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■V 


PHYSICAL     SCIENCE 


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[Frontispiece. 


THE  RECENT  /^^i 
DEVELOPMENT  OF/'^^ 
PHYSICAL     SCIENCE 


BY 

WILLIAM  CECIL  DAMPIER  VVHETHAM 

M.A.,  F.R.S. 

FELLOW   AND   SOMETLME   SENIOR   TUTOR   OF   TRINITY 
COLLEGE,   CAMBRIDGE 


& 


i 


PHILx\DELPHIA 

P.   BLAKISTON'S   SON    &   CO. 

IOI2  WALNUT  STREET 


Printed  in  England 


First  Edition 
Second  Edition 
Third  Edition 
Fourth  Edition 
Fifth  Edition 


August  1904 
September  1 904 
December  1 904 
November  1 909 
August  1924 


PRINTED   IN   GRBAT   BRITAIN    BY 
OLIVER   AND    BOYD,    EDINBURGH 


PREFACE   TO    THE   FIRST 
EDITION 

In  recent  years  we  have  witnessed  a  great 
development  of  physical  science.  The  different 
sections  into  which  natural  knowledge  is,  for  the 
sake  of  convenience,  divided,  have  grown  each 
within  its  own  domain ;  and,  moreover,  have 
shown  increasing  signs  of  extending  beyond  the 
boundaries  arbitrarily  traced  between  them.  The 
methods  of  physics,  in  the  restricted  sense  of 
that  word,  are  being  more  and  more  applied  to 
chemical  and  biological  problems,  while  many 
questions  in  physics  can  only  be  investigated 
by  those  with  mathematical  or  chemical  training. 

Thus  it  happens  that  an  acquaintance  with 
the  knowledge  newly  acquired  in  one  department 
of  science  is  necessary  for  the  study  of  another ; 
indeed,  the  phenomena  which  need  for  their 
interpretation  the  methods  of  two  branches  of 
science  have  proved  often  the  most  fruitful  field 
of  inquiry. 

For  reasons  such  as  these  it  has  been 
thought  possible  that  a  short  account  of  some 
of  the  important  investigations  now  being  carried 
on  in  the  physical  laboratories  of  the  world 
might  prove  useful  to  students  of  science  in 
general ;  while  it  is  hoped  that,  by  treating  the 
subject  as  far  as  possible  without  technical 
language,  the    book    may  also   appeal    to    those 


vl        PREFACE  TO  THE  FIRST  EDITION 

who,  with  Httle  definite  scientific  training,  are 
interested  in  the  more  important  conclusions  of 
scientific  thought. 

The  writer  has  been  fortunate  in  his  surround- 
ings, where  the  knowledge  and  insight  of  one 
worker  are  placed  freely  and  ungrudgingly  at 
the  service  of  another  in  the  day  of  his  need. 
In  the  present  undertaking  he  records  gratefully 
the  help  of  several  friends  who  have  read  the 
proof  sheets  of  the  parts  dealing  with  subjects 
with  which  their  names  are  closely  associated. 
Mr  F.  H.  Neville  criticised  the  chapter  on  The 
Philosophical  Basis  of  Physical  Science,  and  that 
on  Fusion  and  Solidification.  Lord  Berkeley 
read  the  account  of  The  Problems  of  Solution. 
Professor  J.  J.  Thomson  saw  the  manuscript  of 
the  original  article  on  which  is  founded  the 
chapters  on  Conduction  of  Electricity  through 
Gases  and  Radio-Activity.  Professor  Larmor 
revised  the  account  of  Atoms  and  ^ther,  while 
Mr  H.  F.  Newall  read  the  chapter  on  Astro- 
Physics.  For  this  assistance  the  writer  expresses 
his  cordial  gratitude.  He  wishes  especially  to 
thank  his  wife  for  continual  correction  both  of 
the  manuscript  and  of  the  proof  sheets. 

The  editor  of  the  Quarterly  Review  has 
kindly  allowed  use  to  be  made  of  the  article 
on  Matter  and  Electricity  which  appeared  in 
January  1904.  Professor  George  E.  Hale  was 
good  enough  to  permit  some  of  his  photographs 
of  the  sun  to  be  reproduced,  while,  for  other 
illustrations,  acknowledgments  are  due  to  the 
Royal  Society,  to  Mr  Heycock  and  Mr  Neville, 
to  Mr  J.  A.  Ewing,  and  to  Mr  G.  T.  Beilby. 
Lord  Kelvin  kindly  sent  a  signed  portrait,  and 


PREFACE  TO  THE  FIRST  EDITION       vii 

Professor  J.  J,  Thomson  allowed  the  use  of 
a  reproduction  of  Mr  Arthur  Hacker's  admirable 
painting,  which  now  hangs  in  the  Cavendish 
Laboratory. 

In  spite  of  the  generous  help  he  has  received, 
the  author  is  sadly  conscious  of  the  difficulty  of 
his  task.  Although  the  development  of  physical 
science  is  one  of  the  most  powerful  activities  of 
our  time,  a  knowledge  of  its  aims,  methods,  and 
results  has  not  yet  been  recognised  as  a  necessary 
part  of  an  English  liberal  education.  To  give 
a  popular  exposition  of  results,  especially  when 
there  is  an  obvious  practical  application,  is  easy  ; 
to  enable  a  non-scientific  mind  to  follow  and 
appreciate  the  methods  by  which  the  results  are 
reached  is  supremely  difficult.  But  in  science 
methods  are  usually  more  important  than  results, 
while  a  superficial  acquaintance  with  results  with- 
out an  underlying  knowledge  of  method  is  useless, 
or  worse  than  useless. 

In  the  possibility  of  treating  the  wider  and 
deeper  generalisations  of  natural  science  as  fit 
subject-matter  for  current  thought  and  literature, 
the  writer  has  a  profound  belief.  Whether  the 
failure  to  secure  such  treatment  has  been  due  to 
lack  of  adequate  exposition,  or  to  some  radical 
defect  in  the  training  of  the  nation,  is  a  difficult 
and  grave  problem  ;  but,  until  the  point  of  view 
has  been  altered,  it  is  perhaps  hopeless  to  look 
for  a  proper  understanding  of  the  scientific  spirit 
and  of  scientific  method  even  among  the  more 
educated  portion  of  the  community.  For  the 
present,  the  man  of  science  must  perforce  occupy 
a  more  technical  and  isolated  position  than  the 
student  of  history  or   the  lover  of  art.     From 


viii     PREFACE  TO  THE  FIRST  EDITION 

the  point  of  view  of  the  man  of  science,  to  break 
down  this  isolation  would  be,  at  best,  but  sorry 
kindness  ;  but,  from  a  wider  point  of  view,  for 
the  good  of  the  nation  and  of  mankind,  a  more 
general  acceptance  of  a  share  in  the  impersonal 
open-minded  search  for  truth,  which  is  the  essence 
of  science,  is  ardently  to  be  desired. 

With  some  such  thoughts  as  these,  the  writer 
sends  forth  the  following  pages. 


Cambridge,  2^th  June  1904. 


PREFACE   TO    THE    SECOND 

EDITION 

The  need  for  a  reprint  of  this  book,  coming  as 
it  does  within  a  few  weeks  of  publication,  must 
be  set  down  in  part  to  the  exceptional  interest  in 
the  problems  with  which  it  deals  that  has  been 
aroused  by  Mr  Balfour's  Presidential  Address  to 
the  British  Association. 

For,  when  attention  has  been  drawn  to  the 
new  theory  of  matter — to  "the  most  far-reach- 
ing speculation  about  the  physical  universe  which 
has  ever  claimed  experimental  support " — a  state 
of  mind  is  created  that,  in  thoughtful  men,  will 
not  rest  satisfied  without  some  effort  to  under- 
stand the  basis  of  the  speculation,  and  to  weigh 
the  evidence  which  can  be  arraigned  in  its  favour. 
Truly,  the  new  theory  is  concerned,  not  '*  about 
things  remote  or  abstract,  things  transcendental 
or  divine,  but  about  what  men  see  and  handle, 
about  those  *  plain  matters  of  fact '  among  which 
common-sense  daily  moves  with  its  most  confident 
step  and  most  self-satisfied  smile." 

The  importance  of  the  position  now  gained 
for  the  survey  of  the  material  universe  lies  in  the 
unity  of  conception  it  discloses  and  the  resulting 
simplification  of  detail.  Either  instinctively,  or 
as  the  unconscious  result  of  experience,  the  mind 
of  man  naturally  grasps  at  any  plan  thus  to  reduce 


IX 


X      PREFACE  TO  THE  SECOND  EDITION 

and  consolidate  the  questions  which  beset  him 
in  his  journeyings  through  time  and  space.  To 
the  philosophic  import  of  this  mental  attitude 
Mr  Balfour  has  done  well  to  call  attention  in 
words  that  he  kindly  allows  the  writer  to  re- 
produce : — 

''  Now  whether  the  main  outlines  of  the  world- 
picture  which  I  have  just  imperfectly  presented 
to  you  be  destined  to  survive,  or  whether  in 
their  turn  they  are  to  be  obliterated  by  some 
new  drawing  on  the  scientific  palimpsest,  all 
will,  I  think,  admit  that  so  bold  an  attempt  to 
unify  physical  nature  excites  feelings  of  the  most 
acute  intellectual  gratification.  The  satisfaction 
it  gives  is  almost  aesthetic  in  its  intensity  and 
quality.  We  feel  the  same  sort  of  pleasurable 
shock  as  when  from  the  crest  of  some  melancholy 
pass  we  first  see  far  below  us  the  sudden  glories 
of  plain,  river,  and  mountain.  Whether  this 
vehement  sentiment  in  favour  of  a  simple  universe 
has  any  theoretical  justification,  I  will  not  venture 
to  pronounce.  There  is  no  a  priori  reason  that 
I  know  of  for  expecting  that  the  material  world 
should  be  a  modification  of  a  single  medium, 
rather  than  a  composite  structure  built  out  of 
sixty  or  seventy  elementary  substances,  eternal 
and  eternally  different.  Why,  then,  should  we 
feel  content  with  the  first  hypothesis  and  not 
with  the  second  ?  Yet  so  it  is.  Men  of  science 
have  always  been  restive  under  the  multiplication 
of  entities.  They  have  eagerly  noted  any  sign 
that  the  chemical  atom  was  composite,  and  that 
the  different  chemical  elements  had  a  common 
origin.  Nor  for  my  part  do  I  think  such  instincts 
should  be  ignored.  .  .  .  These   obscure  intima- 


PREFACE  TO  THE  THIRD  EDITION       xi 

tions  about  the  nature  of  reality  deserve,  I  think, 
more  attention  than  has  yet  been  given  to  them. 
That  they  exist  is  certain  ;  that  they  modify  the 
indifferent  impartiality  of  pure  empiricism  can 
hardly  be  denied." 

The  principle  of  simplicity  lies  at  the  base 
of  all  our  explanations  of  phenomena,  and 
Mr  Balfour's  address  will  do  much  to  lead  to 
a  clearer  recognition  of  its  importance. 

Advantage  has  been  taken  of  this  opportunity 
to  correct  a  few  verbal  errors  which  appeared 
in  the  first  edition  of  the  book.  The  writer's 
thanks  are  due  to  several  correspondents,  some 
of  them  known  to  him  personally  and  some  not, 
who  were  good  enough  to  send  notes  of  these 
errors. 

Certain  additions,  descriptive  ofwork  published 
within  the  last  few  months,  have  been  made ;  and 
in  places  the  treatment  has  been  modified  in  order 
to  make  the  meaning  clearer.  In  this  task  the 
writer  acknowledges  gratefully  the  help  of  his 
friend,   Mr  Stanley  Leathes. 

22nd  September  1904. 


THIRD    EDITION 

Little  more  than  verbal  changes  have  been 
made  in  transforming  the  second  into  the  third 
edition. 

xoth  November  1904. 


xii     PREFACE  TO  THE  FOURTH  EDITION 


FOURTH    EDITION 

In  the  four  years  which  have  elapsed  since  the 
publication  of  the  third  edition  of  this  book, 
physicists  have  developed  further  the  subjects 
with  which  it  deals,  but  no  striking  new  branches 
of  knowledge  have  appeared.  Hence  it  is  possible 
to  re-issue  the  book,  with  some  additions,  but  with 
no  fundamental  changes  of  plan. 

\Zth  January  1909. 


FIFTH    EDITION 

The  fifteen  years  which  have  passed  since  the 
last  edition  of  this  book  appeared  have  seen 
great  advances  in  many  of  the  subjects  described 
therein,  and  two  new  discoveries  of  fundamental 
importance — the  Principle  of  Relativity  and  the 
Quantum  Theory. 

It  has  been  necessary,  therefore,  completely 
to  revise  the  book ;  many  sections  have  been 
re-written,  and  much  new  substance  has  been 
added,  in  the  attempt  to  give  a  fair  account  of 
the  latest  development  of  physical  science. 

22nd  JU7tt    1924. 


CONTENTS 


CHAPTER  I 

PAGE 

INTRODUCTION  ......  I 


CHAPTER  II 

THE  PHILOSOPHICAL  BASIS  OF   PHYSICAL  SCIENCE  .  II 

CHAPTER  III 

THE  LIQUEFACTION    OF    GASES   AND    THE    ABSOLUTE  ZERO 

OF  TEMPERATURE  .....  4I 

CHAPTER  IV 

FUSION   AND   SOLIDIFICATION  .  .  .  .68 

CHAPTER  V 

THE  PROBLEMS  OF  SOLUTION       .      .      .      -93 

CHAPTER  VI 

THE  CONDUCTION   OF    ELECTRICITY  THROUGH    GASES  .         I25 

CHAPTER  VIJ 

RADIO-ACTIVITY  ,.,...      164 

ziii 


xiv  CONTENTS 


'      CHAPTER  VIII 

PAGE 

MATTER,    SPACE,    AND  TIME      .....        204 


CHAPTER  IX 

ASTRO-PHYSICS  .......        261 


INDEX     ..,..•        307 


LIST  OF  ILLUSTRATIONS 


PORTRAITS 

Sir  Isaac  Newton 

Frontispiece 

Lord  Kelvin 

.     To  face  page     58 

J.   WiLLARD    GiBBS 

„        „         80 

J.  H.  Van't  Hoff 

»         »         97 

Sir  J   J.  Thomson 

»       125 

Sir  E.  Rutherford 

DIAGRAMS 

„         „       164 

PAGE 

Fig.  I    . 

•                        •                         •                         • 

54 

Figs.  2  to  5     . 

• 

To  face    72 

Fig.  6    . 

•                         • 

75 

Fig.  7    . 

•                         • 

.        78 

Fig.  8    . 

■                        • 

79 

Fig.  9    . 

•                        t 

81 

Fig.  10  . 

m                            ■ 

.        83 

Figs,  ii  to  17 

1                            • 

.     To  face    85 

Fig.  18  . 

•                            4 

89 

Figs.  19  to  24 

B                                          •                                         i 

To  face    91 

Fig.  25  . 

•                                          • 

.        95 

Fig.  26  . 

■                                          • 

109 

Fig.  27  . 

•                                           • 

128 

Fig.  28.  Condensation      0 

F     Cloud     ( 

dn      G 

ASEOUS 

Ions 

• 

1                     •                     i 

.     To  face  133 

Fig.  29  . 

• 

• 

•                      • 

140 

Fig.  30.  Deflection-Tube  for  Cathode  Rays    To  face  142 


XV 


XVI 


LIST  OF  ILLUSTRATIONS 


PAGE 

184 
189 


Fig.  31  . 

Fig.  32  . 

Fig.  33  . 

Fig.  34  . 

Fig.  35  . 

Fig.  36  . 

Fig.  37.  C    Line     in    the    Spectrum    of     a    Sun- 
Spot 

Fig,  38.  October  9,  3*^-  30™-     Calcium    Flocculi,  H2 

Level     .....     To  face  281 

Fig.  39.  October     9,   i*'-   04"^-       Hydrogen 

Flocculi  ....     To  face  282 

Fig.  40.  Diagram    to   explain    the  Phenomena   of 

Comets'  Tails  ....     To  face  294 

Figs.  41  and  42.  Regular  and  Spiral  Nebula    To  face  300 


To  face  2^^ 


PHYSICAL  SCIENCE 


CHAPTER    I 

INTRODUCTION 

Not  clinging  to  some  ancient  saw  ; 

Not  mastered  by  some  modern  term  ; 

Not  swift  nor  slow  to  change,  but  firm  : 
And  in  its  season  bring  the  law. 

—Tennyson. 

In  the  early  years  of  the  twentieth  century,  when 
this  book  was  first  published,  physical  science  was 
developing  mainly  In  two  directions.  Although 
these  movements  were  contemporaneous,  it  is  In- 
teresting to  note  that  the  methods  used  by  the 
two  schools  of  research  were,  to  some  extent, 
the  expression  of  opposite  tendencies. 

On  the  one  hand,  we  traced  the  growth  of  the 
study  of  the  conditions  In  which  all  physical  and 
chemical  change  In  a  system  must  cease — the 
conditions  of  physical  and  chemical  equilibrium. 
This  growth  was  due  to  the  thermodynamic 
methods  founded  chiefly  on  the  great  work  of 
the  late  Willard  Gibbs,  of  Yale  University  in 
the  United  States.  On  the  other  hand,  our 
knowledge  of  the  mode  of  the  conduction  of 
electricity  through  gases  was  extended,  mainly 
by  the  efforts  of  Sir  Joseph  John  Thomson, 
Professor  of  Experimental  Physics  at  Cambridge, 
and  now  Master  of  Trinity,  and  of  the  band  of 

1  B 


2  PHYSICAL  SCIENCE 

workers  trained  by  him  and  by  his  pupil  and 
colleague  Sir  Ernest  Rutherford,  in  the  Cavendish 
Laboratory.  Students  from  almost  all  civilised 
countries  have  come  to  Cambridge  as  to  the  centre 
of  this  branch  of  physical  research,  and  many  of 
them  are  now  carrying  forward  their  investigations 
elsewhere,  by  methods  learnt  in  the  University  of 
Newton,  Clerk- Maxwell,  and  Stokes. 

When  we  again  take  up  the  story  twenty 
years  later,  we  see  a  marvellous  increase  in 
both  these  branches  of  knowledge.  In  thermo- 
dynamics much  progress  has  been  made  by 
following  principles  already  laid  down,  and  apply- 
ing them  in  new  directions,  especially  in  the 
subject  of  physical  chemistry. 

On  the  other  hand,  the  study  of  the  con- 
duction of  electricity  through  gases  and  the 
allied  domain  of  radio-activity  has  attracted  most 
of  the  work  of  experimental  physicists.  It  has 
led  to  an  unprecedented  output  of  new  knowledge 
and  an  amazing  new  insight  into  the  secrets  of 
atomic  structure. 

Moreover,  two  fresh  subjects  of  inquiry 
have  appeared.  One  of  these  depends  both  on 
thermodynamics  and  on  atomic  conceptions. 
The  examination  of  the  known  facts  of  optical 
spectra  had  led  Planck  in  1901  to  the  view  that 
radiation  was  emitted  only  in  definite  units, 
the  constant  unit  being  not  energy  but  the 
product  of  energy  and  time — a  quantity  now 
called  "action." 

This  quantum  theory  was  applied  by  Einstein 
in  1907  to  explain  the  specific  heat  of  solids,  and 
by  Bohr  in    191 3  to  give  a   new  picture  of  the 


INTRODUCTION  3 

atom.  In  both  these  applications  it  has  been 
very  successful,  but  it  involves  the  abandonment 
in  problems  of  radiation  and  atomic  structure  of 
hitherto  accepted  dynamical  principles. 

Again,  the  theory  of  relativity,  founded  by 
Einstein  and  Minkowski,  leads  to  another  revolu- 
tion in  scientific  thought.  The  ideas  of  absolute 
space  and  absolute  time  are  held  by  this  new 
view  to  be  unwarranted  though  perhaps  natural 
figments  of  the  imagination.  If  we  keep  to  the 
only  space  and  time  we  really  know,  they  can 
be  but  space  and  time  as  recorded  by  some  ob- 
server ;  they  will  not,  it  appears,  be  the  same  for 
all  observers  ;  they  are  relative  and  not  absolute. 
The  real  unity  is  a  complex  of  both,  and  this  space- 
time  is  absolute  and  not  relative.  Strange  to 
say,  this  theory,  as  followed  by  Einstein,  has  led 
to  a  new  outlook  on  the  agelong  problem  of 
gravitation.  The  motion  of  the  planets,  the 
weight  of  a  stone,  may,  it  seems,  be  due  to 
something  we  must  represent  as  a  curvature  of 
space-time  rather  than  to  the  now  familiar  but  ever 
mysterious  gravitational  attraction  of  Newton. 

As  we  shall  see  in  the  following  pages,  the 
chief  work  of  modern  experimental  physicists  is 
undertaken  and  interpreted  by  the  aid  of  atomic 
and  molecular  conceptions.  The  theory  of  the 
conduction  of  electricity  through  liquids,  based 
originally  on  the  work  of  Faraday,  and  slowly 
matured  by  HIttorf,  Kohlrausch,  Arrhenius,  and 
many  others,  had  accustomed  our  minds  to  the 
conception  of  electric  conduction  by  means  of  the 
motion  of  charged  particles,  called  by  Faraday 
"ions" — the  travellers.     Each  ion  consists  of  an 


4.  PHYSICAL  SCIENCE 

atom,  or  group  of  atoms,  of  the  substance  in 
solution,  associated  with  a  positive  or  negative 
electric  charge ;  it  moves  through  the  liquid 
under  the  action  of  an  applied  electric  force, 
and  gives  up  its  charge  to  the  electrode — that 
is,  the  terminal  by  which  the  current  enters  or 
leaves  the  liquid.  The  conduction,  instead  of 
being  conceived  as  a  river  flowing  uniformly, 
must  figuratively  be  represented  as  taking  place 
by  the  passage  of  discrete  quantities  of  electricity; 
in  much  the  same  way  as  water  is  sometimes 
carried  from  a  lake  to  a  burning  house  by  means 
of  a  chain  of  bucket-bearers. 

By  the  application  of  similar  conceptions,  the 
passage  of  electricity  through  gases  has  received 
a  convincing  explanation.  Differences  appear, 
but  the  fundamental  ideas  are  the  same  in  the 
two  branches  of  the  science  of  electrolytic  conduc- 
tion. It  is,  however,  in  the  newer  side  of  the  subject 
that  the  most  striking  results  have  been  obtained. 
Electrolysis  in  liquids  had  suggested  the  concep- 
tion of  ultimate  units  of  electricity — atoms  of 
electricity,  analogous  to  the  atoms  of  matter. 
Gaseous  conduction  enabled  these  electric  atoms 
to  be  isolated,  separated  from  their  attendant 
material  atoms,  and  studied  independently. 

Great  has  been  the  revelation  which  followed. 
The  isolated  atoms  of  negative  electricity — the 
electrons,  as  they  have  been  named  by  Stoney — - 
have  been  identified  by  the  work  of  Thomson, 
Lorentz,  and  Larmor,  with  one  of  the  physical 
bases  of  matter,  with  the  corpuscles,  or  sub- 
atoms,  by  means  of  which,  combined  in  varying 
numbers  and  in  different  arrangements  with  a 
more   essential    positive    nucleus,    are   composed 


INTRODUCTION  5 

the  chemical  atoms,  for  long  taken  as  ultimate 
indivisible  units. 

Further  light  has  been  thrown  on  these  dark 
places  by  the  remarkable  series  of  discoveries 
through  which  M.  and  Mme.  Curie  and  other 
chemists  gave  us  the  radio-active  elements 
such  as  radium,  and  the  parallel  series  in 
which  Rutherford  and  his  fellow  workers  have 
interpreted  their  properties  as  due  to  the  dis- 
integration of  their  atoms,  as,  one  after  another, 
those  atoms  break  down,  and  are  transmuted 
into  other  substances. 

Throughout  these  investigations  we  deal 
with  atomic  and  molecular  conceptions  in  an 
extreme  form.  We  look  even  within  the  atom, 
and  examine  its  internal  structure  ;  we  trace  the 
electrons  flying  round  the  nucleus  of  the  atom, 
as  we  watch  the  planets  swinging  round  the  sun. 

It  is  remarkable  that,  in  the  other  branch 
of  Physical  Science,  in  which  thermodynamic 
principles  are  used,  the  methods  chiefly  employed 
enabled  us  for  a  time  to  dispense  altogether  with 
atomic  and  molecular  theories. 

At  the  basis  of  the  theory  of  physical  and 
chemical  equilibrium  lies  Lord  Kelvin's  great 
principle  of  the  dissipation  of  energy.  While 
the  total  amount  of  energy  in  an  isolated  system 
is  unchanging  and  unchangeable,  that  energy  is 
tending  always  to  become  less  available  for  the 
performance  of  useful  work.  The  availability  of 
the  energy  tends  continually  to  become  less.  It 
follows  that  permanent  equilibrium  can  only  be 
attained  when  the  limit  has  been  reached  and 
the  availability  is  a  minimum.  Such  a  theorem 
is  independent  of  molecular  hypotheses  ;  in  fact, 


6  PHYSICAL  SCIENCE 

it  expressly  disclaims  such  hypotheses,  for,  as 
Maxwell  showed,  the  chance  collisions  of  the 
individual  molecules  in  a  gas  will  lead  to  differ- 
ing molecular  velocities,  and  to  a  concentration 
of  energy  in  the  fast-moving  molecules.  If  we 
could  follow  the  motions  of  the  individual  mole- 
cules, and  separate  the  fast  from  the  slow,  we 
could  use  this  energy.  The  principle  of  dissipa- 
tion, therefore,  only  holds  while  we  are  obliged,  as 
always  as  yet  in  practice,  to  deal  with  ordinary 
molecules  statistically  and  in  the  aggregate. 

The  principles  thus  applied  to  isolated  systems 
have  been  extended  to  the  visible  universe. 
Predictions  have  been  made  that  ultimately  the 
energy  of  the  universe  will  become  completely 
unavailable,  and  will  settle  down  into  the  energy 
of  heat,  uniformly  distributed.  But  this  final 
sleep  of  the  universe  depends  on  the  assumptions 
that  the  universe  is  an  isolated  system,  finite  in 
extent,  and  that  no  process  of  molecular  concen- 
tration of  energy,  such  as  was  imagined  by 
Maxwell,  is  going  on  anywhere  throughout  the 
depths  of  time  and  space. 

A  more  restricted,  though  more  fruitful, 
application  of  the  dissipation  principle  enabled 
Helmholtz,  and,  in  a  much  more  general  manner, 
Willard  Gibbs,  to  place  on  a  firm  footing  the 
theory  of  non-isolated  but  isothermal  systems — 
systems,  that  is,  maintained  at  a  uniform  and 
constant  temperature  by  the  gain  or  loss  of 
outside  heat.  The  external  work  which  such  a 
system  can  perform,  by  means  of  a  reversible 
change  at  constant  temperature,  tends  to  a 
minimum,  and  the  system  is  in  permanent 
equilibrium  when,  and  when  only,  this  available 


INTRODUCTION  7 

or  free  energy,  as  it  is  called,  becomes  as  small  as 
possible.  By  this  sole  principle,  Willard  Gibbs 
developed  the  complete  theory  of  chemical  and 
physical  equilibrium  ;  as  Sir  Joseph  Larmor  says, 
his  '*  monumental  memoir  made  a  clean  sweep  of 
the  subject ;  and  workers  in  the  modern  experi- 
mental science  of  physical  chemistry  have  re- 
turned to  it  again  and  again  to  find  their  empirical 
principles  forecasted  in  the  light  of  pure  theory,  and 
to  derive  fresh  inspiration  for  new  departures." 

Simultaneously  with  the  development  of  ex- 
perimental research  along  the  several  lines  we 
have  indicated,  there  has  arisen  afresh  an  interest 
in  and  inquiry  into  the  philosophic  basis  on 
which  is  built  the  whole  magnificent  structure 
of  modern  science.  How  far  is  that  basis 
secure  ?  Are  the  conceptions  of  science  life- 
like pictures  of  any  fundamental  reality  behind 
the  phenomena  which  alone  our  senses  can 
apprehend  ?  Such  questions  have  occupied 
periodically  the  ablest  minds  of  certain  epochs  of 
history,  though  in  the  attempts  to  find  answers 
no  such  general  consensus  of  opinion  has  been 
reached  as  we  see  within  the  building  of  science 
itself.  Granted  the  security  of  the  foundations, 
the  edifice  seems  designed  on  a  consistent  plan, 
for  the  relations  of  its  parts  present  themselves 
similarly  to  all  minds  competent  to  judge. 

The  philosophy  of  science  is  intimately  con- 
nected with  its  history;  and  interest  was  stimulated 
afresh  in  the  philosophical  problems  involved  in 
physical  conceptions  by  the  publication  of  Mach's 
great  work  on  the  Science  and  History  of 
Mechanics.  To  many  that  book  put  new  life 
into  the  subject  treated  in  its  pages,  and  it  has 


8  PHYSICAL  SCIENCE 

led  to  a  more  careful  consideration  of  the  funda- 
mental conceptions  of  natural  science  in  general 
by  experimental  and  mathematical  physicists. 

In  more  recent  years  fundamental  changes  of 
outlook  have  been  made  by  the  recondite  re- 
searches of  Bertrand  Russell  and  A.  N.  White- 
head, and  the  less  mathematical  writings  of 
C.   D.  Broad. 

In  the  following  pages  an  attempt  will  be 
made  first  to  consider  the  philosophic  foundations 
of  physics,  and  then  to  trace  some  of  the  more 
important  developments  of  the  experimental  in- 
vestigations for  which  the  last  few  years  have 
been  remarkable. 

The  study  of  physical  equilibrium — the  equi- 
librium between  different  states  or  phases,  solid, 
liquid,  and  gaseous,  of  the  same  substance — 
naturally  opens  with  the  consideration  of  the 
relations  between  the  different  states  of  pure 
chemical  elements  and  compounds.  Here,  the 
most  striking  work  is  the  liquefaction  of  air 
and  hydrogen,  with  which  the  name  of  Dewar 
most  prominently  must  be  associated. 

Next  we  turn  to  mixtures,  and  the  fusion 
and  solidification  of  solutions  and  alloys  claim 
our  attention.  The  microscopic  analysis  of 
metals,  when  elucidated  by  the  theory  of  equi- 
librium, has  had  far-reaching  influence  on  the 
applied  arts  of  metallurgy. 

Then  are  considered  the  problems  of  solution 
in  general,  without  restriction  to  conditions  of 
equilibrium.  Now,  for  the  first  time,  we  come 
in  contact  with  electrical  phenomena ;  and  the 
theory  of  ionic  conduction  throws  light,  not  only 


INTRODUCTION  9 

on    the  nature  of  electrolytic    solutions,  but  on 
many  physiological  questions  of  vital  interest. 

A  natural  step  leads  from  the  conduction  of 
electricity  in  liquids  to  its  conduction  in  gases, 
and,  on  our  stage,  the  ion  is  joined  by  the 
corpuscle  or  electron.  The  dream  of  the  old 
philosophers  of  a  common  basis  for  matter  is 
realised  by  experimental  investigation. 

Arising  from  these  experiments  and  their 
interpretation  comes  the  theory  of  radio-activity, 
the  modern  equivalent  of  the  imagined  trans- 
mutation of  the  mediaeval  alchemist.  We  see 
and  measure  the  gradual  disintegration  of  the 
chemical  elements,  and  draw  on  the  energy 
stored  within  the  atoms  themselves.  Rutherford 
has  even  succeeded  in  inducing  such  changes 
artificially  in  some  few  elements. 

The  vibrations  of  electro-magnetic  systems 
produce  the  aethereal  waves  now  used  in  wireless 
telegraphy,  and  the  vibrations  of  atomic  systems 
give  rise  to  light.  Thus  atoms  must  be  related 
intimately  to  light,  and  light  to  electro-magnetic 
phenomena. 

Our  model  of  an  atom  must  clearly  explain 
not  only  the  facts  of  radio-activity,  but  those  of 
radiation  also.  If  electrons  radiate  energy  as 
they  revolve  in  planetary  orbits,  they  should,  on 
ordinary  dynamical  principles,  move  faster  and 
faster  and  circle  nearer  and  nearer  to  the  nucleus. 
Hence  a  number  of  atoms  should  emit  waves  of 
all  possible  periods  of  vibration  and  wave  lengths. 
But  this  deduction  is  inconsistent  with  the  well- 
known  bright  line  spectra  of  many  elements, 
spectra  showing  vibration  in  one  or  a  few 
definite   periods  only.     This  was  the   origin  of 


lo  PHYSICAL  SCIENCE 

Planck's  theory  that  radiation  can  leave  an  atom 
only  in  definite  units  or  quanta — a  theory  which, 
as  we  shall  see,  explains  bright  line  spectra,  but 
seems  to  be  inexplicable  on  the  principles  of 
classical  mechanics. 

Finally,  we  pass  to  the  bearing  of  all  this 
new  knowledge  on  cosmical  problems.  Physics 
is  rapidly  annexing  the  domain  of  astronomy, 
as  it  has  already  invaded  the  realms  of  chemistry 
and  biology.  By  the  aid  of  the  spectroscope  we 
examine  the  chemical  nature  of  the  sun  and  stars, 
we  measure  the  rates  of  their  motions  and  re- 
volutions, and  obtain  data  from  which  we  may 
speculate  about  their  origin,  development,  and 
decay.  And  the  principle  of  relativity  teaches 
that  time  and  space  are  relative  to  ourselves, 
and  their  mysteries  not  unconnected  with  the 
mystery  of  the  human  mind.  From  the  internal 
structure  of  the  atom  to  the  majestic  progress 
of  the  suns,  the  investigations  of  Physical  Science 
are  surely  and  continuously  gaining  new  know- 
ledge for  mankind. 

We  scatter  the  mists  that  enclose  us. 

Till  the  seas  are  ours  and  the  lands, 
Till  the  quivering  aether  knows  us, 

And  carries  our  quick  commands. 
From  the  blaze  of  the  sun's  bright  glory 

We  sift  each  ray  of  light. 
We  steal  from  the  stars  their  story 

Across  the  dark  spaces  of  night. 

But  beyond  the  bright  search-lights  of  science. 

Out  of  sight  of  the  windows  of  sense, 
Old  riddles  still  bid  us  defiance, 

Old  questions  of  Why  and  of  Whence. 
There  fail  all  sure  means  of  trial, 

There  end  all  the  pathways  we've  trod, 
Where  man,  by  belief  or  denial. 

Is  weaving  the  purpose  of  God. 


CHAPTER    II 

THE    PHILOSOPHICAL    BASIS    OF    PHYSICAL    SCIENCE 

Homo,  naturae  minister  et  interpres,  tantum  facit  et  intelligit 
quantum  de  naturae  ordine  re  vel  mente  observaverit.  .  .  .  Natura 
enim  non  nisi  parendo  vincitur.  .  .  . 

— BacoNj  Novum  Orgamwi. 

Tpie  mind  of  man,  learning  consciously  and 
unconsciously  lessons  of  experience,  gradually 
constructs  a  mental  image  of  its  surroundings — 
as  the  mariner  draws  a  chart  of  strange  coasts 
to  guide  him  in  future  voyages,  and  to  enable 
those  that  follow  after  him  to  sail  the  same  seas 
with  ease  and  safety.  The  chart  may  be  drawn 
to  scale ;  it  may  be  consistent  with  itself  and 
serve  its  purpose — but  it  only  represents  the 
earth's  surface  in  one  limited  and  conventional 
manner ;  it  does  not  give  a  life-like  picture  of 
the  original  in  the  same  sense  as  does  a  photo- 
graph or  a  painting.  So  it  is  with  the  ideas 
that  our  minds  conceive  of  the  world  around  us, 
and  with  the  model  of  that  world  which  our 
minds  construct.  And  this  analogy  may  serve 
to  interpret  to  us  our  attitude  towards  the  con- 
ception that  the  human  race  has  formed  of  the 
world  we  live  in.  If  the  model  be  consistent, 
if  the  various  parts  and  aspects  of  it  do  not 
fail  to  correspond  with  each  other,  it  serves  the 
double  purpose  of  introducing  order  into  what 
would  otherwise  be  mental  confusion,  and  of 
11 


12  PHYSICAL  SCIENCE 

helping    us    to    make    systematic    use    of    the 
resources  of  Nature. 

Confronted  with  the  mystery  of  the  Universe, 
we  are  driven  to  ask  if  the  model  our  minds 
have  framed  at  all  corresponds  with  the  reality  ; 
if,  indeed,  there  be  any  reality  behind  the  image. 
Such  a  question  is  a  proper  study  of  philosophy, 
but  need  not  necessarily  be  answered  for  the 
model  to  be  made  or  used.  The  whole  problem 
mankind  has  to  face  undoubtedly  includes  this 
fundamental  question  of  the  ultimate  nature  of 
reality,  which  would  enter  into  a  complete 
explanation  of  every  fact,  even  of  those  which 
we  regard  as  the  simplest.  This  aspect  of 
the  problem  is  the  subject  of  that  branch  of 
philosophy  usually  known  as  Metaphysics.  But, 
if  we  confine  our  attention  to  the  phenomena 
which  our  senses  apprehend,  and,  thus  restricting 
our  inquiry,  examine  our  mental  picture  of  Nature 
and  the  relation  of  its  parts  to  each  other,  testing 
their  correspondence  or  want  of  correspondence, 
we  are  studying  Natural  Science.  The  limitation 
indicated  has  not  always  been  observed,  and  the 
name  of  Natural  Philosophy  survives  to  remind 
us  that  Natural  Science  is  but  one  part  of  the 
whole  of  conceivable  knowledge. 

Philosophy  may  be  divided  into  two  depart- 
ments. They  are  called  by  Broad  Critical 
Philosophy,  the  analysis  of  our  fundamental 
concepts  and  beliefs,  and  Speculative  Philosophy, 
which  takes  over  the  results  of  science  and  of 
other  modes  of  human  experience,  and,  in  the 
light  of  all  the  evidence,  considers  the  nature 
and  meaning  of  the  Universe. 

The   problem    of  Speculative    Philosophy  is 


THE  PHILOSOPHICAL  BASIS  13 

of  much  greater  difficulty  than  that  of  Critical 
Philosophy  or  of  Natural  Science.  Hence, 
Natural  Science  has  only  begun  to  make  rapid 
progress  since  its  separation  from  Speculative 
Philosophy.  Despite  the  closest  attention  of 
the  acutest  intellects  since  the  age  of  Greece,  no 
general  consensus  of  opinion  has  been  reached  by 
metaphysicians.  Materialism,  Dualism,  Idealism, 
inconsistent  views  of  the  nature  of  reality,  are 
all  of  them  still  held  by  competent  philosophers  : 

Myself  when  young  did  eagerly  frequent 
Doctor  and  saint,  and  heard  great  argument 

About  it  and  about :  but  evermore 

Came  out  by  the  same  door  where  in  I  went. 

The  slow  and  laborious  methods  of  observa- 
tion and  experiment  have  been  pursued  from  the 
earliest  times  for  purposes  of  common  life  and 
technical  industry.  They  were  first  considered 
philosophically  though  inadequately  by  Bacon, 
and  by  their  help  a  firm  ground  has  been 
obtained  for  the  edifice  of  Natural  Science. 
In  contrast  with  the  results  of  Speculative 
Philosophy  or  Metaphysics,  a  general  consensus 
of  scientific  opinion  upon  fundamental  points 
has  been  obtained.  No  physicist  doubts  the 
validity,  within  narrow  limits  of  error,  of  relations 
established  accurately  by  experiment,  though  the 
theories  by  which  those  relations  are  explained 
may  be  subject  to  periodic  revision. 

But  observation  and  experiment  can  be 
directed  only  to  the  examination  of  our  concep- 
tions. In  this  way  we  gain  materials  for  the 
construction  and  examination  of  the  mind's  model 
of  reality ;  we  do  not  touch  reality  itself.  If 
this    be  doubted,  we  must  reflect   that    we  can 


14  PHYSICAL  SCIENCE 

apprehend  the  results  of  experiment  through  our 
senses  alone.  Though,  for  instance,  the  galvano- 
meter seems  at  first  to  supply  us  with  a  new- 
electrical  sense,  on  further  thought  we  see  that 
it  merely  translates  the  unknown  into  a  language 
our  sense  of  sight  can  appreciate,  as  a  spot  of 
light  moves  over  a  scale.  It  is  possible  that 
Philosophy  may  take  into  account  knowledge 
which  reaches  us  by  means  other  than  the 
senses.  Intuitions,  fundamental  assumptions, 
mental  processes  generally,  doubtless  have  an 
external  aspect,  and  may  be  studied  by  the 
science  of  Psychophysics,  but  they  may  have 
also  another  aspect  in  their  internal  relations  to 
consciousness.  Here  they  can  be  examined  by 
Psychology.  But  we  can  only  study  Nature 
through  our  senses — that  is,  we  can  only  study 
the  model  of  Nature  that  our  senses  enable  our 
minds  to  construct ;  we  cannot  decide  whether 
that  model,  consistent  though  it  be,  represents 
truly  the  real  structure  of  Nature ;  whether, 
indeed,  there  be  any  Nature  as  an  ultimate 
reality  behind  its  phenomena. 

In  emphasising  the  essential  distinction 
between  Natural  Science  and  Metaphysics,  we 
must  not  suppose  that  the  results  of  Natural 
Science  have  no  metaphysical  import.  The 
possibility  of  putting  together  a  consistent  mental 
model  of  phenomena  is  a  valid  metaphysical 
argument  in  favour  of  the  view  that  a  consistent 
reality  underlies  those  phenomena,  and  that  the 
reality  is  represented  with  more  or  less  faithful- 
ness by  the  mental  picture  we  have  pieced 
together.  Such  an  argument  must  carry  great 
weight,  and    may,  perhaps,  be   considered   con- 


THE  PHILOSOPHICAL  BASIS  15 

elusive  ;  but  it  is  a  metaphysical  argument,  not 
one  with  which  Natural  Science  is  concerned 
directly.  In  framing  and  attempting  to  answer 
her  own  deeper  questions,  Metaphysics  uses  the 
results  of  Natural  Science,  as  indeed  of  all  other 
branches  of  inquiry.  But  this  does  not  make 
Natural  Science  a  branch  of  Metaphysics,  or 
remove  the  essential  difference  between  the 
subjects  of  the  two  studies. 

The  object  of  Natural  Science,  then,  is  to 
fit  together  a  consistent  and  harmonious  model 
which  shall  represent  to  our  minds  the  phe- 
nomena which  act  on  our  senses.  We  need  not 
fear  that  this  limitation  will  lower  the  dignity  or 
circumscribe  unduly  the  extent  of  our  inquiries. 
Whether  we  look  inwards  or  outwards,  the 
complexity  of  the  phenomena  seems  boundless  : 

Boundless  inward  in  the  atom  ;   boundless  outward  in 
the  whole. 

The  more  we  learn,  the  more  various  and 
intricate  are  the  new  avenues  of  research  which 
open  before  us.  As  has  been  well  said,  the 
larger  grows  the  sphere  of  knowledge,  the  greater 
becomes  its  area  of  contact  with  the  unknown. 

So  complex  would  be  an  entire  mental  picture 
of  phenomena,  that  divisions  of  Natural  Science 
have  arisen,  each  of  them  tending  more  and 
more  to  demand  the  exclusive  attention  of  the 
specialist.  These  divisions  are  purely  arbitrary  ; 
they  have  arisen  partly  from  differences  in 
methods  of  inquiry,  partly  from  historical  reasons. 
Moreover,  they  are  variable,  and  are  shifted  from 
time   to   time   according    to    the    needs    of  each 


i6  PHYSICAL  SCIENCE 

department  and  the  prevalent  direction  of  inquiry, 
while  new  divisions  may  spring  into  existence. 

The  different  sciences  are  not  even  parts 
of  a  whole ;  they  are  but  different  aspects  of 
a  whole,  which  essentially  has  nothing  in  it 
corresponding  to  the  divisions  we  make ;  they 
are,  so  to  speak,  sections  of  our  model  of  Nature 
in  certain  arbitrary  planes,  cut  in  directions  to 
suit  our  convenience.  Thus  a  nerve-impulse 
may  be  considered  in  a  psychological  aspect,  a 
physiological  aspect,  or  a  physical  aspect.  Even 
these  divisions  may  be  sub-divided  ;  the  physics 
of  the  nerve-impulse  may  be  studied  first  from 
the  electrical  side  by  investigating  the  electric 
currents  that  accompany  it,  and  then  from  the 
mechanical  side,  by  correlating  the  electrical 
currents  with  the  movements  of  matter  that 
simultaneously  occur.  No  one  of  these  aspects 
of  the  phenomenon  is  essentially  more  funda- 
mental than  any  other,  and  the  conviction  at 
one  time  prevalent,  and  even  now  by  no  means 
uncommon,  that  a  complete  mechanical  ex- 
planation of  every  phenomenon  is  possible  and 
fundamental,  seems  merely  an  unphilosophical 
fallacy.  Its  origin  is  to  be  sought  in  the 
historical  fact  that  the  section  known  as 
mechanics  was  the  earliest  of  the  physical 
sciences,  and  that  its  methods  and  conclusions 
are  fairly  intelligible  to  the  ordinary  man,  and, 
in  their  elements,  essential  to  his  daily  life. 
The  science  of  mechanics  has  been  more  fully 
developed  from  its  experimental  basis  by  the 
methods  of  mathematical  analysis  than  any  other 
branch  of  Natural  Knowledge,  and  mankind  has 
hence  come  to  believe  that  it  is  essentially  simpler 


THE  PHILOSOPHICAL  BASIS  17 

and  nearer  reality.  But  in  truth  it  is  no  more 
fundamental  than  electricity,  and,  as  we  shall  see 
in  the  following  pages,  there  has  been  a  tendency 
to  conceive  matter  itself  as  an  electrical  mani- 
festation. Indeed,  the  theory  of  relativity  leads 
to  the  view  that  matter  is  a  form  of  energy — 
perhaps  but  a  property  of  a  combined  continuum 
of  space  and  time. 

Again,  it  is  sometimes  argued  that  mechanics 
is  the  fundamental  science  because  its  extension 
is  universal,  while  that  of  physiology,  for  example, 
is  not.  The  contraction  of  a  muscle  has  clearly  a 
mechanical  aspect,  while  the  fall  of  a  stone  to  the 
earth  has  nothing  to  do  with  physiology.  Even 
a  thought,  from  one  side  purely  a  psychological 
phenomenon,  may  have  a  mechanical  aspect  if  we 
could  trace  the  physical  changes  in  the  brain 
which  accompany  it,  whereas,  it  may  be  said, 
the  expansion  of  steam  in  an  engine  has  no 
psychological  significance.  Such  considerations 
certainly  indicate  that  the  arbitrary  plane  cut 
through  our  solid  model  of  the  universe  by 
mechanical  science  is  cut  in  such  a  place  that  it 
traverses  a  large  part  of  the  model — a  larger 
part,  perhaps,  than  any  other  section  which  has 
yet  been  cut.  It  does  not  follow,  however,  that 
it  cuts  through  the  whole ;  still  less  that  a  plane 
section  can  represent  fully  a  solid  model.  Thus 
the  argument  that,  because  of  its  wide  extension, 
mechanics  has  some  fundamental  significance  is 
seen  to  be  a  fallacy.  It  may  be  prima  inter 
pares  of  the  natural  sciences,  but  nothing  more. 
To  go  even  further  than  this,  as  has  sometimes 
been  done,  and  to  suppose  that  the  ultimate 
nature  of  reality  is  the  same  essentially  as  our 

c 


i8  PHYSICAL  SCIENCE 

idea  of  a  single  arbitrary  section,  cut  through  an 
imaginary  model  of  it,  seems  only  to  need  stating 
in  these  terms  to  be  disbelieved. 

The  study  of  physics  enables  us  to  examine 
nature  from  a  broader  standpoint  than  that  used 
by  mechanics.  But  here  again  other  aspects 
must  be  ignored.  As  Mach  has  well  said, 
**  Physical  Science  does  not  pretend  to  be  a 
complete  view  of  the  world  ;  it  simply  claims  that 
it  is  working  towards  such  a  complete  view  in  the 
future.  The  highest  philosophy  of  the  scientific 
investigator  is  precisely  this  toleration  of  an 
incomplete  conception  of  the  world  and  the 
preference  for  it  rather  than  for  an  apparently 
perfect  but  inadequate  conception." 

When  the  experimental  study  of  nature  was 
new,  when  man  first  caught  a  glimpse  of  order 
in  the  multiplicity  of  phenomena,  such  a  view 
of  the  all-comprehending  character  of  physical 
science  seemed  just.  Let  us  again  listen  to 
Mach  :— 

**  The  French  encyclopaedists  of  the  eighteenth 
century  imagined  they  were  not  far  from  a  final 
explanation  of  the  world  by  physical  and  mechani- 
cal principles  ;  Laplace  even  conceived  a  mind 
competent  to  foretell  the  progress  of  nature  for 
all  eternity,  if  but  the  masses,  their  positions,  and 
initial  velocities  were  given.  In  the  eighteenth 
century,  this  joyful  over-estimation  of  the  scope 
of  the  new  physico-mechanical  ideas  is  pardon- 
able. Indeed,  it  is  a  refreshing,  noble,  and 
elevating  spectacle ;  and  we  can  deeply  sym- 
pathise with  this  expression  of  intellectual  joy, 
so  unique  in  history.  But  now,  after  a  century 
has  elapsed,  after  our  judgment  has  grown  more 


THE  PHILOSOPHICAL  BASIS  19 

sober,  the  world-conception  of  the  encyclopaedists 
appears  to  us  as  a  mechanical  mythology  in  con- 
trast with  the  animistic  mythology  of  the  old 
religions.  Both  views  contain  undue  and  fan- 
tastical exaggerations  of  an  incomplete  perception. 
Careful  physical  inquiry  will  lead,  however,"  to 
a  more  complete  philosophy.  "The  direction 
in  which  this  enlightenment  is  to  be  looked  for, 
as  the  result  of  long  and  painstaking  research, 
can  of  course  only  be  surmised.  To  anticipate 
the  result,  or  even  to  attempt  to  introduce  it 
into  any  scientific  investigation  of  to-day,  would 
be  mythology,  not  science." 

Physical  Science,  then,  the  subject  of  the 
present  work,  is  merely  one  aspect  from  which 
we  may  agree  to  look  at  the  model  of  Nature 
that  our  minds  construct.  It  ignores  the  bio- 
logical standpoint,  from  which  phenomena  are 
regarded  in  their  bearing  on  life ;  it  ignores 
the  psychological  standpoint,  from  which  they 
are  studied  in  relation  to  mind.  With  these 
limitations,  let  us  see  what  kind  of  model  of 
Nature  we  are  led  to  build. 

From  the  complex  mystery  that  is  Nature 
the  human  mind  singles  out  certain  relations  of 
parts  of  the  whole  to  itself,  and  thus  at  once 
simplifies  and  formulates  the  problems,  as  it 
simplifies  knowledge  by  the  arbitrary  division 
into  such  sections  as  physics,  chemistry,  and 
biology.  The  ideas  of  length  and  time  may  be 
regarded  from  this  point  of  view  as  primary — 
length  as  the  simplest  form  of  space  conception, 
time  as  a  recognition  of  sequence  in  our  states 
of  consciousness. 


20  PHYSICAL  SCIENCE 

One  of  the  earliest  advances  in  exact  science 
was  the  power  of  counting  and  the  resultant 
method  of  expressing  quantities  as  numbers.  In 
spite  of  its  essential  nature,  the  capacity  for  so 
doing  is  by  no  means  innate  ;  nor  is  it  even  yet 
properly  developed  among  all  the  races  inhabit- 
ing this  globe.  In  order  to  measure  quantities, 
it  is  necessary  to  choose  or  invent  some  unit, 
and  then  to  count  the  number  of  times  that 
unit  is  comprised  in  the  quantity  to  be  measured. 
In  a  civilised  country  the  unit  of  length  is  taken 
as  the  length  between  two  marks  on  a  certain 
standard  metallic  bar.  In  England  there  is  a 
standard  yard,  and  in  France  a  standard  metre. 
In  fact,  both  these  units  are  arbitrarily  selected 
for  their  convenience,  though  the  original  idea 
of  the  metre  was  derived  from  a  connection  with 
the  supposed  dimensions  of  the  earth. 

Like  the  unit  of  length,  the  unit  of  time  is 
arbitrary,  and  ultimately  rests  on  a  measure  of 
our  sequence  of  consciousness.  Again  we  have 
to  choose  some  arbitrary  unit,  which,  in  this 
case,  should  always  contain,  under  similar  con- 
ditions, a  similar  amount  of  human  consciousness. 
For  purposes  of  the  convenience  of  daily  life  the 
obvious  unit  to  select  is  the  day,  while  the 
sequence  of  the  seasons  suggests  another  equally 
arbitrary  unit — the  year.  The  exact  relation 
between  these  two  units  can  only  be  determined 
by  careful  astronomical  observation.  Wrong 
determination  and  consequent  re-determination 
have  led  from  time  to  time  to  necessary  changes 
of  calendar ;  while  the  partial  adoption  of  these 
changes  has  resulted  in  the  inconvenient  differ- 
ences of  date  in  vogue  among  the  various  nations. 


THE  PHILOSOPHICAL  BASIS  21 

That  the  units  of  time  cannot  be  regarded  as 
essentially  fixed  and  unalterable  is  clear  when 
we  remember  that  any  friction  on  the  earth, 
such  as  that  of  the  tides,  is  slowly  prolonging 
the  day,  while  resistance  to  the  bodily  motion 
of  the  earth  round  the  sun  would  gradually 
alter  the  length  of  the  year.  Such  changes  may 
be  appreciable  only  after  millions  of  years  ;  but 
their  possibility  shows  that  our  time-units  are 
as  arbitrary  as  are  those  of  length. 

But,  even  though  our  practical  units  of  time 
and  length  are  arbitrary,  their  statement  assumes 
that  there  are  such  things  as  absolute  time  and 
space  in  which  events  take  place.  The  principle 
of  relativity  has  now  taught  us  that  time  and 
length  are  always  relative  to  some  observer, 
and  that  only  a  continuum  of  space-time  can 
be  considered  as  absolute  and  independent. 
Nevertheless,  our  present  scheme  of  science  has 
been  built  up  on  these  concepts  now  proved  to 
be  relative,  and  we  may  continue  to  use  them 
as  a  matter  of  convenience. 

From  the  conceptions  of  length  and  time,  and 
the  arbitrary  units  chosen  to  measure  them,  may 
be  derived  the  more  complex  ideas  required  for  a 
description  of  motion,and  the  derived  units  needed 
to  investigate  it  quantitatively.  Thus  velocity  is 
measured  by  the  ratio  of  the  number  of  units 
of  length  to  the  number  of  units  of  time,  while 
acceleration,  or  the  rate  of  change  of  velocity, 
is  measured  by  the  number  of  units  of  velocity 
gained  or  lost  per  unit  of  time.  These  relations 
are  expressed  by  saying  that  the  dimensions  of 
the  unit  of  velocity  are  L/T,  while  those  of  the 
unit  of  acceleration  are  v/T  or  L/T^ 


22  PHYSICAL  SCIENCE 

With  metaphysical  theories  of  matter,  Physical 
Science  has  no  direct  concern  ;  and  mechanics,  at 
any  rate,  deals  only  with  matter  as  that  concep- 
tion, which,  in  our  mental  image  of  phenomena, 
is  always  associated  with  another  and  more 
definite  conception,  that  of  mass.  We  need  not 
ask  whether  matter  has  any  objective  existence, 
or  whether  our  conception  of  mass  corresponds 
with  any  actual  property  possessed  by  a  real 
thing- in- itself.  Such  inquiries  are  of  great 
interest  and  importance ;  but  they  are  meta- 
physical inquiries,  not  those  which  the  physicist, 
as  physicist,  must  answer. 

