
Thus  from  the  mean  of  seven  determinations  it  appears  that  100 
parts  of  silver  give  132-8445  parts  of  silver  chloride — that,  is,  that 
32-8445  parts  of  chlorine  are  able  to  combine  with  100  parts  of 
silver  and  with  that  quantity  of  sodium  which  is  contained  in 
54-2078  parts  of  sodium  chloride.  These  observations  show  that 
32-8445  parts  of  chlorine  combine  with  100  parts  of  silver  and 
with  21-3633  parts  of  sodium.  From  these  figures  expressing  the 
relation  between  the  combining  weights  of  chlorine,  silver,  and  sodium, 
it  would  be  possible  to  determine  their  atomic  weights— that  is,  the 
combining  quantity  of  these  elements  with  respect  to  one  part  by 
weight  of  hydrogen  or  16  parts  of  oxygen,  if  there  existed  a  series  of 
similarly  accurate  determinations  for  the  reactions  between  hydrogen 
or  oxygen  and  one  of  these  elements — chlorine,  sodium,  or  silver.  If 
we  determine  the  quantity  of  silver  chloride  which  is  obtained  from 
silver  chlorate,  AgClO3,  we  shall  know  the  relation  between  the 
combining  weights  of  silver  chloride  and  oxygen,  so  that,  taking  the 
quantity  of  oxygen  as  a  constant  magnitude,  we  can  learn  from  this 
reaction  the  combining  weight  of  silver  chloride,  and  from  the  preced- 
ing numbers  the  combining  weights  of  chlorine  and  silver.  For  this 
purpose  it  was  first  necessary  to  obtain  pure  silver  chlorate.  This 
Stas  did  by  acting  on  silver  oxide  or  carbonate,  suspended  in  water, 
with  gaseous  chlorine.26 

26  The  phenomenon  which  then  takes  place  is  described  by  Stas  as  follows,  in  a  manner 
which  is  perfect  in  its  clearness  and  accuracy  :  if  silver  oxide  or  carbonate  be  suspended 
in  water,  and  an  excess  of  water  saturated  with  chlorine  l>e  added,  all  the  silver 


COPPER,  SILVER,  AND  GOLD  437 

The  decomposition  of  the  silver  chlorate  thus  obtained  was  accom- 
plished by  the  action  of  a  solution  of  sulphurous  anhydride  on. 
it.  The  salt  was  first  fused  by  carefully  heating  it  at  243°.  The  solution 
of  sulphurous  anhydride  used  was  one  saturated  at  0°.  Sulphurous 
anhydride  in  dilute  solutions  is  oxidised  at  the  expense  of  silver 
chlorate,  even  at  low  temperatures,  with  great  ease  if  the  liquid  be 
continually  shaken,  sulphuric  acid  -and  silver  chloride  being  formed  : 
AgClO3  +  3SO2-f  3H2O=AgCl  +  3H2SO4.  After  decomposition,  the 
resultant  liquid  was  evaporated,  and  the  residue  of  silver  chloride 
weighed.  Thus  the  process  consisted  in  taking  a  known  weight  of 
silver  chlorate,  converting  it  into  silver  chloride,  and  determining 
the  weight  of  the  latter.  The  analysis  conducted  in  this  manner  gave 
the  following  results,  which,  like  the  preceding,  designate  the  weight 
in  a  vacuum  calculated  from  the  weights  obtained  in  air  :  In  the 
first  experiment  it  appeared  that  138*7890  grams  of  silver  chlorate 
gave  103'9795  parts  of  silver  chloride,  and  in  the  second  experiment 

is  converted  into  chloride,  just  as  is  the  case  with  oxide  or  carbonate  of  mercury, 
and  the  water  then  contains,  besides  the  excess  of  chlorine,  only  pure  hypochlorous 
acid  without  the  least  trace  of  chloric  or  chlorous  acid.  If  a  stream  of  chlorine  be 
passed  into  water  containing  an  excess  of  silver  oxide  or  silver  carbonate  while  the 
liquid  is  continually  agitated,  the  reaction  is  the  same  as  the  preceding;  silver 
chloride  and  hypochlorous  acid  are  formed.  But  this  acid  does  not  long  remain  in  a  free 
state-:  it  gradually  acts  on  the  silver  oxide  and  gives  silver  hypochlorite,  i.e.  AgClO. 
If,  after  some  time,  the  current  of  chlorine  be  stopped  but  the  shaking  continued, 
the  liquid  loses  its  characteristic  odour  of  hypochlorous  acid,  while  preserving  its 
Energetic  decolourising  property,  because  the  silver  hypochlorite  which  is  formed  is  easily 
soluble  in  water.  In  the  presence  of  an  excess  of  silver  oxide  this  salt  can  be  kept  for 
several  days  without  decomposition,  but  it  is  exceedingly  unstable  when  no  excess  of 
silver  oxide  or  carbonate  is  present.  So  long  as  the  solution  of  silver  hypochlorite  is 
shaken  up  with  the  silver  oxide,  it  preserves  its  transparency  and  bleaching  property, 
but  directly  it  is  allowed  to  stand,  and  the  silver  oxide  settles,  it  becomes  rapidly  cloudy 
and  deposits  large  flakes  of  silver  chloride,  so  that  the  black  silver  oxide  which  had 
settled  becomes  covered  with  the  white  precipitate.  The  liquid  then  loses  its  bleaching 
properties  and  contains  silver  chlorate,  i.e.  AgC103,  in  solution,  which  has  a  slightly 
alkaline  reaction,  owing  to  the  presence  of  a  small  amount  of  dissolved  oxide.  In  this 
manner  the  reactions  which  are  consecutively  accomplished  may  be  expressed  by  the 
equations : 

6C12  +  SAg2O  +  8H2O  -  6AgCl  +  6HC1O  ;  6HC1O  +  3Ag2O  =  8H20  +  '6AgC10  ; 

6AgClO  =  4AgCl  +  2AgClO3. 

Hence,  Stas  gives  the  following  method  for  the  preparation  of  silver  chlorate  :  A  slow 
current  of  chlorine  is  caused  to  act  on  oxide  of  silver,  suspended  in  water  which  is  kept 
in  a  state  of  continual  agitation.  The  shaking  is  continued  after  the  supply  of  chlorine 
has  been  stopped,  in  order  that  the  free  hypochlorous  acid  should  pass  into  silver 
hypochlorite,  and  the  resultant  solution  of  the  hypochlorite  is  drawn  off  from  the 
sediment  of  the  excess  of  silver  oxide.  This  solution  decomposes  spontaneously  into 
silver  chloride  and  chlorate.  The  pure  silver  chlorate,  AgClO3,  does  not  change  under 
the  action  of  light.  The  salt  is  prepared  for  further  use  by  drying  it  in  dry  air  at  150°. 
It  is  necessary  during  drying  to  prevent  the  access  of  any  organic  matter ;  this  is  done  by 
filtering  the  air  through  cotton  wool,  and,  passing  it  over  &  layer  of  red-hot  copper  oxide* 


488  PRINCIPLES  OF  CHEMISTRY 

that  259-5287  grams  of  chlorate  gave  194-44515  grams  of  silver 
chloride,  and  after  fusion  194-4435  grams.  The  mean  result  of  both 
experiments,  converted  into  percentages,  shows  that  100  parts  of  silver 
chlorate  contain  74*9205  of  silver  chloride  and  25-0795  parts  of  oxygen. 
From  this  it  is  possible  to  calculate  the  combining  weight  of  silver 
chloride,  because  in  the  decomposition  of  silver  chlorate  there  are 
obtained  three  atoms  of  oxygen  and  one  molecule  of  silver 
chloride:  AgClO3  s=  AgCl  +  3O,  Taking  the  weight  of  an  atom 
of  oxygen  to  be  16,  we  find  from  the  mean  result  that  the  equi- 
valent weight  of  silver  chloride  is  equal  to  143-395.  Thus  if  O=16, 
AgCl= 143-395,  and  as  the  preceding  experiments  show  that  silver 
chloride  contains  32-8445  parts  of  chlorine  per  100  parts  of  silver, 
the.  weight  of  the  atom  of  silver261?^  must  be  107-94  and  that 
of  chlorine  35-45  The  weight  of  the  atom  of  sodium  is  determined 
from  the  fact  that  21*3633  parts  of  sodium  chloride  combine  with 
328445  parts  of  chlorine;  consequently  Na=  23-05.  This  conclusion, 
arrived  at  by  the  analysis  of  silver  chlorate,  was  verified  by  means 
of  the  analysis  of  potassium  chlorate  by  decomposing  it  by  heat 
and  determining  the  weight  of  the  potassium  chloride  formed,  and  also 
by  effecting  the  same  decomposition  by  igniting  the  chlorate  in  a 
stream  of  hydrochloric  acid.  The  combining  weight  of  potassium 
chloride  was  thus  determined,  and  another  series  of  determinations 
confirmed  the  relation  between  chlorine,  potassium,  and  silver,  in  the 
.same  manner  as  the  relation  between  sodium,  chlorine,  and  silver  was 
determined  above.  Consequently,  the  combining  weights  of  sodium, 
chlorine,  and  potassium  could  be  deduced  by  combining  these  data  with 
the  analysis  of  silver  chlorate  and  the  synthesis  of  silver  chloride.  The 
agreement  between  the  results  showed  that  the  determinations  made 
by  the  last  method  were  perfectly  correct,  and  did  not  depend  in  any 
considerable  degree  on  the  methods  which  were  employed  in  the  pre- 
ceding determinations,  as  the  combining  weights  of  chlorine  and  silver 
obtained  were  the  same  as  before.  There  was  naturally  a  difference, 
but  so  small  a  one  that  it  undoubtedly  depended  on  the  errors  inciden- 
tal to  every  process  of  weighing  and  experiment.  The  atomic  weight 
of  silver  was  also  determined  by  Stas  by  means  of  the  synthesis  of 
silver  sulphide  and  the  analysis  of  silver  sulphate.  The  combining 
weight  obtained  by  this  method  .was  107-920.  The  synthesis  of  silver 
iodide  and  the  analysis  of  silver  iodate  gave  the  figure  107-928.  The 

Mbta  The  results  given  by  Stas'  determinations  have  recently  been  recalculated  and 
certain  corrections  have  been  introduced.  We  give  in  the  context  the  average  results  of 
ran  der  Plaat's  and  Thomson's  calculations,  as  well  as  in  Table  IH.  neglecting'  the 
doubtful  thousandths. 


COPPER,  SILVER,  AND  GOLD  439 

synthesis  of  silver  bromide  with  the  analysis  of  silver  broiuate  gave  the 
figure  107-921.  The  synthesis  of  silver  chloride  and  the  analysis  of 
silver  chlorate  gave  a  mean  result  of  107-937.  Hence  there  is  no 
doubt  that  the  combining  weight  of  silver  is  at  least  as  much  as  107-9 
—greater  than  107 '90  and  less  than  107-95,  and  probably  equal  to  the 
mean = 107-92.  Stas  determined  the  combining  weights  of  many  other 
elements  in  this  manner,  such  as  lithium,  potassium,  sodium,  bromine, 
chlorine,  iodine,  and  also  nitrogen,  for  the  determination  of  the 
amount  of  silver  nitrate  obtained  from  a  given  amount  of  silver 
gives  directly  the  combining  weight  of  nitrogen.  Taking  that 
of  oxygen  as  16,  he  obtained  the  following  combining  weights 
for  these  elements  :  nitrogen  14-04,  silver  107-93,  chlorine  35-46, 
bromine  79-95,  iodine  126-85,  lithium  7*02,  sodium  23-04,  potassium 
39-15.  These  figures  differ  slightly  from  those  which  are  usually 
employed  in  chemical  investigations.  They  must  be  regarded  as  the 
result  of.  the  best  observations,  whilst  the  figures  usually  used  in 
practical  chemistry  are  only  approximate — are,  so  to  speak,  round 
numbers  for  the  atomic  weights  which  differ  so  little  from  the  exact 
figures  (for  instance,  for  Ag  108  instead  of  107-92,  for  Na  23  instead 
of  23-04)  that  in  ordinary  determinations  and  calculations  the 
difference  falls  within  the  limits  of  experimental  error  inseparable  from 
such  determinations. 

The  exhaustive  investigations  conducted  by  Stas  on  the  atomic 
weights  of  the  above-named  elements  have  great  significance  in 
the  solution  of  the  problem  as  to  whether  the  atomic  weights  of  the 
elements  can  be  expressed  in  whole  numbers  if  the  unit  taken  be  the 
atomic  weight  of  hydrogen.  Prout,  at  the  beginning  of  this  century, 
stated  that  this  was  the  case,  and  held  that  the  atomic  weights  of  the 
elements  are  multiples  of  the  atomic  weight  of  hydrogen.  The  subse- 
quent determinations  of  Berzelius,  Penny,  Marchand,  Marignac,  Dumas, 
and  more  especially  of  Stas,  proved  this  conclusion  to  be  untenable ; 
since  a  whole  series  of  elements  proved  to  have  fractional  atomic 
weights—for  example,  chlorine,  about  35-5.  On  account  of  this, 
Marignac  and  Dumas  stated  that  the  atomic  weights  of  the  elements 
are  expressed  in  relation  to  hydrogen,  either  by  whole  numbers 
or  by  numbers  with  simple  fractions  of  the  magnitudes  £  and  £;  But 
Stas's  researches  refute  this  supposition  also.  Even  between  the  com- 
bining weight  of  hydrogen  and  oxygen,  there  is  not,  so  far  as  is  yet 
known,  that  simple  relation  which  is  required  by  Front's  hypothesis*1 

37  This  hypothesis,  for  the  establishment  or  refutation  of  which  so  many  researches 
have  been  made,  is  exceedingly  important,  and  fully  deserves  the  attention  which  has 
been  given  to  it.  Indeed,  if-  it  appeared  that  the  atomic  weights  of  all  the  elements  could 

*H 


440  PRINCIPLES  OF  CHEMISTRY 

i.e.,  taking  0=16,  the  atomic  weight  of  hydrogen  is  equalr  not  to  1  but 
to  a  greater  number  somewhere  between  1-002  and  1-008  or  mean 

be  expressed  in  whole  numbers  with  reference  to  hydrogen,  or  if  they  at  least  proved  to 
be  commensurable  with  one  another,  then  it  could  be  affirmed  with  confidence  that  the 
elements,  with  all  their  diversity,  were  formed  of  one  material  condensed  or  grouped  in 
various  manners  into  the  stable,  and,  under  known  conditions,  undecomposable  groups 
which  we  call  the  atoms  of  the  elements.  At  first  it  was  supposed  that  all  the  elements 
were  nothing  else  but  condensed  hydrogen,  but  when  it  appeared  that  the  atomic  weights 
of  the  elements  could  not  be  expressed  in  whole  numbers  in  relation  to  hydrogen, 
it  was  still  possible  to  imagine  the  existence  of  a  certain  material  from  which  both  hydro- 
gen and  all  the  other  elements  were  formed.  If  it  should  transpire  that  four  atoms  of  this 
material  form  an  atom  of  hydrogen,  then  the  atom  of  chlorine  would  present  itself  as 
consisting  of  142  atoms  of.  this  substance,  the  weight  of  whose  atom  would  be  equal  to 
0*25.  But  in  this  case'  the  atoms  of  all  ihe  elements  should  be  'expressed  in  whole 
numbers  with  respect  to  the  weight  of  the  atom  of  this  original  material.  Let  us  sup* 
pose  that  the  atomic  weight  of  this  material  is  equal  to  unity,  then  all  the  atomic  weights 
should  be  expressible  in  whole  numbers  relatively  to  this  unit.  Thus  the  atom  of  one  ele- 
ment, let  us  suppose,  would  weigh  m,  and  of  another  n,  but,  as  both  m  and  n  must  be 
whole  numbers,  it  follows  that  the  atomic  weights  of  all  the  elements  would  be  commen- 
surable. But  it  is  sufficient  to  glance  over  the  results  obtained  by  Stas,  and  to  be 
assured  of  their  accuracy,  especially  for  silver,  in  order  to  entirely  destroy,  or  at  least 
strongly  undermine,  this  attractive  hypothesis.  We  must  therefore  refuse  our  assent  to  the 
doctrine  of  the  building  up  from  a  single  substance  of  the  elements  knqwn  to  us.  This 
hypothesis  is  not  supported  either  by  any  known  transformation  (for  one  element  has  never 
been  converted  into  another  element),  or  by  the  commensurability  of  the  atomic  weights 
of  the  elements.  Although  the  hypothesis  of  the  formation  of  all  the  elements  from  a 
single  substance  (for  which  Crookes  has  suggested  the  name  protyle)  is  most  attractive 
in  its  comprehensiveness,  it  can  neither  be  denied  nor  accepted  for  want  of  sufficient  data. 
Marignac  endeavoured,  however,  to  overcome  Stas's  conclusions  as  to  the  incommensu- 
rability ofj  the  atomic  weights  by  supposing  that  in  his,  as  in  the  detenninations  of  all 
other  observers,  there  were  unperceived  errors  which  were  quite  independent  of  the  mode 
of  observation — for  example,  silver  nitrate  might  be  supposed  to  be  an  unstable  substance 
which  changes,  under  the  heatings,  evaporations,  and  other  processes'  to  which  it  is  sub- 
jected in  the  reactions  for  the  determination  of  the  combining  weight  of  silver.  It  might 
be  supposed,  for  instance,  that  silver  nitrate  contains  some  impurity  which  cannot  be 
removed  by  any  means  ;  it  might  also  be  supposed  that  a  portion  of  the  elements  of  the 
nitric  acid  are  disengaged  in  the  evaporation  of  the  solution  of  silver  nitrate  (owing  to  the 
decomposing  action  of  water),  and  in  its  fusion,  and  that  we  have  not  to  deal  with  normal 
silver  nitrate,  but  with  a  slightly  basic  salt,  or  perhaps  an  excess  of  nitric  acid  which 
cannot  be  removed  from  the  salt.  In  this  case  the  observed  combining  weight  will  n*t 
refer  to  an  actually  definite  chemical  compound,  but  to  some  mixture  for  which  there 
does  not  exist  any  perfectly  exact  combining  relations.  Marignac  upholds  this  proposition 
by  the  fact  that  the  conclusions  of  Stas  and  other  observers  respecting  the  combining 
weights  determined  with  the  greatest  exactitude  very  nearly  agree  with  the  proposition 
of  the  commensurability  of  the  atomic  weights — for  example,  the  combining  weight  of 
silver  was  shown  to  be  equal  to  107'93,  so  that  it  only  differs  by  0'08  from  the  whole 
number  108,  which  is  generally  accepted  for  silver.  The  combining  weight  of  iodine 
proved  to  be  equal  to  126'85 — that  is,  it  differs  from  127  byO'15.  The  combining  weights 
of  sodium,  nitrogen,  bromine,  chlorine,  and  lithium  are  still  nearer  to  the  whole  or  round 
numbers  which  are  generally  accepted.  But  Marignac's  proposition  will  hardly  bear 
criticism.  Indeed  if  we  express  the  combining  weights'of  the  elements  determined  by 
Stas  in  relation  to  hydrogen,  the  approximation  of  these  weights  to  whole  numbers 
disappears,  because  one  part  of  hydrogen  in  reality  does  not  combine  with  16  parts  of 
oxygen,  but  with  15'92  parts,  and  therefore  we  shall  obtain,  taking  H=  1,  not  the  above- 
cited  figures,  but  for  silver  107'38,  for  bromine  79'65,  magnitudes  which  are  btill  further 


COPPER,  SILVER,  AND  GOLD  441 

1  005.  Such  a  conclusion  arrived  at  by  direct  experiment  cannot  but 
be  regarded  as  having  greater  weight  than  Prout's  supposition 
(hypothesis)  that  the  atomic  weights  of  the  elements  are  in  multiple 
proportion  to  each  other,  which  would  gh  e  reason  for  surmising  (but  not 
asserting)  a  complexity  of  nature  in  the  elements,  and  their  com- 
mon origin  from  a  single  primary  material,  and  for  expecting  their 
mutual  conversion  into  each  other.  All  such  ideas  and  hopes  must 

removed  from  whole  numbers.  Besides  which,  if  Marignac'a  proposition  were  true  the 
combining  weight  of  silver  determined  by  one  method — e.g.  .by  the  analysis  of  silveir 
chlorate  combined  with  the  synthesis  of  silver  chloride— would  not  agree  well  with  the 
combining  weight  determined  by  another  method — e.g.  by  means  of  the  analysis  of  silver 
iodate  and  the  synthesis  of  silver  iodide.  If  in  one  case  a  basic  salt  could  be  obtained, 
in  the  other  case  an  acid  salt  might  be  obtained.  Then  the  analysis  of  the  acid  salt 
would  give  different  results  from  that  of  the  basic  salt.  Thus  Marignac's  arguments 
cannot  serve  as  a  support  for  the  vindication  of  Prout's  hypothesis. 

In  conclusion,  I  think  it  will  not  be  out  of  place  to  cite  the  following  passage  from  a 
paper  I  read  before  the  Chemical  Society  of  London  in  1889  (Appendix  II.),  referring  to 
the  hypothesis  of  the  complexity  of  the  elements  recognised  in  chemistry,  owing  to  the 
fact  that  many  have  endeavoured  to  apply  the  periodic  law  to  the  justification  of  this 
idea  'dating  from  a  remote  antiquity,  when  it  was  found  convenient  to  admit  the  existence 
of  many  gods  but  only  one  matter. 

'  When  we  try  to  explain  the  origin  of  the  idea  of  a  unique  primary  matter,  we  easily 
trace  that,  in  the  absence  of  deductions  from  experiment,  it  derives  its  origin  from  the 
scientifically  philosophical  attempt  at  discovering  some  kind  of  unity  in  the  immense 
diversity  of  individualities  which  we  see  around.  In  classical  times  such  a  tendency 
could  only  be  satisfied  by  conceptions  about  the  immaterial  world.  As  to  the  material 
world,  our  ancestors  were  compelled  to  resort  to  some  hypothesis,  and  they  adopted  the 
idea  of  unity  in  the  formative  material,  because  they  were  not  able  to  evolve  the  concep- 
tion of  any  other  possible  unity  in  order  to  connect  the  multifarious  relations  of  matter. 
Responding  to  the  same  legitimate  scientific  tendency,  natural  science  has  discovered 
throughout  the  universe  a  unity  of  plan,  a  unity  of  forces,  and  a  unity  of  matter ;  and 
the  convincing  conclusions  of  modem  science  compel  every  one  to  admit  these  kinds  of 
unity.  But  while  we  admit  unity  in  many  things,  we  none  the  less  must  also  explain 
the  individuality  and  the  apparent  diversity  which  we  cannot  fail  to  trace  everywhere. 
It  was  said  of  old  "  Give  us  a  fulcrum  and  it  will  become  easy  to  displace  the  earth." 
So  also  we  must  say,  "Give  us  something  that  is  individualised,  and  the  apparent 
diversity  will  be  easily  understood."  Otherwise,  how  could  unity  result  in  a  multitude 

'  After  a  long  and  painstaking  research,  natural  science  has  discovered  the  individu- 
alities of  the  chemical  elements,  and  therefore  it  is  now  capable,  not  only  of  analysing, 
but  also  of  synthesising ;  it  can  understand  and  grasp  generality  and  unity,  as  well  as 
the  individualised  and  multifarious.  The  general  and  universal,  like  time  and  space,  like 
force  and  motion,  vary  uniformly.  The  uniform  admit  of  interpolations,  revealing  every 
intermediate  phase;  but  the  multitudinous,  the  individualised — such  as  ourselves,  or  the 
chemical  elements,  or  the  members  of  a  peculiar  periodic  function  of  the  elements,  or 
Dalton's  multiple  proportions — is  characterised  in  another  way.  We  see  in  it — side  by 
side  with  a  general  connecting  principle — leaps,  breaks  of  continuity,  points  which  escape 
from  the  analysis  of  the  infinitely  small — an  absence  of  complete  intermediate  links. 
Chemistry  has  found  an  answer  to  the  question  as  to  the  causes  of  multitudes,  and  while 
retaining  the  conception  of  many  elements,  all  submitted  to  the  discipline  of  a  general 
law,  it  offers  an  escape  from  the  Indian  Nirvana — the  absorption  in  the  universal — re- 
placing it  by  the  individualised.  However,  the  place  for  individuality  is  so  limited  by 
the  all-grasping,  all-powerful  universal,  that  it  is  merely  a  point  of  support  for  the  under* 
standing  of  multitude  in  unity." 


442  PRINCIPLES  OF  CHEMISTRY 

now,  thanks  m&re  especially  to  Stas,  be  placed  in  a  region  void  of  any 
experimental  support  whatever,  and  therefore  not  subject  to  the  dis- 
cipline of  the  positive  data  of  science. 

Among  the  platinum  metals  ruthenium,  rhodium,  and  palladium, 
by  their  atomic  weights  and  properties,  approach  silver,  just  as  iron 
and  its  analogues  (cobalt  and  nickel)  approach  copper  in  all  respects, 
Gold  stands  in  exactly  the  same  position  in  relation  to  the  heavy 
platinum  metals,  osmium,  irid;um,  and  platinum,  as  copper  and 
silver  do  to  the  two  preceding  series.  The  atomic  weight  of  gold  is 
nearly  equal  to  their  atomic  weights  ; 28  it  is  dense  like  these  metals. 
It  also  gives  various  grades  of  oxidation,  which  are  feeble,  both  in 
a  basic  and  an  acid  sense.  Whilst  near  to  osmium,  indium,  and  pla- 
tinum, gold  at  the  same  time  is  able,  like  copper  and  silver,  to  form 
compounds  which  answer  to  the  type  RX — that  is,  oxides  of  the  compo- 
sition R20.  Cuprous  chloride,  CuCl,  silver  chloride,  AgCl,  and  aurous 
chloride,  AuCl,  are  substances  which  are  very  much  alike  in  their 
physical  and  chemical  properties.28  bis  They  are  insoluble  in  water, 
but  dissolve  in  hydrochloric  acid  and  ammonia,  in  potassium  cyanide, 

28  It  might  be  expected  from  the  periodic  law  and  analogies  with  the  series  iron,  cobalt, 
nickel,  copper,  zinc,  that  the  atomic  weights  of  the  elements  of  the  series  osmium, 
indium,  platinum,  gold,  mercury,  would  rise  in  this  order,  and  at  the  time  of  the  esta- 
blishment of  the  periodic  law  (1869),  the  determinations  of  Berzelius,  Rose,  and  ofehers 
gave  the  following  values  for  the  atomic  weights:  Os  =  200,  Ir  =  197,  Pt  =  198,  Au=196, 
Hg  =  200.  The  fulfilment  of  the  expectations  of  the  periodic  law  was  given  in  the  first 
place  by  the  fresh  determinations  (Seubert,  Dittmar,  and  Arthur)  of  the  atomic  weight  of 
platinum,  which  proved  to  be  nearly  196,  if  O  =  16  (as  Marignac,  Brauner,  and  others 
propose) ;  in  the  second  place,  by- the  fact  that  Seubert  proved  that  the  atomic  weight  of 
osmium  is  really  less  than  that  of  platinum,  and  approximately  Os  =  191 ;  and,  in  the 
third  place,  by  the  fact  that  after  the  researches  of  Kriiss,  Thorpe,  and  Laurie  there  was 
no  doubt  that,  the  atomic  weight-  of  gold  is  greater  than  that  of  platinum — namely, 
nearly  197. 

» tii  in  Chapter  XXII.,  Note  40,  we  gave  the  thermal  data  for  certain  of  the  com- 
pounds of  copper  of  the  type  CuXj;  we  will  now  cite  certain  data  for  the  cuprous 
compounds  of  the  type  CuX,  which  present  an  analogy  to  the  corresponding  compounds 
AgX  and  AuX,  some  of  which  were  investigated  by  Thomsen  in  his  classical  work, 
'  Thermochemische  Untersuchungen '  (Vol.  iii.,  1888).  The  data  are  given  in  the  same 
manner  as  in  the  above-mentioned  note : 

R  =  Cu  Ag  Au 

R  +  C1  +33  +29  +6 

RfBi  +25  +28  0 

R  +  I  +16  +14  -6 

R  +  O  +41  +6  -? 

Thus  we  see  in  the  first  place  tbat  gold,  which  possesses  a  much  smaller  affinity  than  Ag, 
evolves  far  less  heat  than  an  equivalent  amount  of  copper,  giving  the  same  compound,  and 
in  the  second  place  that  the  combination  of  copper  with  one  atom  of  oxygen  disengages 
more  heat  than  its  combination  with  one  atom  of  a  halogen,  whilst  with  silver  the  reverse 
is  the  case.  This  is  connected  with  the  fact  that  Cu2O  is  more  stable  under  the  action 
of  beat  than  Ag2O. 


COPPER,  SILVER,  AND  GOLD  443 

sodium  thiosulphate,  &c.  Just  as  copper  forms  a  link  between  the  iron 
metals  and  zinc,  and  as  silver  unites  the  light  platinum  metals  with 
cadmium,  so  also  gold  presents  a  transition  from  the  heavy  platinum 
metals  to  mercury.  Copper  gives  saline  compounds  of  the  types  CuX 
and  CuX 2,  silver  of  the  type  AgX,  whilst  gold,  besides  compounds  of 
the  type  AuX,  very  easily  and  most  frequently  forms  those  of  the  type 
AuCl3.  The  compounds  of  this  type  frequently  pass  into  those  of  the 
lower  type,  just  as  PtX4  passes  into  PtX2,  and  the  same  is  observable 
in  the  elements  which,  in  their  atomic  weights,  follow  gold.  Mercury 
gives  HgX2  and  HgX,  thallium  gives  T1X3  and  T1X,  lead  gives 
PbX4  and  PbX2.  On  the  other  hand,  gold  in  a  qualitative  respect 
differs  from  silver  and  copper  in  the  extreme  ease  with  which  all  its  com- 
pounds are  reduced  to  metal  by  many  means.  This  is  not  only  accom- 
plished by  many  reducing  agents,  but  also  by  the  action  of  heat.  Thus 
its  chlorides  and  oxides  lose  their  chlorine  and  oxygen  when  heated, 
and,  if  the  temperature  be  sufficiently  high,  these  elements  are  entirely 
expelled  and  metallic  gold  alone  remains.  Its  compounds,  therefore, 
act  as  oxidising  agents.29 

In  nature  gold  occurs  in  the  primary  and  chiefly  in  quartzose  rocks, 
and  especially  in  quartz  veins,  as  in  the  Urals  (at  Berezoffsk),  in 
Australia,  and  in  California.  The  native  gold  is  extracted  from  these 
rocks  by  subjecting  them  to  a  mechanical  treatment  consisting  of 
crushing  and  washing.29  bis  Nature  has  already  accomplished  a  similar 

**  Heavy  atoms  and  molecules,  although  they  may  present  many  points  of  analogy,  are 
more  easily  isolated  ;  thus  C16H32,  although,  like  C2H4,  it  combines  with  Br2,  and  has  a 
similar  composition,  yet  reacts  with  much  greater  difficulty  than  C2H4,  and  in  this  it  resem- 
bles gold  ;  the  heavy  atoms  and  molecules  are,  so  to  say,  inert,  and  already  saturated  by 
themselves.  Gold  in  its  higher  grade  of  oxidation,  Au203,  presents  feeble  basic  pro* 
perties  and  weakly-developed  acid  properties,  so  that  this  oxide  of  gold,  Au2O3,  may  be 
referred  to  the  class  of  feeble  acid  oxides,,  like  platinic  oxide.  This  is  not  the  case  in  the 
highest  known  oxides  of  copper  and  silver.  But  in  the  lower  grade  of  oxidation,  aurous 
oxide,  Au20,  gold,  like  silver  and  copper,  presents  basic  properties,  although  they  are. 
not  very  pronounced.  In  this  respect  it  stands  very  close  in  its  properties,  although 
not  in  its  types  of  combination  (AuX  and  AuX3),  to  platinum  (PtX9  and  PtX^)  and  its 
analogues. 

As  yet  the  general  chemical  characteristics  of  gold  and  its  compounds  have  not  been 
fully  investigated.  This  is  partly  due  to  the  fact  that  very  few  researches  have  been 
undertaken  on  the  compounds  of  this  metal,  owing  to  its  inaccessibility  for  working 
in  large  quantities.  As  the  atomic  weight  of  gold  is  high  (Au=197),  the  preparation  of 
its  compounds  requires  that  it  should  be  taken  in  large  quantities,  which  forma  an 
obstacle  to  its  being  fully  studied.  Hence  the  facts  concerning  the  history  of  this  metal 
are  rarely  distinguished  by  that  exactitude  with  which  many  facts  have  been  established 
concerning  other  elements  more  accessible,  and  long  known  in  use. 

19  bu  Sonstadt  (1872)  showed  that  sea  water,  besides  silver,  always  contains  gold. 
Munster  (1892)  showed  that  the  water  of  the  Norwegian  fiords  contains  about  5  milli- 
grams of  gold  per  ton  (or  5  milliardths)— 4.e  a  quantity  deserving  practical  attention,  and 
I  think  it  may  be  already  said  that,  considering  the  immeasurable  amount  of  sea  water, 
in  time  means  will  be  discovered  for  profitably  extracting  gold  from  sea  water  by 


444  PRINCIPLES  OF  CHEMISTBY 

disintegration  of  the  hard  rocky  matter  containing  gold.30  These  dis- 
integrated rocks,  washed  by  rain  and  other  water,  have  formed  gold- 
bearing  deposits,  which  are  known  as  alluvial  gold  deposits.  Gold- 
bearing  soil  is  sometimes  met  with  on  the  surface  and  sometimes  under 

bringing  it  into  contact  with  substances  capable  of  depositing  gold  upon  their  surface. 
The  first  efforts  might  be  made  upon  the  extraction  of  salt  from  sea  water,  and  as  the 
total  amount  of  sea  water  inaybk  taken  as  about  2,000,000,000,000,000,000  tons,  it  follows 
that  it  contains  about  10,000  million  tons  of  gold.  The  yearly  production  of  gold, is  about 
200  tons  for  the  whole  world,  of  which  about  one  quarter  is  extracted  in  Eussia.  It  ia 
supposed  that  gold  is  dissolved  in  sea  water  owing  to  the  presence  of  iodides,  which,  under 
the  action  of  animal  organisms,  yield  free  iodine.  It  is  thought  (as  Professor  Konova- 
loff  mentions  in  his  work  upon  'The  Industries  of  the  United  States,'  1894)  that 
iodine  facilitates  the  solution  of  the  gold,  and  the  organic  matter  its  precipitation. 
These  facts  and  considerations  to  a  certain  extent  explain  the  distribution  of  gold  in 
veins  or  rock  fissures,  chiefly  filled  with  quartz,  because  there  is  sufficient  reason  for 
supposing  that  these  rocks  once  formed  the  ocean  bottom.  R.  Dentrie,  and  subse- 
quently Wilkinson,  showed  that  organic  matter — for  instance,  cork — and  pyrites  are  able 
to  precipitate  gold  from  its  solutions  in  that  metallic  form  and  state  in  which  it  occurs 
in  quartz  veins,  where  (especially  in  the  deeper  parts  of  vein  deposits)  gold  is  frequently 
found  on  the  surface  of  pyrites,  chiefly  arsenical  pyrites.  Kazantseff  (in  Ekaterinburg, 
1891)  evqn  supposes,  from  the  distribution  of  the  gold  in  these  pyrites,  that  it  occurred 
in  solution  as  a  compound  of  sulphide  of  gold  and  sulphide  of  arsenic  when  it  penetrated 
'into  the  veins.  It  is  from  such  considerations  that  the  origin  of  vein  and  pyritic  gold 
Is,  at  the  present  time,  attributed  to  the  reaction  of  solutions  of  this  metal,  the  remains 
Of  which  are  seen  in  the  gold  still  present  in  sea  water. 

30  However,  in  recent  times,  especially  since  about  1870,  when  chlorine  (either -as  a 
solution  of  the  gas  or  as  bleaching  powder)  and  bromine  began  to  be  applied  to  the  extrac- 
tion of  finely-divided  gold  from  poor  ores  (previously  roasted  in  order  to  drive  off  arsenio 
and  sulphur,  and  oxidise  the  iron),  the  extraction  of  gold  from  quartz  and  pyrites, 
by  the  wet  method,  increases  from  year  to  year,  and  begins  to  equal  the  amount 
extracted  from  alluvial  deposits.  Since  the  nineties  th,e  cyanide  process  (Chapter 
XIII.,  Note  IS  bis)  has  taken  an  important  place  among  the  wet  methods  for 
extracting  gold  from  its  ores.  It  consists  in  pouring  a  dilute  solution  of  cyanide  of  potas- 
sium (about  600  parts  of  water  and  1  to  4  parts  of  cyanide  of  potassium  per  1,000  parts 
of  ore,  the  amount  of  cyanide  depending  upon  the  richness  of  the  ore)  and  a  mixture 
of  it  with  NaCN,  (see  Chapter  XIII.,  Note  12)  over  the  crushed  ore  (which  need  not  be 
roasted,  whilst  roasting  is  indispensable  in  the  chlorination  process,  as  otherwise  tho 
chlorine  is  used  up  in  oxidising  the  sulphur,  arsenic,  &c.)  The  gold  is  dissolved 
very  rapidly  even  from  pyrites,  where  it  generally  occurs  on  the  surface  in  such 
fine  and  adherent  particles  that  it  either  cannot  be  mechanically  washed  away,  or, 
more  frequently  is  carried  away  by  the  stream  of  water,  and  cannot  be  caught  by 
mechanical  means  or  by  the  mercury  used  for  catching  the  gold  in  the  sluices. 
Chlorination  had  already  given  the  possibility  of-  extracting  the  finest  particles  of  gold ; 
but  the  cyanide  process  enables  such  pyrites  to  be  treated  as  could  be  scarcely  worked 
by  other  means.  The  treatment  of  the  crushed  ore  by  the  KCN  is  carried  on  in  simple 
wooden  vats  (coated  with  paraffin  or  tar)  with  the  greatest  possible  rapidity  (in  order  that 
the  KCN  solution  should  not  have  tune  to  change)  by  a  method  of  systematic  lixiviation, 
and  is  completed  in  10  to  12  hours.  The  resultant  solution  of  gold,  containing  AuK(CN)2, 
is  decomposed  either  with  freshly-made  zinc  filings  (but  when  the  gold  settles  on  the 
Zn,  the  cyanide  solution  reacts  upon  the  Zn  with  the  evolution  of  H7  and  formation  of 
ZnH2O2)  or  by  sodium  amalgam  prepared  at  the  moment  of  reaction  by  the  action  of  an 
electric  current  upon  a  solution  of  NaHO  poured  into  a  vessel  partially  immersed  in 
mercury  (the  NaCN  is  renewed  continually  by  this  means).  The  silver  in  the  ore  passes 
into  solution,  together  with  the  gold,  as  in  amalgamation. 


COPPER,  SILVER,  AN$  GOLD  445 

the  upper  soil,  but  more  frequently  along  the  banks  of  dried-tip  water- 
courses and  running  streams.  The  sand  of  many  rivers  contains, 
however,  a  very  small  amount  of  gold,  which  it  is  not  profitable  to 
work  ;  for  example,  that  of  the  Alpine  rivers  contains  5  parts  of  gold 
in  10,000,000  parts  of  sand.  The  richest  gold  deposits  are  those  o£ 
Siberia,  especially  in  the  southern  parts  of  the  Government  of  Yeniseisk, 
the  South  Urals,  Mexico,  California,  South  Africa,  and  Australia, 
and  then  the  comparatively  poorer  alluvial  deposits  of  many  countries 
(Hungary,  the  Alps,  and  Spain  in  Europe).  The  extraction  of  the 
gold  from  alluvial  deposits  is  based  on  the  principle  of  levigation  ;  the 
earth  is  washed,  while  constantly  agitated,  by  a  stream  of  water, 
which  carries  away  the  lighter  portion  of  the  earth,  and  leaves  the 
coarser  particles  of  the  rock  and  heavier  particles  of  the  gold,  together 
with  certain  substances  which  accompany  it,  in  the  washing  apparatus. 
The  extraction  of  this  washed  gold  only  necessitates  mechanical  ap- 
pliances,31 and  it  is  not  therefore  surprising  that  gold  was  known  to 
savages  and  in  the  most  remote  period  of  history.  It  sometimes  occurs 
in  crystals  belonging  to  the  regular  system,  but  in  the  majority  of  cases 

51  But  the  particles  of  gold,  are  sometimes  so  small  that  a  large  amount  is  lost  during 
the  washing.  It  is  then  profitable  to  have'  recourse  to  the  extraction  by  chlorine  and 
KCN  (Note  80). 

In  speaking  of  the  extraction  of  gold  the  following  remarks  may  not  be  out  of 
place : 

In  California  advantage  is  taken  of  water  supplied  from  high  altitudes  in  order  to 
have  a  powerful  head  of  water,  with  which  the  rocks  are  directly  washed  away,  thus 
avoiding  the  greater  portion  of  the  mechanical  labour  required  for  the  exploitation  of 
these  deposits. 