The  conception  of  mass,  as  distinct  from  that 
of  weight,  may  arise  from  the  results  of  our  daily 
experience.  Let  us  suppose,  for  instance,  that 
two  fly-wheels  of  the  same  size,  one  of  wood  and 
the  other  of  iron,  were  mounted  on  axles,  and 
were  free  to  revolve.  When  the  wheels  are  set 
spinning,  the  weights  do  not  come  into  play,  for 
neither  wheel  is  raised  or  lowered  as  a  whole. 
Nevertheless,  a  great  difference  will  be  felt  if  we 
try  to  set  the  two  wheels  in  motion  suddenly. 
It  takes  either  a  much  harder  push  or  a  much 
longer  time  to  produce  a  certain  velocity  of 
rotation  in  the  iron  wheel  than  in  the  one  made 
of  wood,  and,  on  the  other  hand,  once  moving, 
the  iron  wheel  is  much  more  difficult  to  stop.  It 
is  these  results  which  lead  us  to  say  that  the 
mass  of  the  iron  wheel  is  the  greater. 

The  idea  of  mass  first  arises  from  the  sense- 
perception  of  force  ;  but,  to  examine  mass  quanti- 
tatively, more  definite  observation  is  necessary. 
The  mutual  action  of  two  bodies,  as  examined 
by  experiment,   is  such  that   our   description  of 


THE  PHILOSOPHICAL  BASIS  23 

their  relative  motion  becomes  greatly  simplified 
by  assigning  to  each  of  them  a  certain  relative 
number  to  express  a  quantity  which  we  may 
term  Its  relative  mass.  Let  us  make  the  two 
bodies,  when  free  to  move,  act  on  each  other  in 
any  way,  excluding  the  possibility  of  rotation, 
for  the  sake  of  simplicity.  Let  us,  for  instance, 
connect  them  by  means  of  a  long,  stretched 
elastic  cord,  and  allow  them  to  move  each  other. 
After  the  action  has  begun,  we  shall  find  that 
one  body  is,  in  general,  moving  faster  than  the 
other,  and  that  the  ratio  of  their  accelerations  is 
constant.  The  inverse  ratio  of  these  accelera- 
tions is  the  measure  of  the  ratio  between  the 
masses  of  the  two  bodies  ;  the  body  with  the 
smaller  mass  is  moved  faster  by  the  mutual 
action  than  Is  the  body  with  the  greater  mass. 

We  now  need  only  to  choose  some  mass  as 
our  unit  with  which  to  compare  other  masses, 
and  to  prove  experimentally  that  the  mass  of  a 
body  as  thus  defined  Is  a  constant  quantity,  to 
complete  our  preparations  for  using  the  concep- 
tion of  mass  in  our  physical  description  of 
observed  phenomena. 

In  all  ordinary  physical  and  chemical  changes, 
mass  is  found  to  be  constant.  But,  when  a 
particle  is  travelling  at  speeds  approaching  that 
of  light,  its  mass,  as  measured  by  an  observer  at 
rest,  increases.  Thus  mass,  like  length  and  time, 
is  not  absolute ;  its  value  depends  on  its  relation 
to  an  observer.  But  with  this  caution  we  may 
use  the  old  concept  of  a  constant  mass  with  those 
of  length  and  time  as  the  basis  of  a  system  of 
physical  units. 

Experience  shows  us  that  we  can  generalise 


24  PHYSICAL  SCIENCE 

the  result  of  our  experiment  on  the  motion  of  the 
two  bodies  connected  with  each  other  by  means 
of  a  string.  We  can  assert  that  no  body  has  an 
acceleration  unless  another  body  is  acting  on  it. 
Thus,  we  cannot  form  a  complete  picture  of  the 
motion  unless  we  consider  both  bodies.  But  it 
is  often  necessary  to  concentrate  our  attention  on 
one  of  them,  and  it  is  then  convenient  to  find  some 
quantity  which  measures  correctly  the  effect  of  the 
other  body  on  the  first.  This  quantity  is  not  the 
acceleration,  for  that  depends  on  the  mass  of  the 
moving  body,  but  it  is  the  product  of  the  mass 
and  the  acceleration,  and  is  independent  of  both. 
This  product  records  completely  the  mechanical 
effect  of  the  second  body  ;  it  may  be  taken  as  an 
accurate  definition  of  that  quantity,  of  which  a 
rough  measure  is  given  by  our  sense-perception 
of  force.  Philosophically,  force  is  the  more 
fundamental  concept,  but  for  physics  it  may  be 
defined  as  mass-acceleration,  and  instead  of  saying 
that  one  body  is  acted  on  by  another,  we  may,  if 
more  convenient,  say  that  it  is  acted  on  by  a 
force.  If  a  force  moves  its  point  of  application, 
work  is  done,  and  the  quantity  of  work  is  measured 
by  the  product  of  the  force  and  the  displacement 
in  the  direction  of  the  force.  The  capacity  for 
doing  work  is  known  as  energy.  A  clear  dis- 
tinction is  to  be  made  between  the  ideas  of  force 
and  energy. 

Together  with  the  conceptions  of  length, 
time,  and  mass,  the  conception  of  force  also  was 
employed  by  Newton  in  his  development  of 
mechanical  theory.  A  simultaneous  and  parallel 
development  of  the  science  was  led  by  Huygens, 
who    used    the    conception   which    we    now   call 


THE  PHILOSOPHICAL  BASIS  25 

work  or  energy  as  a  means  of  co-ordinating  the 
phenomena,  instead  of  stating  them  in  terms 
of  force  as  Newton  did.  Although  it  gave  a 
more  intimate  insight  into  mechanical  processes, 
Newton's  method  was  perhaps  less  general  than 
that  of  Huygens,  which  often  enables  us  to  pass 
directly  from  a  knowledge  of  the  initial  to  a 
prediction  of  the  final  state  of  a  system,  and  to 
avoid  the  difficulties  of  tracing  its  intermediate 
operations.  In  the  history  of  mechanical  science, 
now  one  method  and  now  the  other  has  proved 
the  more  useful;  and,  in  the  wider  field  of  physics, 
the  two  schools  are  still  represented,  on  the  one 
hand,  by  those  who  seek  to  trace  the  intimate 
processes  of  change  by  means  of  molecular 
theories,  and,  on  the  other,  by  those  who  rely 
on  a  more  general  presentment,  which  avoids 
such  hypotheses  by  the  use  of  the  principles  of 
thermodynamics. 

By  simple  experiments,  such  as  those  described 
above,  the  relative  masses  of  two  reacting  bodies 
may  be  measured  by  the  constant  inverse  ratio  of 
their  accelerations.  It  follows  that  the  product  of 
the  mass  and  the  acceleration  is  the  same  for  the 
two  bodies.  Thus  the  force  which  the  first  body 
exerts  on  the  second  is  the  same  as  the  force 
which  the  second  exerts  on  the  first ;  or,  as 
Newton  expressed  it,  action  and  reaction  are 
equal  and  opposite. 

The  conception  of  mass,  in  the  present  sense 
of  the  word,  we  owe  to  Newton  :  before  his  day 
no  clear  distinction  was  made  between  mass  and 
weight.  On  the  principle  of  relativity  mass  and 
weight  are  necessarily  connected,  but,  as  defined 
above,  we  cannot  predict  whether  mass  has  any 


26  PHYSICAL  SCIENCE 

relation  to  weight ;  any  discovery  of  a  connection 
between  them  must  be  a  matter  of  experiment. 

Weight  is  the  force  which  we  must  apply  to 
a  body  to  prevent  it  moving  in  its  natural  path 
towards  the  earth,  the  product  of  the  mass  and 
acceleration  being  the  same  for  the  earth  as  for 
the  body.  If  the  forces  were  equal,  the  accelera- 
tions towards  the  earth  of  two  bodies  would,  by 
our  definition  of  mass,  be  inversely  proportional 
to  their  masses.  By  experiments  on  the  accelera- 
tion, then,  the  forces  may  be  determined.  Now 
it  was  shown  by  Galileo  that,  if  the  resistance  of 
the  air  be  eliminated,  bodies  fall  at  the  same  rate 
to  the  earth  ;  that  is,  that  the  accelerations  of 
all  bodies  to  the  earth  are  the  same.  It  follows 
that  the  forces,  that  is,  the  weights  of  the  bodies, 
must  be  proportional  to  the  masses.  Masses 
can  thus  be  compared  by  weighing,  and  this 
method  is  much  the  most  convenient  in  practice. 
Nevertheless,  it  must  always  be  remembered 
clearly  that  the  proportionality  between  mass  and 
weight,  and  the  consequent  possibility  of  com- 
paring masses  by  means  of  the  balance,  is  not  a 
relation  which  could  be  predicted  a  priori  except 
by  the  recent  and  at  present  unfamiliar  ideas 
of  relativity,  but  one  which  historically  has  been 
established  as  the  result  of  careful  experimental 
investigation. 

When  we  turn  from  mechanics  to  the  other 
branches  of  physics,  it  is  necessary,  in  the  present 
state  of  knowledge,  to  use  certain  new  funda- 
mental conceptions,  such  as  temperature  and 
quantity  of  electricity,  though  it  is  probable  that 
ultimately  these  quantities  will  be  connected  with 
the   mechanical   units.     Again,  in    this   place   it 


THE  PHILOSOPHICAL  BASIS  27 

should  be  remarked  that  such  a  connection  would 
not  show  that  mechanics  is  necessarily  the  more 
fundamental  science  :  it  would  be  quite  as  correct, 
when  the  connection  is  established,  to  express 
mechanical  quantities  in  terms  of  electricity  or 
temperature. 

This  example  leads  us  to  state  in  a  general 
form  the  immediate  object  of  Physical  Science. 
The  physicist  seeks  to  discover  the  relations 
between  different  phenomena,  considered  in  one 
limited  aspect,  and  to  express  those  relations  in 
a  definite  quantitative  way.  Our  minds,  led  by 
the  analogy  with  their  own  volitions,  usually 
think  of  one  of  the  related  phenomena  as  the 
cause,  and  of  the  other  as  the  effect.  The 
physical  equation  which  expresses  the  dependence 
of  A  on  B,  or,  in  symbols,  A  =  f(B),  may  equally 
well  be  written  in  the  inverse  form,  by  which  B 
is  asserted  to  be  a  function  of  A.  In  such  cases, 
there  is  probably  no  philosophical  distinction 
between  cause  and  effect ;  it  is  no  more  rio^ht 
to  say  that  an  increase  of  pressure  produces  a 
decrease  of  volume  in  a  gas  than  to  say  that 
a  decrease  of  volume  produces  an  increase  of 
pressure.  The  student  merely  discovers  by 
experiment  that  the  two  phenomena  accompany 
each  other  in  every  case  investigated,  and  sums 
up  the  results  of  experience  in  conceptual  language 
and  in  a  shorthand  form,  in  order  to  save  the 
detailed  investigation  of  each  future  individual 
case. 

In  these  examples,  the  needlessness  of  the 
ideas  of  cause  and  effect  will  be  fairly  clear,  what- 
ever may  be  thought  about  their  metaphysical 
importance.     It  is  where  the  element  of  time  is 


28  PHYSICAL  SCIENCE 

involved  that  the  idea  of  causation  is  most  vivid. 
When  one  of  the  two  related  phenomena  seems 
to  follow  the  other,  the  mind  instinctively  identifies 
post  hoc  with  propter  hoc.  The  principle  of 
relativity  has  shown  that  there  is  no  absolute 
scale  of  time  in  which  events  may  be  placed  in 
order.  In  some  cases,  one  observer  may  say  that 
A  precedes  B,  while  to  another  B  happens  first. 
But,  even  if  a  distinction  between  cause  and 
effect  is  philosophically  difficult,  as  a  matter  of 
convenience  in  language  it  is  perhaps  justified. 
When  carefully  examined,  however,  the  difficulty 
of  isolating  the  ''cause"  of  any  particular  ''effect" 
will  be  found  to  be  insuperable.  A  long  train 
of  circumstances  has  preceded  the  phenomenon 
considered,  and  the  phenomenon  would  not  have 
appeared  had  any  one  of  those  circumstances 
been  absent.  Each  or  all  of  them  might  equally 
well  have  been  called  the  "cause."  Whether 
the  idea  of  cause  and  effect  represents  a  real 
distinction  in  the  hypothetical  world  which  our 
conceptions  represent,  remains,  like  the  nature 
and  existence  of  that  world  itself,  an  inquiry  for 
the  philosopher. 

Physical  Science,  then,  seeks  to  establish 
general  rules  which  describe  the  sequence  of 
phenomena  in  all  cases.  Underlying  all  such 
attempts  is  the  belief  that  such  an  orderly 
sequence  is  invariably  present,  could  it  only  be 
traced.  This  belief,  which  is  the  result  of 
constant  experience,  is  known  as  the  principle  of 
the  Uniformity  of  Nature.  In  its  absence  no 
organised  knowledge  could  be  obtained,  and  any 
attempt  to  investigate  phenomena  would  be 
perfectly   useless.     Unless,  to   use    the   conven- 


THE  PHILOSOPHICAL  BASIS  29 

tional  language  justified  above  as  a  matter  of 
convenience,  like  causes  always  produce  like 
effects  in  like  circumstances,  science,  and  indeed 
all  organised  knowledge,  would  be  impossible. 

When    fitted     into    our    mental    picture,    a 
generalised   result  of  experience  is   known   as  a 
physical  law,  or,  to  change  the  form  of  a  word 
and  the  size  of  two  letters,  as  a  Law  of  Nature. 
Many  brave  things  have  been  written,  and  many 
capital  letters  expended  in  describing  the  Reign 
of  Law.     The  laws  of  Nature,   however,  when 
the  mode  of  their  discovery  is  analysed,  are  seen 
to  be  merely  the  most  convenient  way  of  stating 
the  results  of  experience  in  a  form  suitable  for 
future  reference.     The  word  'Maw"  used  in  this 
connection    has    had   an    unfortunate   effect.      It 
has  imparted  a  kind  of  idea  of  moral  obligation, 
which  bids  the  phenomena  ''obey  the  law,"  and 
leads  to  the  notion  that,  when  we  have  traced 
a  law,  we  have  discovered  the  ultimate  cause  of 
a  series  of  phenomena.     Newton  and  Ohm  did 
not  first  promulgate  and  then  enforce  the  regula- 
tions   which    are   associated    with    their    names, 
though  it  is  not  only  elementary  students  who 
may  be   heard  saying  that  a  stone  falls  to  the 
ground    "because    of    the   law   of    gravitation." 
We  must  still  ask  why  each  particle  of  one  body 
attracts  each    particle  of  another,  even  if  there 
be  a    force   between    them    proportional    to    the 
product  of  the  masses  divided  by  the  square  of 
the  distance.     We  do  not  necessarily  know  why 
the  electric  current  through  a  conductor  varies  as 
the  applied  electro-motive  force,  when  we  have  dis- 
covered how  these  two  quantities  are  connected. 
The  great  change  in  the  rate  of  progress  of 


30  PHYSICAL  SCIENCE 

Natural  Science  has  occurred  since  men  learned 
to  concentrate  their  immediate  attention  on  the 
question  of  how  phenomena  are  related,  and  to 
cease,  for  the  time  at  any  rate,  to  ask  why  they 
appear.  Before  Galileo's  day  men  sought  to 
explain  the  fall  of  bodies  to  the  earth  by  saying 
that  "every  body  sought  its  natural  place" — 
the  place  of  heavy  bodies  being  below,  and  that 
of  light  ones  above.  Galileo,  exercising  the 
true  scientific  spirit  of  restraint,  set  himself  to 
determine  by  experiment  hozi)  bodies  fell.  He 
thus  discovered  that  the  speed  was  proportional 
to  the  time  of  fall,  and,  by  dropping  bodies  from 
the  leaning  tower  of  Pisa,  showed  that,  contrary 
to  the  received  doctrine  of  tendency  to  seek 
their  natural  place,  heavy  bodies  fell  no  faster 
than  light  ones. 

The  natural  laws  of  falling  bodies  were  thus 
established,  and  the  method  of  their  discovery 
shows  how  such  steps  in  knowledge  are  always 
made.  In  the  first  stage  new  phenomena  are 
observed,  or  old  phenomena  are  brought  under 
accurate  and  quantitative  measurement,  probably 
by  the  light  of  tentative  hypotheses.  -  Here  the 
virtues  of  patience,  accuracy,  incredulity,  and 
conscientious  elimination  of  personal  bias  are  of 
chief  account.  The  classical  example  is  Kepler's 
life-study  of  the  motions  of  the  planets — a  study 
which  led  to  the  establishment  of  general  laws, 
such  as  that  the  planets  move  in  ellipses  having 
the  sun  in  one  focus. 

But  such  laws  alone  are  insufficient  to  satisfy 
our  minds,  which  inevitably  return  to  the  question 
why  such  relations  hold.  The  relations  are  mis- 
interpreted and  re-interpreted,  until  some  Newton 


THE  PHILOSOPHICAL  BASIS  31 

with  the  touch  of  genius  which  often  accompanies 
sober  scientific  insight  and  imagination — some  one 
who  is  able  to  brush  aside  for  a  time  the  non- 
essential, and  to  rise  above  the  confusion  of 
detail — is  inspired  with  a  conception  of  order  in 
the  multiplicity  of  the  phenomena  :  order  to  be 
seen  when  some  simple  principle  is  borne  in  mind, 
and  is  expressed  in  a  formula,  which,  in  terms 
of  our  conceptual  shorthand,  enables  us  to  re- 
member and  to  predict  the  sequence  of  phenomena. 
If  the  formula  is  expressed  in  terms  of  simple  con- 
ceptions, already  known  and  often  used  in  other 
branches  of  knowledge,  the  mind  at  once  looks 
on  it  as  an  *' explanation "  of  the  phenomena, 
though  it  is  evident  on  further  thought  that  the 
phenomena  are  no  more  fully  understood  than 
are  the  fundamental  conceptions — mass,  space, 
time,  whatever  they  be — in  which  the  "explana- 
tion "  is  expressed. 

The  next  step  consists  in  deducing  new  conse- 
quences of  the  hypothesis  ;  and  here  the  methods 
of  mathematical  analysis  are  usefullyapplied.  The 
science  of  mathematics  as  such  has  nothing  to  do 
with  natural  phenomena.  Like  physical  science  it 
is  concerned  with  ideal  conceptions  ;  but  neither 
does  it  seek  to  gain  those  conceptions  from 
an  examination  of  Nature,  nor  to  check  their 
correspondence  by  the  methods  of  experiment. 
Mathematics  may  borrow  subject-matter  from 
observational  science,  or  may  acquire  by  pure 
mental  processes  subject-matter,  such  as  the 
geometry  of  four  dimensional  space,  which  may 
or  may  not  have  a  counterpart  in  Nature.  In 
either  case,  mathematics  deals  with  the  concep- 
tions as  such,  and  traces    their  results  and  the 


32  PHYSICAL  SCIENCE 

relations  between  them  by  the  methods  of  logic, 
with  no  necessary  intention  of  elucidating  the 
phenomena  of  Nature.  Except  when  inventing 
new  methods,  the  mathematician  is  a  calculating 
machine.  His  conclusions  are,  or  ought  to  be, 
contained  implicitly  in  the  premises  he  uses.  He 
develops  the  premises,  discovers  their  full  meaning, 
and  elaborates  their  consequences,  in  a  way  quite 
beyond  the  unaided  power  of  thought,  which, 
without  the  guiding  rules  and  generalisations  of 
mathematical  analysis,  would  be  lost  in  the  maze 
of  complications.  But  the  mathematician  lives  in 
a  purely  conceptual  sphere,  and  mathematics  is 
but  the  higher  development  of  symbolic  logic. 

Taking,  then,  a  new-born  hypothesis,  its  con- 
sequences are  deduced  by  logical  common-sense 
reasoning  ;  and,  where  such  reasoning  cannot  see 
its  way  unaided,  by  the  help  of  mathematical 
analysis.  The  results  thus  obtained  are  then 
used  by  the  observer  or  experimenter,  who  tests 
by  the  use  of  old,  or  the  determination  of  new 
data,  the  truth  of  the  formula  by  every  possible 
means.  Its  relations  to  other  ascertained  prin- 
ciples, its  power  of  correlating  hitherto  uncon- 
nected phenomena,  are  examined  in  turn.  From 
consideration  of  its  significance,  we  gain  sug- 
gestions for  further  observation,  if  possible  for 
future  experiment.  Such  experiments,  undertaken 
with  the  express  purpose  in  view,  are  probably 
better  adapted  to  test  the  formula  than  the 
observations  previously  accumulated.  If  the 
concordance  is  complete  as  far  as  the  accuracy 
of  experiment  can  go,  the  formula  becomes,  in 
the  then  state  of  knowledge,  an  accepted  theory. 
Whatever  this  means,  such  a  generalisation  will. 


THE  PHILOSOPHICAL  BASIS  33 

at  all  events,  prove  a  useful  working  hypothesis, 
by  the  light  of  which  research  may  be  guided  into 
promising  paths.  As  the  range  of  observation 
widens,  and  as  the  accuracy  of  the  old  observations 
is  increased,  the  fate  of  the  new  theory  hangs  in 
the  balance.  The  formula  may,  perhaps,  still  be 
confirmed,  it  may  require  modification,  or  it  may 
have  to  be  abandoned  as  a  theory  which  has 
played  a  useful  and  honourable  part  in  its  day, 
but  has  become  inadequate  to  express  the  develop- 
ing knowledge  of  a  later  time.  If  so,  it  ceases  to 
be  cited  as  an  accepted  theory.  Not  that  Nature 
has  changed,  but  rather  our  attitude  towards  her, 
and  our  conceptual  model  of  her  phenomena. 
Thus  new  theories  replace  the  old  ones. 

Some  years  ago  the  constancy  of  the  chemical 
elements  was,  in  the  then  state  of  knowledge,  an 
accepted  theory.  Latterly,  the  phenomena  of 
radio-activity  have  forced  us  to  believe  that 
radium  is  passing  continuously  and  spontaneously 
into  other  elements — that  true  transmutations  of 
matter  occur.  The  obvious  transmutation  of  one 
kind  of  matter  leads  to  the  possibility  of  the 
gradual  transmutation  of  all ;  since  as  yet  no 
property  of  matter  has  been  noted  which  is  the 
exclusive  possession  of  one  substance  alone.  New 
phenomena,  or  rather  phenomena  for  the  first  time 
appreciated,  are  continually  coming  to  light,  and 
evidence  is  accumulating  from  which  the  profit- 
able construction  of  theories — for  a  time  in  abey- 
ance— may  again  be  pursued.  Nothing  must  be 
ruled  out  of  court  because  contrary  to  received 
views  ;  when  a  prima  facie  case  has  been  made 
out,  everything  must  be  examined  by  experiment, 
induction,  deduction,  and  again  experiment.     This 

D 


34  PHYSICAL  SCIENCE 

is  the  only  sure  road  to  the  understanding  of 
Nature;  and,  in  times  to  come,  it  may  lead  us  into 
regions  now  unknown,  or  considered  to  be  closed 
to  the  investigations  of  science.  The  evolution 
and  disintegration  of  matter,  the  problems  of 
hypnotism  and  of  direct  thought  transference,  are 
questions  which  seem  to  be  coming  rapidly  within 
the  range  of  scientific  inquiry.  It  is  possible  that 
an  advance  has  already  been  made  towards  clear- 
ing away  part  of  the  mystery,  so  attractive  to 
some,  so  repellent  to  others,  that  surrounds  these 
phenomena.  At  any  rate,  in  several  of  the  great 
schools  of  psycho-medicine,  notably  in  France 
and  America,  materials  are  being  accumulated, 
their  trustworthiness  examined,  and  the  results 
systematically  collated.  It  may  be  that  these 
investigations,  so  beset  with  evident  difficulties, 
are  indeed  indefinitely  complicated  in  their  issues 
by  questions  of  racial  predisposition,  of  individual 
temperament  and  mental  condition,  both  of 
observed  and  observers.  Whether  any  or  all 
of  these  problems  will  prove  amenable  to  the 
methods  of  dispassionate  observation  and  experi- 
ment is  a  matter  which  the  years  to  come  alone 
can  show. 

We  must  thus  look  on  natural  laws  merely  as 
convenient  shorthand  statements  of  the  organised 
information  that  at  present  is  at  our  disposal. 
But  when  Physical  Law,  as  understood  in  the 
eighteenth  century,  has  been  dethroned  from  a 
place  that  was  never  rightly  its  own,  let  us  not 
think  that  its  usefulness  has  been  diminished  or  its 
dignity  unduly  lowered.  Without  the  possibility 
of  discovering  such  laws,  and  framing  theories  of 


THE  PHILOSOPHICAL  BASIS  35 

their  meaning,  mankind  would  be  lost  hopelessly 
in  a  wilderness  of  phenomena ;  no  continuous 
progress  could  be  made ;  no  consistent  idea  of 
the  world  around  could  ever  be  attained.  Each 
individual  phenomenon,  as  it  appeared  time  after 
time,  might  still  be  investigated  ;  but,  with  his 
limited  mind  and  short  life,  no  one  man  could  ever 
secure  a  basis  for  adequate  knowledge.  Without 
some  general  way  of  stating  his  experiences,  he 
could  hand  on  neither  his  guesses  after  truth  nor 
his  hard-won  information  :  mankind  would  never 
have  emerged  from  barbarism. 

The  relations  between  an  observer  and  his 
surroundings  may  for  convenience  be  analysed 
into  the  conceptions  of  length,  time,  and  mass. 
From  these,  as  we  have  seen,  the  other  mechanical 
units  can  be  derived,  and  a  mechanical  model  of 
Nature  be  constructed.  It  is  incomplete ;  for 
even  the  simplest  mechanical  fact,  such  as  the 
fall  of  a  body  to  the  ground,  inevitably  has 
other  aspects.  Heat  may  be  developed,  electrical 
manifestations  appear,  and,  if  the  body  be  a  living 
one,  physiological  and  psychological  changes  take 
place.  Neglecting  these  aspects,  however,  a  com- 
plete mechanical  account  of  the  phenomenon  can 
be  given  in  terms  of  the  three  fundamental  concep- 
tions. As  we  have  seen,  new  ideas,  which  may  be 
derived  from  the  primary  ones,  become  necessary 
in  the  course  of  the  investigation.  The  body  falls 
with  a  certain  acceleration,  and,  at  any  instant,  is 
moving  with  a  definite  velocity.  As  it  falls,  it 
acquires  energy  of  motion  and  loses  energy  of 
position. 

During  the  fall  we  find  that  we  can  success- 


36  PHYSICAL  SCIENCE 

fully  describe  what  happens  by  assuming  that  the 
quantity  which  we  call  the  mass  of  the  body  keeps 
constant,  and  that  the  sum  of  the  two  kinds  of 
energy  keeps  constant  also.  If  we  include  in  our 
view  the  complete  physical  and  chemical  aspects 
of  the  phenomena,  we  may  greatly  extend  these 
results.  When  the  body  reaches  the  earth,  it  is 
possible  that  processes  of  decay  set  in,  which 
eventually  result  in  most  of  its  substance  dis- 
appearing in  gases  or  other  products.  The  energy 
of  motion  acquired  by  the  body  during  its  fall 
also  seems  to  disappear,  with  no  corresponding 
gain  of  energy  of  position.  Chemistry,  however, 
generalising  from  many  experimental  results,  tells 
us  that,  if  we  could  trace  all  the  forms  of  matter 
into  which  the  body  is  resolved,  we  should  find 
that  there  was  no  loss.  Every  particle  of  the 
original  body  still  exists  in  one  of  its  products. 
Physics,  on  the  other  hand,  teaches  us  in  the 
same  way  that  the  sum  of  all  the  forms  of  energy, 
heat,  sound,  etc.,  which  appear  as  a  consequence 
of  the  impact  on  the  ground,  could  they  all  be 
taken  into  account,  would  be  exactly  equivalent 
to  the  energy  of  motion  possessed  by  the  body  at 
the  instant  before  contact.  These  great  principles 
of  the  conservation  of  mass  and  the  conservation 
of  energy  are  two  of  the  most  important  practical 
generalisations  ever  reached  by  Physical  Science. 
While  fully  recognising  the  importance  of  these 
generalisations  from  the  physical  point  of  view, 
we  must  be  careful  how  we  give  them  any  meta- 
physical significance  even  under  the  pre-relativity 
theory  of  science.  Under  certain  limiting  con- 
ditions, other  physical  quantities  besides  mass  and 
energy  maybe  conserved.    Thus  in  pure  mechanics 


THE  PHILOSOPHICAL  BASIS  37 

we  recognise  the  conservation  of  momentum — a 
name  for  the  mathematical  quantity  obtained  by 
multiplying  together  the  measures  of  mass  and 
velocity.  Again,  in  reversible  systems,  where 
physical  or  chemical  changes  may  occur  in  either 
direction  with  equal  freedom,  thermodynamics  in- 
dicates the  conservation  of  another  quantity,  named 
by  Clausius,  entropy.  Momentum  and  entropy 
are  only  conserved  under  restricted  conditions  ;  in 
physical  systems  the  momentum  of  visible  masses 
is  often  destroyed,  while  In  irreversible  processes 
entropy  always  tends  to  Increase. 

Mass  and  energy  may  seem  to  be  conserved  In 
the  conditions  known  to  us,  and  we  are  justified 
In  extending  the  principle  of  their  conservation  to 
all  cases  where  those  conditions  apply.  It  does 
not  follow,  however,  that  conditions  unfamiliar 
to  us  do  not  exist,  in  which  mass  and  energy  dis- 
appear or  come  into  existence.  The  persistence 
of  matter,  for  instance,  might  conceivably  be  an 
apparent  persistence.  A  wave,  travelling  over 
the  surface  of  the  sea,  seems  to  persist.  It  keeps 
Its  form  unchanged,  and  the  quantity  of  water  In 
it  remains  unaltered.  We  might  talk  about  the 
conservation  of  waves,  and,  perhaps,  in  so  doing, 
be  as  near  the  truth  as  when  we  talk  of  the  per- 
sistence of  the  ultimate  particles  of  matter.  But 
the  persistence  of  the  wave  Is  an  apparent  phe- 
nomenon. The  form  of  the  wave  indeed  truly 
persists,  but  the  matter  in  It  is  always  changing — 
changing  in  such  a  way  that  successive  portions 
of  matter  take,  one  after  the  other,  an  identical 
form.  Indications  are  not  wanting  that  only  in 
some  such  sense  as  this  is  mass  persistent.  In  a 
later  chapter  we  shall  see  that  there  is  definite 


38  PHYSICAL  SCIENCE 

experimental  evidence  to  show  that  the  mass  of  a 
moving  particle  increases  as  its  velocity  approaches 
that  of  light.  Moreover,  the  principle  of  relativity 
has  changed  profoundly  our  outlook  on  such 
results  as  the  conservation  of  matter  and  energy. 
The  concepts  in  which  they  are  expressed  are 
relative  to  an  observer  and  not  absolute.  We 
may  have  unconsciously  arranged  the  cards,  and 
then  rediscovered  with  enthusiasm  fours  and 
sequences  put  in  by  ourselves. 

Even  if  we  assume  that  some  reality  underlies 
phenomena,  it  is  clear  that  the  reality  must  be 
very  different  from  the  mental  picture  which 
common  sense  frames,  when  unaided  by  the  in- 
ductions of  science.  Our  first  conception  of  a 
wooden  stick  involves  the  ideas  of  a  certain  lonof- 
shaped  form,  of  hardness,  of  weight,  of  a  colour 
more  or  less  brown,  perhaps  of  some  amount  of 
elasticity.  Examination  with  a  microscope  re- 
veals many  appearances  invisible  with  the  unaided 
eye,  and  we  find  that  the  stick  has  a  structure 
much  more  detailed  than  we  imagined.  From  the 
results  of  observation  and  experiment,  physics 
teaches  us  that  the  properties  of  the  stick  can 
only  satisfactorily  be  represented  by  the  hypothesis 
that  the  substance  of  it  is  divisible,  but  not  in- 
finitely divisible  ;  that  it  consists  of  discontinuous 
particles  or  molecules.  Again,  chemistry  assures 
us  that  the  molecules  of  the  stick  are  made  up  of 
still  smaller  parts  or  atoms,  which  separate  from 
each  other  when  chemical  action  occurs,  when,  for 
instance,  the  stick  is  burnt,  and  can  afterwards 
rearranofe  themselves  into  new  molecules. 

When  we  pursue  our  inquiries  into  the  nature 


THE  PHILOSOPHICAL  BASIS  39 

of  these  chemical  atoms,  we  find  that  recent  re- 
search has  shown  that  they  contain  very  much 
smaller  particles  or  corpuscles,  and  we  are  asked 
to  imagine  that  these  are  in  constant  motion 
within  the  atom,  somewhat  as  the  planets  move 
within  the  solar  system.  Intimate  relations  exist 
between  the  properties  of  these  corpuscles  and  the 
phenomena  of  electricity,  and  a  corpuscle  may  be 
regarded  as  an  isolated  electric  charge,  or  electron, 
as  it  is  called,  the  mass  of  the  corpuscle  being 
an  apparent  effect  due  to  electricity  in  motion. 

Thus  we  have  ''  explained "  electricity  in 
terms  of  corpuscles,  and  mass  perhaps  in  terms 
of  electricity.  Adventurous  pioneers  may  strive 
to  reach  more  ultimate  conceptions  by  resolving 
the  electron  into  a  centre  of  intrinsic  strain  in  an 
aether  or  a  kink  in  a  four  dimensional  continuum  of 
space  and  time.  Whatever  fate  may  await  their 
efforts,  we  have  already  travelled  far  in  attempting 
to  construct  a  complete  mental  image  of  the 
wooden  stick  and  all  its  known  properties.  We 
have  reached  ideas  very  different  from  those  of  the 
hard,  continuous  substance  from  which  we  started. 

The  other  properties  of  the  stick  can  be 
analysed  into  physical  conceptions  in  much  the 
same  way.  Thus  the  colour  is  found  to  be  due 
to  a  sorting  action  which  the  particles  of  the 
wood  exert  on  the  complex  system  of  aethereal 
waves,  making  up  white  light.  Some  of  these 
waves  have  their  energy  more  freely  absorbed  by 
the  molecules  of  the  wood  than  have  others  ;  the 
balance  of  light  is  upset,  and  the  reflected  beam 
produces  the  sensation  of  colour.  Here,  again, 
the  most  fundamental  conceptions  into  which 
modern  science  enables  us  to  resolve  our  primitive 


40  PHYSICAL  SCIENCE 

ideas  are  very  different  from  those  in  which  they 
took  their  origin. 

While  Natural  Science  is  not  committed  to 
any  particular  philosophical  system,  while  in  its 
essence  it  is  independent  of  all  such  systems,  the 
language  it  uses  habitually  is  based  on  the 
common-sense  realism,  which  is  the  philosophic 
creed  of  most  men  of  science — indeed,  of  the 
great  bulk  of  mankind,  or  at  all  events,  of  that 
part  of  mankind  belonging  to  the  races  of  Western 
Europe.  The  mass  and  energy  with  which  we 
deal  in  physical  experiments,  and  in  the  mathe- 
matical reasoning  based  on  inductions  from  the 
experiments,  are  purely  conceptual  quantities, 
introduced  to  bring  order  and  simplicity  into  our 
perceptions  of  phenomena.  Perhaps  they  are 
not  absolute  quantities  at  all,  but  merely  relations 
between  ourselves  and  the  systems  we  describe 
in  terms  of  them.  They  may  be  replaced  by  other 
concepts  as  our  changing  knowledge  requires. 

Possibly  the  quantum,  or  unit  of  ''action," 
which  we  are  forced  to  accept  though  it  accords 
ill  with  previous  ideas,  may  be  nearer  reality. 
But  science  still  talks  of  matter  and  energy  as 
though  it  knew  of  the  existence  of  realities  corre- 
sponding with  the  mental  Images  to  which  alone 
these  names  strictly  apply.  In  the  laboratory,  as 
in  practical  life,  there  Is  neither  room  nor  time 
for  philosophic  doubt.  In  periods  of  reflection, 
however,  when  considering  the  theoretical  bearing 
of  the  results  of  our  experiments,  it  is  sometimes 
well  to  remember  the  limitation  of  our  present 
certain  knowledge,  and  the  purely  conceptual 
nature  of  our  scheme  of  Natural  Science  when 
based  merely  on  Its  own  inductions. 


«.! 


CHAPTER  III 

THE    LIQUEFACTION    OF    GASES    AND    THE 
ABSOLUTE    ZERO    OF    TEMPERATURE 

"Scientia    et    potentia    humana    in    idem    coincidunt,    quia 
ignoratio  causae  destituit  efifectum." — Bacon,  Novum  Orga7ium, 

Matter  is  known  to  us  in  three  states — as  solid, 
as  liquid,  and  as  gas.  The  relations  between 
these  three  states  have  been  the  subject  of 
investigation  throughout  the  history  of  Physical 
Science,  and,  indeed,  almost  throughout  the 
history  of  the  human  race.  The  solidification  of 
water  in  a  frost,  and  its  evaporation  by  the  sun 
or  a  fire,  have  been  familiar  to  mankind  from  the 
earliest  times.  But  water  shows  these  changes 
of  state  under  too  favourable  an  aspect  to  be 
taken  as  a  general  example.  It  has  by  no  means 
always  been  clear  that  such  transformations  were 
possible  to  all  kinds  of  matter,  and  it  has  been 
necessary  to  exhaust  the  resources  of  modern 
civilisation  to  liquefy  the  more  permanent  gases. 

Ice,  when  heat  is  supplied,  begins  to  melt  at 
a  definite  temperature,  which  is  called  o°  on  the 
Centigrade  scale,  and  32^  on  the  scale  devised  by 
Fahrenheit.  While  any  ice  remains,  no  change 
of  temperature  occurs  in  the  mixture  of  ice  and 
water.  Heat  is  still  absorbed,  but  its  energy  is 
used  to  effect  a  change  of  state,  not  to  raise  the 


41 


42  PHYSICAL  SCIENCE 

temperature.  The  pure  substance  Ice  has  a  con- 
stant melting-point.  Similarly,  if  water  be  cooled 
at  constant  pressure,  it  begins  and  finishes  to 
freeze  at  the  same  temperature.  It  has  a  constant 
freezing-point,  identical  with  the  melting-point. 

When  water  boils,  a  still  larger  quantity  of 
heat  is  absorbed,  but  the  temperature  again 
remains  unaltered  during  the  whole  process. 
When  the  barometer  stands  at  760  millimetres, 
or  just  under  30  inches  of  mercury,  the  tempera- 
ture of  the  boiling-point  is  taken  as  the  second 
fixed  point  on  our  thermometers,  and  called  100° 
or  212'  according  as  we  use  the  Centigrade  or 
the  Fahrenheit  scale.  If  the  barometer  stands 
higher  or  lower  than  the  standard  height,  the 
boiling-point  of  water  is  found  to  be  above  or 
below  100°  C,  rising  or  falling  through  i°C.  for 
a  change  of  27  millimetres  in  the  barometer. 
The  freezing-point  also  depends  on  the  pressure  ; 
but  the  change  is  much  smaller  than  in  the  case 
of  the  boiling-point,  and  delicate  experiments  are 
necessary  to  determine  It. 

The  variation  with  pressure  of  the  points  of 
transition  from  one  state  of  matter  to  another  are 
connected  with  the  changes  of  volume  which 
simultaneously  occur.  Water  expands  on  freez- 
ing, for  ice  floats  on  the  surface  of  a  lake,  and 
pipes  burst  In  a  frost.  If  this  increase  in  volume 
be  resisted  by  an  external  pressure,  as  by  putting 
the  water  Into  a  strong  closed  vessel,  the  act  of 
freezing  Involves  the  performance  of  external 
work  in  forcing  outwards  the  walls  of  the  vessel 
to  give  room  for  the  Ice  to  form.  It  Is  therefore 
more  difficult  to  produce  Ice  under  pressure,  and 
a  greater  lowering  of  temperature  Is  necessary. 


THE  LIQUEFACTION  OF  GASES  43 

Thus  an  increase  of  pressure  must  lower  the 
melting  or  freezing-point.  On  evaporation,  the 
increase  in  volume  occurs  with  the  change  from 
liquid  to  vapour  ;  an  increase  of  external  pressure 
therefore  makes  evaporation  more  difficult,  and 
consequently  produces  a  rise  in  the  boiling-point. 
If  the  change  in  volume  and  the  amount  of  heat 
required  to  produce  the  change  in  state  are 
known,  the  principles  of  thermodynamics  enable 
us  to  calculate  the  exact  amount  of  alteration  in 
the  freezing  or  boiling-points. 

There  is  reason  to  suppose  that  the  three 
states  of  solid,  liquid,  and  gas,  assumed  within  a 
moderate  range  of  temperature  and  pressure  by 
the  familiar  substance  water,  might  be  obtained 
with  all  bodies  if  we  could  command  temperatures 
and  pressures  high  enough  and  low  enough. 
Metals  melt  and  volatilise  at  high  temperatures, 
while  even  gases  such  as  air  and  hydrogen  have 
now  been  liquefied. 

Several  gases,  previously  unknown  in  any 
other  form,  were  liquefied  by  Faraday.  His 
method  consisted  in  evolving  the  gas  by  heating 
chemical  reagents  in  one  limb  of  a  bent  glass 
tube,  and  cooling  the  other  limb  in  cold  water  or 
a  freezing  mixture.  As  the  gas  is  evolved,  the 
pressure  rises,  and  either  the  gas  is  liquefied  in 
the  cold  limb,  or  the  tube  bursts.  By  this  simple 
means  chlorine,  sulphur  dioxide,  ammonia,  and  a 
few  other  gases  may  be  liquefied. 

The  conditions  necessary  for  liquefaction  were 
not  fully  understood  till  Andrews,  in  1863,  showed 
that  carbonic  acid  gas  could  not  be  liquefied 
unless    its    temperature   was    reduced    below   a 


44  PHYSICAL  SCIENCE 

definite  fixed  point,  which  he  called  the  critical 
point.  The  critical  point  of  carbonic  acid  is 
fairly  high,  about  30°  on  the  Centigrade  scale  ; 
but  for  other  gases,  such  as  air  or  hydrogen,  it  is 
much  lower,  many  degrees  below  the  freezing- 
point  of  water.  However  low  it  be,  unless  a  gas 
is  cooled  to  its  critical  point,  no  pressure,  what- 
ever be  its  intensity,  can  produce  liquefaction. 
Below  their  critical  points,  gases  may  be  con- 
sidered as  vapours,  and  will  liquefy  if  the  pressure 
applied  is  high  enough.  The  problem  of  the 
liquefaction  of  a  refractory  gas  is  thus  solved 
if  we  can  produce  cold  sufficiently  intense  to 
reduce  it  below  its  critical  point. 

Three  methods  have  been  used,  either  singly 
or  in  conjunction,  to  cool  gases  below  their  critical 
points.  The  first  method  depends  on  the  heat 
which  it  is  necessary  to  supply  in  order  to 
evaporate  a  liquid.  A  liquid  boils  when  the 
pressure  of  its  vapour  is  equal  to  the  pressure  of 
the  atmosphere  acting  upon  its  surface,  and,  if  we 
reduce  this  external  pressure,  the  boiling-point  is 
lowered.  Thus,  by  pumping  away  the  vapour  as 
fast  as  it  is  formed,  and  so  keeping  the  pressure 
low,  a  liquid  can  be  boiled  at  a  temperature  much 
below  its  normal  boiling-point.  By  this  method, 
for  example,  it  is  possible  to  make  water  boil  with 
no  outside  supply  of  heat.  The  heat  necessary 
for  evaporation  is  then  taken  from  the  water  itself, 
which  in  this  way  is  gradually  cooled.  If  the  air- 
pump  is  efficient,  and  if  very  little  heat  is  allowed 
to  leak  in,  the  cooling  may  go  so  far  that  the 
remaining  water  is  frozen.  Beginning  at  the 
normal  boiling-point  of  water,  we  should  then 
have  cooled  the  system  by  means  of  evaporation 


THE  LIQUEFACTION  OF  GASES         45 

through  100°.  If,  instead  of  water,  we  had  taken 
some  liquid  of  low  boiling-point,  such  as  liquefied 
sulphur  dioxide,  or,  better  still,  liquefied  carbonic 
acid,  the  same  process  of  cooling  under  exhaustion 
would  have  taken  place  ;  but  the  final  temperature 
reached  would  have  been  much  lower. 

Starting  then  with  some  substance  like  sulphur 
dioxide,  which  is  easily  liquefied  by  pressure  alone 
at  ordinary  temperatures,  we  can  boil  it  away 
under  exhaustion,  and  so  produce  a  low  tempera- 
ture. By  making  a  more  refractory  gas,  such  as 
carbonic  acid,  circulate  through  a  tube  surrounded 
with  the  cold  sulphur  dioxide,  this  new  agent  is 
cooled  below  its  critical  point,  and  liquefied.  In  its 
turn  the  liquid  carbonic  acid  is  boiled  away  under 
low  pressure,  and  used  as  a  refrigerating  agent  to 
cool  the  gas — oxygen,  let  us  say — which  we  are 
attempting  to  conquer.  This,  sometimes  called 
the  cascade  method  of  cooling,  was  the  plan 
adopted  by  the  Swiss  physicist,  Pictet  of  Geneva, 
in  the  experiments  which,  simultaneously  with 
those  of  his  French  contemporary  Cailletet,  first 
liquefied  oxygen.  With  one  of  those  curious 
coincidences  which  the  broad  wave  of  advancing 
knowledge  sometimes  produces,  both  these  results 
were  announced  at  a  memorable  meeting  of  the 
French  Academy,  held  on  the  24th  of  December 

1877. 

Even  when  the  gas  was  thus  cooled,  however, 
Pictet's  process  was  not  entirely  effective.  In 
order  to  pass  the  last  few  degrees  and  reach  the 
critical  point,  a  second  method  of  cooling  had  to 
be  brought  into  play.  To  explain  this  second 
method  other  principles  must  be  taken  into 
account.     When  a  certain  mass  of  gas,  forced  into 


46  PHYSICAL  SCIENCE 

a  closed  vessel  till  the  pressure  rises  to  several 
atmospheres,  is  let  out  suddenly,  its  volume  is,  of 
course,  greatly  increased  by  the  sudden  expansion. 
Room  has  to  be  made  for  the  increase  of  volume, 
and  this  process  requires  the  expenditure  of  work, 
for  the  atmosphere  is  pressing  on  the  gas  on  all 
sides,  and  has  to  be  forced  back  when  the  expan- 
sion occurs.  Moreover,  if  the  particles  of  the  gas 
attract  each  other,  work  must  be  done  in  the 
separation  necessary  for  the  increase  of  volume. 
Thus  internal  as  well  as  external  work  may  be 
performed  during  the  expansion.  Unless  heat  is 
supplied  from  without,  the  energy  needed  to 
perform  all  this  work  must  come  from  the  heat 
supply  of  the  gas  itself,  which  becomes  cooled  in 
the  process.  If  the  expansion  is  sudden  and 
therefore  rapid,  there  is  no  time  for  heat  to  enter 
the  gas,  and  the  cooling  represents  the  full  effect 
of  the  work  done.  By  this  means,  Pictet  finally 
liquefied  his  oxygen.  The  highly  compressed  gas, 
which  had  been  cooled  in  liquid  carbonic  acid 
boiling  under  low  pressure,  was  allowed  suddenly 
to  escape  into  the  atmosphere.  A  large  amount 
of  external  work  was  thus  done,  intense  cooling 
resulted,  and  liquid  oxygen  was  seen  as  spray  in 
the  issuing  jet  of  gas.  It  was  by  a  still  more 
sudden  expansion  that  Cailletet  liquefied  oxygen, 
using  preliminary  cooling  only  to  30""  below  the 
Centigrade  zero. 

In  modern  forms  of  apparatus  for  the  lique- 
faction of  gases  it  is  found  advisable  to  sacrifice 
the  cooling  gained  by  the  performance  of  external 
work,  and  to  rely  on  that  due  to  the  internal  work 
alone.  By  this  means  it  is  possible  to  construct 
much   more  powerful  and   efficient  refrigerating 


THE  LIQUEFACTION  OF  GASES  47 

machines.  The  essential  feature  in  the  process 
of  cooling  by  the  performance  of  external  work  is 
the  expansion  of  the  gas  by  its  own  elastic  force. 
If  the  work  necessary  for  the  increase  of  volume 
under  the  external  pressure  be  supplied  by  an 
engine,  or  if  all  such  work  be  prevented  by  making 
the  gas  expand  into  a  vacuum,  there  is  no  external 
work  to  absorb  the  heat  energy  of  the  gas  itself, 
and  no  cooling  from  this  cause  is  produced.  The 
gas,  however,  still  has  to  supply  any  work  needed 
to  separate  its  own  particles  against  any  mutual 
attractive  forces,  and,  if  such  forces  exist,  cooling 
can  still  be  obtained  at  the  expense  of  the  heat- 
energy  of  the  gas.  On  the  other  hand,  if  the  inter- 
molecular  forces  are  forces  of  repulsion,  expansion 
will  be  aided  by  their  action,  and  will,  in  the 
absence  of  external  work,  be  accompanied  by  an 
increase  of  temperature.  Thus,  by  arranging  for 
free  expansion,  as  It  is  called,  we  can  examine  the 
nature  of  the  inter-molecular  forces  by  observing 
whether  a  gas  is  cooled  or  heated. 

In  such  experiments,  it  is  necessary  to  prevent 
the  performance  of  external  work  by  the  gas  itself, 
and  this  can  be  done  in  either  of  the  two  ways 
indicated  above.  Gay  Lussac,  and  afterwards 
Joule,  filled  one  vessel  with  gas  under  high 
pressure,  and  then  allowed  the  gas  to  expand 
into  another  vessel  previously  exhausted.  Here, 
in  expanding  into  a  vacuum,  no  external  pressure 
has  to  be  overcome,  and  no  external  work  is  done. 
Any  thermal  change  will  be  the  equivalent  of  the 
internal  work.  The  vessels  were  placed  side  by 
side  in  water,  which  was  stirred  after  the  experi- 
ment, and  tested  with  a  sensitive  thermometer. 
At  ordinary   temperatures   no  heating  or   cool- 


48  PHYSICAL  SCIENCE 

ing  could    be  observed  with    any  of  the  gases 
examined. 

The  apparatus  just  described  is  clearly  not 
adapted  to  detect  small  thermal  changes,  and  it 
was  not  till  about  the  year  1850,  when  Thomson 
and  Joule  devised  a  continuous  method,  that 
satisfactory  results  were  obtained.  Instead  of 
preventing  external  work  by  allowing  the  gas  to 
expand  into  a  vacuum,  these  physicists  performed 
the  external  work  needed  to  expand  the  gas 
against  the  pressure  of  the  atmosphere  by  means 
of  an  air-pump  driven  by  an  engine.  By  this 
method  a  continuous  current  of  gas  was  forced 
through  a  porous  plug  of  compressed  wool  or 
silk,  fixed  in  a  wooden  tube.  Here  the  engine 
does  the  external  work,  and  consequently  none 
of  that  work  draws  on  the  heat  energy  of  the 
gas  itself. 

All  the  external  work  is  done  by  the  engine, 
but,  as  we  have  seen,  another  source  of  energy- 
change  exists.  When  a  gas  expands,  whether 
or  not  it  performs  external  work,  the  various 
parts  of  it  become  separated  further  from  each 
other,  since,  on  the  whole,  the  gas  occupies  after 
expansion  a  larger  volume  than  before.  If,  then, 
there  is  any  attraction  between  the  parts  of  the 
gas,  work  must  be  done  in  separating  them ; 
in  terms  of  the  molecular  theory,  work  is  done 
against  the  inter-molecular  forces.  For  the 
performance  of  this  internal  work,  energy  must 
be  drawn  from  the  heat-supply  of  the  gas,  which 
will  therefore  cool,  and  the  amount  of  cooling,  if 
access  of  heat  from  outside  be  prevented,  measures 
the  intensity  of  the  inter-molecular  forces.  On 
the  other  hand,  if  the  inter-molecular  forces  be 


THE  LIQUEFACTION  OF  GASES  49 

repulsive  ones,  they  help  on  the  expansion,  and 
the  energy  so  liberated  appears  as  sensible  heat, 
the  resultant  rise  of  temperature  depending  on  the 
strength  of  the  repulsion  between  the  molecules. 

The  porous  plug  experiment,  to  which  we 
have  referred  on  the  last  page,  was  devised  by 
Professor  William  Thomson,  afterwards  Lord 
Kelvin,  and  the  late  Dr  Joule,  for  the  purpose 
of  examining  the  amount  and  nature  of  these 
inter-molecular  forces,  and  of  determining  the 
amount  of  deviation  of  various  gases  from  the 
ideal  state,  in  which  no  such  forces  exist.  If  a 
thermometer  were  filled  with  such  a  hypothetical 
ideal  gas,  its  indications  would  coincide  exactly 
with  the  absolute  temperature  scale,  deduced  by 
Thomson  from  the  principles  of  thermodynamics. 
The  knowledge  of  the  deviation  of  any  real  gas 
from  the  ideal  state  thus  enables  us  to  compare 
the  absolute  scale  with  the  scale  of  an  actual 
thermometer,  using  the  expansion  of  the  gas  in 
question  as  the  thermometric  property.  The 
great  theoretical  importance  of  the  porous  plug 
experiment  will  now  be  manifest. 

Thomson  and  Joule  found  that  air,  and  all 
other  gases  except  hydrogen,  were  cooled  slightly 
on  passing  the  plug  ;  with  hydrogen,  on  the  other 
hand,  they  obtained  a  still  smaller  heating  effect. 
Thus  in  hydrogen  the  molecules  must  on  the 
whole  repel  each  other,  while  in  air  and  similar 
gases,  the  intermolecular  forces  must  be  attractive 
ones.  The  amount  of  the  effect  was  found  to  in- 
crease in  proportion  to  the  difference  of  pressure 
on  the  opposite  sides  of  the  plug. 

With  air  the  cooling  effect  decreases  as  the 
temperature    is    raised,  and   increases   if  the  air 

E 


50  PHYSICAL  SCIENCE 

be  cooled.  The  change  of  temperature  pro- 
duced, which  was  only  one-fifth  of  a  degree  per 
atmosphere  difference  of  pressure  in  the  original 
experiments,  can  thus  be  increased  to  any  extent 
by  a  preliminary  cooling  of  the  air. 

This  cooling  by  the  performance  of  internal 
work  underlies  the  third  method  adopted  in  the 
liquefaction  of  gases.  It  must  be  distinguished 
clearly  from  the  second  method,  in  which  most 
of  the  cooling  is  effected  by  making  the  gas  do 
external  work. 

Let  us  imagine  that  a  stream  of  air,  previously 
cooled  by  liquid  carbonic  acid,  is  forced  through 
a  spiral  tube  by  aid  of  an  air-pump  and  engine, 
and  that  finally  it  merges  through  a  fine  nozzle 
at  the  end  of  the  tube.  The  nozzle  acts  as  a 
porous  plug,  and  the  air,  cooled  by  free  expansion, 
is  lowered  in  temperature  by  doing  internal  work. 
Let  us  further  suppose  that  the  issuing  air,  so 
cooled,  is  made  to  flow  back  over  the  tube  through 
which  the  stream  of  air  passes.  The  advancing 
current  of  air  is  still  further  cooled,  the  effect  of 
the  expansion  at  the  nozzle  is  increased,  and  a 
temperature  yet  lower  than  before  attained. 
This  cycle  of  operations — the  continual  passage 
of  the  air  just  cooled  by  free  expansion  over  the 
current  of  air  before  it  issues  from  the  nozzle — 
results  in  a  constantly  decreasing  temperature, 
and  eventually  cools  the  air  below  its  critical  point, 
finally  causing  liquefaction.  This  self-intensifying 
action  is  sometimes  referred  to  as  the  regenera- 
tive principle.  It  was  applied  to  the  liquefaction 
of  air  by  Linde  in  Germany,  by  Hampson  and 
Dewar  in  England,  and  by  Tripler  in  America, 
and  is  now  used  on  large  scale  machines. 


THE  LIQUEFACTION  OF  GASES  51 

Liquid  air  can  be  obtained  in  any  quantity 
by  the  expenditure  of  power,  and  the  necessary 
apparatus  has  become  part  of  the  usual  equip- 
ment of  physical  and  chemical  laboratories.  By 
this  means  regions  of  temperature  before  quite 
inaccessible  have  been  opened  up  to  investiga- 
tion, and  the  use  of  liquid  air  promises  to  be  of 
increasing  advantage  in  many  departments  of 
research.  It  would,  of  course,  be  possible  to 
drive  an  engine  by  means  of  liquid  air,  but  such 
a  process  would  be  very  uneconomical.  The  state- 
ments, which  have  sometimes  appeared  in  the 
daily  papers,  announcing  impending  revolutions 
in  methods  of  obtaining  cheap  power  by  the 
application  of  liquid  air,  have  originated  from  an 
imperfect  comprehension  of  the  problems  involved. 

When  air  had  been  successfully  liquefied, 
hydrogen  was  obviously  the  next  gas  to  be 
attacked.  Thomson  and  Joule's  porous  plug 
experiments  had  shown  that,  at  ordinary 
temperatures,  hydrogen  suffers  a  heating  effect 
on  free  expansion.  It  was  therefore  useless  to 
attempt  to  liquefy  it  by  regenerative  cooling 
alone.  But,  just  as  the  cooling  effect  in  the  case 
of  air  increases  as  the  air  is  subjected  to  a  pre- 
liminary cooling,  so  in  hydrogen,  if  it  be  first 
cooled,  the  Thomson-Joule  heating  effect  first 
diminishes  and  then  is  reversed,  becoming  a 
cooling  effect.  This  reversal  was  shown  by 
Olszewski  to  take  place  about  80°  below  the 
Centigrade  zero.  Dewar  then  subjected  hydrogen 
to  a  preliminary  cooling  in  liquid  air  boiling  in  a 
vacuum  at  a  temperature  of  —  205°,  and  afterwards 
forced  the  hydrogen  through  a  regenerative  coil 
under  a  pressure  of  180  atmospheres. 


52  PHYSICAL  SCIENCE 

By  this  means  liquid  hydrogen  was  first 
collected  in  an  open  vessel  on  loth  May  1898, 
though  two  years  before  it  had  been  seen  as 
spray  in  the  jet  of  gas  issuing  from  a  simpler 
apparatus  of  the  same  essential  form.  When 
about  20  cubic  centimetres  of  liquid  had  been 
collected,  the  later  experiment  failed,  owing  to 
the  stoppage  of  the  exit  by  frozen  air — a  very 
common  accident  in  dealing  with  liquid  hydrogen. 

By  working  with  carefully  purified  gas,  much 
larger  volumes  were  soon  obtained,  and  the  writer 
has  a  vivid  memory  of  an  afternoon  in  June  1901, 
when  Professor  Dewar  had  transported  some  five 
litres  of  liquid  hydrogen  from  the  Royal  Institution 
to  the  rooms  of  the  Royal  Society,  and  gave  his 
first  public  demonstration  of  its  extraordinary 
properties.  On  that  occasion  liquid  hydrogen 
flowed  like  water  for  the  first  time.  Its  produc- 
tion in  any  quantity  is  now  simply  a  matter  of 
expense. 

By  carefully  isolating  a  portion  of  liquid 
hydrogen  and  preserving  it,  in  a  manner  shortly 
to  be  described,  from  the  access  of  heat  from 
without,  it  is,  when  suddenly  exhausted  under  an 
air-pump,  transformed  into  a  mass  of  solid  frozen 
foam.  By  immersing  a  tube  containing  the  liquid 
in  this  frozen  foam,  a  quantity  of  the  clear  trans- 
parent ice  of  solid  hydrogen  can  be  obtained. 

Kept  in  an  open  vessel,  liquid  air  and 
liquid  hydrogen  are  analogous  to  the  water  in  a 
saucepan  boiling  over  a  fire.  At  the  normal 
atmospheric  pressure,  water  boils  at  100°  C,  and 
the  rate  at  which  it  evaporates  depends  simply 
on  the  rate  at  which  heat  enters  it — depends,  that 


THE  LIQUEFACTION  OF  GASES  53 

is  to  say,  on  the  fire  below.  In  a  similar  way, 
liquid  air  has  a  definite  boiling-point,  which, 
under  the  normal  pressure  of  the  atmosphere, 
rises  from  —192°  to  -i82°C.  as  evaporation 
proceeds.  This  rise  is  due  to  the  fact  that 
nitrogen  is  more  volatile  than  oxygen  ;  and  thus 
the  liquid,  as  it  boils  away,  gradually  becomes 
richer  in  oxygen.  Liquefied  air  cannot  be  kept 
in  closed  vessels.  Its  vapour  pressure,  equal  to 
the  pressure  of  the  atmosphere  at  —  190°,  becomes 
enormously  great  as  heat  enters  from  surrounding 
objects  and  the  temperature  rises.  In  an  open 
vessel,  as  heat  enters  evaporation  proceeds,  and 
the  heat  is  used  to  effect  the  change  of  state. 
Thus,  owing  to  this  latent  heat  of  evaporation 
which  is  absorbed,  no  rise  of  temperature  (except 
the  very  small  change  already  noted)  occurs. 
But,  in  a  closed  vessel,  as  heat  enters  the  pressure 
will  rise,  and  the  boiling-point  will  rise  with  it. 
The  initial  temperature  being  so  low,  a  large  rise 
of  temperature  is  possible,  and  a  consequent  very 
great  increase  in  pressure.  As  ordinary  tempera- 
tures are  approached  no  vessel  would  withstand 
the  internal  pressure  of  the  evaporating  air. 

In  order  to  preserve  liquid  air  for  any  time  in 
an  open  vessel,  it  is  clearly  necessary  to  prevent 
as  far  as  possible  the  access  of  heat.  Evapora- 
tion must  be  proceeding  continuously,  but,  by 
diminishing  the  rate  at  which  it  goes  on,  the  rate 
of  loss  of  liquid  can  be  retarded. 

Heat  passes  from  one  place  to  another  in 
three  ways  :  by  conduction,  when  heat  flows  from 
one  part  of  a  body  to  another,  or  between  two 
bodies  in  contact ;  by  convection,  when  air  or 
water,  heated  by  contact  with  a  hot  body,  rises 


54 


PHYSICAL  SCIENCE 


through  the  colder  surrounding  fluid,  carrying 
heat  with  It ;  by  radiation,  when  heat  passes 
directly  from  one  body  to  another,  as  from  the 
sun  to  the  earth,  without  warming  the  Interven- 
ing medium.  Bearing  In  mind  these  three  modes 
of    transference.    Professor    Dewar    Invented    a 


Fig.  I. 


vessel  in  which  a  liquid  gas  can  be  kept,  and  the 
effects  of  all  three  of  these  methods  of  heat- 
transfer  be  reduced  to  a  minimum — the  now  well- 
known  thermos  flask. 

A  double-walled  glass  bulb  was  taken,  of  one 
of  the  forms  shown  in  Fig.  i,  and  the  space  be- 
tween the  walls  exhausted  of  air  to  the  completest 
degree  possible.     This  arrangement   diminished 


THE  LIQUEFACTION  OF  GASES  55 

the  effects  of  conduction  and  convection  to  such 
an  extent  that  liquid  air,  placed  within,  evaporated 
at  only  one-fifth  of  the  normal  rate.  An  addi- 
tional device  enabled  the  effects  of  radiation  to 
be  diminished  also.  A  polished  metallic  surface 
is  the  worst  radiator  and  the  worst  absorber  of 
radiation  known,  and,  by  coating  the  opposite 
walls  of  the  vacuum  space  with  a  film  of  bright 
silver  or  mercury,  the  rate  of  evaporation  of  liquid 
air  was  again  reduced  to  the  sixth  part.  By  the 
combined  results  of  the  vacuum  and  the  silvering, 
the  rate  of  loss  of  liquid  was  thus  reduced  to  the 
thirtieth  part  of  its  value  in  an  ordinary  open 
vessel.  Without  the  use  of  these  vessels,  liquid 
air  could  not  be  kept  for  any  length  of  time,  and 
liquid  hydrogen,  at  any  rate,  could  never  have 
been  collected  at  all. 

With  the  liquefaction  of  hydrogen  the  old  class 
of  so-called  permanent  gases  disappeared.  In 
place  of  them,  however,  a  number  of  gases,  pre- 
viously unknown  to  science,  have  been  discovered. 
Argon,  shown  by  the  late  Lord  Rayleigh  and 
Sir  William  Ramsay  to  exist  in  the  atmosphere, 
was  the  first  of  these  gases  to  be  detected.  Its 
name  attempts  to  describe  its  general  chemical 
inertness  ;  and  since  this  discovery  several  other 
new  gases  of  somewhat  similar  chemical  properties 
have  been  detected. 

The  story  of  the  discovery  and  isolation  of 
argon  Is  an  excellent  example  of  the  Importance 
in  science  of  the  infinitely  little,  and  shows  how 
striking  discoveries  may  be  made  as  a  conse- 
quence of  experiments  which  seem  at  first  sight 
simply  adapted  to  Investigate,  with  the  greatest 


56  PHYSICAL  SCIENCE 

attainable  accuracy,  phenomena  already  known 
to  science.  Since  the  days  of  Cavendish,  the 
composition  of  the  air  had  been  looked  upon  as 
an  ascertained  fact  ;  a  certain  proportion  had 
been  shown  to  be  oxygen,  varying  amounts  of 
carbonic  acid  and  aqueous  vapour  were  known  to 
be  present,  while  the  remainder,  as  the  result  of 
careful  investigation,  was  supposed  to  be  nitrogen. 
Cavendish  himself  knew,  so  accurate  was  his 
work,  that  any  undetected  residue  could  not 
exceed  the  yluth  part.  But  in  the  course  of  a 
long  series  of  experiments,  undertaken  to  deter- 
mine afresh  the  densities  of  the  principal  gases. 
Lord  Rayleigh  detected  a  slight  difference  in  the 
density  of  nitrogen  as  prepared  from  ammonia 
and  as  extracted  from  the  air.  This  difference, 
amounting  at  first  to  about  o.  i  per  cent.,  was 
increased  on  subsequent  more  careful  examina- 
tion to  nearly  a  half  per  cent.  It  was  clear  that 
the  gases  prepared  by  these  two  methods  were 
not  identical,  and  that  some  hitherto  unknown 
body  was  responsible  for  the  complication.  The 
existence  of  this  new  body,  the  inert  gas  now 
known  as  argon,  was  announced  by  Rayleigh  and 
Ramsay  in  1894,  and  shortly  afterwards  it  was 
isolated  from  its  companions. 

Argon  is  slightly  more  soluble  in  water  than 
nitrogen,  hence  a  rather  larger  proportion  of  it 
than  might  be  expected  is  found  in  rain  water. 
It  is  also  contained  to  a  small  extent  in  the  gases 
liberated  fromi  certain  thermal  springs.  Traces 
of  three  other  gases,  neon,  krypton,  and  xenon, 
which  much  resemble  argon  In  chemical  properties, 
have  been  detected  in  the  atmosphere.  The  total 
amount  of  these  three  substances  is  almost  im- 


THE  LIQUEFACTION  OF  GASES  57 

measurably  small,  and  does  not  altogether  exceed 
the  four-hundredth  part  of  the  argon  present. 

The  spectrum  of  the  sun  shows  some  lines  which 
do  not  coincide  with  those  of  any  chemical  element 
in  conditions  usually  known  on  the  earth.  Among 
these  lines  many  are  due  to  terrestrial  elements 
in  solar  circumstances,  but  a  bright  line  in  the 
yellow  part  was  detected  in  the  spectrum  of  a 
solar  prominence,  and  was  examined  carefully 
by  Frankland  and  Lockyer  during  the  eclipse  of 
August  1868.  To  explain  its  presence  they  called 
into  existence  a  hypothetical  element,  placed  it  in 
the  sun,  and  gave  to  it  the  name  helium.  For 
many  years  the  line  in  the  sun's  spectrum  was 
the  only  evidence  for  the  existence  of  helium ; 
but  in  1895  its  presence  on  the  earth  was  an- 
nounced by  Ramsay,  who  had  detected  it  in  the 
spectroscopic  analysis  of  the  gases  dissolved  in 
the  mineral  clevite,  together  with  the  other  new 
gases  krypton  and  neon.  Since  this  discovery, 
helium  has  been  isolated  and  collected  in  appreci- 
able quantities,  and  its  physical  and  chemical 
properties  are  now  well  known.  Of  all  substances 
investigated,  helium  has  proved  the  most  difficult  to 
liquefy.  But  in  July  1908,  Professor  Kamerlingh 
Onnes,  of  Leyden,  obtained  liquid  helium  for  the 
first  time  by  the  use  of  a  regenerative  apparatus 
and  a  plentiful  supply  of  liquid  hydrogen. 

It  will  be  seen  from  the  foregoing  account  that 
the  difficulty  of  obtaining  these  low  temperatures 
is  very  great.  While  a  temperature  of  many 
hundred  degrees  above  the  freezing-point  of 
water  is  easily  reached  in  a  common  fire  or  gas 
flame,  to  cool  hydrogen  to  250°  below  that  point 


58  PHYSICAL  SCIENCE 

needs  the  use  of  powerful  engines,  of  elaborate 
and  costly  apparatus.  The  difference  is  very- 
marked.  Moreover,  it  becomes  more  and  more 
difficult  to  cool  a  substance  through  one  degree 
as  we  pass  down  the  scale.  This  fact  suggests 
that  there  is  some  lower  limit  of  temperature 
towards  which  we  may  strive,  but  with  the 
prospect  of  encountering  increasing  difficulty  as 
we  approach ;  it  suggests,  that  is  to  say,  the 
existence  of  an  absolute  zero  of  temperature. 

Our  knowledge  of  an  absolute  scale  of  tempera- 
ture is  due  to  the  genius  of  Lord  Kelvin,  who, 
with  Clausius,  Rankine,  and  Helmholtz,  may 
be  said  to  have  founded  the  modern  science  of 
thermodynamics  about  the  year  1850.  It  can 
be  shown  that  Lord  Kelvin's  absolute  scale  of 
temperature  coincides  with  the  scale  of  an  ideal 
gas — a  gas,  that  is,  such  as  air  would  be  if  its 
molecules  exerted  no  forces  on  each  other,  and, 
consequently,  its  porous-plug-effect  were  nil.  As 
a  matter  of  fact,  at  ordinary  temperatures  and 
pressures,  such  gases  as  air  or  hydrogen  conform 
very  nearly  to  these  conditions — so  nearly  that, 
for  all  ordinary  purposes,  their  deviations  may 
be  neglected.  Now,  if  we  keep  a  gas  at  constant 
pressure,  its  volume  changes  from  i  to  1.366  as 
it  is  heated  from  the  freezing  to  the  boiling-point 
of  water.  Similarly,  if  it  be  kept  at  constant 
volume,  its  pressure  increases  in  the  same  ratio. 
If  we  use  either  of  these  changes  as  our  thermo- 
metric  property,  and  divide  the  interval  between 
the  freezing  and  boiling-points  into  100°  in  the 
Centigrade  manner,   there  will  be  a  change  in 

pressure,  for  example,  of  0.00366  or  of   the 


Photo  ly  Window  &  Grove.'\ 


[To  face  page  58. 


V   THE  LIQUEFACTION  OF  GASES  59 

pressure  at  o^  for  each  degree  through  which  the 
gas  is  heated.      If  we  call  the  pressure  at  o"  unity, 

then  at  1°  the  pressure  will  be  i  + ,  at  2°  it  will 

^  273 

be  I  +-?-,  and  so  on.     Similarly,  if  we  cool  the 

273  ^    , 

gas  below  the  freezing-point,  at  -  1°  the  pressure 

I  .2 

becomes  i ,  at  -  2°  the  pressure  Is  i . 

273  273 

If,  while  we  carry  on  this  process,  the  properties 
of  the  gas  remain  unchanged,  as  they  would  were 
it  the  ideal  gas  we  have  supposed,  at  a  temperature 

of  -  273°  the  pressure  will  fall  to  i —,  that  is, 

273 
I  - 1,    or    zero.      At    -  273°  C,    therefore,    the 

pressure  of  an  ideal  gas  would  vanish  absolutely, 
and  no  further  cooling  could  make  it  smaller. 
On  the  temperature  scale  which  uses  the  pressure 
of  an  ideal  gas  as  the  thermometric  property, 
—  273°  C.  represents  an  absolute  zero,  the  lowest 
conceivable  degree  of  cold.  But,  as  we  said, 
such  a  scale  coincides  exactly  with  the  true 
absolute  or  thermodynamic  scale,  which,  as  can 
be  shown,  unlike  all  other  temperature  scales,  is 
independent  of  the  properties  of  any  particular 
substance,  whether  real  or  imaginary.  On  the 
thermodynamic  scale  also,  then,  —  273"  C.  repre- 
sents the  absolute  zero. 

We  thus  see  that  the  idea  of  an  absolute  zero, 
at  which  all  bodies  would  be  deprived  entirely  of 
heat  energy,  is  not  a  mere  figment  of  the  mathe- 
matical imagination,  derived  from  the  study  of 
a  hypothetical  air  thermometer.  It  has  a  real 
physical  meaning,  and  the  attainment  of  the 
absolute  zero  is,  at  all  events,  theoretically  possible. 


6o  PHYSICAL  SCIENCE 

From  the  practical  side,  however,  difficulties 
accumulate  and  increase  as  the  absolute  zero  is 
approached.  As  Sir  James  Dewar  remarked, 
"the  step  between  the  liquefaction  of  air  and 
that  of  hydrogen  is,  thermodynamically  and 
practically,  greater  than  that  between  the  lique- 
faction of  chlorine  and  that  of  air."  The  boiling- 
points  of  chlorine,  air,  and  hydrogen  under  the 
atmospheric  pressure  are  —  33°f  "193°)  ^^^ 
—  253°  C.  respectively.  If  we  express  these 
temperatures  on  the  absolute  scale,  they  become 
240°,  80°,  and  20°.  The  interval  between  the 
boiling-points  of  chlorine  and  air  is  160°,  but 
the  ratio  of  the  absolute  temperatures  is  240  :  80, 
or  3:1.  On  the  other  hand,  while  the  interval 
between  air  and  hydrogen  is  only  60°,  the  ratio 
of  the  absolute  temperatures  is  80  :  20,  or  4:1. 
The  difficulty  of  the  transition  from  one  to  the 
other  temperature  is  much  more  nearly  pro- 
portional to  the  ratio  than  to  the  difference 
between  them. 

The  absolute  boiling-point  of  hydrogen  is, 
as  we  have  said,  about  20^  and  at  present  this 
temperature  is  the  lowest  which  we  can  con- 
veniently maintain  in  an  ordinary  laboratory. 
Any  further  advance  towards  the  absolute  zero 
must  be  made  by  the  help  of  helium.  By  the 
sudden  expansion  of  gaseous  helium  at  a  pressure 
of  100  atmospheres  and  at  the  temperature  of 
solid  hydrogen,  it  was  estimated  that  a  transient 
temperature  of  9°  or  10°  absolute  was  reached. 
When  that  gas  was  liquefied.  Professor  Onnes 
found  that  its  boiling-point  under  the  normal 
atmospheric  pressure  was  about  4^.5  on  the 
absolute  scale.     This  temperature  is  about  one- 


THE  LIQUEFACTION  OF  GASES  6i 

fourth  the  boiling-point  of  hydrogen,  and  it  has 
proved  at  least  as  hard  to  pass  the  interval 
between  hydrogen  and  helium  as  it  was  to  pass 
from  air  to  hydrogen. 

But,  as  was  foreseen,  the  liquefaction  of  helium 
was  effected  by  an  extension  of  methods  previously 
successful  with  other  gases.  A  preliminary  study 
of  its  properties  showed  that,  after  cooling  in 
liquid  hydrogen,  it  should  cool  further  when  sub- 
jected to  a  regenerative  process.  After  attempts 
by  several  investigators  had  failed.  Professor 
Onnes  succeeded,  and  the  year  1908  saw  the  last 
known  refractory  gas  reduced  to  the  state  of  liquid. 

The  liquefaction  of  helium  gives  command  of 
a  steady  temperature  of  about  4°.  5  absolute,  its 
boiling-point  in  open  vessels.  That  temperature, 
within  5°  of  the  absolute  zero,  is  thus  possible, 
and  Onnes  has  reached  perhaps  a  degree  lower 
by  working  under  low  pressure  ;  but  there,  with 
our  present  methods  and  materials,  seems  to 
come  the  end  of  any  probable  advance. 

We  may  now  pass  to  a  brief  account  of  the 
methods  of  measuring  these  very  low  temperatures. 

Mercury  freezes  at  a  temperature  of  —  40°C., 
and,  at  such  temperatures  as  those  now  under 
consideration,  a  mercury  thermometer  clearly  is 
useless.  The  resistance  of  a  metallic  wire  to  the 
passage  of  an  electric  current  is  a  quantity  which 
can  be  measured  easily  and  accurately.  This 
resistance,  diminishing  as  the  wire  is  cooled, 
depends  on  the  temperature.  With  some  alloys 
the  diminution  of  resistance  with  temperature  is 
very  small,  but  with  pure  metals  it  is  consider- 
able, and  roughly,  at  any  rate,  proportional  to  the 


62  PHYSICAL  SCIENCE 

change  of  temperature.  The  metal  most  usually 
employed  is  platinum,  since  it  is  not  attacked  by 
acids,  and  has  a  very  high  melting  point.  Platinum 
thermometers  are  now  used  extensively  for  physical 
research  ;  they  have  a  very  large  range,  and  are 
probably  susceptible  of  greater  sensitiveness  than 
any  other  form  of  thermometer.  At  ordinary 
temperatures  a  difference  of  temperature  of  one 
ten- thousandth  of  a  degree  can  be  detected  with 
moderate  ease,  while,  with  great  precautions,  the 
hundred-thousandth  of  a  degree  can  be  estimated. 
At  high  or  low  temperatures  such  accuracy  is 
impossible,  but  measurements,  correct  to  the 
nearest  degree,  can  be  made  up  to  about  i  ioo°  C. 
and  as  low  as  —  200°  C.  Below  the  latter  tem- 
perature the  rate  of  change  of  the  resistance 
alters  in  a  manner  to  be  described  below,  and  the 
instrument  ceases  to  be  trustworthy. 

The  standard  to  which  the  readings  of  all 
other  thermometers  are  referred,  as  we  have 
indicated  when  considering  the  absolute  scale  of 
temperature,  is  the  gas  thermometer  containing 
hydrogen  or  helium.  Not  only  is  the  hydrogen 
thermometer  thus  used  for  purposes  of  reference, 
but  it  can  also  be  employed  as  a  practical  instru- 
ment at  temperatures  too  low  to  be  measured  by 
the  platinum  resistance  thermometer.  It  might 
be  thought  that,  as  the  point  of  liquefaction  was 
approached,  a  gas  would  cease  to  be  trustworthy 
as  a  thermometric  substance,  but  experiment  has 
shown  that,  as  long  as  the  pressure  of  the  gas  is 
kept  well  below  the  saturation  value  at  which 
condensation  would  occur,  the  gas  still  expands 
or  contracts  proportionally  to  the  absolute  tem- 
perature.    Dewar  has  found  that  thermometers, 


THE  LIQUEFACTION  OF  GASES  63 

filled  with  oxygen  and  carbonic  acid  at  low 
pressures,  gave  correct  temperatures  as  low  as 
the  boiling-points  of  those  gases  at  the  normal 
atmospheric  pressure.  He  used  therefore  a 
constant  volume  hydrogen  thermometer,  working 
at  low  pressure,  to  determine  the  boiling-point  of 
liquid  hydrogen  itself,  and  confirmed  the  result 
obtained,  —  252°  C,  by  experiments  with  a  similar 
thermometer  filled  with  helium. 

Some  very  remarkable  effects  are  obtained 
with  liquid  hydrogen.  A  vessel  containing  it  is 
so  cold  that  the  air  in  contact  with  it  immediately 
freezes.  A  snow-shower  of  solid  air  is  thus 
produced.  This  process  may  be  applied  to  the 
production  of  very  high  vacua.  If  the  vessel  to 
be  exhausted  be  sealed  to  a  long  tube,  one  end 
of  which  is  plunged  into  liquid  hydrogen,  the  air 
in  the  vessel  is  frozen  out  almost  completely. 
The  air  in  the  cooled  end  of  the  tube  first  con- 
denses, but,  as  it  is  removed,  the  residual  air  in 
the  vessel  expands,  again  fills  the  whole  tube, 
and  again  that  portion  of  it  in  contact  with  the 
cold  part  of  the  tube  is  frozen.  This  process 
continues  till  the  pressure  within  the  tube  falls  to 
the  millionth  of  an  atmosphere  or  less,  a  pressure 
so  low  that  an  electric  discharge  will  only  pass 
through  the  vessel  with  extreme  difficulty.  A 
vacuum  nearly  complete  may  also  be  obtained  by 
using  charcoal  cooled  by  liquid  air  in  place  of  the 
hydrogen. . 

The  liquefaction  of  air  and  hydrogen  has 
led  to  the  making  of  many  experiments  on  the 
influence  of  low  temperature  on  chemical  action, 
and  it  is  found  that  the  rate  of  change  is  very 


64  PHYSICAL  SCIENCE 

greatly  affected  at  these  temperatures.  In  many 
cases,  where  the  reaction  proceeds  rapidly  at 
ordinary  temperatures,  the  rate  is  reduced  to  such 
an  extent  that  in  liquid  air  it  becomes  too  small 
to  be  observed.  In  other  cases  action  may  cease 
altogether,  and  reagents  which  would  otherwise 
undergo  chemical  chancre  are  maintained  in  false 
equilibrium  by  chemical  forces  analogous  to  those 
of  friction.  Fluorine,  for  instance,  which  attacks 
glass  violently  at  ordinary  temperatures,  has  no 
effect  on  it  when  cooled  to  —  i8o°  C. 

It  is  found  that  the  elasticity  of  materials  is 
greatly  affected  by  these  low  temperatures.  On 
the  one  hand,  iron,  lead,  and  tin,  as  well  as  ivory, 
showed  a  considerable  increase  in  this  property, 
balls  of  these  substances  rebounding  to  a  much 
greater  height  than  usual.  On  the  other  hand, 
a  ball  of  india-rubber  became  brittle,  and  was 
broken  by  the  fall.  Connected  with  the  increase 
in  the  elasticity  of  metals  is  their  increased 
strength  ;  wires,  for  example,  will  stand  a  much 
greater  load  without  pulling  out  or  breaking. 

Low  temperatures  also  affect  the  magnetic 
properties  of  iron,  cobalt,  and  other  metals, 
which  are  usually  magnetic  at  ordinary  tempera- 
tures, generally  increasing  the  magnetic  moment. 
Oxygen,  slightly  magnetic  as  a  gas,  as  a  liquid 
becomes  strongly  magnetic.  The  alteration  of 
magnetic  properties  with  temperature  has  been 
studied  in  detail  for  many  years  where  high 
temperatures  are  concerned,  and  this  extension  of 
the  research  has  been  of  great  interest. 

Of  even  more  theoretical  importance  are  the 
experiments  of  Onnes  on  the  electrical  conduc- 
tivities of  metals  at  the  very  low  temperatures 


THE  LIQUEFACTION  OF  GASES  65 

obtained  with  liquid  helium.  As  explained  above, 
the  electrical  resistance  of  a  pure  metal  increases 
generally  nearly  in  proportion  to  the  absolute 
temperature,  and  diminishes  equally  as  the  tem- 
perature falls.  Nevertheless,  most  metals  seem 
to  reach  a  constant  small  resistance  as  they 
approach  the  absolute  zero,  and  this  residual 
resistance  is  increased  by  traces  of  impurities. 

But  Onnes  discovered  that  with  pure  mercury 
he  got  a  sudden  and  almost  complete  destruction 
of  resistance  at  4°.  2  absolute,  a  sudden  drop  in 
resistance  to  about  the  millionth  part.  He  got 
similar  effects  with  tin  at  f.S  and  with  lead  at 
6°  absolute,  but  other  metals  which  he  examined 
gave  no  such  results.  The  conductivity  is  so  high 
that  an  electric  current  once  started  in  a  coil  of 
wire  continues  to  flow  almost  indefinitely,  falling 
in  strength  by  less  than  i  per  cent,  per  hour. 

Applying  a  magnetic  field  of  slowly  increasing 
strength,  Onnes  found  that,  at  a  certain  critical 
magnetic  force,  tin  and  lead  when  cooled  to  these 
temperatures  gave  a  sudden  increase  in  resist- 
ance, an  observation  which  shows  that  this  state 
of  super-conductivity  is  connected  with  magnetic 
phenomena.  Its  real  meaning  is  not  yet  clear, 
though  tentative  theories  of  the  state  have  been 
offered  by  Onnes,  Lindemann,  and  Thomson. 

From  the  point  of  view  of  the  popular  lecture- 
room,  some  of  the  prettiest  effects  given  by  liquid 
air  depend  on  its  power  of  imparting  phosphor- 
escence to  many  substances  which  do  not  usually 
possess  this  property.  Ivory,  egg-shells,  paper, 
cotton-wool,  and  many  other  things  glow  brightly 
in  liquid  air  after  they  have  been  exposed  to  light. 
On  the  other  hand,  certain  sulphides  of  calcium, 

F 


66  PHYSICAL  SCIENCE 

phosphorescent  at  ordinary  temperatures,  cease 
to  be  so  when  cooled.  Some  crystals,  such  as 
those  of  uranium  nitrate,  become  self-luminous 
in  liquid  hydrogen,  apparently  owing  to  intense 
electric  forces  set  up  by  the  cooling.  These 
forces  may  become  so  intense  that  discharges 
take  place  which  are  powerful  enough  to  be 
visible  in  the  dark. 

It  will  be  seen  from  this  account  that  the 
changes  in  physical  properties  are  more  striking 
and  complete  in  the  range  of  temperature  below 
the  freezing-point  of  water  than  in  the  corre- 
sponding range  of  temperature  above  that  point. 
On  the  other  hand,  it  is  very  striking  that  in 
biological  problems,  more  especially  in  those 
connected  with  the  lowliest  forms  of  animal  and 
vegetable  life,  a  hundred  degrees  above  the 
freezing-point  is  productive  of  a  more  complete 
and  destructive  change  than  a  hundred  degrees 
below.  While  exposure  to  the  boiling-point  of 
water,  or  to  a  temperature  a  few  degrees  higher, 
suffices  to  kill  all  known  forms  of  living  organisms, 
many  forms  of  bacteria  merely  have  their  vitality 
temporarily  suspended  in  liquid  air.  Even  seeds 
of  barley,  peas,  etc.,  were  not  permanently  affected; 
in  fact,  they  have  been  placed  for  six  hours  in 
liquid  hydrogen  with  no  effect  on  their  subsequent 
power  of  germination. 

In  closing  this  account  of  low  temperature 
research  it  may  be  of  interest  to  tabulate  some 
of  the  more  important  temperature-constants  now 
known  to  mankind.  In  doing  so,  we  cannot  fail 
again  to  be  struck  by  the  high  temperatures 
easily  obtainable.     On  the  other  hand,  to  cool  an 


THE  LIQUEFACTION  OF  GASES  67 


object  through  250""  of  the  273°  which  separates 
the  freezing-point  of  water  from  the  absolute 
zero  has  taxed  the  skill  of  experimenters  for 
several  generations.  Temperatures,  as  already 
pointed  out,  are  more  justly  compared  by  con- 
sidering their  ratio  on  the  absolute  scale  than  by 
considering  the  number  of  degrees  Centigrade  or 
Fahrenheit  which  separate  them. 


Temperature. 

On  Absolute 

On  Centigrade 

Scale. 

Scale. 

Zero  of  the  absolute  scale    . 

0° 

-273° 

Boiling-point  of  liquid  helium 

4°-5 

-268^5 

Boiling-point  of  liquid  hydrogen. 

20° 

-253° 

Critical-point  of  hydrogen   . 

30° 

-243° 

Boiling-point  of  liquid  air    . 

81° to  91° 

-  192°  to  -  182° 

Boiling-point   of  liquid  carbonic 

acid 

195° 

-78° 

Freezing-point  of  water 

273° 

0° 

Boiling-point  of  water . 

373° 

100° 

Melting-point  of  tin      . 

505° 

23i°7 

Melting-point  of  lead  . 

60  r^ 

327°-7 

Boiling-point  of  sulphur 

718^ 

444°'5 

Melting-point  of  silver 

1234° 

96o°7 

Melting-point  of  gold  . 

1335° 

io6i'-7 

Melting-point  of  copper 

1354° 

io8o°-5 

Melting-point  of  platinum    . 

2073° 

iSoo" 

Approximate  Temperature 

on  Centigrade  Scale. 

Low  red  heat   .... 

500°  to  600° 

White  heat       .... 

.      1500'    „    1800^ 

Temperature  of  furnace   . 

1500°    „    1600" 

Temperature  of  electric  arc     . 

.     3000°    „    4000" 

Estimated  temperature  of  the 

radiating 

layer  of  the  sun    . 

.     5700"    „    7000° 

and  of  the  hottest  stars     . 

23,000'' 

CHAPTER  IV 

FUSION    AND    SOLIDIFICATION 

For  more  is  not  reserved 

To  man,  with  soul  just  nerved 
To  act  to-morrow  what  he  learns  to-day  : 

Here  work  enough  to  watch 

The  Master  work  and  catch 
Hints  of  the  proper  craft,  tricks  of  the  tool's  true  play. 

— Browning,  Rabbi  Ben  Ezra. 

In  the  previous  chapter  we  have  discussed  chiefly 
the  methods  employed  to  bring  about  a  change 
of  state,  especially  that  change  of  state  which 
consists  in  passing  from  the  gaseous  to  the  liquid 
or  solid  condition  in  the  case  of  those  substances 
which  at  ordinary  temperatures  and  pressures 
exist  as  gases.  The  methods  employed  and  the 
principles  underlying  them  were  the  points  of 
interest,  and  the  whole  subject  belonged  to  that 
branch  of  physical  science  which  consists  in  recog- 
nising and  overcoming  difficulties  of  manipula- 
tion, and,  as  it  were,  of  asserting  by  force  the 
superiority  of  mind  over  matter. 

But,  throughout  the  investigations  to  be 
pursued  in  the  present  chapter,  our  attitude  is 
altered.  There  is  no  need  for  such  attempted 
assertion  of  supremacy.  The  changes  of  state 
to  be  examined  are  already  under  our  control, 
and  we  are  able  to  investigate  further  details,  and 
probe  more  deeply  into  the  intimate  nature  of 
the   processes    involved.     We   patiently  seek  to 

68 


FUSION  AND  SOLIDIFICATION  69 

trace  connections  between,  for  example,  the 
mechanical  properties  of  metals  and  their  micro- 
scopic structure  when  solidified  ;  and,  from  the 
complicated  relations  which  declare  themselves, 
we  may  hope  to  throw  light  on  the  processes  of 
fusion  and  solidification,  and  construct  a  theory 
that  will  hereafter  prove  of  some  use  to  the 
enofineer  and  the  metal-worker. 

In  the  first  place  it  is  well  to  remark  that  we 
are  seldom  dealing  with  pure  materials.  Nearly 
the  whole  of  the  phenomena  we  shall  consider 
depend  on  the  admixture  of  two  or  more  sub- 
stances, one  for  the  most  part  predominating.  It 
follows  that  the  result  of  the  inquiry  is  specially 
applicable  in  all  cases  where  traces  of  some 
impurity  are  the  determining  factor ;  that  is,  to 
the  majority  of  cases,  since  the  attainment  of 
chemical  purity  is  more  often  a  pious  hope  than 
an  accomplished  fact. 

Our  investigations  will  lead  us  far  afield,  and 
we  shall  pass  in  review  combinations  of  many  of 
the  principal  metals.  It  is  well,  however,  that 
the  starting-point  should  be  on  familiar  ground  ; 
if,  indeed,  by  such  a  term  it  is  permissible  to 
indicate  the  ice  that  occasionally  covers  our 
ponds  and  perpetually  caps  our  globe. 

It  is  well  known  that  sea-water  remains  liquid 
at  temperatures  low  enough  to  freeze  ponds  and 
lakes,  and,  long  ago,  it  must  have  been  recognised 
that  this  behaviour  was  due  to  the  dissolved  salt, 
though  it  was  not  till  the  year  1788  that  Blagden, 
the  first  worker  in  the  field,  published  a  syste- 
matic series  of  observations  on  the  freezing-points 
of  salt  solutions. 


70  PHYSICAL  SCIENCE 

If  we  cool  the  solution  of  some  substance 
such  as  sodium  chloride,  that  is,  common  salt, 
the  ice  which  freezes  out  is  the  solid  form  of  pure 
water.  The  process  can  be  illustrated  in  a  very 
striking  manner  by  using  the  solution  of  a 
coloured  salt.  If,  for  example,  a  dilute  solution 
of  the  purple-coloured  potassium  permanganate 
be  placed  in  a  glass  bottle  and  be  surrounded  for 
some  hours  by  a  freezing  mixture,  most  of  the 
water  solidifies  to  form  a  hollow  cylinder  of 
perfectly  colourless  ice,  while  the  permanganate 
is  concentrated  in  an  intensely  coloured  liquid 
core  along  the  axis  of  the  cylinder. 

Similar  phenomena  occur  in  other  cases  where 
the  separation  is  not  so  clearly  visible. 

If  the  ice  be  frozen  rapidly,  some  trace  of  salt 
may  be  deposited  also  ;  but  experiment  has  shown 
that  it  does  not  enter  into  the  composition  of  the 
crystals,  and  is  entangled  merely  mechanically 
in  their  interstices.  Essentially,  then,  the  salt 
remains  in  the  liquid  solution,  and,  as  the  solvent 
is  gradually  frozen  out,  the  concentration  of  that 
solution  must  increase.  The  stronger  the  solution 
becomes,  the  lower  is  its  freezing-point ;  but,  if 
the  temperature  at  our  disposal  be  low  enough, 
we  can  go  on  freezing  out  water  till  the  residual 
solution  is  saturated  with  salt  at  the  temperature 
of  its  freezing-point.  Any  further  abstraction  of 
heat,  by  removing  some  of  the  necessary  solvent, 
must  then  be  accompanied  by  the  simultaneous 
deposition  of  salt ;  ice  and  salt  will  be  precipitated 
together,  and  the  residual  solution  will  retain  the 
constant  composition  of  saturation. 

Since,  as  the  process  of  freezing  goes  on  in 
these   conditions,    there    is    no    change    in    the 


FUSION  AND  SOLIDIFICATION  71 

composition  of  the  residual  liquid,  there  can  be 
no  change  in  the  freezing-point.  The  mixture 
of  salt  and  water  of  this  particular  concentration 
will  solidify  completely  at  a  constant  temperature 
into  a  mixture  of  salt  and  ice  of  the  same  com- 
position. But  pure  chemical  elements  like  lead, 
or  pure  compounds  like  water,  also  fuse  and 
solidify  at  constant  temperatures  without  change 
of  composition.  In  these  respects,  then,  the 
particular  mixture  of  salt  and  water  which  we 
are  considering  behaves  like  a  pure  element  or 
compound.  For  this  reason  Guthrie,  who  first 
systematically  examined  such  mixtures,  classed 
them  as  compounds,  and  named  them  cryo- 
hydrates.  It  is,  however,  now  evident  that  their 
properties  are  explicable  in  other  ways. 

The  phenomena  we  have  traced,  and  the 
existence  of  a  cryohydric  point  must  be  borne 
in  mind  if  we  wish  to  understand  the  structure  of 
natural  ice,  the  properties  of  metallic  alloys,  or 
the  processes  which  occur  when,  in  the  cold  of 
an  Arctic  winter,  sea-water  becomes  coated  with 
a  solid  covering. 

Natural  waters,  even  when  known  as  fresh, 
contain  some  amount  of  solids  in  solution.  When 
such  waters  are  cooled  to  the  freezing-point,  how- 
ever, the  crystals  which  appear  form  the  ice  of 
pure  water.  As  the  crystals  grow,  the  dissolved 
salts  become  concentrated  into  the  liquid  which 
remains ;  and  the  freezing-point  of  this  liquid 
falls  as  its  concentration  rises.  Unless  the 
temperature  of  the  cryohydric  point  is  reached, 
some  liquid  must  always  remain,  though,  with 
fairly  pure  water,  it  may  exist  only  as  a  thin  film 
between  the  solid  crystals.     If  the  temperature 


72  PHYSICAL  SCIENCE 

sink  below  the  cryohydric  point,  these  Hquid  films 
themselves  solidify  ;  but,  even  then,  the  mass  is 
not  a  homogeneous  solid,  for  the  cryohydric  con- 
glomerate forms  a  cement-like  connection  between 
the  primary  crystals  of  pure  ice.  We  see  now  the 
explanation  of  the  fact  that  a  block  of  natural 
ice,  taken  from  a  glacier  or  lake,  has  a  definite 
structure,  and  may  be  resolved  into  a  heap  of 
separate  crystals  by  exposure  to  the  sun.  The 
cryohydric  cement  dissolves  first  at  the  lower 
temperature,  and  thus  the  primary  crystals  of 
pure  ice  fall  away  from  each  other  before  the 
temperature  rises  to  their  melting-point. 

Phenomena  precisely  similar  to  those  we  have 
described  appear  when  a  fused  metal  is  allowed 
to  solidify.  Crystalline  structures  of  pure  metal 
form  in  the  liquid,  and  grow  till  the  whole  mass 
becomes  solid.  These  primary  crystals  usually 
start  as  fernlike  forms,  of  which  a  beautiful 
example  is  shown  in  Fig.  2.  This  represents 
the  microscopic  structure  of  a  bronze  ingot, 
suddenly  chilled  from  a  temperature  of  644°  C. 
If  the  crystals  be  allowed  to  grow  by  very  slow 
cooling,  they  may  come  to  fill  nearly  the  whole 
mass,  as  in  the  case  of  the  section  of  iron  shown 
in  Fig.  3.  Even  in  this  case,  with  a  substance 
nearly  as  pure  as  can  be  obtained,  the  lines  of 
separation  between  the  primary  crystals  are 
clearly  visible ;  the  primary  crystals  are  differ- 
ently orientated,  and  their  faces  reflect  the  incident 
light  at  different  angles.  The  crystals  of  zinc  are 
often  remarkably  large  and  well  defined,  and  fine 
specimens  can  be  seen  on  surfaces  of  so-called 
galvanised  iron,  such  as  is  used  for  water-cisterns. 


Fig.  2. 
Magnification  45. 


Fig.  3. 
Magnification  200. 


Fig.  4. 
Magnification  50. 


Fig.  5. 
Magnification  120. 


[To  face  page  72. 


FUSION  AND  SOLIDIFICATION  73 

etc.  When,  instead  of  a  single  metal,  traces  of 
others  are  present,  the  lines  of  separation  between 
the  primary  crystals  are  much  emphasised,  and, 
when  the  quantity  of  other  substances  Is  consider- 
able, there  arise  the  complicated  structures,  which 
we  shall  presently  study  under  the  head  of  alloys. 

The  process  of  the  freezing  of  sea-water  under 
the  Influence  of  the  Intense  cold  of  an  Arctic 
climate  Is  an  interesting  example  of  the  applica- 
tion of  the  same  principles.  The  phenomena 
have  been  described  by  the  explorer,  Weyprecht, 
whose  account  Is  quoted  by  Mr  J.  Y.  Buchanan 
In  his  *'  Chemical  and  Physical  Notes."  When 
a  new  surface  of  sea- water  Is  exposed  to  the  cold 
air,  In  a  short  time  the  surface  of  the  water 
begins  to  get  thick,  threads  like  a  spider's  web 
runnlnor  out  from  the  old  ice.  Brine  is  entano^led 
in  this  structure,  and  its  concentration  constantly 
becomes  greater  as  the  quantity  of  Ice  increases. 
At  this  stage  the  ice  Is  a  pasty  mass,  and  follows 
every  motion  of  the  water  on  which  it  floats. 
With  a  temperature  of  —40^  C.  the  new  Ice,  even 
after  twelve  hours,  is  still  so  soft  that,  in  spite 
of  Its  thickness,  a  stick  can  easily  be  thrust 
through  it. 

As  soon  as  a  layer  of  Ice  is  formed  over  the 
surface,  the  cooling  of  the  underlying  water  pro- 
ceeds much  more  slowly,  and  less  salt  is  entangled 
in  the  crystals.  The  lower  layers  of  sea-water 
ice  give  therefore,  when  melted,  a  much  fresher 
water  than  can  be  obtained  from  the  upper  layers. 
Even  when  strong  enough  to  walk  on,  the  surface 
of  new  sea-Ice,  frozen  by  air  at  —  40,  Is  still 
moist  and  soft,  the  residual  liquid  consisting  of 
a  concentrated  solution  of  various  salts,  chiefly 


74  PHYSICAL  SCIENCE 

calcium  chloride.  The  cryohydric  point  of  calcium 
chloride,  an  extremely  soluble  substance,  is  very 
low,  and  that  of  a  mixture  of  salts  will  be  lower 
than  that  of  either  component.  This  lowering 
of  the  cryohydric  temperature,  which  corresponds 
with  the  lowering  of  the  freezing-point  of  water 
by  the  addition  of  salt,  was  observed  by  Buchanan 
in  experiments  conducted  in  the  Engadine. 

So  far  the  components  of  the  system  we  have 
been  considering  are  not  miscible  with  each  other 
in  all  proportions  ;  only  a  limited  amount  of  salt 
can  be  dissolved  in  a  given  quantity  of  water. 
A  system  not  subject  to  any  such  restriction,  in 
which  the  phenomena  are  as  simple  as  possible, 
is  found  in  mixtures  of  the  metals  silver  and 
copper.  The  equilibrium  of  these  substances  was 
studied  by  Mr  C.  T.  Heycock  and  the  late  Mr 
F.  H.  Neville,  who  determined  the  melting-points, 
or  rather  the  points  of  solidification,  of  mixtures 
of  various  proportions  of  the  two  metals.  At  the 
high  temperatures  involved,  it  would,  of  course, 
be  impossible  to  use  a  mercury  thermometer,  and 
the  measurements  were  consequently  made  by 
means  of  a  platinum  resistance  thermometer, 
with  which  the  temperature  is  determined  by 
observing  the  electrical  resistance  of  a  coil  of 
platinum  wire.  The  metals  in  the  required  pro- 
portion are  fused  in  a  crucible  and  allowed  to 
cool.  As  soon  as  solidification  sets  in,  the  rate 
at  which  the  temperature  falls  always  becomes 
less  ;  and,  in  the  case  of  pure  metals  and  other 
systems  where  the  solid  has  the  same  composition 
as  the  liquid,  the  temperature  remains  constant 
till  solidification  is  complete,  just  as  the  tempera- 


FUSION  AND  SOLIDIFICATION 


75 


ture  of  a  mass  of  ice  and  water  remains  constant 
till  the  whole  is  frozen.  Thus,  by  watching  the 
thermometer,  the  temperature  at  which  solid 
begins  to  form  can  be  estimated. 

The  melting-point  of  silver  is  960"  C.  and  the 
addition  of  copper  lowers  it  just  as  the  addition 
of  salt  lowers  the  freezing-point  of  water.  This 
is  best  shown  by  plotting  the  observations  on  a 
diagram,  as  in  Fig.  6,  in  which  the  horizontal  axis 


0       10 

20 

30 

40 

50 

60 

70 

80 

90 

10 

1 

1 

1 

I 

1 

J 

looo' 

V 

J 

/ 

f 

900° 

\ 

V 

y 

/ 

/ 

800" 

■ 

N 

\ 

sx 

/ 

/ 

Silver 


Copp 


er 


Fig.  6. 


denotes  the  composition  of  the  mixture  expressed 
in  percentage  numbers  of  atomic  equivalents  of 
silver  and  copper,  and  the  vertical  axis  the  tem- 
peratures. On  the  other  hand,  pure  copper  melts 
at  1081°,  and  the  admixture  of  silver  lowers  its 
freezing-point.  The  two  curves  in  the  diagram 
cut  each  other  at  a  point  which  corresponds  with 
a  temperature  of  777',  and  a  composition  of 
40  atomic  percentages  of  silver  and  60  of  copper. 
At  other  points  on  the  curves,  the  process  of 
freezing  consists  in  the  separation  of  primary 
crystals  of  one  or  other  of  the  pure  metals  in  the 


ye^  PHYSICAL  SCIENCE 

manner  we  have  traced  for  solutions  in  water.^ 
The  point  of  intersection  of  the  curves  corre- 
sponds with  the  point  of  saturation  both  of  silver 
with  copper  and  of  copper  with  silver.  When 
the  fused  alloy  has  this  proportion,  crystals  of 
silver  and  copper  freeze  out  together,  just  as 
crystals  of  salt  and  water  freeze  out  together 
when  the  composition  of  the  solution  is  that  of 
the  cryohydrate.  The  point  we  are  considering, 
then,  corresponds  with  the  cryohydric  point  for 
salt  and  water.  The  composition  of  the  solid  is 
here  the  same  as  that  of  the  liquid,  and  therefore, 
as  the  process  of  solidification  goes  on,  the 
residual  liquid  always  has  a  constant  concentra- 
tion. Thus  the  freezing-point  remains  constant 
throughout  the  operation,  and  is  identical  with 
the  melting-point  at  which  liquid  first  appears 
when  the  solid  alloy  is  heated.  Similar  phe- 
nomena constantly  appear  in  the  study  of  other 
metals  ;  and  if  an  alloy  of  this  composition  is 
polished,  etched  with  acid,  and  examined  under  a 
microscope,  it  will  be  seen  to  consist  of  a  uniform 
conglomerate  of  the  two  kinds  of  crystals.  An 
alloy  of  any  other  proportion  exhibits  larger 
primary  crystals  of  that  metal  which  is  present 
in  excess,  and  was  frozen  out  first,  connected  by 
regions  filled  with  the  conglomerate  referred  to 
above.  On  account  of  its  more  uniform  texture, 
this  conglomerate,  which,  as  we  have  seen,  corre- 
sponds with  a  so-called  cryohydrate,  is  named 
the  eutectic  alloy.     Fig.  4,  on  the  plate  facing 

^  Osmond  thinks  that,  in  this  particular  case,  the  primary 
crystals  are  not  perfectly  pure.  He  adduces  evidence  to  show- 
that  a  slight  trace  of  copper  is  dissolved  in  the  solid  crystals  of 
silver.     Any  such  effect,  however,  is  hardly  appreciable. 


FUSION  AND  SOLIDIFICATION  7J 

page  72,  represents  a  microscopic  photograph  of 
the  eutectic  of  gold  and  aluminium;  while  in  Fig.  5 
is  shown  the  structure  of  an  alloy  with  a  com- 
position not  quite  that  of  the  eutectic.  Here 
large  primary  crystals  have  appeared,  the  intervals 
being  filled  with  the  same  eutectic  which  is  seen 
in  Fig.  4.  The  metal  of  Fig.  4  has  been  cooled 
more  slowly  than  that  of  Fig.  5,  and  therefore 
the  eutectic  in  Fig.  4  has  larger  crystals  and  a 
coarser  structure. 

The  eutectic  alloy  has  a  constant  melting  or 
freezing-point ;  but,  during  the  process  of  fusion 
or  solidification  of  other  alloys,  the  temperature 
will  generally  change.  As  the  primary  crystals 
of  one  or  other  pure  metal  form,  they  leave  the 
residual  liquid  richer  in  the  other  constituent,  and 
thus  with  a  lower  freezing-point.  This  process 
continues  till  the  liquid  has  the  composition  of 
the  eutectic  alloy,  when  any  further  loss  of  heat 
will  precipitate  crystals  of  both  metals  side  by 
side.  A  thermometer  immersed  in  the  mixture 
will  show  the  temperature  at  which  primary 
crystals  begin  to  form,  and  the  temperature  at 
which  the  composition  of  the  residual  liquid 
reaches  that  of  the  eutectic,  for  the  rate  at  which 
it  falls  becomes  suddenly  much  slower  when  solid 
first  appears,  and  the  fall  stops  altogether  while 
the  eutectic  is  freezing  out.  Thus,  in  such  a 
simple  case  as  that  of  silver  and  copper,  useful 
information  can  be  obtained  by  merely  drawing 
the  curve  giving  the  observed  relation  between 
the  time  and  the  temperature  for  the  heated  alloy. 
Such  curves  have  forms  more  or  less  resembling 
that  shown  in  Fig.  7. 

With  silver  and  copper  no  chemical  compounds 


7^ 


PHYSICAL  SCIENCE 


are  formed ;  with  many  pairs  of  metals  combination 
occurs,  and  the  phenomena  are  more  complicated. 
A  definite  chemical  compound  plays  a  part  similar 
to  that  of  a  pure  element.  Addition  of  either 
component  lowers  the  freezing-point  of  the 
compound.  Thus  the  point  of  solidification  of 
the  pure  compound  must  correspond  with  a 
maximum  point  on  the  equilibrium  curve.  If 
a  single  compound  is  formed  by  the  two  com- 
ponents, the  curve  must  consist  of  three  branches  ; 


f 


(5; 


Fig.  7. 


a  branch  due  to  the  effect  of  the  compound  being 
interposed  between  two  branches  similar  to  those 
in  the  silver-copper  curve  just  considered.  Copper 
and  antimony  form  a  single  compound  SbCug,  in 
which  two  atoms  of  copper  are  united  with  one 
of  antimony.  The  equilibrium  of  the  solid  and 
liquid  phases  has  been  studied  by  M.  Le  Chatelier, 
whose  results  are  illustrated  in  Fig.  8.  In  this 
case  two  eutectic  alloys  are  formed  ;  one  being 
a  conglomerate  of  crystals  of  the  compound  with 
those  of  copper,  and  the  other  containing  crystals 


FUSION  AND  SOLIDIFICATION 


79 


of  the  compound  and  crystals  of  antimony.  These 
eutectics  are  represented  by  the  points  a  and  c 
in  the  figure,  and  between  them  rises  the  curve 
showing  the  effect  of  the  compound,  which 
exists  in  the  pure  state  at  b,  the  maximum  of 
the  curve. 

In  all  the  cases  yet  considered,  the  crystals 
deposited  consist  either  of  a  pure  metal  or  else 
of  a  pure  chemical  compound.     Whichever  it  be. 


ubo 


lOOO 


900 


800 


700 


eoo 


500 


400 


3  CO 


the  composition  of  any  one  crystalline  species  is 
fixed  and  definite  ;  it  does  not  vary  continuously 
when  the  composition  of  the  mass  of  alloy  is 
altered,  as  does,  for  example,  the  composition  of 
the  fused  liquid.  In  Fig.  6,  p.  75,  the  left-hand 
branch  of  the  curve  gives  the  composition  of  the 
liquid  alloy  which,  at  different  temperatures,  is  in 
equilibrium  with  crystals  of  pure  silver,  while 
the  right-hand  branch  represents  the  liquid  in 
equilibrium  with  pure  copper.     One  phase  only, 


8o  PHYSICAL  SCIENCE 

the  liquid,  can  vary  continuously  in  composition  ; 
the  other,  or  solid,  phase  is  fixed  and  invariable. 
Similarly  in  the  case  illustrated  in  Fig.  8,  the 
crystals  of  the  compound  SbCug  have  a  fixed 
and  constant  composition.  Cases  are  known, 
however,  in  which  the  solid  phase  also  varies 
continuously.  Many  salts,  such  as  the  different 
alums,  are  of  the  same  crystalline  form,  and  can 
replace  each  other  gradually  in  a  crystal,  which 
may  have  any  composition  between  that  of  the 
two  pure  salts.  Such  structures  are  called  mixed 
crystals  or  solid  solutions.  When  they  can  exist, 
the  phenomena  of  equilibrium  become  much  more 
complicated,  for  the  composition  of  the  solid  will 
vary  as  well  as  that  of  the  liquid,  and  will  in- 
troduce a  second  curve  into  the  freezing-point 
diagram. 

It  is  only  of  recent  years  that  it  has  been 
possible  to  interpret  the  complicated  phenomena 
of  solid  solutions.  Now,  however,  we  possess  a 
consistent  theory  of  the  subject,  founded  by 
Professor  Roozeboom  of  Amsterdam,  on  the  work 
of  the  late  Professor  Willard  Gibbs  of  Yale 
University.  Long  ago,  in  the  years  1875  ^^ 
1878,  Gibbs  published  a  series  of  mathematical 
papers  in  the  Transactions  of  the  Connecticut 
Academy.  For  some  time  they  remained  practi- 
cally unknown  to  European  physicists  ;  then  they 
were  discovered  by  Clerk  Maxwell,  who  used  a 
few  of  the  results  in  his  book  on  the  *'  Theory  of 
Heat."  But  even  then  the  time  was  not  ripe,  and 
it  is  only  of  recent  years  that  we  have  realised 
that  the  whole  theory  of  chemical  and  physical 
equilibrium  is  contained  in  Gibbs'  work.  Buried 
for  so  long,  the  seed  has  germinated  in  the  minds 


J.    WiLLARD    GiBBS. 


[To  face  page  80. 


w 


FUSION  AND  SOLIDIFICATION 


8i 


of  many  investigators.  It  has  already  borne  good 
fruit,  and  is  probably  destined  to  bear  still  more 
in  time  to  come.  Happily,  Willard  Gibbs  lived 
to  see  a  general  recognition  of  his  genius,  and 
the  reputations  made  of  younger  men  who  knew 
how  to  extract  and  apply  even  single  results 
taken  from  the  rich  store  hidden  in  his  somewhat 
abstruse  pages. 