The  last  residues  of  gold  are  sometimes  extracted  from  sand  by  washing  them  with 
mercury,  which  dissolves  the  gold.  The  sand  mixed  with  water  is  caused  to  come  into 
contact  with  mercury  during  the  washing.  The  mercury  is  then  distilled. 

Many  sulphurous  ores,  even  pyrites,  contain  a  small  amount  of  gold.  Compounds  of 
gold  with  bismuth,  BiAu2,  tellurium,  AuTe2  (calverite),  &c.,  have  been  found,  although 
rarely. 

Among  the  minerals  which  accompany  gold,  and  from  which  the  presence  of  gold  may 
be  expected,  we  may  mention  white  quartz,  titanic  and  magnetic  iron  ores,  and  also  the 
following,  which  are  of  rarer  occurrence  :  zircon,  topaz,  garnet,  and  such  like.  The  con- 
centrated gold  washings  first  undergo  a  mechanical  treatment,  and  the  impure  gold 
obtained  is  treated  for  pure  gold  by  various  methods.  If  the  gold  contain  a  considerable 
amount  of  foreign  metals,  especially  lead  and  copper,  it  is  sometimes  cupelled,  like  silver, 
so  that  the  oxidisable  metals  may  be  absorbed  by  the  cupel  in  the  form  of  oxides,  but  in 
every  case  the  gold  is  obtained  together  with  silver,  because  the  latter  metal  also  is"  not 
oxidised.  Sometimes  the  gold  is  extracted  by  means  of  mercury,  that  is,  by  amalgama- 
tion (and  the  mercury  subsequently  driven  off  by  distillation),  or  by  smelting  it  with 
lead  (which  is  afterwards  removed  by  oxidation)  and  processes  like  those  employed  for 
the  extraction  of  silver,  because,  gold,  like  silver,  does  not  oxidise,  is  dissolved  by  lead 
and  mercury,  and  is  non-volatile.  If  .copper  or  any  other  metal  contain  gold  and  it  be 
employed  as  an  anode,  pure  copper  will  be  deposited  upon  the  cathode,  while  all  the 
gold  will  remain  at  the  anode  as  a  slime.  This  method  often  amply  repays  the  whole 
cost  of  the  process,  since  it  gives,  besides  the  gold,  a  pure  electrolytic  copper. 


446  PRINCIPLES  OF  CHEMISTRY 

in  nuggets  or  grains  of  greater- or  less  magnitude.  It  always  contains 
silver  (from  very  small  quantities  up  to  30  p.c.,  when  it  is  called 
'  electrum  ')  and  certain  other  metals,  among  which  lead  and  rhodium 
are  sometimes  found. 

The  separation  of  the  silver  from  gold  is  generally  carried  on  with 
great  precision,  as  the  presence  of  the  silver  in  the  gold  does  not 
increase  its  value  for  exchange,  and  it  can  be  substituted  by  other 
less  valuable  metals,  so  that  the  extraction  of  the  silver,  as  a  precious 
inetal,  from  its  alloy  with  gold,  is  a  profitable  operation.  This 
separation  is  conducted  by  different  methods-  Sometimes  the  argenti- 
ferous gold  is  melted  in  crucibles,  together  with  a  mixture  of  common 
salt  and  powdered  bricks.  The  greater  portion  of  the  silver  is  thus 
converted  into  the  chloride,  which  fuses  and  is  absorbed  by  the  slags, 
from  which  it  may  be  extracted  by  the  usual  methods.  The  silver  is 
also  extracted  from  gold  by  treating  it  with  boiling  sulphuric  acid, 
which  does  not  act  on  the  gold  but  dissolves  the  silver.  But  if  the 
alloy  does  not  contain  a  large  proportion  of  silver  it  cannot  be  extracted 
by  this  method  or  at  nil  events  the  separation  will  be  imperfect,  and 
therefore  a  fresh  amount  of  silver  is  added  (by  fusion)  to  the  gold,  in 
such  quantity  that  the  alloy  contains  twice  as  much  silver  as  gold. 
The  silver  which  is  added  is  preferably  such  as  contains  gold,  which  is 
very  frequently  the  case.  The  alloy  thus  formed  is  poured  in  a  thin  . 
stream  into  water,  by  which  means  it  is  obtained  in  a  granulated 
form  ;  it  is  then  boiled  with  strong  sulphuric  acid,  three  parts  of 
acid  being  used  to  one  part  of  alloy.  The  sulphuric  acid  extracts 
all  the  silver  without  acting  on  the  gold.  It  is  best,  however,  to', 
pour  off  the  first  portion  of  the  acid,  which  has  dissolved  the  silver, 
and  then  treat  the  residue  of  still  imperfectly  pure  gold  with  a  fresh 
quantity  of  sulphuric  acid.  The  gold  is  thus  obtained  in  the  form 
of  powder,  which  is  washed  with  water  until  it  is  quite  free  from 
silver.  The  silver  is  precipitated  from  the  solution  by  means  of 
copper,  so  that  cupric  sulphate  and  metallic  silver  are  obtained.  This 
process  is  carried  out  in  many  countries,  as  in  Russia,  at  the  Govern- 
ment mints. 

Gold  is  generally  used  alloyed  with  copper ;  since  pure  gold, 
like  pure  silver,  is  very  soft,  and  therefore  soon  worn  away.  In 
assaying  or  determining  the  amount  of  pure  gold  in  such  an  alloy 
it  is  usual  to  add  silver  to  the  gold  in  order  to  make  up  an  alloy 
containing  three  parts  of  silver  to  one  of  gold  (this  is  known  as 
quartation  because  the  alloy  contains  £  of  gold),  and  the  resultant 
alloy  is  treated  with  nitric  acid.  If  the  silver  be  not  in  excess  over 
the  gold,  it  is  not  all  dissolved  by  the  nitric  acid,  and  this  is  the  reason 


COPPER,  .SILVER,  AtfD  GOLD  447 

for  the  quartation.  The  amount  of  pure  gold  (assay)  is  determined  by 
weighing  the  gold  which  remains  after  this  treatment.  English  gold 
(=22  carats)  coinage  is  composed  of  an  alloy  containing  91 '66  p.c.  of 
gold,  but  for  many  articles  gold  is  frequently  used  containing  a  larger 
amount  of  foreign  metals. 

Pure  gold  may  be  obtained  from  gold  alloys  by  dissolving  in  aqua 
iregia,  and  then  adding  ferrous  sulphate  to  the  solution  or  heating  it 
•with  a»  solution  of  oxalic  acid.  These  deoxidising  agents  reduce  the 
gold,  but  not  the  other  metals.  The  chlorine  combined  with  the  gold 
then  acts  like  free  chlorine.  The  gold,  thus  reduced,  is  precipitated  as 
an  exceedingly  fine  brown  powder.31  bis  It  is  then  washed  with  water, 
and  fused  with  nitre  or  borax.  Pure  gold  reflects  a  yellow  light,  and 
in  the  form  of  very  thin  sheets  (gold  leaf),  into  which  it  can  be 
hammered  and  rolled,31  tri  it  transmits  a  bluish-green  light.  The 
specific  gravity  of  gold  is  about  19*5,  the  sp.  gr.  of  gold  coin  is  about 
17'1.  It  fuses  at  1090° — at  a  higher  temperature  than  silver — and  can 
be  drawn  into  exceedingly  fine  wires  or  hammered  into  thin  sheets. 
With  its  softness  and  ductility,  gold  is  distinguished  for  its  tenacity, 
and  a  gold  wire  two  millimetres  thick  breaks  only  under  a  load  of  68 
kilograms^  Gold  vaporises  even  at  a  furnace  heat,  and  imparts  a 
greenish  colour  to  a  flame  passing  over  it;  in  a  furnace.  Gold  .alloys 
with  copper  almost  without  changing  its  volume.32  In  its  chemical 

si  bis  Schottlander  (1893)  obtained  gold  in  a  soluble  colloid  form  (the  solution  ia  violet) 
by  the  fiction  of  a  mixture  of  solutions  of  cerium  acetate  .and  NaHO  upon  a  solution  of 
AuCl3.  The  gold  separates  out  from  such  a  .solution  in  exactly  ^the  same  manner  as  Ag 
does  from  the  solution  of  colloid,  silver  mentioned  above.  There  always  remains  » 
certain  amount  of  a  higher  oxide  of  cerium,  Ce02,  in  the.  solution — i.e.  the  gold  ia 
reduced  by  converting  -the  cerium  into  a  higher  grade  of  oxidation.  Besides  „  which 
Kriiss  and  Hofmann  show'ed  that  sulphide  of  gold  precipitated  by  the  action  of  H2S  upon 
a  solution  of  AuKCy3  mixed  with  HC1  easily  passea  into  a  colloid  solution  after  being 
properly  washed  (like  AsaS5,  CuS,  &c.,  Chapter  I.,  Note  57). 

5! tfl  Gold-leaf  is  used  for  gilding  wood  (leather,  cardboard,  and  suchlike,  upon  which 
tt  is  glued  by  means  of  varnish,  &c.),  and  is  about  O'OOS  millimetre  thick.  It  is  obtained 
from  thin  sheets  (weighing  at  first  about  £  grm.  to  a  square  inch),  rolled  between  gold 
rollers,  by  gradually  hammering  them  (in  packets  of  a  number  at  once)  between  sheets 
of  moist  (but  not  wet)  parchment,  and  then,  after  cutting  them  into  four  pieces,  between 
&  specially  prepared  membrane,  which,  when  at  the  right  degree  of  moisture,  does  not 
tear  or  stick  together  under  the  blows  of  the  hammer. 

3»  The  formation  of  the  alloys  Cu  +  Zn,  Cu  +  Sn,  Cu  +  Bi,  Cu  +  Sb,  Pb  +  Sb,  Ag  +  Pb, 
Ag+.Sn,'  Au-^-Zn,  Au  +  Sn,  &c.,  is  accompanied  by  a  contraction  (and  evolution  of  heat). 
The  formation  of  the  alloys  Fe  +  Sb,  Fe  +  Pb,  Cu-fPb,  Pb-i-Sn,  Pb  +  Sn,  Pb  +  Sb, 
Zn  +  Sb,  Ag  +  Cu,  Au  +  Cu,  'Au  +  Pb,  takes  place  with  a  certain  increase  in  volume. 
With  regard  to  the  alloys  of  gold,'  it  may  be  mentioned  that  gold  is  only  slightly 
dissolved  by  mercury  (about  0'06  p.c.,  Dudley,  1890) ;  the  remaining  portion  forms  a 
granular  alloy,  whose  composition  has  not  been  definitely  determined.  Aluminium  (and 
silicon)  also  have  the  capacity  of  forming  alloys  with  gold.  The  presence  of  a  small 
amount  of  aluminium  lowers  the  melting  point  of  gold  considerably  (Roberts-Austen, 
1692)  i  thus  the  addition  of  4  p.o.  of  aluminium  lowers  it  by  14°-28,  the  addition  of  10  P.O. 


448  PRINCIPLES  OF  CHEMISTRY 

aspect,  gold  presents,  as  is  already  seen  from  its  general  characteristics 
:given  above,  an  example  of  the  so-called  noble  metals — i.e.  it  is 
incapable  of  being  oxidised  at  any  temperature,  and  its  oxide  is 
decomposed  when  calcined.  Only  chlorine  and  bromine  combine 
directly  with  it  at  the  ordinary  temperature,  but  many  other  metals 
and  non-metals  combine  with  it  at  a  red  heat — for  example,  sulphur, 
phosphorus,  and  arsenic.  Mercury  dissolves  it  with  great  ease.  It 
dissolves  in  potassium  cyanide  in  the  presence  of  air ;  a  mixture  of 
sulphuric  acid  with  nitric  acid  dissolves  it  with  the  aid  of  heat, 
although  in  small  quantity.  It  is  also  soluble  in  aqua  regia  and  in 
selenic  acid.  Sulphuric,  hydrochloric,  nitric,  and  hydrofluoric  acids 
and  the  caustic  alkalis  do  not  act  on  gold,  but  a  mixture  of  hydro- 
chloric acid  with  such  oxidising  agents  as  evolve  chlorine  naturally 
dissolves  it  like  aqua  regia.32  b". 

As  regards  the  compounds  of  gold,  they  belong,  as  was  said 
above,  to  the  types  AuX3  and  AuX.  Auric  chloride  or  gold  tri- 
chloride, AuCl3,  which  is  formed  when  gold  is  dissolved  in  aqua  regia, 
belongs  to  the  former  and  higher  of  these  types.  The-solution  of  this 
substance  in  water  has  a  yellow  colour,  and  jt  may  be  obtained  pure  by 
evaporating  the  solution  in  aqua  regia  to  dryness,  but  not  to  the  point 
of  decomposition.  If  the  evaporation  proceed  to  the  point  of  crystal- 
lisation, a  compound  of  gold  chloride  and  hydrochloric  acid,  AuHCl4,  is 
obtained,  like  the  allied  compounds  of  platinum  ;  but  it  easily  parts 
with  the  acid  and  leaves  auric  chloride,  which  fuses  into  a  red-brown 
liquid,  and  then  solidifies  to  a  crystalline  mass.  If  dry  chlorine  be 
passed  over  gold  in  powder  it  forms  a  mixture  of  aurous  and  auric 
chlorides,  but  the  aurous  chloride  is  also  decomposed  by  water  into 
gold  and  auric  chloride.  Auric  chloride  crystallises  from  its  solutions 
as  AuCl3,2H2O,  which  easily,  loses  water,  and  the  dry  chloride  loses 
two-thirds  of  its  chlorine  at  185°,  forming  aurous  chloride,  whilst 

Al  by  41°-7.  The  latter  alloy  is  white.  The  alloy  AuAl2  has  a  characteristic  purple 
colour,  and  its  melting  point  is  32°'5  above  that  of  gold,  which  shows  it  to  be  a  definite 
compound  of  the  two  metals.  The  melting  points  of  alloys  richer  in  AJ  gradually  fall 
to  660°— that  is,  below  that  of  aluminium  (665°). 

Heycock  and  Neville  (1892),  in  studying  the  triple  alloys  of  Au,  Cd,  and  Sn,  observed 
a  tendency  in  the  gold  to  give  compounds  with  Cd,  and  by  sealing  a  mixture  of  Au  and  Cd 
in  a  tube,  from  which  the  air  had  been  exhausted,  and  heating  it,  they  obtained  a  grey 
crystalline  brittle  definite  alloy  AnCd. 

5JbU  Calderon  (1892),  at  the  request  of  some  jewellers,  investigated  the  cause  of  a 
peculiar  alteration  sometimes  found  on  the  surface  of  dead-gold  articles,  there  appearing 
brownish  and  blackish  spots,  which  widen  and  alter  their  form  in  course  of  time.  He 
came  to  the  conclusion  that  these  spots  are  due  to  the  appearance  and  development  of 
peculiar  micro-organisms.  (Aspergillus  niger  and  Micrococcus  cimbareus)  on  the  gold, 
epores  of  which  were  found  in  abundance  on  the  cotton-wool  in  which  the  gold  article* 
had  been  kept.  \ 


COPPER,  SILVER,  AND  GOLD  449 

above  300°  the  latter  chloride  also  loses  its-  chlorine  and  leaves 
metallic  gold.  Auric  chloride  is  the  usual  form  in  which  gold  occurs  in 
solutions,  and  in  which  its  salts  are  used  in  the  arts  and  for  chemical 
purposes.  It  is  soluble  in  water,  alcohol,  and  ether.  Light  has  a  reduc- 
ing action  on  these  solutions,  and  after  a  time  metallic  gold  is  deposited 
upon  the  sides  of  vessels  containing  the  solution.  Hydrogen  when 
nascent,  and  even  in  a  gaseous  form,  reduces  gold  from  this  solution 
'to  a  metallic  state.  The  reduction  is  more  conveniently  and  usually 
effected  by  ferrous  sulphate,  and  in  general  by  the  action  of  ferrous 
salts.3' 

If  a  solution  of  potassium  hydroxide  be  added  to  a  solution  of  auric 
chloride,  a  precipitate  is  first  formed,  which  re-dissolves  in  an  excess  of 
the  alkali.  On  being  evaporated  under  the  receiver  of  an  air-pump, 
this  solution  yields  yellow  crystals,  which  present  the  same  composition 
as  the  double  salts  AuMCl4,  with  the  substitution  of  the  chlorine  by 
oxygen — that  is  to  say,  potassium  aurafe,  AuKO2,  is  formed  in  crystals 
containing  3H2O.  The  solution  has  a  distinctly  alkaline  reaction. 
Auric  oxide,  Au2O3,  separates  when  this  alkaline  solution  is  boiled  with 
an  excess  of  sulphuric  acid.  But  it  then  still  retains  some  alkali ;  how- 
ever, it  may  be  obtained  in  a  pure  state  as  a  brown  powder  by 
dissolving  in  nitric  acid  and  diluting  with  water.  The  brown  powder 
decomposes  below  250°  into  gold  and  oxygen.  It  is  insoluble  in  water 
and  in  many  acids,  but  it  dissolves  in  alkalis,  which  shows  the  acid 
character  of  this  oxide.  An  hydroxide,  Au(OH)v)  may  be  obtained  as  a 
brown  powder  by  adding  magnesium  oxide  to  a  solution"  of  auric  chlo- 
ride and  treating  the  resultant  precipitate  of  magnesium  aurate  with 
nitric  acid.  This  hydroxide  loses  water  at  100°.  and  gives  auric  oxide.34 

83  Stannous  chloride  as  a  reducing  agent  also  acts  on  auric  chloride,  and  gives  &  red 
precipitate  known  as  purple  of  Cassius.  This  substance,  which  probably  contains  a 
.mixture  or  compound  of  aurous  oxide  and  tin  oxide,  is  used  as  a  red  pigment  for  china 
And  glass.  Oxalic  acid,  on  heating,  reduces  metallic  gold  from  its  salts,  and  this  property 
may  be  taken  advantage  of  for  separating  it  from  its  solutions.  The  oxidation  which 
then  takes  place  in  the  presence,  pf  water  may  be  expressed  by  the  following  equation  : 
i^AuGls+8C2H2O4  =  2Au  +  (5HCl+ 6CO2.  Nearly  all  organic  substances  have  a  reducing 
action  on  gold,  and  solutions  of  gold  leave  a  violet  stain  on  the  skin. 

Auric  chloride,  like  platinic  chloride,  is  distinguished  for  its  clearly-developed 
property  of  forming  double  salts.  These  double  salts,  as  a  rule,  belong  to  the  type 
AuMCl4.  The  compound  of  auric- chloride  with  hydrochloric  acid  mentioned  above 
evidently  belongs  to  the  same  type.  The  compounds  2KAuCl4,5H20,  NaAuCl4,2H2O, 
AuNH4Cl4,H2O,  Mg'AuCl4)2,2H2O,  and  _the  like  are.  easily  crystallised  in  well-formed 
Crystals..  Wells,  Wheeler,  and  Penfield  (1892)  obtained  EbAuCl4  (reddish  yellow)  and 
CsAuCl4  (golden  yellow),  and  corresponding  bromides  (dark  coloured).  AuBr3  is  ex- 
tremely like  the  chloride.  Auric  cyanide  is  obtained  easily  in  the  form  of  a  double  salt 
of  potassium,  KAu(CN)4,  by  mixing  saturated  and  hot  solutions  pf  potassium  cyanide 
with  auric  chloride"  and  then  cooling. 

31  .If  ammonia  be  added  to  a  solution  of  auric  chloride,  it  forms  a  yellow  precipitate 


450  PRINCIPLES  OF  CHEMISTRY 

The  starting-point  of  the  compounds  of  the  type  AuX  35  is  gold 
tnonochloride  or  aurous  chloride,  AuCl,  which  is  formed,  as  mentioned 
above,  by  heating  auric  chloride  at  185°.  Aurous  chloride  forms  a 
yellowish -white  powder  ;  this,  when  heated  with  water,  is  decomposed 
into  metallic  gold  and  auric  chloride,  which  passes  into  solution  • 
3AuCl  =  AuCl3  -f  2 Au.  This  decomposition  is  accelerated  by  the  action 
of  light.  Hence  it  is  obvious  that  the  compounds  corresponding  with 
aurous  oxide  are  comparatively  unstable.  But  this  only  refers  to  the 
simple  compounds  AuX  ;  some  of  the  complex  compounds,  on  the 
contrary,  form  the  most  stable  compounds  of  gold.  Such,  for  ex- 
ample, is  the  cyanide  of  gold  and  potassium,  AuK(CN)2.  It  is  formed, 
for  instance,  when  finely-divided  gold  dissolves  in  the  presence  of 
air  in  a  solution  of  potassium  cyanide:  4KCN  +  2Au  +  H2O  +  0 
=  2KAu(CN)2-f  2KHO  (this  reaction  also  proceeds  with  solid  pieces 
of  gold,  although  very  slowly).  The  same  compound  is  formed  in 
solution  when  many  compounds  of  gold  are  mixed  with  potassium 
cyanide,  because  if  'a  higher  compound  of  gold  be  taken,  it  is  reduced 

of  the  so-called  fulminating  gold,  which  contains  gold,  chlorine,  hydrogen,  nitrogen, 
and  oxygen,  but  its  formula  is  not  known  with  certainty.  It  is  probably  a  sort  of  am- 
TOonio-metallic  compound,  Au2OS)4jNH.v  or  amide  (like  the  mercury  compound).  This 
precipitate  explodes  at  140°,  but  when  left  in  the  presence  of  solutions  containing  am- 
monia it  loses  all  its  chlorine  and  becomes  non-explosive.  In  this  form  the  composition 
AuiO3,2NH;>;,H2O  is  ascribed  to  it,  but  this  is  uncertain.  Auric  sulphide,  Au2S3,  is 
obtained  by  the  action  of  hydrogen  sulphide  on  a  solution  of  auric  chloride,  and  also 
directly  by  fusing  sulphur  with  gold.  It  has  an  acid  character,  and  therefore  dissolves 
fn  sodium  and  ammonium  sulphides. 

55  Many  double  salts  of  suboxide.of  gold  belong  to  the  type  AuX — for  instance,  the 
cyanide  corresponding  to  the  type  AuKX2)  like  PtK2X4,  with  which  we  became-acquainted 
in  the  last  chapter.  We  will  enumerate  several  of  the  representatives  of  this  class  of 
compounds.  If  auric  chloride,  AuCl3,  be  mixed  with  a  solution  of  sodium  thiosulphate, 
the  gold  passes  into  a  colourless  solution,  which  deposits  colourless  crystals,  con- 
taining a  double  thiosulphate  of  gold  and  sodium,  which  are  easily  soluble  in  water 
but  are  precipitated  by  alcohol.  The  composition  of  this  salt  is  Na3Au(S2O5)2,2H2O.' 
If  the  sodium  thiosulphate  be  represented  as  NaS2O3Na,  the  double  salt  in  question 
will  be  AuNa(S2O3Na)2,2H2O,  according  to  the  type  AuNaX2.  The  solution  of  this 
colourless  and  easily  crystallisable  salt  has  a  sweet  taste,  and  the  gold  is  not  separated 
from  it  either  by  ferrous  sulphate  or  oxalic  acid.  This  salt,  which  is  known  as  Fordoa 
and  Gelis's  salt,  is  used  in  medicine  .and  photography  In  general,  aurous  oxide 
exhibits  a  distinct  inclination  to  the  formation  of  similar  double  salts,  as  we  saw  also 
with  PtX2 — for  example,  it  forms  similar  salts  with  sulphurous  acid.  Thus  if  a  solution 
of  sodium  sulphite  be  gradually  added  to  a  solution  of  oxide  of  gold  in  sodium 
hydroxide,  the  precipitate  at  first  formed  re-dissolves  to  a  colourless  solution,  which 
contains  the  double  salt  Na3Au(SO3)2=AuNa(SO3Na)2,  The  solution  of  this  salt, 
when  mixed  with  barium  chloride,  first  forms  a  precipitate  of  barium  sulphite,  and 
then  a  red  barium  double  salt  which  corresponds  with  the  above  sodium  salt. 

The  oxygen  compound  of  the  type  AuX,  aurous  oxide,  Au2O,  is  obtained  as  a  greenish 
violet  powder  on  mixing  aurous  chloride  with  potassium  chloride  in  the  cold.  With 
hydrochloric  acid  this  oxide  gives  gold  and  auric  chloride,  and  when  heated  it  easily 
splits  up  into  oxygen  and  metallic  gold. 


COPPER,  SILVEB,  AND  GOLD  451 

t>y  the  potassium  cyanide  into  aarous  oxide,  which  dissolves  in  potas- 
sium cyanide  and  forms  KAu(CN)2.  This  substance  is  soluble  in 
water,  and  gives  a  colourless  solution,  which  can  be  kept  for  a  long 
time,  and  is  employed  in  electro-gilding — that  is,  for  coating  other 
metallic  objects  with  a  layer  of  gold,  which  is  deposited  if  the  object 
be  connected  with  the  negative  pole  of  a  battery  and  the  positive  pole 
consist  of  a  gold  plate.  When  an  electric  current  is  passed  between 
them,  the  gold  from  the  latter  will  dissolve,  whilst  a  coating  of  go 
from  the  solution  will  be  deposited  on  the  object. 


APPENDIX  I 


AN  ATTEMPT  TO  APPLY  TO  CHEMISTRY  ONE  OF  THE 
PRINCIPLES  OF  NEWTON'S  NATURAL  PHILOSOPHY 

BY  PKOFESSOR  MENDELEEFF 


A  LECTURE   DELIVERED  AT   THE   ROYAL  INSTITUTION   OF   GREAT  BRITAIN 
ON  FRIDAY,   MAY  31,  1889 

NATURE,  inert  to  the  eyes  of  the  ancients,  has  been  revealed  to  us  as  full  of 
Jife  and  activity.  The  conviction  that  motion  pervaded  all  things,  which  was 
first  realised  with  respect  to  the  stellar  universe,  has  now  extended  to  the 
unseen  world  of  atoms.  No  sooner  had  the  human  understanding  denied  to 
the  earth  a  fixed  position  and  launched  it  along  its  path  in  space,  than  it  was 
sought  to  fix  immovably  the  sun  and  the  stars.  But  astronomy  has  demon- 
strated that  the  sun  moves  with  unswerving  regularity  through  the  star- set 
-Universe  at  the  rate  of  about  50  kilometres  per  second.  Among  the  so-called 
iixed  stars  are  now  discerned  manifold  changes  and  various  orders  of  move-- 
raent.  Light,  heat,  electricity— like  sound— have  been  proved  to  be  modes 
of  motion ;  to  the  realisation  of  this  fact  modern  science  is  indebted  for 
powers  which  have  been  used  with  such  brilliant  success,  and  which  have  been 
expounded  so  clearly  at  this  lecture  table  by  Faraday  and  by  his  successors, 
^As,  in  the  imagination  of  Dante,  the  invisible  air  became  peopled  with  spiritual 
beings,  so  before  the  eyes  of  earnest  investigators,  and  especially  before  those 
of  Clerk  Maxwell,  the  invisible  mass  of  gases  became  peopled  with  particles : 
their  rapid  movements,  their  collisions,  and  impacts  became  so  manifest  that 
it  seemed  almost  possible  to  count  the  impacts  and  determine  many  of 
'the  peculiarities  or  laws  of  their  collisions.  The  fact  of  the  existence  of 
these  invisible  motions  may  at  once  be  made  apparent  by  demonstrating  the 
difference  in  the  rate  of  diffusion  through  porous  bodies  of  the  light  and 
rapidly  moving  atoms  of  hydrogen  and  the  heavier  and  more  sluggish  par- 
ticles of  air.  Within  the  masses  of  liquid  and  of  solid  bodies  we  have  been 
forced  to  acknowledge  the  existence  of  persistent  though  limited  motion  of 
their  ultimate  particles,  for  otherwise  it  would  be  impossible  to  explain,  for 
example,  the  celebrated  experiments  of  Graham  on  diffusion  through  liquid 
and  colloidal  substances.  If  there  were,  in  our  times,  no  belief  in  the 


454  PRINCIPLES  OF  CBEM1STRY 

molecular  motion  in  solid  bodies,  could  the  famous  Spring  have  hoped  to 
attain  any  result  by  mixing  carefully -dried  powders  of  potash,  saltpetre  and 
sodium  acetate,  in  order  to  produce,  by  pressure,  a  chemical  reaction  between 
these  substances  through  the  interchange  of  their  metals,  and  have  derived, 
for  the  conviction  of  the  incredulous,  a  mixture  of  two  hygroscopic  though 
solid  salts — sodium  nitrate  and  potassium  acetate  ? 

In  these  invisible  and  apparently  chaotic  movements',  reaching  from  the 
stars  to  the  minutest  •atoms,  there  reigns,  however,  a  harmonious  order  which 
is  commonly  mistaken  for  complete  rest,  but  which  is  really  a  consequence 
of  the  conservation  of  that  dynamic  equilibrium  which  was  first  discerned 
by  the  genius  of  Newton,  and  which  has  been  traced  by  his  successors  in  the 
detailed  analysis  of  the  particular  consequences  of  the  great  generalisation, 
namely,  relative  immovability  in  the  midst  of  universal  and  active  movement. 
But  the  unseen  world  of  chemical  changes  is  closely  analogous  to  the 
visible  world  of  the  heavenly  bodies,  since  our  atoms  form  distinct  portions 
of  an  invisible  world,  as  planets,  satellites,  and  comets  form  distinct  portions 
of  the  astronomer's  universe ;  our  atoms  may  therefore  be  compared  to  the 
eolar  systems,  or  to  the  systems  of  double  or  of  single  stars :  for  example, 
ammonia  (NH3)  may  be  represented  in  the  simplest  manner  by  supposing 
the  sun,  nitrogen,  surrounded  by  its  planets  of  hydrogen ;  and  common  salt 
(NaCl)  may  be  looked  on  as  a  double  star  formed  of  sodium  and  chlorine. 
Besides,  now  that  the  indestructibility  of  the  elements  has  been  acknow* 
lodged,  chemical  changed  cannot  otherwise  be  explained  than  as  changes  of 
motion,  and  the  production  by  chemical  reactions  of  galvanic  currents,  of 
light,  of  heat,  of  pressure,  or  of  steam  power,  demonstrates  visibly  that  the 
processes  of  chemical  reaction  are  inevitably  connected  with  enormous  though 
unseen  displacements,  originating  in  the  movements  of  atoms  in  molecules, 
Astronomers  and  natural  philosophers,  in  studying  the  visible  motions  of  the 
heavenly  bodies  .and  of  matter  on  the  earth,  have  understood  and  have  esti- 
mated the  value  of  this  store  of  energy.  But  the  chemist  has  had  to  pursue 
a  contrary  course.  Observing  in  the  physical  and  mechanical  phenomena 
which  accompany  chemical  reactions  the  quantity  of  energy  manifested  by 
the  atoms  and  molecules,  he  is  constrained  to  acknowledge  that  within  the 
molecules  there  exist  atoms  in  motion,  endowed  with  an  energy  which,  like 
matter  itself,  is  neither  being  created  nor  capable  of  being  destroyed.  There- 
fore, in  chemistry,  we  must  seek  dynamic  equilibrium  not  only  between  the 
molecules,  but  also  in  then:  midst  among  their  component  atoms.  Many 
conditions  of  such  equilibrium  have  been  determined,  but  much  remains  to  be 
done,  and  it  is  not  uncommon,  even  in  these  days,  to  find  that  some  chemists 
forget  that  there  is  the  possibility  of  motion  in  the  interior  of  molecules,  and 
therefore  represent  them  as  being  in  a  condition  of  death-like  inactivity. 

Chemical  combinations  take  place  with  so  much  ease  and  rapidity, 
possess  so  many  special  characteristics,  and  are  so  numerous,  that  their  sim- 
plicity and  order  were  for  a  long  time  hidden  from  investigators.  Sympathy, 
relationship,  all  the  caprices  or  all  the  fancifulness  of  human  intercourse, 
seemed  to  have  found  complete  analogies  in  chemical  combinations,  but  with 
this  difference,  that  the  characteristics  of  the  material  substances — such  as 
•ilver,  for  example,  or  of  any  other  body— remain  unchanged  in  every  sub- 


APPENDIX  I.  455 

division  from  the  largest  masses  to  the  smallest  particles,  and  consequently 
these  characteristics  must  be  properties  of  the  particles.  But  the  world  of 
heavenly  luminaries  appeared  equally-  fanciful  at  man's  first  acquaintance 
\vith  it,  so  much  so,  that  the  astrologers  imagined  a  connection  between  the 
individualities  of  men  and  the  conjunctions  of  planets.  Thanks  to  the  genius 
of  Lavoisier  and  of  Dalton,  man  has  been  able,  in  the  unseen  world  of  che- 
mical combinations,  to  recognise  laws  of  the  same  simple  order  as  those 
which  Copernicus  and  Kepler  proved  to  exist  in  the  planetary  universe.  Man 
discovered,  and  continues  every  hour  to  discover,  what  remains  unchanged 
in  chemical  evolution,  and  how  changes  take  place  in  combinations  of  the 
unchangeable.  He  has  learned  to  predict,  not  only  what  possible  combina- 
tions may  take  place,  but  also  the  very  existence  of  atoms  of  unknown  elemen- 
tary substances,  and  has  besides  succeeded  in  making  innumerable  practical 
applications  of  his  knowledge  to  the  great  advantage  of  his  race,  and  has 
accomplished  this  notwithstanding  that  notions  of  sympathy  and  affinity 
still  preserve  a  strong  vitality  in  science.  At  present  we  cannot  apply 
Newton's  principles  to  chemistry,  because  the  soil  is  only  being  now  prepared. 
The  invisible  world  of  chemical  atoms  is  still  waiting  for  the  creator  of  che- 
mical mechanics.  For  him  our  age  is  collecting  a  mass  of  materials,  the 
inductions  of  well-digested  facts,  and  many-sided  inferences  similar  to  those 
which  existed  for  Astronomy  and  Mechanics  in  the  days  of  Newton.  It  is 
well  also  to  remember  that  Newton  devoted  much  time  to  chemical  experi- 
ments, and  while  considering  questions  of  celestial  mechanics,  persistently 
kept  in  view  the  mutual  action  of  those  infinitely  small  worlds  which  are 
concerned  in  chemical  evolutions.  For  this  reason,  and  also  to  maintain  the 
unity  of  laws,  it  seems  to  me  that  we  must,  in  the  first  instance,  seek  to 
harmonise  the  various  phases  of  contemporary  chemical  theories  with  the 
immortal  principles  of  the  Newtonian  natural  philosophy,  and  so  hasten  the 
advent  of  true  chemical  mechanics.  Let  the  above  considerations  serve  as 
my  justification  for  the  attempt  which  I  propose  to  make  to  act  as  a  champion 
of  the  universality  of  the  Newtonian  principles,  which  I  believe  are  com-, 
petent  to  embrace  every  phenomenon  in  the  universe,  from  the  rotation  of 
the  fixed  stars  to  the  interchanges  of  chemical  atoms. 

In  the  first  place  I  consider  it  indispensable  to  bear  in  mind  that,  up  to 
quite  recent  times,  only  a  one-sided  affinity  has  been  recognised  in  chemical 
reactions.    Thus,  for  example,  from  the  circumstance  that  red-hot  iron  de-ji 
composes  water  with  the  evolution  of  hydrogen,  it  was  concluded  that  oxygeif*ir       •—* 
had  a  greater  affinity  for  iron  than  for  hydrogen.    But  hydrogen,  in  presence 
of  red-hot  iron  scale,  appropriates  its  oxygen  and  forms  water,  whence  an 
exactly  opposite  conclusion  may  be  formed. 

During  the  last  ten  years  a  gradual,  scarcely  perceptible,  but  most 
important  change  has  taken  place  in  the  view?,  and  consequently  in  the 
researches,  of  chemists.  They  have  sought  everywhere,  and  have  always, 
found,  systems  of  conservation  or  dynamic  equilibrium  substantially  similar 
to  those  which  natural  philosophers  have  long  since  discovered  in  the  visible 
world,  and  in  virtue  of  which  the  position  of  the  heavenly  bodies  in  th? 
Universe  is  determined.  There  where  one-sided  affinities  only  were  at  first 
detected,  not  only  secondary  or  lateral  ones  have  been  found,  but  even  those 


456  PRINCIPLES  OF  CHEMISTRY 

Which  are  diametrically  opposite ;  yet  among  these,  dynamical  equilibrium 
establishes  itself  not  by  excluding  one  or  other  of  the  forces,  but  regulating 
them  all.  So  the  chemist  finds  in  the  flame  of  the  blast  furnace,  in  the 
formation  of  every  salt,  and,  with  especial  clearness,  in  double  salts  and  iu 
the  crystallisation  of  solutions,  not  a  fight  ending  in  the  victory  of  one  side, 
as  used  to  be  supposed,  but  the  conjunction  of  forces ;  the  peace  of  dynamic 
eqxailibrium  resulting  from  the  action  of  many  forces  and  affinities.  Car- 
bonaceous matters,  for  example,  burn  at  the  expense  of  the  oxygen  of  the 
air,  yielding  a  quantity  of  heat,  and  forming  products  of  combustion,  in 
which  it  was  thought  that  the  affinities  of  the  oxygen  with  the  combustible 
elements  were  satisfied.  But  it  appeared  that  the  heat  of  combustion  was 
competent  to  decompose  these  products,  to  dissociate  the  oxygen  from  the 
combustible  elements,  and  therefore  to  explain  combustion  fully  it  is  neces- 
sary to  take  into  account  the  equilibrium  between  opposite  reactions,  betweeo 
those  which  evolve  and  those  which  absorb  heat. 

In  the  same  way,  in  the  case  of  the  solution  of  common  salt  in  water,  it 
is  necessary  to  take  into  account,  on  the  one  hand,  the  formatibn  of  compound 
particles  generated  by  the  combination  of  salt  with  water,  and,  on  the  other, 
the  disintegration  or  scattering  of  the  new  particles  formed,  as  well  as  of 
these  originally  contained.  At  present  we  find  two  currents  of  thought, 
apparently  antagonistic  to  each  other,  dominating  the  study  of  solutions : 
according  to  the  one,  solution  seems  a  mere  act  of  building  up  or  association  ; 
according  to  the.  other,  it  is  only  dissociation  or  disintegration.  The  truth 
lies,  evidently,  between  these  views  ;  it  lies,  as  I  have  endeavoured  to  prove 
by  my  investigations  into  aqueous  solutions,  in  the  dynamic  equilibrium  of 
particles  tending  to  combine  and  also  to  fall  asunder.  The  large  majority  of 
chemical  reactions  which  appeared  to  act  victoriously  along  one  line  have 
been  proved  capable  of  acting  as  victoriously  even  along  an  exactly  opposite 
line.  Elements  which  utterly  decline  to  combine  directly  may  often  be 
formed  into  comparatively  stable 'compounds  by  indirect  means,  as,  for  ex- 
ample, in  the  case  of  chlorine  and  carbon  ;  and  consequently  the  sympathies 
and  antipathies  which  it  was  thought  to  transfer  from  human  relations  to 
those  of  atoms  should  be  laid  aside  until  the  mechanism  of  chemical  rela- 
tions is  explained.  Let  us  remember,  however,  that  chlorine,  which  does  not 
form  with  carbon  the  chloride  of  carbon,  is  strongly  absorbed,  or,  as  it  were, 
dissolved,  by  carbon,  which  leads  us  to  suspect  incipient  chemical  action  even 
in  an  external  and  purely  surface  contact,  and  involuntarily  gives  rise  to 
conceptions  of  that  unity  of  the  forces  of  nature  which  has  been  so  ener- 
getically insisted  on  by  Sir  "William  Grove  and  formulated  in  his  famous 
paradox.  Grove  noticed  that  platinum,  when  fused  in  the  oxyhydrogen 
flame,  during  which  operation  water  is  formed,  when  allowed  to  drop  into 
water  decomposes  the  latter  and  produces  the  explosive  oxyhydrogen  mixture. 
The  explanation  of  this  paradox,  as  of  many  others  which  arose  during  the 
period  of  chemical  renaissance,  has  led,  in  our  time,  to  the  promulgation  by 
Henri  Sainte-Claire  Deville  of  the  conception  of  dissociation  and  of  equili. 
brium,  and  has  recalled  the  teaching  of  Berthollet,  which,  notwithstanding  its 
brilliant  confirmation  by  Heinrich  Rose  and  Dr.  Gladstone,  had  not,  up  to 
|hat  period,  been  included  in  received  chemical  views. 