By  the  use  of  Gibbs'  thermodynamic  principles, 
Roozeboom  was  able  to  trace  the  various  possible 


ConcentraJiorv 

Fig.  9. 

forms  which  can  be  assumed  by  the  two  curves, 
representing  the  compositions  of  the  liquid  and 
solid  phases  in  equilibrium  with  each  other.  The 
simplest  case  indicated  by  the  theory  is  shown  in 
Fig.  9.  In  regions  above  the  higher  curve,  acb, 
which  is  called  the  ''liquidus,"  all  points  represent 
states  completely  liquid,  while  below  the  curve 
adb,  or  *'solidus,"  the  alloy  is  entirely  solid. 
Between  these  curves  exist  both  liquid  and  solid 
in  various  proportions.     At  a  definite  temperature, 

G 


82  PHYSICAL  SCIENCE 

a  liquid  of  one  composition,  say  c,  is  in  equilibrium 
with  a  solid  of  another  composition,  such  as  d. 
As  the  process  of  solidification  proceeds,  the 
composition  of  both  liquid  and  solid  changes 
continuously.  In  the  light  of  these  theoretical 
curves,  the  complicated  experimental  curves,  found 
by  observing  the  freezing-points  of  mixtures  of 
metals  and  of  other  substances,  are  now  being 
interpreted  in  a  manner  which  otherwise  would 
have  been  quite  impossible. 

One  of  the  most  successful  examples  of  such  an 
interpretation  is  given  by  the  very  thorough  study 
which  was  made  by  Heycock  and  Neville  of  the 
bronzes,  that  is,  of  alloys  consisting  of  copper 
and  tin.  The  curves  in  Fig.  lo  show  the  results 
of  their  own  experiments  and  of  previous  work  by 
Roberts- Austen.  Heycock  and  Neville  examined 
microscopically  the  structure  of  various  alloys  of 
the  two  metals  in  conjunction  with  the  equilibrium 
curves,  and  gave  us  a  knowledge  of  the  bronzes 
more  complete  than  that  which  we  then  possessed 
for  any  other  series  of  alloys  showing  phenomena 
of  an  equal  degree  of  complexity. 

Fig.  lo  shows  the  equilibrium  curves,  from 
pure  copper  on  the  left  to  an  alloy  containing  80 
atomic  percentages  of  tin  on  the  right.  Above 
the  "liquidus"  abcdefgh  the  alloys  consist  of  a 
homogeneous  liquid,  in  which  solid  first  begins 
to  form  when  the  temperature  falls  to  points 
represented  on  the  curve.  The  "solidus"  curve, 
below  which  the  whole  mass  is  solid,  is  the 
complicated  curve  hblcmef'£.^Y.^^!'u. 

It  has  long  been  known  that  the  physical 
properties  of  metals,  especially  of  alloys,  depend 
on  the  way  in  which  they  are  cooled  from  a  state 


FUSION  AND  SOLIDIFICATION 


83 


of  fusion.  The  whole  process  of  the  annealing  or 
tempering  of  steel  depends  on  a  perception  of  this 
fact.  Many  observers  had  studied  the  changes  of 
physical  properties  thus  produced  by  examining 
microscopically  the  solid  alloys  obtained  by 
different  treatments,  and  relations  between  the 
properties  of  the  alloy  and  its  microscopic  structure 


Percentage   ty  Wei^t    of    Tin 


A' 


oc 


300   ■ 


a  +  8 


j/y*"] 

n  +  li^ 

6  +  r) 

-— ^ 

E4 

Y]  +II  +  li«j. 

\ 
\ 
\ 
\ 

\ 

M 

r)  +  II  t   Tin 

Fig.  10. 

had  been  traced.  But  for  the  first  time  a  com- 
plete investigation  was  made  by  Heycock  and 
Neville  of  the  changes  in  microscopic  structure 
produced  by  different  methods  of  cooling,  and 
studied  in  conjunction  with  the  equilibrium  curves 
by  the  light  of  the  theory  of  solid  solutions.  The 
work  was  rendered  possible  by  the  fact  that,  if  a 
hot  metal  be  cooled  suddenly  from  any  tempera- 
ture by  chilling  it  in  cold  water,  the  microscopic 


84  PHYSICAL  SCIENCE 

structure  it  possessed  at  that  temperature  is  stereo- 
typed almost  perfectly  by  the  process  of  sudden 
chilling,  and  can  be  examined  at  leisure  in  the 
cold  metal  by  polishing  and  etching  it  with  acid 
in  the  usual  manner. 

In  this  way  equilibrium  curves  lying  below 
the  solidus  were  detected  and  traced.  Such 
curves  represent  changes  of  structure  which  occur 
in  a  mass  completely  solid,  and  quite  explain  the 
changes  in  physical  properties  caused  by  anneal- 
ing or  chilling.  Take  as  an  example  the  two 
curves  /x  and  e'x,  which  cut  each  other  in  the 
point  X,  and  recall  in  their  general  form  and 
relations  the  simple  curves  of  equilibrium  between 
liquid  and  solid  for  alloys  of  silver  and  copper 
already  described  and  illustrated  in  Fig.  6  (p.  75). 
The  analogy  is  more  than  one  of  mere  form.  Just 
as  crystals  of  silver  or  copper  separate  out  of  the 
homogeneous  liquid  of  Fig.  6,  so  crystals  of  new 
substances  separate  out  of  the  homogeneous  solid 
solution  which  exists  within  the  triangular  space 
Ixflm  Fig.  10;  and,  as  the  crystals  of  silver  or 
copper  are  in  equilibrium  with  the  liquid  alloy  in 
states  represented  by  points  on  the  freezing-point 
curves  of  Fig.  6,  so  the  new  crystalline  structures 
are  in  equilibrium  with  the  homogeneous  mother 
substance  lying  within  our  present  triangle. 

The  positions  of  these  curves  of  equilibrium 
between  solid  phases  are  investigated  chiefly  by 
the  microscopic  examination  of  ingots  of  metal, 
which  are  fused,  allowed  to  cool  very  slowly  to 
the  temperature  to  be  investigated,  in  order  that, 
as  far  as  possible,  equilibrium  may  be  reached, 
and  then  suddenly  chilled  by  immersion  in  cold 
water.     A  section  of  the  ingot  is  polished,  and 


P'iG.  II. — Magnification  1 8. 


Fig.  12. — INIagnihcation  45. 


F 


1    f 


Fig.  13. — AJagnitication  18.     FiG.  14. — MagnilKation  18.     FiG.  15. — Magniticaiion  18. 


Fig.  16. — Magnification  18. 


Fig.  17. — Magnification  18. 


[To  face  page  85. 


FUSION  AND  SOLIDIFICATION  85 

etched  with  acid  or  other  suitable  Hquid,  in  order 
to  bring  out  the  structure-pattern.  Each  pure 
metal,  compound,  or  solid  solution,  crystallising 
from  the  mother  liquid,  possesses  a  characteristic 
appearance,  which  can  readily  be  recognised 
after  some  practice  In  interpretation  of  the 
micro-photographs.  Such  photographs  enable 
us  to  trace  the  formation,  development,  and 
decay  of  new  crystal-species  in  a  liquid  or  in  a 
solid  matrix. 

The  effect  on  the  microscopic  structure  of 
differences  in  the  rate  of  cooling  is  well  shown 
in  Figs.  II,  12,  and  13.  The  same  alloy  Is 
represented  in  all  these  photographs,  and  was, 
in  each  case,  chilled  from  about  the  same  tempera- 
ture. The  differences  In  structure  depend  solely 
on  the  differences  in  the  rate  of  cooling  from  a 
liquid  condition  to  the  temperature  at  which  the 
ingot  was  chilled  in  cold  water. 

The  alloy  contained  13.5  atomic  percentages 
of  tin,  and  is  represented  by  the  vertical  dotted 
line  in  Fig.  10.  When  this  alloy  in  cooling 
passes  the  liquidus  abc,  crystal  skeletons  of 
a  solid  solution  called  a  appear  mixed  with 
the  mother  liquid.  These  skeletons  somewhat 
resemble  the  larger  fern-like  structures  of  Fig.  2 
on  p.  72,  which,  however,  chosen  chiefly  for  its 
beauty,  was  taken  from  a  bronze  of  another 
composition. 

When  the  alloy  we  are  now  considering 
passes  the  line  /c  (Fig.  10),  a  new  kind  of 
crystalline  solid  solution,  called  /3,  begins  to 
form  ;  and,  if  time  is  given  it  by  keeping  the 
ingot  hot,  the  /^  substance  gradually  eats  up 
the   existing    crystals    of    a.       This    process    is 


86  PHYSICAL  SCIENCE 

illustrated  in  Figs,  ii,  12,  and  13.  In  Fig.  11 
the  residual  a  is  seen  as  white  cores  within  the 
grey  /3,  which  follows  the  arrangement  of  the 
original  a  structures,  while,  in  the  particular 
illumination  employed,  the  part  that  was  liquid 
at  the  instant  of  chilling  shows  as  a  dark  back- 
ground. In  Fig.  12,  where  the  ingot  was  cooled 
more  slowly,  the  change  has  gone  farther  ;  the  /5 
substance  ceases  to  follow  the  original  skeletons 
of  a,  a  higher  magnification  brings  out  the 
characteristic  striated  appearance  of  the  1^,  while, 
owing  to  a  different  illumination,  the  mother 
liquid  shows  as  a  light  background.  Fig.  13  is 
taken  from  an  ingot  which  had  been  cooled  to 
the  same  chill  point  exceedingly  slowly,  and  kept 
many  hours  just  above  that  temperature.  The 
whole  ingot  is  now  filled  with  uniform  striated 
/5,  a  tiny  speck  of  a,  seen  towards  the  lower  side 
of  the  photograph,  alone  remaining.  In  the  light 
of  these  three  photographs  it  is  not  surprising 
that  the  physical  and  mechanical  properties  of 
metals  are  modified  profoundly  by  differences  in 
the  rates  at  which  they  have  been  cooled  from 
a  fused  condition. 

Following  the  dotted  line  in  Fig.  10  still 
further,  we  see  that,  in  ingots  chilled  from 
temperatures  about  750°,  /5  alone  should  exist. 
Fig.  14  shows  a  chill  from  740°,  which  was 
cooled  to  that  temperature  almost  slowly  enough 
to  destroy  all  the  primary  crystals  of  a,  which 
now  only  show  as  scattered  specks  of  white. 

Again  following  the  dotted  line  in  the  equi- 
librium curve  of  Fig.  10,  we  pass  the  boundary 
/x,  and  again  enter  a  region  where  a  and  P  exist 
together.     The    facts    on    which    this    curve    is 


FUSION  AND  SOLIDIFICATION  87 

based  are  illustrated  In  Fig.  15.  Here  a  new 
or  secondary  crop  of  a  crystals  has  begun  to 
grow.  This  ingot  was  chilled  at  558  ,  and  there 
is  no  doubt  that  the  new  growth  of  a  took 
place  in  a  mass  which  had  solidified  completely 
long  before. 

The  further  growth  of  the  new  a  is  seen  in 
Fig.  16,  which  represents  an  alloy  of  slightly 
higher  content  of  tin  (14  atomic  per  cents.)  chilled 
from  a  temperature  of  530  .  As  the  alloy  in 
cooling  passes  the  temperature  of  500',  the  whole 
of  the  /3  substance  is  transformed  into  a  complex 
consisting  of  a  crystals  intimately  mixed  with  a 
new  solid  solution  called  S.  This  complex  is 
shown  in  Fig.  17  as  a  light  background;  while, 
in  contrast  with  it,  the  a  crystals  come  out  dark 
after  the  treatment  adopted. 

These  changesagain  occur  inamassthoroughly 
solid  throughout,  and  explain  in  a  most  striking 
manner  the  effect  of  such  processes  as  annealing 
and  tempering,  in  which  the  properties  of  a  metal 
are  altered  by  heating  it  to  a  temperature  well 
below  its  fusion-point  and  then  cooling  it  either 
slowly  or  rapidly. 

Heycock  and  Neville's  investigation  of  the 
bronzes  was  a  very  laborious  undertaking.  One 
hundred  micro-photographs  were  published,  and 
these  represent  only  a  selection  of  those  taken  ; 
many  observations  of  freezing-points  were  also 
made.  But  the  labour  of  the  work  is  well  repaid 
by  the  magnificent  results  finally  obtained. 

Iron  and  steel,  as  used  in  the  arts  and  in- 
dustries, consist  of  pure  iron  alloyed  with  various 
substances,  chiefiy  carbon.    Solid  solutions,  similar 


88  PHYSICAL  SCIENCE 

to  those  we  have  studied  In  other  cases,  are 
formed  between  iron  and  carbon,  and  the  phe- 
nomena of  equilibrium  between  the  Hquid  and 
solid  phases,  even  when  no  other  component  is 
present,  are  very  complicated. 

Owing  to  their  industrial  importance,  the 
alloys  of  iron  have  been  investigated  more  ex- 
tensively than  those  of  any  other  metal,  and  the 
various  compounds  and  solid  solutions  identified 
have  received  definite  names,  which,  in  many 
cases,  were  given  long  before  the  application 
by  Roozeboom  of  the  theory  of  solid  solutions 
enabled  the  true  phenomena  of  equilibrium  to  be 
understood.  Roozeboom's  diagram  for  alloys  of 
iron  and  carbon,  containing  less  than  7  per  cent, 
of  carbon,  is  reproduced  in  Fig.  18.  Its  general 
meaning  will  be  clear  in  the  light  of  what  has 
been  said  in  the  case  of  the  bronzes.  Here  again 
changes  occur  at  definite  temperatures,  even  in 
alloys  which  are  completely  solid.  The  viscosity 
of  the  material  makes  these  changes  very  slow, 
and  very  different  proportions  of  the  various 
possible  constituents  will  be  found  in  alloys  that 
have  been  cooled  quickly  and  slowly.  The  effects 
of  tempering  steel  and  iron  thus  receive  a  physical 
explanation. 

By  heating  iron  above  one  of  the  transforma- 
tion temperatures  indicated  in  the  diagram,  and 
maintaining  it  at  a  high  temperature  for  some 
time,  it  will  obviously  be  possible  to  produce 
extensive  changes  in  the  physical  nature  of  the 
metal.  Experiment  by  Mr  J.  E.  Stead  has  shown, 
that  when  steel  rails  have  become  dangerously 
brittle  and  crystalline  by  long  use,  they  can  be 
reconverted  into  a  toucrh,  elastic,  and   therefore 


FUSION  AND  SOLIDIFICATION 


89 


safe  condition  by  prolonged  heating  at  tempera- 
tures from  850°  to  900"^  C.  This  improvement  in 
properties  has  been  traced  to  the  development 
of  a  constituent  of  the  alloy  known  as  sorbite. 
It    is  this  constituent  which    gives    the   peculiar 


1600 


1500  - 


14 00  - 


1300 


.03  .04  .OS 

Fig.  18. 


tenacious    properties    to    iron    which    has   been 
specially  prepared  for  drawing  into  wire. 

Microscopic  studies  of  the  alloys  composing 
iron  and  steel  have  been  very  numerous.  The 
work  of  Sorby,  Andrews,  Osmond,  Le  Chatelier, 
and  Stead  should  particularly  be  mentioned.  It 
is  by  such  microscopic  investigations  that  the 
different  constituents  of  the  alloys  have  been  for 


90  PHYSICAL  SCIENCE 

the  most  part  distinguished,  the  crystals  of  each 
constituent  having  a  characteristic  appearance, 
which  usually  persists  throughout  a  series  of 
changes. 

The  investigations  we  have  described  all 
emphasise  one  point — the  fact  that  metals  possess 
a  structure  essentially  crystalline.  In  some  cases, 
such  as  that  of  the  surfaces  of  zinc  deposited  on 
so-called  galvanised  iron,  this  crystalline  structure 
is  readily  visible,  but  most  of  the  metallic  objects 
in  common  use  possess  polished  surfaces  on  which 
no  trace  of  crystals  can  be  seen.  The  possibility 
of  polishing  a  surface  to  such  a  state  of  perfection 
that  it  will  act  as  a  mirror  and  reflect  a  ray  of 
light  without  appreciable  scattering,  is  a  matter 
of  considerable  interest.  Any  irregularities  on 
such  a  surface  must  be  small  compared  with  the 
wave-length  of  light,  and  it  is  difficult  to  see  how 
any  such  surface  could  be  obtained  by  the  use  of 
ordinary  polishing  materials,  if  the  action  of  these 
materials  be  regarded  as  a  mere  mechanical 
grinding  away  of  projections  after  the  manner 
of  a  file. 

Many  careful  observations  have  been  made  on 
the  process  of  polishing.  Among  them  should 
be  noted  those  published  in  August  1903  in  the 
Proceedings  of  the  Royal  Society,  by  Sir  George 
Beilby.  He  investigated  the  subject  microscop- 
ically, and  found  reason  to  believe  that  the  passage 
over  the  surface  of  a  scratched  metal  of  a  polish- 
ing substance  like  wash  leather  covered  with  rouge 
produces  a  kind  of  surface  flow,  the  outer  layers 
of  the  metal  flowing  like  a  viscous  liquid  under 
the  action  of  the  pressure  on  the  polishing  tool, 


Fig.  19. — Magnification  775' 


-  I -ill 


Fig.  23. — Magnification  77; 


.1 


ill 
11 


:m 


Fig.  20. — Magnification  775. 


i  iu.  22. — Magiuricuiiuii  775. 


Fig.  24. — Magnification  775. 


•ic/> 


[To  foxe  page  91. 


FUSION  AND  SOLIDIFICATION  91 

and  assuming  an  optically  perfect  surface  under 
the  Influence  of  surface  tension.  In  this  way  a 
film  Is  formed  over  the  surface  of  a  metal,  which 
film  is  in  a  state  essentially  different  from  that  of 
the  bulk  of  the  substance  below.  Inside  the  metal 
the  crystalline  forces  have  full  play  ;  at  its  surface, 
the  controlling  influences  consist  in  part  of  surface 
tension,  which,  under  the  pressure  of  a  polishing 
tool,  is  able  to  overcome  the  tendency  to  assume 
a  crystalline  structure.  In  Figs.  19  to  24  are 
shown  six  of  Sir  George  Beilby's  photographs. 
Fig.  19  shows  the  surface  of  crystalline  antimony 
after  rubbing  with  fine  emery  paper.  The  magnifi- 
cation is  such  that  the  photograph  Is  775  times 
life-size.  Fig.  20,  which  represents  the  same 
surface  after  polishing  with  rouged  leather,  shows 
the  gradual  dragging  of  a  film  of  metal  over  the 
pits  and  furrows  of  the  first  surface.  The  larger 
pits  get  filled  with  filings  of  metal,  and  the  film 
seems  to  bridge  them  over,  forming  a  continuous 
sheet  over  the  loosely-packed  fragments  below. 
When  an  acid  or  other  liquid  capable  of  dissolving 
the  metal  is  placed  on  the  surface,  the  film  Is  dis- 
solved, and  the  pits  and  furrov/s  reappear.  This 
comes  out  In  Fig.  21,  in  which  the  antimony 
previously  polished  has  been  etched  with  a  solution 
of  potassium  cyanide.  Fig.  22  shows  a  polished 
surface  of  speculum  metal,  an  alloy  used  for  the 
reflectors  of  telescopes.  Here  the  underlying 
crystalHne  structure  Is  faintly  visible.  The  surface 
film  has,  In  Fig.  23,  been  removed  with  potassium 
cyanide,  and  the  structure  is  now  plain,  the 
primary  crystals,  separated  by  channels  of  eutectic 
alloy,  being  clearly  brought  out.  Finally,  in 
Fig.    24,  the  same  surface  has  been  repolished, 


92  PHYSICAL  SCIENCE 

and  the  channels  bridged  over  with  the  flowing 
film  of  viscous  metal. 

These  experiments  have  an  interest  which 
extends  further  than  the  immediate  subject  to 
elucidate  which  they  were  undertaken — an  experi- 
ence not  uncommon  in  physical  research.  The 
existence  of  this  viscous  metallic  film  under 
certain  conditions  suggests  that,  when  minute 
quantities  of  a  solid  alone  exist — when  there  is  in 
effect  inside  the  surface  film  no  substance  beyond 
the  range  of  molecular  action  —  all  crystalline 
structure  must  disappear.  The  initial  formation 
of  solid  in  the  body  of  a  saturated  solution  or  of  a 
fused  material  will,  on  this  view,  be  co-ordinated 
exactly  with  the  deposition  of  drops  of  water  from 
a  mass  of  air  saturated  with  aqueous  vapour,  and 
the  possibility  of  super-saturation  will,  in  each 
case,  depend  on  the  work  required  to  form  a 
new  surface  of  separation  under  the  influence 
of  surface  tension  alone.  It  is  only  when  the 
individual  solid  structures  attain  a  considerable 
size  that  crystalline  forms  begin  to  appear. 


CHAPTER   V 

THE    PROBLEMS    OF    SOLUTION 

"  If  we  accept  the  hypothesis  that  the  elementary  substances 
are  composed  of  atoms,  we  cannot  avoid  concluding  that  electricity 
also  ...  is  divided  into  definite  elementary  portions,  which  behave 
like  atoms  of  electricity."  —  H.  vON  Helmholtz,  "Faraday 
Lecture,"  1881. 

To  one  inexperienced  in  the  problems  which 
confront  the  workers  in  the  world  of  natural 
science,  the  whole  question  of  solution  and  its 
attendant  phenomena  may  appear,  at  first  sight, 
of  small  account.  Yet  the  study  of  these  same 
phenomena,  and  the  unravelling  of  their  intricate 
connections,  are  of  fundamental  importance. 
Furthermore,  as  the  work  of  the  last  twenty 
years  has  shown,  the  problems  involved  are  of 
increasing  interest,  not  only  from  the  point  of 
view  of  physics  and  chemistry,  but  also,  and 
perhaps  especially,  from  the  physiological  stand- 
point. More  and  more  the  reactions  of  inorganic 
substances,  whether  liquid  or  solid,  are  referred 
to  their  properties  in  a  state  of  solution,  while 
every  process  of  life  to  be  investigated  by  the 
biologist  seems  capable  of  interpretation  only 
through  attention  to  the  conditions  thereby  in- 
volved. Moreover,  most  chemical  actions,  especi- 
ally those  examined  easily  in  the  laboratory,  occur 
between  substances  one  or  more  of  which  are 
actually  in  the  liquid  state ;  while  the  application 

98 


94  PHYSICAL  SCIENCE 

of  physical  conceptions  to  the  problems  of  living 
matter  chiefly  depends  on  the  knowledge  we 
possess  of  the  physics  and  chemistry  of  ordinary 
solutions. 

The  earliest  Investigations  of  the  subject  were 
of  a  chemical  nature,  and,  till  the  passage  of 
electric  currents  through  liquids  came  to  be 
examined  at  the  beginning  of  the  nineteenth 
century,  little  systematic  study  of  the  physical 
properties  of  solutions  was  made.  But  since  that 
period  there  has  been  constant  progress,  and  many 
new  fields  of  research  have  been  opened  up. 

It  happens  constantly  that  light  Is  thrown 
on  the  dark  places  of  one  science  by  work 
undertaken  to  elucidate  those  of  another  ;  and, 
in  this  case,  the  starting-point  for  the  modern 
theory  of  solution  Is  found  In  some  experiments 
made  by  Pfeffer  in  1877  In  a  botanical  laboratory. 
Ten  years  earlier,  Traube,  In  studying  the  modes 
of  formation  of  the  organic  cells  of  plants  and 
animals,  had  discovered  how  to  construct  artificial 
membranes  permeable  to  water  but  not  to  solu- 
tions of  certain  substances  dissolved  therein. 
Pfeffer  made  a  further  examination  of  these 
semi-permeable  membranes,  as  they  have  been 
called,  and  by  their  use  obtained  results  of  great 
importance  in  the  study  of  biology. 

A  porous  pot  of  unglazed  earthenware,  6  to 
8  centimetres  high  and  2  or  3  centimetres  In 
diameter,  Is  sealed  by  means  of  sealing-wax  to 
a  glass  tube,  as  shown  in  Fig.  25.  Having  been 
thoroughly  washed.  It  Is  filled  with  the  solution 
of  a  salt,  such  as  potassium  ferrocyanlde,  and 
the  outside  is  then  surrounded  with  the  solution 


THE  PROBLEMS  OF  SOLUTION 


95 


of  another  salt,  such  as  copper  sulphate  or  ferric 
chloride,    which    gives    an    insoluble   precipitate 


Fig.  25. 


when  in  contact  with  the  first  salt.  The  two 
solutions  gradually  diffuse  from  opposite  sides 
into  the  walls  of  the  cell,  and  form  an  insoluble 


96  PHYSICAL  SCIENCE 

membrane,  indicated  by  a  dotted  line,  where  they 
meet  inside  the  thickness  of  the  walls.  This 
process  can  be  hastened,  and  the  resulting 
membrane  improved,  by  forcing  the  salts  into 
the  porous  material  by  means  of  an  electric 
current.  The  solutions  are  washed  away,  and 
the  wide  glass  tube  is  drawn  out  and  sealed  to 
a  smaller  tube  in  the  manner  shown  in  the  figure. 

Inside  a  cell  thus  prepared  let  us  place  the 
solution  of  some  substance,  such  as  sugar  in 
water,  and  surround  the  outside  with  a  large 
volume  of  the  pure  solvent,  in  this  case,  water. 
Water  will  gradually  force  its  way  into  the  cell, 
and,  by  placing  mercury  in  the  glass  tube  to  use 
as  a  pressure  gauge,  it  will  be  found  that  this 
influx  will  continue  till  a  definite  internal  pressure 
is  reached — a  pressure  greater  than  that  without. 
This  gives  a  measure  of  what  is  called  the  osmotic 
pressure  of  the  solution  as  it  finally  exists  in  the 
cell  after  the  entrance  of  the  additional  quantity 
of  water. 

Pfeffer  found  that  this  osmotic  pressure  was 
proportional  to  the  concentration  of  the  solution, 
at  all  events  between  the  concentrations  of  i  and 
6  per  cent,  of  sugar.  For  a  i  per  cent,  solution, 
the  excess  of  pressure  at  6°. 8  C.  was  equal  to 
that  of  a  column  of  mercury  505  millimetres  high, 
the  normal  atmospheric  pressure  being  equivalent 
to  760  millimetres. 

Many  membranes  within  animal  and  vegetable 
organisms  are  semi-permeable,  or,  at  all  events, 
are  more  permeable  to  solvent  than  to  solution. 
The  permanent  or  temporary  differences  of 
pressure,  which  are  thus  set  up,  are  being 
investigated    extensively    by    physiologists,    and 


[To  face  page  97. 


THE  PROBLEMS  OF  SOLUTION  97 

have  already  been  shown  to  play  important  parts 
in  the  processes  of  living  structures. 

Attention  was  first  called  to  the  interest  and 
importance  of  osmotic  pressure  from  a  physical 
standpoint  by  the  distinguished  Dutch  chemist, 
the  late  professor  Van't  Hoff.  In  1885  Van't 
Hoff  pointed  out  that  Pfeffer's  numbers  showed  : 
(i)  that  the  osmotic  pressure  was  inversely 
proportional  to  the  volume  in  which  a  given 
mass  of  sugar  was  confined ;  and  (2)  that  the 
absolute  value  of  the  pressure  in  the  case  of  the 
solution  of  sugar  was  the  same  as  that  which 
would  be  exerted  by  an  equal  number  of  molecules 
of  a  gas  when  placed  in  a  vessel  having  a  volume 
equal  to  that  of  the  solution.  For  instance,  a 
quantity  of  gas  of  the  same  molecular  concentra- 
tion as  a  I  per  cent,  solution  of  sugar  would,  at 
6°.S  C,  exert  a  pressure  equivalent  to  that  of 
508  millimetres  of  mercury,  a  number  identical, 
within  the  limits  of  experimental  error,  with 
Pfeffer's  observed  value  for  the  osmotic  pressure 
quoted  above.  The  first  result  is  equivalent  to 
the  extension  to  dilute  solutions  of  Boyle's  law 
for  gases,  a  law  which  states  the  experimental 
result  that  the  volume  of  a  gas  is  inversely 
proportional  to  its  pressure.  The  second  result 
shows  that,  in  a  dilute  solution,  the  pressure 
depends  only  on  the  number  of  molecules  present, 
and  not  on  their  nature — a  statement  which, 
applied  to  gases,  is  known  as  Avogadro's  law. 

But  Van't  Hoff  did  not  alone  call  attention  to 
the  experimental  basis  of  the  new  subject.  He 
also  placed  the  theory  of  it  on  a  sound  footing. 
The  amount  of  a  gas  which  dissolves  in  a  given 
quantity  of  water  is  proportional  to  the  pressure, 

H 


98  PHYSICAL  SCIENCE 

and  from  this  experimental  result  Van't  Hoff 
showed  mathematically  by  the  principles  of 
thermodynamics,  that,  when  in  solution,  this 
same  gas  must  exert  an  osm.otic  pressure  of  the 
observed  value.  The  proof  involves  no  assump- 
tion as  to  the  physical  mechanism  by  which  the 
osmotic  pressure  is  produced.  Whether  it  be 
due  to  the  impacts  of  the  dissolved  molecules  on 
the  semi-permeable  walls,  in  the  same  way  that 
the  molecules  of  a  gas  exert  pressure  on  the  walls 
of  the  containing  vessel  ;  whether  it  be  due  to 
chemical  affinity  between  the  dissolved  substance 
and  the  solvent,  affinity  which  causes  more 
solvent  to  enter  the  cell ;  or  whether  some  other 
hitherto  untraced  effects  come  into  play,  remains 
an  open  question.  The  thermodynamic  argument 
simply  shows  that,  from  the  experimental  solubility 
relations  of  gases,  the  observed  osmotic  results 
follow  for  the  gases  when  dissolved  ;  but  the 
physical  modus  operajtdi  of  the  pressure  remains 
uncertain. 

The  extension  of  the  theoretical  result  to  the 
case  of  non-gaseous  solutes  like  sugar  involves 
some  amount  of  assumption.  However,  since 
substances  of  all  degrees  of  volatility  are  known, 
the  extension  seems  reasonable  ;  and  it  is 
abundantly  justified  by  Pfeffer's  experimental 
measurements. 

Another  method  of  applying  the  principles 
of  thermodynamics  to  this  problem  has  been 
developed  by  Willard  Gibbs,  Von  Helmholtz, 
and  Larmor.  Whatever  view  we  take  of  the 
fundamental  nature  of  a  solution,  we  must 
imagine  the  dissolved  substance  scattered  as  a 
number    of    discrete    particles    throughout    the 


THE  PROBLEMS  OF  SOLUTION  99 

volume  of  the  solvent.  The  nature  of  the  inter- 
action which  occurs  between  the  solute  and  the 
solvent  is  unknown,  possibly  unknowable;  but, 
whatever  it  may  be,  each  particle  of  solute 
will  affect  only  a  minute  sphere  of  solvent  lying 
round  it.  The  solution,  then,  may  be  regarded 
as  containing  a  number  of  little  systems,  each 
composed  of  a  solute  particle  surrounded  by  an 
atmosphere  of  solvent  in  some  way  influenced  by 
its  nucleus. 

While  the  solution  is  concentrated,  the  little 
spheres  will  intersect  each  other,  and  the  addition 
of  further  solvent  will  involve  some  change  in  the 
interaction  between  solute  and  solvent.  But,  in 
the  process  of  dilution,  a  time  will  come  when  the 
spheres  are  beyond  each  other's  reach,  and  the 
addition  of  more  solvent  merely  increases  their 
mutual  separation  without  affecting  their  internal 
structure. 

Thus,  in  a  dilute  solution,  the  energy-change 
of  further  dilution  is  merely  the  energy-change 
involved  in  separating  the  particles  of  the  solute  ; 
it  will  not  depend  on  the  nature  of  any  possible 
interaction  between  the  solute  and  the  solvent. 
The  change  of  energy  is  thus  independent  of 
the  nature  of  the  solvent,  and  will  be  the  same 
whether  that  solvent  be  water,  alcohol,  or  any 
other  liquid.  It  will  even  be  the  same  when, 
in  cases  where  that  is  possible,  the  solvent  is 
removed  altogether,  and  the  solute  is  obtained 
in  the  gaseous  state. 

If  we  imagine  that  the  bottom  of  a  frictionless 
engine  cylinder  is  made  of  a  semi-permeable  mem- 
brane, separating  a  solution  within  the  cylinder 
from  a  solvent   without,   it   is  easy  to  see   that 


lOO  PHYSICAL  SCIENCE 

osmotic  pressure  may  be  made  to  do  work,  which 
will  be  measured  by  the  pressure  multiplied  by  the 
change  of  volume.  Thus  the  osmotic  pressure  is 
measured  by  the  change  of  the  available  energy 
per  unit  increase  of  volume  ;  that  is,  by  the  rate 
of  change  in  the  available  energy  of  dilution. 

In  this  manner  we  arrive  again  at  the  con- 
clusion, that  the  osmotic  pressure  must  be  equal 
in  amount  to  the  gaseous  pressure  exerted  by 
the  same  number  of  molecules  when  vaporised, 
and  must  conform  to  the  laws  which  describe 
the  temperature,  pressure,  and  volume  relations 
of  gaseous  matter.  The  result  is  seen  clearly  to 
be  independent  of  any  hypothesis  concerning  the 
mechanism  of  the  pressure  or  the  nature  of  the 
solution. 

In  the  last  chapter  we  have  traced  the 
phenomena  of  fusion  and  solidification,  and,  in 
the  course  of  our  inquiry,  studied  the  equilibrium 
of  liquid  solutions  with  the  different  solid  phases 
which  may  exist  in  contact  with  the  liquids.  The 
fundamental  problem  of  the  nature  of  a  solution 
was  untouched  ;  indeed,  from  the  point  of  view 
then  adopted,  such  a  problem  did  not  arise. 

Until  the  last  quarter  of  the  nineteenth  century, 
it  was  generally  assumed  that  the  forces  which 
were  brought  into  play  when  a  solid  dissolved  in 
water  were  of  the  same  nature  as  those  involved 
in  chemical  action  ;  and  the  resulting  solution 
was  looked  on  simply  as  a  chemical  compound 
in  which  there  happened  to  be  no  fixed  relation 
between  the  masses  of  the  components.  The  study 
of  dilute  solutions,  and,  in  particular,  the  examina- 
tion of  their  osmotic  pressures,  showed  that,  in 
many  respects,  a  dilute  solution  was  analogous 


THE  PROBLEMS  OF  SOLUTION         loi 

to  a  gas,  and  conformed  to  the  same  laws  of 
pressure,  volume,  and  temperature.  Such  results 
emphasised  the  analogy  between  the  dissolution 
of  a  solid  and  the  diffusion  of  a  gas  through 
a  space  in  which  it  was  not  originally  present, 
and  sometimes  led  to  the  idea  that  the  osmotic 
pressure  of  a  solution,  like  the  pressure  of  a  gas, 
was  due  to  the  impact  of  its  molecules  on  the 
containing  wall.  As  an  extreme  case  of  this 
aspect  of  the  phenomena,  the  view  has  been 
expressed  that  the  solvent  should  simply  be 
regarded  as  giving  room  for  the  diffusion  of  the 
molecules  of  the  solid ;  any  possible  interaction, 
of  a  chemical  nature  or  otherwise,  between  the 
solvent  and  solute  being  disregarded. 

The  similarity  between  the  laws  of  gases  and 
those  of  dilute  solutions,  however,  does  not  neces- 
sarily connote  identity  in  physical  nature ;  the 
account  of  the  subject  given  by  thermodynamics 
shows  clearly  that  the  essential  feature,  common 
to  both  cases,  on  which  the  similarity  depends,  is 
the  dilution.  In  a  gas  the  molecules  are,  on  the 
average,  too  far  from  each  other  to  exert  appreci- 
able intermolecular  forces,  and  the  change  in 
energy  produced  by  further  dilution  does  not 
involve  such  intermolecular  forces.  In  the  same 
way  the  dissolved  molecules  in  a  dilute  solution 
are  so  far  from  each  other  that,  whatever  be  their 
action  on  the  solvent,  they  exert  none  on  each 
other.  Here  again,  the  change  of  energy  on 
further  dilution  does  not  involve  the  forces 
between  those  molecules  which  alone  from  this 
point  of  view  are  to  be  considered,  that  is, 
the  molecules  of  the  dissolved  substance.  The 
essential  point   is  the  distant  separation  of  the 


102  PHYSICAL  SCIENCE 

molecules  in  each  case  from  each  other ;  any 
interaction  between  solvent  and  solute  would  not 
affect  the  result,  and  the  result  therefore  cannot  be 
used  as  evidence  for  or  against  such  interaction. 

The  similarity  in  pressure-volume  laws,  then, 
cannot  be  regarded  as  determining  the  question 
whether  solution  is,  in  its  essential  nature, 
chemical  or  physical.  To  settle  such  a  problem 
other  evidence  must  be  sought.  Very  little  such 
evidence  is  yet  available ;  what  little  there  is 
seems  rather  to  favour  the  chemical  view,  which 
regards  a  solution,  say  of  salt  and  water,  as 
in  some  way  a  chemical  compound  of  these 
components  ;  a  compound  in  which  the  relative 
proportion  between  the  components  can  vary 
continuously  between  certain  wide  limits. 

The  results  in  this  case  are  characteristic  of 
the  methods  of  thermodynamic  theory  as  applied 
in  physical  science.  Thermodynamics  is  not 
concerned  with  the  physical  7nodus  operandi  of 
the  phenomena.  It  does  not  involve  molecular 
hypotheses ;  it  is  free  from  any  doubt  which 
accompanies  such  hypotheses,  though  it  gives 
less  insight  into  the  intimate  processes  of 
the  phenomena  than  do  successful  molecular 
conceptions. 

In  the  development  of  several  branches  of 
physics  and  chemistry  two  stages  can  be  traced. 
It  has  sometimes  happened  that  the  earliest 
theoretical  account  of  a  subject  has  been  given 
from  the  mechanical  or  molecular  standpoint.  In 
this  way  a  definite  working  hypothesis  has  arisen, 
on  the  lines  of  which  much  investigation  has  been 
undertaken.  Gradually,  however,  this  preliminary 
scaffolding  has  been  found  to  be  unnecessary,  and 


THE  PROBLEMS  OF  SOLUTION  103 

a  thermodynamic  theory  has  been  developed, 
which  connects  the  phenomena  directly,  and 
brings  out  their  relations  with  similar  phenomena 
in  other  branches  of  science. 

The  two  methods  may  perhaps  be  illustrated 
in  some  such  way  as  the  following.  In  looking 
at  the  face  of  a  watch,  certain  relations  are 
observed  between  the  positions  of  the  two  hands 
at  different  times.  In  order  to  explain  these 
phenomena  we  make  hypotheses  concerning  the 
structure  of  the  inside  of  the  watch.  We  imagine 
various  arrangements  of  springs,  wheels,  and 
levers  till  we  hit  on  one  particular  system  which 
consideration  shows  us  will  give  the  observed 
result.  Here  we  have  an  intimate  picture  of  the 
inside  of  the  watch,  which  may  or  may  not 
represent  the  only  possible  arrangement,  and 
may  or  may  not  correspond  with  the  reality. 
Such  a  picture  is  analogous  to  a  molecular  theory 
of  a  physical  problem. 

One  day,  however,  we  notice,  in  the  course  of 
our  studies  of  the  watch,  that,  whatever  be  the 
position  of  the  hands,  one  of  them  always  moves 
twelve  times  as  fast  as  the  other.  We  have 
discovered  a  necessary  relation  between  the 
phenomena,  which  enables  us,  if  we  will,  to 
dispense  with  all  hypotheses  about  the  wheels 
and  springs  which  drive  the  mechanism.  The 
observed  connection  between  the  rates  of  motion 
allows  us  to  evade  all  such  complications,  and 
to  calculate  directly  the  relative  positions  of  the 
two  hands  at  any  future  time. 

So  with  thermodynamics.  Lord  Kelvin's  great 
principle  of  the  dissipation  of  energy,  especially 
in  its  modern  form,  which  states  that  the  available 


104  PHYSICAL  SCIENCE 

energy  of  an  isothermal  system  tends  constantly 
to  decrease,  enables  us  in  many  cases  to  evade 
all  molecular  considerations,  and  to  trace  directly 
the  connections  between  various  physical  and 
chemical  phenomena.  By  this  method  it  is 
possible  to  -develop  the  theoretical  relations  of 
many  subjects  without  involving  the  molecular 
hypothesis.  Such  treatment,  using  as  its  sole 
principle  of  co-ordination  the  law  of  available 
energy,  ultimately  rests  on  the  experimental 
impossibility  of  perpetual  motion. 

This  way  of  treating  physical  science  was  at 
one  time  adopted  by  a  certain  number  of  chemists, 
as  a  means  of  presenting  their  subject  without 
applying  to  it  the  language  or  conceptions  of  the 
atomic  theory,  in  terms  of  which  even  its  simplest 
experimental  facts  have  come  to  be  expressed. 
In  particular  Franz  Wald  and  Ostwald  have 
explained  the  phenomena  of  chemical  combination 
in  definite  proportions  from  the  standpoint  of 
energetics.  They  have  shown  that  the  existence 
of  the  two  types  known  to  us  as  elements  and 
compounds  may  be  deduced  from  the  thermo- 
dynamic theory  of  equilibrium  without  reference 
to  atomic  hypotheses.  But,  in  the  present  state 
of  knowledge,  such  a  doctrine  seems  limited  in 
its  scope,  and  cases  in  which  it  ceases  to  be 
sufficient  will  constantly  recur  in  this  volume. 
For  instance,  the  phenomena  of  highly  rarified 
gases  have  only  been  interpreted  successfully  by 
the  aid  of  strictly  molecular  conceptions.  The 
passage  of  electricity  through  gases,  which  will 
be  considered  in  a  future  chapter,  again  suggests 
molecular  hypotheses,  and,  in  conjunction  with 
the  phenomena  of  radio-activity,  gives  an  extended 


THE  PROBLEMS  OF  SOLUTION  105 

insight  into  the  intimate  structure  of  atoms  and 
molecules.  In  such  matters  we  are  driven  back 
to  molecular  theory,  which  offers  an  alternative 
method  of  correlating  other  phenomena  also, 
equally  definite,  and  supported  by  an  ever  in- 
creasing number  of  experimental  concordances. 

Thermodynamic  theory,  as  well  as  practical 
experiment,  thus  indicates  that  the  osmotic 
pressure  of  a  solution  depends  only  on  the 
number  of  dissolved  particles,  and  not  on  their 
nature  or  on  the  nature  of  the  solvent.  The 
phenomena  of  gases  show  that  the  number  of 
molecules  in  two  systems  may  be  compared  by 
a  knowledge  of  the  total  masses  and  of  the 
chemical  molecular  weights.  Thus,  two  solutions, 
one  of  sugar,  let  us  suppose,  and  one  of  alcohol, 
which  are  prepared  so  as  to  contain  the  same 
number  of  molecules  in  the  same  volume,  both 
in  theory  and  practice,  possess  equal  osmotic 
pressures.  But,  if  equimolecular  solutions  of 
sugar  and  salt  be  examined,  the  osmotic  pressure 
of  the  salt  is  found  to  be  greater,  and,  if  the 
solutions  be  dilute,  nearly  twice  as  great  as  that 
of  the  sugar.  These  abnormally  great  osmotic 
pressures  were  discovered  at  an  early  date  in 
the  history  of  the  subject ;  and  further  investiga- 
tion showed  that,  at  all  events  when  the  solvent 
was  water,  they  occurred  in  the  cases  of  those 
solutions  which  were  conductors  of  electricity. 

When  Van't  Hoff  formulated  the  physical 
theory  of  the  osmotic  pressure,  he  treated  these 
abnormal  values  as  exceptions  to  the  usual  law. 
It  was  reserved  for  the  physicists  Arrhenius  of 
Stockholm  and   Planck  of   Berlin  to   point  out 


io6  PHYSICAL  SCIENCE 

that  the  extension  of  Van't  Hoff's  principles  to 
these  cases  required  the  assumption  of  the  dis- 
sociation of  the  molecules  of  salt  in  order  that 
the  total  number  of  particles  in  solution  should 
still  be  the  number  indicated  by  the  observed 
phenomena.  According  to  this  hypothesis,  in  a 
dilute  solution  of  common  salt,  the  solute  does 
not  exist  as  molecules  of  sodium  chloride,  but  as 
the  dissociated  parts,  sodium  and  chlorine,  which, 
since  the  solution  conducts  a  current  of  electricity, 
must  be  associated  with  electric  charges.  Each 
salt  molecule  thus  gives  two  pressure-producing 
particles  in  solution,  and  the  double  value  of 
the  osmotic  pressure  is  explained.  In  stronger 
solutions  this  dissociation  is  not  complete,  and 
the  osmotic  pressure  is  less  than  twice  the  normal 
value  ;  but  no  exact  correlation  of  pressure  and 
dissociation  can  be  made,  for  the  thermodynamic 
theory  as  formulated  above  is  only  valid  for  very 
dilute  solutions. 

Like  the  thermodynamic  theory  of  osmotic 
pressure  generally,  this  extension  of  it  does  not 
involve  any  particular  view  as  to  the  cause  of  the 
pressure  or  the  nature  of  solution.  The  dissocia- 
tion hypothesis  is  concerned  simply  with  the 
difference  between  solutions  of  electrolytes  and 
non-electrolytes,  and  leaves  entirely  open  the 
more  fundamental  question,  whether  solution  is 
essentially  chemical  or  physical  in  its  nature. 

The  dissociation  theory  of  aqueous  solutions 
of  electrolytes,  originally  indicated  by  osmotic 
phenomena,  is  supported  perhaps  even  more 
clearly  and  strongly,  by  the  study  of  the  electrical 
properties.       During   the   years     1830   to    1840, 


THE  PROBLEMS  OF  SOLUTION         107 

Faraday  made  a  series  of  experiments  on  the 
passage  of  electricity  through  liquids,  in  this  way 
laying  the  foundations  of  our  quantitative  know- 
ledge of  that  subject.  He  showed  that  the 
transfer  of  a  given  quantity  of  electricity  was 
always  accompanied  by  the  liberation  of  a  definite 
quantity  of  one  of  the  constituents  of  the  solution, 
a  quantity  proportional  to  the  total  electric 
transfer,  and  to  the  chemical  equivalent  weight 
of  the  substance  liberated.  The  quantity  of 
electricity  which  passed,  then,  depended  on  the 
number  of  chemical  equivalents  of  substance 
liberated,  and  not  on  their  nature.  These  results 
led  to  a  definite  view  as  to  the  nature  of  the 
process  of  electrolysis.  We  must  regard  the 
passage  of  an  electric  current  through  a  solution 
as  due  to  the  carriage  by  moving  parts  of  the  salt 
of  opposite  electric  charges  in  opposite  directions 
through  the  liquid.  Under  the  influence  of 
applied  electric  forces,  these  carriers  drift  through 
the  solution,  and  finally  give  up  their  charges  to 
the  electrodes,  as  the  terminals  by  which  the 
current  enters  and  leaves  the  solution  are  called. 
With  common  salt,  for  example,  a  stream  of 
positively  electrified  sodium  drifts  with  the  electric 
current,  while  negatively  electrified  chlorine  passes 
in  the  opposite  direction.  The  moving  parts  of 
the  salt,  with  their  accompanying  electric  charges, 
were  named  Ions  by  Faraday  ;  the  positive  ion 
which  moves  down  the  electric  current  is  termed 
the  cation,  and  the  negative  ion  which  travels  up 
the  electric  stream  is  called  the  anion.  The 
electrodes  to  which  they  travel  are  known  as  the 
cathode  and  anode  respectively.  The  electric 
charge  on  a  single  ion  of  a  substance  like  sodium 


io8  PHYSICAL  SCIENCE 

or  chlorine  constitutes  a  true  natural  unit  of 
electricity.  No  smaller  quantity  seems  capable  of 
existing.  As  Helmholtz  has  insisted,  electricity, 
like  matter,  is  not  infinitely  divisible  ;  it  possesses 
an  atomic  structure. 

In  the  year  1855  Hittorf  examined  the  changes 
In  the  concentration  of  a  solution  which  occur  on 
the  passage  of  an  electric  current,  and  explained 
them  by  supposing  that  the  two  ions  moved  at 
unequal  rates.  It  is  evident  that  more  salt  will 
be  taken  from  that  end  of  the  solution  from  which 
comes  the  more  mobile  ion,  and,  on  the  assump- 
tion that  this  is  the  only  cause  at  work,  Hittorf 
calculated  the  ratio  between  the  velocities  of  the 
two  ions  in  many  cases. 

The  next  great  step  was  made  by  Kohlrausch, 
in  1873.  The  conductivity  of  a  solution  is 
measured  by  the  total  quantity  of  electricity 
which  passes  through  the  solution  per  second 
under  the  action  of  a  given  electric  force  ;  and, 
since  the  current  is  carried  by  the  motion  of 
charged  ions,  the  conductivity  must  depend  on 
the  number  of  the  ions,  that  Is,  on  the  con- 
centration of  the  solution,  and  on  the  velocity 
with  which  the  opposite  ions  move  through  the 
liquid.  Thus,  by  measuring  the  conductivity, 
the  velocities  of  the  ions  under  a  given  electric 
force  can  be  calculated. 

So  far  the  movement  of  the  Ions  was  visible 
to  the  mind's  eye  only.  Their  passage  through 
a  solution  seemed  necessary  to  explain  the  facts, 
and,  in  an  indirect  way,  their  velocities  could  be 
calculated,  but  no  direct  evidence  of  the  reality 
of  these  hypothetical  phenomena  was  forthcoming. 
However,  In  the  year    1886    Sir  Oliver   Lodge, 


THE  PROBLEMS  OF  SOLUTION 


109 


and  shortly  afterwards  by  a  somewhat  different 
method  the  present  writer,  showed  how  to 
render  these  molecular  processes  visible,  and  how 
to  watch  the  motion  of  the  ions  as  they  drift 
through  the  solution  under  the  action  of  the 
electric  forces. 

One  apparatus,  as  improved  by  Nernst,  for 
this  purpose  is  represented  in  Fig.  26. 
Let  the  solution  of  a  colourless  salt  be 
first  placed  in  the  tube  and  a  heavier 
coloured  solution  then  run  in  below,  so 
that  a  fairly  sharp  line  of  demarcation 
is  produced  between  them.  The  solu- 
tions should  be  of  the  same  molecular 
concentration,  the  same  conductivity, 
and  the  denser  solution  must,  of 
course,  be  placed  below  the  lighter. 
Let  us  take,  as  an  example,  the  case 
of  solutions  of  potassium  bichromate 
and  potassium  carbonate,  which  fulfil 
the  necessary  conditions.  The  colour 
of  the  former  salt  is  due  to  the  acid 
part,  the  bichromate  ion,  which  has 
the  chemical  composition  represented 
by  Cr207 ;  the  potassium  ion  is  colour- 
less. When  a  current  of  electricity  is 
passed  across  the  junction  between  the 
liquids,  the  colour  boundary  is  seen  to 
move,  and,  from  the  rate  at  which  it  creeps  along 
the  tube,  the  velocity  of  the  bichromate  ion  under 
a  given  electric  force  can  be  determined. 

The  conductivity  of  a  salt  solution,  made 
solid  by  the  addition  of  gelatine  or  some  similar 
substance,  is  nearly  the  same  as  that  of  the 
liquid   solution  without   the  jelly,  and   this    fact 


Fig.  26. 


no  PHYSICAL  SCIENCE 

justifies  the  use  of  such  solid  solutions  in  ex- 
periments on  the  migration  of  ions.  Lodge 
determined  the  velocity  of  the  hydrogen  ion  by 
watching  the  rate  at  which,  passing  along  a  glass 
tube,  it  changed  the  colour  of  an  indicator,  while 
the  present  writer  has  measured  the  velocity  of 
many  other  ions  by  tracing  the  formation  of 
opaque  precipitates,  formed  in  minute  quantity 
by  the  ions  in  their  path. 

These  methods  have  been  improved  and 
extended  by  Orme-Masson,  B.  D.  Steele,  G.  N. 
Lewis,  and  Lash  Miller.  The  result  of  the 
experiments  is  to  confirm  the  values  for  the  ionic 
velocities  calculated  from  the  theories  of  Kohl- 
rausch  and  Hittorf. 

The  velocities  with  which  the  ions  travel,  even 
when  driven  forward  by  intense  electric  forces, 
are  very  small.  Hydrogen,  the  most  mobile  ion 
known,  moves  over  a  distance  of  lo  centimetres, 
or  4  inches,  in  one  hour,  when  the  applied  electro- 
motive force  is  i  volt  per  centimetre.  Most 
other  ions  travel  at  about  one-tenth  this  rate. 

These  comparatively  small  velocities  must  not 
be  confounded  with  an  entirely  different  thing  : 
the  velocity  with  which  an  electric  impulse,  started 
at  one  end  of  a  tube  filled  with  an  electrolyte, 
reaches  the  other  end.  This  velocity  is  very 
great,  closely  approaching  the  rate  at  which  an 
electro-magnetic  wave  travels  through  free  space, 
that  is,  the  velocity  of  light,  about  180,000 
miles  a  second. 

If  we  accept  for  the  moment  the  common  con- 
ception of  an  electric  current  as  analogous  to  the 
flow  of  a  liquid  through  a  conducting  pipe,  the 


THE  PROBLEMS  OF  SOLUTION  iii 

connection  between  the  two  modes  of  motion  may 
be  illustrated  by  a  familiar  example.  Suppose 
that  a  long  wooden  rod  is  lying  on  the  surface  of 
the  ground,  and  that  a  push  is  given  to  one  end  of 
it.  The  motion  of  the  rod  may  be  quite  slow,  an 
inch  an  hour  if  we  like.  But,  after  moving  one 
end,  the  other  end  begins  to  move  an  extremely 
minute  fraction  of  a  second  after  the  starting  of 
the  impulse.  Perhaps  it  never  has  occurred  to 
us  that  any  appreciable  time  elapses  between  the 
starting  of  the  two  ends.  Yet,  if  we  think  for  a 
moment,  it  is  clear  that  the  initial  push  must  travel 
as  a  wave  of  compression  along  the  rod,  and  that 
the  far  end  can  only  begin  to  move  when  the  wave 
front  reaches  it.  The  bearing  of  the  analogy  is 
now  obvious.  The  slow  movement  of  the  rod  as 
a  whole  when  once  started  corresponds  with  the 
slow  drift  of  the  ions  ;  the  almost  instantaneous 
passage  of  the  wave  of  compression  along  the  rod 
corresponds  with  the  velocity  of  electricity  in  the 
electrolytic  solution. 

A  picture  of  the  phenomena,  more  nearly 
corresponding  with  the  facts,  is  obtained  by 
considering  that  the  rapid  electric  impulse  travels 
as  an  electric  wave  through  the  surrounding 
insulating  medium.  On  this  view,  due  to  Faraday 
and  Maxwell,  and  now  universally  accepted,  the 
electric  forces  always  travel  through  the  medium. 
When  thev  act  on  electric  charofes  free  to  move, 
as  in  metallic  conductors,  or  on  charges  attached 
to  matter  as  in  electrolytic  solutions,  they  produce 
a  drift  of  the  charges — a  drift  which  constitues  a 
current.  Along  the  line  of  the  drift,  that  is,  along 
a  conductor,  energy  is  lost,  and  thus  along  that 
line,  and  there  alone,  energy  is  constantly  flowing. 


112  PHYSICAL  SCIENCE 

being  carried  forward  by  the  medium  to  supply  the 
place  of  the  energy  dissipated  by  the  current. 