APPENDIX  I.  457 

Chemical  equilibrium  in  general,  and  dissociation  in  particular,  are  now 
being  so  fully  worked  out  in  detail,  and  supplied  in  such  various  ways,  that  I 
do  not  allude  to  them  to  develop,  but  only  use  them  as  examples  by  which 
to  indicate  the  correctness  of  a  tendency  to  regard  chemical  combinations 
from  points  of  view  differing  from  those  expressed  by  the  term  hitherto  ap- 
propriated to  define  chemical  forces,  namely,  '  affinity.'  Chemical  equilibria, 
dissociation,  the  speed  of  chemical  reactions,  thermochemistry,  spectroscopy, 
and,  more  than  all,  the  determination  of  the  influence  of  masses  and  the 
search  for  a  connection  between  the  properties  and  weights  of  atoms  and 
molecules — in  one  word,  the  vast  mass  of  the  most  important  chemical  re- 
searches of  the  present  day--clearly  indicate  the  near  approach  of  the  time 
when  chemical  doctrines  will  submit  fully  and  completely  to  the  doctrine 
which  was  first  announced  in  th«  Principia  of  Newton. 

In  order  that  the  application  of  these  principles  may  bear  fruit  it  is  evi- 
dently  insuflicienfr  to  assume  that  statical  equilibrium  reigns  alone  in  chemical 
systems  or  chemical  molecules:  it  is  necessary  to  grasp  the  conditions  of 
possible  states  of  dynamical  equilibria,  and  to  apply  to  them  kinetic  prin- 
ciples. Numerous  considerations  compel  us  to  renounce  the  idea  of  statical 
equilibrium  in  molecules,  and  the  recent  yet  strongly- supported  appeals  to 
dynamic  principles  constitute,  in  my  opinion,  the  foundation  of  the  modern 
teaching  relating  to  atomicity,  or  the  valency  of  the  elements,  which  usually 
forms  the  basis  of  investigations  into  organic  or  carbon  compounds. 

This  teaching  has  led  to  brilliant  explanations  of  very  many  chemical 
relations  and  to  cases  of  isomerism,  or  the  difference  in  the  properties  of 
substances  having  the  same  composition.  It  has  been  so  fruitful  in  its  many 
applications  and  in  the  foreshadowing  of  remote  consequences,  especially 
respecting  carbon  compounds,  that  it  is  impossible  to  deny  its  claims  to  be 
ranked  as  a  great  achievement  of  chemical  science.  Its  practical  application 
to  the  synthesis  of  many  substances  of  the  most  complicated  composition 
entering  into  the  structure  of  organised  bodies,  and  to  the  creation  of  an  un- 
limited number  of  carbon  compounds,  among  which  the  colours  derived  from 
coal  tar  stand  prominently  forward,  surpass  the  synthetical  powers  of  Nature 
itself.  Yet  this  teaching,  as  applied  to  the  structure  of  carbon  compounds, 
is  not  on  the  face  of  it  directly  applicable  to  the  investigation  of  other  ele- 
ments, because  in  examining  the  first  it  is  possible  to  assume  that  the  atoms 
of  carbon  have  always  a  definite  and  equal  number  of  affinities,  whilst  in  the 
combinations  of  other  elements  this  is  evidently  inadmissible.  Thus,  for 
example,  an  atom  of  carbon  yields  only  one  compound  with  four  atoms  of 
hydrogen  and  one  with  four  atoms  of  chlorine  in  the  molecule,  whilst  the 
atoms  of  chlorine  and  hydrogen  unite  only  in  the  proportions  of  one  to  one. 
Simplicity  is  here  evident,  and  forms  a  point  of  departure  from  which  it  is 
easy  to  move  forward  with  firm  and  secure  tread.  Other  elements  are  of  a 
different  nature.  Phosphorus  unites  with  three  and  with  five  atoms  of 
chlorine,  and  consequently  the  simplicity  and  sharpness  of  the  application  of 
structural  conceptions  are  lost.  Sulphur  unites  only  with  two  atoms  of 
hydrogen,  but  with  oxygen  it  enters  into  higher  orders  of  combination.  The 
periodic  relationship  which  exists  among  all  the  properties  of  the  elements — 
euch,  for  example,,  as  their  ability  to  enter  into  various  combinations — and 


458  PRINCIPLES  OF  CHEMISTRY 

their-atomic  weights,  indicate  that  this  variation  in  atomicity  is  subject  to 
one  perfectly  exact  and  general  law,  and  it  is  only  carbon  and  its  near" 
analogues  which  constitute  cases  of  permanently  preserved  atomicity.  It  ia 
impossible  to  recognise  as  constant  and  fundamental  properties  of  atoms, 
powers  which,  in  substance,  have  proved  to  be  variable.  But  by  abandoning- 
the  idea  of  permanence,  and  of  the  constant  saturation  of  affinities— that  is 
to  say,  by  acknowledging  the  possibility  of  free  affinities— many  retain  & 
comprehension  of  the  atomicity  of  the  elements  '  under  given  conditions ; ' 
and  on  this  frail  foundation  they  build  up  structures  composed  of  chemical 
molecules,  evidently  only  because  the  conception  of  manifold  affinities  gives, 
at  once,  a  simple  statical  method  of  estimating  the  composition  of  the  most 
complicated  molecules. 

I  shall  enter  neither  into  details,  nor  into  the  various-consequences  follow- 
ing from  these  views,  nor  into  the  disputes  which  have  sprung  up  respecting- 
them  (and  relating  especially  to  the  number  of  isomerides  possible  on  the 
assumption  of  free  affinities),  because  the  foundation  or  origin  of  theories  of 
this  nature  suffers  from  the  radical  defect  of  being  in  opposition  to  dynamics. 
The  molecule,  as  even  Laurent  expressed  himself,  is  represented  as  an  archi- 
tectural structure,  the  style  of  which  is  determined  by  the  fundamental 
arrangement  of  a  few  atoms,  whilst  the  decorative  details,  which  are  capable 
of  being  varied  by  the  same  forces,  are  formed  by  the  elements  entering  into 
the  combination.  It  is  on  this  account  that  the  term  '  structural '  is  so  appro- 
priate to  the  contemporary  views  of  the  above  order,  and  that  the  '  struc- 
turalists '  seek  to  justify  the  tetrahedric,  plane,  or  prismatic  disposition  of 
the  atoms'  of  carbon  in  benzene.  It  is  evident  that  the  consideration  relates 
•to  the  statical  position  of  atoms  and  molecules  and  not  to  their  kinetic  rela- 
tions. The  atoms  of  the  structural  type  are  like  the  lifeless  pieces  on  a  chess 
board :  they  are  endowed  but  with  the  voices  of  living  beings,  and  are  not 
those  living  beings  themselves ;  acting,  indeed,  according  to  laws,  yet  each 
possessed  of  a  store  of  energy  which,  in  the  present  state  of  our  knowledge, 
must  be  taken  into  account. 

In  the  days  of  Hatty,  crystals  were  considered  in  the  same  statical  and 
structural  light,  but  modern  crystallographers,  having  become  more  tho- 
roughly acquainted  with  their  physical  properties  and  their  actual  formation, 
have  abandoned  the  earlier  views,  and  have  made  their  doctrines  dependent 
on  dynamics. 

The  immediate  object  of  this  lecture  is  to  show  that,  starting  with 
Newton's  third  law  of  motion,  it  is  possible  to  preserve  to  chemistry  all  the 
•  advantages  arising  from  structural  teaching,  without  being  obliged  to  build  . 
up. molecules  in  solid  and  motionless  figures,  or  to  ascribe  to  atoms  definite 
limited  valencies,  directions  of  cohesion,  or  affinities.  The  wide  extent  of 
the  subject  obliges  me  to  treat  only  a  small  portion  of  it,  namely  of  substitu- 
tions, without  specially  considering  combinations  and  decompositions,  and 
even  then  limiting  myself  to  the  simplest  examples,  which,  however,  will 
throw  open  prospects  embracing  all  the  natural  complexity  of  chemical  rela- 
tions. For  this  reason,  if  it  should  prove  possible  to  form  groups  similar,  for 
example,  to  H4  or  CH0  as  the  remnants  of  molecules  CH4  or  C^R,  we  shall 
not  pause  to  consider  them,  because,  as  far  as  we  know,  they  fall  asunder  into 


APPENDIX  I.  459 

two  parts,  H3  •»•  H0  or  CH4  *  H^fcs  soon  as  they  are  even  temporarily  formed, 
and  are  incapable  of  separate  existence,  and  therefore  can  take  no  part  itt 
the  elementary  act  of  substitution.  With  respect  to  the  simplest  molecules 
tvhich  we  shall  select — that  is  to  say,  those -of  whicH  the  parts  have  no  sepa- 
rate existence,  and  therefore  cannot  appear  in  substitutions— we  shall  con* 
eider  them  according  to  the  periodic  law,  arranging  them  in  direct  dependence 
on  the  atomic  weight  of  the  elements. 

Thus,  for  example,  the  molecules  of  the  simplest  hydrogen  compounds- 
HP  H,0  H3N  H4C 
hydrofluoric  acid             water                    ammonia                   methane 

correspond  with  elements  the  atomic  weights  of  which  decrease  consecutively 
F  =  19,          0  =  10,          N  =  14,          C  =  12. 

Neither  the  arithmetical  order  (1,  2,  3,  4  atoms  of  hydrogen)  nor  the  total' 
information  we  possess  respecting  the  elements  will  permit  us  to  interpolate 
into  this  typical  series  one  more  additional  element ;  and  therefore  we  have 
here,  for  hydrogen  compounds,  a  natural  base  on  which  are  built  up  thosd 
eimple  chemical  combinations  which  we  take  as  typical.  But  even  they  ard 
competent  to  unite  with  each  other,  as  we  see,  for  instance,  in  the  property 
which  hydrofluoric  acid  has  of  forming  a  hydrate — that  is,  of  combining  with, 
water ;  and  a  similar  attribute  of  ammonia,  resulting  in  the  formation  of  a 
caustic  alkali,  NH3,H,0,  or  NH^OH. 

Having  made  these  indispensable  preliminary  observations,  I  may  now 
,Bttack.the  problem  itself  and  attempt  to  explain  the  so-called  structure  or 
Bather  construction,  of  molecules— that  is  to  say,  their  constitution  and  trans- 
formations— without  having  recourse  to  the  teaching  of '  structuralists,'  but  <?n 
Newton's  dynamical  principles. 

Of  Newton's  three  laws  of  motion,  only  the  third  can  be  applied  directly 
to  chemical  molecules  when  regarded  as  systems  of  atoms  among  which  it 
must  be  supposed  that  there  exist  common  influences  or  forces,  and  resulting 
compounded  relative  motions.  Chemical  reactions  of  every  kind  are  un- 
doubtedly accomplished  by  changes  in  these  internal  movements,  respecting 
the  nature  of  which  nothing  is  known  at  present,  but  the  existence  of  which 
the  mass  of  evidence  collected  in  modern  tunes  forces  us  to  acknowledge  as 
forming  part  of  the  common  motion  of  the  universe,  and  as  a  fact  further 
established  by  the  circumstance  that  chemical  reactions  are  always  charac- 
terised by  changes  of  volume  .or  the  relations  between  the  atoms  or  the 
molecules.  Newton's  third  law,  which  is  applicable  to  every  system,  declares 
that,  '  action  is  also  associated  with  reaction,  and  is  equal  to  it.'  The 
brevity  of  conciseness  of  this  axiom  was,  however,  qualified  by  Newton  in- 
a  more  expanded  statement,  'the  action  of  bodies  one  upon  another  are 
always  equal,  and  in  opposite  directions.'  This  simple  fact  constitutes  the 
point  of  departure  for  explaining  dynamic  equilibrium— that  is  to  say,  systems 
of  conservancy.  It  is  capable  of  satisfying  even  the  dualists,  and  of  explain- 
ing, without  additional  assumptions,  the  preservation  of  those  chemical  types 
.which  Dumas,  Laurent,  and  Gerhardt  created  unit  types,  and  those  views  of 
atomic  combinations  which  the  structuralists  express  by  atomicity  or  the 


460  PRINCIPLES  OF  CHEMISTRY 

valency  of  (he  elements,  and,  in  connection  with  them,  the  various  numbers 
of  affinities.  In  reality,  if  a  system  of  atoms  or  a  molecule  be  g'ven,  then  in 
it,  according  to  the  third  law  of  Newton,  each  portion  of  atoms  acts  on  the 
remaining  portion  in  the  same  manner,  and  with  the  same  force  as  the 
second  set  of  atoms  acts  on  the  first.  We  infer  directly  from  this  considera- 
tion that  both  sets  of  atoms,  forming  a  molecule,  are  not  only  equivalent  with, 
regard  to  themselves,  as  they  must  be  according  to  Dalton's  law,  but  also  that 
they  may,  if  united,  replace  each  other.  Let  there  be  a  molecule  containing 
atoms  A  B  C,  it  is  clear  that,  according  to  Newton's  law,  the  action  of  A  on. 
B  C  must  be  equal  to  the  action  .of  B  C  on  A,  and  if  the  first  action  is  directed 
on  B  C,  then  the  second  must  be  directed  on  A,  and  consequently  then,  where 
A  can  exist  in  dynamic  equilibrium,  B  C  may  take  its  place  and  act  in  a  like 
manner.  In  the  same  way  the  action  of  C  is  equal  to  the  action  of  A  B.  In 
one  word  every  two  sets  of  atoms  forming  a  molecule  are  equivalent  to  each 
other,  and  may  take  each  other's  place  in  other  molecules,  orf  having  the 
power  of  balancing  each  other,  the  atoms  or  their  complements  are  endowed 
with  the  power  of  replacing  each  other.  Let  us  call  this  consequence  of  an 
evident  axiom  '  the  principle  of  substitution,'  and  let  us  apply  it  to  those 
typical  forms  of  hydrogen  compounds  which  we  have  already  discussed,  and 
which,  on  account  of  their  simplicity  and  regularity,  have  served  as  starting-- 
points of  chemical  argument  long  before  the  appearance  of  the  doctrine  of 
structure. 

In  the  type  of  hydrofluoric  acid,  HP,  or  in  systems  of  double  stars,  are 
included  a  multitude  of  the  simplest  molecules.  It  will  be  sufficient  for  our 
purpose  to  recall  a  few :  for  example,  the  molecules  of  chlorine,  Cij,  and  of 
hydrogen,  Hj,  and  hydrochloric  acid,  HC1,  which  is  familiar  to  all  in  aqueous 
solution  as  spirits  of  salt,  and  which  has  many  points  of  resemblance  with 
HF,  HBr,  HI.  In  these  cases  division  into  two  parts  can  only  be  made  in 
one  way,  and  therefore  the  principle  of  substitution  renders  it  probable  that 
exchanges  between  the  chlorine  and  the  hydrogen  can  take  place,  if  they  are 
competent  to  unite  with  each  other.  There  was  a  time  when  no  chemist 
would  even  admit  the  idea  of  any  such  action  ;  it  was  then  thought  that  the 
power  of  combination  indicated  -a  polar  difference  of  the  molecules  in  com- 
bination, and  this  thought  set  aside  all  idea  of  the  substitution  of  one  com- 
ponent element  by  another. 

Thanks  to  the  observations  and  experiments  of  Dumas  and  Laurent  fifty 
years  ago,  such  fallacies  were  dispelled,  and  in  this  manner  the  principle 
of  substitution  was  exhibited.  Chlorine  and  bromine  acting  on  many 
hydrogen  compounds,  occupy  immediately  the  place  of  their  hydrogen,  and 
the  displaced  hydrogen,  with  another  atom  of  chlorine  or  bromine,  forms 
hydrochloric  acid  or  bromide  of  hydrogen.  This  takes  place  in  all  typical 
hydrogen  compounds.  Thus  chlorine  acts  on  this  principle  on  gaseoua 
hydrogen — reaction,  under  the  influence  of  light,  resulting  in  the  formation 
of  hydrochloric  acid.  Chlorine  acting  on  the  alkalis,  constituted  similarly  to 
water,  and  even  on  water  itself — only,  however,  under  the  influence  of  light 
and  only  partially  because  of  the  instability  of  HC10— forms  by  this  principle 
bleachirg  salts,  which  are  the  same  as  the  alkalis,  but  with  their  hydrogen 
replaced  by  chlorine.  In  ammonia  and  in  methane,  chlorine  can  also  replace 


APPENDIX  I  461 

the  hydrogen.  From  ammonia  is  formed  in  this  manner  the  so-called 
chloride  of  nitrogen,  NC13,  which  decomposes  very  readily  with  violent  explo- 
sion on  account  of  the  evolved  gases,  and  falls  asunder  as  chlorine  and 
nitrogen.  Out  of  marsh  gas,  or  methane,  CH4,  may  be  obtained  consecu- 
tively, by  this  method,  every  possible  substitution,  of  which  chloroform, 
CHOI,,  is  the  best  known,  and  carbon  tetrachloride,  CC14,  the  most  instruc- 
tive. But  by  virtue  of  the  fact  that  chlorine  and  bromine  act,  in  the  manner 
shown,  on  the  simplest  typical  hydrogen  compounds,  their  action  on  the 
more  complicated  ones  may  be  assumed  to  be  the  same.  This  can  be  easily 
demonstrated.  The  hydrogen  of  benzene,  CaH6,  reacts  feebly  under  the  influ- 
ence of  light  on  liquid  bromine,  but  Gustavson  has  shown  that  the  addition 
of  the  smallest  quantity  of  metallic  aluminium  causes  energetic  action  and 
the  evolution  of  large  volumes  of  hydrogen  bromide. 

If  we  pass  on  to  the  second  typical  hydrogen  compound — that  is  to  say, 
water— its  molecule,  HOH,  may  be  split  up  in  two  ways :  either  into  an  atom 
of  hydrogen  and  a  semi-molecule  of  hydrogen  peroxide,  HO,  or  into  oxygen, 
0,  and  two  atoms  of  hydrogen,  H ;  and  therefore,  according  to  the  principle 
of  substitution,  it  is  evident  that  one  atom  of  hydrogen  can  exchange 
with  hydrogen  oxide,  HO,  and  two  atoms  of  hydrogen,  H,  with  one  atom  of 
oxygen,  0. 

Both  these  forms  of  substitution  will  constitute  methods  of  oxidation — 
that  is  to  say,  of  the  entrance  of  oxygen  into  the  -compound— a  .reaction 
which  is  so  common  in  nature  as  well  as  in  the  arts,  taking  place  at  the 
expense  of  the  oxygen  of  the  air  or  by  the  aid  of  various  oxidising  sub- 
stances or  bodies  which  part  easily  with  their  oxygen.  There  is  no  occasion 
to  reckon  up  the  unlimited  number  of  cases  of  such  oxidising  reactions.  It 
is  sufficient  to  state  that  in  the  first  of  these  oxygen  is  directly  transferred, 
and  the  position,  the  chemical  function,  which  hydrogen  originally  occupied, 
is,  after  the  substitution,  occupied  by  the  hydroxyl.  Thus  ammonia,  NH3, 
yields  hydroxylamine,  NH,2(OH),  a  substance  which  retains  many  of  the 
properties  of  ammonia. 

Methane  and  a  number  of  other  hydrocarbons  yield,  by  substitution  of 
the  hydrogen  by  its  oxide,  methyl  alcohol,  CH3(OH),  and  other  alcohols.  The 
substitution  of  one  atom  of  oxygen  for  two  atoms  of  hydrogen  is  equally 
common  with  hydrogen  compounds.  By  this  means  alcoholic  liquids  con. 
taming  ethyl  alcohol,  or  spirits  of  wine,  C.2HS(OH),  are  oxidised  until  they 
become  vinegar,  or  acetic  acijd,  C8H30(OH).  In  the  same  way  caustic 
ammonia,  or  the  combination  of  ammonia  with  water,  NH^H./),  or  NH4(OH), 
which  contains  a  great  deal  of  hydrogen,  by  oxidation  exchanges  four  atoms 
of  hydrogen  for  two  atoms  of  oxygen,  and  becomes  converted  into  nitric  acid, 
NO,(OH).  This  process  of  conversion  of  ammonium  salts  into  saltpotre  goes 
on  in  the  fields  every  summer,  and  with  especial  rapidity  in  tropical  countries. 
The  method  by  which  this  is  accomplished,  though  complex,  though  involving 
the  agency  of  all -permeating  micro-organisms,  is,  in  substance,  the  same  aa 
that  by  which  alcohol  is  converted  into  acetic  acid,  or  glycol,  C2H4(OH)2,  into 
oxalic  acid,  if  we  view  the  process  of  oxidation  in  the  light  of  the  Newtonian 
principles. 

But  while  speaking  of  the  application  of  the  principle  of  substitution  to 


462  PRINCIPLES  OP  CHEMISTRY 

water,  we  need  not  multiply  instances,  tut  must  turn  our  attention  to  two 
special  circumstances  which  are  closely  connected  with  the  very  mechanism, 
of  substitutions. 

In  the  first  place,  the  replacement  of  two  atoms  of  hydrogen  by  one  atom 
of  oxygen  may  take  place  in  two  ways,  because  the  hydrogen  molecule  is 
composed  of  two  atoms,  and  therefore,  under  the  influence  of  oxygen,  the 
molecule  forming  water  may  separate  before  the  o.  gen  has  time  to  take  its 
place.  It  is  for  this  reason  that  we  find,  during  the  conversion  of  alcohol 
into  acetic  acid,  that  there  is  an  interval  during  which  is  formed  aldehyde» 
C2H4O,  which,  as  its  very  name  implies,  is  '  alcohol  dehydrogenatum,'  or  > 
alcohol  deprived  of  hydrogen.  Hence  aldehyde  combined  with  hydrogen 
yields  alcohol ;  and  united  to  oxygen,  acetic  acid. 

For  the  same  reason  there  should  be,  and  there  actually  are,  intermediate 
products  between  ammonia  and  nitric  acid,  KOS(HO),  containing  either  less 
hydrogen  than  ammonia,  less  oxygen  than  nitric  acid,  or  less  water  than 
caustic  ammonia.  Accordingly  we  find,  among  the  products-  of  the  deoxida- 
tion  of  nitric  acid  and  the  oxidation  of  ammonia,  not  only  hydroxylamine, 
but  also  nitrous  oxide,  nitrous  and  n'tric  anhydrides.  Thus,  the  production 
of  nitrous  acid  results  from  the  removal  of  two  atoms  of  hydrogen  from 
caustic  ammonia  and  the  substitution  .of  the  oxygen  for  the  hydrogen, 
NO(OH) ;  or  by  the  substitution,  in  ammonia,  of  three  atoms  of  hydrogen  by 
hydroxyl,  N(OH)3,  and  by  the  removal  of  water:  N(OH)3-HaO  =  NO(OH). 
The  peculiarities  and  properties  of  nitrous  acid — as,  for  instance,  its  action  on 
ammonia  and  its  conversion,  by  oxidation,  into  nitric  acid— are  thus  clearly 
revealed 

On  the  other  hand,  in  speaking  of  the  principle  of  substitution  as  applied 
to  water,  it  is  necessary  to  observe  that  hydrogen  and  hydroxyl,  H  and  OH, 
are  not  only  competent  to  unite,  but  also  to  form  combinations  with  them- 
selves, and  thus  become  H7  and  H.,0.^ ;  and  such  are  hydrogen  and  the 
peroxide  thereof.  In  general,  if  a  molecule  A  B  exists,  then  molecules  A  A 
and  B  B  can  exist  also.  A  direct  reaction  of  this  kind  does  not,  however, 
take  place  in  water,  therefore  undoubtedly,  at  the  moment  of  formation, 
hydrogen  reacts  on  hydrogen  peroxide,  as  we  can  show  at  once  by 
experiment ;  and  further  because  hydrogen  peroxide,  H202,  exhibits  a 
structure  containing  a  molecule  of  hydrogen,  II,,  and  one  of  oxygen,  02, 
either  of  which  is  capable  of  separate  existence.  The  fact,  however,  may 
now  be  taken  as  thoroughly  established,  that,  at  the  moment  of  combustion 
of  hydrogen  or  of  the  hydrogen  compounds,  hydrogen  peroxide,  is  always 
formed,  and  not  only  so,  but  hi  all  probability  its  formation  invariably  pre- 
cedes the  formation  of  water.  This  was  to  be  expected  as  a  consequence  of 
the  law  of  Avogadro  and  Gerhardt,  which  leads  us  to  expect  this  sequence 
in  the  case  of  equal  interactions  of  volumes  of  vapours  and  gases ;  and  in 
hydrogen  peroxide  we  actually  have  such  equal  volumes  of  the  elementary 
gases. 

The  instability  of  hydrogen  peroxide — that  is  to  say,  the  ease  with 
•which  it  decomposes  into  water  and  oxygen,  even  at  the  mere  contact  of 
porous  substances — accoitnts  for  the  circumstance  that  it  does  not  form  a  per- 
manent product  of  combustion,  and  is  not  produced  during  the  decomposition 


APPENDIX  I.  463 

cf  water.  I  may  mention  this  additional  consideration  that,  with  respect 
to  hydrogen  peroxide,  we  may  look  for  its  effecting  still  further  substitu- 
tions of  hydrogen  by  means  of  which  we  may  expect  to  obtain  still  more 
highly  oxidised  water  compounds,  such  as  H803  and  H204.  These  Schonbein 
and  Bunsen  have  long  been  seeking,  and  Berthelot  is  investigating  them 
at  present.  It  is  probable,  however,  that  the  reaction  will  stop  at  the 
last  compound,  because  we  find  that,  in  a  number  of  cases,  the  addition  of 
four  atoms  of  oxygen  seems  to  form  a  limit.  Thus,  Os04,  KC104,  KMn04, 
KjS04,  Na3P04,  and  such  like,  represent  the  highest  grades  of  oxidation.1 

As  for  the  last  forty  years,  from  the  times  of  Berzelius,  Dumas,  Liebig, 
Oerhardt,  Williamson,  Frankland,  Kolbe,  Kekule",  and  Butleroff,  most  theo- 
retical generalisations  have  centred  round  organic  or  carbon  compounds, 
we  will,  for  the  sake  of  brevity,  leave  out  the  discussion  of  ammonia  deriva- 
tives, notwithstanding  their  simplicity  with  respect  to  the  doctrine  of  substi- 
tutions ;  we  will  dwell  more  especially  on  its  application  to  carbon  compounds, 
•starting  from  methane,  CH4,  as  the  simplest  of  the  hydrocarbons,  containing 
in  its  molecule  one  atom  of  carbon.  According  to  the  principles  enumerated 
we  may  derive  from  CH4  every  combination  of  the  form  CH3X,  CH2Xj, 
CHX3,  and  CX4,  in  which  X  is  an  element,  or  radicle,  equivalent  to  hydrogen — 
that  is  to  say,  competent  to  take  its  place  or  to  combine  with  it.  Such  are 
the  chlorine  substitutes  already  mentioned,  such  is  wood-spiritT  CH3(OH),  in 
which  X  is  represented  by  the  residue  of  water,  and  such  are  numerous  other 
carbon  derivatives.  If  we  continue,  with  the  aid  of  hydroxyl,  further  substi- 
tutions of  the  hydrogen  of  methane  we  shall  obtain  successively  CH2(OH)2, 
CH(OH)3,  and  C(OH)4.  But  if,  in  proceeding  thus,  we  bear  in  mind  that 
CH.^OH^  contains  two  hydroxyls  in  the  same  form  as  hydrogen  peroxide, 
H/Xj  or  (OHJ.j,  contains  them — and  moreover  not  only  in  one  molecule,  but 
together,  attached  to  one  and  the  same  atom  of  carbon — so  here  we  must 
look  for  the  same  decomposition  as  that  which  we  find  in  hydrogen. peroxide, 
And  accompanied  also  by  the  formation  of  water  as  an  independently 
existing  molecule  ;  therefore  CH<j(OH)3  should  yield,  as  it  actually  does,  im- 
mediately water  and  the  oxide  of  rnethylene,  CH.^0,  which  is  methane  with 

1  Because  more  than  four  atoms  of  hydrogen  never  unite  with  one  atom  of  the  ele- 
ments, and  because  the  hydrogen  compounds  (e.g.  HC1,  HSS,  H3P,  H4Si)  always  form 
their  highest  oxides  with  four  atoms  of  oxygen,  and  as  the  highest  forms  of  oxides  (OsO4, 
BuO4)  also  contain  four  of  oxygen,  and  eight  groups  of  the  periodic  system,  corresponding 
to  the  highest  basic  oxides  K3O,  RO,  Ro03,  RO2)  R805,  RO5,  R2O7,  and  RO4,  imply  the 
above  relationship,  and  because  of  the  nearest  analogues  among  the  elements — such  as 
Mg,  Zn,  Cd,  and  Hg ;  or  Cr,  Mo,  W,  and  U ;  or  Si,  Ge,  Sn,  and  Pt;  or  F,  Cl,  Br,  and  I, 
and  so  forth — not  more  than  four  are  known,  it  seems  to  me  that  in  these  relationships 
there  lies  a  deep  interest  and  meaning  with  regard  to  chemical  mechanics.  But  because, 
to  my  imagination,  the  idea  of  unity  of  design  in  Nature,  either  acting  in  complex 
celestial  systems  or  among  chemical  molecules,  is  very  attractive,  especially  because  the 
atomic  teaching  at  once  acquires  its  true  meaning,  I  will  recall  the  following  facts  re- 
lating to  the  solar  system.  There  are  eight  major  plarets,  of  which  the  four  inner  ones 
are  not  only  separated  from  the  'four  outer  by  asteroids,  but  differ  from  them  in  many 
respects,  as,  for  example,  in  the  smallness  of  their  diameters  and  their  greater  density. 
Saturn  with  his  ring  has  eight  satellites,  Jupiter  and  Uranus  have  each  four.  It  is  evi- 
dent that  in  the  solar  systems  also  we  meet  with  these  higher  numbers  four  and  eight 
which  appear  in  the- combination  of  chemical  molecules. 

*I 


464  PRINCIPLES  OF  CHEMISTRY 

oxygen  substituted  for  two  atoms  of  hydrogen.  Exactly  in  the  same  manner 
put  of  CH(OH)3  are  formed  water  and  formic  acid,  CHO(OH),  and  out  of 
C(OH)4  is  produced  water  ani  carbonic  acid,  or  directly  carbonic  anhydride, 
C02,  which  will  therefore  be  nothing  else  than  methane  with  the  double  re- 
placement  of  pairs  of  hydrogen  by  oxygen.  As  nothing  leads  to  the  supposi- 
tion  that  the  four  atoms  of  hydrogen  in  methane  differ  one  from  the  other, 
BO  it  does  not  matter  by  what  means  we  obtain  any  one  of  the  combinations 
indicated  —  they  will  be  identical  ;  that  is  to  say,  there  will  be  no  case  of 
actual  isomerism,  although  there  may  easily  be  such  cases  of  isomerisni  as 
have  been  distinguished  by  the  term  metamerism. 

Formic  acid,  for  example,  has  two  atoms  of  hydrogen,  o~ne  attached  to  the< 
carbon  left  from  the  methane,  and  the  other  attached  to  the  oxygen  which 
has  entered  in  the  form  of  hydroxyl,  and  if  one  of  them  be  replaced  by  some 
substance  X  it  is  evident  that  we  shall  obtain  substances  of  the  same  composi- 
tion, but  of  different  construction,  or  of  different  orders  of  movement  among 
the  molecules,  and  therefore  endowed  with  other  properties  and  reactions.  If 
X  be  methyl,  CH3—  that  is  to  say,  a  group  capable  of  replacing  hydrogen 
because  it  is  actually  contained  with  hydrogen  in  methane  itself  —  then  by 
substituting  this  group  for  the  original  hydrogen  we  obtain  acetic  acid, 
CCHS0(OH),  out  of  formic,  and  by  substitution  of  the  hydrogen  in  its  oxide  or 
hydroxyl  we  obtain  methyl  formate,  CHO(OOH3).  These  substances  differ  so 
much  from  each  Other  physically  and  chemically  that  at  first  sight  it  is  hardly 
possible  to  admit  that  they  contain  the  same  atoms  in  identically  the  same 
proportions.  Acetic  acid,  for  example,  boils  at  a  higher  temperature  than 
water,  and  has  a  higher  specific  gravity  than  it,  whilst  its  metamerido, 
methyl  formate,  is  lighter  than  water,  and  boils  at  30°—  that  is  to  say,  it 
evaporates  very  easily. 

Let  us  now  turn  to  carbon  compounds  containing  two  atoms  of  carbon  to 
the  molecule,  as.  in  acetic  acid,  and  proceed  to  evolve  them  from  methane  by 
the  principle  of  substitution.  This  principle  declares  at  once  that  methane 
can  only  be  split  up  in  the  four  following  ways  :  — 

1.  Into  a  group  GH3  equivalent  with  H.    Let  us  call  changes  of  this 
nature  methylation. 

2.  Into  a  group  CH2  and  H2.    We  will  call  this  order  of  substitutions 
methylenation. 

3.  Into  CH  and  H3,  which  commutations  we  will  call  acetylenation. 

4.  Into  C  and  H4,  which  may  be  called  carbonation. 

It  is  evident  that  hydrocarbon  compounds  containing  two  atoms  of  carbon 
can  only  proceed  from  methane,  CH4,  which  contains  four  atoms  of  hydrogen 
by  the  first  three  methods  of  substitution  ;  carbonation  would  yield  free  carbon 
if  it  could  take  place  directly,  and  if  the  molecule  of  free  carbon—  which  is  in 
reality  very  complex,  that  is  to  say  strongly  polyatomic,  as  I  have  long  since 
been  proving  by  various  means  —  could  contain  only  C2  like  the  molecules 
0.^,  H,;  Na,  and  so  on. 

By  methylation  we  should  evidently  obtain  from   marsh  gas,  ethane, 


By  methylenation—  that  is,  by  substituting  group  CH,  for  Ha—  methane 
forms  ethylene,  CH^CII,  =  C2H<. 


APPENDIX  I.  465 

By  acetylenation— that  is,  by  substituting  three  atoms  of  hydrogen,  H3,  in 
methane— by  the  remnant  CH,  we  g*t  acetylene,  CHCH  =  C,H2. 

If  we  have  applied  the  principles  of  Newton  correctly,  there  should  not  be 
any  other  hydrocarbons  containing  two  atoms  of  carbon  in  the  molecule. 
All  these  combinations  have  long  been  known,  and  in  each  of  them  we  can 
not  only  produce  those  substitutions  of  which  an  example  has  been  given  in 
the  case  of  methane,  but  also  all  the  phases  of  other  substitutions,  as  we  shall 
find  from 'a  few  more  instances,  by  the  aid  of  which  I  trust  that  I  shall  be 
able  to  show  the  great  complexity  of  those  derivatives  which,  on  the  principle 
of  substitution,  can  be  obtained  from  each  hydrocarbon.  Let  us  content  our- 
eelves  with  the  case  of  ethane,  CH3CH3,  and  the  substitution  of  the  hydrogen 
by  hydroxyl.  The  following  are  the  possible  changes  :— 

1.  CH3CH2(OH) :   this  is  nothing  more  than  spirit  of  wine,  or  ethyl 
alcohol,  C9H5(OH)  or  C,H60. 

2.  CH2(OH)CH.2(OH) :  this  is  the  glycol  of  Wiirtz,  which  has  shed  so 
much  light  on  the  history  of  alcohol.     Its  isomeride  may  be  CH3CH(OH)2, 
but  as  we  have  seen  in  the  case  of  CH(OH)2,  it  decomposes,  giving  off  water, 
and  forming  aldehyde,  CH3CHO,  a  substance'  capable  of  yielding  alcohol  by 
uniting  with  hydrogen,  and  of  yielding  acetic  ac.id  by  uniting  with  oxygen. 

If  glycol,  CH.2(C)H)CEL(OH),  loses  its  water,  it  may  be  seen  at  once  that 
it  will  not  now  yield  aldehyde,  CH3CHO,  but  its  isomeride,  ^CH?,  the 

oxide  of  ethylene.  I  have  here  indicated  in  a  special  manner  the  oxygen 
which  has  taken  the  place  of  two  atoms  of  'the  hydrogen  of  ethane  taken 
from  different  atoms  of  the  carbon. 

8.  CH3C(OH)3  decomposed  as  CH(OH)3,  forming  water  and  acetic  acid, 
CHjCO(OH).  It  is  evident  that  this  acid  is  nothing  else  than  formic  acid, 
CHO(OH),  with  its  hydrogen  replaced  by  methyl.  Without  examining 
further  the  vast  nu,mber  of  possible  derivatives,  I  will  direct  your  attention 
to  the  circumstance  that  in  dissolving  acetic  acid  in  water  we  obtain  the 
maximum  contraction  and  the  greatest  viscosity  when  to  the  molecule 
CH3CO(OH)  is  added  a  molecule  of  water,  which  is  the  proportion  which 
would  form  the  hydrate  CH3C(OH)3.  It  is  probable  that  the  doubling  of 
the  molecule  of  acetic  acid  at  temperatures  approaching  its  boiling-point 
has  some  connection  with  this  power  of  uniting  with  one  molecule  of 
water. 

4.  CH2(OH)C(OH)3  is  evidently  an  alcoholic  acid,  and  indeed  this  com- 
pound,  after    losing  water,    answers  to  glycolic    acid,  CH^OH^CO (OH). 
Without  investigating  all  the  possible  isomerides,  we  will  note  only  that  the 
hydrate  CH(OH)2CH(OH)2  has  the  same  composition  as  CH^OHjCtOH)^ 
and  although  corresponding  to  glycol,  and,being  a  symmetrical  substance,  it 
becomes,  on  parting  with  its  water,  the  aldehyde  of  oxalic  acid,  or  the  glyoxal 
of  Debus,  CHOCHO. 

5.  CH(OH)2C(OH8),  from.the  tendency  of  all  the  preceding,  corresponds 
with  glyoxylic  acid,,  an   aldehyde  acid,  CHOCO(OH),  because   the  gronp 
CO(OH),  or  carboxyl,  enters  into  the  compositions  of  organic  acids,  and  thei 
group  CHO  defines  the  aldehyde  function. 

6.  C(OH)3C(OH)3  through  the  loss  of  2B,0  yields  the  bibasio  oxalic  acidl 


466  PRINCIPLES  OF  CHEMISTRY 

CO(OH)CO(OH),  which  generally  .crystallises  with  2H.20,  following  thus  the 
normal  type  of  hydration  characteristic  of  ethane.2 

Thus,  by  applying  the  principle  of  substitution,  we  can,  in  the  simplest 
manner,  derive  not  only  every  kind  of  hydrocarbon  compound,  such  as  the 
alcohols,  the  aldehyde-alcohols,  aldehydes,  alcohol-acids,  and  the  acids,  but 
also  combinations  analogous  to  hydrated  crystals  which  usually  are  dis- 
regarded. 

But  even  those  unsaturated  substances,  of  which  ethylene,  CH.jCH5,  and 
acetylene,  CHCH,  are  types,  may  be  evolved  with  equal  simplicity.  With 
respect  to  the  phenomena  of  isornerism,  there  are  many  possibilities  among 
the  hydrocarbon  compounds  conta;ning  two  atoms  of  carbon,  and  without 
going  into  details  it  will  be  sufficient  to  indicate  that  the  following  formulae, 
though  not  identical,  will  be  isomeric  substantially  among  themselves : — 
CH3CHX2  and  CH2XCH,X,  although  both  contain  C,H4X2 ;  or  CH,CXa  and 
CHXCHX,  although  both  contain  C.^Xj,  if  by  X  we  indicate  chlorine  or 
generally  an  element  capable  of  replacing  one  atom  of  hydrogen,  or  capable 
of  uniting  v/ith  it.  To  isomerism  of  this  kind  belongs  the  case  of  aldehyde 
and  the  oxide  of  ethylene,  to  which  we  have  already  referred,  because  both 
have  the  composition  C2H40. 