The  mobility  of  any  one  ion  is,  in  dilute 
solutions,  independent  of  the  nature  of  the  other 
ion  present,  at  all  events  in  simple  salts,  such  as 
the  chlorides  of  sodium,  potassium,  and  lithium. 
This  independence  itself  indicates  that  the  ions 
are  free  from  each  other,  and  again  suggests 
some  form  of  dissociation. 

The  phenomena  of  conductivity  also  point  to 
the  same  idea.  To  set  free  an  ion  or  its  products 
at  the  electrodes  requires  the  expenditure  of  a 
certain  amount  of  electric  work,  and  at  the 
electrodes  an  equivalent  reverse  electro-motive 
force  exists.  When,  however,  this  reverse  force 
is  overcome,  the  passage  of  the  current  through 
the  solution  is  opposed  by  no  other  reversible 
forces,  and  it  is  found  that  the  work  expended 
is  that  required  to  force  the  current  against  the 
frictional  resistance  of  the  electrolyte  alone.  The 
current  is  proportional  to  the  excess  of  the  electric 
force  applied  beyond  what  is  needed  to  overcome 
the  effect  at  the  electrodes  ;  this  part  of  the  con- 
duction conforms  to  Ohm's  law,  which  describes 
the  process  in  metallic  conductors.  In  the  body 
of  the  solution,  then,  as  distinct  from  the  transition 
layer  in  contact  with  the  electrodes,  the  electric 
forces  do  no  reversible  work,  such  as  would  be 
needed  to  separate  the  ions  from  each  other. 
Whatever  freedom  is  requisite  between  the  ions 
for  the  purpose  of  conduction,  must  necessarily 
exist  whether  the  electric  forces  act  or  not ;  the 
function  of  the  electric  forces  when  applied  is 
simply  to  force  the  ions,  already  separated  from 


THE  PROBLEMS  OF  SOLUTION         113 

each  other,  against  the  frictional  resistance  of  the 
liquid  medium.  A  certain  freedom  of  interchange, 
at  all  events,  is  thus  indicated  between  the  ions, 
and  the  freedom  of  interchange  exists  whether  the 
current  passes  or  not.  Such  freedom,  indeed,  had 
been  inferred  long  ago  from  the  phenomena  of 
double  decomposition  observed  in  the  chemical 
reactions  between  solutions  of  different  salts. 

So  far  the  conductivity  relations  indicate  the 
possibility  of  ionic  interchange  between  the  parts 
of  the  dissolved  molecules,  though  the  conformity 
of  solutions  with  Ohm  s  law  does  not,  of  itself, 
necessitate  the  idea  of  permanent  ionic  freedom. 
But  on  any  other  view  the  possibility  of  inter- 
change must  be  secured  by  collisions  between  the 
dissolved  molecules,  and  consequent  interchanges 
between  their  ions,  which  would  thus  work  their 
way  through  the  solution  by  a  series  of  such 
collisions.  The  velocity  with  which  this  process  is 
effected  must  depend  on  the  frequency  of  collision, 
which  would  be  proportional  to  the  square  of  the 
concentration.  The  ionic  velocities,  then,  on  this 
supposition,  would  increase  in  proportion  to  the 
square  of  the  concentration  of  the  solution,  and  the 
conductivity,  which  depends  on  the  product  of  the 
ionic  velocities  and  the  concentration,  would  vary 
as  the  cube  or  third  power  of  the  concentration. 

But  the  facts  are  quite  inconsistent  with  this 
hypothesis.  The  conductivity  is  proportional  at 
the  most  to  the  first  power  of  the  concentration  ; 
and  the  ionic  velocities,  instead  of  increasing  as 
the  square,  are,  in  dilute  solution,  independent 
of  the  concentration,  and  in  more  concentrated 
solutions  decrease  with  increasing  concentration. 
Thus  again  we  are  driven  to  the  belief  that  the 


114  PHYSICAL  SCIENCE 

ions  are  free  from  each  other,  and  move  in- 
dependently of  each  other  through  the  Hquid 
under  an  electric  force :  free  from  union  with 
each  other,  let  us  observe,  not  necessarily  free 
from  combination,  chemical  or  other,  with  the 
solvent.  As  already  indicated,  the  dissociation 
theory  does  not  depend  on  any  particular  view 
as  to  the  nature  of  solution  in  general. 

For  aqueous  solutions,  then,  the  evidence 
in  favour  of  the  dissociation  hypothesis  is  very 
strong,  and  it  can  safely  be  used  as  a  working 
hypothesis  to  co-ordinate  the  known  phenomena, 
and  to  guide  future  research.  For  solutions  in 
other  solvents,  less  evidence  is  yet  available ; 
though  for  solutions  of  certain  salts  in  alcohol, 
the  laws  of  the  electrolysis  seem  to  be  similar 
to  those  of  aqueous  solutions  and  to  indicate 
a  similar  theory.  In  fused  salts,  which  also 
conduct  electricity  and  suffer  chemical  decom- 
position at  the  electrodes,  the  conditions  are 
perhaps  different,  and  we  must  wait  for  further 
light  before  we  can  profitably  theorise  about  the 
nature  of  the  conduction  process. 

« 

Besides  explaining  the  electrical  and  osmotic 
properties  of  solutions,  the  dissociation  theory, 
in  the  domain  of  chemistry,  has  proved  one  of 
the  most  fruitful  generalisations  that  has  ever 
been  formulated.  Solutions  of  salts  and  acids, 
electrolytes  in  fact,  are  the  solutions  which 
exhibit  chemical  activity  In  the  highest  degree. 
In  them,  the  ions  alone  are  concerned  in  chemical 
action,  and  so  clearly  is  this  the  case,  that,  as 
soon  as  the  subject  is  examined,  the  ordinary 
chemical  tests  for  the  presence  of  salts  are  seen 


THE  PROBLEMS  OF  SOLUTION         115 

at  once  to  be,  in  reality,  tests  for  the  individual 
ions  of  those  salts.  At  one  time  it  seemed  likely 
that  all  cases  of  rapid  chemical  action  might  be 
reduced  to  reactions  between  electrolytic  ions, 
but  experiments  by  Kahlenberg  and  others  seem 
to  show  that  in  non-aqueous  solvents  rapid 
reactions  may  occur  not  in  any  way  correlated 
with  electrolytic  conductivity.  However  this 
may  be,  in  water  many  chemical  actions  are 
certainly  connected  in  a  very  intimate  way  with 
the  electrical  properties,  and  the  dissociation 
theory  gives  a  satisfactory  method  of  co-ordinating 
the  two  sets  of  properties.  In  some  reactions  the 
actual  electric  charges  on  the  ions  seem  to  be  the 
determining  factors  of  the  whole  process. 

There  is  a  marked  difference  in  chemical  and 
physical  properties  between  bodies  of  definite  crys- 
talline form,  such  as  most  inorganic  salts,  and  soft 
or  amorphous  substances,  such  as  albumen  and  the 
various  kinds  of  jelly.  Long  ago  Graham  distin- 
guished the  two  groups  as  crystalloids  and  colloids 
respectively,  and  particularly  examined  them  with 
regard  to  their  relative  powers  of  diffusion  through 
water.  He  found  that,  while  crystalloids  diffuse 
comparatively  rapidly,  the  motion  of  colloids  is 
so  slow  that  it  is  often  almost  inappreciable. 

Many  different  kinds  of  chemical  compounds 
show  colloidal  properties.  Besides  a  vast  number 
of  animxal  and  vegetable  substances,  some  of  which 
are  of  fundamental  importance  in  the  phenomena 
of  living  matter,  many  of  the  precipitates  which 
are  formed  in  the  course  of  inorganic  chemical 
reactions  appear  in  an  amorphous  or  colloidal 
state.     The  sulphides  of  such  metals  as  antimony 


ii6  PHYSICAL  SCIENCE 

and  arsenic  are  good  examples.  If  a  solution  of 
arsenious  acid  be  allowed  to  flow  into  water  kept 
saturated  with  sulphuretted  hydrogen  by  means 
of  a  current  of  that  gas,  a  colloidal  hydrosulphide 
is  formed.  Many  hydrates,  too,  are  colloids, 
ferric  hydrate,  for  instance,  which  can  readily  be 
prepared  from  the  corresponding  salts  of  iron. 
By  treating  dilute  solutions  of  gold  chloride  with 
reducing  agents,  such  as  a  few  drops  of  a  solution 
of  phosphorus  in  ether,  the  gold  is  set  free  in 
the  colloidal  condition,  forming  a  ruby-coloured 
solution.  Silver,  bismuth,  and  mercury  can  also 
be  obtained  in  colloidal  solution. 

Crystalloids  diffuse  much  more  rapidly  through 
water  and  other  solvents  than  do  colloids.  If 
a  mixture  of  crystalloids  and  colloids  be  placed 
in  a  drum  covered  with  a  colloidal  membrane, 
such  as  bladder  or  parchment,  complete  separa- 
tion can  be  effected,  for  the  dissolved  colloids 
seem  quite  incapable  of  passing  through  such 
membranes.  This  process  probably  plays  a  great 
part  in  animal  and  vegetable  physiology. 

Solutions  of  colloids  in  crystalloid  solvents, 
such  as  water  or  alcohol,  seem  to  be  divisible  into 
two  classes.  Both  classes  appear  to  mix  with 
warm  water  in  all  proportions,  and  the  mass  will 
solidify  under  certain  conditions  to  form  a  solid 
which  may  be  called  a  gel.  One  class,  represented 
by  gelatine  and  agar  jelly,  will,  when  solidified,  re- 
dissolve  on  warming  or  dilution,  while  the  other 
class,  containing  such  substances  as  hydrated 
silica,  albumen,  and  metallic  hydro-sulphides,  will, 
under  the  influence  of  heat  or  on  the  addition  of 
electrolytes,  form  gels  which  cannot  be  redissolved. 
The  solidification  of  members  of  the  first  class  into 


THE  PROBLEMS  OF  SOLUTION         117 

redlssolvable  substances  is  termed  setting,  that 
of  substances  in  the  second  class,  which  form  in- 
soluble precipitates,  is  termed  coagulation. 

The  mechanism  of  gelation  in  the  first,  or 
reversible  class  of  colloidal  systems,  has  been 
studied  experimentally  by  Van  Bemmelen  and  by 
W.  B.  Hardy.  The  process  of  solidification  seems 
to  consist  in  the  growth  of  a  solid  framework 
containing  more  liquid  portions.  The  tempera- 
ture at  which  this  separation  into  two  phases 
occurs  depends  on  the  amount  of  water  present. 

The  coagulation  of  irreversible  colloidal 
solutions,  as  already  stated,  can  be  effected  by 
the  addition  of  small  quantities  of  the  solution 
of  an  electrolyte,  such  as  an  ordinary  salt  or  acid. 
Graham,  who  originally  investigated  the  subject, 
found  that  a  minute  trace  of  salt  was  often 
sufficient.  Thus,  hydrated  alumina,  prepared  from 
a  solution  of  the  chloride  in  distilled  water,  was  so 
unstable  that  a  few  drops  of  well-water  produced 
coagulation,  and  the  same  change  was  brought 
about  by  pouring  the  colloidal  solution  into  a  new 
glass  vessel,  unless  the  vessel  had  previously 
been  washed  repeatedly  with  distilled  water. 

Several  experimenters,  including  Schulze, 
Linder  and  Picton,  and  Hardy,  have  investigated 
this  coagulative  power  of  electrolytes,  with  very 
curious  and  interesting  results.  The  coagulative 
power  of  a  salt  is  found  to  vary  in  a  remarkable 
manner  with  the  chemical  valency  of  one  of  its 
ions.^     The  average  of  the  coagulative  powers  of 

^  The  valency  of  a  chemical  atom  may  be  defined  as  the  number 
of  hydrogen  atoms  it  will  combine  with  or  replace.  Thus  the 
normal  valency  of  oxygen  is  two,  since  two  hydrogen  atoms  unite 
with  one  oxygen  atom  to  form  water.  Faraday's  work  showed  that 
the  electric  charge  carried  by  an  ion  is  proportional  to  its  valency. 


ii8  PHYSICAL  SCIENCE 

salts  of  univalent,  divalent,  and  trivalent  metals 
were  found  to  be  proportional  to  the  numbers 
1:35:  1023  respectively.  Most  properties  which 
depend  on  the  valency  vary  in  the  ratios  i  :  2  :  3, 
and  the  great  difference  in  the  numbers  now 
under  consideration  is  very  striking.  An  attempt 
at  a  preliminary  explanation  of  these  unusual 
relations  has  been  made  by  the  present  writer. 

Let  us  frame  a  mental  picture  of  a  solution  as 
it  is  represented  by  the  dissociation  theory.  A 
certain  number  of  the  dissolved  molecules  are 
regarded  as  dissociated  into  charged  ions,  which 
wander,  free  from  each  other,  through  the  liquid, 
perhaps  by  successive  combinations  with  solvent 
molecules  in  their  path.  When  an  electric  force 
is  applied,  though  still  moving  sometimes  in  one 
direction  and  sometimes  in  another,  the  ions,  on 
the  whole,  drift  in  the  direction  indicated  by  the 
force,  and  we  may  imagine,  therefore,  that  two  pro- 
cessions of  oppositely  charged  ions  pass  each  other, 
drifting  in  opposite  directions  through  the  solution. 

When  there  is  no  electric  force,  the  ions  are 
subject  to  no  steady  drift,  and  must  move  some- 
times in  one  direction,  sometimes  in  another,  as 
the  chances  of  their  life  direct.  Any  one  ion  will 
be  passing  sometimes  from  one  solvent  molecule 
to  another,  carrying  its  electric  charge  with  it ; 
sometimes  it  will  come  across  an  ion  of  the  opposite 
kind  in  such  a  way  that  combination  occurs,  and, 
for  a  time,  an  electrically  neutral  molecule  is  formed. 
By  collisions  of  unusual  violence,  or  by  other 
means,  soon  this  molecule  will  be  dissociated,  and 
its  ions  again  set  free  from  each  other,  to  be  handed 
backwards  and  forwards  by  the  solvent  molecules 
as  already  described. 


THE  PROBLEMS  OF  SOLUTION         119 

Let  us  suppose  that,  in  order  to  produce  the 
aggregation  of  colloidal  particles  which  constitute 
coagulation,  a  certain  minimum  electric  charge  has 
to  be  brought  within  reach  of  a  colloidal  group, 
and  that  such  conjunctions  must  occur  with  a 
certain  minimum  frequency  throughout  the  solu- 
tion. Since  the  electric  charge  on  an  ion  is 
proportional  to  its  valency,  we  shall  get  equal 
charges  by  the  conjunction  of  211  triads,  37^  diads, 
or  6n  monads,  where  n  is  any  whole  number. 

The  chance  conjunctions  of  a  large  number 
of  particles  moving  like  the  ions  of  an  electrolytic 
solution  can  be  investigated  by  the  principles  of 
the  kinetic  theory  of  gases.  If  ifx  denote  the 
chance  of  one  ion  colliding  with  a  colloidal  particle, 
the  chance  that  two  ions  should  collide  with  it  is 
the  product  of  their  separate  chances,  or  \\x^,  and 
so  on.  When  applied  to  the  case  in  hand,  these 
principles  lead  to  the  conclusion  that  the  relative 
coagulative  powers  of  univalent,  divalent,  and 
trivalent  ions  will  be  proportional  to  the  ratios 
I  \  n  \  n^.  The  value  of  n,  which  depends  on  a 
number  of  unknown  factors,  remains  arbitrary. 
If  we  assume  that  n  is  32,  n^  is  1024,  and  we  get 
the  numbers  i  :  32  :  1024  to  compare  with  the 
experimental  values  of  the  relative  coagulative 
powers  I  :  35  :  1023. 

This  theoryis,  of  course,  only  a  first  approxima- 
tion. It  takes  no  account  of  the  action  of  the  other 
ion,  or  of  differences  in  the  effect  of  different  ions 
of  the  same  valency.  Experiments  by  Oden  on 
colloidal  sulphur  show  these  differences  to  a  degree 
that  in  some  instances  masks  the  effect  of  valency. 
Butthisextremespecific  effecthas  not  been  found  in 
any  other  case,  and  it  seems  that  the  simple  theory 


I20  PHYSICAL  SCIENCE 

given  above  supplies  a  foundation  on  which  a  more 
detailed  explanation  may  some  day  be  built. 

The  particles  in  solutions  of  colloids  in  water 
generally  move  slowly  when  acted  on  by  electric 
forces,  the  direction  of  motion  depending  on  the 
nature  of  the  colloid  and  on  that  of  the  solvent. 
Hardy  found  that  the  direction  of  movement  of 
certain  proteins  could  be  changed  by  changing 
the  solvent  from  a  very  dilute  acid  to  a  very  dilute 
alkali.  This  reversal  implied  a  change  in  the  sign 
of  the  charges  on  the  colloid  particles  ;  and,  if 
the  solvent  was  very  carefully  neutralised,  an  iso- 
electric point  was  reached  at  which  the  solution 
became  very  unstable,  and  coagulation  seemed 
to  occur  spontaneously.  The  same  observer  also 
found  that,  in  the  case  of  colloids  travelling  with 
the  current,  it  is  the  acid  ion  which  is  active  in 
causing  coagulation,  and  not  the  metallic  ion  as 
in  the  work  of  the  older  experimenters,  who 
all  used  colloids  which  travel  against  the  electric 
current.  Thus  it  is  always  the  ion  possessing  a 
chargeof  oppositekindto  that  onthecolloid  particle 
which  is  effective  in  producing  coagulation. 

Burton  has  found  a  similar  change  in  velocity 
in  an  electric  field  when  to  a  colloidal  solution  of 
silver  increasing  amounts  of  aluminium  sulphate 
are  added.  The  velocity  of  the  silver  decreases, 
and  vanishes  at  or  near  the  coagulating  point. 
With  more  aluminium,  coagulation  is  prevented 
for  a  time,  and  the  unstable  colloid  moves  in  the 
opposite  direction,  showing  that  its  electric  charge 
has  been  reversed  by  the  absorption  of  excess  of 
aluminium  ions. 

These  results  are  of  great  importance  from 
the   point  of  view  of  physiology,    and   also   as 


THE  PROBLEMS  OF  SOLUTION         121 

throwing-  light  on  the  nature  of  colloid  solution 
— perhaps,  indeed,  of  solution  in  general.  It  looks 
as  though  colloid  particles,  at  any  rate,  could  exist 
in  solution  only  when  charged  electrically.  If,  by 
the  conjunction  of  more  mobile  ions,  their  charge  is 
neutralised,  or  perhaps  reduced  to  a  critical  value, 
an  iso-electric  point  is  reached,  and  coagulation 
must  immediately  follow. 

It  is  probable  that  these  effects  depend  on 
changes  in  the  surface  of  separation  between  the 
colloidal  particles  and  the  more  liquid  phase  which 
surrounds  them.  Such  a  surface  of  separation 
must  exhibit  thewell-known  phenomena  of  surface- 
tension,  and  will  possess  an  amount  of  available 
energy  proportional  to  its  area,  which  therefore 
tends  to  become  as  small  as  possible.  A  number 
of  separate  particles  would,  in  these  conditions, 
tend  to  coagulate  into  larger  ones,  just  as  small 
raindrops  tend  to  coalesce  into  larger  ones.  If 
the  colloidal  particles  are  electrified,  the  electric 
energy  is  greater  when  the  charge  is  concentrated 
on  a  small  area,  and,  on  this  account,  the  area  will 
tend  to  increase.  The  effect  of  the  electric  charge 
is  thus  opposite  to  that  of  the  natural  surface- 
tension,  and  diminishes  the  tendency  to  coagulate. 
Thus  an  electric  charge  may  enable  the  colloid 
to  dissolve,  while  neutralisation  of  the  charge  may 
result  in  coagulation. 

Much  discussion  has  taken  place  about  the 
nature  of  liquid  colloidal  solutions,  and  their 
relations  with  ordinary  solutions  of  mineral  salts 
and  other  crystalloids.  They  may  either  be 
regarded  as  ordinary  solutions,  in  which  the 
dissolved  particles  are  similar  in  kind  to  those 
of  crystalloid  solutions,  though  of  much  higher 


122  PHYSICAL  SCIENCE 

molecular  weight,  or  they  may  be  considered  to  be 
systems  of  two  phases,  composed  of  suspensions  of 
particles  In  the  liquid,  the  particles  being  different 
in  kind  from  the  liquid,  and  of  much  greater  than 
molecular  dimensions. 

In  some  colloid  solutions  the  presence  of  sus- 
pended particles  can  be  detected  readllyby ordinary 
means.  Sometimes  they  are  visible  under  a  good 
microscope  ;  In  other  cases,  while  too  small  to  be 
directly  visible,  they  are  large  enough  to  scatter 
and  polarise  a  beam  of  light.  This  means  that  their 
size  must  be  comparable  with  the  wave-length  of 
light,  about  5  x  io~^  cm.  Such  particles  would  be 
too  few  in  number  to  exert  a  measurable  osmotic 
pressure,  and  the  absence  of  such  pressure  does 
not  necessarily  mean  that  solutions  of  colloids  are 
different  in  kind  from  solutions  of  crystalloids. 

It  is  worthy  of  note  that  turbid  suspensions 
of  clay,  kaolin,  etc..  In  water  are  rapidly  cleared 
by  the  addition  of  small  quantities  of  metallic 
salts.  This  action,  which  is  almost  certainly  of 
the  same  nature  as  the  coagulation  described 
above,  probably  helps  in  the  formation  of  sand- 
banks at  the  mouths  of  rivers  ;  the  salts  of  the 
sea-water  clear  the  suspensions  of  clay  brought 
down  with  the  fresh  water,  and  precipitation  is 
then  aided  by  the  diminished  velocity. 

The  conditions  which  determine  the  colloid  or 
crystalloid  nature  of  a  substance  are  still  not  fully 
understood.  The  persistence  of  colloid  properties, 
when  a  substance  passes  from  the  dissolved  to  the 
non-dissolved  state,  shows  that  the  determining 
conditions  must  be  of  fundamental  importance. 
The  molecular  forces  seem  to  be  much  less  active 
in  colloids,  but  the  freedom  with  which  some  of 


THE  PROBLEMS  OF  SOLUTION         123 

them  disintegrate  and  dissolve  in  presence  of 
water  and  other  liquids  indicates  that  some  inter- 
action between  them  and  their  solvent  must  occur. 
It  seems  likely  that  the  forces  which  are  involved 
in  crystalloid  solution  are  of  the  nature  of  those 
classed  as  chemical  or  molecular,  while,  when 
colloids  dissolve,  the  actions  between  solvent  and 
solute  are  conditioned  also  by  the  phenomena 
studied  under  the  names  of  capillarity  and  surface 
tension.  It  is  not  likely  that  any  sharp  line  of 
demarcation  can  be  drawn  ;  though,  as  the  size 
of  the  dissolved  particles  increases,  the  importance 
of  the  chemical  forces  probably  diminishes,  and 
that  of  the  capillary  forces  grows. 

If  colloid  and  crystalloid  solution  are  but 
the  extreme  limits  of  a  continuous  series  of 
phenomena,  the  study  of  dissolved  colloids  of 
varying  degrees  of  aggregation  should  throw 
much  light  on  the  general  problem  of  the  funda- 
mental nature  of  solution. 

A  study  of  the  colloidal  state  is  primarily  the 
affair  of  physics  and  chemistry.  But  that  study  has 
led  to  many  technical  applications,  as,  for  instance, 
in  dyeing,  of  great  industrial  importance.  More- 
over, colloids  play  a  supreme  part  in  the  phenomena 
of  living  matter.  Protoplasm,  the  material  basis  of 
life  which  fills  all  living  cells,  is  essentially  a  colloid, 
and  in  physiology  and  biochemistry  colloidal 
problems  continually  arise.  Again,  the  soil  of  our 
fields,  so  simple  to  the  eyes  of  the  pioneers  in 
agricultural  chemistry,  is  now  known  to  be  a 
complex  containing  many  colloids,  with  a  flora 
and  fauna  of  its  own.  But  this  Is  not  the  place 
to  follow  further  these  fascinating  developments — 
the  physics  of  colloids  contains  enough  of  interest. 


124  PHYSICAL  SCIENCE 

The  explanation  of  the  coagulation  of  colloidal 
solutions  as  an  effect  on  the  surface  conditions  at 
the  junction  between  colloid  and  solvent,  brought 
about  by  the  chance  conjunctions  of  dissociated 
electric  ions,  is  an  illustration  of  a  course  of 
history  which  indeed  constantly  repeats  itself 
in  scientific  inquiry.  An  observation  is  made, 
perhaps  long  series  of  experiments  are  carried 
out,  before  the  general  state  of  knowledge  enables 
a  satisfactory  explanation  of  the  phenomena  to 
be  formed,  or  a  theoretical  co-ordination  of  them 
with  other  phenomena  to  be  traced.  Even 
Graham's  acute  and  powerful  mind,  in  the  absence 
of  the  dissociation  theory  of  electrolytes,  and  of 
the  knowledge  of  the  surface  relations  of  two 
phases  which  we  now  possess,  could  frame  no 
explanation  of  the  coagulation  effects  which  he 
examined  with  such  skill.  By  experiments  on 
coagulation  alone  it  is  probable  that  an  explana- 
tion could  never  have  been  reached.  But  by  the 
advance  of  other  observers,  led  by  Gibbs  on  one 
far-off  flank,  and  by  Van't  Hoff  and  Arrhenius 
on  the  other,  almost  out  of  touch  with  the 
original  attack,  the  position  of  the  adversary — 
ignorance — was  turned  ;  and  when,  at  a  later 
time,  a  new  frontal  assault  was  made,  the  way 
lay  open  to  an  approximate  theory,  and  probably 
in  the  future  will  lead  to  a  complete  explanation. 

For,  while  the  tired  waves,  vainly  breaking, 

Seem  here  no  painful  inch  to  gain. 
Far  back,  through  creeks  and  inlets  making. 

Comes,  silent,  flooding  in,  the  main. 

And  not  by  eastern  windows  only, 

When  daylight  comes,  comes  in  the  light ; 

In  front  the  Sun  climbs  slow,  how  slowly  ! 
But  westward,  look  !  the  land  is  bright. 


?■? 


J  K.cv**-^-a^^ 


[To  face  page  125. 


CHAPTER    VI 

THE    CONDUCTION    OF    ELECTRICITY    THROUGH 

GASES 

"  It  is  difficult  to  think  of  a  single  branch  of  the  physical  sciences 
in  which  these  advances  are  not  of  fundamental  importance.  .  .  . 
The  physicist  sees  the  relations  between  electricity  and  matter  laid 
bare  in  a  manner  hardly  hoped  for  hitherto.  .  .  .  But  it  is  the 
philosopher  that  these  researches  will  affect  most  profoundly.  As 
much  by  the  aid  of  a  perfect  mastery  over  the  properties  of  materials 
as  by  the  sheer  intellectual  power  of  abstract  reasoning,  some 
of  the  fundamental  problems  of  the  constitution  of  matter  are  here 
presented  as  on  the  verge  of  solution." — Times^  22nd  January  1904. 

Unlike  the  liquid  solutions  and  other  electrolytes 
studied  in  the  last  chapter,  gases,  in  normal 
conditions,  are  almost  perfect  insulators  of  elec- 
tricity. Telegraph  wires  are  insulated  by  the  air 
which  surrounds  them,  and,  if  leakage  occurs  to 
any  measurable  extent,  it  can  always  be  traced  to 
the  solid  supports  to  which  the  wires  are  attached. 
Nevertheless,  by  delicate  instruments,  a  slight 
leakage  of  electricity  through  air  can  be  detected. 
This  air  leakage  is  usually  extremely  small,  but  it 
can  be  increased  greatly  in  many  ways.  The 
passage  of  Rontgen  rays,  the  incidence  of  ultra- 
violet light  on  a  metal  plate,  the  neighbourhood 
of  flames,  incandescent  metals,  or  of  radio-active 
bodies  such  as  radium,  are  among  the  agencies 
whereby  the  condition  of  the  surrounding  air  is 
modified  so  that  it  can  rapidly  conduct  away  the 
electric  charge. 

In  general,  the  currents  through  gases  are  too 

125 


1-26  PHYSICAL  SCIENCE 

small  to  be  investigated  by  means  of  a  galvano- 
meter. By  the  aid  of  an  electrometer,  however, 
or  by  the  use  of  some  form  of  gold  leaf  electro- 
scope, the  passage  of  electricity  may  be  detected, 
and  the  amount  of  the  current  determined. 

The  quadrant  electrometer  consists  of  a  light 
but  rigid  strip  of  aluminium  or  silvered  paper, 
suspended  horizontally  by  a  fine  quartz  fibre. 
This  strip  is  kept  permanently  charged  with 
electricity,  and  is  therefore  deflected  when  other 
charges  are  given  to  brass  quadrants  which 
surround  it.  By  the  rate  at  which  the  deflection 
diminishes,  it  is  possible  to  estimate  the  rate  at 
which  the  charge  on  the  quadrants,  and  on  any 
conductor  connected  with  them,  disappears  or 
increases. 

Still  simpler  and  yet  more  sensitive  is  the  gold 
leaf  electroscope,  in  which  a  thin  strip  of  gold  leaf 
is  attached  to  a  brass  plate,  and  charged  with 
electricity.  Owing  to  the  repulsive  forces  between 
portions  of  the  same  charge,  the  gold  leaf  is 
repelled  from  the  plate  and  stands  out  at  an 
angle.  By  observing  through  a  microscope  the 
rate  at  which  the  leaf  falls,  we  can  determine  the 
rate  at  which  its  charge  leaks  away. 

Whichever  apparatus  be  adopted,  the  natural 
leak,  due  to  the  apparatus  itself  and  the  air 
surrounding  it,  must  first  be  determined,  and 
subtracted  from  the  leakage  afterwards  found 
under  the  influence  of  an  ionizing  agency. 

In  the  last  chapter  we  have  seen  that  the 
properties  of  conducting  solutions  have  been 
successfully  co-ordinated  and  explained  on  the 
hypothesis  that  the  passage  of  a  current  is  effected 
by  the  motion  of  charged  particles   called  ions. 


CONDUCTION  THROUGH  GASES       127 

A  similar  supposition  has  been  adopted  to  explain 
the  conductivity  of  gases,  although  it  will  be  clear 
that,  in  many  respects,  the  ions  in  the  case  of 
electric  discharge  through  gases  must  be  endowed 
with  properties  different  from  those  which  pertain 
to  the  ions  of  liquid  solutions. 

After  a  period  of  activity  on  the  part  of 
some  ionizing  agency,  such  as  Rontgen  rays, 
the  resultant  conductivity  does  not  cease  simulta- 
neously with  the  action  of  the  rays.  It  persists 
for  some  little  time ;  it  can  be  blown  about  with 
currents  of  air  ;  and  in  all  respects  acts  as  though 
it  were  due  to  the  presence  of  material  particles, 
formed  somehow  in  the  gas  through  which  the 
rays  had  passed.  The  conductivity  is  destroyed 
if  the  gas  be  passed  through  a  plug  of  glass  wool 
or  bubbled  through  water  ;  it  is  also  removed  if 
the  gas  be  subjected  to  the  action  of  an  electric 
field.  Such  experiments,  and  many  others  of 
somewhat  similar  nature,  are  readily  explained 
by  the  conception  of  charged  particles,  which, 
produced  in  some  way  by  the  action  of  the 
ionizing  agency  on  the  molecules  of  the  gas,  are 
afterwards  driven  through  the  gas  by  an  electric 
force,  just  as  the  ions  of  a  salt  solution  are  driven 
through  the  liquid.  Unlike  the  ions  of  liquids, 
however,  those  of  gases  do  not  long  persist  after 
the  cessation  of  the  outside  ionizing  agency. 
Left  to  themselves,  the  ions  gradually  disappear. 
Such  a  disappearance  might  be  anticipated  on 
the  view  that  the  opposite  ions  recombine  and 
neutralise  each  other,  and  also  on  the  assumption 
that  they  give  up  their  charges  to  the  solid  objects 
with  which  they  come  in  contact  as  they  move 
about  under  their  own  motions  of  diffusion,  and 


128  PHYSICAL  SCIENCE 

that  they  are  driven  towards  an  electrode  by  the 
action  of  an  electric  force. 

The  non-persistence  of  gaseous  ions  and  the 
consequent  need  of  their  perpetual  renewal  explains 
the  relation  between  current  and  electro-motive 
force — a  relation  different  from  that  observed  in 
liquid  solutions.  In  solutions,  as  we  saw,  the 
conduction  conforms  to  Ohm's  law — the  current 
is  proportional  to  the  electro-motive  force.     In 


Electromotive    Force 
Fig.  27. 

gases  this  Is  not  the  case.  For  an  ionizing  agency 
of  constant  intensity,  such  as  a  layer  of  oxide  of 
uranium,  the  current  at  first  rises  with  the  applied 
electro-motive  force,  but  soon  it  tends  towards  a 
limit,  and  finally  reaches  a  maximum,  when,  till 
we  approach  the  sparking  point,  no  further  increase 
of  electro-motive  force  will  produce  any  appreci- 
able increase  of  current.  This  saturation  current, 
as  it  is  called,  is  represented  by  the  horizontal  part 
of  the  curve  in  Fig.  27.     Obviously  it  corresponds 


CONDUCTION  THROUGH  GASES        129 

to  a  state  in  which  all  the  ions  are  removed  to 
the  electrodes  as  fast  as  they  are  produced  by  the 
ionizing  agency. 

As  the  sparking  point  is  approached,  the  curve 
shows  that  the  current  again  rises  rapidly  ;  the 
applied  electric  force  being  strong  enough  to 
produce  ions  in  the  gas  by  its  own  action. 
Townsend  has  shown  that  this  process  is  effected 
by  the  collision  with  the  gas  molecules  of  ions 
already  present,  which  are  driven  forward  by 
the  electric  force  with  high  velocity.  In  this 
way  are  formed  most  of  the  ions  which  carry 
the  current  in  an  electric  spark,  or  in  the  arc 
discharge. 

We  have  described  already  the  methods  of 
calculating  the  velocities  with  which  the  ions  of 
liquids  move  under  known  electric  forces,  and  of 
determining  those  velocities  by  direct  experiment. 
For  gaseous  ions,  the  corresponding  velocities 
are  much  higher.  They  have  been  determined 
in  several  indirect  ways,  with  concordant  results. 
For  instance,  Zeleny  measured  the  electric  force 
required  to  push  an  ion  against  a  stream  of  gas, 
moving  with  a  known  and  uniform  velocity  in  the 
opposite  direction  to  the  natural  motion  of  the 
ion.  Langevin,  in  1902,  attacked  the  problem  in 
another  way.  The  gas  between  two  parallel  elec- 
trodes was  exposed  momentarily  to  the  action  of 
Rontgen  rays.  The  ions  thus  produced  may  dis- 
appear in  two  ways.  Opposite  ions  may  recombine 
with  each  other,  or  they  may  pass  to  the  electrodes 
under  the  influence  of  an  electric  force.  If  the 
force  be  great,  the  latter  method  alone  is  operative, 
the  number  of  ions  recombining  before  reaching 
the  electrodes    being   very  small.     If,   then,   the 

K 


130  PHYSICAL  SCIENCE 

electric  field  be  kept  acting  in  one  direction,  all 
the  positive  ions  produced  by  the  Rontgen  rays 
will  go  to  one  electrode,  and  all  the  negative  ions 
to  the  other.  But  if  the  electric  force  be  reversed 
before  all  the  ions  get  across,  the  charge  received 
by  an  electrode  would  be  less  than  before.  Thus, 
measurement  of  the  charges  received  by  the  elec- 
trodes with  different  speeds  of  reversal  will  give  a 
means  of  calculating  the  velocities  of  the  ions.  At 
atmospheric  pressure,  under  a  potential  gradient 
of  I  volt  per  centimetre,  the  velocities  of  different 
ions  vary  from  about  three-quarters  of  a  centi- 
metre per  second  in  the  case  of  carbon  dioxide,  to 
about  7  centimetres  per  second  in  the  case  of 
hydrogen.  The  velocity  of  the  negative  ion  is, 
in  general,  appreciably  greater  than  that  of  the 
positive  ion,  the  ratio,  unity  for  carbon  dioxide, 
rising  to  1.24  for  air  and  oxygen. 

We  should  expect  the  velocity  of  an  ion  to  be 
inversely  proportional  to  the  pressure  of  the  gas, 
and  this  has  been  found  to  be  the  case  with  the 
positive  ions.  The  mobility  of  the  negative  ions, 
on  the  other  hand,  increases  with  decreasing  pres- 
sure much  faster  than  this  expectation  justifies, 
and  at  low  pressures,  100  millimetres  of  mercury 
and  less,  the  change  is  very  marked.  This  result 
indicates  an  alteration  in  the  nature  of  the  ions 
themselves,  and  justifies  the  belief  that  they  must 
possess  more  complex  structures  at  high  than  at 
low  pressures. 

We  shall  see  later  that,  at  the  very  low 
pressures  which  exist  in  good  vacuum  tubes,  it 
is  possible  to  estimate  the  absolute  mass  of  the 
ions,  with  the  remarkable  result  that,  whereas 
the  mass  of  the  positive  ion  appears  to  be  much 


COXDLXTIOX  THROUGH  GASES        131 

the  same  as  the  mass  of  an  atom,  the  mass  of 
the  nesfative  ion  is  about  the  eioi'hteen  hundredth 
part  of  the  mass  of  the  Hghtest  atom  known 
to  chemistry,  that  of  hydrogen.  The  decrease 
of  the  ionic  velocity  at  low  pressures  probably 
indicates  an  approach  to  this  state  of  low  ionic 
mass. 

A  similar  decrease  in  the  size  of  the  negative 
ion,  compared  with  that  of  the  positive,  is  produced 
by  raising  the  temperature.  H.  A.  Wilson  found 
that,  at  2000  C,  the  velocity  of  the  negative 
ions,  produced  by  salts  volatilised  in  flames,  was 
seventeen  times  greater  than  the  velocity  of  the 
positive  ions.     - 

The  problem  of  determining  the  dimensions 
of  the  ions  at  atmospheric  pressure  has  been 
attacked  by  measuring  their  rates  of  diffusion 
into  non-ionized  gases.  The  rate  of  diffusion  of 
a  gas  depends  on  the  mass  of  its  molecule,  and 
experiments  show  that  the  mass  of  an  ion  at 
atmospheric  pressure  is  considerably  greater  than 
that  of  the  molecule  of  an  ordinary  gas. 

All  these  results  may  be  explained  by  the 
theory  that  the  normal  process  of  gaseous  ioniza- 
tion consists  in  the  detachment  from  an  atom  of 
the  gas  of  a  minute  particle,  called  by  Sir  J.  J. 
Thomson  a  corpuscle.  At  extremely  low  pressures 
the  corpuscle  constitutes  the  negative  ion,  and  the 
atom  or  molecule  from  which  it  has  been  separated 
forms  the  positive  ion.  As  the  pressure  rises, 
neutral  molecules  become  attached  to  the  ions, 
probably  by  virtue  of  the  electric  forces,  and 
collect  round  the  original  ion,  which  constitutes 
the  nucleus.  These  complex  systems  form  the 
ions  of  gases  at  atmospheric  pressures. 


132  PHYSICAL  SCIENCE 

The  presence  of  gaseous  ions  may  be  inferred 
from  the  phenomena  of  current  conduction  through 
the  gases,  but  the  existence  of  charged  particles 
of  greater  than  molecular  dimensions  has  been 
demonstrated  directly  by  Mr  C.  T.  R.  Wilson  in 
a  very  striking  manner.  Long  ago  Aitken  showed 
that  the  condensation  of  drops  of  water  from  air 
saturated  with  aqueous  vapourwas  much  helped  by 
the  presence  of  particles  of  dust ;  in  the  absence 
of  dust,  considerable  supersaturation  could  be 
attained  before  condensation  set  in.  Each  particle 
of  dust  forms  a  nucleus,  round  which  collect 
molecules  of  water  ;  and,  when  the  drops  have 
grown  to  a  sufficient  size,  they  fall,  carrying 
down  the  dust  particle  also.  In  this  way  the 
air  is  freed  from  the  presence  of  dust,  and  to  this 
action,  on  a  large  scale,  we  must  attribute  partially 
the  clearness  of  the  atmosphere  after  a  downfall 
of  rain. 

Wilson  devised  an  apparatus  whereby  air 
could  be  subjected  to  a  sudden  expansion.  By 
this  means  it  was  cooled ;  and,  if  previously 
saturated  with  water  vapour,  any  desired  degree 
of  supersaturation  could  be  obtained  by  adjusting 
the  amountof  expansion.  By  repeated  expansions, 
the  dust  particles  were  removed,  and  any  further 
expansion  then  produced  only  a  few  drops  of  water. 
If,  however,  when  the  air  had  thus  been  depleted 
of  possible  nuclei,  Rontgen  rays  or  other  ionizing 
agency  were  allowed  to  act  on  the  gas,  instead 
of  these  few  drops,  a  dense  cloud  was  once  more 
obtained  by  the  same  expansion.  This  cloud  was 
not  formed  if  the  ions  were  removed  previously 
by  an  electric  field,  or  by  some  other  means. 

Fig.  28  is  a  photograph  of  one  of  Mr  Wilson's 


Fig.  28.— Condensation  of  Cloud  on  Gaseous  Ions 

(J/r  C.  T.  R.  Wilsoii). 


[To  face  page  133. 


CONDUCTION  THROUGH  GASES        133 

clouds,  illuminated  by  a  beam  of  light  from  an 
electric  lantern.  The  nuclei  in  this  case  were 
the  ions  produced  by  a  piece  of  radium  contained 
in  the  tube  seen  to  the  right  of  the  glass  cloud- 
chamber.  The  cloud  has  settled  down  to  the 
lower  part  of  the  hemispherical  chamber,  and  its 
sharply-defined  upper  surface  is  clearly  visible. 
The  expansion  is  effected  by  the  movement  of  a 
piston  within  the  vertical  brass  cylinder,  the  lower 
part  of  which  is  put  suddenly  into  communication 
with  the  exhausted  vessel  seen  lying  on  the  table. 

In  1893,  Professor  Thomson  had  shown  that, 
in  causing  condensation,  negative  electrification 
was  more  effective  than  positive,  and  Wilson,  in 
1899,  further  examined  this  point.  He  found 
that,  while  negative  ions  produced  condensation 
of  a  cloud  when  the  volume  of  the  gas  was  in- 
creased in  the  ratio  of  i  :  1.28,  positive  ions  did 
not  cause  an  equal  effect  till  the  expansion  reached 
1.3 1.  It  is  possible  that  this  difference  may  have 
an  important  meteorological  significance.  If,  as 
there  is  reason  to  suppose,  the  atmosphere  some- 
times contains  a  considerable  number  of  gaseous 
ions,  an  expansion  or  fall  of  temperature  would 
result  in  the  formation  of  drops  of  water  round 
the  negative  ions  sooner  than  round  the  positive 
ions.  The  negative  ions  thus  would  be  removed 
first,  and  the  air  would  be  left  with  an  excess  of 
positive  electrification.  It  is  not  unlikely  that 
the  origin  of  the  commonly  observed  potential 
of  the  atmosphere,  positive  relative  to  that  of 
the  earth,  is,  partially  at  any  rate,  to  be  found  in 
this  selective  withdrawal  of  the  negative  ions. 

If  the  ionization  be  not  too  intense,  it  is  possible 
to  remove  completely  the  ions  from  air  by  means 


134  PHYSICAL  SCIENCE 

of  a  single  expansion.  Each  ion  will  then  be  the 
nucleus  of  a  water-drop  ;  and,  since  the  amount 
of  water  left  in  the  air  must  be  just  that  required 
for  the  equilibrium  of  saturation,  the  quantity  of 
water  removed  by  the  falling  cloud  can  be  calcu- 
lated. This  amount  of  water  is  constant  for  a 
given  expansion,  and  the  number  of  ions  present 
must  therefore  be  the  factor  which  determines  the 
size  of  the  drops.  Minute  drops,  the  constituent 
parts  of  the  artificial  cloud  or  fog  under  considera- 
tion, fall  very  slowly,  and  Sir  George  Stokes 
showed  long  ago  how  their  size  may  be  calculated 
from  the  rate  of  their  fall.  The  cloud  settles  down 
at  a  steady,  well-marked  pace,  which  can  readily 
be  observed  by  watching  the  upper  surface  as  seen 
in  Fig.  28.  This  measurement  gives  the  average 
size  of  each  drop  ;  and,  since  the  total  mass  of  all 
the  drops  can  be  calculated  from  the  expansion, 
the  total  number  of  drops,  and  therefore  of  ions, 
can  be  deduced  approximately. 

Sir  J.  J.  Thomson  used  this  method  to  deter- 
mine the  electric  charge  on  a  gaseous  ion.  The 
current  through  the  gas  is  given  by  the  product 
of  the  number  of  ions,  the  charge  carried  by  each, 
and  the  velocity  with  which  they  move.  The 
velocity,  as  we  have  said,  can  be  determined  for  a 
known  electro-motive  force  ;  and,  by  measuring 
the  resultant  current  with  an  electrometer,  and 
finding  the  number  of  ions  by  Wilson's  method,  the 
ionic  charge  was  estimated  as  3.4  x  lO"^^  electro- 
static units.  Within  the  limits  of  experimental 
error  it  was  found  to  be  the  same  as  the  charge 
on  an  ion  in  liquid  electrolysis,  and  this  result  was 
obtained  also  by  Townsend  in  another  way.  The 
importance  of  this  conclusion  will  appear  later. 


CONDUCTION  THROUGH  GASES        135 

An  electric  machine  capable  of  yielding  sparks 
was  invented  manyyears  ago  during  the  eighteenth 
century;  and  the  question  soon  arose  whether  such 
sparks  were  of  the  same  nature  as  the  lightning 
flash — whether  the  roll  of  the  thunder  was  but 
the  reiterated  crackle  of  the  stupendous  electric 
machine  of  the  atmosphere,  echoing  amid  the  con- 
volutions of  theclouds.  The  questionwas  answered 
in  the  year  1752  by  Franklin,  who  floated  a  kite 
in  the  air,  and,  when  the  string  was  made  a  con- 
ductor by  a  shower  of  rain,  was  able  to  draw  the 
confirming  sparks  from  its  lower  end. 

A  very  great  electric  force  is  required  to  main- 
tain a  visible  discharge  through  a  few  centimetres 
of  air  at  the  atmospheric  pressure,  and  the  initial 
force  needed  to  start  the  process  is  still  larger.  It 
was  soon  found,  however,  that  a  reduction  of 
pressure  facilitated  the  passage  of  the  spark,  and 
that  it  was  much  easier  to  send  the  discharge 
throuofh  a  vessel  from  which  the  air  had  been 
partially  exhausted  by  means  of  an  air-pump.  To 
illustrate  this,  platinum  wires,  to  act  as  electrodes, 
are  sealed  into  little  glass  tubes  containing  air  at 
low  pressure.  For  many  years  these  vacuum 
tubes,  as  they  are  called,  were  the  electrical  play- 
things of  the  laboratory  and  popular  lecture-room. 
Recent  discoveries  have  raised  them  from  the 
position  of  scientific  toys  to  the  rank  of  pieces  of 
apparatus,  whereby  have  been  made  some  of  the 
greatest  discoveries  in  physical  knowledge  that 
the  present  generation  has  seen. 

Through  such  a  tube,  in  which  the  pressure  of 
the  air  is  only  a  small  part  of  an  atmosphere,  a 
discharge  may  readily  be  passed  by  the  aid  of  a 
voltaic  battery  and  an  induction  coil,  or  by  the  use 


136  PHYSICAL  SCIENCE 

of  an  influence  electric  machine.  As  in  liquid 
conductors,  the  electrode  by  which  the  current 
enters  is  called  the  anode,  and  that  by  which  it 
leaves,  the  cathode.  Starting  from  the  cathode, 
we  first  see  a  bright  glow  covering  its  surface,  then 
a  dark  space,  succeeded  by  a  second  dark  space, 
beyond  which  is  a  luminous  column  reaching  to 
the  anode.  Within  certain  limits  of  pressure  and 
strength  of  current,  this  positive  column,  as  it  has 
been  called,  shows  fluctuating  striations.  If  the 
length  of  the  tube  be  increased,  it  is  this  positive 
column  alone  which  increases  with  it ;  the  two 
dark  spaces,  and  the  negative  glow,  vary  very 
little  with  the  length  of  the  tube. 

The  effect  of  very  high  vacua  on  the  electric 
discharge  was  first  systematically  investigated  by 
Sir  William  Crookes.  As  the  air  is  gradually 
removed,  it  is  found  that  the  dark  space  nearest 
the  cathode,  known  as  Crookes'  dark  space, 
gradually  extends,  until  eventually  it  fills  the  whole 
tube.  At  this  stage,  green  phosphorescent  effects 
begin  to  appear  on  the  anode  and  on  the  glass 
opposite  the  cathode.  If  a  solid  object,  such  as  a 
screen  of  mica,  be  interposed  between  the  glass 
and  the  cathode,  a  sharp  shadow  is  seen,  showing 
from  its  position  that  rays  capable  of  producing 
phosphorescence  proceed  in  straight  lines  from  the 
cathode.  These  cathode  rays  possess  momentum, 
for  a  light  windmill  placed  in  their  path  can  be 
made  to  rotate  ;  moreover,  they  are  deflected  by 
a  magnet,  in  the  same  direction  as  would  be 
negatively  electrified  particles,  travelling  in  the 
course  of  the  rays.  For  these  reasons,  the  cathode 
rays  must  be  regarded  as  a  flight  of  negatively 
electrified  material  particles. 


CONDUCTION  THROUGH  GASES        137 

In  the  year  1895,  Professor  Rontgen  of  Munich 
made  the  first  of  the  sensational  discoveries  in 
physical  science  for  which  the  last  thirty  years 
have  been  remarkable.  Many  other  recent  in- 
vestigations have  been  as  interesting,  and  several 
have  more  profoundly  modified  our  outlook  on 
Nature,  but  few  have  struck  so  readily  the 
imagination  of  the  plain  man  as  the  revelation 
of  the  skeleton  within  the  living  flesh. 

The  origin  of  this  discovery  may  be  said  to 
have  been  almost  accidental.  Rontgen  noticed 
that  photographic  plates,  kept  under  cover  in 
the  neighbourhood  of  a  highly  exhausted  tube 
through  which  electric  discharges  were  passing, 
became  fogged,  as  though  they  had  been  exposed 
to  light.  He  investigated  this  effect,  and  found 
that,  when  cathode  rays  impinged  either  on  the 
glass  of  the  tube,  or  on  the  anode,  or  on  any 
metallic  plate  within  the  tube,  a  type  of  radiation 
was  produced  which  would  penetrate  many  sub- 
stances opaque  to  ordinary  light.  Dense  bodies, 
like  metal  or  bone,  absorbed  the  rays  more  fully 
than  did  lighter  materials,  such  as  leather  or 
flesh,  and  Rontgen,  at  once  putting  this  discovery 
to  some  purpose,  was  able  to  photograph  the 
coins  in  his  purse  and  the  bones  in  his  hand. 

Given  the  rays,  the  mechanical  contrivances 
required  to  demonstrate  their  effects  are  not 
elaborate.  Rontgen  rays  produce  phosphor- 
escence on  screens  of  barium  platino-cyanide  and 
other  similar  salts,  and,  by  using  these  screens 
in  place  of  a  photographic  plate,  objects,  usually 
hidden  from  our  eyes,  may  be  made  visible. 

A  remarkable  property  of  the  rays  is  their 
power   of   converting   the   air   and   other   gases 


138  PHYSICAL  SCIENCE 

through  which  they  pass  into  conductors  of 
electricity.  In  ordinary  circumstances,  as  was 
pointed  out  in  the  earlier  part  of  this  chapter,  air 
is  an  almost  perfect  insulator  ;  and  an  electrified 
body  exposed  to  it,  while  shielded  from  other 
sources  of  leakage,  loses  its  charge  with  extreme 
slowness.  If,  however,  Rontgen  rays  are  passing 
through  the  air  in  the  neighbourhood  of  the 
electrified  body,  the  charge  quickly  disappears. 

For  several  years  after  their  discovery,  the 
physical  nature  of  the  Rontgen  rays  was  widely 
discussed,  and,  for  a  long  time,  no  general  con- 
sensus of  opinion  was  reached.  Their  photo- 
graphic effects  and  the  fluorescence  they  produced 
on  suitable  screens  suggested  that,  like  ordinary 
light,  they  were  to  be  regarded  as  waves  in  the 
luminiferous  aether.  The  power  they  possess  of 
penetrating  some  opaque  substances  does  not 
forbid  such  an  assumption  ;  for  a  difference  in 
the  wave-length,  or  in  the  period  of  vibration,  is 
sufficient  to  produce  marked  differences  in  the 
penetration  of  ordinary  light.  Glass,  transparent 
to  the  visible  rays,  is  opaque  to  those  invisible 
rays  of  longer  wave-length,  which  possess  great 
heating  power  —  hence  its  use  in  fire-screens; 
while  a  solution  of  iodine  in  bisulphide  of  carbon 
is  opaque  to  luminous  radiation,  but  allows  the 
long  waves  to  pass. 

Rontgen  rays  are  not  refracted  like  ordinary 
light,  and  very  little  trace  of  regular  reflection 
has  been  detected.  Moreover,  it  was  only  with 
great  difficulty  that  they  were  persuaded  to  show 
signs  of  such  a  typical  property  as  polarisation. 
Two  plates  ol  tourmaline  seem  to  be  as  trans- 
parent to  the  rays  when  the  axes  of  the  crystals 


CONDUCTION  THROUGH  GASES        139 

are  crossed  as  when  the  axes  are  parallel.  Such 
indications  as  these  did  not  suggest  an  identity  in 
nature  between  Rontgen  rays  and  ordinary  light. 

On  the  other  hand,  the  rays  suffer  no  devia- 
tion when  acted  on  by  a  magnetic  or  by  an  electric 
field  of  force,  a  result  which  indicates  that  they  are 
not  projected  particles  carrying  electric  charges. 
In  this  particular,  they  must  be  distinguished 
carefully  from  their  creative  agency — from  the 
flight  of  negative  particles  or  cathode  rays  which, 
by  impact  on  glass  or  metal,  give  rise  to  this  new 
type  of  radiation. 

In  the  year  1896,  Sir  George  Stokes  suggested 
that  an  explanation  should  be  sought  in  the  hypo- 
thesis that  Rontgen  rays  were  single  pulses  travel- 
ling through  the  aether.  Ordinary  light  is  to  be 
represented  as  a  series  of  regular  waves,  succeed- 
ing each  other  at  periodic  intervals,  many  thousand 
waves,  almost  exactly  similar  to  each  other,  follow- 
ing in  order  in  a  minute  fraction  of  a  second. 
According  to  this  view,  Rontgen  rays  must  be 
regarded  as  single  disturbances,  propagated  with 
the  same  velocity  as  light,  but  not  followed  by 
a  train  of  waves.  The  thickness  of  the  pulse,  in 
which  the  whole  disturbance  is  concentrated,  was 
supposed  to  be  considerably  smaller  than  the 
wave-length  of  any  visible  light. 

But  this  ingenious  theory  of  single  pulses  had 
to  be  discarded.  Evidence  accumulated  that 
X-rays  were  light,  of  very  short  wave-length,  and 
that  interpretation  was  placed  beyond  doubt  by 
Laue  in  191 2  and  soon  after  by  Sir  William  and 
W.  L.  Bragg,  who  showed  that  X-rays  could  be 
diffracted  by  crystals,  as  light  is  by  a  diffraction 
grating. 