What  I  have  said  appears  to  me  sufficient  to  show  that  the  principle  of 
eubstitution  adequately  explains  the  composition,  the  isomerism,  and  all  the 
diversity  of  combination  of  the  hydrocarbons,  and  I  shall  limit  the  further 
development  of  these  views  to  preparing  a  complete  list  of  every  possible 
hydrocarbon  compound  containing  three  atoms  of  carbon  in  the  molecule. 
There  are  eight  in  all,  of  which  only  five  are  known  at  present.3 

Among  those  possible  for  C3H6  there  should  be  two  isomerides,  propylene 
and  trimethyleue,  and  they  are  both  already  known.  For  C3H4  there  should 
be  three  isomerides :  allylene  and  allene  are  known,  but  the  third  has  not 
yet  been  discovered ;  and  for  CaH2  there  should  be  two  isomerides,  though 
neither  of  them  is. known  as  yet.  Their  composition  and  structure  are  easily 

2  One  more  isomeride,  CH2CH(OH),  is  possible— that  is,  secondary  vinyl  alcohol, 
•which  is  related  to  ethylene,  CH2CH2,  but  derived  by  the  principle  of  substitution  from 
CH4.  Other  isomerides,  of  the  composition  C2H4O,  such,  for  example,  AS  CCH5(OH), 
are  impossible,  because  it  would  correspond  with  the  hydrocarbon  CHCH3=C2H4,  which 
is  isomeric  with  ethylene,  and  it  cannot  be  derived  from  methan.e.  If  such  an  isomeride 
existed  it  would  be  derived  from  CH2,  but  such  products  are,  up  to  the  present,  unknown. 
In  such  cases  the  insufficiency  of  the  points  of  departure  of  the  statical  structural  teach- 
ing is  shown.  It  first  admits  constant  atomicity  and  then  rejects  it,  the  facts  serving  to 
establish  either  one  or  the  other  view ;  and  therefore  it  seems  to  me  that  we  must  come 
to  the  conclusion  that  the  structural  method  of  reasoning,  having  done  a  service  to 
science,  has  outlived  the  age,  and  must  be  regenerated,  as  in  their  time  was  the  teaching 
of  the  electro-chemists,  the  radicalists,  and  the  adherents  of  the  doctrine  of  types.  An 
we  cannot  now  lean  on  the  views  above  stated,  it  is  time  to  abandon  the  structural 
theory  They  will  all  be  united  in  chemical  mechanics,  and  the  principle  of  substitution 
must  be  looked  on  only  as  a  preparation  for  the  coming  epoch  in  chemistry,  where 
such  cases  as  the  isomerism  of  fumaric  and  inaleic  acids,  when  explained  dynamically,  ae 
proposed  by  Le  Bel  and  Van't  Hoff,  may  yield  points  of  departure. 

5  Conceding  variable  atomicity,  the  structuralists  must  expect  an  incomparably  larger 
number  of  isomerides,  and  they  cannot  now  decline  to  acknowledge  the  change  of 
atomicity,  were  it  only  for  the  examples  HgCl  and  HgClj,  CO  and  CO2,  PC13  and  PC15. 


APPENDIX  I.  467 

deduced  from  ethane,  ethylene,  and  acetylene,  by  methylation,  by  methylena* 
tion,  by  acetylenation  and  by  carbonation. 

1.  C3H8  =  CH3CH4CH3  out  of  CH3CH3  by  methylation.    This  hydro* 
carbon  is  named  propane. 

2.  C3H6  =  CH3CHCH.;  out   of  CH3CH3  by  methylenation.    This  sub* 
stance  is  propylene. 

8.  C3H6  =  CHSCH8CH2  out  of  CH3CH3  by  methylenation.  This  sub. 
stance  is  trimethylene. 

4.  C3H4  -  CH3CCH  out  of-  CH3CH3  by  acetylenation  or  from  CHCH  by 
methylation.  This  hydrocarbon  is  named  allylene. 


6.  C3H4   =         ij1    outofCH3CH3by  acetylenation,  or  from  CHjCH,  by 
methylenation,  because  CHvjJH  =  C^£  H     This  body  is  as  yet  unknown. 

6.  C8H4  =  CH2CCH2  out  of  CH^H.^  by  methylenation.    This  hydro- 
carbon  is  named  allene,  or  iso-allylene. 


p-rrpTT 

7.  C8H,  =    w«**»   out  Of  CH3CHj  by  symmetrical  carbonation,  or  out 

of  CH2CH2  by  acetylenation.    This  compound  i»  unknown. 

PP 

8.  CsILj   =  ^g.    out  of  CH3CH3  by  carbonatioii,  or  out  of  CHCH  by 

rnethylenation.     This  compound  is  unknown. 

If  we  bear  in  mind  that  for  each  hydrocarbon  serving  as  a  type  in  the 
above  tables  there  are  a  number  of  corresponding"  derivatives,  and  that  every 
compound  obtained  may,  by  further  methylation,  methylenation,  acetylena- 
tion, and  carbonation,  produce  new  hydrocarbons,  and  these  may  be  followed 
by  a  numerous  suite  of  derivatives  and  an  immense  number  of  isomeric 
eubstances,  it  is  possible  to  understand  the  limitless  number  of  carbon  com- 
pounds, although  they  all  have  the  one  substance,  methane,  for  their  origin, 
The  number  of  substances  is  so  enormous  that  it  is  no  longer  a  question  of 
enlarging  the  possibilities  of  discovery,  but  rather  of  finding  some  means  of 
testing  them  analogous  to  the  well-known  two  which  for  a  long  time  have 
served  as  gauges  for  all  carbon  compounds. 

I  refer  to  the  law  of  even  numbers  and  to  that  of  limits,  the  first  enunciated 
by  Gerhardt  some  forty  years  ago,  with  respect  to  hydrocarbons,  namely, 
that  their  molecules  always  contain  an  even  number  of  atoms  of  hydrogen. 
But  by  the  method  which  I  have  used  of  deriving  all  the  hydrocarbons  from, 
methane,  CH4,  this  law  may  be  deduced  as  a  direct  consequence  of  the 
principle  of  substitutions.  Accordingly,  in  methylation,  CH3  takes  the  place 
of  H,  and  therefore  CH.,  is  added.  In  methylenation  the  number  of  atoms  of 
hydrogen  remains  unchanged,  and  at  each  acetylenation  it  is  reduced  by  two, 
and  in  carbonation  by  four,  atoms—  that  is  to  say,  an  even  number  of  atoms 
of  hydrogen  is  always  added  or  removed.  And  because  the  fundamental 
hydrocarbon,  methane,  CH4,  contains  an  even  number  of  atoms  of  hydrogen, 
all  its  derivative  hydrocarbons  will  also  contain  even  numbers  of  hydrogen, 
and  this  constitutes  the  law  of  even  numbers. 

The  principle  of  substitutions  explains  with  equal  simplicity  the  conception 
of  the  limiting  compositions  of  hydrocarbons  C,,H.,,,+2,  which  I  derived,  in 


468  PRINCIPLES  t>F  CHEMISTRY 

1861,4  in  an  empirical  manner  from  accumulated  materials  available  at  that 
time,  and  on  the  basis  of  the  limits  to  combinations  worked  out  by  Dr.  Frank- 
land  for  other  elements. 

Of  all  the  various  substitutions  the  highest  proportion  of  hydrogen  is 
yielded  by  methylation,  because  in  that  operation  alone  does  the  quantity  of 
hydrogen  increase ;  hence,  taking  methane  as  a  point  of  departure,  if  we 
imagine  methylation  effected  (n  -  1)  time  -we  obtain  hydrocarbon  compounds 
containing  the  highest  quantities  of  hydrogen/  It  is  evident  that  they  will 
contain  CH4  +  (n  -  IJCHj,  or  CnHjn  +  2,  because  methylation  leads  to  the  addi- 
tion of  CH2  to  the  compound. 

It  will  thus  be  seen  that  by  the  principle  of  substitution — that  is  to  say, 
by  the  third  law  of  Newton— we  are  able  to  deduce,  in  the  simplest  manner, 
not  only  the  individual  composition,  the  isomerism,  and  relations  of  sub- 
stances, but  also  the  general  laws  which  govern  their  most  complex  combina- 
tions without  having  recourse  either  to  statical  constructions,  to  the  definition 
of  atomicities,  to  the  exclusion  of  free  affinities,  or  to  the  recognition  of  those 
single,  double  or  treble  bonds  which  are  so  indispensable  to  structuralists  in  the 
explanation  of  the  composition  and  construction  of  hydrocarbon  compounds. 
And  yet,  by  the  application  of  the  dynamical  principles  of  Newton,  we  can 
attain  to  that  chief  and  fundamental  object,  the  comprehension  of  isomerism 
in  hydrocarbon  compounds,  and  the  forecasting  of  the  existence  of  combina- 
tions as  yet  unknown,  by  which  the  edifice  raised  by  structural  teaching  is 
strengthened  and  supported.  Besides— and  I  count  this  for  a  circumstance 
of  special  importance — the  process  which  I  advocate  will  make  no  difference 
in  those  special  cases  which  have  been  already  so  .well  worked  out,  such  as, 
for  example,  the  isomeriam  of  the  hydrocarbons  and  alcohols,  even  to  the 
extent  of  not  interfering  with  the  nomenclature  which  has  been  adopted,  and 
the  structural  system  will  retain  all  the  glory  of  having  worked  up,  in  a 
thoroughly  scientific  manner,  the  store  of  information  which  Gerhardt  had 
accumulated  about  the  middle  of  the  fifties,  and  the  still  higher  glory  of 
establishing  the  rational  synthesis  of  organic  substances.  Nothing  will  be 
lost  to  the  structural  doctrine  except  its  statical  origin ;  and  as  soon  as  it 
will  embrace  the  dynamic(  principles  of  Newton,  and  suffer  itself  to  be  guided 
by  them,  I  believe  that  we  shall  attain  for  chemistry  that  unity  of  principle, 
which  is  now  wanting.  Many  an  adept  will  be  attracted  to  that  brilliant  and 
fascinating  enterprise,  the  penetration  .into  the  unseen  worjd  of  the  kinetic< 
relations  of  atoms,  to  the  study  of  which  the  last  twenty-five  years  have  con- 
tributed so  much  labour  and  such  high  inventive  faculties. 

D'Alembert  found  in  mechanics  that  if  inertia  be  taken  to  represent  force, 
dynamic  equations  may  be  applied  to  statical  questions,  which  are  thereby 
rendered  more  simple  and  more  easily  understood. 

The  structural  doctrine  in  chemistry  has  unconsciously  followed  the  same 
course,  and  therefore  its  terms  are  easily  adopted  ;  they  may  retain  their 
present  forms  provided  that  a  truly  dynamical— that  is  to  say,  Newtonian — 
meaning  be  ascribed  to  them. 

Before  finishing  my  task  and  demonstrating  the  possibility  of  adapting 

Essai  d'une  theorie  sur  les  limites  des  combinaisons  organiques,'  par  D.  Mendel^eff, 
g/11  aofit  1801,  Bulletin  de  I'Academie  i.  d.  Sc.  de  St.  Pttersbourg,  t.  v 


APPENDIX  I.  469 

structural  doctrines  to  the  dynamics  of  Newton,  I  consider  it  indispensable 
to  touch  on  one  question  which  naturally  arises,  and  which  I  have  heard 
discussed  more  than  once.  If  bromine,  the  atom  of  which  is  eighty  times 
heavier  than  that  of  hydrogen,  takes  the  place  of  hydrogen,  it  would  eeem 
that  the  whole  system  of  dynamic  equilibrium  must  be  destroyed. 

Without  entering  into  the  minute  analysis  of  this  question,  I  think  it 
will  be  sufficient  to  examine  it  by  the  light  of  two  well-known  phenomena, 
one  of  which  will  be  found  in  the  department  of  chemistry  and  the  other  in 
that  of  celestial  mechanics,  and  both  will  serve  to  demonstrate  the  existence 
of  that  unity  in  the  plan  of  creation  which  is  a  consequence  of  the  Newtonian 
doctrines.  Experiments  demonstrate  that  when  a  heavy  element  is  substi- 
tuted for  a  light  one  in  a  chemical  compound — for  example,  for  magnesium, 
in  the  oxide  of  that  metal,  an  atom  of  mercury,  which  is  8£  times  heavier — 
the  chief  chemical  characteristics  or  properties  are  generally,  though  not 
always,  preserved. 

The  substitution  of  silver  for  hydrogen,  than  which  it  is  108  times  heavier, 
does  not  affect  all  the  properties  Of  the  substance,  though  it  does  some. 
Therefore  chemical  substitutions  of  this  kind— the  substitution  of  light  for 
heavy  atoms — need  not  necessarily  entail  changes  in  the  original  equilibrium  ; 
and  this  point  is  still  further  elucidated  by  the  consideration  that  the  periodic 
law  indicates  the  degree  of  influence  of  an  increment  of  weight  in  the  atom 
as  affecting  the  possible  equilibria,  and  also  what  degree  of  increase  in  the 
•weight  of  the  atoms  reproduces  some,  though  not  all,  of  the  properties  of  the 
substance. 

This  tendency  to  repetition — these  periods — may  be  likened  to  those 
annual  or  diurnal  periods  with  which  we  are  so  familiar  on  the  earth.  Days 
and  years  follow  each  other,  but,  as  they  do  so,  many  things  change  ;  and  in 
like  manner  chemical  evolutions,  changes  in  the  masses  of  the  elements, 
permit  of  much  remaining  undisturbed,  though  many  properties  undergo 
alteration.  The  system  is  maintained  according  to  the  laws  of  conservation 
in  nature,  but  the  motions  are  altered  in  consequence  of  the  change  of  parts, 

Next,  let  us  take  an  astronomical  case — such,  for  example,  as  the  earth  and 
the  moon — and  let  us  imagine  that  the  mass  of  the  latter  is  constantly 
increasing.  The  question  is,  what  will  then  occur  ?  The  path  of  the  moon 
in  space  is  a  wave-line  similar  to  that  which  geometricians  have  named  epi- 
cycloidal,  or  the  locus  of  a  point  in  a  circle  rolling  round  another  circle.  But 
in  consequence  of  the  influence  of  the  moon  it  is  evident  that  the  path  of  the 
earth  itself  cannot  be  a  geometric  ellipse,  even  supposing  the  sun  to  be  im- 
movably fixed ;  it  must  be  an  epicycloiclal  curve,  though  not  very  far  removed 
from  the  true  ellipse — that  is  to  say,  it  will  be  impressed  with  but  faint  un- 
dulations. It  Is  only  the  common  centre  of  gravity  of  the  earth  and  the 
moon  which  describes  a  true  ellipse  round  the  sun.  If  the  moon  were  to 
increase,  the  relative  undulations  of  the  earth's  path  would  increase  in  ampli- 
tude, those  of  the  moon  would  also  change,  and  when  the  mags  of  the  moon 
bad  increased  to  an  equality  with  that  of  the  earth,  the  path  would  consist  of 
epicycloidal  curves  cro'ssing  each  other,  and  having  opposite  phases.  But  a 
similar  relation  e.xists  between  the  sun  and  the  earth,  because  the  former  is 
also  moving  in  space.  "We  .way  apply  these  views  to  the  world  of  atoms,  and 


470  PRINCIPLES  OF  CHEMISTRY 

suppose  that  in  their  movements,  when  heavy  ones  take  the  place  of  those 
that  are  lighter,  similar  changes  take  place,  provided  that  the  svstem  or  the 
molecule  is  preserved  throughout  the  change. 

It  seems  probable  that  in  the  heavenly  systems,  during  incalculable 
astronomical  periods,  changes  have  taken  place  and  are  still  going  on  similar 
tor  those  which  pass  rapidly  before  our  eyes  during  the  chemical  reaction  of 
molecules,  and  the  progress  of  molecular  mechanics  may — we  hope  will— in 
course  of  time  permit  us  to  explain  those  changes  in  the  stellar  world  which 
have  more  than  once  been  noticed  by  astronomers,  and  which  are  now  so 
carefully  studied.  A  coming  Newton  will  discover  the  laws  of  these  changes. 
Those  laws,  when  applied  to  chemistry,  may  exhibit  peculiarities,  but  these 
will  certainly  be  mere  variations  on  the  grand  harmonious  theme  which 
reigns  in  nature.  The  discovery  of  the  laws  which  produce  this  harmony  in 
chemical  evolution  will  only  be  possible,  it  seems  to  me,  under  the  banner 
of  Newtonian  dynamics,  which  has  so.  long  waved  over  the  domains  of 
mechanics,  astronomy,  and  physics.  In  calling  chemists  to  take  their  stand 
under  its  peaceful  and  catholic  shadow  I  imagine  that  I  am  aiding  in  estab- 
lishing that  scientific  union  which  the  managers  of  the  Royal  Institution 
wish  to  effect,  who  have  shown  their  desire  to  do  so  by  the  flattering  invita- 
tion which  has  given  me— a  Russian — the  opportunity  of  laying  before  the 
countrymen  of  Newton  an  attempt  to  apply  to  chemistry  one  of  his  immortal 
principles. 


APPENDIX    II 

THE   PERIODIC    LAW    OF    THE    CHEMICAL    ELEMENTS 

BY  PROFESSOR  MENDELEEFF 

FARADAY   LECTURE   DELIVERED   BEFORE   THE   FELLOWS   OF 
THE   CHEMICAL  SOCIETY   IN   THE   THEATRE   OF  THE   ROYAL   INSTITUTION, 
ON   TUESDAY,   JUNE   4,   1889 

THE  high  honour  bestowed  by  the  Chemical  Society  in  inviting  me  to  pay  a 
tribute  to  the  world-famed  name  of  Faraday  by  delivering  this  lecture  has 
induced  me  to  take  for  its  subject  the  Periodic  Law  of  the  Elements— this 
being  a  generalisation  in  chemistry  which  has  of  late  attracted  much 
attention. 

While  science  is  pursuing  a  steady  onward  movement,  it  is  convenient 
from  time  to  time  to  cast  a  glance  back  on  the  route  already  traversed,  and 
especially  to  consider  the  new  conceptions  which  aim  at  discovering  the 
general  meaning  of  the  stock  of  facts  accumulated  from  day  to  day  in  our 
laboratories.  Owing  to  the  possession  of  laboratories,  modern  science  now 
bears  a  new  character,  quite  unknown,  not  only  to  antiquity,  but  even  to  the 
preceding  century.  Bacon's  and  Descartes'  idea  of  submitting  the  mechanism 
of  science  simultaneously  to  experiment  and  reasoning  has  been  fully  realised 
in  the  case  of  chemistry,  it  having  become  not  only  possible  but  always 
customary  to  experiment.  Under  the  all-penetrating  control  of  experiment, 
a  new  theory,  even  if  crude,  is  quickly  strengthened,  provided  it  be  founded 
on  a  sufficient  basis ;  the  asperities  are  removed,  it  is  amended  by  degrees, 
and  soon  loses  the  phantom  light  of  a  shadowy  form  or  of  one  founded  on 
mere  prejudice ;  it  is  able  to  lead  to  logical  conclusions,  and  to  submit  to  ex- 
perimental proof.  Willingly  or  npt,  in  science  we  all  must  submit  not  to  what 
seems  to  us  attractive  from  one  point  of  view  or  from  another,  but  to  what 
represents  an  agreement  between  theory  and  experiment ;  in  other  words,  to 
demonstrated  generalisation  and  to  the  approved  experiment.  Is  it  long 
since  many  refused  to  accept  the  generalisations  involved  in  the  law  of  Avo- 
gadro  and  Ampere,  so  widely  extended  by  Gerhardt  ?  We  still  may  hear  the 
voices  of  its  opponents ;  they  enjoy  perfect  freedom,  but  vainly  will  their 
voices  rise  so  long  as  they  do  not  use  the  language  of  demonstrated  facts 


472  PRINCIPLES  OF  CHEMISTRY 

The  striking  observations  with  the  spectroscope  which  have  permitted  us  to 
analyse  the  chemical  constitution  of  distant  worlds,  seemed,  at  first,  appli- 
cable to  the  task  of  determining  the  nature  of  the  atoms  themselves ;  but  the 
working  out  of  the  idea  in  the  laboratory  soon  demonstrated  that  the  charac- 
ters of  spectra  are  determined,  not  directly  by  the  atoms,  but  by  the  mole- 
cules into  which  the  atoms  are  packed ;  and  so  it  became  evident  that  more 
verified  facts  must  be  collected  before  it  will  be  possible  to  formulate  new 
generalisations  capable  of  taking  their  place  beside  those  ordinary  ones  based 
upon  the  conception  of  simple  substances  and  atoms.  But  as  the  shade  of  the 
leaves  and  roots  of  living  plants,  together  with  the  relics  of  a  decayed  vege- 
tation, favour  the  growth  of  the  seedling  and  serve  to  promote  its  luxurious 
development,  in  like  manner  sound  generalisations— together  with  the  relics 
of  those  which  have  proved  to  be  untenable — promote  scientific  productivity, 
and  ensure  the  luxurious  growth  of  science  under  the  influence  of  rays  ema- 
nating from  the  centres  of  scientific  energy.  Such  centres  are  scientific 
associations  and  societies.  Before  one  of  the  oldest  and  most  powerful  of 
these  I  am  about  to  take  the  liberty  of  passing  in  review  the  twenty  years'  life 
of  a  generalisation  which  is  known  under  the  name  of  the  Periodic  Law.  It 
Was  in  March  1869  that  I  ventured  to  lay  before  the  then  youthful  Bussian 
Chemical  Society  the  ideas  upon  the  same  subject  which  I  had  expressed  in 
my  just  written  '  Principles  of  Chemistry.' 

Without  entering  into  details,  I  will  give  the  conclusions  I  then  arrived 
at  in  the  very  words  I  used : — 

'  1.  The  elements,  if  arranged  according  to  their  atomic  weights,  exhibit 
en  evident  periodicity  of  properties. 

4  2.  Elements  which  are  similar  as  regards  their  chemical  properties  hav« 
atomic  weights  which  are  either  of  nearly  the  Same  value  (e.g.  platinum, 
indium,  osmium)  or  Which  increase  regularly  (e.g.  potassium,  rubidium, 
caesium). 

'  3.  The  arrangement  of  the  elements,  or  of  groups  of  elements,  in  the 
order  of  their  atomic  weights,  corresponds  to  their  so-called  valencies  as  well 
as,  to  some  extent,  to  their  distinctive  chemical  properties— as  is  apparent, 
among  other  series,  in  that  of  lithium,  beryllium,  barium,  carbon,  nitrogen, 
oxygen,  and  iron. 

'  4.  The  elements  which  are  the  most  widely  diffused  have  small  atomic 
•weights. 

*fl.  The  magnitude  of  the  atomic  weight  determines  the  character  of  the 
element,  just  as  the  magnitude  of  the  molecule  determines  the  character  of 
a  compound. 

1 6.  We  must  expect  the  discovery  of  many  yet  unknown  elements— for 
example,  elements  analogous  to  aluminium  and  silicon,  whose  atomic  weight 
would  be  between  65  and  75. 

4  7.  The  atomic  weight  of  an  element  may  sometimes  be*  amended  by  a 
knowledge  of  those  of  the  contiguous  elements.  Thus,  the  atomic  weight  of 
tellurium  must  lie  between  123  and  126,  and  cannot  be  128. 

*  8.  Certain  characteristic  properties  of  the  elements  can  be  foretold  from 
their  atomic  weights. 


APPENDIX  II.  473 

'  The  aim  of  this  communication  will  be  fully  attained  if  I  succeed  in 
drawing  the  attention  of  investigators  to  those  relations  which  exist  between 
the  atomic  weights  of  dissimilar  elements,  which,  so  far  as  I  know,  have 
hitherto  been  almost  completely  neglected.  I  believe  that  the  solution  of 
some  of  the  most  important  problems  of  our  science  lies  in  researches  of  this 
kind.' 

To-day,  twenty  years  after  the  above  conclusions  were  formulated,  they 
may  still  be  considered  as  expressing  the  essence  of  the  now  well-known 
periodic  law. 

Beverting  to  the  epoch  terminating  with  the  sixties,  it  is  proper  to  indi- 
cate three  series  of  data  without  the  knowledge  of  which  the  periodic  law 
could  not  have  been  discovered,  and  which  rendered  its  appearance  natural 
and  intelligible. 

In  the  first  place,  it  was  at  that  time  that  the  numerical  value  of  atomic 
weights  became  definitely  known.  Ten  years  earlier  such  knowledge  did  not, 
exist,  as  may  be  gathered  from  the  fact  that  in  1860  chemists  from  all  parts 
of  the  world  met  at  Karlsruhe  in  order  to  come  to  some  agreement,  if  not 
with  respect  to  views  relating  to  atoms,  at  any  rate  as  regards  their  definite 
representation.  Many  of  those  present  probably  remember  how  vain  were 
the  hopes  of  corning  to  an  understanding,  and  how  much  ground  was  gained' 
at  that  Congress  by  the  followers  of  the  unitary  theory  so  brilliantly  repre- 
sented by  Cannizzaro.  I  vividly  remember  the  impression  produced  by  his 
speeches,  which  admitted  of  no  compromise,  and  seemed  to  advocate  truth 
itself,  based  on  the  conceptions  of  Avogadro,  Gerhardt,  and  Regnault,  which 
at  that  time  were  far  from  being  generally  recognised.  And  though  no 
understanding  could  be  arrived  at,  yet  the  objects  of  the  meeting  were  attained, 
for  the  ideas  of  Cannizzaro  proved,  after  a  few  years,  to  be  the  only  ones 
which  could  stand  criticism,-  and  which  represented  an  atom  as— 'the 
smallest  portion  of  an  element  which  enters  into  a  molecule  of  its  compound.' 
Only  such  real  atomic  weights — not  conventional  ones— could  afford  a  basis 
for  generalisation.  It  is  sufficient,  by  way  of  example,  to  indicate  the 
following  cases  in  which  the  relation  is  seen  at  once  and  is  perfectly  clear : — 
K  =30  Kb  =  85  Cs  -133 

Ca  =  40  Sr=87  Ba  =  137 

whereas  with  the  equivalents  then  in  use— 

K  =39  Rb  =  85  Cs=133 

Ca  =  20  Sr  =  43'5  Ba  =  68'5 

the  consecutiveness  of  change  in  atomic  weight,  which  with  the  true  values 
is  so  evident,  completely  disappears. 

Secondly,  it  had  become  evident  during  the  period  1860-70,  and  even 
during  the  preceding  decade,  that  the  relations  between  the  atomic/ weights 
of  analogous  elements  were  governed  by  some  general  and  simple  laws. 
Coolce,  Cremers,  Gladstone,  Grnelin,  Lenssen,  Pettenkofer,  and  especially 
Dumas,  had  already  established  many  facts  bearing  on  that  view.  Thus 
"Dumas  compared  the  following  groups  of  analogous  elements  with  organic 
radicles : — 


474  PRINCIPLES  OF  CHEMISTRY 


U-7, 


Diff.  Diff.  Diff.  Diff. 

Mg=12j8  P-81J44  0-818 

Ca  =  20>  As  =  75*  S  =16, 

16  3-x8  44  3x8 

Na-28!  Sr  ««4  »»!»  Se  =  40> 

16  13x8  12x44  }3x8 

K  =39'                  Ba  =68)  Bi=207)  Te  =  64) 

and  pointed  out  some  really  striking  relationships,  such  as  the  following  :— 
F   =19. 

Cl  =35-5  =  19  +  16-5 
Br=80    =19  +  2x16-5  +  28. 
I    =  127  = 


A.  Strecker,  in  his  work  '  Theorien  und  Experimente  zor  Bestimmung 
der  Atomgewichte  der  Elemente  '  (Braunschweig,  1859),  after  summarising 
the  data  relating  to  the  subject,  and  pointing  out  the  remarkable  series  of 
equivalents  — 

Cr  =  26-2       Mn»27-6       Fe  =  28        Ni  =  29       Co  =  30       Cu  =  81'7 
Zn  =  32-5 

remarks  that:  It  is  hardly  probable  that  all  the  above-mentioned  relations 
between  the  atomic  weights  (or  equivalents)  of  chemically  analogous  elements 
are  merely  accidental.  We  must,  however,  leave  to  the  future  the  discovery 
of  the  law  of  the  relations  which  appears  in  these  figures.'  * 

In  such  attempts  at  arrangement  and  in  such  views  are  to  be  recognised 
the  real  forerunners  of  the  periodic  law  ;  the  ground  was  prepared  for  it 
between  1860  and  1870,  and  that  it  was  not  expressed  in  a  determinate  form 
before  the  end  of  the  decade  may,  I  suppose,  be  ascribed  to  the  fact  that  only 
analogous  elements  had  been  compared.  The  idea  of  seeking  for  a  relation 
between  the  atomic  weights  of  all  the  elements  was  foreign  to  the  ideas  then 
current,  so  that  neither  the  vi*  tellurique  of  De  Chancourtois,  nor  the  law  of 
octaves  of  Newlands,  could  secure  anybody's  attention.  And  yet  both  De 
Chancourtois  and  Newlands  like  Dumas  and  Strecker,  more  than  Lenssen 
and  Pettenkofer,  had  made  an  approach  to  the  periodic  law  and  had  dis- 
covered its  germs.  The  solution  of  the  problem  advanced  but  slowly,  because 
the  facts,  but  not  the  law,  stood  foremost  in  all  attempts  ;  and  the  law  could 
not  awaken  a  general  interest  so  long  as  elements,  having  no  apparent  con- 
nection with  each  other,  were  included  in  the  same  octave,  as  for  example  :  — 

1st    octave    of 


Newlands  .  . 
7th  Ditto 


Cl 
Fe 


Co&Ni  Br 

Se          Eh'&Ru 


Pd 
Te 


Au 


Pt&Ir 
Os  or  Th 


Analogies  of  the  above  order  seemed  quite  accidental,  and  the  more  so  as 
the  octave  contained  occasionally  ten  elements  instead  of  eight,  and  when  two 

1  'Es  ist  wohl  kaum  anzunehmen,  class  alle  im  Vorhergehenden  hervorgehobenen 
Beziehungen  zwischen  den  Atomgewichten  (oder  Aequivalenten)  in  chemischen  Verhiilt- 
nissen  einander  a'hnliche  Elemente  bloss  zulallig  sind.  Die  Auffindung  der  in  diesen 
Zahlen  geaetziichen  Beziehungen  miissen  wir  jedoch  der  Zukunft  iiberlasaen.' 


APPENDIX  IJ.  475 

such  elements  as  Ba  and  V,  Co  and  N.i,  or  Rh  and  Ru,  occupied  one  place  in 
the  octave.3  Nevertheless,  the  fruit  was  ripening,  and  I  now  see  clearly  that 
Strecker,  De  Chancourtois,  and  Newlands  stood  foremost' in  the  way  towards 
the  discovery  of  the  periodic  law,  and  that  they  merely  wanted  the  boldness 
necessary  to  place  the  whole  question  at  such  a  height  that  its  reflection  on 
the  facts  could  be  clearly  seen. 

A  third  circumstance  which  revealed  the  periodicity  of  chemical  elements 
was  the  accumulation,  by  the  end  of  the  sixties,  of  new  information  respecting 
the  rare  elements,  disclosing  their  many-sided  relations  to  the  other  elements 
and  to  each  other.  The  researches  of  Marignac  en  niobium,  and  those  of 
Roscoe  on  vanadium,  were  of  special  moment.  The  striking  analogies  between 
vanadium  and  phosphorus  on  the  one  hand,  and  between  vanadium  and 
chromium  on  the  other,  which  became  so  apparent  in  the  investigations  con- 
nected with  that  element,  naturally  induced  the  comparison  of  V  =  51  with 
Cr  =  52,  Kb  =  94  with  Mo  =  96,  and  Ta  =  192  with  W  =  194;  while,  on  the 
Other  hand,  P  =  31  could  be  compared  with  S  -  32,  As  =  75  with  Se  «•  79,  and 
Sb  =  120  with  Te  - 125.  From  such  approximations  there  remained  but  one 
step  to  the  discovery  of  the  law  of  periodicity. 

The  law  of  periodicity  was  thus  a  direct  outcome  of  the  stock  of  generali- 
sations and  established  facts  which  had  accumulated  by  the  end  of  the  decade 
1860-1870 :  it  is  an  embodiment  of  those  data  in  a  more  or  less  systematic 
expression.  Where,  then,  lies  the  secret  of  the  special  importance  which  has 
since  been  attached  to  the  periodic  law,  and  has  raised  it  to  the  position  of  a 
generalisation  which  has  already  given  to  chemistry  unexpected  aid,  and 
which  promises  to  be  far  more  fruitful  in  the  future  and  to  impress  upon 
several  branches  of  chemical  research  a  peculiar  and  original  stamp?  The 
remaining  part  of  my  communication  will  be  an  attempt  to  answer  this 
question. 

In  the  first  place  we  have  the  circumstance  that,  as  soon  as  the  law  made 
its  appearance,  it  demanded  a  revision  of  many  facts  which  were  considered 
by  chemists  as  fully  established  by  existing  experience.  I  shall  return,  later 
on,  briefly  to  this  subject,  but  I  wish  now  to  remind  you  that  the  periodic 
law,  by  insisting  on  the  necessity  for  a  revision  of  supposed  facts,  exposed 
itself  at  once  to  destruction  in  its  very  origin.  Its  first  requirements,  how- 
ever; have  been  almost  entirely  satisfied  during  the  last  20  years ;  the  sup- 
posed  facts  have  yielded  to  the  law,  thus  proving  that  the  law  itself  was  a 
legitimate  induction  from  the  verified  facts.  But  our  inductions  from  data 
have  often  to  do  with  such  details  of  a  science  so  rich  in  facts,  that  only 
generalisations  which  cover  a  wide-  range  of  important  phenomena  can  attract 
general  attention.  What  were  the  regions  touched  on  by  the  periodic  Jaw  ? 
This  is  what  we  shall  now  consider. 

The  most  important  point  to  notice  is,  that  periodic  functions,  used  for 
the  purpose  of  expressing  changes  which  are  dependent  on  variations  of  time 
ftnd  space,  have  been  long  known.  They  are  familiar  to  the  mind  when  we 
have  to  deal  with  motion  in  closed  cycles,  or  with  any  kind  of  deviation  from 

8  To  judge  from  J.  A.  R.  Newlands's  work,  On  the  Discovery  of  the  Periodic  Law, 
London,  3884,  p.  149;  'Qo  the  Law  of  Octaves'  (from  the  Chemical  News,  12,  88, 
August  18, 1965). 


476  PRINCIPLES  OF  CHEMISTRY 

a  stable  position,  snch  as  occurs  in  pendulum-oscillations.  A  like  periodic 
function  became  evident  in  the  case  of  the  elements,  depending  on  the  mass 
of  the  atom.  The  primary  conception  of  the  masses  of  bodies,  or  of  the  masses 
of  atoms,  belongs  to  a  category  which  the  present  state  of  science  forbids  us 
to  discuss,  because  as  yet  we  have  no  means  of  dissecting  or  analysing  the 
conception.  All  that  was  known  of  functions  dependent  on  masses  derived 
its  origin  from  Galileo  and  Newton,  and  indicated  that  such  functions 
either  decrease  or  increase  with  the  increase  of  mass,  like  the  attraction  of 
celestial  bodies.  The  numerical  expression  of  the  phenomena  was  always 
found  to  be  proportional  to  'the  mass,  and  in  no  case  was  an  increase  of  mass 
followed  by  a  recurrence  of  properties  such  as  is  disclosed  by  the  periodic  law 
of  the  elements.  This  constituted  such  a  novelty  in  the  study  of  the  phenomena 
of  nature  that,  although  it  did  not  lift  the  veil  which  conceals  the  true  concep- 
tion of  mass,  it  nevertheless  indicated  that  the  explanation  of  that  conception 
must  be  searched  for  in  the  masses  of  the  atoms  ;  the  more  so,  as  all  masses 
are  nothing  but  aggregations,  or  additions,  of  chemical  atoms  which  would  be 
best  described  as  chemical  individuals.  Let  me  remark,  by  the  way,  that 
though  the  Latin  word  '  individual '  is  merely  a  translation  of  the  Greek  word 
'  atom,'  nevertheless  history  and  custom  have  drawn  a  sharp  distinction 
between  the  two  words,  and  the  present  chemical  conception  of  atoms  is 
nearer  to  that  defined  by  the  Latin  word  than  by  the  Greek,  although  this 
latter  also  has  acquired  a  special  meaning  which  was  unknown  to  the  classics. 
The  periodic  law  has  shown  that  our  chemical  individuals  display  a  harmonic 
periodicity  of  properties  dependent  on  their  masses.  Now  natural  science 
has  long  been  accustomed  to  deal  with  periodicities  observed  in  nature,  to 
seize  them  with  the  vice  of  mathematical  analysis,  to  submit  them  to  the 
rasp  of  experiment.  And  these  instruments' of  scientific  thought  would, 
surely,  long  since,  have  mastered  the  problem  connected  with  the  chemical 
elements,  were  it  not  for  a  new  feature  which  was  brought  to  light  by  the 
!  periodic  law,  and  which  gave  a  peculiar  and  original  character  to  the  periodic 
function. 

If  we  mark  on  an  axis  of  absciss®  a  series  of  lengths  proportional  to 
angles,  and  trace  ordinates  which  are  proportional  to  sines  or  other  trigono- 
metrical functions,  w«  get  periodic  curves  of  a  harmonic  character.  So  it 
might  seem,  at  first  sight,  that  with  the  increase,  of  atomic  weights  the  funct 
tion  of  the  properties  of  .the  elements  should  also  vary  in  the  same  harmonious 
way.  But  in  this  case  there  is  no  such  continuous  change  as  in  the  curves 
just  referred  to,  because  the  periods  do  not  contain  the  infinite  number  of 
,  points  constituting  a  curve,  but  a  finite  number  only  of  such  points.  An 
]  example  will  .better  illustrate  this  view.  The  atomic  weights— 

Ag  =  108        Cd  =  J12        In  =113        Sn  =  118        Sb  =  120 

Te  =  125         I  =  127 

steadily  'increase,  and  their  increase  is  accompanied  by  a  modification  of 
many  properties  which  constitutes  the  essence  of  the  periodic  law.  Thaa, 
for  example,  the  densities  of  the  above  elements  decrease  steadily,  being 
respectively — 

10-5  8-G  7-4  7-2  6-7  6'4  4'9 


APPENDIX  II.  477 

while  their  oxides  contain  an  increasing  quantity  of  oxygen-* 

Ag2O        Cdj>02         In2O3         Siifc04         Sb20a         Te^,,        1,0, 

But  to  connect  by  a  curve  the  summits  of  the  ordinates  expressing  any 
fcf  these  properties  would  involve  the  rejection  of  Dalton's  law  of  multiple 
proportions.  Not  only  are  there  no  intermediate  elements  between  silver, 
which  gives  AgCl,  and  cadmium,  which  gives  CdCl.,,  but,  according  to  the 
very  essence  of  the  periodic  law,  there  can  be  none  ;  in  fact  a  uniform  curve 
would  be  inapplicable  in  such  a  case,  as  it  would  lead  us  to  expect  elements 
possessed  of  special  properties  at  any  point  of  the  curve.  The  periods  of  the 
elements  have  thus  a  character  very  different  from  those  which  are  so  simply 
represented  by  geometers.  They  correspond  to  points,  to  numbers,  to  sudden 
changes  of  the  masses,  and  not  to  a  continuous  evolution.  In  these  sudden 
changes  destitute  of  intermediate  steps  or  positions,  in  the  absence  of 
elements  intermediate  between,  say,  silver  and  cadmium,  or  aluminium 
and  silicon,  we  must  recognise  a  problem  to  which  no  direct  application 
of  the  analysis  of  the  infinitely  small  can  be  made.  Therefore,  neither  the 
trigonometrical  functions  proposed  by  Eidberg  and  Flavitzky,  nor  the  pen- 
dulum-oscillations  suggested  by  Crookes,  nor  the  cubical  curves  of  the  Eev. 
Mr.  Haughton,  which  have  been  proposed  for  expressing  the  periodic  law, 
from  the  nature  of  the  case,  can  represent  the  periods  of  the  chemical 
elements.  If  geometrical  analysis  is  to  be  applied  to  this  subject,  it  will  re» 
quire  to  be  modified  in  a  special  manner.  It  must  find  the  means  of  repre- 
senting in  a  special  way,  not  only  such  long  periods  as  that  comprising 

K    Ca    Sc    Ti    V    Cr    Mn    Fe   Co    Ni    Cu    Zn    Ga    Ge    As    Se    Br, 

but  short  periods  like  the  following  :— 

Na        Mg        Al        Si        P        S        Cl. 

In  the  theory  of  numbers  only  do  we  find  problems  analogous  to  ours, 
and  two  attempts  at  expressing  the  atomic  weights  of  the  elements  by  alge- 
braic  formula*  seem  to  b6  deserving  of  attention,  although  neither  of  them 
can  be  considered  as  a  complete  theory,  nor  as  promising  finally  to  solve  the 
problem  of  the  periodic  law.  The  attempt  of  E.  J.  Mills  (1886)  does  not 
even  aspire  to  attain  this  end.  He  considers  that  all' atomic  weights  can  be 
expressed  by  a  logarithmic  function, 

15(71-0-93750, 

in  which  the  variables  n  and  t  are  whole  numbers.  Thus,  for  oxygen,  n  =  2, 
and  t  =  1,  whence  its  atomic  weight  is  =  15'94 ;  in  the  case  of  chlorine, 
bromine,  and  iodine,  n  has  .respective  values  of  3,  6,  and  9,  whilst  t  =  7,  6, 
and  9  ;  in  the  case  of  potassium,  rubidium,  and  caesium,  n  =  4,  6,  and  9,  and 
t  =  14,  18,  and  20. 

Another  attempt  was  made  in  1888  by  B.  N.  Tchitche'rin.  Its  author 
places  the  problem  of  the  periodic  law  in  the  first  rank,  but  as  yet  he  has 
investigated  the  alkali  metals  only  Tchitche'rin  first  noticed  the  simple 


478  PRINCIPLES  OF  CHEMISTRY 

relations  existing  between  the  atomic  volumes  of  all  alkali  metals ;   they 
can  be  expressed,  according  to  his  views,  by  the  formula 
A(2-  0-00535  Aw), 

where  A  is  the  atomic  weight,  and  n  is  equal  to  8  for  lithium  and  sodium,  to 
4  for  potassium,  to  3  for  rubidium,  and  to  2  for  ceesiuin.  If  n  remained  equal 
to  8  during  the  increase  of  A,  the  volume  would  become  zero  at  A  =  43jJ, 
and  it  would  reach  its  maximum  at  A  =-  23£.  The  close  approximation  of 
the  number  46§  to  the  differences  between  the  atomic  weights  of  analogous 
elements  (such  as  Cs  —  Rb,  I  —  Br,  and  so  on) ;  the  close  correspondence  of 
the  number  23£  to  the  atomic  weight  of  sodium ;  the  fact  of  n  being  neces- 
sarily a  whole  number,  and  several  other  aspects  of  the  question,  induce 
Tchitche'rin  to  believe  that  they  afford  a  clue  to  the  understanding  of  the 
nature  cf  the  elements ;  we  must,  however,  await  the  full  development  of 
his  theory  before  pronouncing  judgment  on  it.  What  we  can  at  present  only 
be  certain  of  is  this :  that  attempts  like  the  two  above  named  must  be  re- 
peated and  multiplied,  because  the  periodic  law  has  clearly  shown  that  the 
masses  of  the  atoms  increase  abruptly,  by  steps,  which  are  clearly  connected 
in  some  way  with  Dalton's  law  of  multiple  proportions ;  and  because  the 
periodicity  of  the  elements  finds  expression  in  the  transition  from  RX  to 
RXj,  RX3,  RX4,  and  so  on  till  RX.,,  at  which  point,  the  energy  of  the  com- 
bining forces  being  exhausted,  the  series  begins  anew  from  RX  to  RX2,  and 
so  on. 

While  connecting  by  new  bonds  the  theory  of  the  chemical  elements  with 
Dalton's  theory  of  multiple  proportions,  or  atomic  structure  of  bodies,  the 
periodic  law  opened  for  natural  philosophy  a  new  and  wide  field  for  specula- 
tion. Kant  said  that  there  are  in  the  world  '  two  things  which  never  cease 
to  call  for  the  admiration  and  reverence  of  man :  the"  moral  law  within 
ourselves,  and  the  stellar  sky  above  us.'  But  when  we  turn  our  thoughts 
towards  the  nature  of  the  elements  and  the  periodic  law,  we  must  add  a  third 
subject,  namely, '  the  nature  of  the  elementary  individuals  which  we  discover 
everywhere  around  us.'  Without  them  the  stellar  sky  itself  is  inconceiv- 
able ;  and  in  the  atoms  we  see  at  once  their  peculiar  individualities,  the  in- 
finite multiplicity  of  the  individuals,  and  the  submission  of  their  seeming 
freedom  to  the  general  harmony  of  Nature. 

Having  thus  indicated  a  new  mystery  of  Nature,  which  does  not  yet  yield 
to  rational  conception,  the  periodic  law.  together  with  the  revelations  of 
spectrum  analysis,  have  contributed  to  again  revive  an  old  but  remarkably 
long-lived  hope— that  of  discovering,  if  not  by  experiment,  at  least  by  a 
mental  effort,  the  primary  matter — which  had  its  genesis  in  the  minds  of 
the  Grecian  philosophers,  and  has  been  transmitted,  together  with  many 
other  ideas  of  the  classic  period,  to  the  heirs  of  their  civilisation.  Having 
grown,  during  the  times  of  the  alchemists  up  to  the  period  when  experimental 
proof  was  requh  ed,  the  idea  has  rendered  good  service  ;  it  induced  those 
•careful  observations  and  experiments  which  later  on  called  into  being  the 
works  of  Scheele,  Lavoisier,  Priestley,  and  Cavendish.  It  then  slumbered 
awhile,  but  was  soon  awakened  by  the  attempts  either  to  confirm  or  to  refute 
the  ideas  of  Prout  as  to  the  multiple  proportion  relationship  of  the  atomic 


APPENDIX.  IL  479 

weights  of  all  the  elements.  And  once  again  the  inductive  or  experimental 
method  of  studying  Nature  gained  a  direct  advantage  from  the  old  Pytha- 
gorean idea :  becaxise  atomic  weights  were  determined  with  an  accuracy 
formerly  unknown.  But  again  the  idea  could  not  stand  the  ordeal  of  expert- 
mental  test,  yet  the  prejudice  remains  and  has  not  been  uprooted,  even  by 
'Stas ;  nay,  it  has  gained  a  new  vigour,  for  we  see  that  all  which  is  imperfectly 
worked  out,  new  and  unexplained,  from  the  still  scarcely  studied  rare  rnetala 
to  the  hardly  perceptible  nebulse,  have  been  used  to  justify  it.  As  eoon  as 
8pectrum  analysis  appears  as  a  new  and  powerful  weapon  of  chemistry,  the 
idea  of  a  primary  matter  is  immediately  attached  to  it.  From  all  sides  we 
see 'attempts  to  constitute -the  imaginary  substance  helium3  the.  so  much 
longed  for  primary  matter..  No  attention  is  paid  to  the  circumstance  that 
the  helium  line  is  only  seen  in  the  spectrum  of  the  solar  protuberances,  so 
that  its  universality  in  Nature  remains  as  problematic  as  the  primary  matter 
itself ;  nor  to  the  fact  that  the  helium  line  is  wanting  amongst  the  Fraun- 
hofer  lines  of  the  solar  spectrum,  and  thus  does  not  answer  to  the  brilliant 
fundamental  conception  which  gives  its  real  force  to  spectrum  analysis. 

And  finally,  no  notice  is  even  taken  of  the  indubitable  fact  that  the  bril* 
liancies  of  the  spectral  lines  of  the  simple  substances  vary  under  different  tem- 
peratures and  pressures ;  so  that  all  probabilities  are  in  favour  of  the  helium 
line  simply  belonging  to  some  long  since  known  element  placed  under  such 
conditions  of  temperature,  pressure,  and  gravity  as  have  not  yet  been  realised 
in  our  experiments.  Again,  the  idea  that  the  excellent  investigations  of 
Lockyer  of  the  spectrum  o£iron  can  be  interpreted  in  favour  of  the  compound 
nature  of  that  element,  evidently,  must  have  arisen  from  some  misunder- 
standing. The  spectrum  of  a  compound  certainly  does  not  appear  as  a 
sum  of  the  spectra  of  its  components ;  and  therefore  the  observations  of 
Lockyer  can  be  considered  precisely  as  a  proof  that  iron  undergoes  no  other 
changes  at  the  temperature  of  the  sun  than  those  which  it  experiences  in  the 
•voltaic  arc— provided  the  spectrum  of  iron  is  preserved.  As  to  the  shifting 
of  some  of  the  lines  of  the  spectrum  of  iron  while  the  other  lines  maintain 
their  positions,  it  can  be  explained,  as  shown  by  M.  Kleiber  ('  Journal  of  the 
Russian  Chemical  and  Physical  Society,  1885, 147),  by  the  relative  motion 
of  the  various  strata  of  the  sun's  'atmosphere,  and  by  Zollner's  laws  of  the 
relative  brilliancies  of  different  lines  of  the  spectrum.  Moreover,  it  ought 
not  to  be  forgotten  that  if  iron  were  really  proved  to  consist  of  two  or  more, 
unknown  elements,  we  should  simply  have  an  increase  in  the  number  of  our 
elements— not  a  reduction,  and  still  less  a  reduction  of  all  of  them  to  one 
single  primary  matter. 

Feelipg  that  spectrum  analysis  will  not  yield  a  support  to  the  Pythagorean 
conception,  its.  modern  promoters  are  so  bent  upon  its  being  confirmed  by 
the  periodic  law,  that  the  illustrious  Berthelot,  in  his  work  '  Les  origines  de 
1'Alchimie,'  1885,  313,  has  simply  mixed  up  the  fundamental  idea  of  the  law 
of  periodicity  with  the  ideas  of  Prout,  the  alchemists,  and  Democritus  about 
primary  matter,4  But  the  periodic  law,  based  as  it  is  on  the  solid  and  whole- 

3  That  is,  a  substance  having  a  wave-length  equal  to  0-0005875  millimetre. 

4  He  maintains  (on  p.  809)  that  the  periodic  law   requires  two  new  analogous 
elements,  having  atomic  weights  oi  48  and  64,  occupying  positions  between  sulphur 


480  PRINCIPLES  OF  CHEMISTRY 

some  ground  of  experimental  research,  has  been  evolved  independently  of 
any  conception  as  to  the  nature  of  the  elements ;  it  does  not  in  the  least 
driginate  in  the  idea  of  a  unique  matter ;  and  it  has  no  historical  connec- 
tion with  that  relic  of  the  torments  of  classical  thought,  and  therefore  it 
affords  no  more  indication  of  the  unity  of  matter  or  of  the  compound  character 
of  our  elements,  than  the  law  of  Avogadro,  or  the  law  of  specific  heats,  or 
even  the  conclusions  of  spectrum  analysis.  None  of  the  advocates  of  a 
unique  matter  have  ever  tried  to  explain  the  law  from  the  standpoint  of  ideas 
taken  from  a  remote  antiquity  when  it  was  found  convenient  to  admit  the 
existence  of  nianj  gods — and  of  a  unique  matter. 

When  we  try  to  explain  the  origin  of  the  idea  of  a  unique  primary 
matter,  we  easily  trace  that  in  the  absence  of  inductions  from  experiment  it 
derives  its  origin  from  the  scientifically  philosophical  attempt  at  discovering 
some  kind  of  unity  in  the  immense  diversity  of  individualities  which  we  see 
around.  In  classical  times  such  a  tendency  could  only  be  satisfied  by  con- 
ceptions about  the  immaterial  world.  As  to  the  material  world,  our  ancestors 
were  compelled  to  resort  to  some  hypothesis,  and  they  adopted  the  idea  of 
unity  in  the  formative  material,  because  they  were  not  able  to  evolve  tho 
conception  of  any  other  possible  unity  in  order  to  connect  the  multifarious 
relations  of  matter.  Responding  to  the  same  legitimate  scientific  tendency, 
natural  science  has  discovered  throughout  the  universe  a  unity  of  plan,  a 
unity  of  forces,  and  a  unity  of  matter,  and  the  convincing  conclusions  of 
modern  science  compel  every  one  to  admit  these  kinds  of  unity.  But  while 
we  admit  unity  in  many  things,  we  none  the  less  must  also  explain  the 
individuality  and  the  apparent  diversity  which  we  cannot  fail  to  trace  every- 
where. It  has  been  said  of  old,  '  Give  us  a  fulcrum,  and  it  will  become  easy  to 
displace  the  earth.'  So  also  we  must  say,  '  Give  us  something  that  is  individu- 
alised, and  the  apparent  diversity  will  be  easily  understood.'  Otherwise,  how 
could  unity  result  in  a  multitude  ? 

After  a  long  and  painstaking  research,  natural  science  has  discovered  the 
individualities  of  the  chemical  elements,  and  therefore  it  is  now  capable  not 
only  of  analysing,  but  also  of  synthesising ;  it  can  understand  and  grasp 
generality  and  unity,  as  well  as  the  individualised  and  the  multifarious. 
The  general  and  universal,  like  time  and  space,  like  force  and  motion,  vary  uni- 
formly ;  the  uniform  admit  of  interpolations,  revealing  every  intermediate 
phase.  But  the  multitudinous,  the  individualised — such  as  ourselves,  or  the 
chemical  elements,  or  the  members  of  a  peculiar  periodic  function  of  the 
elements,  or  Dalton's  multiple  proportions — is  characterised  in  another 
way:  We  see  in  it,  side  by  side  with  a  connecting  general  principle,  leaps, 
breaks  of  continuity,  points  which  escape  from  the  analysis  of  the  infinitely 
small — an  absence  of  complete  intermediate  links.  Chemistry  has  found  an 
answer  to  the  question  as  to  the  causes  of  multitudes ;  and  while  retaining 
the  conception  of  many  elements,  all  submitted  to  the  discipline  of  a  general 
law,  it  offers  an  esc.ape  from  the  Indian  Nirvana — the.  absorption  in  the 
universal,  replacing  it  by  the  individualised.  However,  the  place  for  indi- 

and  selenium,  although  nothing  of  the  kind  results  from  any  of  the  different  readings  of 
the  law. 


APPENDIX  IL  481 

viduality  is  so  limited  by  the  all-grasping,  all-powerful'  universal,  that  it.  is 
merely  a  point  of  support  for  the  understanding  of  multitude  in  unity. 

Having  touched  upon  the  metaphysical  bases  of  the  conception  of  a 
unique  matter  which  is  supposed  to  enter  into  the  composition  of  all  bodies 
I  think  it  necessary  to  dwell  upon  another  theory,  akin  to  the  above  concep- 
tion—the theory  of  the  compound  character  of  the  elements  now  admitted  by 
some  —  and  especially  upon  one  particular  circumstance  which,  being  related 
to  tfie  periodic  law,  is  considered  to  be  an  argument  in  favour  of  that  hypo- 
thesis. 

Dr.  Pelopidas,  in  1883,  made  a  communication  to  the  Russian  Chemical 
and  Physical  Society  on  the  periodicity  of  the  hydrocarbon  radicles,  pointing 
out  the  remarkable  parallelism  which  was  to  be  noticed  in  the  change  of 
properties  of  hydrocarbon  radicles  and  elements  when  classed  in  groups. 
Professor  Carnelley,  in  1886,  developed  a  similar  parallelism.  The  idea  of 
M.  Pelopidas  will  be  easily  understood  if  we  consider  the  series  of  hydro- 
carbon radicles  which  contain,  say,  6  atoms  of  carbon  :— 

I.  II.  III.  IV.          V.  VI.         VII,  VIII. 

CflHl9       C6H12       C6HU        C6H1Q       C6H9        C6H8        06H7 


The  first  of  these  radicles,  like  the  elements  of  the  1st  group,  combines  with 
Cl,  OH,  and  so  on,  and  gives  the  derivatives  of  hexyl  alcohol,  C6H13(OH)  ; 
but,  in  proportion  as  the  number  of  hydrogen  atoms  decreases,  the  capacity 
of  the  radicles  of  combining  with,  say,  the  halogens  increases.  C6H12  already 
combines,  with  2  atoms  of  chlorine  ;  C6HU  with  3  atoms,  and  so  on.  The 
last  members  of  the  series  comprise  the  radicles  of  aoids  :  thus  CeHg,  which 
belongs  to  the  6th  group,  gives,  like  sulphur,  a  bibasic  acid,  C6H802(OH)2, 
which  is  homologous  with  oxalic  acid.  The  parallelism  can  be  traced  still 
further,  because  C6H5  appears  as  a  monovalent  radicle  of  benzene,-  and  with 
it  begins  a  new  series  of  aromatic  derivatives,  so  analogous  to  the  derivatives  of 
the  aliphatic  series.  Let  me  also  mention  another  example  from  among  those 
which  have  been  given  by  M.  Pelopidas.  Starting  from  the  alkaline  radicle 
of  monomethylammonium,  N(CH3)H3,-  or  -NCH6,  which  presents  many 
analogies  with  the  alkaline  metals  of  the  1st  group,  he  arrives,  by  successively 
diminishing  the  number  of  the  atoms  of  hydrogen,  at  a  7th  group  which 
contains  cyanogen,  ON,  which  .has  long  since  been  compared  to  the  halogens 
of  the  7th  group. 

The  most  important  consequence  which,  in  my  opinion,  can  be  drawn 
from  the  above  comparison  is  that  the  periodic  law,  so  apparent  in  the 
elements,  has  a  wider,  application  than  might  appear  at  first  sight  ;  it  opens 
up  a  new  vista  of  chemical  evolutions.  But,  while  admitting  the  fullest 
parallelism  between  the  periodicity  of  the  elements  and  that  of  the  compound 
radicles,  we  must  not  forget  that  in  the  periods  of  the  hydrocarbon  radicles 
we  have  a  decrease  of  mass  as  we  pass  from  the  representatives  of  the  first 
group  to  the'  next,  while  in  the  periods  of  the  elements  the  mass  increases 
during  the  progression.  It  thus  becomes  evident  that  we  cannot  speak  of  an 
identity  of  periodicity  iri  both  cases,  unless  we  put  aside  the  ideas  of  mass 
and  attraction,  which  are  the  real  corner-stones  of  the  whole  of  natural 
science,  and  «ven  enter  intothose  very  conceptions  of  simple  substances  which 


482  PRINCIPLES  OF  CHEMISTRY 

came  to  light  a  fuU  hundred  years  later  than  the  immortal  principles  of 
Newton.0 

From  the  foregoing,  as  well  as  from  the  failures  of  so  many  attempts  at 
finding  in  experiment  and  speculation  a  proof  of  the  compound  character  of 
the  elements  and  of  the  existence  of  primordial  matter,  it  is  evident,  in  my 
opinion,  that  this  theory  must  be  classed  among  mere  Utopias.  But  Utopias 
can  only  be  combated  by  freedom  of  opinion,  by  experiment,  and  by  new 
Utopias.  In  the  republic  of  scientific  theories  freedom  of  opinions  is  guaran- 
teed. It  is  precisely  that  freedom  which  permits  me  to  criticise  openly  the 
widely-diffused  idea  as  to  the  unity  of  matter  in  the  elements.  Experiments 
and  attempts  at  confirming  that  idea  have  been  so  numerous  that  it  really 
would  be  instructive  to  have  them  all  collected  together,  if  only  to  serve  as  a 
warning  against  the  repetition  of  old  failures.  And  now  as  to  new  Utopias 
which  may  bo  helpful  in  the  struggle  against  the  old  ones,  I  do  not  think  it 
quite  useless  to  mention  &  phantasy  of  one  of  my  students  who  imagined  that 
the  weight  of  bodies  does  not  depend  upon  their  mass,  but  upon  the  character 
of  the  motion  of  their  atoms.  The  atoms,  according  to  this  new  Utopian,  may 
all  be  homogeneous  or  heterogeneous,  we  know  not  which ;  we  know  them 
in  motion  only,  and  that  motion  they  maintain  with  the  same  persistence  as 
the  stellar  bodies  maintain  theirs.  The  weights  of  atoms  differ  only  in  con- 
sequence of  their  various  modes  and  quantity  of  motion  ;  the  heaviest  atoms 
may  be  much  simpler  than  the  lighter  ones :  thus  an  atom  of  mercury  may 
be  simpler  than  an  atom  of  hydrogen— the  manner  in  which  it  moves  causes 
it  to  be  heavier.  My  interlocutor  even  suggested  that  the  view  which 
attributes  the  greater  complexity  to  the  lighter  elements  finds  confirmation 
in  the  fact  that  the  hydrocarbon  radicles  mentioned  by  Pelopidas,  while 
becoming  lighter  as  they  lose  hydrogen,  change  their  properties  periodically 
in  the  same  manner  as  the  elements  change  theirs,  according  as  the  atoms 
grow  heavier. 

The  French  proverb,  La  critique  eat  facile,  mais  Vart  est  difficile,  how- 
ever, may  well  be  reversed  in  the  case  of  all  such  ideal  views,  as  it  is  much 
easier  to  formulate  than  to  criticise  them.  Arising  from  the  virgin  soil  of 
newly-established  facts,  the  knowledge  relating  to  the  elements,  to  their 
masses,  and  to  the  periodic  changes  of  their  properties  has  given  a  motive 
for  the  formation  of  Utopian  hypotheses,  probably  because  they  could  not  be 
foreseen  by  the  aid  of  any  of  the  various  metaphysical  systems,  and  exist, 
like  the  idea  of  gravitation,  as  an  independent  outcome  of  natural  science, 
requiring  the  acknowledgment  of  general  laws,  when  these  have  been  estab- 
lished with  the  same  degree  of  persistency  as  is  indispensable  for  the  accept- 
ance of  a  thoroughly  established  fact.  Two  centuries  have  elapsed  since  the 
theory  of  gravitation  was  enunciated,  and  although  we  do  not  understand  its 
cause,  we  still  must  regard  gravitation  as  a  fundamental  conception  of  natural 
philosophy,  a  conception  which  has  enabled  us  to  perceive  much  more  than 
the  metaphysicians  did  or  could  with  their  seeming  omniscience.  A  hundred 

*  It  is  noteworthy  that  the  year  in  which  Lavoisier  was  born  (1748)— the  author  of 
the  idea  of  elements  and  of  the  indestructibility  of  matter— is  later  by  exactly  one 
century  than  the  year  in  which  the  author  of  the  theory  of  gravitation  and  mass  was  born 
/1643  N.S.).  The  affiliation  of  the  ideas  of  Lavoisier  and  those  of  Newton  is  beyond  doubt. 


APPENDIX  II.  483 

years  later  the  conception  of  the  elements  arose ;  it  made  chemistry  what  it 
now  is ;  and  yet  we  have  advanced  as  little  in  our  comprehension  of  simple 
substances  since  the  times  of  Lavoisier  and  Dalton  as  we  have  in  our  und^r- 
standing  of  gravitation.  The  periodic  law  of  the  elements  is  only  twenty 
years  old;  it  is  not  surprising,  therefore,  that,  knowing  nothing  about  the- 
onuses  of  gravitation  and  mass,  or  about  the  nature  of  the  elements,  we  do 
not  comprehend  the  rationale  of  the  periodic  law.  It  is  only  by  collecting 
established  laws — that  is,  by  working  at  the  acquirement  of  truth — that  \\  o 
can  hope  gradually  to  lift  the  veil  which  conceals  from  us  the  causes  of  the 
mysteries  of  Nature  and  to  discover  their  mutual  dependency.  Like  the 
telescope  and  the  microscope,  laws  founded  on  the  basis  of  experiment  are 
the  instruments  and  means  of  enlarging  our  mental  horizon. 

In  the  remaining  part  of  my  communication  I  shall  endeavour  to  show, 
and  as  briefly  as  possible,  in  how  far  the  periodic  law  contributes  to  enlarge 
our  range  of  vision.  Before  the  promulgation  of  this  law  the  chemical 
elements  were  mere  fragmentary,  incidental  facts  in  Nature ;  there  was  no 
special  reason  to  expect  the  discovery  of  new  elements,  and  the  new  ones 
•which  were  discovered  from  time  to  time  appeared  to  be  possessed  of  quite 
novel  properties.  The  law  of  periodicity  first  enabled  us  to  perceive  undis- 
covered elements  at  a  distance  which  formerly  was  inaccessible  to  chemical 
vision ;  and  long  ere  they  were  discovered  new  elements  appeared  before  oui- 
eyes  possessed  of  a  number  of  well-defined  properties.  We  now  know  threo 
cases  of  elements  whose  existence  and  properties  were  foreseen  by  the  instru- 
mentality of  the  periodic  law.  I  need  but  mention  the  brilliant  discovery  of 
gallium,  which  proved  to  correspond  to  eka-aluminium  of  the  periodic  law,  by 
Lecoq  de  Boisbaudran ;  of  scandium,  corresponding  to  ekaboron,  by  Nilson  ; 
and  of  germanium,  which  proved  to  correspond  in  all  respects  to  ekasilicoi% 
by  Winkler.  "When,  in  1871, 1  described  to  the  Bussian  Chemical  Society 
the  properties,  clearly  defined  by  the  periodic  law,  which  such  elements 
ought  to  possess,  I  never  hoped  that  I  should  live  to  mention  their  discovery 
to  the  Chemical  Society  of  Great  Britain  as  a  confirmation  of  the  exactitude 
and  the  generality  of  the  periodic  law.  Now  that  I  have  had  the  happiness 
of  doing  so,  I  unhesitatingly  say  that,  although  greatly  enlarging  our  vision, 
even  now  the  periodic  law  needs  further  improvements  in  order  that  it  may 
become  a  trustworthy  instrument  in  further  discoveries.6 

I  will  venture  to  allude  to  some  other  matters  which  chemistry  has  dis- 
cerned by  means  of  its  new  instrument,  and  which  it  could  not  have  made 

6  I  foresee  some  more  new  elements,  but  not  wvith  the  some  certitude  as  before.  I 
shall  give  one  example,  and  yet  I  do  not  see  it  quite  distinctly.  In  the  series  which  con- 
tains Hg=204,  Pb=»206,  and  Bi  =  208,  we  can  imagine  the  existence  (at  the  place  VI— 11) 
Of  an  element  analogous  to  tellurium,  which  we  can  describe  as  dvi-tellurium,  Dt,  having 
an  atomic  weight  of  212,  and  the  property  of  forming  the  oxide  DtOj.  If  this  element 
.really  exists,  it  ought  in  the  free  state  to  be  an  easily  fusible,  crystalline,  non-volatile 
metal  of  a  grey  colour,  having  a  density  of  about  9'S,  capable  of  giving  a  dioxide,  Dt02, 
Squally  endowed  with  feeble  acid  and' basic  properties.  This  dioxide  must  give  on  active 
Oxidation  an  unstable  higher  oxide,  DtOs,  which  should  resemble  in  its  pro'perties  PbOj 
and  Bi2O6.  Dvi-tellurium  hydride,  if  it  be  found  to  exist,  will  be  a  less  stable  compound 
than  even  H2Te.  The  compounds  of  dvi-tellurium  will  be  easily  reduced,  and  it  will  form 
oIiaracteriBtio  definite  alloya  with  other  metals. 


484  PRINCIPLES   OF  CHEMISTRY 

out  without  a  knowledge  of  the  law  of  periodicity,  and  I  will  confine  myself 
to  simple  substances  and  to  oxides. 

Before  the  periodic  law  was  formulated  the  atomic  weights  of  the  elements 
were  purely  empirical  numbers,  so  that  the  magnitude  of  the  equivalent,  and 
the  atomicity,  or  the  value  in  substitution  possessed  by  an  atom,  could  only 
be  tested  by  critically  examining  the  methods  of  determination,  but  never 
directly  by  considering  the  numerical  values  themselves  ;  in  short,  we  were 
compelled  to  move  in  the  dark,  to  submit  to  the  facts,  instead  of  being  masters 
of  them.  I  need  not  recount  the  methods  which  permitted  the  periodic  law 
at  last  to  master  the  facts  relating  to  atomic  weights,  and  I  would  merely 
call  to  mind  that  it  compelled  us  to  modify  the  valencies  of  indium  and 
cerium,  and  to  assign  to  their  compounds  a  different  molecular  composition. 
Determinations  of  the  specific  heats  of  these  two  metals  fully  confirmed  the 
change.  The  trivalency  of  yttrium,  which  makes  us  now  represent  its  oxide 
as  Y203  instead  of  as  YO,  was  also  foreseen  (hi  1870)  by  the  periodic  law,  and 
it  has  now  become  so  probable  that  Cleve,  and  all  other  subsequent  investi- 
gators of  the  rare  metals,  have  not  only  adopted  it,  but  have  also  applied  ifc 
without  any  new  demonstration  to  substances  so  imperfectly  known  as  those. 
of  the  cerite  and  gadolinite  group,  especially  since  Hillebrand  determined  the 
specific  heats  of  lanthanum  and  didymium  and  confirmed  the  expectations 
suggested  by  the  periodic  law.  But  here,  especially  in  the  case  of  didymium, 
we  meet  with  a  series  of  difficulties  long  since  foreseen  through  the  periodic 
law,  but  only  now  becoming  evident,  and  chiefly  arising  from  the  relative 
rarity  and  insufficient  knowledge  of  the  elements  which  usually  accompany 
didymium. 

Passing  to  the  results  obtained  in  the  case  of  the  rare  elements  beryllium, 
scandium,  and  thorium,  it  is  found  that  these  have  many  points  of  contact 
with  the  periodic  law.  Although  Avde"eff  long  since  proposed  the  magnesia 
formula  to  represent  beryllium  oxide,  yet  there  was  so  much  to  be  said  in 
favour  of  the  alumina  formula,  on  account  of  the  specific  heat  of  the  metals 
and  the  isomorphism  of  the  two  oxides,  that  it  became  generally  adopted 
and  seemed  to  be  well  established.  The  periodic  law,  however,  as  Brauner 
repeatedly  insisted  ('Berichte,'  1878,872;  1881,  53),  was  against  the  formula 
Be203 ;  it  required  the  magnesia  formula  BeO — that  is,  an  atomic  weight 
of  9 — because  there  was  no  place  in  the  system  for  an  element  like  beryllium 
having  an  atomic  weight  of  13'5.  This  divergence  of  opinion  lasted  for 
years,  and  I  often  heard  that  the  question  as  to  the  atomic  weight  of  beryllium 
threatened  to  disturb  the  generality  of  the  periodic  law,  or,  at  any  rate,  to 
require  some  important  modifications  of  it.  Many  forces  were  operating  in 
the  controversy  regarding  beryllium,  evidently  because  a  much  more  im- 
portant question  was  at  issue  than  merely  that  involved  in  the  discussion  of 
the  atomic  weight  of  a  relatively  rare  element :  and  during  the  controversy 
the  periodic  law  became  better  understood,  and  the  mutual  relations  of  the 
elements  became  more  apparent  than  ever  before.  It  is  most  remarkable  that 
the  victory  of  the  periodic  law  was  won  by  the  researches  of  the  very  observers 
who  previously  had  discovered  a  number  of  facts  in  support  of  the  tri- 
valency of  beryllium.  Applying  the  higher  law  of  Avogadro,  Nilson  and 
Petterson  have  finally  shown  that  the  density  of  the  vapour  of  the  beryl- 


APPENDIX  II.  485 

Hum  chloridej/'BeClj,  obliges  us  to  regard  beryllium  as  bivalent  in 
conformity  with  the  periodic  law.7  I  consider  the  confirmation  of  Avdeeff's 
and  Brauner's  view  as  important  in  the  history  of  the  periodic  law  as  the 
discovery  of  scandium,  which,  in  Nilson's  hands,  confirmed  the  existence  of 
ekaboron. 

The  circumstance  that  thorium  proved  to  be  quadrivalent,  and  Th  =  232, 
in  accordance  with  the  views  of  Chydenius  and  the  requirements  of  th4 
periodic  law",  passed  almost  unnoticed,  and  was  accepted  without  opposition, 
and  yet  both  thorium  and  uranium  are  of  great  importance  in  the  periodic 
system,  as  they  are  its  last  members,  and  have  the  highest  atomic  weights  of 
all  the  elements. 

The  alteration  of  the  atomic  weight  of  uranium  from  U  =  120  into  U  =  240 
attracted  more  attention,  the  change  having  been  made  on  account  of  the 
periodic  law,  and  for  no  other  reason.  Now  that  Roscoe,  Bammelsberg, 
Zimmermann,  and  several  others  have  admitted  the  various  claims  of  the 
periodic  law  in  the  case  of  uranium,  its  high  atomic  weight  is  received  .with- 
out objection,  and  it  endows  that  element  with  a  special  interest. 

While  thus  demonstrating  the  necessity  for  modifying  the  atomic  weights 
of  several  insufficiently  known  elements,  the  periodic  law  enabled  us  also  to 
detect  errors  in  the  determination  of  the  atomic  weights  of  several  elements 
whose  valencies  and  true  position  among  other  elements  were  already  well 
known.  Three  such  cases  are  especially  noteworthy :  those  of  tellurium, 
titanium  and  platinum.  Berzelius  had  determined  the  atomic  weight  of 
tellurium  to  be  128,  while  the  periodic  law  claimed  for  it  an  atomic  weight 
below  that  of  iodine,  which  had  been  fixed  by  Stas  at  126-5,  and  which  was 
certainly  not  higher  than  127.  Brauner  then  undertook  the  investigation, 
and  he  has  shown  that  the  true  atomic  weight  of  tellurium  is  lower  than  that 
of  iodine,  being  near  to  125.  For  titaniiim  the  extensive  researches  of 
Thorpe  have  confirmed  the  atomic  weight  of  Ti  =  48,  indicated  by  the  law, 
and  already  foreseen  by  Rose,  but  contradicted  by  the  analyses  of  Pierre  and 
several  other  chemists.  An  equally  brilliant  confirmation  of  the  expectations 
based  on  the  periodic  law  has  been  given  in  the  case  of  the  series  osmium, 
iridium,  platinum,  and  gold.  At  the  time  of  the  promulgation  of  the  periodic 
law,  the  determinations  of  Berzelius,  Rose,  and  many  others  gave  the  follow- 
ing figures : — 

Os  =  200;  Ir  =  197;  Pt  =  198;  Au  =  196. 

7  Let  me  mention  another  proof  of  the  bivalency  of  beryllium  which  may  have  passed 
unnoticed,  as  it  was  only  published  in  the  Russian  chemical  literature.  Having  remarked" 
(ia  1884)  that  the  density  of  such  solutions  of  chlorides  of  metals,  MCln,  as  contain  200 
mols.  of  water  (or  a  large  and  constant  amount  of  water)  regularly  increases  as  the  mole- 
cular weight  of  the  dissolved  salt  increases,  I  proposed  to  one  of  our  young  chemists, 
M.  Burdakoff,  that  he  should  investigate  beryllium  chloride.  If  its  molecule  be  BeCl2 
its  weight  must  be  =  80 ;  and  in  such  a  case  it  must  be  heavier  than  the  molecule  of 
KC1  =  74'5,  and  lighter  than  that  of  MgCl2  =  93.  On  the  contrary,  if  beryllium  chloride  is 
a  trichloride,  BeCl3  =  120,  its  molecule  must  be  heavier  than  that  of  CaCl2  =  lll,  and 
lighter  than  that  of  MnCl2  =  126.  Experiment  has  shown  the  correctness  of  the  former 
formula,  the  solution  BeCl2  +  200H2O  having  (at  15°/4°)  a  density  of  T0138,  this  being  a 
higher  density  than  that  of  the  solution  KC1  +  200H2O  (  =  1-0121),  and  lower  than  that  of 
MgCl2+200H3O  (  =  1-0203).  The  bivalency  of  beryllium  was  thus  confirmed  in  the  case 
both  of  the  dissolved  and  the  vaporised  chloride. 


486  PRINCIPLES  OF  CHEMISTHY 

The  expectations  of  the  periodic  law 8  have  been  confirmed,  first,  by  net? 
determinations  of  the  atomic  weight  of  platinum  (by  Seubert,  Dittmar,  and 
M4  Arthur,  which  proved  to  be  near  to  196  (taking  0 « 16,  as  proposed  by 
Marignac,  Brauner,  and  others)  j  secondly,  by  Seubert  having  proved  that 
the  atomic  weight  of  osmium  is  really  lower  than  that  of  platinum,  being 
near  to  191 ;  and  thirdly,  by  the  investigations  of  Krttss,  Thorpe  and 
Laurie,  proving  that  the  atomic  weight  of  gold  exceeds  that  of  platinum, 
and  approximates  to  197.  The  atomic  weights  which  were  thus  found  to 
require  correction  were  precisely  those  which  the  periodic  law  had  indicated 
as  affected  with  errors;  and  it  has  been  proved,  therefore,  that  the  periodic 
law  affords  a  means  of  testing  experimental  results.  If  we  succeed  in  dis- 
covering the  exact  character  of  the  periodic  relationships  between  the 
increments  in  atomic  weights  of  allied  elements  discussed  by  Bidberg  in 
1885,  and  again  by  Bazaroff  in  1887,  we  may  expect  that  our  instrument 
will  give  us  the  means  of  still  more  closely  controlling  the  experimental  data 
relating  to  atomic  weights. 

Let  me  next  call  to  mind  that,  while  disclosing  the  variation  of  chemical 
properties,9  the  periodic  law  has  also  enabled  us  to  systematically  discuss 
many  of  the  physical  properties  of  elementary  bodies,  and  to  show  that  these 
properties  are  also  subject  to  the  law  of  periodicity.  At  the  Moscow  Congress 
of  Kussian  Naturalists  in  August,  1869,  I  dwelt  upon  the  relations  which 
existed  between  density  and  the  atomic  weight  of  the  elements.  The  follow- 
ing  year  Professor  Lothar  Meyer,  in  his  well-known  paper,10  studied  the 
same  subject  in  more  detail,  and  thus  contributed  to  spread  information, 
about  the  periodic  law.  Later  on,  Camelley,  Laurie,  L.  Meyer,  Eoberts- 
Austen,  and  several  others  applied  the  periodic  system  to  represent  the  order 
in  the  changes  of  the  magnetic  properties  of  the  elements,  their  melting 
points,  the  heats  of  formation  of  their  haloid  compounds,  and  even  of  such 
mechanical  properties  as  the  co-efficient  of  elasticity,  the  breaking  stress,  &c., 
Ac.  These  deductions,  which  have  received  further  support  in  the  discovery 
of  new  elements  endowed  not  only  with  chemical  but  even  with  physical 
properties,  which  were  foreseen  by  the  law  of  periodicity,  are  well  known ; 
ao  I  need  not  dwell  upon  the  subject,  and  may  pass  to  the  consideration  of 
oxides.11 

8  I  pointed  them  out  in  the  LieUg^s  Annal&n,  Supplement  Band.,  viii.  1871,  p.  211. 

9  Thus,  in  the  typical  small  period  of 

Li,  Be,  B,  C,  N,  O,  P, 

we  see  at  once  the  progression  from  the  alkali  metals  to  the  acid  non-metals,  such  ft» 
are  the  halogens. 

1U  Liebig's  Annalen,  Supplement  Band.,  vii.  1870. 

11  A  distinct  periodicity  can  also  be  discovered  in  the  spectra  of  the  elements.  Thus. 
the  researches  of  Hartley,  Ciamician,  and  others  have  disclosed,  first,  the  homology 
of  the  spectra  of  analogous  elements:  secondly,  that  the  alkali  metals  have  simpler 
spectra  than  the  metals  of  the  following  groups ;  and  thirdly,  that  there  is  a  certain  like- 
ness between  the  complicated  spectra  of  manganese  and  iron  on  the  one  hand,  and  thd 
no  less  complicated  spectra  of  chlorine  and  bromine  on  the  other  hand,  and  their  likeness 
corresponds  to  the  degree  of  analogy  between  those  elements  which  is  indicated  by  the 
periodic  law. 


APPENDIX  II.  487 

In  indicating  that  the  gradual  increase  of  the  power  of  elements  of  com- 
tuning  with  oxygen  is  accompanied  by  a  corresponding  decrease  in  their 
power  of  combining  with  hydrogen,  the  periodic  law  has  shown  that  there  is 
a  limit  of  oxidation,  just  as  there  is  a  well-known  limit  to  the  capacity  of 
elements  for  combining  with  hydrogen.  A  single  atom  of  an  element  com- 
bines with  at  most  four  atoms  of  either  hydrogen  or  oxygen  ;  and  while  CH4 
and  SiH4  represent  the  highest  hydrides,  so  Ku04  and  Os04  are  the  highest 
oxides.  We  are  thus  led  to  recognise  types  of  oxides,  just  as  we  have  had  to 
recognise  types  of  hydrides.13 

The  periodic  law  has  demonstrated  that  the  maximum  extent  to  which 
different  non-metals  enter  into  combination  with  oxygen  is  determined  by  the 
extent  to  which  they  combine  with  hydrogen,  and  that  the  sum  of  the  number 
of  equivalents  of  both  must  be  equal  to  8.  Thus  chlorine,  which  combines 
with  1  atom  or  1  equivalent  of  hydrogen,  cannot  fix  more  than  7  equivalents 
of  oxygen,  giving  CljO,  ;  while  sulphur,  which  fixes  2  equivalents  of  hydrogen, 
cannot  combine  with  more  than  6  equivalents  or  3  atoms  of  oxygen.  It  thus 
becomes  evident  that  we  cannot  recognise  as  a  fundamental  property  of  the 
elements  the  atomic  valencies  deduced  from  their  hydrides;  and  that  we 
must  modify,  to  a  certain  extent,  the  theory  of  atomicity  if  we  desire  to  raise 
it  to  the  dignity  of  a  general  principle  capable  of  affording  an  insight  into  the 
constitution  of  all  compound  molecules.  In  other  words,  it  is  only  to  carbon, 
which  is  quadrivalent  with  regard  both  to  oxygen  and  hydrogen,  that  we  can 
apply  the  theory  of  constant  valency  and  of  bond,  by  means  of  which  so  many 
still  endeavour  to  explain  the  structure  of  compound  molecules.  But  I  should 
go  too  far  if  I  ventured  to  explain  in  detail  the  conclusions  which  can  be 
drawn  from  the  above  considerations.  Still,  I  think  it  necessary  to  dwell 
upon  one  particular  fact  which  must  be  explained  from  the  point  of  view  of 
the  periodic  law  in  order  to  clear  the  way  to  its  extension  in  that  particular 
direction. 