I40  PHYSICAL  SCIENCE 

The  usual  form  of  diffraction  grating  consists 
of  a  transparent  or  reflecting  surface,  on  which 
a  large  number  of  parallel  scratches  are  ruled 
very  accurately,  so  near  together  that  the  distance 
between  them  is  comparable  with  the  wave-length 
of  light.  By  allowing  light  to  fall  on  such  a 
surface,  a  spectrum  is  formed,  like  that  given  by 
a  prism  or  a  rainbow.  A  similar  effect  could  be 
obtained  by  a  number  of  very  thin  glass  plates, 
P  P  P  P  ^^  F ^S-  29,  piled  closely  one  upon  another. 
Let  A  A'  A''  N'^  denote  a  wave-front  of  homo- 


FiG.  29. 

geneous  light,  such  as  the  yellow  rays  from  a 
colourless  gas  flame  in  which  a  sodium  salt  is 
placed.  This  light  is  reflected  at  B  B'  B^'  B^^ 
and,  in  one  particular  direction  BC,  all  these 
reflected  rays  coalesce.  If  BC  is  in  such  a 
direction  that  the  difference  in  path  between 
ABC  and  A'  B'  C  is  just  one  wave-length,  the 
crest  of  one  wave  will  coincide  with  the  crest  of 
the  next.  All  the  little  waves,  therefore,  produce 
similar  effects,  and  the  resultant  effect  is  large — 
a  bright  yellow  line  appears  along  BC.  Else- 
where there  will  be  no  such  coincidence.  Crests 
and  troughs  of  the  wavelets  mix  together,  inter- 


CONDUCTION  THROUGH  GASES        141 

ference  results,  and  the  resultant  effect  is  negligible. 
If  instead  of  homogeneous  sodium  light  we  used 
white  light,  the  different  coloured  components 
would  produce  bright  lines  at  different  angles, 
and  a  coloured  spectrum  would  be  formed  at  C 
at  right  angles  to  BC. 

Now,  if  X-rays  be  regular  wave-trains  at  all, 
their  wave-lengths  must  be  much  shorter  than 
those  of  visible  light.  No  glass  plates  would  be 
thin  enough  to  give  a  reflection  spectrum.  But, 
if  the  atoms  in  a  crystal  be  situated  in  regular 
layers,  it  is  possible  (i)  that  they  might  act  as 
superposed  reflecting  plates,  and  (2)  that  the 
wave-lengths  of  X-rays  might  be  of  the  same 
order  of  size  as  the  distance  between  the  layers 
of  atoms. 

This  was  found  to  be  the  case.  In  particular, 
the  Braggs  have  thus  proved  the  wave-lengths 
of  X-rays  to  be  about  io~^  or  one  hundred 
millionth  of  a  centimetre,  and  have  discovered 
many  interesting  facts  about  the  structure  of 
crystals. 

For  instance,  a  photograph  of  the  X-ray 
spectrum  from  a  crystal  of  rock  salt,  shows  that 
layers  of  high  reflection  are  interspersed  with 
layers  of  low  reflection.  Hence  it  is  concluded 
that  layers  of  sodium  atoms  lie  between  layers  of 
chlorine  atoms.  It  is  the  atam  and  not  the 
molecule  which  is  important  in  crystal  structure. 
Indeed,  the  crystal  must  be  regarded  as  one 
enormous  molecule  of  formula  Na^Cl,^. 

Again,  the  X-ray  spectrum  from  a  diamond 
shows  that  the  carbon  atoms  each  lie  at  the 
centre  of  a  tetrahedron,  and  are  linked  together 
in  six-membered  rings,  corresponding  exactly  to 


142  PHYSICAL  SCIENCE 

the  ring  formula  of  benzene  which  is  inferred  from 
ordinary  chemical  evidence. 

Direct  evidence  of  the  negative  charge  carried 
by  the  cathode  rays  was  given  by  experiments  of 
Perrin.  He  showed  that,  when  the  rays  were 
deflected  by  a  magnet  so  that  they  fell  on  an  in- 
sulated metal  cylinder  placed  within  the  discharge- 
bulb  and  connected  with  an  electrometer,  a  strong 
negative  electrification  was  imparted  to  the  system. 
When  the  rays  fell  on  other  parts  of  the  bulb,  this 
electrification  was  not  observed. 

A  less  direct  but  more  interesting  method  was 
used  by  Thomson  in  1897,  and  led  to  one  of  the 
great  discoveries  of  modern  science.  In  the  glass 
apparatus  shown  in  Fig.  30,  the  left-hand  terminal 
of  the  induction  coil  is  connected  with  the  cathode, 
the  right-hand  terminal  with  a  thick  metallic  disc 
which  acts  as  the  anode.  Through  the  anode, 
and  through  a  second  thick  disc  connected  with 
the  earth  by  the  wire  going  to  the  bottom  of  the 
photograph,  are  bored  in  sequence  two  holes  about 
a  millimetre  in  diameter.  A  thin  pencil  of  cathode 
rays  is  thus  obtained  beyond  the  second  disc. 
These  rays  pass  between  the  two  metallic  plates, 
seen  in  the  wider  part  of  the  tube,  which  can  be 
connected  with  the  poles  of  a  voltaic  battery  by 
means  of  the  wires  passing  to  the  right.  An 
electric  force  of  known  amount  can  thus  be  applied 
to  the  cathode  rays.  When  that  force  is  sufficient, 
the  path  of  the  rays  is  deflected,  and  the  magnitude 
of  this  effect  can  be  determined  by  observing  the 
deflection  of  the  spot  of  fluorescent  light  on  the 
screen  at  the  right-hand  end  of  the  apparatus.  It 
is  well  known  that  the  cathode  rays  are  deflected 


[To  face  page  142'. 


■  CONDUCTION  THROUGH  GASES        143 

by  a  magnetic  field  also,  and  this  effect  too  can 
be  measured  in  the  same  apparatus.  Both  these 
deflections  are  to  be  expected  if  the  rays  consist 
of  moving  electrified  particles  ;  and  the  directions 
of  the  deflections  are  such  that  the  electrification 
must  be  that  to  which  is  conventionally  given  the 
negative  sign. 

The  conclusions  drawn  from  these  experi- 
ments are  of  extreme  importance.  In  analysing 
the  deflections  of  the  particles  three  things  are 
involved:  (i)  the  velocity;  (2)  the  mass;  and 
(3)  the  electric  charge.  For  both  deflections,  the 
electric  and  magnetic,  the  two  last  quantities 
appear  as  the  ratio  ejin — that  is,  the  charge 
divided  by  the  mass.  If  we  treat  this  ratio  as 
a  single  quantity,  we  find  ourselves  with  two 
unknown  values  to  be  determined  by  the  two 
experiments,  the  one  on  the  magnetic,  and  the 
other  on  the  electric  deflections.  Both  the  un- 
known quantities — to  wit,  the  velocity  and  the 
ratio  ejm — can  therefore  be  found  from  the  results 
of  the  experiments. 

When  a  magnetic  force  is  applied,  the  spot 
of  phosphorescent  light  in  the  tube  of  Fig.  30 
is  drawn  out  into  a  band  of  appreciable  length. 
This  result  is  a  consequence  of  a  difference  in 
velocity  of  the  rays  :  in  any  one  discharge,  rays 
are  found  with  a  considerable  range  of  velocity, 
and  therefore  these  rays  are  deflected,  according 
to  their  velocities,  through  a  series  of  different 
angles. 

The  following  table  gives  some  of  the  results 
of  Sir  J.  J.  Thomson's  experiments,  and  shows 
the  mean  values  of  the  velocity,  Vy  in  centimetres 
per  second,  and  of  the  ratio  nije  for  cathode  rays, 


144  PHYSICAL  SCIENCE 

m  being  expressed  in  grams,  and  e  in   electro- 
magnetic units  of  electricity. 


Gas. 

V. 

m/e. 

Air 

.    2.8xio9 

I-2X  IO-7 

Hydrogen 

.      2-5  X  lO^ 

1-5  X  IO-' 

Carbonic  acid 

.      2-2  X  10^ 

i'5  X  lo-^ 

Thus,  within  the  limits  of  experimental  error, 
the  values  of  m/e  are  independent  of  the  nature 
of  the  residual  gas  left  in  the  vacuum  tube. 
Moreover,  in  these  experiments,  and  in  a  further 
series  due  to  H.  A.  Wilson,  the  results  were 
shown  to  be  the  same  whatever  metal  was  used 
to  form  the  cathode.  In  all  circumstances  the 
mean  velocity  is  very  high,  being  about  one- 
twelfth  that  of  light,  and  the  mean  value  of  m/e 
is  1.3  X  io~^  which  makes  the  reciprocal  ratio 
e/m  about  7.7  x  lO^ 

Since  the  date  of  Thomson's  original  in- 
vestigation, these  measurements  have  often  been 
repeated.  More  recent  results,  especially  those 
of  Millikan,  give  values  for  7;i/e  of  5.64  x  io~^  and 
for  e//u  of  1.77  X  lo"^. 

Now  in  liquid  electrolytes,  the  passage  of 
one  electro-magnetic  unit  of  electricity  evolves 
io~^  gram  of  hydrogen.  Thus,  in  this  case,  the 
ratio  vi/e  is  about  IO~^  or  about  eighteen  hundred 
times  more  than  5.64  xio~^  its  value  for  the 
negative  particle  in  a  cathode  ray. 

But,  as  we  have  already  seen  (p.  134),  by  an 
application  of  C.  T.  R.  Wilson's  beautiful  experi- 
ments on  the  electric  formation  of  clouds,  Thomson 
has  proved  that  the  individual  charge  on  all  the 
gaseous  ions  examined  is  the  same  as  the  charge 
on  the  ions  in  liquid  electrolysis,  and  this  result 
has  been  confirmed  by  other  methods.     Although 


CONDUCTION  THROUGH  GASES        145 

the  cathode  ray  particles  themselves  could  not  be 
investigated  in  this  way,  there  seems  no  reason 
to  suppose  that  they  are  exceptions  to  a  rule  other- 
wise universal.  If,  then,  e  is  the  same  both  for 
gases  and  for  liquids,  m  must  be  different ;  the 
cathode  ray  particle  must  have  a  mass  which  is 
only  the  one  eighteen-hundredth  part  of  that  of 
the  hydrogen  atom. 

Similar  values  have  been  obtained  for  the  mass 
of  the  negative  particles  when  produced  in  other 
ways.  In  one  case,  that  of  the  ions  due  to  the 
incidence  at  a  low  pressure  of  ultra-violet  light 
on  metals,  both  e  and  ejni  have  been  measured  for 
the  same  particles.  A  zinc  plate  is  illuminated 
with  ultra-violet  light,  and  placed  opposite  to  and 
parallel  with  a  second  metallic  plate  connected 
with  an  electrometer,  the  gas  surrounding  the 
apparatus  being  exhausted  to  a  very  low  pressure. 
An  electric  force  is  established  between  the  two 
plates,  and  the  negative  ions,  produced  at  the  zinc 
plate,  are  by  this  force  urged  towards  the  second 
plate.  If  no  other  agency  were  at  work,  all  the 
negative  ions  would  reach  the  second  plate,  and 
transfer  their  charges  to  the  electrometer.  Now 
let  us  imagine  that  a  magnetic  force  is  applied  at 
right  angles  to  the  electric  force  and  parallel  to 
the  planes  of  the  plates.  The  magnetic  force  will 
deflect  the  negative  particles  from  their  original 
straight  course,  and  their  path  becomes  a  cycloid. 
They  travel  out  from  the  zinc  plate,  curve  round, 
and  approach  it  again.  If  the  second  plate  is 
placed  near  enough  to  the  first  to  intercept  this 
curved  orbit,  all  the  ions  will  still  reach  the  plate 
connected  with  the  electrometer,  and  the  rate  at 
which   it   gains    negative   electricity   will    not   be 

L 


146  PHYSICAL  SCIENCE 

affected  by  the  presence  of  the  magnetic  field.  If, 
however,  the  electrometer  plate  be  moved  away 
from  the  zuic  plate  till  it  lies  beyond  the  path  of 
the  ions,  it  will  receive  none  of  them,  and  the 
establishment  of  the  magnetic  force  should  stop 
completely  the  supply  of  negative  electricity  to  the 
electrometer.  If  AT  be  the  electric  force  and  H 
the  magnetic  force,  theory  shows  that  no  ions 
should  cross  the  space  between  the  plates  if 
the  distance  between  them  exceeds  2XmjeH'^, 
while  below  that  distance  the  addition  of  the 
magnetic  force  H  should  produce  no  effect  on 
the  rate  of  gain  of  negative  charge  by  the 
electrometer. 

The  experiments  which  Thomson  carried  out 
by  this  method  showed  that  no  such  sudden 
change  could  be  produced.  As  the  distance  was 
diminished,  or  the  magnetic  field  increased,  at  first 
the  effect  of  putting  on  or  taking  off  the  magnetic 
force  was  small.  Then  a  stage  was  reached  at 
which  a  considerable  effect  was  produced  ;  while 
finally,  in  a  third  stage,  the  magnetic  force  cut  off 
almost  all  the  ions  from  the  electrometer  plate. 
This  somewhat  gradual  change  is  explained  if  we 
suppose  that  the  negative  ions  are  not  all  formed 
at  the  surface  of  the  zinc  plate,  but  that,  as  the 
primary  ions  there  produced  move  forward  under 
the  action  of  the  electric  force,  they  produce  new 
ions  by  their  collisions  with  the  molecules  of  the 
gas.  The  ions  are  thus  formed,  not  exclusively 
at  the  surface  of  the  plate,  but  throughout  a  thin 
layer  of  gas  near  the  plate.  This  secondary  pro- 
duction of  ions  by  primary  ions  moving  with  high 
velocities  occurs  in  many  other  cases,  and  has 
been    studied    systematically    by    Townsend.      It 


CONDUCTION  THROUGH  GASES        147 

explains  the  large  currents  which  can  be  carried 
by  the  electric  arc  or  spark  discharge. 

These  considerations  indicate  that,  in  the  ex- 
periments we  are  now  describing,  the  limit  of  the 
second  stage,  in  which  some  but  not  all  of  the 
negative  ions  are  stopped  by  the  magnetic  field, 
gives  the  distance  at  which  those  ions  coming 
from  the  surface  of  the  zinc  plate  just  fail  to  get 
across  the  space  between  the  plates.  The  expres- 
sion given  above  then  leads  directly  to  a  value  for 
ejin,  the  ratio  of  the  ionic  charge  to  the  ionic  mass. 
Thomson  found  as  the  result  7.3  x  lo^  a  number 
which  agreed  well  with  that  which  he  deduced 
for  cathode  rays,  namely,  J.J  y>  lo^ 

With  the  negative  ions  produced  by  the  inci- 
dence of  ultra-violet  light  on  a  zinc  plate,  it  is  easy 
to  repeat  C.  T.  R.  Wilson's  experiments  on  the 
formation  of  clouds  round  ions  as  nuclei,  and  thus 
to  determine  the  value  of  e,  the  electric  charge 
associated  with  the  same  ions  for  which  ejin  has 
already  been  obtained.  The  result  shows  that, 
as  always,  the  charge  is  the  same  as  the  charge 
on  an  ion  in  liquid  electrolytes  ;  and  therefore  for 
the  ions  due  to  ultra-violet  light,  as  for  the  cathode 
ray  particles,  the  mass  must  be  much  less  than 
that  of  the  hydrogen  atom.  The  result  has  been 
confirmed  by  Lenard,  who  used  a  somewhat 
different  type  of  apparatus. 

In  all  these  investigations  the  existence  of 
particles  much  smaller  than  the  smallest  of  the 
hitherto  indissoluble  chemical  atoms  is  clearly 
indicated.  Since  the  beginning  of  the  nineteenth 
century  the  chemical  atom  has  been  the  ultimate 
unit  in  which  our  conception  of  matter  has  been 
expressed.     The  sixty,  seventy,  or  eighty  different 


148  PHYSICAL  SCIENCE 

elements,  progressively  known  to  the  chemist, 
seemed  to  be  essentially  different  in  kind,  though 
certain  likenesses  between  them,  and  periodic  rela- 
tions between  their  properties  and  masses,  vaguely 
pointed  to  a  common  origin.  Now,  after  a  hundred 
years,  the  atom  yields  place  to  Thomson's  corpuscle 
as  the  ultimate  known  particle  of  matter  ;  while 
the  phenomena  of  radio-activity,  as  w^e  shall  see 
hereafter,  have  shaken  the  belief  in  the  immuta- 
bility of  the  elements,  and  are  leading  to  a  new- 
faith  in  their  transmutation. 

Speculation,  it  is  true,  from  the  days  of 
Democritus  to  those  of  Sir  William  Crookes, 
has  been  busy  with  imaginings  anent  ultimate 
particles,  which  should  be  common  to  all  types  of 
matter,  and  should  compose  the  different  elements 
by  differences  in  their  number  or  arrangement. 
But  Professor  Thomson  has  not  followed  the  facile 
and  barren  paths  of  speculation.  He  has  first 
found  the  particles,  and  has  weighed  and  timed 
them  before  theorising  on  their  origin  and  destiny. 

We  are  now  in  a  position  to  estimate  the 
importance  of  the  experiments  which  have  shown 
that  the  mass  of  the  corpuscle  is  independent  both 
of  the  nature  of  the  gas  in  w^hich  it  is  found,  and 
also  of  the  material  of  the  electrode  used  in  pro- 
ducing it.  Not  only  must  we  conceive  atoms  to 
contain  these  more  minute  particles,  but  it  is 
necessary  to  suppose  that  in  all  atoms,  whatever 
be  their  nature,  these  particles  are  similar.  The 
dream  of  an  ultimate  particle,  common  to  all  kinds 
of  matter,  has  thus  at  length  come  true. 

The  relation  between  the  corpuscles  and  the 
electric  charges  associated  with  them  must  next  be 
considered.     These  isolated  particles  have  never 


CONDUCTION  THROUGH  GASES        149 

been  observed  with  positive  charges  ;  positive  ions 
are  found  to  have  masses  equal  to  those  of  some 
chemical  atoms.  The  facts  may  provisionally  be 
explained  by  the  hypothesis  that  the  corpuscle  con- 
stitutes the  isolated  negative  unit  of  electricity. 

Now  the  existence  of  electric  units  as  a  basis 
of  matter  had  been  suggested  already  by  Lorentz 
and  Larmor.  The  light  and  radiant  heat  emitted 
by  incandescent  substances  are  electro-magnetic 
waves,  and  must  therefore  arise  from  the  vibration 
of  electric  charges.  The  periods  of  vibration  are 
too  quick  to  be  due  to  the  motion  of  atoms  as 
wholes,  and  we  must  therefore  look  within  the 
atom  for  the  source  of  radiation.  Hence  it  again 
follows  that  atoms  must  be  complex  structures 
with  more  minute  internal  parts  containing 
electric  charges.  Those  parts  themselves  have 
been  pictured  as  electric  units  and  given  the 
name  of  electrons — a  word  invented  by  Stoney. 
They  may  be  identifiedwith  Thomson's  corpuscles, 
and  these  are  indeed  now  generally  called  electrons. 

The  ordinary  phenomena  of  electrification  may 
be  described  in  these  new  terms. 

An  atom  of  ordinary  matter,  with  one  electron 
beyond  its  proper  number,  is  an  atom  negatively 
electrified  ;  an  atom  with  the  electron  detached 
from  it  is  an  atom  positively  electrified.  These 
charged  atoms  act  as  ions,  negative  and  positive 
respectively,  in  accordance  with  the  usual  con- 
vention about  signs. 

A  moving  electrified  body  acts  like  an 
electric  current,  and  therefore  must  be  associated 
with  electro-magnetic  energy  and  electro-magnetic 
momentum  in  the  surrounding  dielectric  medium. 
To  change  the  velocity,  therefore,  requires  the  ex- 


ISO  PHYSICAL  SCIENCE 

penditure  of  electro-magnetic  energy,  and  thus  the 
electrified  bodypossesseselectric  inertia  in  addition 
to  its  ordinary  dynamical  inertia.  As  long  as  the 
velocity  is  small,  this  electric  inertia  is  constant, 
but  an  electrified  body  moving  rapidly  can  be 
shown  mathematically  to  behave  as  though  its 
inertia,  that  is,  its  mass,  were  increased  ;  and,  as 
the  velocity  of  light  is  approached,  this  apparent 
electric  mass  grows  very  rapidly.  Now  some 
experiments  by  Kaufmann,  in  which  the  masses 
of  the  negative  corpuscles  emitted  by  radium  were 
investigated,  are  of  intense  interest  in  this  con- 
nection. The  radium  electrons  move  much  more 
rapidly  than  those  found  in  cathode  rays,  though 
in  other  respects  electrons  from  the  two  sources 
appear  to  be  identical.  With  radium  the  veloci- 
ties are  so  great  that  they  approach  closely  that  of 
light.  A  speed  of  2.85  x  10^^  centimetres  a  second 
has  been  observed,  that  of  light  itself  being 
3.0  xio^^  At  these  enormous  velocities,  Kauf- 
mann found  that  the  value  of  efm,  determined 
from  the  magnetic  and  electric  deflections,  was  con- 
siderably diminished,  a  value  of  about  one-third 
the  normal  being  obtained.  Assuming  that  the 
charge  be  constant,  this  means  a  threefold  in- 
crease in  My  the  effective  mass  of  the  corpuscles. 

From  the  theory  of  electrons  it  is  possible  to 
calculate  what  the  increase  of  apparent  mass 
should  be,  on  the  assumption  that  the  whole  of 
the  mass  of  the  corpuscle  is  an  electrical  manifesta- 
tion, and,  as  we  shall  see  in  a  future  chapter,  the 
results  of  these  and  later  experiments  agree  with 
the  calculated  numbers.  Such  results  are  of 
fundamental  importance,  both  physically  and 
philosophically.     It  seems  that  the  whole  of  the 


CONDUCTION  THROUGH  GASES        151 

observed  mass  of  the  electron  may  be  regarded 
as  an  effect  due  to  the  electro-magnetic  inertia 
of  its  electric  charge.  Representing  the  atoms  of 
ordinary  matter  as  made  up  of  negative  electrons 
scattered  in  space  round  some  central  positive 
nucleus,  it  becomes  possible  to  explain  their  mass 
by  the  electro-magnetic  properties  of  an  electric 
charge.  To  explain  other  phenomena,  it  is 
supposed  that  the  electrified  corpuscles  —  the 
electrons — are  in  rapid  orbital  or  oscillatory 
motion  within  the  atom  :  that,  for  example,  the 
electrons  whirl  round  the  nucleus  in  their  orbits 
as  the  planets  swing  round  the  sun,  and  thus  we 
get  a  first  picture  of  the  atom  which  has  been 
filled  in  in  much  detail  by  later  research. 

Mass  or  inertia  is  the  most  permanent  and 
characteristic  property  of  matter,  and  having 
explained  mass  as  due  to  electricity  in  motion,  the 
physicist  may  well  ask  the  metaphysical  question  : 
has  matter  any  objective  reality  ;  may  not  its  very 
essence  be  but  a  form  of  disembodied  energy  ? 
And  here  the  philosophical  speculation  of  1904 
is  in  accord  with  the  mathematical  principle  of 
relativity  of  to-day.  On  that  principle,  matter 
and  energy  are  of  the  same  nature,  and  both 
intimately  bound  up  with  the  properties  of  that 
combined  space-time  which  is  more  real  than 
either  time  or  space  independently. 

An  attempt  to  obtain  a  more  vivid  picture  of 
the  electro-magnetic  field  was  made  by  J.  J. 
Thomson  by  means  of  the  conception  of  tubes  of 
force,  a  conception  which  we  owe  to  the  instinctive 
insight  of  Faraday.  A  small  electrified  body, 
carrying,  let  us  suppose,  a  negative  charge,  is  well 
known  to  attract  other  bodies  in  the  neighbour- 


152  PHYSICAL  SCIENCE 

hood  when  those  bodies  are  positively  electrified, 
and  to  repel  them  if  their  charges  be  negative. 
Rejecting  the  idea  of  action  at  a  distance,  Faraday 
regarded  these  electric  forces  as  transmitted  by 
stresses  and  strains  in  the  dielectric  or  Insulatlno- 
medium,  and  represented  the  state  of  that  medium 
by  a  series  of  lines,  drawn  everywhere  so  as  to 
lie  in  the  direction  of  the  force  on  a  positively 
electrified  particle. 

The  distribution  of  these  electric  lines  of  force 
can  be  investigated  theoretically,  the  laws  of  force 
being  known,  but  it  is  not  easy  to  illustrate  them 
experimentally.  On  the  other  hand,  the  corre- 
sponding magnetic  lines  can  be  rendered  visible 
and  mapped  out  by  a  familiar  experiment,  which, 
indeed,  first  suggested  to  Faraday  his  conception 
of  lines  or  tubes  of  force.  If  the  poles  of  a 
horse-shoe  magnet  be  placed  beneath  a  sheet  of 
cardboard,  over  which  iron  filings  are  sprinkled, 
a  picture  of  the  magnetic  lines  of  force  is  formed 
by  the  filings  (Fig.  31).  Under  the  influence  of 
the  magnetic  field,  each  filing  becomes  a  little 
magnet,  and  attracts  others,  forming  chains  of 
filings  which  lie  everywhere  in  the  direction  of 
the  magnetic  force.  Where  the  force  is  strong, 
the  filings  cluster  thickly ;  where  the  force  is 
weak,  few  filings  are  to  be  seen.  Thus  a 
complete  representation  of  the  lines  of  magnetic 
force  is  obtained. 

The  laws  of  force  are  similar  for  electric 
charges  and  for  magnetic  poles,  and  the  lines  of 
force  will  possess  the  same  form.  Thus  the 
filings  in  Fig.  31  represent  also  the  direction  and 
distribution  of  the  electric  lines  or  tubes  of  force 
in  the  neighbourhood  of  two  electric  charges  of 


CONDUCTION  THROUGH  GASES        153 


opposite  signs.  Here  we  have  two  charges  ;  but, 
for  an  isolated  charged  body,  the  Hnes  of  electric 
force  must  evidently  be  radial. 

Now  Thomson  explained  electro-magnetic 
momentum  as  an  effect  of  the  Faraday  tubes  of 
force  in  pulling  after  them  as  they  move  some  of 
the  surrounding  medium.  A  solid  body  moving 
through  water  drags  some  of  the  liquid  with  it, 
and,  in  this  way,  its  effective  mass  is  increased. 


«  » 


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::::--:-v>n\;  ;  /  /  /  /    /' 
— :::-.>\\\  i  ;  /  /  /  /  / 

,----    .'■■.''>7'^W^r\ :.^^>v77?n^?^^V':--.     - -- 

.'-     .-'.'// /,*;\  \-.\  •.;-.;- ---''•'-''//!  1  \^  ^'v  '%  ^-. 

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////■■    \  ••■;-C ■■:::■<//  ■    \  \  \  \   '-. 


Fig.  31. 

A  vortex  filament,  too,  carries  with  it  some  of  the 
fluid  of  which  the  vortex  is  composed.  So  with 
the  F'araday  tubes  if  we  look  on  them  as  physical 
realities.  Maxwell  showed  that  the  same  medium 
would  explain  both  light  and  electric  waves,  and 
we  may  perhaps,  as  an  illustration,  think  of  tubes 
of  force  as  vortex  filaments  in  the  luminiferous 
aether.  We  may  then  suppose  that  they  move 
some  of  the  surrounding  aether  with  them.  If  the 
aether  possess  mass,  it  will  endow  the  moving 


154  PHYSICAL  SCIENCE 

tubes  with  effective  momentum.  In  this  way, 
Thomson  regarded  electric  momentum  as  similar 
in  kind  to  ordinary  dynamical  momentum.  Should 
the  inertia  of  material  objects  be  electrical  in  its 
nature,  then,  on  this  view,  the  mass  and  kinetic 
energy  of  ordinary  bodies  is  to  be  regarded  as  the 
mass  and  kinetic  energy  of  the  aether  bound  to 
the  Faraday  tubes  which  emanate  from  the  con- 
stituent electrons.  If  such  a  scheme  be  accepted, 
the  problem  of  the  material  universe  is  referred 
completely  to  the  problem  of  the  nature  and 
properties  of  the  luminiferous  aether.  A  great 
simplification  in  our  conception  of  the  world  is 
thus  effected,  but  again,  as  always,  an  ultimate 
explanation  eludes  us.  Moreover,  some  of  the 
consequences  of  the  theory  of  relativity,  which  we 
shall  trace  in  Chapters  VIII.  and  IX.,  show  that 
caution  and  restraint  are  needed  in  dealing  with 
the  luminiferous  aether.  We  probably  know  less 
about  it  than  our  fathers  did. 

Instead  of  stating  matter  in  terms  of  electricity, 
it  is  simpler,  and  perhaps  less  ambitious,  to  express 
electricity  in  terms  of  matter,  as  Thomson  did  at 
first,  and  say  that  electrified  atoms  contain  one 
or  more  corpuscles  in  excess  or  defect  of  their 
normal  number.  Nevertheless,  the  electron  theory 
of  matter,  formerly  supported  on  mathematical 
grounds,  has  been  strengthened  greatly  by  these 
developments  of  experimental  science.  Moreover, 
from  the  point  of  view  of  radio-activity,  which  we 
shall  consider  in  the  next  chapter,  that  theory  is 
of  supreme  importance,  for  it  gave  the  first  indica- 
tion that  an  atom  was  a  complex  and  possibly 
unstable  body.  Now  the  occasional  instability 
of  a  complex  chemical  atom,  and  its  disintegration 


CONDUCTION  THROUGH  GASES        155 

into  simpler  bodies,  as  we  shall  presently  see, 
is  the  universally  accepted  explanation  of  the 
phenomena  of  radio-activity. 

Having  now  dealt  with  the  phenomena  of 
cathode  rays  and  the  theoretical  results  which 
have  followed  their  discovery,  we  must  turn  to 
the  corresponding  positive  rays,  which  are  emitted 
from  the  anode  of  an  exhausted  tube  through 
which  an  electric  discharge  is  passed.  If  holes 
be  bored  in  a  cathode  placed  opposite  an  anode, 
positive  rays  will  be  found  to  have  passed  through 
the  holes  into  the  space  behind  the  cathode.  They 
may,  if  necessary,  be  sent  through  a  window  of 
thin  aluminium  foil,  and  thus  examined  outside 
the  discharge  tube. 

The  magnetic  and  electric  deflections  of  these 
anode  rays  are  much  less  marked  than  the  deflec- 
tions of  cathode  rays,  and  stronger  fields  must  be 
used  to  examine  them.  The  results  show  that 
they  are  positively  electrified  particles,  with  masses 
corresponding  to  those  of  known  chemical  atoms, 
instead  of  the  sub-atomic  electrons  of  cathode  rays. 

It  was  again  Sir  J.  J.  Thomson  who  first  made 
an  extensive  investigation  of  these  positive  rays. 
He  passed  them  through  both  an  electric  and  a 
magnetic  field,  so  that  they  fell  on  a  photographic 
plate  in  such  a  way  that  all  projected  particles 
having  the  same  value  of  mje  formed  a  single  line. 

In  hydrogen,  for  instance,  the  chief  line  is  found 
in  a  position  which  indicates  a  value  for  mje  of 
IO"^  the  same  as  for  the  hydrogen  ion  in  liquid 
electrolytes.  Another  line  showing  a  doubled  value 
for  mje  indicates  a  hydrogen  molecule  carrying  a 
single  charge.     In  oxygen,  atoms  appeared  carry- 


156  PHYSICAL  SCIENCE 

ing  two  unit  charges.  In  neon  (atomic  weight 
20.2)  two  Hnes  were  found  very  near  together, 
suggesting  atomic  weights  of  20  and  22. 

This  last  indication  was  carried  further  by 
F.  W.  Aston,  who  improved  the  apparatus,  so 
that  it  gave  a  photographic  ''mass  spectrum,"  and 
obtained  most  interesting  and  important  results. 
The  double  nature  of  ordinary  neon  was  confirmed. 
It  consists  of  a  mixture  of  two  types  of  atom, 
identical  in  chemical  properties,  but  of  different 
atomic  weight.  Such  bodies  were  named  isotopes 
by  Soddy  who  discovered  instances  in  another 
way.  Aston,  extending  the  work  on  mass  spectra, 
proved  that  manyelements  as  known  to  the  chemist 
consist  of  mixed  isotopes.  Thus  chlorine  has  an 
atomic  weight  of  35.46,  and,  with  similar  cases,  was 
always  a  stumbling  block  in  the  road  of  those  who 
sought  to  reduce  the  elements  to  different  com- 
binations of  hydrogen  atoms  as  units  of  atomic 
structure.  Aston  showed  at  once  that  its  mass 
spectrum  gave  two  lines,  corresponding  to  atomic 
weights  of  35  and  ^J.  The  riddle  of  chlorine  was 
solved  :  it  consists  of  two  isotopes,  each  atomic 
weight  being  a  whole  number.  Similar  results 
were  obtained  with  many  other  elements. 

Amoncr  the  various  ao^encies  enumerated  at  the 
beginning  of  this  chapter  for  the  production  of 
gaseous  ions,  special  interest  attaches  to  the  action 
of  incandescent  metals  and  carbon.  Elster  and 
Geitel,  Richardson,  H.  A.  Wilson,  and  others  have 
shown  that,  as  a  platinum  wire  is  heated  gradually, 
it  begins  to  emit  positive  ions  at  a  temperature 
corresponding  to  a  low  red  heat.  The  investiga- 
tion of  the  influence  of  a  magnetic  force  shows 


CONDUCTION  THROUGH  GASES        157 

that  these  ions  vary  In  size,  some  probably  being 
molecules  of  the  gas,  and  others  molecules  of  the 
metal  or  even  dust  disintegrated  from  its  surface. 
As  the  platinum  is  still  further  heated,  negative 
ions  also  come  off,  ultimately  in  large  excess.  In 
vacuo  the  negative  leak  from  platinum  and  carbon 
filaments  is  very  large — from  carbon  it  may  even 
amount  to  as  much  as  an  ampere  of  current  from 
each  square  centimetre  of  surface.  The  negative 
ions  are  then  of  sub-atomic  dimensions,  and  are 
identical  with  the  electrons  otherwise  obtained. 
H.  A.  Wilson  has  shown  that,  at  the  lower  tem- 
peratures at  which  the  negative  leak  occurs,  it  is 
very  largely  due  to  the  effect  of  hydrogen  absorbed 
in  the  platinum,  and  liberated  under  the  action  of 
the  heat.  At  the  highest  temperatures,  however, 
the  electrons  due  to  the  wire  itself  seem  to  be 
much  more  numerous  than  those  depending  on 
the  presence  of  hydrogen,  and  to  the  metal  itself 
we  must  then  look  for  their  source. 

The  emission  of  electrons  at  high  temperatures 
is  not  confined  to  solids.  Thomson  finds  that 
sodium  vapour  also  gives  off  a  large  supply,  and 
the  effect  seems  to  be  common  to  all  kinds  of 
matter  at  a  white  heat.  Carbon  is  particularly 
efficacious,  perhaps  because  it  can  be  raised  to  a 
higher  temperature  than  can  most  metals.  It  is 
easy  to  demonstrate  the  existence  of  a  measur- 
able current  from  one  limb  of  the  carbon  filament 
of  an  ordinary  incandescent  electric  lamp  to  an 
insulated  plate  placed  between  the  limbs. 

Owing  to  the  emission  of  electrons  by  an 
incandescent  wire  or  carbon  filament  along  which 
a  current  flows,  the  effective  current-carrying  area 
of  the  wire  is  increased.     In  vacuo  a  considerable 


158  PHYSICAL  SCIENCE 

fraction  of  the  current  might  pass  through  the 
space  surrounding  the  wire,  which  must  become 
filled  with  electrons.  Although  in  gases  at 
ordinary  pressures  the  emission  of  electrons  is 
less  copious,  still,  ionization  will  occur  to  an 
appreciable  extent  just  round  the  wire,  and  a  part, 
though  perhaps  a  small  part,  of  the  current  will 
pass  along  outside  the  substance  of  the  wire. 

The  phenomena  we  are  now  considering  have 
a  practical  application  in  the  art  of  wireless  tele- 
graphy and  telephony  (see  Chapter  VIII.).  But 
they  also  have  an  important  bearing  on  cosmical 
processes.  The  photosphere  of  the  sun  contains 
large  quantities  of  glowing  carbon,  and  this  carbon 
will  emit  electrons  until  the  resultant  positive 
charge  left  on  the  sun  exerts  an  electro-static 
force  great  enough  to  prevent  further  emission. 
In  this  way  a  condition  of  equilibrium  would  be 
reached.  Any  local  elevation  of  temperature 
would  then  cause  a  stream  of  electrons  to  leave 
the  sun  and  pass  into  the  surrounding  space. 
When  electrons  pass  through  a  gas  with  high 
velocity,  they  make  it  luminous,  and  Arrhenius 
and  others  have  explained  many  of  the  periodic 
peculiarities  of  the  Aurora  Borealis  by  the 
supposition  that  electrons  from  the  sun,  due 
either  to  incandescence  or  to  some  other  cause, 
stream  through  the  upper  regions  of  the  earth's 
atmosphere. 

The  phenomena  of  electrolytic  conduction 
through  liquids,  and  of  non-electrolytic  conduction 
through  metallic  substances,  must  now  be  inter- 
preted in  terms  of  this  electronic  theory.  The 
chemical  decomposition  of  electrolytic  solutions, 


CONDUCTION  THROUGH  GASES        159 

which  we  have  described  in  Chapter  V.,  indicates 
that  an  electric  transfer  through  such  liquids 
involves  a  movement  of  the  chemical  constituents 
of  the  substance  decomposed.  In  fact,  as  we  have 
seen,  that  movement  has  been  experimentally 
demonstrated,  and  the  passage  of  the  ions  rendered 
visible.  We  must  suppose,  then,  that  the  electron 
forming  the  effective  negative  essence  of  the  anion, 
is,  in  liquid  electrolytes,  attached  to  an  atom  of 
matter.  This  atom  may  possibly  be  associated 
with  other  atoms  or  molecules  forming  a  complex 
ion,  but  the  point  is  that  the  isolated  electron 
cannot  slip  from  one  atom  to  another,  and  thus 
carry  an  electric  current  through  the  liquid  ;  the 
electron  cannot  move  without  a  corresponding 
movement  of  matter — of  matter,  that  is,  in  its 
atomic  or  molecular  sense. 

Here  again  the  motion  of  the  positive  ion 
involves  the  simultaneous  passage  of  a  particle 
of  matter  of  at  least  atomic  dimensions.  The 
positive  ion  consists  of  an  atom  of  the  electrolyte 
with  one  of  its  electrons  missing.  In  this  way, 
a  unit  of  negative  electricity  is  removed  from  it, 
that  is,  it  is  left  with  a  positive  charge. 

In  metals  an  electric  current  flows  without 
chemical  change  in  the  substance  of  the  con- 
ductor, so  that,  in  this  case,  we  must  imagine 
the  electrons  to  be  freely  mobile.  They  pass 
from  atom  to  atom,  and  thus  carry  the  current 
when  an  electromotive  force  acts.  In  the  presence 
or  absence  of  such  a  force,  they  may  be  regarded 
as  existing  within  the  metal  in  a  state  resembling 
in  many  ways  the  state  of  a  gas  in  a  closed 
vessel.  Estimates  have  been  made  of  the 
number  of  electrons  present  in  a  given  volume ; 


i6o  PHYSICAL  SCIENCE 

of  the  velocity  with  which  they  move  under  an 
electric  force  ;  and  of  their  mean  free  path  within 
the  metal,  that  is,  of  the  average  distance  an 
electron  moves  between  its  collisions  with  other 
electrons.  As  we  have  seen,  when  the  metal  is 
heated,  the  electrons  begin  to  leave  it,  and  stream 
away  into  the  surrounding  space.  At  any  constant 
temperature  equilibrium  is  set  up  between  the 
electrons  leaving  the  metal  owing  to  the  effect 
of  temperature,  and  those  drawn  back  again  by 
the  residual  positive  charge  on  the  metal.  We 
may  look  on  the  system  as  analogous  to  a  liquid 
in  equilibrium  with  its  own  vapour. 

In  the  last  chapter  we  saw  that  it  was  necessary 
clearly  to  distinguish  the  electric  current  and  the 
heating  effect  of  the  current  from  the  flow  of  the 
energy  by  which  the  current  was  maintained. 
The  energy  passes  through  the  surrounding 
medium,  through  the  luminiferous  sether.  The 
current  is  merely  the  line  along  which  the  energy 
of  the  aether  can  be  dissipated  as  heat.  Faraday 
and  Maxwell  showed  that  the  medium  invoked  to 
explain  the  phenomena  of  light  was  also  competent 
to  explain  electric  and  magnetic  manifestations. 
An  electric  force  is  a  state  of  strain  in  the  aether, 
and  the  immediate  function  of  an  electric  machine 
or  voltaic  battery  is  to  set  up  such  a  state  of  strain. 
If  the  poles  of  the  battery  are  insulated  from  each 
other,  the  state  of  strain  is  maintained,  the  poles 
are  attracted  towards  each  other  with  a  small 
force,  but  nothing  else  happens.  Faraday,  as 
we  have  seen,  represented  this  state  of  strain  by 
drawing  lines  of  force,  or  tubes  of  force,  which 
map  out  the  electric  field,  and  everywhere  follow 
the  direction  of  the  electric  force.     The  tubes  of 


CONDUCTION  THROUGH  GASES        i6i 

electric  force  end  on  the  surfaces  of  conductors, 
and  the  opposite  ends  of  each  tube,  where  they 
touch  the  conductors,  constitute  unit  electric 
charges  of  opposite  sign.  The  state  of  strain  in 
the  field  Is  such  that  we  must  imagine  the  tubes 
of  force  as  tending  to  shorten  in  length  and  to 
push  each  other  apart ;  and,  when  the  poles  of 
a  battery  are  disconnected,  the  tubes  of  force 
will  be  in  equilibrium  under  these  forces.  The 
distribution  of  the  electric  tubes  will  then  be 
very  similar  to  that  of  the  magnetic  lines,  made 
visible  by  the  filings  shown  in  Fig.  31  on  p.  153. 
A  conducting  wire  must  be  regarded  as  a 
channel  along  which  the  free  ends  of  a  line  or 
tube  of  force  can  move,  and,  when  the  poles  of 
the  battery  are  connected  by  means  of  a  wire,  the 
tubes  of  force  in  the  surrounding  air  run  their 
opposite  ends  on  to  the  wire,  pull  those  ends 
towards  each  other,  and  shut  up.  Other  tubes 
are  then  pushed  into  the  wire  by  their  mutual 
transverse  pressure,  and  are  obliterated  in  turn. 
The  tubes  of  force  In  the  dielectric  field  are  thus 
inclined  to  disappear,  and  the  state  of  sethereal 
strain  In  that  field  tends  to  be  relieved.  Simulta- 
neously, however,  the  battery  endeavours  to 
reassert  the  original  distribution  of  tubes,  and 
once  more  to  set  up  the  strain.  In  this  way 
new  tubes  are  constantly  forming  between  the 
terminals  of  the  battery,  and  are  as  constantly 
pushed  into  the  connecting  wire,  where  they 
vanish.  When  the  connection  is  metallic,  it  is 
only  the  negative  ends  of  the  tubes,  attached  to 
the  electrons,  that  move,  the  positive  ends 
remain  at  rest.  If,  on  the  other  hand,  part  of 
the  circuit  is  composed  of  an  electrolyte,  In  that 

M 


i62  PHYSICAL  SCIENCE 

part  the  positive  ends  of  the  tubes  are  also  mobile. 
Now  it  is  this  continual  process  of  establishment 
of  aethereal  strain  by  a  battery,  and  the  compensa- 
ting process  of  its  obliteration  along  a  conductor 
that,  according  to  the  views  of  Faraday  and 
Maxwell,  now  accepted  as  one  aspect  of  the  truth, 
constitute  an  electric  current. 

The  ionic  theory  of  electrolysis  gave  a  clear 
idea  of  the  mechanism  by  which  the  slipping  of 
the  ends  of  the  tubes  of  force  occurred  in  con- 
ducting liquids,  and  the  electronic  hypothesis 
gives  us  an  equally  vivid  insight  into  the  nature 
of  the  process  within  metallic  circuits.  The  tubes, 
anchored  by  their  ends  to  an  ion  in  electrolytes 
or  to  an  electron  in  metals,  drag  their  anchors. 
It  is  the  slip  of  the  anchors  that  constitutes  the 
current,  and  the  heat  developed  by  the  passage 
of  the  current  is  to  be  explained  by  the  frictional 
resistance  to  the  drag  of  the  anchor,  or  to  some 
other  means  of  dissipating  energy,  such  as  internal 
radiation,  not  yet  fully  understood. 

Faraday  had  no  skill  in  mathematical  analysis, 
and  his  insight  into  physical  principles  is  one  of 
the  best  examples  of  scientific  instinct  found  in 
history.  As  was  well  said  by  Von  Helmholtz  in 
the  Faraday  Lecture  for  the  year  1881,  *' Now 
that  the  mathematical  interpretation  of  Faraday's 
conceptions  regarding  the  nature  of  electric  and 
magnetic  forces  has  been  given  by  Clerk  Maxwell, 
we  see  how  great  a  degree  of  exactness  and 
precision  was  really  hidden  behind  the  words, 
which  to  Faraday's  contemporaries  appeared 
either  vague  or  obscure  ;  and  it  is  in  the  highest 
degree  astonishing  to  see  what  a  large  number 
of  general  theorems,  the  mathematical  deduction 


CONDUCTION  THROUGH  GASES        163 

of  which  requires  the  highest  powers  of  mathe- 
matical analysis,  he  formed  by  a  kind  of  intuition, 
with  the  security  of  instinct,  without  the  help  of  a 
single  mathematical  formula.  I  have  no  intention 
of  blaming  his  contemporaries,  for  I  confess  that 
many  times  I  have  myself  sat  hopelessly  looking 
upon  some  paragraph  of  Faraday's  descriptions 
of  lines  of  force,  or  of  the  galvanic  current  being 
an  axis  of  power." 

Such  a  confession  from  a  man  of  the  com- 
manding ability  of  Von  Helmholtz  shows  how  far 
the  instinctive  genius  of  Faraday  had  carried  him 
in  advance  of  his  age.  "  We  must  also  in  his 
case  acquiesce  in  the  fact  that  the  greatest  bene- 
factors of  mankind  usually  do  not  obtain  a  full 
reward  during  their  lifetime,  and  that  new  ideas 
need  the  more  time  for  gaining  general  assent 
the  more  really  original  they  are,  and  the  more 
power  they  have  to  change  the  broad  path  of 
human  knowledge." 


CHAPTER   VII 

RADIO-ACTIVITY 

To  watch  the  abysm-birth  of  elements. 

— Keats,  Efidymion. 

Scientific  investigation,  which  usually  proceeds 
unmarked  by  most  of  those  not  directly  engaged 
in  it,  is  from  time  to  time  forced  on  the  attention 
of  the  public  by  some  discovery  of  immediate  and 
striking  advantage  to  mankind,  or  by  the  attain- 
ment of  some  theoretical  result,  which,  from  its 
novelty  and  interest,  fires  the  imagination  of 
every  thinking  man. 

To  those  who  follow  closely  the  course  of 
research,  these  brilliant  advances  in  knowledge 
rarely  come  suddenly.  The  slow  and  patient 
work  of  many  observers  through  long  years  often 
leads  up  to  and  suggests  the  particular  step  from 
which  follows,  almost  of  necessity,  the  practical 
application  or  the  far  -  reaching  theory.  The 
mathematical  genius  of  Clerk  Maxwell,  the  experi- 
mental skill  of  Hertz,  laid  the  foundations  on 
which,  years  afterwards,  was  reared  the  super- 
structure of  wireless  telegraphy.  The  observa- 
tions of  Crookes,  Lenard,  J.  J.  Thomson,  and 
many  others,  on  electric  discharges  through 
rarified  gases,  had  given  to  the  physicist  an 
extended  insight  into  the  nature  of  these  pheno- 
mena, before  Rontgen's  almost  accidental  dis- 
covery— that    photographically  active  rays    thus 

1(34 


L  ^t-^H^^iU^fyi'^i^ 


[To  face  page  164. 


RADIO-ACTIVITY  '         165 

obtained  could  traverse  certain  substances  opaque 
to  light — revealed  the  bones  in  his  hand  to  the 
man  in  the  street. 

General  attention  was  first  directed  to  the 
subject  of  radio-activity  when  in  1903  M.  Curie 
demonstrated  that  the  stream  of  energy  proceed- 
ing constantly  from  the  newly-discovered  element 
radium  could  be  detected  by  a  measurable  rise  of 
temperature  in  a  small  quantity  of  the  substance 
protected  from  loss  of  heat.  From  then  onwards, 
an  unbroken  series  of  successful  experimental 
researches  and  brilliant  theoretical  generalisations 
have  together  vastly  extended  our  knowledge  of 
nature  and  revolutionised  the  outlook  of  physical 
science. 

In  this  case  also  the  essential  phenomena  have 
been  under  investigation  longer  than  is  generally 
known  ;  and  their  detection  naturally  arose  from 
a  knowledge  of  the  properties  of  Rontgen  rays. 
These  rays  produce  fluorescent  effects  on  suitable 
screens  ;  and  it  was  natural  to  examine  phosphor- 
escent and  fluorescent  substances,  to  determine  if 
they  were  the  source  of  similar  radiation.  For 
some  time  no  definite  results  were  obtained  ;  but, 
in  the  year  1896,  M.  Henri  Becquerel  discovered 
that  compounds  of  the  metal  uranium,  whether 
phosphorescent  or  not,  affected  a  photographic 
plate  through  an  opaque  covering  of  black  paper, 
and  rendered  the  air  in  their  neighbourhood  a 
conductor  of  electricity. 

Such  were  the  first  observations  on  the 
property  of  radio-activity  ;  but  the  rapid  develop- 
ment of  the  subject  which  has  followed  could  only 
have  taken  place  with  the  aid  of  our  previous 
knowledge  of  the  electrical  properties  of  gases. 


i66  PHYSICAL  SCIENCE 

Although  the  superficial  similarity  between 
Becquerel  rays  and  Rontgen  rays  has  proved  for 
the  most  part  misleading,  the  relations  between 
the  two  branches  of  the  subject  are  so  intimate 
that  it  is  impossible  to  study  satisfactorily  the 
phenomena  of  radio-activity  without  a  knowledge 
of  the  results  previously  and  simultaneously 
reached  by  the  investigation  of  electric  discharge 
through  gases. 

After  Becquerel's  discovery  of  the  photographic 
and  electric  activity  of  uranium,  it  was  found 
that,  like  Rontgen  rays,  the  rays  from  uranium 
produced  electric  conductivity  in  air  and  other 
gases  through  which  they  passed.  Compounds 
of  thorium,  too,  were  found  to  possess  similar 
properties.  In  the  year  1900,  M.  and  Mme. 
Curie  made  a  systematic  search  for  these  effects 
in  a  great  number  of  chemical  elements  and  com- 
pounds, and  in  many  natural  minerals.  They 
found  that  several  minerals  containing  uranium 
were  more  active  than  that  metal  itself.  Pitch- 
blende, for  instance,  a  substance  consisting  chiefly 
of  an  oxide  of  uranium,  but  containing  also  traces 
of  many  other  metals,  was  especially  active. 
When  obtained  from  Cornwall  its  activity  was 
about  equal  to  that  of  the  same  weight  of  uranium, 
but  Samples  from  the  Austrian  mines  were  found 
to  be  three  or  four  times  as  effective.  The 
presence  of  some  more  active  constituent  was  thus 
suggested.  To  examine  this  point,  the  various 
components  of  pitch-blende  were  separated  chemi- 
cally from  each  other  and  their  radio-activities 
determined.  In  this  way  three  different  sub- 
stances,   radium,    polonium,    and    actinium,    all 


RADIO-ACTIVITY  167 

previously  unknown,  were  quickly  isolated  by 
different  observers.  Of  these  three  the  most 
famous  is  the  now  well-known  radium,  discovered 
by  M.  and  Mme.  Curie,  working  with  M.  Bemont. 
Radium  is  obtained  from  pitch-blende  in  com- 
pany with  the  metal  barium  ;  and  the  two  seemed 
at  first  to  be  connected  chemically  so  intimately 
that  the  new  substance  was  for  a  time  called 
*' active  barium."  However,  a  slight  difference  in 
the  solubilities  of  some  of  their  salts  allows  them 
to  be  separated  gradually  by  a  process  of  repeated 
fractionisation,  the  radium  chloride  and  bromide 
crystallising  out  more  readily  than  the  correspond- 
ing compounds  of  barium. 

These  processes  of  chemical  separation  were 
remarkable  for  their  use  of  the  new  property  of 
radio-activity  as  a  sole  guide  in  the  operations. 
After  each  reaction  the  activities  of  both  the 
product  and  the  residue  were  determined.  It  was 
thus  settled  whether  the  reaction  just  tried  was 
effective,  and  in  which  of  the  substances  separated 
by  the  reaction  the  property  of  radio-activity  had 
been  concentrated. 

The  quantity  of  radium  present  in  pitch-blende 
is  extremely  small,  many  tons  of  the  mineral 
yielding,  after  long  and  tedious  work,  only  a  small 
fraction  of  a  gram  of  an  impure  salt  of  radium. 
Its  extraction  is  consequently  a  matter  of  great 
labour  and  high  cost.  Radium  salts  of  fair  purity 
have  now  become  articles  of  commerce,  though 
the  supply  is  insufficient  to  meet  the  demand  ; 
and  radium  is  at  present  worth  many  thousand 
times  its  weight  in  gold. 

An  interesting  point  in  these  investigations  is 
the  extreme  sensitiveness  of  the  property  of  radio- 


i68  PHYSICAL  SCIENCE 

activity  as  a  test  for  the  presence  of  those  sub- 
stances which  possess  it.  A  deHcate  electroscope 
will  show  easilya  leak  of  electricity  with  a  substance 
having  an  activity  of  about  the  one-hundredth  part 
of  that  possessed  by  uranium.  The  activity  of 
pure  radium  has  been  estimated  as  about  two 
million  times  that  of  uranium  ;  and  such  radium  is 
a  definite,  well-marked  chemical  element,  like  other 
elements,  forming  salts  and  other  chemical  com- 
pounds, and  giving  strong  bright  lines  when  heated 
and  examined  with  a  spectroscope.  Spectrum 
analysis  has  hitherto  been  the  most  delicate  means 
at  our  disposal  for  detecting  the  presence  of  the 
chemical  elements  ;  but  in  the  preparation  of 
radium  from  pitch-blende  its  spectrum  only  began 
to  appear  when,  in  the  prolonged  process  of 
fractionisation,  the  product  had  reached  an  activity 
of  about  fifty  times  that  of  uranium. 

It  appears  from  these  figures  that  the  electro- 
scopic  method  of  detecting  radio-active  matter  is 
several  thousand  times  more  sensitive  than  the 
most  refined  methods  of  spectrum  analysis,  and  in 
other  cases  a  still  greater  sensitiveness  seems  to 
have  been  reached.  History  has  again  repeated 
itself.  When  the  spectroscope  was  first  placed  in 
the  hands  of  chemists,  it  revealed  the  existence  of 
several  elements  which  occurred  in  quantities  too 
small  to  be  detected  by  any  other  means  then 
known.  In  a  similar  way  additional  elements  have 
now  been  detected  and  isolated  by  the  help  of  the 
newer  and  more  powerful  method  of  research. 

In  the  year  1899  Professor  Rutherford  of 
Montreal,  now  Sir  Ernest  Rutherford,  Cavendish 
Professor  at  Cambridge,  discovered  that  the  radia- 
tion from  uranium  consists  of  two  distinct  parts. 


RADIO-ACTIVITY  169 

One  part  was  found  to  be  unable  to  pass  through 
more  than  about  four  layers  of  thin  aluminium 
foil,  while  the  other  part  would  pass  through  about 
one  hundred  layers  before  its  intensity  was  reduced 
by  one  half.  The  first  named,  or  a  rays,  produce 
the  most  marked  electric  effects,  while  the  more 
penetrating,  or  /5  rays,  are  those  which  affect  a 
photographic  plate  through  opaque  screens.  At 
a  later  date  was  detected  a  third  type  of  still  more 
penetrating  radiation,  known  as  7  rays,  which 
can  traverse  plates  of  lead  a  centimetre  thick,  and 
still  produce  photographs  and  discharge  electro- 
scopes. In  proportion  to  its  general  activity, 
radium  evolves  all  three  types  of  radiation  much 
more  freely  than  uranium,  and  is  best  employed 
for  their  investigation. 