The  higher  oxides  yielding  salts  the  formation  of  which  was  foreseen  by 
the  periodic  system  —  for  instance,  in  the  short  series  beginning  with  sodium— 

Na,0,  MgO,  A1A,  Si02,  P205>  S03,  Cl,07, 


must  be  clearly  distinguished  from  the  higher  degrees  of  oxidation  which  cor« 
respond  to  hydrogen  peroxide  and  bear  the  true  character  of  peroxides.  Per- 
oxides such  as  Naa02,  Ba03,  and  the  like  have  long  been  known.  Similar 

12  Formerly  it  was  supposed  that,  being  a  bivalent  element,  oxygen  can  enter  into  any 
grouping  of  the  atoms,  and  there  was  no  limit  foreseen  as  to  the  extent  to  which  it  could 
further  enter  into  combination.  We  could,  not  explain  why  bivalent  sulphur,  which  forms. 
compounds  such  as 


could  not  also  form  oxides  such  as— 


while  other  elements,  as,  for  instance,  chlorine,  form  compounds  such  as— 

Cl-O—Q-O—  O—  K 
*J 


488  PKINCIPLES  OF  CHEMISTRY 

peroxides  have  also  recently  become  known  in  the  case  of  chromium,  sulphur, 
titanium,  and  many  other  elements,  and  I  have  sometimes  heard  it  said  that 
discoveries  of  this  kind  weaken  the  conclusions  of  the  periodic  law  in  so  far 
as  it  concerns  the  oxides.  I  do  not  think  so  in  the  least,  and  I  may  remark, 
in  the  first  place,  that  all  these  peroxides  are  endowed  with  certain  properties 
obviously  common  to  all  of  them,  which  distinguish  them  from  the  actual, 
.higher,  salt-forming  oxides,  especially  tbejr  easy  decomposition  by  means  of 
simple  contact  agencies  ;  their  incapability  of  forming  salts  of  the  common 
type  ;  and  their  capability  of  combining  with  other  peroxides  (like  the  faculty 
which  hydrogen  peroxide  possesses  of  combining  with  barium  peroxide,  dis- 
covered by  Schoene).  Again,  we  remark  that  some  groups  are  especially 
Characterised  by  their  capacity  of  generating  peroxides.  Such  is,  for  instance, 
the  case  in  the  sixth  group,  where  we  find  the  well-known  peroxides  of 
sulphur,  chromium,  and  uranium  ;  so  that  further  investigation  of  peroxides 
will  probably  establish  a  new  periodic  function,  foreshadowing  that  molyb- 
denum and  tungsten  will  assume  peroxide  forms  with  comparative  readiness. 
To  appreciate  the  constitution  of  such  peroxides,  it  is  enough  to  notice  that 
the  peroxide  form  of  sulphur  (so-called  persulphuric  acid)  stands  in  the  same 
relation  to  sulphuric  acid  as  hydrogen  peroxide  stands  to  water  :  — 

H(OH),  or  ELjO,  responds  to  (OH)(OH),  or  H^, 
and  so  also  — 

,  or  H,S04,  responds  to  (HS04)(HS04),  or  H2S208. 


Similar  relations  are  seen  everywhere,  and  they  correspond  to  the  principle 
of  substitutions  which  I  long  since  endeavoured  to  represent  as  one  of  the 
chemical  generalisations  called  into  life  by  the  periodic  law.  So  also 
sulphuric  acid,  if  considered  with  reference,  to  hydroxyl,  and  represented  as 
follows— 

HO(S02OH), 

has  its  corresponding  compound  in  dithionic  acid  — 
(S02OH)(S02OH),  or  H2S206. 


Therefore,  also,  phosphoric  acid,  'H.O^OB.^,  has,  in  the  same  sense,  its 
corresponding  compound  in  the  subphosphoric  acid  of  Saltzer  :  — 

(POH802)(POH,02),  or  H4PaOe  ; 

and  we  must  suppose  that  the  peroxide  compound  corresponding  to  phosphoric 
acid,  if  it  be  discovered,  will  have  the  following  structure  :  — 

(H2P04)2  or  H4P208  =  2H20  +  2P03.13 
So  far  as  is  known  at  present,  the  highest  form  of  peroxides  is  met  with  in 

15  In  this  sense,  oxalic  acid,  (COOH)2,  also  corresponds  to  carbonic  acid,  OH(COOH), 
in  the  same  way  that  dithionic  acid  corresponds  to  sulphuric  acid,  and  snbphosphoric 
acid  to  phosphoric;  hence,  if  a  peroxide  corresponding  to  carbonic  acid  be  obtained, 
.it  will  have  the  structure  of  (HCO3)3,  or  H8C8O6  =  H8O  +  C2O5,  So  also  lead  must  have 
a  real  peroxide,  Pb2O5. 


APPENDIX  II.  489 

the  peroxide  of  uranium,  U04,  prepared  by  Fairley ; 14  while  Os04  is  the 
highest  oxide  giving  salts.  The  line  of  argument  which  is  inspired  by  the 
periodic  law,  so  far  from  being  weakened  by  the  discovery  of  peroxides,  is 
thus  actually  strengthened,  and  we  must  hope  that  a  further  exploration  of 
the  region  under  consideration  will  confirm  the  applicability  to  chemistry 
generally  of  the  principles  dediiced  from  the  periodic  law. 

Permit  me  now  to  conclude  my  rapid  sketch  of  the  oxygen  compounds  by 
the  observation  that  the  periodic  law  is  especially  brought  into  evidence  in 
the  case  of  the  oxides  which  constitute  the  immense  majority  of  bodies  at  our 
disposal  on  the  surface  of  the  earth. 

The  oxides  are  evidently  subject  to  the  law,  both  as  regards  their  chemical 
and  their  physical  properties,  especially  if  we  take  into  account  the  cases  of 
polymerism  which  are  so  obvious  when  comparing  C02  with  Sin02n.  In  order 
to  prove  this  I  give  the  densities  s  and  the  specific  volumes  v  of  the  higher 
oxides  of  two  short  periods.  To  render  comparison  easier,  the  oxides  are  all 
represented  as  of  the  form  B.20n.  In  the  column  headed  A  the  differences 
are  given  between  the  volume  of  the  oxygen  compound  and  that  of  the  parent 
element,  divided  by  n— that  is,  by  the  number  of  atoms  of  oxygen  in  the 
compound ; — 15 


2W2' 


5.  V.  A 

Na20  2-6  24  -22 

3-6  22  -3 

A1A 4-0  26 

Bi204  2-65  45  5-2 

P205   2-39  59  6-2 

S,0«   ,                ..  1-96  82  8-7 


8.  V.  A 

2-7  35  -55 

3-15  36  -7 

,  3-86  35  0 

Li204 4-2  38  +5 

V205  3-49  52  6-7 

Cr206 2-74  73  9'5 


I  have  nothing  to  add  to  these  figures,  except  that  like  relations  appear  in 
other  periods  as  well.  The  above  relations  were  precisely  those  which  made 
it  possible  for  me  to  be  certain  that  the  relative  density  of  ekasilicon  oxide 
would  be  about  4-7 ;  germanium  oxide,  actually  obtained  by  Winkler,  proved, 
in  fact,  to  have  the  relative  density  4-703. 

The-foregoing  account  is  far  from  being  an  exhaustive  one  of  all  that  has 
already  been  discovered  by  means  of  the  periodic  law  telescope  in  the  bound- 
less realms  of  chemical  evolution.  Still  less  is  it  an  exhaustive  account  of  all 
that  may  yet  be  seen,  but  I  trust  that  the  little  which  I  have  said  will  account 


14  The  compounds  of  uranium  prepared  by  Fairley  seem,  to  me  especially  instructive 
in  understanding  the  peroxides.    By  the  action  jof  hydrogen  peroxide  on  uranium  oxide, 
UO3,  a  peroxide  of  uranium,  UO4,4H2O,  is  obtained  (U  =  240)  if  the  solution  be  .acid;  but 
if  hydrogen  peroxide  act  on  uranium  oxide  in  the  presence  of  caustic  soda,  a  crystalline 
deposit  is  obtained  which  has  the  composition  Na4U08,4H2O,  and  evidently  is  a  combina- 
tion of  sodium  peroxide,  Na-jOa,  with  uranium  peroxide,  UO4.    It  is  possible  that  the 
former  peroxide,  UO4,4H2O,  contains  the  elements  of  hydrogen  peroxide  and  uranium 
peroxide,  U2O7,  or  even  U(OH)6,H2O2,  like  the  peroxide  of  tin  recently  discovered  by 
Spring,  which  has  the  constitution  SngC^HgOs. 

15  A  thus  represents  the  average  increase  of  volume  for  each  atom  of  oxygen  con- 
tained in  the  higher  salt-forming  oxide.    The  acid  oxides  give,  as  a  rule,  a  higher  value 
of  A,  while  in  the  case  of  the  strongly  alkaline  oxides  ita  value  is  usually  negative. 


490  PRINCIPLES  OF  CHEMISTRY 

for  the  philosophical  interest  attached  in  chemistry  to  this  law.  Although 
but  a  recent  scientific  generalisation,  it  has  already  stood  the  test  of  laboratory 
verification,  and  appears  as  an  instrument  of  thought  which  has  not  yet  been 
compelled  to  undergo  modification ;  but  it  needs  not  only  new  applications, 
but  also  improvements,  further  development,  and  plenty  of  fresh  energy.  All 
this  will  surely  come,  seeing  that  such  an  assembly  of  men  of  science  as  the 
Chemical  Society  of  Great  Britain  has  expressed  the  desire  to  have  the  his- 
tory of  the  periodic  law  described  in  a  lecture  dedicated  to  the  glorious  name 
of  Faraday. 


491 


APPENDIX  III 

ARGON,  A  NEW  CONSTITUENT  OF  THE  ATMOSPHERE 
WRITTEN  BT  PBOFESSOB  MENDELEEFF  IN  FEBBUABY  1895. 

THE  remarks  made  in  Chapter  V.,  Note  16  bis  respecting  the  newly  discovered 
constituent  of  the  atmosphere  are  here  supplemented  by  data  (taken  from 
the  publications  of  the  Eoyal  Society  of  London)  given  by  the  discoverers 
Lord  Eayleigh  and  Professor  Bamsay  in  January  1895,  together  with  obser* 
vations  made  by  Crookes  and  Olszewsky  upon  the  same  subject, 

This  gas,,  which  was  discovered  by  Eayleigh  and  Bamsay  in  atmo- 
spheric nitrogen,  was  named  argon l  by  them,  and  upon  the  supposition  of 
-its  being  an  element,  they  gave  it  the  symbol  A.  But  its  true  chemical 
nature  is  not  yet  fully  known,  for  not  only  has  no  compound  of  it  been  yet 
obtained,  but  it  has  not  even  been  brought  into  any  reaction.  From  all  that 
is  known  about  it  at  the  present  time,  we  may  conclude  with  the  discoverers 
that  argon  belongs  to  those  gases  which  are  permanent  constituents  of  the 
atmosphere,  and  that  it  is  a  new  element.  The  latter  statement,  however, 
requires  confirmation.  We  shall  presently  see,  however,  that  the  negative, 
chemical  character  of  argon  (its  incapacity  to  react  with  any  substance),  and 
the  small  amount  of  it  present  in  the  atmosphere  (about  1£  per  cent,  by 
volume  in  the  nitrogen  of  air,  and  consequently  about  1  per  cent,  by  volume 
in  air),  as  well  as  the  recent  date  of  its  discovery  (1894)  and  the  difficulty 
pf  its  preparation,  are  quite  sufficient  reasons  for  the  incompleteness  of  the 
existing  knowledge  respecting  this  element.  But  since,  so  far  as  is  yet  known, 
we  are  dealing  with  a  normal  constituent  of  the  atmosphere  *  *u,  the 

*  Prom  the  Greek  Ap-ybj/— inert, 

»  w»  In  Note  16  bis,  Chapter  V.,  I  mentioned  that,  judging  from  the  specific  gravity 
of  argon,  it  might  possibly  be  polymerised  nitrogen,  Nj,  bearing  the  same  relationship  to 
nitrogen,  N2,  that  ozone,  O3,  bears  to  ordinary  oxygen.  If  this  idea  were  confirmed,  still, 
one  would  not  imagine  that  argon  -was  formed  from  the  atmospheric  nitrogen  by  those: 
reactions  by  which  it  was  obtained  by  Bayleigh  and  Bamsay,  bat  rather  that  it  arises 
from  the  nitrogen  of  the  atmosphere  under  natural  conditions.  Although  this  proposition 
is  not  quite  destroyed  by  the  more  recent  results,  still  it  is  contradicted  by  the  fact  that 
the  ratio  of  the  specific  heats  of  argon  was  found  to  be  T66,  which,  as  far  as  is  now  known, 
could  not  be  the  case  for  a  gas  containing  8  atoms  in  its  molecule,  since  such  gases,  (sad 
Chapter  XIV.,  Note  7)  give  the  ratio  approximately  1'S .  (for  example,  CO,).  In  abstain* 
ing  from  further  conclusions,  for  they  must  inevitably  be  purely  conjectural,  I  consider 
it  advisable  to  suggest,  that  in  conducting  further  researches  upon  argon  it  might  be  well. 


492  PRINCIPLES  OF  CHEMISTRY 

existing  data,  notwithstanding  their  insufficiently  definite  nature,  should 
find  a  place  even  in  such  an  elementary  work  as  the  present,  all  the  more  as 
the  names  of  Rayleigh,  Ramsay,  Crookes  and  Olszewsky,  who  have  worked 
upon  argon,  are  among  the  highest  in  our  science,  and  their  researches  among 
jthe  most  difficult.2  These  researches,  moreover,-  were  directed  straight  to 
|the  goal,  which  was  only  partly  reached  owing  to  the  unusual  properties  of 
Wgon  itself. 

When  it  became  known  (Chapter  V.,  Note  4  bis)  that  the  nitrogen  obtained 
from  air  (by  removing  the  oxygen,  moisture  and  C02  by  various  reagents) 
has  a  greater  density  than  that  obtained  from  the  various  (oxygen,  hydrogen 
and  metallic)  compounds  of  nitrogen,  it  was  a  plausible  explanation  that  the 
latter  contained  an  admixture  of  hydrogen,  or  of  some  other  light  gas  lower- 
ing the  density  of  the  mixture.  But  such  an  assumption  is  refuted  not  only 
by  the  fact  that  the  nitrogen  obtained  from  its  various  compounds  (after 
purification)  has  always  the  same  density  (although  the  supposed  impurities 
mixed  with  it  should  vary),  but  also  by  Rayleigh  and  Ramsay's  experiment 
of  artificially  adding  hydrogen  to  nitrogen,  and  then  passing  the  mixture  over 
red-hot  oxide  of  copper,  when  it  was  found  that  the  nitrogen  regained  its 
original  density,  i.e.  that  the  whole  of  the  hydrogen  was  removed  by  this 
treatment.  Therefore  the  difference  in  the  density  of  the  two  varieties  of 
nitrogen  had  to  be  explained  by  the  presence  of  a  heavier  gas  in  admixture 
with  the  nitrogen  obtained  from  the  atmosphere.  This  hypothesis  was  con- 
firmed by  the  fact  that  Rayleigh  and  Ramsay  having  obtained  purified  nitrogen 
(by  removing  the  O.,,  CO.,  and  H20),  both  from  ordinary  air  and  from  air 
which  had  been  previously  subjected  Jo  atmolysis,  that  is  which  had  been 
passed  through  porous  tubes  (of  burnt  clay,  e.g.  pipe-stem),  surrounded  by  a 
rarefied  space,  and  so  deprived  of  its  lighter  constituents  (chiefly  nitrogen), 
found  that  the  nitrogen  from  the  air  which  had  been  subjected  to  atmolysis 
was  heavier  than  that  obtained  from  air  which  had  not  been  so  treated.  This 
experiment  showed  that  the  nitrogen  of  air  contains  an  admixture  of  a  gas 
which,  being  heavier  than  nitrogen  itself,3  diffuses  more  slowly  than  nitrogen 

•to  subject  it  to  as  high  a  temperature  as  possible.  And  the  possibility  of  nitrogen 
polymerising  is  all  the  more  admissible  from  the  fact  that  the  aggregation  of  its  atoms 
in  the  molecule  is  not  at  all  unlikely,  and  that  polymerised  nitrogen,  judging  from  many 

^examples,  might  be  inert  if  the  polymerisation  were  accompanied  by  the  evolution  of 
teat.  In' the  following  footnotes  I  frequently  return  to  this  hypothesis,  not  only  because 
I  have  not  yet  met  any  facts  definitely  contradictory  to  it,  but  also  because  the  chief 
properties  of  argon  agree  with  it  to  a  certain  extent. 

2  The  chief  difficulty  in  investigating  argon  lies  in  the  fact  that  its  preparation  requires 
the  employment  of  a  large  quantity  of  air,  which  has  to  be  treated  with  a  number  of 
different  reagents,  whose  perfect  purity  (especially  that  of  magnesium)  will  always  be 
doubtful,  and  argon  haslnot  yet  been  transferred  to  a  substance  in  which  it  could  be  easily 
purified.  Perhaps  the  considerable  solubility  of  argon  in  water  (or  in  other  suitable 
liquids,  which  have  not  apparently  yet  been  tried)  may  give  the  means  of  doing  so,  and  it 
may  be  possible,  by  collecting  the  air  expelled  from  boiling  water,  to  obtain  a  richer  source 
of  argon  than  ordinary  air. 

.• .  5  II  might  also  be  supposed  that  this  heavy  gas  is  separated  by  the  copper  when  the 
latter  absorbs  the  oxygen  of  the  air ;  but  such  a  supposition  is  not  only  improbable  in 
itself,  but  does  not  agree  with  the  fact  that  nitrogen  may  be  obtained  from  air  by  absorb- 
ing the  oxygen  by  various  other  substances  in  solution  (for  instance,  by  the  lower  oxides 


APPENDIX  HI.  498 

through  the  porous  material.  It  remained,  therefore-,  to  separate  this  irn- 
purity  from  the  nitrogen.  To  do  this  Rayleigh  and  Ramsay  adopted  two 
methods,  converting  the  nitrogen  into  solid  and  liquid  substances,  either 
by  absorbing  the  nitrogen  by  heated  .magnesium  (Chapter  V.,  Note  6,  and 
Chapter  XIV.,  Note  14),  with  the  formation  of  nitride  of  magnesium,  or  else 
by  converting  it  into  nitric  acid  by  the  action  of  electric  sparks  or  the  presence 
of  an  excess  of  air  and  alkali,  as  in  Cavendish's  method.3  bis  In  both  cases 
the  nitrogen  entered  into  reaction,  while  the  heavie.r  gas  mixed  with  it 
remained  inert,  and  was  thus  able  to  be  isolated.  That  is,  the  argon  could  be 
separated  by  these  means  from  the  excess  of  atmospheric  nitrogen  accom- 
panying  it.4  As  an  illustration  we  will  describe  how  argon  was  obtained 
from  the  atmospheric  nitrogen  by  means  of  magnesium.5  To  begin  with, 
it  was  discovered  that  when  atmospheric  nitrogen  was  passed  through. a  tube 
containing  metallic  magnesium  heated  to  redness,' its  specific  gravity  rose  to 
14-88.  As  this  showed  that  part  of  the  gas  was  absorbed  by  the  magnesium, 
a  mercury  gasometer  filled  with  atmospheric  nitrogen  was  taken,  and  the 
gas  drawn  over  soda-lime,  P205,  heated  magnesium 6  and  then  through 
tubes  containing  red-hot  copper  oxide,  soda-lime  and  phosphoric  anhydride 
to  a  second  mercury  gasometer.  Every  time  the  gas  was  repassed  through 
the  tubes,  it  decreased  in  volume  and  increased  in  density.  After  repeating 

of  the  metals,  like  FeO)  besides  red-hot  copper,  and  that  the  nitrogen  obtained  is  always 
just  as  heavy.  Besides  which,  nitrogen  is  also  set  free  from  its  oxides  by  copper,  and  the 
nitrogen  thus  obtained  is  lighter.  Therefore  it  is  not  the  copper  which  produces  the 
.heavy  gas — i.e.  argon. 

8  *>is  n  js  worthy  of  note  that  Cavendish  obtained  a  small  residue  of  gas  in  con- 
verting nitrogen  into  nitric  acid;  but  he  paid  no  attention  to  it,  although  probably  he 
had  in  his  hands  the  very  argon  recently  discovered. 

4  When  in.  these  experiments,  instead  of  atmospheric  nitrogen  the  gas  obtained  from 
its  compound  was  taken,  an  inert  residue  of  a  heavy  gas,  having  the  properties  of  argon, 
was  also  remarked,  but  its-amount  was  very  small.    Rayleigh  and  Ramsay  ascribe  the 
formation  of  this  residue  to  the  fact  that  the  gas  in  these  experiments  was  collected  over 
water,  and  a  portion  of  the  dissolved  argon  in  it  might  have  passed  into  the  nitrogen.    4.3 
the  authors  of  this  supposition  did  not  prove  it  by  any  special  experiments,  it  forms  a 
weak  point  in  their  classical  research.    If  it  be  admitted  that  argon  is  N3,  the  fact  of  its 
being  obtained  from  the  nitrogen  of  compounds  might  be  explained  by  the  polymerisation 
of  a  portion  of  the  nitrogen  in  the  act  of  reaction,  although  it  is  impossible  to  refute 
Rayleigh  and  Ramsay's  hypothesis  of  its  being  evolved  from  the  water  employed  in  the 
manipulation  of  the  gases.     Three  thousand  volumes  of  nitrogen  extracted  from  its 
compounds  gave  about  three  volumes  of  argon,  while  thirty  volumes  were  yielded  by  the 
same  amount  of  atmospheric  nitrogen. 

5  The  preparation  of  argon  by  the  conversion  of  nitrogen  int.o  nitric  acid  is  complicated 
by  the  necessity  of  adding  a  large  proportion  of.  oxygen  and  alkali,  of  passing ^an  electric 
discharge  through  the  mixture  jor  a  long  period,  and  then  removing  the  remaining 
oxygen.    All-this  was  repeatedly  done  by  the  authors,  but  this  method  is  far  more 
complex,  Both  in  practice  and  theory,  than  the  preparation  of  argon  by  means  of 
magnesium.    From  100  volumes  of  air  subjected  to  conversion  into  HN03,  0'76  volume 
of  argon  were  obtained  after  absorbing  the  excess  of  oxygen. 

6  In  these  and  the  following  experiments  the  magnesium  was  placed  in  an  ordinary 
hard  glass  tube,  and  heated  in  a  gas  furnace  to  a  temperature  almost  sufficient  to  soften 
the  glass.    The  current  of  gas  must  be  very  slow  (a  tube  containing  a  small  quantity  of 
sulphuric  acid  served  as  a  meter),  as  otherwise  the  heat  evolved  in  the  formation  of  the 
Mg3N2  (Chapter  XIV.,  Note  14)  will  .melt  the  tube, 


494  PRINCIPLES  OF  OHEMISTKY 

this  for  teA  Hays  1,600  c.c.  of  gas  were  reduced  to  200  cc.,  and  the  density 
increased  to  16*1  (if  that  of  R,  - 1  and  N2  - 14).  Further  treatment  of  the 
remainder  brought  the  density  up  to  19*09.  After  adding  a  small  quantity 
of  oxygen  and  repassing  the  gas  through  the  apparatus,  the  density  rose  to 
20*0.  To  obtain  argon  by  this  process  Ramsay  and  Rayleigh  (employing  a 
mercury  air  pump  and  mercury  gasometers)  once  treated  about  150  litres  of 
atmospheric  nitrogen.  On  another  occasion  they  treated  7,925  c.c.  of  air  by 
the  oxidation  method  and  obtained  65  c.c.  of  argon,  which  corresponds  to 
0-82  per  cent.  The  density  of  the  argon  obtained  by  this  means  was  nearly 
19*7,  while  that  obtained  by  the  magnesium  method  varied  between  19-09 
and  20-38. 

Thus  the  first  positive  and  very  important  fact  respecting  argon  is  that 
its  specific  gravity  is  nearly  20 — that  is,  that  it  is  20  times  heavier  than 
hydrogen,  while  nitrogen  is  only  14  times  and  oxygen  16  times  heavier  than 
hydrogen.  This  explains  the  difference  observed  by  Bayleigh  between  the 
densities  of  nitrogen  obtained  from  its  compounds  and  from  the  atmosphere 
^Chapter  V.,  Note  4  bis).  At  0°  and  760  mm.  a  litre  of  the  former  gas  weighs 
1*2505  grm.,  while  a  litre  of  the  latter  weighs  1-2572,  or  taking  H  -  1,  the 
density  of  the  first  -  13-916,  and  of  the  latter  -  13-991.  If  the  density  of 
argon  be  taken  as  20,  it  is  contained  in  atmospheric  nitrogen  to  the  extent  of 
about  1-23  per  cent,  by  volume,  whilst  air  contains  about  0-97  per  cent,  by 
volume. 

When  argon  had  been  isolated  the  question  naturally  arose,  was  it  a  new 
homogeneous  substance  having  definite  properties  or  was  it  a  mixture  of 
gases  ?  The  former  may  now  be  positively  asserted,  namely,  that  argon  is  a 
peculiar  gas  previously  unknown  to  chemistry.  Such  a  conviction  is  in  the 
first  place  established  by  the  fact  that  argon  has  a  greater  number  of  nega- 
tive properties,  a  smaller  capacity  for  reaction,  than  any  other  simple  or 
compound  body  known.  The  most  inert  gas  known  is  nitrogen,  but  argon 
far  exceeds  it  in  this  respect.  Thus  nitrogen  is  absorbed  at  a  red  heat  by  many 
metals,  with  the  formation  of  nitrides,  while  argon,  as  is  seen  in  the  mode 
of  its  preparation  and  by  direct  experiment,  does  not  possess  this  property. 
Nitrogen,  under  the  action  of  electric  sparks,  combines  with  hydrogen  in  -the 
presence  of  acids  and  with  oxygen  in  the  presence  of  alkalis,  while  argon  is 
unable  to  do  so,  as  is  seen  from  the  method  of  separation  from  nitrogen. 
Rayleigh  and  Ramsay  also  proved  that  argon  is  unable  to  react  with  chlorine 
(dry  or  moist)  either  directly  or  under  the  action  of  an  electric  discharge,  or 
with  phosphorus  or  sulphur,  at  a  red  heat.  Sodium,  potassium,  and  tellurium 
may  be  distilled  in  an  atmosphere  of  argon  without  change.  Fused  caustic 
soda,  incandescent  soda-lime,  molten  nitre,  red-hot  peroxide  of  sodium, 
and  the  polysulphides  of  calcium  and  sodium  also  do  not  react  with  argon. 
Platinum  black  does  not  absorb  it,  and  spongy  platinum  is  unable  to  excite  its 
reaction  with  oxygen  or  chlorine.  Aqua  regia,  bromine  water,  and  a  mixture 
of  hydrochloric  acid  and  KMn04  were  also  without  action  upon  argon.  Besides 
which  it  is  evident  from  the  method  of  its  preparation  that  it  is  not  acted  upon 
by  red-hot  oxide  of  copper.  All  these  facts  exclude  any  possibility  of  argon  con- 
taining any  already  known  body,  and  prove  it  to  be  the  most  inert  of  all  the 
gases  known.  But  besides  these  negative  points,  the  independency  of  argon  is 


APPENDIX  III.  495 

confirmed  by  four  observed  positive  properties  possessed  by  it,  which 
are:— 

1.  The  spectrum  of  argon  observed  by  Crookes  under  a  low  pressure  (in 
Geissler-Pliicker  tubes)  distinguishes  it  from  other  gases.7  It  was  proved 
by  this  means  that  the  argon  obtained  by  means  of  magnesium  is  identical 
with  that  which  remains  after  the  ..conversion  of  the  atmospheric  nitrogen 
into  nitric  acid.  Like  nitrogen,  argon  presents  two  spectra  produced  at 
different  potentials  of  the  induced  current,  one  being  orange-red,  the  other 
steel-blue  ;  the  latter  is  obtained  under  a  higher  degree  of  rarefaction  and 
with  a  battery  of  Leyden  jars.  Both  the  spectra  of  argon  (in  contradistinction 
to  those  of  nitrogen)  are  distinguished  by  clearly  defined  lines.8  The  red 
(ordinary)  spectrum  of  argon  has  two  particularly  brilliant  and  characteristic 
red  lines  (not  far  from  the  bright  red  line  of  lithium,  on  the  opposite  side  to 
the  orange  band)  having  wave-lengths  705-64  and  696-56  (see  Vol.  I., 
p.  565).  Between  these  bright  lines  there  are  in  addition  lines  with  wave 
lengths  603-8,  565-1,  561-0,  555-7,  518-58,  516-5,  450-95,  420-10,  415-95  and 
894-85.  Altogether  80  lines  have  been  observed  in  this  spectrum  and  119  in 
the  blue  spectrum,  of  which  26  are  common  to  both  spectra.9 

2>  According  to  Rayleigh  and  Ramsay  the  solubility  of  argon  in  water 
is  approximately  4  volumes  in  100  volumes  of  water  at  13°.  Thus  argon 
is  nearly  2£  times  more  soluble  than  nitrogen,  and  its  solubility  ap- 
proaches that  of  oxygen.  Direct  experiment  proves  that  nitrogen  obtained 
from  air  from  boiled  water  is  heavier  than  that  obtained  straight  from  tho 
atmosphere.  This  again  is  an  indirect  proof  of  the  presence  of  argon  in 
air. 

3.  The  ratio  Jc  of  the  two  specific  heats  (at  a  constant  pressure  and  at 

7  The  greatest  brilliancy  of  the  spectrum  of  argon  is  obtained  at  a  tension  of  3  mm., 
while  for  nitrogen  it  is  about  75  mm.  (Crookes).    In  Chapter  V.,  Note  16  bis,  it  is  said 
that  the  same  blue  line  observed  in  the  spectrum  of  argon  is  also  observed  in  the  spectrum 
of  nitrogen.    This  is  a  mistake,  since  there  is  no  coincidence  between  the  blue  lines  of 
the  argon  and  nitrogen  spectra.    However,  we  may  add  that  for  nitrogen  the  following 
moderately  bright  lines  are  known  of  wave-lengths  585,  574,  544,  516,  457,  442,^36,  and 
426,  which  are  repeated  in  the  spectra  (red  and  blue)  of  argon,  judging  by  Crookes' 
researches  (1895) ;  but  it  is  naturally  impossible  to  assert  that  there  is  perfect  identity 
Until  some  special  comparative  work  has  been  done  in  this  subject,  which  is  very  desirable, 
and  more  especially  for  the  bluish-violet  portion  of  the   spectrum,  more  particularly 
between  the  lines  442-436,  as  these  lines  are  distinguished  by  their  brilliancy  in  both  the 
argon  and  nitrogen  spectra.    The  above-mentioned  supposition  of  argon  being  polymerised 
nitrogen  (N3),  formed  from  nitrogen  (N2),  with  the  evolution  of  heat,  might  find  some 
support  should  it  be  found  after  careful  comparison  that  even  a  limited  number  of 
spectral  lines  coincided. 

8  At  first  the  spectrum  of  argon  exhibits  the  nitrogen  lines,  but  after  a  certain  time 
these  lines  disappear  (under  the  influence  of  the  platinum,  and  also  of  Al  and  Mg,  but 
with  the  latter  the  spectrum  of  hydrogen  appears)  and  leave  a  pure  argon  spectrum.    It 
floes  not  appear  clear  to  me  whether  a  polymerisation  here  takes  place  or  a  simple 
absorption.    Perhaps  the  elucidation  of  this  question  would  prove  important  in  the 
history  of  argon.    It  would  be  desirable  to  know,  for  instance,  whether  the  volume  of 
argon  changes  when  it  is  first  subjected  to  the  action  of  the  electric  discharge. 

9  Crookes  supposes  that  argon  contains  a  mixture  of  two  gases,  but  as  he  gives  no 
reasons  for  this,  beyond  certain  peculiarities  of  a  spectroscopio  character,  we  will  not 
consider  this  hypothesis  further. 


496  PRINCIPLES  OF  CHEMISTRY 

a  constant  volume)  of  argon  was  determined  by  Rayleigh  and  Ramsay  *by  the 
method  of  the  velocity  of  sound  (see  Chapter  XIV.,  Note  7  and  Chapter  VII., 
Note  26)  and  was  found  to  be  nearly  1-66,  that  is  greater  than  for  those  gases 
whose  molecules  contain  two  atoms  (for  instance,  CO,H2,N2,  air,  &c.,  for 
which  Tt  is  nearly  1*4)  or  those  whose  molecules  contain  three  atoms  (for 
instance,  C02,N20,  &c.,  for  which  &  is  about  T3),  but  closely  approximate 
to  the  ratio  of  the  specific  heats  of  mercury  vapour  (Kundt  and  Warburg, 
fc  =  T67).  And  as  the  molecule  of  mercury  vapour  contains  one  atom,  so  it 
may  be  said  that  argon  is  a  simple  gaseous  body  whose  molecule  contains 
one  atom.10  A  compound  body  should  give  a  smaller  ratio.  The  experi- 
ments upon  the  liquefaction  of  argon,  which  we  shall  presently  describe,  speak 
against  the  supposition  that  argon  is  a  mixture  of  two  gases.  The  import- 
ance of  the  results  in  question  makes  one  wish  that  the  determinations  of  the 
ratio  of  the  specific  heats  (and  other  physical  properties)  might  be  confirmed 
with  all  possible  accuracy.11  If  we  admit,  as  we  are  obliged  to  do  for  the 
present,  that  argon  is  a  new  element,  its  density  shows  that  its  atomic  weight 
must  be  nearly  40,  that  is,  near  to  that  of  K  =  39  and  Ca  =  40,  which  does 
not  correspond  to  the  existing  data  respecting  the  periodicity  of  the  properties 

10  This  portion  of  Rayleigh  and  Ramsay's  researches  deserves  particular  attention  as, 
80  far,  no  gaseous  substance  is  known  whose  molecule  contains  but  one  atom.    Were  it 
not  for  the  above  determinations,  it  might  be  thought  that  argon,  having  a  density  20, 
has  a  complex  molecule,  and  may  be  a  compound  or  polymerised  body,  for  instance,  N5 
or  NXn,  or  in  general  Xn ;  but  as  the  matter  stands,  it  can  only  be  said  that  either  (1) 
argon  ia  a  new,  peculiar,  and  quite  unusual  elementary  substance,  since  there  is  no 
reason  for  assuming'  it  to  contain  two  simple  gases,  or  (2)  the  magnitude,  k  (the  ratio  of 
the  specific  heats)  does  not  only  depend  upon  the  number  of  atoms  contained  in  the 
molecules,  but  also  upon  the  store  of  internal  energy  (internal  motion  of  the  atoms  in 
the  molecule).    Should  the  latter  be  admitted,  it  would  follow  that  the  molecules  of  very 
active  gaseous  elements  would  correspond  to  a  smaller  k  than  those  of  other  gases  having 
an  equal  number  of  atoms  in  their  molecule.     Such  a  gas  is  chlorine,  for  which  A  =  1'33 
(Chapter  XTV.,  Note  7).    For  gases  having  a  small  chemical  energy,  on  the  contrary,  a 
larger  magnitude  would  be  expected  for  A;.     I  think  these  questions  might  be  partially 
settled  by  determining  k  for  ozone  (O3)  and  sulphur  (S6)  (at  about  500°).    In  other  words, 
I  would   suggest,  though   only  provisionally,  that  the  magnitude,  A  =  1'6,  obtained  for 
argon  might  prove  to  agree  with  the  hypothesis  that  argon  is  N3,  formed  from  N2  with 
the  evolution  of  heat  or  loss  of  energy.    Here  argon  gives  rise  to  questions  of  primary 
importance,  and  it  is  to  be  hoped  that  further  research  will  throw  some  light  upon  them. 
In  making  these  remarks,  I  only  wish  to  clear  the  road  for  further  progress  in  the  study 
of  argon,  and  of  the  questions  depending  on  it.    I  may  also  remark  that  if  argon  is  Nj 
formed  with  the  evolution  of  heat,  its  conversion  into  nitrogen,  N2,  and  into  nitride 
compounds  (for  instance,  boron  nitride  or  nitride  of  titanium)  might  only  take  place  at  a, 
very  high  temperature. 

11  Without  having  the  slightest  reason  for  doubting  the  accuracy  of  Rayleigh  and 
Ramsay's  determinations,  I  think  it  necessary  to  say  that  as  yet  (February  1895)  I  am 
only  acquainted  with  the  short  memoir  of  the  above  chemists  in  the  '  Proceedings  of  the 
Royal  Society,'  which  does  not  give  any  description  of  the  methods  employed  and  results 
obtained,  while  at  the  end  (in  the  general  conclusions)  the  authors  themselves  express 
some  doubt  as  to  the  simple  nature  of  argon.    Moreover,  it  seems  to  me  that  (Note  10) 
there  must  be  a  dependence  of  k  upon  the  chemical  energy.    Besides  which,  it  is  not 
clear  what  density  of  the  gas  Rayleigh  and  Ramsay  took  in  determining  k.     (If  argon  be 
N8,  its  density  would  be  near  to  21.)    Hence  I  permit  myself  to  express  some  doubt  as  to 
whether  the  molecule  of  argon  contains  but  one  atom. 


APPENDIX  III.  497 

of  the  elements  in  dependence  upon  their  atomic  weights,  for  there  is  no 
reason  on  the  basis  of  existing  data,  for  admitting  any  intermediate  elements 
between  Cl  =  35'5  and  K  =  39,  and  all  the  positions  above  potassium  in  the 
periodic  system  are  occupied.  This  renders  it  very  desirable  that  the  velocity 
of  sound  in  argon  should  be  re-determined.12 

4.  Argon  was  liquefied  by  Professor  Olszewsky,  who  is  well  known  for  his 
classical  researches  upon  liquefied  gases.  These  researches  have  an  especial 
interest  since  they  show  that  argon  exhibits  a  perfect  constancy  in  its 

12  If  it  should  be  found  that  k  for  argon  is  less  than  1*4,  or  that  Jc  is  dependent  upon 
the  chemical  energy,  it  would  be  possible  to  admit  that  the  molecule  of  argon  contains 
not  one,  but  several  atoms— for  instance,  either  N3  (then  the  density  would  be  21,  which 
is  near  to  the  observed  density)  or  X6)  if  X  stand  for  an  element  with  an  atomic  weight 
near  to  6'7.  No  elements  are  known  between  H  =  1  and  Li  =  7,  but  perhaps  they  may 
exist.  The  hypothesis  A  =  40  does  not  admit  argon  into  the  periodic  system.  If  the 
molecule  of  argon  be  taken  as  A^— i.e.  the  atomic  weight  as  A  =  20— argon  apparently 
finds  a  place  in  Group  VIII.,  between  F  =  19  and  Na=23 ;  but  such  a  position  could  only 
be  justified  by  the  consideration  that  elements  of  small  atomic  weight  belong  to  the 
category  of  typical  elements  wnich  offer  many  peculiarities  in  their  properties,  as  is 
seen  on  comparing  N  with  the  other  elements  of  Group  V.,  or  0  with  those  of  Group  VI. 
Apart  from  this  there  appears  to  me  to  be  little  probability,  in  the  light  of  the  periodic 
law,  in  the  position  of  an  inert  substance  like  argon  in  Group  VIII.,  between  such  active 
elements  as  fluorine  and  sodium,  as  the  representatives  of  this  group  by  their  atomic 
weights  and  also  by  their  properties  show  distinct  transitions  from  the  elements  of  the 
last  groups  of  the  uneven  series  to  the  elements  of  the  first  groups  of  the  even  series— for 
instance, 

Group       vi.         vn.  vin.  i.  n. 

Cr  Mn  Fe,Co,Ni  Cu  Zn 

While  if  we  place  argon  in  a  similar  manner, 

VI.  VII.  VIII.  I.  II. 

O  =  16  F  =  19  A=20  Na=23  Mg=24 

although  from  a  numerical  point  of  view  there  is  a  similar  sequence  to  the  fljbove,  still 
from  a  chemical  and  physical  point  of  view  the  result  is  quite  different,  as  there  is  no 
such  resemblance  between  the  properties  of  O,  F  and  Na,  Mg,  as  between  Cr,  Mn,  and 
Cu,  Zn.  I  repeat  that  only  the  typical  character  of  the  elements  with  small  atomic 
weights  can  justify  the  atomic  weight  A =20,  and  the  placing  of  argon  in  Group  VIII 
amongst  the  typical  elements ;  then  N,  O,  F,  A  are  a  series  of  gases. 

It  appears  to  me  simpler  to  assume  that  argon  contains  N3,  especially  as  argon  is 
present  in  nitrogen  and  accompanies  it,  and,  as  a  matter  of  fact,  none  of  the  observed 
properties  of  argon  are  contradictory  to  this  hypothesis. 