The  moderately  penetrating  or  fi  rays  can  be 
deflected  easily  by  a  magnet ;  and  Becquerel,  who 
deflected  them  by  an  electric  field  as  well,  con- 
clusively proved  that  they  were  projected  particles, 
charged  with  electricity.  M.  and  Mme.  Curie  had 
shown  previously  by  direct  experiment  the  exist- 
ence of  a  negative  charge  associated  with  these 
rays.  Owing  to  their  ionizing  action,  it  is  im- 
possible to  demonstrate  that  a  body  surrounded 
by  air  gains  a  charge  when  exposed  to  the  rays. 
Such  a  charge  would  leak  away  as  fast  as  it  was 
acquired.  But,  by  working  in  a  very  good  vacuum, 
or  by  surrounding  the  body  with  a  solid  dielectric 
such  as  paraffin,  the  acquisition  of  a  negative 
charge  can  be  demonstrated  by  means  of  an 
electrometer.  Further  investigation  showed  that 
the  i^  rays  behave  in  all  respects  like  cathode  rays, 
although  they  possess  greater  velocities  than  any 
cathode  rays  hitherto  examined,  velocities  which 


170  PHYSICAL  SCIENCE 

have  different  values  ranging  from  60  to  95  per 
cent,  of  the  velocity  of  light.  The  ^  rays,  then, 
are  negative  corpuscles,  or  negative  electrons. 

Magnetic  and  electric  fields  which  are  strong 
enough  to  deflect  considerably  the  /5  rays,  produce 
no  effect  on  the  easily  absorbed  a  rays.  R.  J. 
Strutt,  now  Lord  Rayleigh,  suggested  in  the  year 
1900,  that  the  a  rays  were  positively  charged 
particles,  of  mass  greater  than  that  of  the  negative 
/5  particles,  but  it  was  not  till  some  time  after- 
wards that  their  magnetic  and  electric  deviations 
were  demonstrated  experimentally,  and  shown  to 
be  in  the  direction  opposite  to  that  observed  with 
/5  rays.  The  mass  of  the  carriers  in  the  a  rays, 
as  calculated  from  the  deviations,  is  the  same  as 
that  of  helium  atoms — more  than  four  thousand 
times  that  of  the  negative  electrons — and  the 
positive  charge  associated  with  the  particles  is 
found  to  be  double  that  on  a  univalent  ion.  The 
velocity  is  about  one-tenth  of  that  of  light. 

The  very  penetrating  or  7  rays  have  never 
been  deflected,  and  from  this  fact  it  has  been 
supposed  that  they  are  different  in  kind  to  the 
other  types,  and,  like  the  X-rays  discovered  by 
Rontgen,  consist  of  electro-magnetic  waves  similar 
in  nature  to  light  but  of  shorter  wave-length.  On 
the  analogy  of  the  cathode  rays,  we  should  expect 
that  such  pulses  would  be  started  as  a  secondary 
effect  of  the  /5  rays.  In  1903,  Strutt  published 
experiments  which  show  that,  as  with  the  a  and 
^  rays,  and  also  with  the  cathode  rays,  different 
gases  absorb  the  7  rays  in  direct  proportion  to  the 
density.  The  absorption  phenomena  exhibited 
by  ordinary  Rontgen  rays  are  of  an  entirely 
different  kind.     But  very  *'hard"  Rontgen  rays 


RADIO-ACTIVITY  171 

— that  IS,  the  extremely  short  electro-magnetic 
waves  produced  by  the  cathode  rays  of  very 
high  vacua — show  absorption  phenomena  similar 
to  those  of  the  7  rays  of  radium.  Forasmuch  as 
the  P  rays  travel  with  velocities  higher  than  those 
of  any  ordinary  cathode  rays,  we  should  naturally 
expect  the  resulting  waves  to  have  shorter  wave- 
lengths than  ordinary  Rontgen  rays.  The  nature 
of  7  rays  was  finally  placed  beyond  dispute  when 
the  same  experiments  on  crystals  were  carried  out 
with  them  as  had  been  so  successful  with  X-rays. 
The  7  rays  also  showed  diffraction  phenomena, 
and  gave  several  spectral  lines  of  wave-length 
about  io~^  centimetres.  It  seems  certain  then 
that  the  7  rays  are  identical  in  nature  and  origin 
with  very  **hard"  Rontgen  rays. 

All  the  three  types  of  radiation,  when  they 
pass  through  air  or  any  other  gas,  render  the  gas 
a  conductor  of  electricity,  so  that  the  charge  of 
an  electroscope  or  of  an  electrometer  leaks  away. 
The  charged  particles  of  atomic  mass  which 
constitute  the  a  rays,  the  negative  corpuscles  or 
electrons  which  form  the  /3  rays,  and  the  7  rays, 
short  electric  waves,  are  all  able  to  convert  some 
of  the  molecules  of  a  gas  into  electrified  ions. 
The  a  and  (3  projectiles  probably  effect  this 
change  by  the  energy  of  their  collisions  with  the 
molecules  of  gas,  and  it  is  possible  to  estimate 
the  number  of  ions  produced  by  each  shot.  It 
has  been  reckoned  that  this  number  is  sufficient 
to  give  air  a  measurable  conductivity  when  one 
positive  particle  per  second  is  emitted  by  the 
radio-active  substance.  Even  if  one  atom  of 
radium  emits  only  one  such  particle,  this  estimate 
means    that  the  electroscope  is  able   to   detect 


172  PHYSICAL  SCIENCE 

effects  which  depend  on  one  atom  coming  into 
action  each  second.  We  may  well  be  astonished 
at  the  delicacy  of  this  means  of  research. 

Again,  all  three  kinds  of  rays  produce  phos- 
phorescent and  photographic  effects,  though  the 
penetrating  powder  of  the  /3  and  7  rays  makes 
the  phenomena  due  to  them  more  remarkable. 

Radium  salts  are  self-luminous,  owing  either 
to  the  direct  emission  of  light  by  their  agitated 
atoms,  or  to  some  phosphorescent  effect  of  the 
internal  bombardment  produced  by  their  radio- 
activity. The  spectrum  of  this  spontaneous 
luminosity  was  photographed  by  Sir  William  and 
Lady  Huggins,  and  shown  to  correspond  with 
the  spectrum  obtained  by  passing  electric  sparks 
through  nitrogen.  Sir  William  Crookes  and  Sir 
James  Dewar  found  that  this  spectrum  vanished 
when  the  radium  compound  was  placed  in  a  high 
vacuum.  Probably,  therefore,  it  Is  due  to  the 
effect  of  the  activity  of  the  radium  on  atmospheric 
nitrogen  surrounding  the  radium  salt  or  occluded 
within  It. 

A  screen  of  the  phosphorescent  substance,  zinc 
sulphide,  when  placed  In  the  neighbourhood  of 
a  radium  compound,  glows  brightly,  and  Crookes 
has  used  this  property  In  a  most  striking  and 
beautiful  experiment.  A  tiny  fragment  of  a 
radium  salt  is  fixed  at  the  distance  of  a  fraction 
of  a  millimetre  in  front  of  a  plate  covered  with 
zinc  sulphide.  On  looking  through  a  lens  or 
a  low-power  microscope  In  a  dark  room,  brilliant 
scintillations  are  seen,  and  the  effect  of  the  atomic 
projectiles  of  the  a  radiation  as  they  strike  the 
target  Is  thus  made  visible  to  the  human  eye. 
In  1908  Rutherford  used  this  effect  to  count  the 


RADIO-ACTIVITY  173 

number  of  a  particles  in  a  narrow  pencil  of  the 
rays,  and  recalculated  from  his  results  several 
radio-active  constants. 

In  the  year  1900  Rutherford  made  another 
striking  discovery.  The  radiation  from  thorium 
was  known  to  be  very  capricious,  being  affected 
especially  by  slight  currents  of  air  passing  over  the 
surface  of  the  active  material.  Rutherford  traced 
this  effect  to  the  emission  of  a  substance  which 
behaved  like  a  heavy  gas  having  temporary 
radio-active  properties.  This  emanation,  as  it 
was  named,  is  to  be  distinguished  clearly  from 
the  radiations  previously  described,  which  travel 
in  straight  lines  with  velocities  approaching  that 
of  light.  The  emanation  diffuses  slowly  through 
the  atmosphere,  as  would  the  vapour  of  a  volatile 
liquid.  It  acts  as  an  independent  source  of 
straight  line  radiations,  but  suffers  a  decay  of 
activity  with  time. 

Similar  emanations  are  given  off  by  radium 
and  actinium,  but  not  by  polonium  or  uranium. 
The  emanations  seem  to  be  very  inert  chemically, 
in  this  resembling  gases  of  the  argon  group. 
They  pass  unchanged  through  acids  or  hot  tubes, 
but  are  condensed  at  the  temperature  of  liquid 
air,  evaporating  again  as  the  tube  is  warmed. 
By  taking  advantage  of  this  property,  many  pretty 
lecture-room  experiments  may  be  performed.  For 
example,  a  quantity  of  radium  emanation  is  con- 
densed in  a  tube  surrounded  with  liquid  air.  The 
tube  is  connected  with  others,  and,  if  the  liquid 
air  be  removed,  the  emanation  can  be  traced  as 
it  diffuses,  by  the  fluorescence  it  excites  on  the 
glass,  or  on  small  pieces  of  paper  covered  with 
zinc  sulphide,  which   are  placed  here  and  there 


174  PHYSICAL  SCIENCE 

within  the  tubes.  By  measuring  the  rates  of 
diffusion  of  the  emanations  into  other  gases, 
their  densities  have  been  determined  approxi- 
mately and  found  to  be  of  the  order  of  two 
hundred  times  that  of  hydrogen. 

When  the  emanations  come  into  contact  with 
solid  bodies,  they  cause  these  bodies  themselves 
to  become  temporarily  radio-active.  This  radio- 
activity, which,  in  some  cases,  is  found  to  be 
acquired  more  readily  by  negatively  electrified 
surfaces,  has  been  traced  to  radio-active  deposits 
clinging  to  the  surfaces.  Whatever  the  effective 
substance  may  be,  it  may  be  treated  chemically, 
and  can  be  dissolved  in  some  acids  and  regained 
as  a  radio-active  residue  on  evaporation. 

All  the  three  types  of  radiation  considered 
above,  and  known  as  a,  1^,  and  y  rays,  have  one 
remarkable  property  which,  at  first  sight,  is  not 
shared  by  the  emanations  just  described.  The 
radio-activity  of  any  element,  with  regard  to  the 
emission  of  these  rays,  is  independent  of  the  com- 
pound in  which  that  element  is  contained.  Thus, 
for  a  mass  containing  the  same  amount  of  the 
element  radium,  the  activity  of  radium  chloride 
is  the  same  as  that  of  radium  bromide ;  while 
uranium,  the  metal,  has  the  same  activity  as  it  has 
when  combined  chemically  in  uranium  nitrate. 
Moreover,  an  alteration  in  the  physical  con- 
ditions, such  as  temperature,  which  always  largely 
influence  the  course  of  ordinary  physical  and 
chemical  changes,  seems,  throughout  an  extended 
range,  to  be  entirely  without  effect  on  the  processes 
involved  in  radio-activity.  Heating  to  redness, 
or  exposure  to  the  extreme  cold  of  liquid  air  or 
liquid   hydrogen,  equally  leave   the  activities  we 


RADIO-ACTIVITY  175 

are  considering  untouched.  In  liquid  hydrogen 
most  chemical  activities  are  entirely  suspended, 
and  these  results,  to  whatever  cause  they  may  be 
due,  are  very  remarkable.  It  seems  certain  that, 
even  when  we  approach  the  absolute  zero,  all 
the  activities  of  radium  are  quite  independent  of 
temperature.  Such  extraordinary  results  as  these 
point  to  a  deep-seated  difference  in  kind  between 
the  radio-active  processes  and  all  chemical  and 
physical  operations  hitherto  investigated.  We 
shall  presently  examine  this  point  more  closely. 

Unlike  the  "straight  line"  radiations  of  the 
types  a,  /3,  and  7,  the  emanations  discovered  by  Sir 
Ernest  Rutherford  are  emitted  much  more  freely 
from  some  compounds  of  the  radio-active  element 
than  from  others,  while  the  rate  of  emission  is 
largely  dependent  on  physical  conditions,  such  as 
the  temperature  of  the  system.  By  a  striking 
series  of  experiments,  however,  Rutherford  traced 
these  differences  to  variations  in  the  ease  with 
which,  after  formation,  the  emanation  escapes 
from  the  generating  substance. 

Let  us  consider  these  results  in  more  detail. 
It  is  found,  for  example,  that  while  the  emanation 
is  given  off  very  slowly  from  dry  and  solid  radium 
chloride,  it  is  emitted  freely  from  the  same  salt  in 
solution.  This  allows  the  problem  to  be  submitted 
to  the  test  of  quantitative  experiment.  The  rate 
of  decay  of  the  radium  emanation  is  known  ;  its 
activity  falls  to  half  value  in  '^.^  days.  Thus,  the 
activity  of  the  emanation  stored  in  a  solid  radium 
salt  reaches  a  limit,  when  its  rate  of  decay  becomes 
equal  to  the  constant  rate  at  which  the  emanation 
is  produced  by  the  radium.  On  the  hypotheses 
that  the  emanation  is  formed  at  the  same  rate  in 


176  PHYSICAL  SCIENCE 

the  solid  as  in  the  solution,  that  it  escapes  from 
the  solution  as  fast  as  it  is  formed,  and  that  it  does 
not  appreciably  escape  from  the  solid  at  all,  it  is 
clearly  possible  to  calculate  the  amount  of  emana- 
tion that  should  be  stored  in  the  solid,  as  compared 
with  the  amount  produced  and  emitted  by  the 
solution  in  a  given  time. 

The  calculation  shows  that  463,000  times  more 
should  be  stored  in  the  solid  than  is  emitted  by 
the  solution  in  one  second.  Now  if,  as  supposed, 
the  emanation  is  stored  in  the  solid,  this  large 
amount  will  be  liberated  instantaneously  when  that 
solid  is  dissolved  in  water.  Rutherford  and  Soddy 
measured  this  rush  of  emanation  by  its  effect  on  an 
electroscope,  and  found  that  it  was  477,000  times 
greater  than  the  quantity  afterwards  developed 
by  the  solution  in  one  second :  a  remarkable 
confirmation  of  the  several  hypotheses  given 
above. 

The  effect  of  raising  the  temperature  is  similar 
to  that  of  solution.  When  a  solid  radium  com- 
pound is  brought  to  a  red  heat,  a  rush  of  emana- 
tion takes  place,  which  makes  the  initial  emanating 
power  some  hundred  thousand  times  greater  than 
that  of  the  cold  solid.  This  high  rate  of  emission, 
however,  does  not  last ;  it,  also,  is  due  to  the 
rapid  escape  of  stored  material. 

By  experiments  such  as  these,  the  emanating 
power  of  radio-active  elements  has  been  brought 
into  line  with  their  other  radio-active  properties, 
and  has  been  shown  to  depend  only  on  the  mass 
of  the  element  present,  whatever  be  the  state  of 
combination  in  which  that  element  exists,  and 
whatever  be  the  physical  conditions  under  which 
the  process  occurs. 


RADIO-ACTIVITY  177 

Soon  after  appreciable  quantities  of  radium 
were  available  for  investigation,  Giesel  drew  atten- 
tion to  the  fact  that  a  radium  compound  gradually 
increases  in  activity  after  formation,  and  only 
reaches  a  constant  state  after  a  month's  interval. 
Similar  phenomena  were  observed  by  Curie  and 
Dewar  for  the  heat  effect.  These  results  are 
readily  explained  if  we  consider  the  properties  of 
the  emanation  as  elucidated  by  the  experimental 
evidence  that  has  now  accumulated. 

When  a  salt  of  radium  is  dissolved  in  water, 
and  the  solution  boiled,  the  emanation  previously 
stored  in  the  salt  is  evolved  and  removed.  The 
residual  activity  of  the  salt  is  then  found  to  be 
much  diminished.  This  activity  must  include 
that  due  to  the  radium  itself,  and  also  that  of 
the  active  deposit,  which  has  been  developed  by 
the  emanation,  but  is  not  removed  with  it.  The 
effect  of  the  active  deposit  decays  rapidly ;  after 
a  few  hours  it  will  nearly  have  vanished,  and  we 
then  get  the  true  activity  of  the  pure  radium  salt 
alone,  uncomplicated  by  that  of  the  emanation, 
or  by  that  of  the  active  deposit  which  is  produced 
by  the  emanation. 

This  residual,  non-separable  activity  is  found 
to  consist  entirely  of  a  rays,  and,  measured 
electrically,  is  about  25  per  cent,  of  the  normal 
activity  of  a  radium  compound  after  a  month's 
existence ;  a  normal  activity  which  comprises 
the  combined  effects  of  radium,  of  the  radium 
emanation,  and  of  the  active  deposit. 

Rutherford  and  Soddy  studied  these  relations 
in  detail.  They  dissolved  a  radium  compound, 
removed  the  emanation,  and  waited  till  the  activity 
of  the  deposit  had  subsided.     The  solution  was 

N 


178  PHYSICAL  SCIENCE 

then  evaporated,  and  the  recovery  of  the  activity  of 
the  solid  crystals  of  salt  was  traced  by  measuring 
at  intervals  the  ionizing  power.  The  results  are 
shown  in  Fig.  32,  where,  neglecting  the  residual 
activity,  the  recovery  curve  of  the  activity  of  the 
salt  is  compared  with  the  curve  of  decay  of 
activity  of  the  separated  emanation.  It  will  be 
seen  that  the  two  curves  are  complementary  to 
each  other ;    the  activity  of  the   emanation   falls 


€  8  H)  fZ 

LcLys 

Fig.  32. 

to  half  its  initial  value  in  a  little  less  than  four 
days,  and  the  purified  radium  salt  recovers  half 
its  final  activity  in  the  same  time.  If  the  activity 
of  the  emanation  at  any  instant  be  added  to  that 
of  the  recovering  radium,  the  result  is  equal  to 
the  normal  activity  of  the  radium  when  fully 
recovered.  Thus  the  total  activity  of  the  residual 
radium  and  its  separated  emanation,  considered 
together,  remains  constant  throughout,  though 
resolved  into  constituent  portions.  This  result 
again  illustrates  the  characteristic  feature  of  radio- 


RADIO-ACTIVITY  179 

active  processes  :  the  impossibility  of  changing 
the  amount  of  activity  by  any  ordinary  chemical 
or  physical  operations. 

Since  the  phenomena  of  radio-activity  have 
been  well  known,  and  the  various  types  of  radia- 
tion and  emanation  which  proceed  from  radio- 
active materials  clearly  distinguished,  traces  of  the 
property  have  been  found  to  be  disseminated  very 
widely.  Mr  C.  T.  R.  Wilson,  for  example, 
detected  radio-activity  in  newly-fallen  rain  and 
snow ;  when  evaporated  they  leave  a  residue 
which  discharges  an  electroscope.  Again,  Sir 
J.  J.  Thomson  found  that  when  air  is  bubbled 
through  various  samples  of  water  from  deep 
wells,  or  when  the  water  is  boiled  and  the  dis- 
solved air  driven  off  and  collected,  there  is  present 
in  the  air  a  radio-active  gas,  which  behaves  as 
though  it  were  the  emanation  from  some  active 
substance  of  which  slight  traces  are  contained  in 
the  water.  The  air  loses  its  active  properties, 
while  the  water  regains  a  small  part,  and  after 
some  days  will  again  yield  a  supply  of  active  gas. 
The  rate  of  recovery  and  decay  seem  to  be 
about  the  same  as  for  the  radium  emanation, 
and  this  suggests  that  the  active  material  is 
radium  in  minute  quantity. 

Again,  M'Lennan,  Rutherford  and  Cooke,  and 
Strutt  found  that  the  rate  of  leak  in  a  closed 
vessel  depends  on  the  nature  of  the  walls  of  the 
vessel.  But  Strutt  detected  some  variation  in 
the  rate  of  leak  with  different  samples  of  the 
same  material,  and  Cooke  diminished  the  rate  of 
leak  in  a  brass  electroscope  by  carefully  cleaning 
the  walls.     Probably  this  result  is  to  be  explained 


i8o  PHYSICAL  SCIENCE 

by  the  presence  of  slight  traces  of  some  active 
emanation  in  the  atmosphere,  and  the  consequent 
active  deposit  on  solid  materials,  which  active 
deposit  is  removed  by  cleaning.  Nevertheless, 
it  seems  that  a  few  elements  such  as  potassium, 
not  classed  as  radio-active,  show  the  effect  to  an 
extent  just  measurable. 

The  air  of  the  atmosphere  itself,  when  tested 
with  a  sensitive  electroscope,  is  found  to  possess 
a  slight  conductivity.  It  seems  likely  that  this 
effect  is  due  to  traces  of  some  radio-active  sub- 
stance, whence  issue  the  radiations  which  ionize 
the  air.  The  rate  of  leak  of  electricity  through 
air  has  been  shown  by  Elster  and  Geitel  to  be 
greater  in  a  cave  or  cellar  than  in  the  open ; 
while  air  drawn  from  a  clay  soil  contained  a 
radio-active  emanation.  From  such  experiments 
we  know  that  traces  of  some  radio-active  sub- 
stance are  present  in  many  places  in  the  earth  ; 
on  the  other  hand,  we  know  that  some  active 
bodies  emit  radiations  of  an  extremely  penetrating 
nature.  It  thus  seems  reasonable  to  believe  that 
the  slight  conductivity  which  appears  to  exist  at 
all  times  in  the  atmosphere  is  due  to  the  pro- 
duction of  gaseous  ions  by  the  action  of  stray 
radiations  proceeding  from  some  radio-active 
material,  near  or  far. 

It  was  hoped  at  first  that  radium  might  play 
a  useful  part  in  the  curative  treatment  of  certain 
diseases.  Rontgen  rays  have  occasionally  been 
employed  as  a  means  of  checking  the  spread  of 
cancer,  and  the  radiations  from  radium  also 
appeared  to  be  effective,  besides  being  applied 
far   more    easily    locally,    and    for    considerable 


RADIO-ACTIVITY  i8i 

periods.  But  there  are  grave  difficulties  in  the 
use  of  radium,  for  we  are  as  yet  very  ignorant  of 
its  entire  physiological  action  ;  its  after-effects  on 
those  who  have  handled  any  large  quantity  for 
some  time  are  far  from  reassuring. 

The  medicinal  springs  of  Bath  and  Buxton 
contain  radio-active  emanations,  while  radium 
itself  has  been  detected  in  the  solid  deposits  at 
Bath.  It  is  possible  that  the  curative  effects  of 
these  waters  is  caused  by  their  radio-activity,  and 
if  so,  the  uselessness  of  drinking  the  water,  when 
kept  and  removed  to  a  distance,  may  be  due, 
more  to  the  decay  of  the  activity  of  the  emana- 
tions, than  to  the  provident  imagination  of  the 
local  authorities. 

In  seeking  an  explanation  of  these  physio- 
logical effects,  some  experiments,  due  to  Mr  W.  B. 
Hardy,  must  be  noticed.  As  we  have  seen  in 
Chapter  V.,  solutions  of  salts  and  acids,  which 
are  conductors  of  electricity,  possess  the  power 
of  coagulating  clear  solutions  of  colloidal  or  jelly- 
like substances  such  as  albumen  or  sulphide  of 
arsenic,  and  this  action  is  readily  explained  by 
referring  the  coagulative  action  to  the  electric 
charges  on  the  ions. 

The  influence  of  charged  ions  on  colloidal 
solutions  being  thus  made  clear,  Hardy  tried  the 
effect  of  exposing  a  very  sensitive  solution  of 
globulin,  a  substance  contained  in  the  living  tissue 
of  animals,  to  the  charged  particles  emitted  from 
radium,  which  produce  ions  so  readily  when  pass- 
ing through  a  gas.  The  penetrating  /3  rays  were 
without  action,  but  the  easily  absorbed  a  rays, 
which  enter  a  film  of  the  liquid  when  it  is  placed 
near  a    radium  salt  with   no   screen   interposed, 


i82  PHYSICAL  SCIENCE 

immediately  coagulated  the  globulin.  On  the 
other  hand,  the  /5  and  7  rays  were  found  to 
induce  certain  chemical  reactions,  liberating  iodine 
from  iodoformin  presence  of  oxygen.  This  change 
is  also  produced  by  ordinary  light  and  by  Rontgen 
rays,  but  not  by  the  a  radiation.  These  results, 
physical  and  chemical,  may  explain  some  of 
the  curious  physiological  effects  of  radio-active 
substances. 

It  seems  unlikely  that  radio-activity  will  ever 
be  cheap  enough  for  us  to  use  its  energy  to  develop 
mechanical  power,  but  it  is  just  possible  that  the 
phosphorescence  of  sensitive  screens  in  the  neigh- 
bourhood of  a  radio-active  body  may  some  day 
be  employed  as  an  effective  source  of  light.  I  n  this 
way  luminous  effects  would  be  obtained  directly 
from  a  store  of  energy  self-contained  and  practi- 
cally inexhaustible,  whereas,  in  all  our  present 
arrangements,  light  is  derived  from  a  hot  body, 
and  large  quantities  of  energy  are  necessarily 
wasted  in  maintaining  the  incandescence. 

In  order  to  gain  some  insight  into  the  cause  of 
radio-activity,  we  must  now  examine  another  series 
of  phenomena  of  fundamental  importance,  which 
were  discovered  in  the  case  of  uranium  by  Crookes 
and  by  Becquerel,  and  in  the  case  of  thorium  by 
Rutherford  and  Soddy.  By  definite  processes  of 
chemical  fractionisation,  somewhat  like  those  by 
which  radium  was  isolated  from  pitch-blende,  pro- 
ducts can  be  obtained  in  minute  quantities  from 
uranium  and  thorium  many  times  more  active  than 
those  substances  themselves.  The  uranium  and 
thorium  from  which  those  products  have  been 
separated  lose  much  of  their  activity  ;  the  radiation 


RADIO-ACTIVITY  183 

they  then  emit  seems  to  be  an  inseparable  property 
of  the  elements  themselves,  and  is  of  the  a  type  only. 
To  the  separated  products  the  names  of  uranium- A' 
and  thorium-^  have  been  given.  They  may  be 
analogous  to  emanations  as  far  as  the  series  of 
radio-active  changes  is  concerned,  being,  however, 
solid  instead  of  gaseous  at  ordinary  temperatures. 

The  important  point  is  this  :  if  these  X  pro- 
ducts be  kept  for  some  weeks  or  months,  they  will 
be  found  to  have  lost  their  radio-active  properties, 
while  the  original  samples  of  uranium  or  thorium 
will  have  become  as  active  as  they  were  before 
the  separation,  and  will  again  emit  all  three  types 
of  radiation.  The  rates  at  which  the  processes  of 
loss  and  gain  of  activity  occur  have  been  studied 
carefully  by  Rutherford  and  Soddy,  and  shown 
to  correspond  accurately  with  each  other.  This 
correspondence  is  clearly  shown  by  the  curves  in 
Fi^-  33^  which  give  the  decay  of  activity  of  the 
separated  uranium-X,  and  the  recovery  of  the 
residual  uranium.  Again  we  see  that  the  total 
amount  of  activity  remains  constant,  and  is  not 
affected  by  the  processes  of  chemical  action. 

These  experiments  lead  to  a  definite  view  as 
to  the  source  of  the  radiations.  Whenever  radio- 
activity exists,  the  active  material  is  always  slowly 
changing  into  some  other  substance,  which  has 
distinct  chemical  properties,  and  can  be  separated 
by  chemical  means  from  the  original  material. 
Thus,  in  the  case  of  thorium  compounds,  the  radio- 
active body  producing  most  of  the  effects  usually 
observed  is  not  really  thorium,  but  a  definite 
substance  we  may  call  thorium-X,  which  is  being 
formed  at  a  constant  rate  from  the  bulk  of  the 
thorium,  and,  after  its  formation,  gradually  loses 


i84  PHYSICAL  SCIENCE 

its  activity.  The  radio-activity  of  the  pure  thorium 
seems  to  be  a  consequence  of  its  change  into 
thorium-X  and  to  accompany  that  change.  The 
activity  of  the  thorium-J^,  in  a  similar  way, 
accompanies,  and  is  a  consequence  of,  its  continual 
change  into  other  bodies,  in  this  case,  the  thorium 
emanation.  The  constant  activity  of  a  thorium 
compound,  as  ordinarily  found,  is  thus  due  to  a 


URAM/UM 


60  so  wo  tZO 

Time'     irv     days 

Fig.  33. 

balance  in  the  rate  of  production  of  the  active 
thorium-^  and  the  rate  of  its  loss  of  radio-activity. 
What  view  are  we  to  take  of  the  changes  In  the 
thorium  or  uranium  which  result  in  the  formation 
of  the  X  products,  and  what  further  changes  must 
we  suppose  to  go  on  when  the  X  products  give 
rise  to  emanations  or  other  bodies  ?  Are  these 
changes  of  the  nature  of  ordinary  chemical  action, 
in  which  atomic  or  molecular  combinations,  or 
rearrangements  of  the  atoms  in  a  molecule,  are 


RADIO-ACTIVITY  185 

involved,    or    must   we    look    deeper    for    their 
causes  ? 

Three  essential  pieces  of  evidence  should 
be  considered  in  this  connection.  The  rate  at 
which  radio-active  power  is  gained  or  lost 
depends  only  on,  and  is  always  proportional  to, 
the  total  amount  of  active  material  at  any  instant 
remaining  effective ;  it  does  not  depend  on  the 
concentration  of  that  material.  For  instance,  if 
the  activity  of  a  quantity  of  thorium-^,  or  of 
radium  emanation,  be  examined,  it  will  be  found 
to  decrease  during  each  unit  of  time  by  the  same 
fraction  of  the  value  it  had  at  the  beginning  of 
that  interval.  If,  in  the  first  four  days,  the 
activity  falls  to  half  its  initial  value,  during  the 
second  four  days  it  will  fall  to  half  that  half- 
value,  or  to  one  quarter  of  the  initial  value ; 
during  each  successive  four  days  the  remaining 
activity  is  halved,  the  process  being  represented 
by  a  curve  of  the  type  of  those  in  Figs.  32,  2)3- 
The  rate  of  decay  does  not  depend  on  the 
volume  which  the  material  occupies.  This  mode 
of  change  in  a  geometrical  progression,  depending 
only  on  the  total  amount  of  effective  material 
present  at  the  instant,  is  well  known  in  chemical 
processes.  In  such  processes  it  always  indicates 
that  the  reaction  is  an  alteration  going  on  in  the 
individual  molecules,  which  may  either  be  dis- 
sociating into  simpler  molecules,  or  be  suffering 
a  rearrangement  of  their  constituent  atoms.  Each 
molecule  undergoes  this  change  alone,  and  does 
not  react  with  other  molecules.  If,  on  the  other 
hand,  a  change  is  going  on,  in  which  combination 
or  rearrangement  between  two  reacting  systems 
is  involved,  whether  the  systems  consist  of  atoms 


i86  PHYSICAL  SCIENCE 

or  molecules,  another  law  holds  ;  and  the  rate  of 
change  is  found  to  increase  when  the  material  is 
concentrated  into  a  smaller  space,  so  that  the  two 
systems  are  more  closely  within  reach  of  each 
other.  In  the  phenomena  we  are  considering, 
then,  the  change  involves  one  system  only,  what- 
ever that  system  may  be. 

In  examining  the  further  question  thus  raised, 
we  are  confronted  at  once  with  the  remarkable 
fact  that  the  radio-activity  of  a  series  of  com- 
pounds of  any  radio-active  element  is  simply 
proportional  to  the  amount  of  the  element  which 
they  contain.  The  activity  of  the  element  is  not 
affected  by  its  state  of  combination,  or  by  very 
great  changes  in  the  physical  conditions,  such  as 
temperature,  which  play  a  large  part  in  determin- 
ing ordinary  physical  or  chemical  equilibrium. 
As  we  have  seen,  this  remarkable  result  applies 
not  only  to  the  emission  of  the  "rays,"  but  also 
to  the  formation  of  the  emanations  which  proceed 
from  some  of  the  radio-active  elements  ;  the  differ- 
ences in  emanating  power  have  been  traced  to 
differences  in  the  rate  at  which  the  emanations 
can  escape  from  the  various  compounds  under 
various  conditions.  The  law  of  decay  of  activity 
shows  that  one  reacting  system  only  is  involved  ; 
these  further  phenomena  show  that  the  system 
does  not  alter  with  the  changing  conditions  which 
are  found  to  affect  all  known  molecular  processes, 
or  with  the  state  of  combination  which  affects  the 
physical  and  chemical  properties  that  control  the 
behaviour  of  the  elements  in  all  other  respects. 
Moreover,  as  we  shall  see  later,  it  is  possible  to 
calculate  the  energy  liberated  by  a  given  amount 
of  radio-active  change.     This  energy  is  several 


RADIO-ACTIVITY  187 

million  times  greater  than  that  involved  in  the 
most  energetic  chemical  action  known.  ^ 

The  conclusion  is  thus  forced  on  us  that, 
in  radio-active  processes,  we  are  dealing  with 
changes  in  the  atoms  themselves,  and  are  watching 
the  phenomena  which  accompany  a  true  trans- 
mutation of  the  elements.  The  continuity  of  the 
problems  which  present  themselves  to  the  human 
intellect  is  once  more  strikingly  demonstrated, 
for  surely  the  imagination  must  be  deficient 
which  does  not  see  in  these  transformations  of 
matter  a  partial  fulfilment  of  the  dreams  of  the 
mediaeval  alchemist. 

The  strength  of  any  hypothesis  lies  in  its 
power  of  co-ordinating  observed  facts,  and  of  fore- 
casting intelligently  the  discoveries  of  the  future. 
If,  then,  we  accept  this  new  revelation,  and  in  its 
light  reconsider  the  phenomena  we  have  already 
discussed,  we  shall  be  able  to  marshal  our  facts 
in  orderly  array,  while  the  few  privileged  pioneers 
alone  can  tell  how  much  assistance  they  received 
from  it  in  their  brilliant  achievements. 

Let  us  then,  in  terms  of  this  new  theory,  re- 
state the  results  which  we  have  already  described. 
All  radio-active  elements  have  very  high  atomic 
weights,  the  atom  of  radium,  for  instance,  being 
226  times  as  heavy  as  that  of  hydrogen.  Radio- 
active atoms  are  therefore  very  complex  structures, 
and,  on  the  theory  we  are  considering,  are  capable 
of  breaking  down  into  simpler  and  lighter  systems. 
The  elements  thorium  and  uranium  contain  some 
few  atoms  which,  at  any  moment,  are  disintegrat- 
ing. As  we  have  seen,  the  activity  of  the  pure 
separated  thorium  or  uranium  consists  of  a  rays 
only.      Thus,  the  essential  process  of  the  radio- 


i88  PHYSICAL  SCIENCE 

activity  of  these  bodies  consists  in  the  emission  of 
a  rays,  the  disintegration  of  each  atom  resulting 
in  the  projection  of  one  or  more  a  particles  with  a 
velocity  about  one-twelfth  that  of  light,  while  the 
residues  break  down  into  new  and  simpler  atoms, 
which  are  themselves  in  a  state  of  instability,  and 
are  known  to  us  as  thorium- J^  and  uranium-JY. 

The  further  transformation  of  these  bodies  is 
very  rapid,  their  activity  disappearing  in  a  time  to 
be  measured  in  days.  In  radium  we  possess  an 
analogous  substance,  also  an  intermediate  product 
in  a  state  of  instability,  the  life  of  which  is  enor- 
mously longer.  The  primary  substance,  standing 
to  radium  as  thorium  stands  to  thorium-A^,  was 
discovered  by  Boltwood  and  named  ionium.  It  is 
itself  derived  from  uranium  through  uranium-X 
and  two  other  intermediate  products. 

In  compounds  of  radium  and  thorium,  we  get 
the  emanations  as  a  step  in  the  process  of  atomic 
dissociation.  These  bodies  also  are  unstable, 
that  is,  radio-active.  They  emit  new  a  rays,  and 
produce  the  radio-active  deposit  which  generally 
appears  on  the  walls  of  the  containing  vessel. 
This  again  breaks  down,  with  the  usual  accom- 
paniment of  a  radiation.  The  decay  of  the  active 
deposit  on  a  rod,  exposed  for  a  very  short  time 
to  the  radium  emanation,  is  shown  in  Fig.  34. 
The  curve  is  a  complicated  one,  and  may  profit- 
ably be  compared  with  the  simple  curves  giving 
the  rate  of  decay  of  the  activity  of  uranium-J\^, 
the  curve  of  Fig.  33  on  page  184,  and  with  the 
curve  of  decay  of  the  radium  emanation.  Fig.  32 
on  page  178.  Rutherford  has  shown,  however, 
that  the  complex  curve  of  the  decay  of  the  excited 
activity  of  radium  can  be  made  up  by  the  con- 


RADIO-ACTIVITY 


189 


junction  of  several  constituent  curves,  of  the  usual 
typical  form  shown  by  uranium-X.  At  least  three 
successive  changes  in  the  radio-active  matter 
are  indicated  :  the  actual  curve  is  the  resultant 
of   these   three   processes,   which  are   going   on 


'^O  60  80  100 

Time  in  MunjjZes 

Fig.  34. 

simultaneously.  If  the  measurements  be  confined 
to  the  /5  and  7  rays,  it  is  found  that  the  activity 
rises  from  zero  to  a  maximum  before  it  begins 
to  decay.  Radium,  radium  emanation  and  its 
first  product  emit  a  rays  only,  the  second  product 
gives  P  and  7  rays,  and  the  third  product  all  three 
types  of  radiation. 


190  PHYSICAL  SCIENCE 

Evidence  of  further  changes  is  also  forth- 
coming. Surfaces  exposed  to  the  emanation  of 
radium  retain  a  small  part  of  their  residual 
activity  for  several  years  without  appreciable 
diminution.  By  taking  advantage  of  differences 
in  volatility  and  other  properties,  Rutherford  has 
traced  three  more  stages  in  the  transmutation  of 
the  radium  products.  The  first  is  half  accom- 
plished in  about  sixteen  years,  and  involves  /8  and 
7  rays  ;  the  second  takes  six  days,  and  is  also 
accompanied  by  the  emission  of  ^  and  7  rays  ; 
while  the  third,  marked  by  a  radiation,  needs 
136  days  to  sink  to  half  its  initial  activity. 
Rutherford  calls  the  deposited  radio-active  matter 
radium  A,  B,  etc.,  and  writes  these  eight  genera- 
tions of  the  radium  pedigree  as  :  Radium,  radium 
emanation,  radium  A,  radium  B,  radium  C, 
radium  D,  radium  E,  radium  F. 

Radium  F  has  been  shown  by  Rutherford  to 
be  identical  with  the  substance  separated  by 
Madame  Curie  from  pitch-blende  and  called  by 
her  polonium.  Could  it  be  prepared  pure,  it 
should  be  several  hundred  times  as  active  as 
radium,  but,  as  half  of  it  would  vanish  in  about 
143  days,  the  labour  and  expense  needed  for  its 
separation  would  afford  but  a  short-lived  specimen 
for  the  investigator. 

A  somewhat  similar  series  of  changes  has  been 
made  out  in  the  case  of  thorium.  Another  radio- 
active constituent  too  has  been  separated  from 
pitch-blende  and  named  actinium  by  Debierne. 
It  is  derived  indirectly  from  uranium  and  forms 
an  X  product,  an  emanation,  and  several  solid 
deposits  distinguished  as  actinium  A,  B,  C,  and  D. 

The  quantities  of  matter  involved  in  any  radio- 


RADIO-ACTIVITY  191 

active  change  are  excessively  minute,  and  no  other 
method  at  present  known  enables  us  to  detect  the 
final  inactive  products  as  they  are  formed.  It 
is,  however,  not  improbable  that,  by  the  slow 
accumulation  of  material  which  must  of  necessity 
go  on  when  a  radio-active  body  is  kept  for  a 
long  time,  the  inactive  products  will  be  obtained 
eventually  in  amounts  sufficient  to  be  distin- 
guished by  the  spectroscope  or  even  by  ordinary 
chemical  analysis.  In  this  connection  attention 
was  soon  called  to  the  fact  that  in  all  radio-active 
minerals  lead  is  found  and  considerable  quantities 
of  helium  gas  are  occluded. 

Sir  William  Ramsay  and  Mr  Soddy,  by 
spectroscopic  methods,  detected  helium  in  the 
gases  evolved  from  a  sample  of  radium,  originally 
prepared  from  pitch-blende  and  kept  as  a  solid 
for  some  months.  The  spectrum  of  helium  was 
invisible  when  the  emanation  was  first  collected 
and  examined,  but  soon  appeared  and  gradually 
increased  in  intensity  with  the  lapse  of  time. 

Similar  results  were  then  obtained  by  Dewar 
and  Curie,  who,  moreover,  traced  the  disappear- 
ance of  a  minute  volume  of  the  emanation.  This 
has  been  explained  by  the  idea  that  the  resulting 
helium,  being  projected  in  the  atomic  state  with 
great  velocity,  penetrated  the  glass  walls  of  the 
vessel  and  thus  occupied  no  volume.  The  decrease 
in  the  volume  of  a  minute  quantity  of  emanation 
was  also  observed  by  Ramsay  and  Soddy. 

This  question,  however,  has  been  settled  finally 
by  the  researches  of  Rutherford  and  his  fellow- 
workers.  Measurements  of  the  maofnetic  and 
electric  deflection  of  the  a  rays  had  indicated  that 
the  a  particles  ejected  by  any  single  active  sub- 


192  PHYSICAL  SCIENCE 

stance  had  approximately  the  same  velocity,  but 
that  velocity  depended  on  the  substance  used> 
Radium  emanation,  for  instance,  emits  a  particles 
with  a  velocity  of  1.62  x  lo^  while  those  from 
radium  C  move  with  a  speed  of  1.92  x  10^  centi- 
metres per  second. 

On  the  other  hand,  for  all  a  particles,  whatever 
their  source,  the  ratio  e/m  of  the  charge  to  the 
mass  is  constant,  and  equal  to  about  4820  electro- 
magnetic units.  The  value  of  e/m  for  the  hydrogen 
ion  in  liquid  electrolytes  is  9649  in  the  same  units. 
If,  therefore,  the  a  particle  carries  the  same  unit 
electric  charge  as  the  hydrogen  ion,  its  mass  must 
be  twice  as  great ;  but  if  it  carries  a  double  charge 
its  mass  will  be  four  times  that  of  the  hydrogen 
atom  and  equal  to  that  of  helium. 

Clearly,  then,  the  determination  of  the  charge 
carried  by  the  a  particle  was  a  problem  of  funda- 
mental importance,  and  this  problem  was  attacked 
by  Rutherford  and  Geiger. 

We  have  already  described  the  scintillations 
produced  on  a  screen  of  zinc  sulphide  by  the  impact 
of  the  a  rays  from  a  minute  speck  of  a  radium  com- 
pound. Rutherford  and  Geiger  proved  that  each 
particle  causes  a  visible  scintillation,  and  then 
counted  the  particles  emitted  by  a  measured 
amount  of  a  radium  product.  They  also  used 
another  method.  A  quantity  of  gas  at  a  low 
pressure  exposed  to  an  electric  field  of  force  just 
not  strong  enough  to  give  a  spark,  is  in  a  very 
sensitive  state.  An  a  particle  shot  through  it 
produces  a  large  number  of  ions  by  collision  with 
its  molecules,  and  each  of  these  ions  is  set  in 
motion  by  the  electric  force,  producing  other 
ions  in  its  turn.     Each  a  particle  in   this   way. 


RADIO-ACTIVITY  193 

directly  or  Indirectly,  gives  rise  to  some  40,000 
ions  or  more,  enough  to  give  the  gas  a  measurable 
conductivity.  If,  therefore,  an  electrometer  be 
inserted  in  the  circuit,  the  flight  of  each  a  particle 
is  shown  by  a  transient  deflection — by  a  visible 
movement  of  a  spot  of  light  reflected  from  the 
needle  of  the  instrument  on  to  a  scale.  It  will  be 
noted  that,  in  these  two  ways,  Rutherford  and 
Geiger  detected  the  effect  of  an  individual  atom — 
a  crowning  verification  for  Dalton's  atomic  theory. 

The  number  of  a  particles  emitted  persecond  by 
the  product  radium  C  in  radio-active  equilibrium 
with  I  gram  of  radium  was  thus  estimated  at 
3.4  X  lo^^  The  same  number  must  be  emitted 
by  the  gram  of  radium  itself,  and  by  each  of  its 
three  a  ray  products — radium  emanation,  radium 
A,  and  radium  C.  Consequently  i  gram  of  radium 
and  its  products  in  equilibrium  emit  13.6  x  10^^  a 
particles  per  second. 

Having  counted  the  number,  the  next  thing 
was  to  estimate  the  total  charge  of  the  particles. 
The  a  rays  are  intercepted  by  an  insulated  plate, 
and  the  gain  in  electric  charge  by  the  plate  is 
measured  by  an  electrometer.  The  charge  carried 
by  each  a  particle  was  found  to  be  9.3  x  io~^^ 
electrostatic  units.  This  result  was  confirmed  by 
Regener,  who  obtained  the  figure  9.6  x  io~^*^  from 
radium  F. 

Now  Mllllkan's  value  for  the  single  unit 
charge  carried  by  electrons  on  univalent  ions  is 
4.77  X  io~"^°  electrostatic  units.  Thus  it  is  clear 
that  the  a  particle  carries  two  positive  electric 
units,  and  has  a  mass  four  times  that  of  a  hydrogen 
atom — that  it  is,  in  fact,  a  helium  atom  with  two 
unit  charges,  shot  forth  with  high  velocity. 

o 


194  PHYSICAL  SCIENCE 

To  verify  this  result,  Rutherford  and  Royds 
repeated  and  improved  the  earlier  spectroscopic 
experiments.  A  quantity  of  radium  emanation 
was  compressed  into  a  tiny  thin  walled  capillary 
glass  tube.  The  a  particles,  shot  through  the  thin 
walls,  were  collected  in  another  glass  tube  which 
surrounded  the  inner  one.  After  a  few  days  the 
complete  spectrum  of  helium  was  seen  by  sparking 
the  gas  from  this  outer  tube.  The  a  particle  has 
an  atomic  mass  of  4  and  a  charge  +  2e,  When  its 
velocity  is  destroyed  by  passing  through  matter, 
it  absorbs  two  negative  electrons,  and  becomes 
an  ordinary  neutral  helium  atom.  Helium  is  one 
final  product  of  radio-active  atomic  disintegration. 

In  the  course  of  this  investigation,  we  have 
seen  that  the  number  of  a  particles  emitted  per 
second  by  a  gram  of  radium  was  estimated  as 
3.4  X  10^^.  That  result  enabled  Rutherford 
incidentally  to  calculate  the  rate  of  decay  of  the 
radio-activity  of  radium,  and  therefore  the  life  of 
radium  itself.  The  activity  would  fall  to  half- 
value  in  1730  years,  and  we  may  therefore  con- 
clude that  a  mass  of  radium  would  disintegrate 
at  a  speed  which  would  destroy  half  of  it  in  that 
time. 

Each  exploding  radium  atom  emits  an  a 
particle — a  helium  atom  of  atomic  mass  4  with 
two  positive  electric  charges.  Hence  the  atomic 
weight  of  the  residue  is  reduced  by  4  and  radium, 
with  an  atomic  weight  of  226,  passes  into  radium 
emanation  with  an  atomic  weight  of  222.  So 
radium  A  must  have  an  atomic  weight  of  2 1 8,  and 
radium  B  of  2 14.  But  radium  B  passes  into  radium 
C  by  ejecting  p  and  7  rays  only,  hence  it  suffers 


RADIO-ACTIVITY  195 

no  appreciable  change  in  mass  and  the  atomic 
weight  of  radium  D  is  also  214. 

Now  let  us  consider  the  changes  in  electrical 
charge.  When  radium  loses  an  a  particle,  it  loses 
also  two  units  of  positive  charge.  We  shall  see 
later  that  there  is  reason  to  believe  that  what  is 
called  the  atomic  number  of  radium,  a  number 
which  measures  the  essential  electric  charge  on  the 
atomic  nucleus,  is  ZS.  Hence  the  nuclear  charge 
or  atomic  number  of  the  emanation  is  ^6,  of  radium 
A  is  84,  and  of  radium  B,  82.  Since  in  each  case 
the  whole  atom  is  neutral,  it  has  to  discard  also 
two  of  the  electrons  from  the  outer  orbits.  The 
whole  change  involves  a  complete  rearrangement, 
and  consequently  a  new  atom. 

When  radium  B  explodes,  it  emits  only  p  and 
y  rays,  and  in  losing  a  /3  particle  it  loses  a  negative 
electron  with  a  single  unit  charge.  The  high  speed 
of  the  /5  particle  shows  that  it  comes  from  the 
nucleus,  and  thus  the  residual  nucleus,  though  it 
has  practically  the  same  mass  as  its  parent,  has 
increased  its  charge  by  one  unit  and  become  a  new 
atom  with  quite  different  properties.  And  so  the 
whole  series  of  radio-active  changes  down  to 
radium  F  or  polinium  has  been  traced. 

The  question  of  the  ultimate  fate  of  the  radio- 
active matter  remains.  What  becomes  of  radium 
F  when  it  in  turn  disintegrates  ?  No  product  has 
been  detected  by  radio-activity,  and,  if  the  sub- 
stance formed  is  not  active,  we  can  only  investigate 
it  by  examining  minerals,  where  the  slow  accumu- 
lation of  ages  has  gone  on.  Now  Boltwood  pointed 
out  that  in  minerals  from  the  same  geological 
formations,  and  therefore  presumably  of  about  the 
same  age,  the  contents  of  lead  are  proportional  to 


196 


PHYSICAL  SCIENCE 


the  contents  of  uranium  and  radium.     Thus  lead 
was  indicated  as  the  last  traceable  stage  in  the 

Pedigree  of  the  Raditan  Family. 


Atomic 
Number. 

Atomic 
Weight. 

Time  of 
Half-decay. 

Radio- 
activity. 

Uranium 

4-. 

Uranium  Xj    . 

4' 
Uranium  X2    . 

^. 

Uranium  II     . 
1 

92 

238 

5  X  10^  years 

a 

90 

234 

23.5  days 

A  7 

91 

234 

I '17  minutes 

/S,7 

92 

?90 

234 
?230 

2  X  10^  years 
25-5  hours 

a 
/3 

?4 

Uranium  Y   . 

loni 

< 
um    . 

90 

230 

2  X  10^  years 

a 

Radium  .... 

Radium  Emanation . 

Radium  A       .         .         . 
J- 

88 

226 

1730 

a 

86 

222 

3.85  days 

a 

84 

218 

3'05  minutes 

a 

Rad 
Rad 

ium  B       .         .         . 

ium  C       .         .         . 

1 

82 
83 

81 

84 
82 

214 

214 

210 
214 
210 

26-8 
19-5 

1-38       „ 
?  10"^  seconds 
1 6 '5  years 

^,7 
a,^)7 

A  7 

a 

i 

^,7 

Rad 
Rac 

j8            Radium  C.  . 
ium  Ci      .         .         . 

\ 

lium  D       .         .         . 
1. 

Y 
Radium  E       .         .         . 

Radium  F  (Polonium)     . 

Lead       .... 

83 
84 

210 
210 

4' 8  5  days 
136 

i3,7 
a 

•  •  • 

206 

... 

... 

series   of  disintegrations  we  have  followed.     In 
recent   years,    Soddy  has  proved   that   the  lead 


RADIO-ACTIVITY  197 

from  radio-active  minerals  has  an  atomic  weigfht 
appreciably  different  from  that  of  other  lead.  To 
such  elements  with  different  atomic  weiofhts  but 
identical  chemical  properties,  he  gave  the  name  of 
isotopes.  We  have  seen  in  Chapter  VI.  how  Aston 
has  discovered  many  other  isotopes  by  a  quite 
different  method. 

It  is  now  time  to  put  together  the  complete 
pedigree  of  the  radium  family  as  investigated  by 
our  radio-active  genealogists.  No  place  is  found 
for  thorium  and  its  derivatives.  They  seem  to 
form  a  separate  and  independent  radio-active 
family.  Another  radio-active  family  of  some  ten 
generations  is  that  of  actinium.  It  is  probable 
that  this  is  a  collateral  branch  of  the  radium  family 
derived  from  uranium  Y. 

Such  is  the  theory  of  radio-activity  indicated 
by  the  remarkable  series  of  investigations  that 
have  followed  Becquerel's  original  discovery.  We 
are  led  to  refer  the  energy  liberated  to  transforma- 
tions in  the  chemical  atoms,  and  to  recognise 
clearly,  what  has  long  been  suspected,  that  the 
store  of  energy  in  the  atoms  themselves  enor- 
mously transcends  the  energy  involved  in  ordinary 
physical  or  chemical  changes,  in  which  the  atoms 
suffer  no  alteration.  This  internal  atomic  energy, 
then,  must  be  looked  on  as  the  source  of  the  heat 
detected  experimentally  by  Curie  in  the  neighbour- 
hood of  a  radium  compound.  Its  immediate  cause 
may  be,  partly  at  least,  the  internal  bombardment 
of  the  a  particles,  which,  shot  off  by  the  radium 
and  the  emanation  stored  in  it,  are  for  the  most 
part  absorbed  by  the  substance  itself  Rutherford 
has  traced  the  increase  of  the  heat  effect  in  radium 


198  PHYSICAL  SCIENCE 

bromide  newly  precipitated  from  solution,  and  has 
shown  that  it  grows  pari  passu  w^ith  the  radio- 
activity as  measured  electrically — a  method  which, 
as  we  have  seen,  depends  chiefly  on  the  a  radiation. 

The  greater  part  of  the  radiation  coming  from 
a  solid  radium  compound  is  emitted  by  the  stored 
emanation  and  its  products,  the  active  deposits. 
The  emanation  can  be  extracted  only  in  such 
minute  quantities  that,  except  in  most  exceptional 
conditions,  its  radio-activity  alone  reveals  to  us 
its  existence.  As  we  have  seen,  the  emanation 
is  of  the  nature  of  a  dense  gas,  half  of  any 
quantity  of  which  would  be  transformed  into 
other  substances  in  about  four  days.  Owing  to 
this  process  of  change,  only  a  limited  amount 
of  emanation  can  be  obtained  from  a  given 
quantity  of  radium,  and  the  bubble  which  can 
be  evolved  from  the  small  supply  of  radium 
possessed  by  any  experimenter  is  too  minute  to 
be  visible,  except  by  the  most  refined  and  sensitive 
methods  of  investigation.  Could  a  cubic  inch 
of  the  radium  emanation  be  obtained,  the  radia- 
tion from  it  would  be  so  powerful  that  the  vessel 
used  to  contain  the  gas  would,  in  all  probability, 
be  fused  instantly. 

By  the  methods  we  have  already  described,  it 
is  possible  to  determine  the  mass  and  the  velocity 
of  the  projected  particles,  and  therefore  to  calculate 
their  kinetic  energy.  From  the  principles  of  the 
molecular  theory,  we  know  that  the  number  of 
atoms  in  a  gram  of  a  solid  material  is  about 
lO^^  Eight  successive  a  ray  stages  in  the  dis- 
integration of  radium  have  been  recognised  ;  and, 
since  each  of  these  involves  the  emission  of  one 
particle,  the   total  energy  of  radiation   which    i 


RADIO-ACTIVITY  199 

gram  of  radium  could  furnish  If  entirely  dis- 
integrated seems  to  be  enough  to  raise  the 
temperature  of  about  1.5  x  10^^  grams,  or  about 
15,000  tons  of  water,  through  one  degree  centi- 
grade. We  may  say  that,  in  mechanical  units, 
the  energy  available  for  radiation  in  one  ounce 
of  radium  is  sufficient  to  raise  a  weight  of  some- 
thing like  five  thousand  tons  one  mile  high. 

It  will  now  be  clear  that,  on  the  theory  which 
has  been  put  forward,  we  are,  while  investigating 
a  radio-active  body,  in  reality  watching  the  process 
of  the  transmutation  of  matter.  Radio-active 
substances,  themselves  unstable,  may  have  been 
formed  by  the  disintegration  of  parent  atoms, 
which  are  unknown  to  us,  and,  indeed,  may  now 
be  non-existent  on  our  globe.  Radio-activity 
denoting  an  unstable  state,  it  is  probable  that 
the  total  amount  of  it  in  the  world  is  constantly 
diminishing,  as  the  atoms  of  the  active  elements 
pass  gradually  into  inactive  forms.  Perhaps  in 
former  ages  nearly  all  matter  was  intensely  radio- 
active ;  and  mankind  has  discovered  these  phe- 
nomena only  in  the  last  cosmical  moments  of  a 
few  thousand  or  million  years  before  they  cease 
for  ever  to  manifest  their  existence  in  the  striking 
manner  which  has  made  radium  so  remarkable. 