These  observations  were  written  by  me  in  the  beginning  of  February  1895,  and  on 
the  29th  of  that  month  I  received  a  letter,  dated  February  25,  from  Professor  Kamsay 
informing  me  that '  the  periodic  classification  entirely  corresponds  to  its  (argon's)  atomic 
weight,  and  that  it  even  gives  a  fresh  proof  of  the  periodic  law,'  judging  from  the 
researches  of  my  English  friends.  But  in  what  these  researches  consisted,  and  how  the 
above  agreement  between  the  atomic  weight  of  argon  and  the  periodic  system  was  arrived 
at,  is  not  referred  to  in  the  letter,  and  we  remain  in  expectation  of  a  first  publication  of 
the  work  of  Lord  Rayleigh  and  Professor  Ramsey.  [For  more  complete  information  see 
papers  read  before  the  Royal  Society,  January  81, 1895,  February  13,  March  10,  and 
May  21,  1896,  and  a  paper  published  in  the  Chemical  Society's  Transactions,  1895, 
p.  684.  For  abstracts  of  these  and  other  papers  on  argon  and  helium,  and  correspon- 
dence, see  'Nature,'  1895  and  1896.] 


498  PKINCIPLES   OF  CHEMISTRY 

properties  in  the  liquid  and  critical  states,  which  almost 13  disposes  of  the  sup- 
position that  it  contains  a  mixture  of  two  or  more  unknown  gases.  As  the 
first  experiments  showed,  argon  remains  a  gas  under  a  pressure  of  100 
atmospheres  and  at  a  temperature  of  -  90° ;  this  indicated  that  its  critical 
temperature  was  probably  below  this  temperature,  as  was  indeed  found  to 
be  the  case  when  the  temperature  was  lowered  to  -  1280>6  14  by  means  of 
liquid  ethylene.  At  this  temperature  argon  easily  liquefies  to  a  colourless 
liquid  under  38  atmospheres.  The  meniscus  begins  to  disappear  at  between 

15  There  only  remains  the  very  remote  possibility  that  argon  consists  of  a  mixture  of 
two  gases  having  very  nearly  the  same  properties. 

14  The  following  data,  given  by  Olszewsky,  supplement  the  data  given  in  Chapter  II., 
.Note  29,  upon  liquefied  gases. 

(tc)  (pc)  t  tl  s 

N2  -146°  35  -194°'4  -214°  0'885 

CO  -139°'5  85-5  -190°  -207  ? 

A  -121°  50-6  -187°  -189°-6  1'5 

O2  -1180>8  50-8  -182°'7  ?  1124 

NO  -  98°'5  71-2  -153°'6  -167°  ? 

CH4          -  81°'8  54-9  -164°  -158°'8  0'415 

•where  tc  ia  the  absolute  (critical)  boiling  point,  pc  the  pressure  (critical)  in  atmospheres 
corresponding  to  itj  t  the  boiling  point  (under  a  pressure  of  760  mm.),  tl  the  melting  point, 
and  s  the  specific  gravity  in  a  liquid  state  at  t. 

The  above  shows  that  argon  in  its  properties  in  a  liquid  state  stands  near  to  oxygen 
(as  it  also  does  in  its  solubility),  but  that  all  the  temperatures  relating  to  it  (tc,  t,  and  ti) 
are  higher  than  for  nitrogen.  This  fully  answers,  not  only  to  the  higher  density  of  argon, 
but  also  to  the  hypothesis  that  it  contains  N3.  And  as  the  boiling  point  of  'argon  differs 
from  that  of  nitrogen  and  oxygen  by  less  than  10°,  and  its  amount  is  small,  it  is  easy  to 
understand  how  Dewar  (1894),  who  tried  to  separate  it  from  liquid  air  and  nitrogen  by 
fractional  distillation,  was  unable  to  do  so.  The  first  and  last  portions  were  identical, 
and  nitrogen  from  air  showed  no  difference  in  its  liquefaction  from  that  obtained  from 
its  compounds,  or  from  that  which  had  been  passed  through  a  tube  containing  incandescent 
magnesium.  Still,  it  is  not  quite  clear  why  both  kinds  of  nitrogen,  after  being  passed 
over  the  magnesium  in  Dewar's  experiments,  exhibited  an  almost  similar  alteration  in 
their  properties,  independent  of  the  appearance  of  a  small  quantity  of  hydrogen  in  them. 

Concluding  Remarks  (March  81,  1895). — The  '  Comptes  rendus '  of  the  Paris 
Academy  of  Sciences  of  March  18, 1895,  contains  a  memoir  by  Berthelot  upon  the  reaction 
of  argon  with  the  vapour  of  benzene  under  the  action  of  a  silent  discharge.  In  his  ex- 
periments, Berthelot  succeeded  in  treating  83  per  cent,  of  the  argon  taken  for  the 
purpose,  and  supplied  to  him  by  Ramsay  (87  c.c.  in  all).  The  composition  of  the  product 
could  not  be  determined  owing  to  the  small  amount  obtained,  but  in  its  outward 
appearance  it  quite  resembled  the  product  formed  under  similar  conditions  by  nitrogen. 
This  observation  of  the  famous  French  chemist  to  some  extent  supports  the  supposition 
that  argon  is  a  polymerised  variety  of  nitrogen  whose  molecule  contains  N3,  while  ordinary 
nitrogen  contains  N2.  Should  this  supposition  be  eventually  verified,  the  interest  in 
argon  will  not  only  not  lessen,  but  become  greater.  For  this,  however,  we  must  wait  for 
further  observations  and  detailed  experimental  data  from  Rayleigh  and  Ramsay. 

The  latest  information  obtained  by  me  from  London  is  that  Professor  Ramsay,  by 
treating  cleveite  (containing  PbO,  UO3,  Y2O3,  &c.)  with  sulphuric  acid,  obtained  argon, 
and,  judging  by  the  spectrum,  helium  also.  The  accumulation  of  similar  data  may,  after 
detailed  and  diversified  research,  considerably  increase  the  stock  of  chemical  knowledge 
•which,  constantly  widening,  cannot  be  exhaustively  treated  in  these  'Principles  of 
Chemistry,'  although  rery  probably  furnishing  fresh  proof  of  the  'periodicity  of  the 
elements.' 


APPENDIX  III.  499 

-  119°-8  and  -  121°'6,  mean  -  121°  at  a  pressure  of  50'6  atmospheres.  The 
vapour  tension  of  liquid  argon  a$  —  128°'G,  is  38'0  atmospheres,  at  -  187° 
it  is  one  atmosphere,  and  at  -  18$°'6  it  solidifies  to  a  colourless  substance 
like  ice.  The  specific  gravity  of  liquid  argon,  at  about  -  187°  is  nearly  1-5, 
which  is  far  above  that  of  other  liqxiefied  gases  of  very  low  absolute  boiling 
point. 

The  discovery  of  argon  is  one  of  the  most  remarkable  chemical  acquisi- 
tions of  recent  times,  and  we  trust  that  Lord  Eayleigh  and  Professor  Ramsay, 
who  made  this  wonderful  discovery,  will  further  elucidate  the  true  nature  of 
argon,  as  this  should  widen  the  fundamental  principles  of  chemistry,  to  which 
the  chemists  of  Great  Britain  have  from  early  times  made  such  valuable 
contributions.  It  would  be  premature  now  to  give  any  definite  opinions 
upon  so  new  a  subject.  Only  one  thing  can  be  said  ;  argon  is  so  inert  that 
its  role  in  nature  cannot  be  considerable,  notwithstanding  its  presence  in  the 
atmosphere.  But  as  the  atmosphere  itself  plays  such  a  vast  part  in  the 
life  of  the  surface  of  the  earth,  every  addition  to  our  knowledge  of  its  compo- 
sition must  directly  .x>r  indirectly  react  upon  the  sum  total  of  our  knowledge 
of  nature. 


INDEX   OF   AUTHOEITIES 


ABASHEFF,  i.  75 
Abel,  ii.  56,  326,  410 
Acheson,  ii.  107 
Adie,  ii.  186 
Alexe"eff,  i.  75,  94 
Alluard,  i.  458 
Amagat,  i.  132, 135, 140 
Amat,  ii.  171 
Ammermiiller,  i.  604 
Ampere,  i.  309 
Andreeff,  i.  ,251 
Andrews,  i.  136,  203 
Angeli,  i.  266 
Ansdell,  i.  451 
Arfvedson,  i.  575 
Arrhenius,  i.  89,  92,  889 
Aschoff,  ii.  313 
Askenasy,  i.  508 
Aubel,  ii.  45 
Aubin,  i.  238 
Avdeeff.i.  618;  ii.  484 
Avogadro,  i.  309 


BABO,  v.,  i.  9*  SOO,  208 

Bach,  i.  39" 

Baohmetiefi,  ii.  81 

Baeyer,  v.,  i.  507 

Bagouski,  i.  384 

Bailey,  i.  449;  ii.  29,  538 

Baker,  i.  318,  403 

Balard,  i.  480,  494,  495,  605 

Ball,  ii.  414 

Bannoff,  i.  506 

Barfoed,  ii.  53 

Baroni,  i.  331 

Barreswill,  ii.  282, 

Baudrimont,  ii.  35 

Baume",  i.  193 

Baumgau  :i  ii.  20 

Baumhauer,  i,  495 

Bayer,  ii.  76, 159 

Bazaroff.  i.  409  ;  ii.  24,  68,  486 


BOI 

Becher,  i.  17 

Becker,  i.  16 

Beckmann,  i.  91,  496  ;  ii.  166 

Becquerel,  i.  228 ;  ii.  97,  220 

Beilby,  i.  71 

Beilstein,  i.  373  ;  ii.  188 

Beketoff,  i.  120,  122, 124,  146,  403,  459, 

466,  534,  641,  674,  577 ;  ii.  87,  102, 

289,  429 
Bender,  i.  476 
Benedict,  ii.  65 
Berglund,  ii.  229 
Bergman,  i.  27,  435 ;  ii.  100 
Berlin,  i.  95 
Bernouilli,  i.  81 
Bernthsen,  ii.  228 
Bert,  i.  86, 153 
Bertheim,  ii.  337 
Berthelot,  i.  171, 173,189,199,229,230, 

258,  264,  266,  267,  272,  283,  289,  351, 

372,  393,  394,  405,  415,424,  438,  457, 

463,  502,  506,  507,  518,  529,  537,  582; 

ii.  23,  57,  207,  209,  251,  253, 259, 345. 

367 

Berthier,  ii.  8 
Berthollet,  i.  27,  31, 105,  433,  434,  459, 

470,  502,  609 
Berzelius,  i.  131, 148, 194,  255,  379;  ii. 

8, 100, 102, 147, 148, 219, 281, 300, 48C 
Besson,  i.  288 ;  ii.  67,  70, 105, 179 
Beudant,  ii.  7,  8 

Bineau,  i.  100,  271,  452,  604;  ii.  239 
Binget,  i.  75 
Blaese,  ii.  188 
Blagden,  i.  91.  428 
Blake,  ii.  30 
Blitz,  ii.  184 
Blomstrand,  ii.  299 
Boerwald,  ii.  279 
B5ttger,  ii.  595 
Bogorodsky,  i. 
BoiUeau,  i.  415 
Boisbaudran.L.  de,  i.  97, 102,  572,  600; 

ii.  6,  26,  90,  82,  284,  483 


602 


PRINCIPLES   OF  CHEMISTRY 


Bornemann,  i.  509 

Botkin,  ii.  30 

Bouchardat,  ii.  45 

Boullay,  ii.  55 

Bourdiakoff,  i.  584,  617 

Boussingault,  i.  131,  157,  233,  235,  525, 

615 

Boyle,  i.  124 
Brand,  ii.  150 
Brandau,  i.  481 
Brandes.  i.  72 
Bravais,  i.  233 
Brauner,  i.  490,  491 ;  ii.  26,  69,  94,  96, 

97,134,144,194,271,483 
Brewster,  i.  669 
Brigham,  ii.  193 
Brodie,  i.  212,  351,  405  ;  ii.  252 
Brooke,  ii.  357 
Brown,  i.  81,  88 
Brugelltoann,  i.  616 
Brunn,  ii.  182,  189 
Bruyn,  i.  262 
Briihl,  I  263,  337 

Brunner,  i.  124, 146, 263 ;  ii.  230, 309, 534 
Buchner,  i.  615 
Buckton,  ii.  143 
Buff,  iL  103 
Bunge,  i.  288 
Bunsen,  i.  43,  69,  78,  117,  180,  465,  568, 

575,  576,  577  ;  ii.  27,  289 
Bussy,  i.  75,  594,  619 
Butleroff,  i.  143 
Bystrom,  i.  585 


CAONIARD  DE  LATOUB,  i.  135,  345 

Cahours,  ii.  143,  173 

Cailletet,  i.  132,  138  ;  ii.  45 

Callender,  i.  134 

Calvert,  i.  484  ;  ii.  45 

Cannizzaro,  i.  584,  587 

Carey-Lea,  ii.  420,  424,  425,  432 

Carius,  i.  69,  481 

Carnelley,  i.  483,  515,  555  ;  ii.  22, 29,  30, 

81,  64,  143,  486 
Carnot,  ii.  294,  361 
Caron,  i.  595, 604,  610 ;  ii.  336 
Carrara,  i.  213 
Cass,  ii.  85 

Castner,  i.  431,  535,  541 
Cavazzi,  ii.  160,  172,  182 
Cavendish,  i.  113, 125,  228 ;  ii.  493 
Chabrie,  i.  229 

Chappuis,  i.  50, 199,  205,  264 
Chapuy,  i.  59 

Cheltzoff,  i.  393,  457  ;  ii.  41,  247,  582 
Cherikoff,  ii.  102 
Chertel,  ii.  245 
Chevillot,  ii.  311 


Chevreul,  i.  530 

Chigoffsky,  ii.  62 

Christomanos,  i.  511 

Chroustchoff,  i.  353,  444 ;  ii.  122 

Chydenius,  ii.  148,  485 

Ciamician,  i.  565,  573 ;  ii.  486 

Clark,  i.  26 

Classen,  ii.  146 

Clausius,  i.  81,  93,  140,  142,  212,  309, 

491 

Clement,  i.  494 
Cleve,  ii.  26,  94,  97,  484 
Cloez,  i.  207,  246,  377 
Clowes,  i.  242 
Calderon,  i.  596 
Collendar,  i.  134 
Comaille,  i.  596 
Comb,  ii.  81 
Connell,  i.  508 
Coppet,  i.  91,  428,  601 
Corenwinder,  i.  501 
Cornu,  i.  565 
Courtois,  i.  494 
Cracow,  ii.  380 
Crafts,  i.  380  ;  ii.  80,  83 
Cremers,  ii.  100 
Croissier,  i.  251 
Crompton,  i.  247 
Crookes,  i.  229,  617;    ii.  20,  91,  96, 

440,  491 

Crum,  ii.  79,  311 
Cundall,  i.  611 
Curtius,  i.  258,  265 


DAHL,  ii.  59 

Dalton,  i.  29,  78,  81,  109,  206,  271,  322 

Dana,  ii.  8 

Davies,  i.  484 

Davy,  i.  37, 114,  195,  255,  364,460,463, 
484,  489,  494,  533,  541,  594,  604,  617 

Deacon,  i.  599 

Debray,  i.  609 ;  ii.  45,  122,  291,  293, 
384,  385 

De  Chancourtois,  ii.  20,  26 

De  Forcrand,  ii.  106,  211 

De  Haen,  ii.  189 

De  Keen,  i.  140 

Delafontaine,  ii.  9T,  148, 198 

De  la  Rive,  i.  198  ;  ii.  226 

Del-Rio,  ii.  197 

De  Saussure,  i.  235,  240 

De  Schulten,  ii.  48 

Deville,  St.-Claire,  i.  4, 36, 118, 143, 179, 
180,  227,  239,  Ji80,  281,  301,  320,  392, 
393,  399,  459,  467,  476,  477,  500,  534, 
595,  608,  609 ;  ii.  48,  80,  83,  85,  102, 
147,  156,  198,  289,  309,  321,  352,  373 
374,  429 


INDEX  OF  AUTHORITIES 


503 


De  Vries,  i.  62,  64,  429 

Dewar,  i.  3,  5,  135,  139,  163,  297,  563, 

565,  569,  585  ;  ii.  176,  220 
Dick,  ii.  414 
Dingwall,  i.  486 
Ditte,  i.  72,  403,  430, 457,  509,  539,  618  ; 

ii.  64,  65,  85,  189,  249 
Dittmar,  i.  100,  452  ;  ii.  240 
Divers,  i.  274.  294  ;  ii.  54 
Dixon,  i.  171 
Dobereiner,  i.  145 
Dokouchaeff,  i.  344 
Donny,  i.  534 
Dossios,  i.  502 
Draper,  i.  465 
Drawe,  ii.  161 
Drebbel,  i.  294 
Dulong,  i.  131,  148,  437 
Dumas,  i.  28,  131,  148,  150,  233,  302, 

320,  379,  471,  476,  584,  586,  604  ;  ii. 

22,  37,  62,  101,  156,  420 
Dumont,  ii.  197 


EBELMANN,  ii.  65 
Eder,  i.  566 
Edron,  ii.  95 
Edwards,  ii.  311 
Egoreff,  i.  569 
Eissler,  i.  553 
Elbers,  ii.  221 
Emich,  i.  286,  287 
Emilianoff,  ii.  126 
Engel,  i.  457  ;  ii.  130,  132,  189,  206 
Engelhardt,  i.  530 
Eotvos,  i.  333 
.  Erdmann,  i.  150 
Ernst,  i.  399 
Erofeeff,  i.  352 
Esson,  ii.  314 

tard,  i.  72,  516,  615  ;  ii.  238,  335,  356 
Ettinger,  i.  53,  312 


FAMINTZIN,  i.  611 

Faraday,  i.  134,  177,  296,  385,  463,  464 

Favorsky,  i.  373 

Favre,  i.  120,  172,  267  ;  ii.  83,  259, 284, 

380,  582 
Fick,  i.  62 
Fisher,  ii.  424 
Fizeau,  ii.  31,  429 
Flavitzky,  i.  21 
Fleitmann,  ii.  170 
Foerster,  ii.  375,  389 
Forchhammer,  ii.  311 
Fordos,  ii.  257 
Fortmann,  ii.  230,  366 
Fourcroy,  i.  114 


Fowler,  i.  449 

Frank,  ii.  88 

Franke,  ii.  311,  313 

Frankel,  ii.  294 

Frankenheim,  ii.  7 

Frankland,  i.  178,  357,  486 ;  ii.  16,  143 

Fraunhofer,  i.  563 

Fremy,  i.  228, 489,  492;  ii.  74,  131,  133, 

142,  229,  290,  359 
Freyer,  i.  171,  488 
Friedheim,  ii.  197,  294 
Friedel,  i.  353,   472  ;   ii.   80,   83,   103, 

122 

Friedrich,  i.  49  ;  ii.  144 
Fritzsche,  i.  94,  285,  600,  612  ;  ii.  125, 

218,  280,  341 
Fromherz,  ii.  313 
Fiirst,  i.  484 


GALILEO,  i.  7 

Garni,  i.  582 

Garzarolli-Thurnlackh,  i.  481 

Gatterrnann,  i.  596  ;  ii.  102,  104 

Gautier,  i.  585 

Gavaloffsky,  i.  160 

Gay-Lussac,  i.  40,  61,  71,  93,  170,  302, 

307,  406,  412,  460,  463,  464,  467,  500, 

506,  508,  511,  515,  534,  539  ;  ii.  8,  56, 

256 

Geber,  i.  17 
Gelis,  ii.  257 
Genth,  ii.  359 
Georgi,  ii.  197 
Georgiewics,  ii.  64 
Gerberts,  i.  528 
Gerhardt,  i.  196,  309,  357,  388 
Gerlach,  i.  525 
Gernez,  i.  97  ;  ii.  205 
Geuther,  i.  281,  283,  285  ,  ii.  176 
Gibbs,  i.  140,  464  ;  ii.  293,  410 
Girault,  i.  498 

Gladstone,  i.  337,  438,  573 ;  ii.  213 
Glatzel,  ii.  213,  289,  309 
Glauber,  f.  17,  26,  193,  432 
Glinka,  i.  607 
Goldberg,  i.  93 
Gooch,  i.  484 
Gore,  i.  489,  492,  493 
Graham,  i.   62,  63,  98,  143,  155,  388, 

429,  518,  601 ;  ii.  77,  114,  131,  163, 

170,  296,  307 
Granger,  ii.  157,  410 
Grassi,  i.  88 
Green,  ii.  310 
Greshoff,  i.  403 
Griffiths,  i.  135 
Grimaldi.i.  537 
Groth,  ii.  10 


504 


PRINCIPLES  OF  CHEMISTRY 


ORO 

Grouven,  i.  615 
Grove,  i.  118,  119 
Griinwald,  i.  573 
Grutzner,  ii.  296 
Guckelberger,  ii.  84 
Guibourt,  ii.  53 
Guldberg,  i.  439,  464 
Giintz,  i.  575  ;  ii.  430 
Gustavson,  i.  443,  444,  472,  505,  547 ; 

ii.  29,  175 

Guthrie,  i.  99,  428,  601 
Guy,  i.  136 


HABERMANN,  ii.  210 

Hagebach,  i.  573 

Hagen,  i.  337 

Haitinger,  i.  593 

Hammed,  i.  613 

Hanisch,  ii.  233 

Hannay,  i.  352  ;  ii.  135 

Harcourt,  ii.  314 

Hargreaves,  i.  515 

Harris,  ii.  52 

Hartley,  i.  573  ;  ii.  486 

Hartog,  ii.  268 

Hasselberg,  i.  566 

Haiiy,  ii.  7 

Haughton,  ii.  20 

Haussermann,  i.  483 

Hautefeuille,  i.  199,«205,.264,  409,  414, 

476,  477,  501,  538  ;  ii.  102,  122,  379 
Hayter,  ii.  175 
Hemilian,  i.  132 
Hempel,  i.  59,  524 
Henkoff,  i.  530 
Henneberg,  ii.  170 
Henning,  ii.  3 
Henry,  i.  78,  81 
Herard,  ii.  191 
Hermann,  ii.  8,  47,  197 
Hermes,  i.  529 
Hertz,  ii.  156 
Hess,  i.  178,  588 
Heycock,  i.  537  ;  ii.  128,  448 
Hillebrand,  ii.  26,  93,  94,  484 
Hintze,  ii.  10 
Hirtzel,  ii.  55 
Hittorf,  ii.  155 
Hodgkinson,  ii.  432 
Hoglund,  ii.  94 

Hofmann,  i.  302  ;  ii.  146,  218,  447 
Holtzmann,  i.  505 
Hoppe-Seyler,  i.  611 
Horstmann,  i.  408 
Houzeau,  i.  202 
Hughes,  ii.  212 
Hugo,  ii.  21 
Humboldt,  i.  170 


Humbly,  i.  493 ;  ii.  311 
Hutchinson,  i.  491 
Huth,  ii.  20 
Huyghens,  i.  569 


IKEDA,  ii.  152 
Ilosva,  i.  202 

Inostrantzeff,  i.  345  ;  ii.  4 
Isambert,  i.  250,  257,  408  ;  ii.  41 
Ittner,  i.  412 


JANSSEN,  i.  569 

Jawein,  ii.  170 

Jay,  i.  258 

Jeannel,  i.  104 

Joannis,  i.  251,  255,  405,  537,  559 

Jorgensen,  i.  498  ;  ii.  359,  361,  376 

Johnson,  ii.  45 

Jolly,  i.  233 

Joly,  ii.  384,  385 


KAMENSKY,  ii.  414 

Kammerer,  i.  286,  402,  509  ;  ii.  297 

Kane,  ii.  57 

Kapoustin,  i.  403 

Karsten,  i.  427,  428,  541,  599 

Kassner,  i.  158 

Kayander,  i.  133,  384;  ii.  46 

Reiser,  i.  150 

Kekule,  i.  358,  309,  507  ;  ii.  294 

Keyser,  ii.  33 

Khichinsky,  i.  440 

Kimmins,  i.  510 

Kirchhoff,  i.  567 

Kirmann,  ii.  268 

Kiipieheff,  i.  132 

Kjeldahl,  i.  249 ;  ii.  •-'!!> 

Klaproth,  ii.  7,  145,  147,  301 

Kleiber,  i.  570 

Klimenko,  i.  465 

Klobb,  ii.  357 

Klodt,  i.  426 

Knopp,  ii.  338 

Knox,  i.  489 

Kobb,  ii.  125 

Kobell,  ii.  197 

Koch,  i.  44 

Kohlrausch,  i.  245,  525 

Kolbe,  i.  506 

Konovaloff,  i.  39,  65,  90,  93,  100,  140, 

142,  172,  322 ;  ii.  235,  268 
Kopp,  i.  586,  587,  612  ;  ii.  3,  37 
Koucheroff,  i.  373 
Kolotoff,  i.  263 

Kournakoff,  i.  393  ;  ii.  294,  365,  396 
Kouriloff,  i.  209,  247,  274 ;  ii.  41 


INDEX   OF  AUTHOKITIES 


505 


Oft 

Kraevitoh,  i.  133, 134 
Kraft,  i.  65,  88,  537 
Krafts,  i.  393 
Kreisler,  i.  233 

Kremers,  i.  87,  443  ;  li.  244,  437 
Kreider,  i.  484 
Kronig,  i.  81 
Kruger,  ii.  282,  284 
Kriiss,  ii.  355,  442,  447, 486 
fcubierschky,  ii.  213 
Kuhlmann,  i.  608 
Kuhnheim,  i.  612 
Kundt,  i.  328,  589 ;  ii.  498 
Kvasnik,  ii.  57 
Kynaston,  i.  522 


LACHINOFF,  i.  116,  457  ;  ii.  410 

Ladenburg,  ii,  103 

Lamy,  ii.  91 

Landolt,  i.  7,  837 

Lang,  i.  399 

Langer,  i.  226,  459,  462 

Langlois,  i.  570  ;  ii.  257 

Latchinoff  (see  Lachinoff),  i.  103,  352 

Laurent,  i.  28,  196,  388,  471,  526  ;  ii.  7, 

9,  10,  117,  292 

Laurie,  i.  106 ;  ii.  32,  442, 486 
Lavenig,  i.  140 
Lavoisier,  i.  7,  29,  49,  114,   131,   155, 

379,  459 
Leblanc,  ii.  8 
Le  Chatelier,  i.  158,  172,  350,  393,  399, 

585,  588,  611 ;  ii.  51,  65,  420 
Le  Due,  i.  131,  170 
Lemery,  i.  125 
Lemoine,  i.  501 ;  ii.  155 
Lerch,  i.  405 
Leroy,  i.  285 
Lescoeur,  i.  103 
Leton,  ii.  425 
Levy,  ii.  102 
Lewes,  i.  371 
Lewy,  i.  232 
Lidoff,  ii.  209 

Liebig,  i.  195,  388,  495,  527 ;  ii.  50 
Linder,  ii.  223 
Lies-Bodart,  i.  604,  612 
Lisenko,  i.  373 
Liveing,  i.  563,  569 
Lockyer,  i.  565,  569 
Loevv,  ih.376 

L6wel*j»525,  600;  ii.  45,  284,  286 
LoewigTi.  528 ;  ii.  77 
Loewitz,  i.  96 
Lessen,  i.  262 
Louget,  i.  489 
Louginine,  i.  360 
Louise,  ii.  81 


Lovel,  i.  515  ;  ii.  338 
Lubavin,  i.  593 
Lubbert,  ii.  85,  170 
Ludwig,  i.  463 
Luedeking,  ii.  194 
Luff,  ii.  321 
Lunge,  ii.  244,  246 
Liipke,  ii.  157 
Lvotf ,  i.  358 


MAACK,  i.  596 

Mac  Cobb,  i.  612 

Mac  Laurin,  i.  553 

McLeod,  ii.  180 

Magnus,  i.  93,  510 

Mailfert,  i.  199 

Malaguti,  i.  437, ;  ii.  300 

Mallard,  i.  172,  393,  588  ;  ii.  4 

Mallet,  i.  493 

Maquenne,  i.  349,  620,  621 

Marchand,  i.  150 

Marchetti,  ii.  288 

Maresca,  i.  534 

Marguerite,  ii.  292 

Marignac,  i.  198,  233,  428,  430,  453,  454, 

518,  525,  600,  601 ;  ii.  6,  9,  95,  101, 

194,  197,  198,  199,  234,  239,  241,  244, 

292,  293,  295,  357,  440,  486 
Markleffsky,  i.  273 
Markovnikoff,  i.  373 
Maroffsky,  ii.  138 
Marshall,  ii.  253,  365 
Matigon,  i.  258,  266 
Maumene,  i.  258 
Maxwell,  i.  81 
Mayow,  i.  17 
Mendeleeff.i.  99,  132,133,  136, 141,275, 

321,  357,  373,  377,  406,  426,  427,  428, 

506,  587,  596  ;  ii.  27,  33,  93,  94 
Menschutkin,  i.  171 
Mente,  ii.  270 
Merme,  i.  462 
Merz,  i.  505 
Meselan,  i.  463 
Metzner,  ii.  189 
Metchikoff,  i.  44 
Meusnier,  i.  114 
Meyer  (Lothar),  i.  226,  321,  403;  ii.  21, 

24,  26,  29,  33,  486 
Meyer  (Victor),  i.  135, 171, 294, 303,  320, 

427,  459,  462,  467,  488,  506,  508, 558; 

ii.  43,  48,  52,  80,  129, 184 
Meyerhoffer,  ii.  410 
Miasnikoff,  i.  372 
Michaelis,  ii.  175 
Michel,  i.  65,  88 
Millon,  i.  481,  484,  508 
Mills,  ii.  20 


506 


PRINCIPLES  OF  CHEMISTKY 


tor 

Mitchell,  i.  156 

Mitscherlich,  i.  428,  527 ;  ii.  1, 5,  6, 156,. 
184,  311,  313 

Moissan,  i.  202,  349,  353,  490,  564,  585, 
621 ;  ii.  66,  67,  70,  88,  100,  107,  147, 
174, 196,  289,  295,  309,  311,  313,  321 

Mond,  i.  129,  400,  405 ;  ii.  345,  367- 

Monge,  i.  114 

Monnier,  i.  611 

Montemartini,  i.  279 

Moraht,  ii.  384 

Moreau,  ii.  298 

Morel,  i.  549 

Mosander,  ii.  97 

Miihlhauser,  ii.  66, 107 

Muir,  ii.  193 

Mulder,  i.  515 

Miiller-Erzbach,  i.  103 

Muller,  i.  427  ;  ii.  425 

Munster,  ii.  443 

Miintz,  i.  238,  241,  420,  553 

Muthmann,  ii.  273 

Mylius,  ii.  375,  389 

NASCHOLD,  i.  483 

Nasini,  i.  496 ;  ii.  156 

Natanson,  i.  282,  409 

Natterer,  i.  132, 135, 141,  385 

Naumann,  i.  399,  408 

Nernst,  i.  62,  148 ;  ii.  3,  50 

Nensky,  i.  245 

Neville,  i.  537 ;  ii.  128,  448 

Newlands,  ii.  21,  26 

Newth,  i.  505 

Newton,  i.  7,  29 

Nickles,  ii.  10 

Nikolukin,  i.  491 ;  ii.  144 

Nilson,  i.  618  ;  ii.  26,  37,  80,  83,  W«  94, 

95,  271,  378,  483 
Nordenskiold,  i.  241 
Norton,  i.  76 ;  ii.  94 
Nuricsan,  ii.  264 


ODLING,  ii.  52 

Offer,  i.  99 

Ogier,  i.  321,  509  ;  ii.  159,  182 

Olszewski,  i.  139,  569 ;  ii.  491,  491 

Oppenheim,  i.  506 

Ordway,  ii.  80 

Osmond,  ii.  326 

Ossovetsky,  ii.  137 

Ostwald,  i.  89,  92,  389,  441,  443 

Ournoff,  i.  62 


PALLARD,  i.  491  ,  ii.  83 

Panfeloff,  i.  603 

Paracelsus,  i.  17, 125,  129,  379 


QtJl 

Parkinson,  i.  596, 
Pashkoffsky,  i.  595 
Pasteur,  i.  44,  241,  242 
Paterno,  i.  496  ;  ii.  156 
Pattison  Muir,  i.  436 
Pebal,  i.  315,  484 
Pechard,  ii.  282,  294,  296,  297 
Pekatoros,  i.  465 
Peligot,  ii.  299,  301 
Pelopidas,  ii.  22,  481 
Pelouze,  i.  463,  464,  480,  610 ;  ii.  229 
Penfield,  i.  545  ;  ii.  370 
Perkin,  i.  558 ;  ii.  244 
Perman,  i.  537 
Personne,  i.  75,  506,  537 
Petit,  i.  584,  586 
Petrieff,  i.  440 
Pettenkofer,  ii.  22 
Pettersson,  i.  618,  619 ;  ii.  37,  80,  83, 91, 

197,  484 

Pfaundler,  i.  445  ;  ii.  241,  430 
Pfeiffer,  i.  64 
Pfordten,  V.  der,  ii.  420 
Phipson,  i.  596 ;  ii.  59 
Piccini,  ii.  23,  146,  197,  288,  298 
Pici,  ii.  57 

Pickering,  i.  88,  91,  99,  104,  106,- 272, 
333,  452,  517,  525,  529,  613  ;  ii.  241, 
245,  246,  247 

Pictet,  i.  81,  129,  137  ;  ii.  31,  241 
Picton,  ii.  223 

Pierre,  i.  452,  495  ;  ii.  226,  485 
Pierson,  i.  93 
Pigeon,  ii.  377 
Pionchon,  i.  585 
Pistor,  i.  399 
Plantamour,  ii.  5 
Plaset,  ii.  289 
Plessy,  ii.  257 
Pliicker,  i.  572 
Poggiale,  i.  427 
Poiseuille,  i.  355 
Poleck,  ii.  296 
Poluta,  ii.  30 
Popp,  ii.  232 
Potilitzin,  i.  96,  97,  98,  445,  486,  499, 

502,  509,  612  ;  ii.  29,  357 
Pott,  ii.  100 
Pouleno,  ii.  174,  289 
Prange,  ii.  422 
Prelinger,  ii.  310 

Priestley,  i.  17,  154, 159,  297,  379,  402 
Pringsheim,  i.  465 
Prost,  i.  98,  486 
Prout,  i.  31 ;  ii.  439 
Puchot,  i.  452 
Pullinger,  ii.  389 

QUIKCKE,  i.  427,  495 


INDEX  OF  AUTHORITIES 


607 


RAMMELSBERG,  i.  430,  510,  525 ;  ii.  26, 

161,  485 
Ramsay,  i.  133,  140,  141,  232,  247,  333, 

495,  496,  581 ;  ii.  128,  491 
Rantsheff,  ii.  20 

Raoult,  i.  91,  274,  330,  331,  332,  429 
Rascher,  ii.  85 
Raschig,  i.  263 ;  ii.  229 
Rathke,  i.  399 
Ray,  i.  17 

Rayleigh,  i.  226,  232,  491 
Rebs,  ii.  213,  217 
Recoura,  i.  332 ;  ii.  289 
Regnault,  i.  40,  53,  54,  90,  93,  181,  133, 

297,  443,  495,  584,  587,  588 ;  ii.  60, 

208,  238 
Reich,  ii.  91 
Reiset,  i.  238 
Remsen,  ii.  335 
Retgers,  ii.  157,  158,  180 
Reychler,  ii.  65 
Reynolds,  i.  581 

Richards,  i.  526,  585  ;  ii.  32,  432 
Riche,  i.  509  ;  ii.  127,  292 
Richter,  i.  193,  194  ;  ii.  91 
Ridberg,  ii.  21,  24,  486 
Riddle,  i.  135 
Rideal,  ii.  297 
Roberts-Austen,  ii.  486 
Robinson,  i.  515 
Rodger,  ii.  213,  263 
Rodwell,  i.  17 
Roebuck,  i.  294 
Roggs,  ii.  119 
Rohrer,  ii.  343 
Roozeboom,  i.  106,  452,  453,  464,  496, 

506,  511,  599,  613 ;    ii.  3,  226,  341, 

410 
Roscoe,  i.  80,  100,  101,  379,  452,  463, 

485,  486,  568,  572 ;  ii.  26,  194,  196, 

197,  297,  303,  485 
Rose,  i.  436,  437,  518,  525,  608,  612  ; 

ii.  8,  230,  235,  248,  281,.  363,  428,  485 
Rosenberg,  ii.  351 
Rossetti,  i.  428 
Rouart,  Le,  ii.  86 

Rousseau,  i.  354  ;  ii.  337,' 366,  378 
Roux,  ii.  81 
Rudberg,  ii.  136 
Riicker,  i.  142 
RudorfiYi.  91, 428,  598,  601 
Rybalkin,  i.  455 


SABANEEFF,  i.  371 
Sabatier,  i.  284 ;  ii. 
Saint  Edme,  ii.  335 
Saint  Gilles,  i.  431 
Sakurai,  i.  331 


Salzer,  ii.  161 

Sarasin,  ii.  122 

Sarrau,  i.  140,  142 

Saunders,  ii.  189 

Scharples,  i.  576 

Scheele,  i.  155,  161,  412,  459,  462,  6.08 ; 

ii.  100,  150,  291 
Scheffer,  i.  453 
Scheibler,  ii.  292,  296 
Scherer,  ii.  8 
Schiaparelli,  ii.  318 
Schidloffsky,  i.  238 
Schiloff,  i.  212 
Schlamp,  i.  332 
Schiflf,  i.  430,  588  ;  ii.  106,  267 
Schloesing,  i.  238,  239,  240,  553,  610 
Schmidt,  i.  539 
Schneider,  i.  89 
Schone,  i.  208,  209,  211, 394, 617  ;  ii.  15, 

72,  219,  251,  488 
Schonebein,  i.  198,  202,  208,  212,  509  ; 

ii.  228,  463 
Schottlander,  ii.  447 
Schroder,  i.  75 
Schroederer,  ii.  366 
Schrotter,  ii.  153,  284 
Schiitzenberger,  i/Sll,  579  ;  ii.  102, 107, 

228,  367,  389 
Schuliachenko,  i.  608 
Schuller,  ii.  180 
Schultz,  i.  518 ;  ii.  273 
Schulze,  i.  98 ;  ii.  215 
Schuster,  i.  572 
Schwicker,  ii.  227,  230 
Scott,  i.  405,  537,  558 
Sechenoff,  i.  80,  86 
Seelheim,  ii.  379 
Sefstrom,  ii.  197 
Selivanoff,  i.  476,  507,  508 
Senderens,  i.  284 
Serullas,  i.  485 
Setterberg,  i.  576 
Seubert,  ii.  27,  83,  343,  442 
Sewitsch,  i.  372 
Shaffgotsch,  i.  565 
Shapleigh,  ii.  95 
Shenstone,  i.  611 
Shields,  i.  333 
Shishkoff,  i.  276 ;  ii.  56 
Silbermann,  i.  120,  172  ;  ii.  259 
Sims,  ii.  268 
Skraup,  ii.  346 
Smith,  i.  271 
Smithson,  ii.  100 
Snyders,  ii.  100 
Sokoloff,  ii.  85,  122 
Solet,  i.  5Q9 
Sonstadt,  ii.  443 
Sorby,  i.  88 


508 


PRINCIPLES  OF  CHEMISTRY 


son 

Soret,  i.  66,  202,  203,  427 
Spring,  i.  38,  98,  434,  486 ;  ii.  46,  50, 

133,  223,  258,  288,  314,  423,  427 
Btadion,  i.  485 
Stahl,  i.  16 
fcjtas,  i.  7,  233,  379,  428,  498,  581 ;  ii. 

420,  434,  485 
Staudenmaier,  ii.  168 
Stcherbakoff,  i.  97,  428,  458,  601 
Btohmann,  i.  359,  360,  396 
Stokes,  i.  355 
Stortenbeker,  i.  511 
Stromeyer,  ii.  47 
Struve,  i.  208,  612 

TAIT,  i.  203 

Tammann,  i.  91, 148 ;  ii.  170)  247 

Tanatar,  i.  511 

Tchitcherin,  ii.  21 

Terreil,  ii.  313 

Than,  i.  317 

Thenard,  i.  207,  229, 460, 464,  534, 539  ; 

ii.  251 

Thillot,  ii.  170 
Thilorier,  i.  385 
Thomsen,  i.  Ill,  120, 124, 131, 173, 189, 

267,  359,  389,  396,  441,  453,  466,  472, 

494,  502,  515,  529,  555,  582 ;  ii.  9,  32, 

50,  55,  105,  165,  208,  224,  264,  368, 

370,  438,  442 
Thorpe,  i.  142,  285, 445, 493 ;  ii.  27, 160, 

173,  213,  259,  263,  268,  301,  313,  442, 

486 

Thoune,  i.  294,  295 
Tiemman,  i.  213 
Tilden,  i.  516 
Timeraseeff,  i.  170 
Timofeeff,  i.  78 
Tessi6  du  Motay,  i.  158 
Tissandier,  i.  78 
Titherley,  i.  539 
Tivoli,  ii.  183 
Tomassi,  ii.  339 
Topsoe,  i.  506 
Tourbaba,  i.  88  ;  ii.  247 
Trapp,  i.  511 

Traube,  i.  312,  611 ;  ii.  270 
Troost,  i.  64,  274,  281,  320,  409,  414, 

500,  538 ;    ii.  80,  83,  102,  147,  156, 

254,  379 

Tscherbacheff,  i.  577 
Tutton,  i.  543  ;  ii.  160, 174,  412 

UMOFF,  i.  429 
Unverdorben,  ii.  280 
Urlaub,  ii.  301 

VALENTINE,  i.  17 
Van  der  Heyd,  i.  599 


wn, 

Van  der  Plaats,  i.  496 ;  ii.  439 

Van  der  Waals,  i.  82, 140 

Van  Deventer,  i.  599 

Van  Helmont,  i.  379 

Van  Marum,  i.  198 

Van  't  Hoff,  i.  64,  65,  331,  599  5  ii.  8 

Vare,  ii.  55 

Vauquelin,  i.  114,  619 ;  ii,  7 

Veeren,  i.  612  ;  u.  45 

Veley,  i.  279 

Verneuille,  ii.  225 

Vernon,  ii.  151 

Vezes,  ii.  391 

Viard,  ii.  285 

Viguon,  ii.  126,  131 

Villard,  i.  106,  296,  297 

Villiers,  ii.  259 

Violette,  i.  342,  345 

Violle,  i.  301 

Vogt,  i.  611 

Volkovitch,  ii.  201 

Voskresensky,  i.  345 


WAAOE,  i.  439 

Wachter,  i.  508 

Wagner,  i.  357 

Wahl,  ii.  310 

Walden,  ii.  57 

Walker,  ii.  143 

Walmer,  i.  573 

Walter,  ii.  256 

Walters,  ii.  234 

Wanklyn,  i.  100,  539 

Warburg,  i.  589  ;  ii.  496 

Warder,  i.  450 

Warren,  ii.  102 

Watson,  i.  527  ;  ii.  169 

Watts,  i.  526 

Weber,  i.  280,  583  ;  ii.  83, 129, 131, 186, 

230,  233,  234,  249 
Weith,  ii  502 
Weitz,  ii.  57 
Welch,  ii.  425 
Weller,  ii.  146 

Wells,  i.  477,  545  ;  ii.  57,  370 
Welsbach,  ii.  96,  97 
Weltzien,  i.  204,  595 
Wenzel,  i.  193 

Weruboif  (see  Wyruboff),  ii.  4 
Weselski,  i.  507 
Weyl,  i.  255 
Wheeler,  i.  545 
Wichelhaus,  ii.  179 
Wiedemann,  i.  439,  688 
Wilhelmj,  ii.  315 
Willgerodt,  i.  608 ;  ii.  29 
Williamson,  ii.  268 
Wilm,  ii.  376,  388 


INDEX  OF  AUTHORITIES 


$09 


Winkler,  i.  78,  79,  677,  594, 621 ;  ii.  25, 

30,  66,  97,  102,  124,  125,  147.  234, 

246,  355,  483 
Wischin,  ii.  384 
Wislicenus,  i.  267,  294 
Witt,  ii.  3 
Wohler,  i.  410,  619;  ii.  85,  103,  107, 

146,  285,  289,  420,  425 
Wollaston,  i.  8 
Wreden,  i.  507 
Wright,  ii.  321 
Wroblewski,  i.  79,  80,  10&  139,  387  ; 

ii.  226 

Wulfing,  ii.  119 
Wiillner,  i.  91,  572 


ZOB 

Wiirtz,  i.  301,  476 ;   ii.  171,  173,  213, 

267 
Wyruboff,  ii.  4,  9 


YOUNG,  i.  134,  136,  140,  141,  247,  494- 
496  ^* 


ZABOUDSKY,  i.  354 
Zaencheffsky,  i.  140 
Zimmermann,  ii.  26,  303,  355,  485 
Zinin,  i.  276 
Zorensen,  i.  284 
Zorn,  i.  295 


SUBJECT    INDEX 


ACID,  acetic  sp.  gr.  of  solutions  of,  i.  59 

—  arsenic,  ii.  181 

—  bismutbic,  ii.  190 
-~  boric,  ii.  64 

—  carbamic,  i.  408 

—  chamber,  i.  294 

—  chloric,  i.  482 

•>—  chloro-platino-phosphorous,  ii.  890 

—  chlorosulphonic,  ii.  268 

—  chlorous,  i.  481 

—  chromic,  i.  208  ;  ii.  282 

—  chromo-sulphuric,  ii.  288 

—  cyanic,  i.  409 

—  cyanuric,  i.  409 

—  dithionic,  ii.  256 

—  ferric,  ii.  344 

—  fluoboric,  ii.  69 

—  graphitic,  i.  351     - 

—  hydriodic,  i.  501,  503,  505,  506 

—  hydro-boro-fluoric,  ii.  69 

—  hydrobromic,  i.  80,  503,  505,  506 

—  hydrochloric,  i.  448,  451,  453 

—  hydrocyanic,  i.  406,  411 

—  hydro-ferro-cyanic,  ii.  348 

—  hydrofluoric,  i.  49 

—  hydrofluosilic,  ii.  106 

—  hydro-platino-cyanic,  ii.  386 

—  hydrosulphurous,  ii.  228 

—  hydro-rutheno-cyanic,  ii.  388 

—  hypochlorous,  i.  479,  481 

—  hyponitrous,  i.  265,  294 

—  hypophosphoric,  ii.  101 

—  hypophosphorous,  ii.  172 

—  iodic,  i.  100,  508 

—  isethionic,  ii.  250 

—  metantimonic,  ii.  188 

—  metaphosphoric,  ii.  162,  169 

—  metastannic,  ii.  131 

—  molybdic,  ii.  292 

—  nitric,  i.  268,  272 

—  Nordhausen,  ii.  233 

—  orthophosphoric,  ii.  162 

—  osmic,  ii.  384 

—  pentathionic,  ii.  257 


Acid,  percarbonic,  i.  394 

—  perchloric,  i.  484 

—  periodic,  i.  510 

—  permanganic,  ii.  313 

—  permolybdic,  ii.  297 

—  pernitric,  i.  264 

—  persulphurio,  ii.  251 

—  pertungstic,  ii,  297 

—  phosphamic,  ii.  179 

—  phosphamolybdic,  ii.  293 

—  phosphorous,  ii.  171 

—  polysilicic,  ii.  117 

—  pyrophosphoric,  ii.  169 

—  pyrosulphuric,  ii.  234 

—  silenic,  ii.  272 

—  silico-tungstic,  ii.  295 

—  stannic,  ii.  130 

—  sulphonic,  ii.  249 

—  sulphuric,  i.  76,  77,89,  111,  290,  294; 

ii.  235,  238,  241 

—  telluric,  ii.  272 

—  tetrathionic,  ii.  257 

—  thipcarbonic,  ii.  263 

—  thiocyanic,  ii.  263 

—  thionic,  ii.  255 

—  thiosulphuric,  ii.  230 

—  trithionic,  ii.  257 

—  tungstic,  ii.  292,  294 

—  vanadic,  ii.  196 
Acids,  i.  185 

—  avidity  of,  i.  389,  442 

—  basicity  of,  i.  387 

—  complex,  i.  197  ;  ii.  293 

—  fuming,  i.  102 

—  organic,  i.  394,  396,  405 
Acetylene,  i.  372 
Actinium,  ii.  59 

Affinity,  chemical,  i.  26,  389 
Air,  i.  131,  231,  233 
Alchemy,  i.  14 
Alcohol,  i.  53,  88 
Alkali,  metals,  i.  558,  577 

—  waste,  ii.  204 
Alkalis,  i.  186 


SUBJECT  INDEX 


511 


Allotropism,  i.  207 
Alloys,  ii.  128,  537 
Alumina,  ii.  75 
Aluminium,  ii.  70,  85 

—  bromide,  ii.  84 

—  bronze,  ii.  88 

—  carbide,  ii.  88 

—  chloride,  ii.  80,  83 

—  double  chlorides,  ii.  84 

—  fluoride,  ii.  83 

—  hydroxide,  ii.  75 

—  iodide,  ii.  85 

—  nitrate,  ii.  80 

—  sulphate,  ii.  82 
Alums,  ii.  5,  82,  343 
Alunite,  ii.  80 
Amalgams,  ii.  58 
Amides,  i.  258,  406 
Amidogen,  i.  258 

—  hydrate,  i.  258 
Amines,  i.  416 
Ammonia,  i.  229,  246 

—  of  crystallisation,  i.  257 

—  heat  of  solution  of,  i.  74 

—  in  air,  i.  240 

—  liquefaction  of,  i.  250 

—  salts,  i.  254 

—  soda  process,  i.  524 

—  solutions  of,  i.  80,  252 
Ammonium,  i.  254 

—  amalgam,  i.  255 

—  bicarbonate,  i.  527 

—  carbamate,  i.  407,  408 

—  carbonate,  i,  407 

—  cobalt  salts,  ii.  359 

—  dichromate,  ii.  279 

—  molybdate,  ii.  292 

—  nitrate,  i.  273,  274 

—  nitrite,  i.  284 

—  phosphates,  ii.  167 

—  sulphate,  ii.  269 

—  sulphide,  ii.  218 

Analogy  of  elements,  i.  573,  578 
Anthracite,  i.  345 
Antimoniuretted  hydrogen,  ii.  180; 
Antimony,  ii.  186 

—  chlorides,  ii.  189 

—  oxides,  ii.  187,  188 

—  sulphides,  ii.  221 
Aqua  Eegia,  i.  467 
Aqueous  radicle,  i.  213 
Argon,  i.  226,  232  ;  App.  III. 
Arsenic,  ii.  179 

—  anhydride,  ii.  181 
~  sulphides,  ii.  221 

—  tribromide,  ii.  181 

—  trichloride,  ii.  180 

—  trifluoride,  ii.  18.1 
Arsenious  anhydride,  ii,  184 

*K 


Arsenious  oxychloride,  ii.  180 
Arsenites,  ii.  185 
Arseniuretted  hydrogen,  ii.  182 
Astrakhanite,  i.  59 
Atmolysis,  i.  156 
Atomic  theory,  i.  210 

—  volumes,  ii.  33 

—  weights,  i.  21 

Atoms  and  molecules,  i.  322 


BABITJM,  i.  614,  617 

—  chlorate,  i.  483 

—  chloride,  i.  615 

—  hydroxide,  i.  616 

—  metatungstate,  ii.  295 

—  nitrate,  i.  615 

—  oxide,  i.  616 

—  peroxide,  i.  157,  160,  209,  617 

—  sulphate,  i.  614,  615 
Bauxite,  ii.  76 
Benzalazine,  i.  258 
Berthollet's  doctrine,  i.  433 
Beryllium,  i.  618 

—  atomic  weight  of,  i.  325,  618 

—  chloride,  i.  584 

—  oxide,  i.  619 
Binary  theory,  i.  195 
Bismuth,  ii.  189 

—  nitrates,  ii.  192 

—  oxides,  ii.  190,  191 
Blast  furnace,  ii.  324 
Bleaching,  i.  469 
-^•powder,  i.  162,  477 
Boiling  point,  absolute,  i.  130 
Borates,  ii.  63 

Borax,  ii.  61 

Boric  anhydride,  ii.  64 

Boron,  ii.  60,  66 

—  chloride,  ii.  69 

—  fluoride,  ii.  67,  68 

—  iodide,  ii.  70 

—  nitride,  i.  227  ;  ii.  67 
-    oxide,  ii.  60 

—  specific  heat  of,  i.  585 

—  sulphide,  ii.  62 
Bromides,  ii.  32 
Bromine,  i.  494 
Bronze,  ii.  127 

Butyl  alcohol,  solubility  of,  i.  75 


CADMIUM,  ii.  47 

—  iodide,  ii.  48 

—  oxide,  ii.  48 

—  sulphide,  ii.  47 
Caesium,  i.  576 
Calcium,  i.  590,  604 

—  carbonate-,.!.  592,  608,  609,  610 


512 


PRINCIPLES  OF  CHEMISTRY 


Calcium  chloride,  i.  237,  612 
crystallohydrates  of,  i.  613 

—  fluoride,  i.  491 

—  hypochlorite,  i.  162 

—  iodide,  i.  604 

—  peroxide,  i.  607 

—  phosphate,  ii.  167 

—  sulphate,  i.  611 

—  sulphide,  ii.  220 
Calomel,  ii.  64 
Carbamide,  i.  409 
Carbides,  i.  349,  853 
Carbon,  i.  338 

—  bisulphide,  ii.  258 
• —  molecule  of,  i.  354 

—  oxysulphide,  ii.  264 

—  tetrachloride,  i.  473 
Carbonic  anhydride,  i.  370 

assimilation  of  by  plants,  i.  393 

dissociation  of,  i.  392,  393,  899 

in  air,  i.  238,  242 

—  —  liquid,  i.  385 

—  —  solutions  of,  i.  80,  86 

specific  heat  of,  i.  393 

Carbonic  oxide,  i.  396 

and  nickel,  i.  405 

Carborundum,  ii.  107 
Carboxyl,  i.  395 
Carnallite,  i.  421,  644,  560 
Catalytic  phenomena,  i.  211 
Caustic  potash,  i.  550 

—  soda,  i.  529 
Cements,  ii.  122 
Cerite  metals,  ii.  93 
Cerium,  ii.  93 

Chamber  crystals,  i.  290 ;  ii.  230 

Charcoal,  i.  343 

Chemical  change,  rate  of,  ii.  314 

—  transformations,  i.  3 
Chloranhydrides,  i.  468 ;  ii.  174,  176, 

177 

Chlorates,  i.  482 
Chlorides,  i.  455,  466;.  ii.  31 
Chlorine,  i.  4G3 

—  compounds,  heat  of  formation  of,  i.  44 

—  crystallohydrates  of,  i.  464 

—  oxides,  i.  479 

—  preparation  of,  i.  460 

—  solubility  of,  i.  463 
Chloroform,  i.  473 
Chlorophosphamide,  ii.  179 
Chloryl  compounds,  i.  476 
Chrome  alum,  ii.  283 
Chromic  acid,  i.  208 

—  anhydride,  ii.  280 

—  oxide,  ii.  284,  285 
Chromium,  ii.  276,  289 

—  chlorides,  ii.  285 

—  fluorides,  ii.  280,  289 


Chromyl  chloride,  ii.  281 
Chryseone,  ii.  108 
Clay,  ii.  70 
Coal,  i.  345 
Cobalt,  ii.  353 

—  dioxide,  ii.  366 

—  fluoride,  ii.  358 
Cobaltamine  salts,  ii.  359 
Cobaltic  oxide,  ii.  362 
Cobalto-amine,  ii.  359 
Cobaltous  hydroxide,  ii.  358 
Cohesion  of  liquids,  i.  52 
Coke,  i.  345 

Collodion  cotton,  i.  275 
Colloids,  i.  63  ;  ii.  77,  423 
Combination,  chemical,  i.  3 
Combining  weights,  i.  21 ;  ii.  439 
Combustion,  imperfect,  i.  341 

—  heat  of,  i.  172,  176,  399,  400 
Compounds,  definite  and  indefinite,  i.  31 

—  types  of,  ii.  10 
Compressibility  of  solutions,  i.  88 
Conductivity,,  electro-molecular,  i.  389 
Contact  reactions,  i.  163,  290 
Copper,  ii.  400 

—  carbonate,  ii.  411 

—  complex  salts  of,  ii.  412 

—  nitrate,  ii.  411 

—  nitride,  ii.  409 

—  sulphate,  ii.  413 
Corundum,  ii.  75 
Critical  points,  i.  141 
Cryohydrates,  i.  99 

Cryoscopio  investigations  of  solutions 

i.  90,  332 
Crystals,  i.  51 
Crystalline  form,  ii.  7 
Crystallo-hydrates,  i.  102 
Crystalloids,  i.  63 
Cupellation,  ii.  417 
Cyanides,  i.  406 
Cyanogen,  i.  406,  414 

—  chloride,  ii.  176 


DECOMPOSITION,  chemical,  i.  4 

Deliquescence,  i.  104 

Delta  metal,  ii.  414 

Desiccaior,  i.  68 

Detonating  gas,  i.  115, 170, 178 

Depression  of  freezing  point  of  sola- 

tions,  i.  90,  92,  330 
Dialysis,  i.  63  ;  ii.  114 
Diamond,  i.  350,  353 
Didymium,  ii.  93 
Diffusion,  rate  of,  i.  63 
Dimorphism,  i.  610,  ii.  178 
Disinfectants,  i.  245 
Diaodium  orthpphosphate,  ii.  166 


SUBJECT  INDEX 


513 


Dissociation,  i.  30,  282,008 
Distillation,  dry,  i.  4,  247,  342 
Dust,  atmospheric,  i.  241 


EFFLORESCENCE,  i.  103 
Ekacadmium,  ii.  59 
Ekasilicon,  ii.  25 
Electro-chemical  theory,  i.  195 
Electric  energy  and   thermal   units,  i. 

682 

Electrolysis,  i.  110 
Elements,  i.  20 

—  grouping  of,  ii.  1 

—  typical,  ii.  19 
Emulsions,  i.  98 
Energy,  chemical,  i.  29 
Equations,  chemical,  i.  278 
Equivalents,  law  of,  i.  194 
Equivalent  weights,  i.  581 
Ethane,  i.  300 

Ether,  critical  points  of,  i.  141 

Ethylene,  i.  370 

Ethyl  silicates,  i.  101 

Euchlorine,  i.  -I  Ml 

Eudiometer,  i.  169 

Expansion,  linear,  of  elements,  ii.  31 

Explosion,  rate*  of  transmission  of,  i. 

171 
Explosives,  i.  275,  270 


FF,LSPAR,  ii.  122 

Fermentation,  i.  242 

Ferric  chloride,  i.  558  ;  ii.  340 

—  hydrates,  ii.  339 

—  nitrate,  ii.  340 

—  orthophosphate,  ii.  342 

—  oxide,  ii.  339 
Ferrous  chloride,  ii.  335 

—  sulphate,  ii.  335 
solubility  of,  i.  72 

—  sulphide,  ii.  210 
Flame,  i.  177,  179 
Fluoborates,  ii.  09 
Fluorides,  i.  491,  493 
Fluorine,  i.  203,  489 
Fluorspar,  i.  4'.)L 

Formula,  chemical,  i.  151,  326 
Freezing  mixtures,  i.  76 
Fuel,  calorific  capacity  of,  i.  360 
Furnace,  electrical,  i.  352 
Fusco-cobaltic  salts,  ii.  360 


OADOLINITE  METALS,  ii.  93 
Gallium,  ii.  88,  90 
Gas,  illuminating,  i.  361 
—  producers,  i.  397 


Gases,  absorption  of,  i.  :548 

—  diffusion  of,  i.  83 

—  expansion  of,  i.  133 

—  liquefaction  of,  i..  134,  135,  137 

—  measurement  of,  i.  78,  300 

—  solution  of,  i.  68,  78,  80 

—  theory  of,  i.  81,  83,  140 
Germanium,  ii.  20,  124 

—  chloride,  ii.  125 

—  oxide,  ii.  125 
Glass,  i.  123 

—  soluble,  ii.  110 
Glauber's  salt,  i.  517 
Glycols,  ii.  117 
Gold,  ii.  442 

—  alloys,  i.  440,  447 

—  chlorides,  ii.  448,  450 

—  colloid,  ii.  447  . 

—  cyanide,  ii.  450 

—  extraction  of,  ii.  -144,  445 

—  fulminating,  ii.  450 

—  oxides,  ii.  448 

—  refining,  ii.  446 
Graduators,  i.  424 
Graphite,  i.  350,  351 
Gros'  salt,  ii.  39:» 
Guignet's  green,  ii.  285 
Gunpowder,  i.  557 
Gypsum,  i.  S'.i.'i,  Oil 


HAMXJKNS,  i.  445,  487,  499 
Halogen  compounds,  heat  of  formation 
of,  i.  494,  502  ;  ii.  .'52 

boiling-points  of,  i.  502 

Ilausmannite,  ii.  10 

Helium,  i.  570;  ii.  498 

Hemimorphism,  ii.  9 

Homeomorphism,  ii.  8 

Homologous  compounds,  i.  368 

Humus,  i.  344 

Hydrates,  i.  109,  1K5 

Hydrazine,  i.  25H 

Hydrides,  i.  021  ;  ii.  2:5 

Hydrocarbons,  i.  .'555,  359 

Hydrogen,  i.  123, 129, 130,  142, 143, 14G 

—  pentasulphide,  ii.  217 

—  peroxide,  i.  207,  312 
Hydrosols,  i.  98 
Hydroxyl,  i.  192,  213 
Hydroxylamine.  i.  262 
Hypochlorit.es,  i.  481 
Hyponitrites,  i.  294 

IMIDES,  i.  258 
Indium,  ii.  27,  37,  88,  97 
lodates,  i.  609 
Iodides,  ii.  32 

—  of  nitrogen,  i.  507 


514 


PRINCIPLES"   OF  CHEMISTRY 


IOD 

Iodine,  i.  320.  321,  496,  497,  498 

—  chlorides  of,  i.  611 
lodosobenzol,  i.  508 
Iridious  oxide,  ii.  382 
Iridium,  ii.  382 
Iron,  ii.  317,  322,  585 

—  and  carbonic  oxide,  ii.  345 

—  cast,  ii.  325 

—  nitride,  ii.  346 

—  ores,  319 

—  sulphate,  ii.  335 
Isethionic  acid,  ii.  250 
Isomorphism,  i.  203,  368  ;  ii.  1,  4, 8 


KAOLIN,  ii.  70 


LAKES,  ii.  77 

Lanthanum,  ii.  93 

Laughing  gas,  ii.  297 

Law  of  Avogadro-Gerhardt,  i.  309 

— .—  Berthollet,  i.  445 

— '  —  Boyle  and  Mariotte,  i.  132 

-^  —  combining  weights,  i.  221 

Dulong  and  Petit,  i.  584 

—  r—  equivalents,  i.  1  94 
even  numbers,  i.  357 

—  —  Gay  Lussac,  i.  133,  304,  307 

Guldberg  and  Waage,  i.  441 

Henry  and  Dalton,  i.  78 

— » : —  indestructibility  of  matter,  i.  6 

Kirchoff,  i.  568 

limits,  i.  357 

—  —  maximum  work,  L  120 

multiple  proportions,  i.  109,  214 

-^ partial  pressures,  i.  82 

periodic,  ii.  17 

phases,  ii.  410 

reversed  spectra,  i.  568 

specific  heats,  i.  584 

substitution,  i.  260,  365 

volumes,  i.  304 

Lead,  ii.  134 

—  acetate,  ii.  137 

—  carbonate,  ii.  140 

—  'chloride,  ii.  139 

—  chromate,  ii.  136,  279 
i —  dioxide,  ii.  142 

—  nitrate,  ii.  139 

—  oxide,  ii.  137 

—  red,  ii.  142 

—  salts  of,  i.  491 

—  tetrachloride,  ii.  144 

—  tetrafluoride,  ii.  144 

—  white,  ii.  140 
Leucone,  ii.  107 
Levigation,  ii.  72 

Light,  chemical  action  of,  i.  465 


MOL 

Lime,  i.  605 

Liquids,  boiling  points  of,  i.  135 

Lithium,  i.  574 

—  carbonate,  i.  575 

Litharge,  ii.  137 

Litmus,  i.  185 

Lixiviation,  methodical,  i.  521 

Luteo-cobaltic  salts,  ii.  359 


MAGNUS'  salt,  ii.  392 
Magnesia,  i.  597 
Magnesium,  i.  590,  594 

—  carbonate,  i.  592,  602 

—  chloride,  i.  602 

—  crystallohydrates  of,  i.  601 

—  double  salts  of,  i.  597 

—  nitride,  i.  595 

—  silicide,  ii.  102 

—  sulphate,  i.  600 
Manganese,  ii.  303 

—  nitrides,  ii.  310 

—  oxides,  ii.  306,  307,  308,  313 

—  peroxide,  i.  159  ;  ii.  305 

—  sulphate,  ii.  307 

Mass,  influence  of,  i.  32,  436 
Matches,  ii.  154 
Matter,  primary,  ii.  440 

—  transmutability  of,  i.  14 
Mercury,  ii.  48 

—  ammonia  compounds,  ii.  57 

—  basic  salts  of,  ii.  54 

—  chlorides,  ii.  52,  53,  54 

—  compounds,  heat  of  formation,  ii.  50 

—  cyanide,  ii.  55 

—  fulminating,  ii.  56 

—  iodide,  ii.  55 

—  nitrates,  ii.  51 

—  nitrides,  ii.  56 

—  oxides,  ii.  53 

—  sulphate,  ii.  57 

—  sulphides,  ii.  221 
Metalepsis,  i.  28,  471 
Metalloids,  i.  23 
Metals,  i.  23 

of  alkaline  earths,  i.  64,  590,  591 

of  alkalis,  i.  543 

displacement  of,  ii.  427 
Methane,  i.  360 

Moisture,  determination  of,  in  gases,  i. 
40 

influence  upon  reaction,  i.  403 
Molecular  volumes,  ii.  37 

—  weight  and  boiling  point,  i.  331 

coefficient  of  refraction,  i.  336 

latent  heat,  i.  329 

—  specific  gravity  of  solutions,  i. 

335 
surface  tension,  i.  334 


SUBJECT  INDEX 


515 


Molecules,  i.  319,  322 
Molybdates,  ii.  292 
Molybdenum,  ii.  290 

—  anhydride,  ii.  291 

—  fluo-compounds,  ii.  298 

—  sulphides,  ii.  297 
Monophosphamide,  ii.  178 
Monosodium  orthophosphate,  ii.  107 
Morphotropy,  ii.  10 

NAPHTHA,  i.  373,  377 
Nascent  state,  i.  33,  145,  146 
Neodymium,  ii.  97 
Nickel,  ii.  353 

—  alloys,  ii.  367 

—  and  carbonic  oxide,  ii.  367 

—  fluoride,  ii.  358 

—  hydroxide,  ii.  358 

—  oxide,  ii.  365 

—  sulphate,  i.  97  ;  ii.  359 

—  tetra-carboxyl,  ii.  367 
Niobium,  ii.  194,  198,  199 
Nitrates,  i.  273 

Nitres,  i.  268,  555 
Nitric  anhydride,  i.  280 

—  oxide,  i.  286 
Nitrides,  i.  227,  258,  620 
Nitriles,  i.  406 
Nitrites,  i.  284 
Nitro-cellulose,  i.  275 
Nitro-compounds,  i.  274 
Nitrogen,  i.  223,  225,  475 

—  chloride,  i.  476 

—  iodide,  i.  507 

—  oxides  of,  i.  267,  280,  284,  294, 298 

—  sulphide,  ii.  270 
Nitro-prussides,  ii.  351 
Nitroso-compounds,  i.  288 
Nitrosulphates,  ii.  229 
Nitrosyl  chloride,  ii.  176 
Norwegium,  ii.  59 

OCCLUSION,  i.  143 
Olefiant  gas,  i.  370 
Organo-metallic  compounds,  i.  358 
.Osmium,  ii.  372,  382,  384 
Osmotic  pressure,  i.  64 
Osmuridium,  ii.  383 
,0xamide,  i.  406 
Oxidation,  i.  16 
Oxides,  i.  183  ;  ii.  36 
Oxycobaltamine  salts,  ii.  359 
Oxygen,  i.  152,  157,  158,  163 

—  compounds,  heat  of  formation  of,  i. 

120,  466 
Ozone,  i.  198,  229 

PAIJADITJM,  ii.  369 

—  hydride,  i.  143 ;  ii.  380 


POT 
Palladous  chloride,  ii.  379 

—  iodide,  ii.  379 
Paracyanogen,  i.  414 
Paramorphism,  ii.  9 
Parasulphatammon,  ii.  269 
Peat,  i.  344 

Peligot's  salt,  ii.  281 
Percentage  composition,  i.  326 
Perchloric  anhydride,  ii.  282 
Periodates,  i.  510 
Permanganic  anhydride,  ii.  313 
Permolybdates,  ii.  297 
Peroxide,  chloric,  i.  484 
Peroxides,  i.  159  ;  ii,  15/23 
Perstannic  oxide,  ii.  133 
Persulphates,  ii.  253 
Petroleum,  i.  373 
Phenol,  solubility  of,  i.  75 
Phlogiston,  i.  17 
Phosgene  gas,  ii.  175 
Phospham,  ii.  178 
Phosphides,  ii.  157 
Phosphine,  ii.  158,  160 
Phosphonium  iodide,  ii.  159 
Phosphoric  anhydride,  ii.  161 
Phosphorous  anhydride,  ii.  160 
Phosphorus,  ii.  149 

—  ammonium  compounds,  ii.  178 

—  chlorides,  ii.  174 

—  fluorides,  ii.  173 

—  iodides,  i.  505,  506  ;  ii.  172 

—  oxychlorides,  ii;  175 

—  sulphides,  ii.  213 

—  sulpho-chloride,  ii.  213 

—  thermo-chemical  data  for,  ii.  153 
Phosphuretted  hydrogen,  ii.  158,  160 
Photography,  ii.  431 
Photo-salts,  ii.  432 

Plants,  chemical  reactions  in,  i.  547 
• —  and  nitrogen,  i.  230 
Platinic  chloride,  ii.  377 

—  hydroxide,  ii.  379 
Platino-ammonium  compounds,  ii.  391 

—  chlorides,  i.  467  ;  ii.  378 

—  cyanides,  ii.  386 

—  nitrites,  ii.  390 

—  sulphites,  ii.  390 
Platinous  chloride,  ii.  379 
Platinum,  ii.  376 

—  alloys,  ii.  373 

—  black,  ii.  376 

—  metals,  ii.  369,  375 

—  oxide,  ii.  378 
Poly-haloid  salts,  i.  545 
Polymerism,  i.  207,  367 
Polysulphides,  ii.  217 
Potassium,  i.  544,  558 

—  aurate,  ii.  449 

—  bromide,  i.  550 


516 


PRINCIPLES  OF  CHEMISTRY 


POT 

Potassium  carbonate,  i.  549 

—  chlorate,  i.  161,  482 

—  chloride,  i.  72,  543 

—  chromate,  ii.  280 

—  cyanide,  i.  412,  551 

—  dichromate,  ii.  278 

—  ferricyanide,  ii.  346 

—  ferrocyanide,  i.  346,  412 

—  hydrosulphide,  ii.  219 

—  hydroxide,  i.  548 

—  iodide,  i.  550 

—  manto'anate,  ii.  310 

—  nitrate,  i.  553 

—  oxides,  i.  559 

—  permanganate,  ii.  311 

—  stannate,  ii.  133 

—  sulphate,  i.  72,  549 

—  sulphide,  ii.  219 

—  telluride,  ii.  274 
Praseocobaltic  salts,  ii.  361 
Praseodidymium,  ii.  97 
Proteid  substances,  i.  224 
Prout's  hypothesis,  ii.  439 
Prussian  blue,  i.  419  :  ii.  349 
Purpureo-cobaltic  salts,  ii.  361 
Purpureo-tctramine  salts,  ii.  361 
Pyrocollodion,  i.  275 
Pyronaphtha,  i.  375 
Pyrosulphuryl  chloride,  i.  821  j  IL  839 


REACTIONS,  chemical,  i.  9 

—  —  conditions  for,  i.  34 
contact,  i.  39 

—  —  endothermal,  i.  30 

—  —  exothermal,  i.  30 

—  -  •  limit  of,  i.  437 

—  —  rate  of,  ii.  152 
Eecalescence,  ii.  333 
Reduction,  i.  16 
Refraction  equivalent,  i.  836 
Regenerative  furnaces,  i.  898 
Reiset's  salts,  ii.  394 
Respiration,  i.  152, 154,  887 
Rhodium,  ii.  381 

Rock  salt,  i.  421 
Roseocobaltic  salts,  ii.  360 
Rosetetramine  salts,  ii.  361 
Rubidium,  i.  576 
Ruthenium,  ii.  372,  382,  384 

SALAMMONIAC,  i.  248,  318,  457 

—  solubility  of,  i.  458 

—  vapour  density  of,  i.  317 
Salts,  i.  187,  419 

—  acid,  i.  193,  533 

—  basic,  i.  193,  533 ;  ii.  54 

—  double,  i.  598 


Salts,  electrolysis  of,  i.  191 

—  heat  of  formation,  i.  189 

—  melting  points  of,  i.  135 

—  pyro,  i.  193 

—  theory  of,  i.  193 
Saponification,  i.  530 
Scandium,  ii.  94 
Selenium,  ii.  273 

—  chlorides,  ii.  275 
Selenious  anhydride,  ii.  271 
Silica,  ii.  100  ;  ii.  108 

—  soluble,  ii.  113 
Silicates,  i.  544  ;  ii.  116 
Silicon,  ii.  99 

—  chloride,  ii.  103,  104 

—  chloroform,  ii.  103 

—  bromide,  ii.  104 

—  fluoride,  ii.  105 

—  hydride,  ii.  102,  103 

—  iodide,  ii.  105 

—  iodoform,  ii.  105 
Silver,  ii.  418 

—  allotropic  varieties  of,  ii.  421 

—  bromide,  ii.  429 

—  chlorate,  ii.  437 

—  chloride,  ii.  429 

—  cyanide,  ii.  433 

—  fluoride,  ii.  430 

—  fulminating,  ii.  426 

—  hyponitrite,  i.  294 

—  iodide,  ii.  429 

—  nitrate,  ii.  426 

—  nitrite,  i.  284 

—  orthophosphate,  ii.  164 

—  oxides,  ii.  424 

—  peroxide,  ii.  422 

—  plating,  ii.  434 

—  soluble,  ii.  420 

—  subchloride,  ii.  432 
Slags,  ii.  323 
Smalt,  ii.  354 
Soaps,  i.  531 

Soda  ash,  i.  519 

—  caustic,  i.  527 

—  manufacture  of,  i.  459 

—  waste,  i.  522 
Sodamide,  i.  539 
Soda  lime,  i.  237 
Sodium,  i.  513,  533 

—  alloys,  i.  559 

—  amalgams,  i.  537 

—  bicarbonate,  i.  526 

—  carbonate,  i.  519,  525 

crystal lohydrates  of,  i.  108 

manufacture  of,  i.  623 

solutions  of,  i.  525 

—  chloride,  i.  419 

double  salts  of,  i.  430 

solutions  of,  i.  88,  99,  429 


SUBJECT  INDEX 


517 


Sodium  hydride,  i.  537 

—  hydroxide,  i.  528,  529 
solutions  of,  i.  529 

—  nitrate,  i.  269 

—  —  solutions  of,  1.  72 

—  organo  compounds  of,  i.  540 

—  oxides,  i.  540,  541 

—  phosphates,  ii.  166 

—  platinate,  ii.  378 

—  pyrosulphate,  i.  518 

—  sesquicarbonate,  i.  526 

—  stannate,  ii.  133    • 

—  subchloride,  i.  540 

—  sulphate,  i.  513 

• acid  salt,  i.  518 

—  —  crystallohydrates  of,  i.  615 

solutions  of,  i.  73,  515,  516 

-  -  sulphite,  ii.  226 

—  thiosulphate,  ii.  230 

solutions  of,  i.  74 

<—  tungstate,  ii.  294 
Soils,  i.  344  ;  ii.  73 
Solubility  coefficient  of,  i.  67,  71 
Solutions,  i.  330 

—  aqueous,  i.  59 

—  boiling  points  of,  i.  94,  100 

—  crystallisation  of,  i.  427 

—  colour  of,  i.  95 

—  diffusion  of,  i.  61,  429 

—  of  double  salts,  i.  599 

—  formation  of  ice  from,  i.  91,  429 

—  heat  of  formation  of,  i.  74.  75,  76 

—  of  gases,  i.  68 
• —  igotonic,  i.  64 

—  saturated,  i.  65 

—  specific  gravity  of,  i.  429,  584 
— •  supersaturated,  i.  96 

—  theory  of,  i.  64,  89,  92,  97,106,  215, 
323,  608  ;  ii.  3,  164 

—  vapour  tension  of,  i.  90,  92 

—  volumes  of,  i.  87 

—  Specific  heat,  i.  585,  586,  588 
Spectra  absorption,  i.  566 
Spectrum  analysis,  i.  560,  561 
Stannic  chloride,  ii.  132 

—  fluoride,  ii.  132 

—  oxide,  ii.  130 

—  sulphide,  ii.  132 
Stannous  chloride,  ii.  130 

—  oxide,  ii.  129 

—  salts,  ii.  129 

Steam,  vapour  tension  of.  i.  54 
Steel,  ii.  327,  328,  330 
Strontium,  i.  615 

—  chloride,  i.  615 

—  hydroxide,  i.  615 

—  nitrate,  i.  615 

—  oxide,  i.  617 
Substitution  chemical,  i.  5 


Sulphamide,  ii.  270 
Sulphatammon,  ii.  269 
Sulphates,  ii.  248 
Sulphides,  i.  98  ;  ii.  213 
Sulphonitrites,  ii.  229 
Sulphoxyl,  ii.  250 
Sulphur,  ii.  200 

—  chlorides  of,  ii.  264 
Sulphuretted  hydrogen,  ii.  209 
Sulphuric  anhydride,  ii.  232 

—  peroxide,  ii.  251 
Sulphurous  anhydride,  ii.  224 
Sulphuryl  chloride,  ii.  268 
Superphosphates,  ii.  168 


TANTALUM,  iii  194,  199 
Tellurium,  ii.  274 
* —  bromide,  il.  275 
• —  chlorides,  ii.  275 
Tellurious  anhydride,  ii.  271 
Temperature,  critical,  i.  131 
Test  papers,  i.  185 
Thallium,  ii.  88,  91 
Thallic  oxide,  ii.  93 
Thallous  hydroxide,  ii.  92 

—  oxide,  ii.  92 
Thiocarbonates,  ii.  262 
Thionyl  chloride,  ii.  267 
Thiophosgene,  ii.  262 
Thiophosphoryl  fluoride,  ii.  263 
Theory,  atomic,  i.  216 

—  unitary,  i.  195 

—  vortex,  i.  217 
Thermochemistry,  i.  173 
Thorium,  ii.  148 

Tin,  ii.  125 

—  alloys,  ii.  127 
Titanium,  ii.  144 

—  chloride,  ii.  145 

—  nitride,  ii.  146 

—  nitrocyanide,  ii.  146 

—  oxides,  ii.  145 
Tripoli,  ii.  110 

Trisodium  orthophosphate,  ii.  166 
Tungstates,  ii.  292 
Tungsten,  ii.  290 

—  anhydride,  ii.  291 

—  nitride,  ii.  297 

—  sulphide,  ii.  297 
TurnbulPs  blue,  ii.  350 
Types  of  combination,  ii.  10 


ULTKAMABINE,  ii.  84 
Uranium,  ii.  30,  297 

—  atomic  weight  of,  ii.  26 

—  dioxide,  ii..301 

—  oxides,  ii.  298 


518 


PRINCIPLES  OF  CHEMISTRY 


TTftA 

Uranium  tetrachloride,  ii.  301 
Urano-alkali  compounds,  ii.  298 
Uranyl,  ii.  301 

—  ammonium  carbonate,  ii.  300 

—  nitrate,  ii.  300 

—  phosphate,  ii.  300 
Urea,  i.  409 


VALENCY  of  elements,  i.  404,  418,  581 
Van  der  Waal's  formula,  i.  82,  140 
Vanadic  anhydride,  ii.  196 
Vanadium,  ii.  194 

—  oxychloride,  ii.  195 

Vapour  density,  determination  of,  i.  301 
Ventilation,  i.  244 
Viscosity,  i.  355 
Volumes,  molecular,  ii.  4 

—  gases,  i.  300 


WATEB,  i.  40 

—  composition  of,  i.  114, 118, 148, 169, 
305,  333 

—  compressibility  of,  i.  53 

—  of  constitution,  i.  109 

—  of  crystallisation,  i.  95,  510 

—  dissociation  of,  i.  118 

—  expansion  of,  i.  53 


ZIR 

Water  gas,  i.  129,  400,  401 

—  hard,  i.  47 

—  hygroscopic,  i.  56 
• —  mineral,  i.  45 

—  rain,  i.  43 

—  river,  i.  43 

—  sea,  i.  46 

—  specific  heat  of,  i.  52 
gravity  of,  i.  50 

—  spring,  i.  44 
Wave  lengths,  i.  564 
Wood,  i.  339 


YTTERBIUM,  ii.  93- 
Yttrium,  ii.  93 


ZINC,  ii.  39 

—  ammonia-chlorides,  ii.  41 

—  chloride,  ii.  40,  41 

—  compounds,  heat  of  formation  of,  ii. 
51 

—  oxide,  ii.  39,  40 

—  sulphate,  ii.  39 
Zirconium,  ii.  146 

—  chloride,  ii.  147 

—  hydroxide,  ii.  147 

—  oxide,  ii.  147 


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