When  we  trace  in  this  way  the  creation  and 
evolution  of  new  elements,  it  is  impossible  to  resist 
wondering  whether  the  process  of  change,  so  far 
observed  to  an  appreciable  extent  only  in  a 
few  radio-active  bodies,  may  not  in  reality  be  a 
general  property  of  matter,  though  in  other  cases 
possessed  in  such  an  infinitesimal  degree  that  it 
almost  transcends  the  delicate  means  of  detection 


200  PHYSICAL  SCIENCE 

that  are  now  at  our  disposal.  As  we  have  seen, 
experimental  evidence  is  not  altogether  wanting 
in  favour  of  such  a  supposition.  We  must,  at  any 
rate,  cease  to  regard  matter  as  essentially  eternal 
and  unalterable  ;  the  possibility  of  its  undergoing 
a  continual  though  slow  process  of  disintegration 
is  clearly  before  us. 

A  striking  property  of  radio-active  change  is 
our  inability  to  produce  it,  or  even  to  modify  its 
course,  by  any  of  the  usual  means  within  the 
resources  of  modern  physical  science.  The 
highest  temperature  we  can  employ  on  the  in- 
tense cold  of  liquid  air,  the  most  complete 
vacuum  attainable  on  a  pressure  of  a  thousand 
atmospheres,  are  equally  useless.  The  observa- 
tion that  the  activity  of  radium  is  independent 
of  the  concentration  of  the  material  shows  that 
the  disintegration  of  one  part  of  this  substance 
is  not  accelerated  by  the  radiation  from  another 
part.  Even  under  the  fierce  and  continuous 
bombardment  of  the  atomic  projectiles  hurled 
forth  by  radium,  and  the  sharp  musketry  of  its 
corpuscular  1^  rays,  the  residual  atoms  of  radium 
are  unaffected.  They  remain  unchanged  by  the 
action  of  any  internal  agency,  till,  in  the  fullness 
of  time,  their  own  internal  processes  result  in 
instability,  and,  from  the  shattered  fragments 
of  each  radium  atom,  as,  in  its  turn,  it  breaks 
asunder,  new  elements  emerge. 

But  in  1922  Rutherford  announced  the  result 
of  some  experiments  in  which  the  atoms  of  certain 
elements  had  been  broken  up  by  the  bombard- 
ment of  a  particles.  Heavy  atoms  such  as  those  of 
radium  seem  to  resist  all  such  attempts,  but  with 
some  types  of  lighter  atoms  success  was  attained. 


RADIO-ACTIVITY  201 

That  bombardment  by  a  rays  is  the  most 
promising  mode  of  attack,  is  clear  from  figures. 
These  particles  are  projected  with  speeds  some 
twenty  thousand  times  greater  than  that  of  a  rifle 
bullet,  while  mass  for  mass  their  kinetic  energy 
is  four  hundred  million  times  that  of  a  bullet. 

If  a  particles,  that  is  helium  nuclei  of  atomic 
mass  4,  be  fired  into  hydrogen  gas,  occasional 
collisions  give  rise  to  fast  moving  hydrogen  nuclei, 
that  is  charged  hydrogen  atoms,  which  will 
penetrate  some  30  centimetres  of  air  and  produce 
scintillations  on  a  zinc  sulphide  screen.  If  oxygen 
or  carbon  dioxide  be  substituted  for  hydrogen, 
a  few  of  these  scintillations,  due  to  traces  of 
hydrogen  impurities,  are  still  seen.  But,  if  dry 
air,  or  still  better  nitrogen,  be  used,  many  more 
scintillations  appear,  and  still  persist  if  the  screen 
be  moved  far  beyond  the  30  centimetres,  or  be 
covered  with  a  screen  equivalent  to  this  thickness 
of  air,  that  is,  if  the  screen  is  out  of  shot  for  the 
nuclei  produced  by  collision  between  a  rays  and 
hydrogen  molecules. 

Their  magnetic  deflections  indicated  that  the 
projectiles  were  still  singly  charged  hydrogen 
nuclei,  but  moving  with  much  higher  speeds  than 
those  obtained  by  collision  in  the  free  hydrogen 
molecules,  thus  confirming  the  evidence  of  their 
great  power  of  penetration.  It  seems  to  follow 
that  these  fast  moving  hydrogen  atoms  can  only 
be  obtained  by  an  explosion  of  nitrogen  atoms, 
induced  by  the  impact  of  the  a  particles. 

The  effect  of  a  particles  on  solids  was  then 
investigated  by  firing  them  at  very  thin  films 
with  oxygen  gas  beyond.  Of  all  the  elements 
examined,    the   following    alone   give    the    same 


202  PHYSICAL  SCIENCE 

result  as  nitrogen  (14):  boron  (11),  fluorine  (19), 
sodium  (23),  aluminium  (27),  and  phosphorus  (31). 
All  these  elements,  and  possibly  a  few  others, 
give  hydrogen  scintillations  beyond  the  range  of 
ordinary  hydrogen,  and  are  therefore  thought  to 
be  broken  up  by  the  bombardment. 

This  interpretation  is  confirmed  by  the 
evidence  of  the  atomic  weights.  The  atomic 
weights  of  the  elements  named  are  given  by 
the  general  formulae  ^.n  +  2  or  4;^  +  3,  where 
71  is  a  whole  number.  Other  elements  such  as 
oxygen  (16)  or  carbon  (12),  which  have  atomic 
weights  represented  by  /[n,  are  not  active.  Now 
when  we  come  to  deal  with  the  modern  theory 
of  the  atom,  we  shall  see  that  these  latter  elements 
may  be  supposed  made  up  of  n  helium  nuclei  each 
of  weight  4,  while  the  active  elements  have  two 
or  three  units  in  addition,  which  may  well  be 
hydrogen  nuclei  each  weighing  one  unit.  Hence 
all  the  evidence  is  in  favour  of  the  view  that  the 
nuclei  of  the  atoms  of  the  elements  boron,  nitrogen, 
fluorine,  sodium,  aluminium,  and  phosphorus  are 
built  up  of  helium  and  hydrogen,  and  that  the 
nucleus  is  shattered  by  a  rays,  with  the  ejection 
of  hydrogen  particles. 

We  shall  consider  the  problem  of  the  structure 
of  the  atom  in  greater  detail  in  the  next  chapter  ; 
here  we  are  concerned  only  with  the  broad  result 
that  Sir  Ernest  Rutherford  has  not  only  taught 
us  that  radio-activity  is  due  to  the  spontaneous 
explosion  of  atoms,  but  has  now  shown  us  how 
to  produce  disintegration  in  elements  usually  in- 
active, by  using  the  concentrated  energy  of  an 
a  ray  projectile. 

Thus  we  approach  even  nearer  to  the  hope 


RADIO-ACTIVITY  203 

of  the  alchemist.  But  it  is  easier  to  destroy  than 
to  build  up,  and  it  does  not  follow  that,  because 
we  can  knock  to  pieces  one  atom  in  a  million,  we 
shall  ever  be  able  to  put  together  a  more  complex 
atom  from  simpler  ones. 

Moreover,  the  number  of  atoms  affected  is 
almost  infinitesimally  small.  About  two  a  particles 
in  a  million  dislodge  hydrogen  nuclei.  If  all  the 
a  particles  from  a  gram  of  radium  were  steadily  fired 
into  aluminium,  only  about  the  one-thousandth 
part  of  a  cubic  millimetre  of  hydrogen  would  be 
produced  in  a  year.  The  modern  philosopher's 
stone  falls  far  short  of  medieval  requirements. 

By  investigating  radio-active  changes,  we  can 
trace  the  transmutation  of  the  elements  ;  we  can 
watch  the  disintegration  of  matter ;  we  can  even 
knock  to  pieces  a  few  atoms  of  certain  elements  ; 
but  we  are  far  from  bringing  these  processes  fully 
under  our  control.  It  would  be  rash  to  predict 
that  our  impotence  will  last  for  ever.  It  is  con- 
ceivable, too,  that  some  means  may  one  day  be 
found  for  inducing  radio-active  change  in  elements 
which  are  not  normally  subject  to  it — means  more 
effective  than  bombardment  by  the  comparatively 
few  a  particles  which  have  yet  been  used. 
Sir  Ernest  Rutherford  once  playfully  suggested 
to  the  writer  the  disquieting  idea  that,  could  a 
proper  detonator  be  discovered,  an  explosive  wave 
of  atomic  disintegration  might  be  started  through 
all  matter  which  would  transmute  the  whole  mass 
of  the  globe,  and  leave  but  a  wrack  of  helium 
behind.  Such  a  speculation  is,  of  course,  only  a 
nightmare  dream  of  the  scientific  imagination,  but 
it  serves  to  show  the  illimitable  avenues  of  thought 
opened  up  by  the  study  of  radio-activity. 


CHAPTER   VIII 

MATTER,    SPACE,    AND    TIME 

Oh,  dear  !  what  can  the  matter  be  ? 

— Old  Song. 

Our  primary  conception  of  matter  as  continuous 
In  time  and  space  fails  to  correspond  with  phe- 
nomena which  are  perceived  as  soon  as  inquiry 
passes  beyond  the  most  elementary  stages.  The 
expansion  of  a  quantity  of  gas  without  assignable 
limit  can  hardly  be  represented  mentally  if  the 
gas  is  thought  of  as  a  homogeneous  substance 
filling  completely  the  space  in  which  It  exists. 
We  cannot  imagine  that  the  same  amount  of 
substance  fills  equally  at  different  times  volumes 
different  from  each  other.  The  immediate 
difficulty  disappears  if  we  suppose  the  gas  to 
consist  of  a  number  of  discrete  particles,  which 
can  be  pressed  nearer  together  or  allowed  to  move 
farther  apart. 

The  phenomena  of  diffusion,  too,  clearly 
indicate  that  liquids  and  gases  must  consist  of 
particles  in  motion  relatively  to  each  other, 
capable  of  penetrating  the  interspaces  between 
the  similar  particles  of  contiguous  bodies.  A 
vessel  filled  with  hydrogen  and  a  vessel  filled 
with  oxygen,  when  opened  into  each  other,  soon 
contain  an  equal  mixture  of  the  two  gases,  while 
two  solutions  in  contact  gradually  become  of 
uniform  concentration  throughout.     Nor  are  such 

204 


MATTER,  SPACE,  AND  TIME  205 

processes  confined  to  fluids.  Sir  William  Roberts- 
Austen  has  shown  that  gold,  If  placed  in  intimate 
contact  with  lead,  will  diffuse  at  ordinary  tempera- 
tures to  such  an  extent  that,  after  the  lapse  of 
some  years,  it  can  be  detected  in  the  lead  by 
chemical  analysis  at  distances  of  a  millimetre  or 
more  from  the  surface  of  contact.  Chemical 
analysis  is  by  no  means  one  of  the  most  sensitive 
methods  of  research,  and  to  be  discovered  in 
this  way  the  gold  must  have  migrated  to  a  con- 
siderable extent. 

Again,  our  usual  conceptions  of  the  nature 
of  heat  rest  on  the  view  that  it  is  a  form  of 
energy — the  energy  of  agitation  of  particles  too 
small  to  be  recognised  or  controlled  individually 
by  ordinary  mechanical  means.  No  hypothesis 
previously  proposed  fulfils  the  needs  of  a  satis- 
factory explanation. 

While  the  most  obvious  phenomena  thus  point 
consistently  to  the  conception  of  the  grained 
structure  of  matter,  the  more  recondite  branches 
of  physical  science  indicate  the  same  conclusion 
by  evidence  which,  In  its  cumulative  effect,  is 
irresistible.  The  phenomena  of  liquid  electrolysis, 
no  less  than  those  of  gaseous  discharge  and  radio- 
activity, have  been  successfully  co-ordinated  and 
explained  by  ionic  hypotheses  which  are  an  extreme 
form  of  molecular  theory.  Indeed,  we  have  now 
several  methods  by  which  the  Individual  atom 
may  be  made  manifest  to  our  imperfect  senses. 
Crookes  and  Rutherford  have  shown  that  the 
molecular  bombardment  of  each  individual  a 
particle  from  radium  may  be  rendered  visible  by 
the  scintillation  produced  on  a  fluorescent  screen 
of  zinc  sulphide. 


206  PHYSICAL  SCIENCE 

Rutherford  and  Geiger,  as  explained  on  p.  192, 
have  detected  by  the  throw  of  the  needle  of  an 
electrometer  the  flight  of  each  a  particle  shot 
through  a  gas  in  a  strong  electric  field.  C.  T.  R. 
Wilson  has  shown  how  to  make  the  tracks  of 
individual  a  particles  visible  by  the  lines  of  cloud 
formed  upon  them.  And,  as  we  know,  a  particles 
are  positively  electrified  helium  atoms. 

Turning  to  chemistry,  we  are  again  impelled 
to  molecular  conceptions  by  the  familiar  evidence 
on  which  rests  Dalton's  atomic  theory.  It  is 
true  that  at  one  time  an  alternative  explanation, 
based  on  the  principles  of  energetics  alone,  was 
put  forward.  As  we  have  seen  in  Chapter  IV., 
mixtures  possessing  a  maximum  or  minimum 
melting  or  boiling-point  change  their  state  with- 
out change  in  composition  of  either  phase.  The 
particular  composition  at  which  this  mode  of 
change  occurs  depends  in  general  on  the  physical 
conditions,  such  as  pressure.  If,  however,  as  a 
limiting  case,  variation  in  conditions  is  without 
effect,  the  system  would  be  classed  as  a  compound 
or  an  element — a  compound  ii  the  constancy 
extends  only  over  a  limited  range,  an  element  if 
no  known  variation  of  conditions  will  alter  the 
composition.  This  attempt  to  connect  matter  and 
energy  was  premature  and  has  been  discarded. 
But  modern  views  of  atomic  structure  and  other 
physical  problems  have  suggested  another,  and 
more  deep-seated  relation  between  the  two  funda-^ 
mental  concepts  of  matter  and  energy.  To  this 
point  we  shall  return. 

Truth  may  possess  many  aspects,  but,  since 
the  time  of  Dalton,  it   has  been  safe  to  accept 


MATTER,  SPACE,  AND  TIME  207 

provisionally  the  idea  of  the  molecular  structure 
of  matter.  The  next  point  to  consider  was 
whether  it  is  possible  to  obtain  any  exact  know- 
ledge of  the  dimensions  of  this  structure,  that  is, 
of  the  number  of  molecules  we  must  suppose  to 
exist  in  a  cubic  centimetre,  and  of  the  size  of  the 
molecular  individuals. 

It  is  clear  that  the  molecules  must  be  at  least 
as  small  as  the  most  minute  piece  of  matter  we 
can  prepare  and  recognise,  and  in  many  ways  it  is 
possible  to  obtain  substances  in  a  very  fine  state 
of  division.  Gold  leaf  can  be  beaten  out  till  its 
thickness  does  not  exceed  the  millionth  part  of 
an  inch,  while  the  deep  blue  colour  of  thin  smoke 
coming  from  a  wood  fire  shows  that  the  particles 
therein  are  able  to  distinguish  selectively  the 
various  waves  making  up  a  beam  of  white  light, 
and  must  therefore  be  comparable  in  minuteness 
with  the  lengths  of  those  waves. 

Such  results  as  these,  while  fixing  an  upper 
limit  to  the  size  of  molecules,  are  powerless  to 
assist  in  the  determination  of  a  lower  limit,  smaller 
than  which  the  inter-molecular  distances  cannot 
be.  Such  inferior  limits  can,  however,  be  deter- 
mined, and  to  one  of  the  methods  by  which  they 
have  been  obtained — one  due  to  Lord  Kelvin — 
^e  will  now  turn. 

A  soap  bubble  always  tends  to  contract  and 
diminish  its  area,  and  therefore,  in  order  to 
increase  its  size,  it  is  necessary  to  do  work  against 
the  force  of  contraction  to  an  amount  which  may 
be  calculated  by  measuring  the  surface  tension  of 
the  film.  Adding  the  energy  required  to  prevent 
the  film  from  cooling  during  its  extension,  we  can 
calculate  the  total  work  absorbed  per  unit  increase 


2o8  PHYSICAL  SCIENCE 

of  area.  By  continual  extension  it  would  be 
possible  to  expend  an  unlimited  quantity  of  work, 
as  long  as  no  change  in  the  nature  of  the  film 
took  place  under  the  influence  of  the  progressive 
expansion  and  consequent  attenuation  of  the  film. 
The  point  at  which  it  is  natural  to  expect  that 
some  change  would  occur  is  that  moment  when 
the  two  sides  of  the  film  have  been  brought  so 
near  to  each  other,  by  the  process  of  continual 
thinning,  that  the  outside  faces  confining  the  film 
come  within  range  of  each  other's  molecular 
forces.  But,  however  far  the  film  be  extended, 
it  is  evident  that,  as  long  as  it  remains  a  film, 
less  work  must  be  used  than  could  otherwise  be 
expended  in  evaporating  the  film  and  converting 
its  substance  into  steam,  since  by  this  means  its 
molecules  would  be  separated  completely  from 
each  other's  sphere  of  influence.  The  value  of 
this  latter  amount  of  work  is  known  from  other 
experiments,  and  is  measured  by  the  latent  heat 
of  evaporation  of  the  substance  of  the  film,  which 
is  composed  almost  entirely  of  water.  It  is 
possible,  therefore,  to  calculate  for  the  film  a 
hypothetical  thickness,  certainly  less  than  the 
critical  thickness  at  which  it  would  begin  to 
show  new  properties  owing  to  the  approach  of 
the  opposite  faces  within  molecular  distances. 
Numerical  results  show  that  this  limiting  thick- 
ness may  be  put  at  about  io~^  of  a  centimetre. 
There  are  thus  not  more  than  ten  million  molecules 
in  a  row  in  a  length  of  a  millimetre,  and  two 
hundred  million  in  the  space  of  an  inch.  The 
numbers  in  the  corresponding  volumes  will  be 
found  by  cubing  these  values  ;  a  cubic  centimetre 
of  water  contains  not  more  than  lo'"^  molecules. 


MATTER,  SPACE,  AND  TIME  209 

This,  as  we  have  indicated,  is  a  maximum 
estimate ;  it  is  possible  than  the  number  is 
less. 

As    already   suggested,   the    interdiffusion   of 
gases  also  leads  to  a  molecular  conception  of  their 
structure,  and   from  the  observed  values  of  the 
coefficients  of  diffusion,  and  of  the  allied  property 
viscosity,   it   is  possible,   from   the  principles   of 
the  kinetic  theory,  to  calculate  more  exactly  the 
number  of  molecules  in  a  cubic  centimetre  of  a 
gas.     The  results   of  the  investigation   indicate 
about  2.5  X  10^^  molecules  per  cubic  centimetre. 
Since   water,    the   liquid,    is    about    1200    times 
denser  than  its  vapour,   it  follows  that  a  cubic 
centimetre    of    water    contains    about    3  x  10^^ 
molecules,   a    number  which    may  profitably    be 
compared    with    the    maximum    estimate    given 
above.     Such  figures  do  indeed  convey  little  to 
the  mind ;  but  it  may  be  useful  to  remember  that 
the  thinnest  line  clearly  visible  in  a  good  micro- 
scope— a  line  with  a  thickness  approaching  the 
hundred-thousandth  of  a  centimetre — would  need 
about  three  hundred  molecules  to  stretch  across  it 
from  side  to  side.     Thus  the  molecular  structure  of 
matter  is  not  immeasurably  finer  than  magnitudes 
which,  with  the  aid  of  modern  instruments,  our 
senses  are  enabled  to  apprehend. 

Our  mental  picture  of  matter,  then,  is  that 
of  a  discontinuous  substance;  we  can,  moreover, 
form  some  notion  of  the  number  of  grains  in  a 
given  volume,  and  we  know  some  of  the  chemical 
properties  of  the  individual  grains.  But  what  is 
the  nature  of  these  particles  ?  Are  they  similar 
in  kind  to  the  matter-in-bulk  they  compose,  or 

p 


210  PHYSICAL  SCIENCE 

do  the  properties  of  matter-in-bulk  appear  as  a 
consequence  of  the  collaboration  of  vast  numbers 
of  particles  essentially  different  in  nature  from  any 
lump  of  matter  we  can  touch  or  see  ?  Again, 
are  the  particles  which  make  up  different  kinds 
of  matter  different  from  each  other,  or  has  all 
matter  a  common  constituent  ?  Are  the  different 
elements  composed  of  identical  particles  of  which 
the  number  and  arrangement  form  the  determining 
factors  of  the  chemical  atoms  ? 

Such  questions  have  puzzled  mankind  from 
early  times,  and,  until  theories  began  to  be 
founded  on  facts  and  tested  by  experiment,  the 
track  of  history  is  strewn  with  the  speculative 
hypotheses  of  the  metaphysicians  and  the  poets. 
Here  and  there  a  lucky  guess  or  shrewd  suggestion 
chances  to  agree  with  the  views  which  represent, 
temporarily  it  may  be,  the  conclusions  of  experi- 
mental science.  It  is  curious  and  interesting 
that,  to  many  highly  educated  people,  the 
problems  connected  with  the  constitution  of 
matter  are  better  known  by  such  triumphant 
proofs  of  the  sagacity,  scientific  insight  and  good 
luck  of  some  Greek  philosopher  than  from  the 
definite  theories,  slowly  put  together  by  Kelvin, 
J.  J.  Thomson,  Rutherford,  and  Bohr,  on  the  firm 
basis  of  experimental  knowledge. 

The  problems  at  issue  could  not  even  be 
formulated  profitably  till  the  work  of  Dalton  and 
Avogadro  had  fixed  our  ideas  of  atoms  and 
molecules.  In  the  light  of  present  knowledge, 
we  define  an  atom  to  be  the  smallest  particle  of 
matter  which  can  take  part  in  chemical  action, 
or  enter  into  the  chemical  structure  of  a  compound. 
It   is    the    ultimate  chemical   unit ;    the  particles 


MATTER,  SPACE,  AND  TIME  211 

smaller  than  an  atom  were  discovered  by  physical 
means,  and  are  but  parts  of  the  structures  which 
take  part  In  ordinary  chemical  action.  The  atom 
is,  moreover,  defined  as  the  unit  of  the  chemical 
elements.  An  atom  of  a  compound  is  a  meaning- 
less term  ;  the  atoms  of  water,  for  instance,  would 
be,  not  water,  but  hydrogen  and  oxygen. 

As  to  the  outward  chemical  nature  of  atoms, 
physics  has  had  till  lately  little  to  say  ;  but 
molecules,  on  the  other  hand,  from  the  first  had 
to  be  regarded  either  In  a  chemical  or  in  a  physical 
aspect.  Chemically  they  are  the  ultimate  units 
of  the  compound,  the  smallest  parts  of  that 
compound  which  can  exist  and  still  retain  the 
properties  of  the  compound.  Any  further  sub- 
division would  result  in  the  liberation  of  the 
elements.  Physically,  on  the  other  hand,  mole- 
cules are  the  smallest  particles  of  matter  which 
act  as  wholes  in  the  incessant  irregular  move- 
ments which  the  particles  of  matter  are  always 
undergoing.  The  energy  of  these  molecular 
movements  Is  the  energy  of  heat ;  and.  In  the 
most  striking  case,  that  of  a  gas,  the  impact  of 
the  molecules  on  the  walls  of  the  containing 
vessel  gives  the  physical  explanation  of  the 
pressure  which  the  gas  exerts.  It  is  evident 
that  the  physical  molecule  may  contain  one  or 
more  chemical  atoms.  Clear  evidence  shows 
that  in  well-known  gases  such  as  oxygen  and 
hydrogen,  the  molecule  consists  of  two  atoms, 
while  some  gases  like  argon  and  the  vapours 
of  some  metals,  mercury,  for  example,  possess 
monatomic  molecules. 

Thus  the  relations  between  atoms  and  mole- 
cules are  ascertained,  and   further  inquiry  must 


212  PHYSICAL  SCIENCE 

deal  with  the  intimate  structure  of  the  atom,  as 
the  more  fundamental  unit. 

The  essence  of  Dalton's  great  conception  was 
that  the  relative  chemical  combining  weights  of  the 
different  elements  lead  directly  to  a  knowledge  of 
the  relative  weights  of  the  atoms  of  those  elements. 
Since  Dalton's  time  it  has  been  recognised  that 
the  atoms,  in  the  chemical  sense  of  the  word,  of 
different  elements  must  have  different  weights 
and  different  properties.  If,  then,  we  look  for 
some  common  constituent  composing  the  different 
elementary  substances  known  to  chemistry,  we 
must  look  within  the  atom  ;  we  must  cease  to 
regard  it  as  the  ultimate  unit,  and  examine  the 
internal  structure  of  the  atom  itself;  we  must 
abandon  the  etymological  meaning  of  the  word, 
retaining  it  only  for  its  historic  associations. 

On  arranging  the  elements  in  order  of  their 
atomic  weights,  Mendeleeff  discovered  that  periodic 
relations  become  apparent  between  the  physical 
and  chemical  properties,  elements  with  similar 
properties  recurring  at  constant  intervals.  This 
periodicity  was  so  marked  a  feature  that  it  was 
possible  to  arrange  the  elements  in  groups,  in 
which  the  various  properties  were  possessed  by 
the  individual  members  to  a  greater  or  less  extent, 
according  to  their  position  in  the  groups.  It  was 
even  possible  successfully  to  predict  the  atomic 
weight,  properties,  and  compounds  of  undiscovered 
elements  from  knowledge  of  the  behaviour  of  their 
neighbours,  which  were  situated  round  empty 
spaces  in  the  periodic  table. 

The  periodic  law  suggests  a  common  origin 

for  the  elements,  and  indicates  that,  as  we  pass 

rom  light  to  heavy  atoms,  we  are  going  from 


MATTER,  SPACE,  AND  TIME  213 

simple  to  complex  structures  containing  different 
numbers  of  some  common  sub-atom.  The  atomic 
weights  of  many  elements  are  nearly  simple  mul- 
tiples of  that  of  hydrogen,  and  Prout  suggested 
that  hydrogen  was  the  ultimate  basis  of  other 
elements.  More  accurate  chemical  experiments 
did  not  eliminate  the  divergence  of  some  atomic 
weights  from  whole  numbers,  and  Prout's  hypo- 
thesis for  many  years  was  discarded  ;  but  the 
idea  of  some  common  constituent  in  the  different 
elements  has  a  deep  scientific  instinct  and  even 
then  some  experimental  evidence  in  its  favour, 
and  only  waited  for  definite  confirmation  to  be 
received  as  the  natural  conclusion  of  many 
promising  speculations. 

For  the  first  time,  in  1897,  such  definite 
experimental  confirmation  was  given  by  Professor 
J.  J.  Thomson,  who,  in  the  remarkable  series  of 
researches  described  on  pages  142  to  147,  clearly 
showed  that,  in  the  cathode  rays  of  a  vacuum 
tube,  we  can  detect  corpuscles  with  about  the  one 
thousand  eight-hundredth  part  of  the  mass  of  the 
lightest  atom  known,  that  of  hydrogen.  These 
corpuscles  were  shown  to  be  identical,  whatever 
the  nature  of  the  residual  gas  in  the  tube,  and 
whatever  the  metal  employed  as  electrode.  The 
corpuscles  are  common  to  all  kinds  of  matter, 
and  the  mind  at  once  sees  in  them  a  common 
constituent  of  all  the  chemical  atoms. 

To  explain  the  phenomena  of  radiation,  that 
is  the  emission  of  electro-magnetic  waves,  we  must 
suppose  with  Lorentz  and  Larmor  that  the  parts 
of  atoms  which  vibrate  are  electrical  in  nature. 
As  explained  above,  we  thus  reach  the  idea  of 
electric    units    on    electrons    as    components    of 


214  PHYSICAL  SCIENCE 

matter  and  may  identify  them   with  Thomson's 
corpuscles. 

Then  came  the  discovery  of  radio-activity, 
throwing  a  new  Hght  on  the  problem  of  atomic  con- 
stitution. Atoms  appeared  as  complex  structures, 
some  of  the  heaviest  of  which  break  down  spon- 
taneously, leaving  a  new  and  somewhat  lighter 
element  as  residue,  and  ejecting  charged  helium 
atoms  in  the  form  of  a  particles  and  electrons  as 
/5  particles. 

The  first  detailed  picture  of  a  modern  atom 
was  drawn  by  J.  J.  Thomson,  who  published  in 
March  1904  a  mathematical  investigation  of  the 
conditions  of  stability  of  systems  of  revolving 
corpuscles,  and  thereby  deduced  in  a  most  re- 
markable manner  many  of  the  properties  of  the 
different  chemical  atoms.  He  supposed  any  one 
atom  to  consist  of  a  uniform  sphere  of  positive  elec- 
trification, the  structure  of  which  is  not  specified, 
and  of  a  number  of  negatively  charged  corpuscles 
revolving  in  orbits  within  that  positive  sphere, 
under  the  influence  of  the  attraction  of  the  positive 
electricity  and  of  their  own  mutual  repulsions. 

A  similar  problem  was  long  ago  attacked  by 
Mayer  by  means  of  experiment.  A  number  of 
little  magnetised  needles  were  thrust  through 
corks,  and  were  allowed  to  float  on  the  surface  of 
water  with  their  axes  vertical.  The  similar  poles  of 
all  the  magnets  were  directed  upwards,  and  thus  the 
resultant  force  between  the  magnets  was  a  repul- 
sion. High  above  the  water  wa^  placed  a  powerful 
bar  magnet,  with  that  pole  downwards  of  which 
the  magnetisation  was  opposite  in  kind  to  that  of 
the  upward  poles  of  the  little  floating  magnets. 
This  large  pole  attracted   inwards  all  the  little 


MATTER,  SPACE,  AND  TIME  215 

poles  pointing  upward,  and  thus  the  magnets 
were  drawn  towards  the  centre  by  the  attraction  of 
the  big  magnet  suspended  above  them,  and  at  the 
same  time  were  repelled  from  the  centre  by  their 
mutual  repulsions.  Under  the  influence  of  these 
two  forces  they  assumed  positions  of  equilibrium. 
Mayer  found  that  as  long  as  the  number  of 
little  magnets  was  not  more  than  five,  they 
arranged  themselves  in  a  single  ring,  but  that, 
on  increasing  the  number  to  six,  a  discontinuity 
of  arrangement  was  observed ;  the  single-ring 
structure  ceased  to  be  stable,  and  the  magnets 
placed  themselves  with  five  in  a  ring  and  one  at 
the  centre.  This  two-ring  configuration  persisted 
as  more  magnets  were  added,  till  the  number  rose 
to  fourteen,  with  five  in  the  middle  ring  and  nine 
in  the  outer  circle.  With  fifteen  magnets  this 
arrangement  in  its  turn  became  unstable,  and  a 
three-ring  system  appeared. 

Thomson  overcame  the  difficulties  of  the 
mathematical  analysis,  and  has  shown  that  similar 
phenomena  of  disposition  must  appear  in  the 
system  which,  as  described  above,  he  imagines  to 
correspond  with  the  atom.  Here  also,  discon- 
tinuities in  arrangement  will  appear,  and,  when 
certain  definite  numbers  of  electrons  have  come 
together,  an  additional  ring  will  be  formed. 
Periodic  likenesses  in  structure  also  arise  and  will 
give  to  the  system  in  which  they  occur  similarities 
of  periods  of  vibration,  and,  it  was  thought,  might 
explain  the  homologous  series  of  lines  which  are 
found  in  the  spectra  of  elements  lying  in  the 
same  group  of  Mendeleeffs  periodic  classification. 

So  far  the  theory  had  been  carried  when  this 
book  first  appeared.     But,  as  we  shall  see  below, 


2l6 


PHYSICAL  SCIENCE 


the  central  force  is  now  supposed  to  be  connected 
with  a  minute  positively  electrified  nucleus  and 
not  with  a  sphere  of  positive  electricity  as  large 
as  the  atom. 

Thomson  thus  accepted  the  nuclear  atom,  and 
revised  his  theory  in  its  terms.  He  imagines 
that  within  a  given  distance  from  the  nucleus  its 
attraction  for  an  electron  changes  to  a  repulsion. 
At  this  critical  distance  a  single  electron  will 
rest  in  equilibrium.  Two  electrons  will  rest  on 
opposite  sides  of  the  nucleus,  three  at  the  corners 
of  an  equilateral  triangle,  four  at  the  corners  of 
a  regular  tetrahedron,  and  so  on  up  to  eight. 
With  nine  electrons,  eight  will  form  an  inner 
shell  round  the  nucleus,  and  one  will  stay  outside, 
further  from  the  centre  of  the  atom.  This  outer 
layer  increases  regularly  in  number  as  we  pass 
to  heavier  atoms  and  more  electrons  are  added, 
till  it  too  contains  eight  electrons. 

Now,  while  radio-activity  is  an  affair  of  the 
nucleus,  the  chemical  properties  of  the  atom 
depend  on  the  outer  electrons.  As  we  shall  see 
below,  the  atoms  of  hydrogen  and  lithium  have  each 
one  electron,  and  the  number  will  rise  as  we  pass 
up  Mendeleeff's  Periodic  Table,  as  shown  in  the 
following  list  of  a  few  of  the  lighter  elements  : — 


Number  of 

• 

a 

3 
3 

a 

1— < 

(-1 

• 

§ 

o 

u 

o 

• 

bD 
O 

-£3 

O 

o 

a 
o 

a 

o 

CO 

• 

a 

'm 
a> 
(3 
fcO 

a 
a 

o 
m 

• 

W3 
hi 

2 
o 

1— ( 

oJ 

a 

'C 

o 

i— < 

1 
o 

o 

• 

a 

03 
03 

o 

Free  electrons 

in  the  atom 

I 

2 

3 

4 

5 

6 

7 

8 

9 

10 

II 

12 

13 

14 

15 

i6 

17 

Electrons     in 

the      outer 

layer  . 

I 

2 

3 

4 

5 

6 

7 

8 

I 

2 

3 

4 

5 

6 

7 

8 

1 

I 

MATTER,  SPACE,  AND  TIME  217 

Thus  the  periodicity  in  the  properties  of  the 
elements  is  reproduced  in  the  number  of  electrons 
in  the  outer  layer.  It  is  clear  that  these  electrons 
are  specially  concerned  in  chemical  reaction.  The 
valencies  of  the  elements,  that  is,  the  number  of 
simple  atoms  like  hydrogen  or  chlorine  with  which 
these  atoms  will  combine,  rise  from  one  to  four 
as  we  pass  from  lithium  to  carbon  or  from  sodium 
to  silicon.  Nitrogen  and  phosphorus  can  be  both 
pentavalent  and  trivalent,  oxygen  and  sulphur 
are  divalent,  fluorine  and  chlorine  monovalent, 
while  neon  and  argon  do  not  react.  Hence  it 
seems  that  the  valencies  depend  on  the  number 
of  these  outer  electrons  from  i  to  4,  and  decrease 
again  as  the  numbers  rise  higher. 

We  may  picture  chemical  combination  between 
two  atoms  as  due  to  the  forces  between  their 
nuclei  and  the  free  electrons 
of  their  outer  layers.  The 
simplest  case,  the  union  of 
two  hydrogen  atoms  to  form 
a  hydrogen  molecule,  may  be 
represented  by  Fig.  35,  in 
which  A  and  B  are  the  positive  nuclei,  and  a  and 
/3  two  negative  electrons.  A  repells  B  and  a 
repells  A  while  A  and  B  attract  both  a  and  ^. 
The  electrons  may  then  be  regarded  as  common 
to  both  atoms,  and,  on  this  theory,  that  arrange- 
ment is  the  meaning  of  chemical  combination. 

At  first  sight  we  may  well  say  that  Thomson's 
corpuscle — one  of  the  latest  conceptions  of  science 
— does  but  carry  us  back  to  the  ideas  and  specu- 
lations of  Democritus,  and  justify  the  glorification 
of  those  ideas  in  the  poem  of  Lucretius,  though 


2x8  PHYSICAL  SCIENCE 

internal  evidence  seems  to  show  that  Lucretius 
himself  did  not  find  the  explanation  easy  to 
reproduce  : — 

Nee  me  animi  fallit  Graiorum  obscura  reperta 
Difficile  inlustrare  Latinis  versibus  esse. 

If,  however,  in  one  aspect  these  modern 
corpuscles  may  resemble  the  hard,  impenetrable 
atoms  of  the  Greek  philosopher  and  the  Latin 
poet,  such  a  resemblance  vanishes  when  we 
identify  them  with  the  disembodied  charges  of 
electricity,  mathematically  studied  by  Larmor  and 
Lorentz.  If  the  corpuscle  is  a  negative  electron 
— a  disembodied  ghost — an  electric  charge — we 
enter  a  region  of  knowledge  the  bare  existence 
of  which  was  unknown  to  the  ancients. 

The  hard  particle  of  Democritus,  which,  as 
late  as  the  age  of  Newton,  still  served  as  a 
working  hypothesis,  gradually  failed  to  respond 
to  the  demands  made  on  its  constitution  by  both 
philosophers  and  physicists,  in  their  search  for  a 
conceptual  model  of  the  chemical  atom.  Pictures 
of  mere  lumps  of  stuff,  similar  in  kind  to  the 
perception  of  matter-in-bulk  given  by  our  senses, 
were  no  help  to  the  theories  of  the  metaphysician, 
while  the  complexity  of  structure,  demanded  by 
the  facts  of  radiation  as  disclosed  by  the  spectro- 
scope, showed  that  an  atom  must  be  capable  of 
many  and  various  modes  of  vibration. 

In  extreme  opposition  to  the  hard  Impenetrable 
sphere  of  Democritus,  we  have  Boscovlch's  ideal- 
istic conception  of  atoms  as  centres  of  force.  This 
theory  gave  too  little  scope  for  definite  develop- 
ment to  serve  permanently  as  a  useful  working 
hypothesis,    and,    in    face    of  the    phenomena  of 


MATTER,  SPACE,  AND  TIME  219 

atomic  radiation,  it  too  seemed  insufficient.  It 
is  worthy  of  note,  however,  that  Faraday,  in  his 
day,  and  Lord  Kelvin,  in  more  recent  years,  have 
advocated  views  differing  but  Httle  from  those  of 
Boscovich ;  while  the  school  of  chemists,  who 
tried  to  banish  from  their  ken  all  atomic  theories, 
regarded  energy  as  the  only  physical  reality  known 
to  us,  and  matter  as  "  a  complex  of  energies  which 
we  find  together  in  the  same  place." 

It  seemed  at  first  that  a  real  advance  had  been 
made  when  Lord  Kelvinapplied  the  theory  of  vortex 
rings,  developed  by  Von  Helmholtz  and  himself, 
to  explain  the  properties  of  the  atoms  of  matter. 
A  smoke  ring,  blown  in  air,  soon  dies  away,  but 
even  this  evanescent  thing,  while  it  lasts,  shows 
a  definite  separation  fromthe  surrounding  medium, 
and  maintains  an  independent  existence.  Air  is 
an  imperfect  fluid,  and  movement  in  it  is  resisted 
by  the  frictional  forces  due  to  its  viscosity,  but,  if 
we  imagine  the  air  to  be  replaced  by  a  hypothetical 
perfect  fluid,  in  which  there  is  no  viscosity,  vortex 
rings,  once  formed,  will  persist  for  ever.  In  a  fluid 
not  quite  perfect,  their  life  will  be  long,  though 
not  eternal. 

Here  then  was  a  striking  representation  of 
some  of  the  most  important  properties  of  the 
chemical  atoms.  The  structure  of  interlacing 
systems  of  vortex  rings  gave  sufficient  complexity 
to  explain  radiation,  the  infinite  possibilities  of 
variation  in  number  and  arrangement  of  the  rings 
would  account  for  the  relations  between  different 
atoms  as  manifested  in  the  periodic  law,  while 
the  persistence  of  matter  could  be  explained  if 
a  perfect  or  nearly  perfect  fluid  were  postulated 
as  the  basis  of  the  vortex  motion. 


220  PHYSICAL  SCIENCE 

At  this  point  we  reach  for  the  first  time  in 
our  inquiry  the  idea  of  an  all-pervading  medium 
• — an  idea  which  has  played  such  a  large  part  in 
the  development  of  physical  science,  that  a  con- 
siderable digression  will  be  necessary.  Newton 
explained  the  phenomena  of  light  by  a  corpuscular 
theory.  He  supposed  that  streams  of  corpuscles 
were  projected  from  luminous  objects,  and  pro- 
duced the  sensation  of  sight  by  impinging  on  the 
nerves  of  the  eye.  Ultimately  Newton's  theory 
was  abandoned,  mainly  for  two  reasons.  The 
phenomena  of  refraction  could  only  be  explained  by 
it  on  the  assumption  that  the  corpuscles  travelled 
more  quickly  in  dense  media  than  in  air,  and  this, 
always  improbable,  was  eventually  disproved.  On 
the  other  hand,  the  theory  failed  to  explain  the 
phenomena  of  interference  and  diffraction  of 
light,  except  by  the  addition  of  so  many 
arbitrary  supplementary  hypotheses,  that,  in  the 
end,  it  was  borne  down  by  the  weight  of  its 
own  superstructure. 

This  illustrates  a  case,  oft  recurring,  not  only 
in  the  realm  of  science,  where  men  have  been 
deceived  and  led  to  form  opinions  wide  of  the 
truth  through  the  agency  of  certain  resemblances 
to  that  truth.  The  corpuscular  theory  of  light 
was  put  aside,  but  not  before  it  had  appreciably 
retarded  the  progress  of  science.  The  master- 
mind, the  originator  of  the  theory,  had  been 
withdrawn  before  altered  circumstances  and  in- 
creased knowledge  reversed  the  weight  of  evidence. 
He  who  would  have  been  the  first  to  detect 
the  want  of  harmony,  the  first  to  move  on  to 
new  conceptions  in  the  search  for  truth,  by  the 
irony  of  fate,  became  for  a  time,  in  virtue  of  his 


MATTER,  SPACE,  AND  TIME  221 

intellectual  supremacy,  a  stumbling  block  to  his 
weaker  brethren,  and  an  impediment  to  the  cause 
he  had  most  at  heart. 

In  recent  years  the  discovery  of  radio-activity 
has  revealed  to  us  particles  very  like  those  that 
Newton  used  to  explain  ordinary  light.  The  /3 
rays  from  radium  are  projected  particles  moving 
with  velocities  nearly  approaching  that  of  light 
itself.  Newton's  inscrutable  insight,  amounting 
almost  to  an  instinctive  knowledge  of  Nature,  has 
again  been  demonstrated.  His  corpuscles  cannot, 
indeed,  explain  the  phenomena  of  ordinary  light ; 
but  similar  corpuscles  we  find  do  exist,  and  their 
properties  as  set  forth  by  Newton  are  not  so 
unlike  those  actually  occurring  in  the  working  of 
Nature  as  men  have  assumed  throughout  the  years 
which  separate  the  establishment  of  the  undulatory 
theory  of  light  from  the  discovery  of  radio-activity. 

The  corpuscular  theory  of  light  was  replaced 
by  a  theory  of  waves  in  a  medium  which  already 
had  been  recognised  by  Newton  as  a  necessary 
addition  to  his  idea  of  corpuscles.  Newton's 
difficulty,  which  caused  him  to  reject  the  undula- 
tory hypothesis,  namely,  the  rectilinear  propaga- 
tion of  light,  and  the  consequent  possibility  of 
sharp  shadows,  was  finally  overcome  by  Fresnel 
and  Young,  who  showed  that  shadows  were  the 
result  of  the  minuteness  of  the  wave-lengths  of 
light  as  compared  with  the  dimensions  of  ordinary 
obstacles.  This  cleared  the  way  for  the  wave 
theory  as  already  formulated  by  Huygens,  and 
there  arose  a  definite  physical  need  for  the  exact 
specification  of  an  aether  or  luminiferous  medium, 
pervading  all  space,  and  the  interstices,  if  not  the 
substance,  of  material  objects.     Such  a  medium. 


222  PHYSICAL  SCIENCE 

indeed,  had  long  been  imagined  by  philosophers, 
as  a  means  of  transmitting  actions  from  one  body 
to  another,  but  its  use  as  a  physical  explanation 
of  the  phenomena  of  light  first  indicated  some  of 
its  necessary  properties.  The  reflection  of  light 
from  the  surface  of  a  glass  plate,  or  its  passage 
through  certain  doubly  refracting  crystals,  such 
as  tourmaline,  modifies  the  light,  which  acquires 
properties  not  the  same  on  all  sides  of  the  emergent 
beam,  and  is  then  said  to  be  polarised.  No  wave 
system  in  which  the  direction  of  vibration  is  in 
the  direction  of  propagation  can  show  such  differ- 
ences, for  in  such  a  system  the  waves  must  be 
alike  on  all  sides  of  their  path.  It  follows  that 
the  luminous  vibrations  must  be  transverse  to 
the  direction  in  which  the  rays  are  travelling. 
Transverse  waves,  if  we  are  to  regard  them  as 
mechanical  motion  in  a  real  medium  with  ordinary 
dynamical  properties,  imply  a  certain  amount  of 
rigidity  or  elasticity  of  shape  in  the  medium — 
such  elasticity  as  is  possessed  by  solids  alone. 
No  fiuid  when  distorted  has  any  tendency  to 
return  to  its  original  form  ;  it  cannot  transmit 
waves  which  depend  on  mere  distortional  dis- 
placements. Waves  in  a  fluid  must  be  waves  of 
compression  and  expansion,  in  which  the  direction 
of  vibration  is  in  the  direction  of  propagation. 

If,  then,  it  is  to  carry  a  transverse  wave-motion 
of  an  ordinary  mechanical  kind,  the  luminiferous 
aether  must  possess  some  of  the  properties  of  a 
solid,  and  at  one  time  the  great  problem  of  ^ethereal 
physics  consisted  in  formulating  a  medium  possess- 
ing the  necessary  rigidity.  Any  elastic  jelly  theory 
leads  to  obvious  difficulties  when  the  passage  of 
matter  through  the  aether  is  considered,  a  passage 


MATTER,  SPACE,  AND  TIME  223 

which  often  proceeds  with  high  velocity,  but,  as  far 
as  observation  goes,  is  entirely  unimpeded.  Rays 
of  light  from  the  stars  appear  to  reach  the  earth  in 
straight  lines,  suffering  no  deflection  on  passing 
through  the  aether  outside  the  atmosphere  near  the 
earth.  This  result  suggests  that  the  luminiferous 
medium  is  not  disturbed  by  the  movement  through 
it  of  the  earth  with  a  velocity  of  eighteen  miles  a 
second — the  speed  with  which  the  earth  moves 
round  the  sun.  On  the  other  hand,  the  passage 
of  light  over  the  surface  of  the  earth  is  not  affected 
by  a  change  in  direction  relative  to  the  earth's 
total  motion,  the  velocity  of  the  light  is  the  same 
whether  it  is  passing  with  or  against  the  motion 
of  the  earth.  This  result  indicates  at  first  sight 
a  conclusion  opposed  to  that  formerly  reached,  and 
suggests  that  the  aether  is  at  rest  relatively  to  the 
surface  of  the  earth  and  is  dragged  along  with  the 
ground  as  it  moves.  This  seeming  discrepancy 
led  at  a  later  date  to  Einstein's  theory  of 
Relativity.  The  general  dynamical  problem  of 
constructing  a  model  of  the  aether  on  ordinary 
mechanical  ideas  of  wave  propagation  never  was 
accomplished  satisfactorily  during  the  years  when 
it  appeared  to  be  perhaps  possible. 

As  long  as  the  aether  was  invoked  only  to 
explain  the  phenomena  of  light,  the  difficulties  of 
interpretation  might  well  suggest  doubts  about  the 
fundamental  hypothesis  as  to  its  existence,  but 
when  Clerk  Maxwell  showed  that  it  was  possible 
to  explain  the  phenomena  of  the  electro-magnetic 
field  by  an  aether  having  properties  identical  with 
those  of  the  luminiferous  medium,  the  evidence 
for  both  theories  was  strengthened  very  greatly. 
Maxwell  proved  mathematically  that  the  velocity 


224  PHYSICAL  SCIENCE 

of  an  electro-magnetic  wave  through  free  space 
determined  the  relative  magnitudes  of  certain 
electric  units,  so  that  by  comparing  the  values  of 
the  units  the  velocity  could  be  calculated.  Experi- 
ment showed  that  the  velocity  was  the  same  as 
that  of  light ;  light  became  an  electro-magnetic 
phenomenon,  and  optical  science  a  branch  of  elec- 
tricity. Many  years  afterwards,  Maxwell's  great 
work  was  confirmed  by  the  direct  experiments  of 
Hertz,  who  detected  the  existence,  and  measured 
the  speed  of  electro-magnetic  waves,  thus  laying 
the  foundations  on  which  the  practical  art  of 
wireless  telegraphy  is  based. 

Details  of  the  technical  applications  of  science 
are  outside  the  scope  of  this  work,  but  a  short 
description  of  the  theory  of  wireless  telegraphy 
and  telephony  may  not  be  out  of  place. 

The  work  of  main  theoretic  interest  was  done 
by  Maxwell  and  Hertz,  but  much  development  by 
other  men  was  needed  before  their  achievements 
could  be  turned  to  practical  account.  The  radia- 
tion and  reception  of  sufficient  energy  for  signal- 
ling at  a  distance  was  first  made  possible  by 
Marconi's  introduction  of  the  aerial  wire,  which  is 
used  to  emit  the  waves  at  one  station  and  catch 
them  at  another. 

Each  single  electric  spark  from  an  induction 
coil  consists  of  a  few  electric  oscillations,  rapidly 
dying  away.  It  was  these  damped  oscillations 
which  were  used  by  Hertz  and  in  all  the  early 
methods  of  wireless  telegraphy.  But  nowadays 
continuous  waves,  the  vibrations  of  which  are 
maintained  constant  except  when  purposely 
interrupted,  are  used. 


MATTER,  SPACE,  AND  TIME  225 

The  continuous  or  undamped  waves  may  be 
produced  by  means  of  what  is  called  a  thermionic 
valve.  A  hot  wire  of  tungsten,  such  as  is  used 
in  electric  light  bulbs,  as  explained  on  page  157, 
is  found  to  emit  negative  corpuscles  or  electrons. 
Ordinarily,  the  wire  is  thus  left  positively  elec- 
trified, and,  owing  to  electric  attraction,  a  state  of 
equilibrium  is  reached  and  no  more  electrons  are 
emitted.  But  if  the  hot  wire  be  connected  with 
the  negative  terminal  of  a  battery,  and  a  metal 
plate  within  the  bulb  with  the  positive  terminal, 
a  large  continuous  negative  current  will  pass  from 
the  hot  wire  to  the  plate,  carried  by  the  continually 
escaping  electrons.  On  the  other  hand,  if  the 
battery  terminals  be  reversed,  no  appreciable 
current  will  flow,  since  the  electrons  now  tend  to 
be  driven  back  into  the  hot  tungsten. 

Thus  the  first  use  of  the  thermionic  valve  is 
as  a  rectifier.  It  will  allow  to  pass  that  half  of  an 
alternating  current  which  flows  in  one  direction, 
and  will  stop  the  half  which  tries  to  flow  in 
the  other. 

Next,  let  a  grid,  made  of  a  piece  of  wire  gauze, 
be  put  between  the  hot  wire  and  the  plate.  When 
the  grid  is  positively  electrified,  it  will  help  the 
emission  of  electrons  and  increase  the  thermionic 
current ;  when  it  is  negative  it  will  decrease  the 
current.  Hence,  if  it  alternate  in  potential,  it  will 
cause  the  valve  of  the  current  to  oscillate — in 
effect  superposing  an  alternating  current  on  a 
direct  or  one-directional  current.  If  the  primary 
coil  of  an  induction  coil  be  inserted  in  the  plate 
circuit,  oscillations  will  be  set  up  in  it  when  the 
grid  potential  alternates.  These  oscillations  will 
be  reproduced  in  the  potential  at  the  ends  of  the 

Q 


226  PHYSICAL  SCIENCE 

secondary  of  the  induction  coil,  and,  if  these  ends 
be  coupled  back  with  the  grid  and  the  filament 
in  the  right  direction,  the  oscillations  will  give 
to  the  grid  the  proper  alternating  potential  to 
maintain  the  oscillations  and  the  plate  circuit. 
The  apparatus  is  thus  self-supporting  when  a 
current  is  passed  through  it,  and  will  continue  to 
produce  oscillations,  the  period  of  which  depends  on 
the  induction  of  the  coils  and  the  electric  capacity 
of  the  system.  By  adjusting  the  induction  and 
capacity,  the  period  of  the  oscillations  can  be  tuned. 

Hence  a  thermionic  valve  may  be  used,  both 
to  emit  continuous  waves  and  to  rectify  alternating 
currents  when  received.  By  starting  and  stopping 
such  a  train  of  waves  at  appropriate  intervals,  a 
series  of  long  and  short  signals  may  be  emitted 
at  one  station  and  received  at  another  in  the  so- 
called  Morse  code. 

Continuous  waves  may  also  be  used  to  transmit 
speech  by  telephony.  The  alternations  of  these 
waves  are  much  too  rapid — in  the  neighbourhood 
of  a  million  a  second — to  pass  through  a  telephone 
or  make  it  sound.  But  if  the  currents  they  produce 
in  an  aerial  wire  be  rectified,  and  then  interrupted 
from  loo  to  10,000  times  a  second,  a  sound  of 
corresponding  pitch  is  heard  in  the  telephone. 

That  is  the  principle  of  broadcasting  speech 
or  other  sounds.  A  steady,  continuous  undamped 
wave  is  emitted  from  the  sending  station.  This 
*' carrier  wave"  has  some  definite  wave-length — 
say  465  metres,  which  gives  a  frequency  of  645,000 
alternations  per  second. 

On  this  carrier  wave,  changes  and  interrup- 
tions are  superposed  by  speaking  into  a  micro- 
phone connected  with  the  circuit.     At  the  receiving 


MATTER,  SPACE,  AND  TIME 


227 


station,  the  carrier  wave  is  caught  by  the  aerial 
wire,  and  passed  through  a  thermionic  valve,  which 
rectifies  it,  and  a  telephone,  which  reproduces  the 
interruptions  and  variations  of  the  rectified  wave 
as  audible  sounds. 


If  we  accept  the  view  that  an  atom  is  composed 
of  corpuscles  or  electrons  spaced  round  a  nucleus, 
the  electro-magnetic  radiation  which  constitutes 
light,  if  it  be  emitted  in  accordance  with  ordinary- 
dynamical  principles, 
might    naturally    be  ^ 

supposed  to  take  its 
rise  from  the  ac- 
celerations of  these 
corpuscles  as  they 
revolve  in  their  or- 
bits, though,  as  we 
shall  see  later,  this 
view  has  not  pre- 
vailed in  its  simple 
form. 

Faraday's  con- 
ception of  tubes  of 
electric  force,  may 
here  be  revived  in  order  to  explain  the  radiation 
of  light  on  the  hypothesis  of  electronic  accelera- 
tion. As  long  as  an  electron  is  moving  forward 
with  uniform  velocity,  it  carries  its  attendant 
tubes  with  it  in  a  steady  manner,  and  no  radia- 
tion occurs.  When  it  is  stopped  suddenly,  as 
at  the  point  O  in  Fig.  36,  an  electro-magnetic  pulse 
spreads  out  from  it,  travelling  with  the  velocity  of 
light.  Within  the  sphere  covered  by  this  pulse, 
a  tube  of  force  such  as  0/  is  stopped,  so  as  to 


Fig.  ^6. 


228  PHYSICAL  SCIENCE 

correspond  with  the  new  position  of  the  electron 
at  rest,  while  outside  it,  in  regions  as  yet  unaffected 
by  the  change  in  velocity,  the  tube  ^Q  is  still 
moving  forward  with  the  original  speed  of  the 
electron.  In  the  pulse  itself,  then,  the  electric 
tube/^  is  bent  more  or  less  at  right  angles  to  the 
direction  of  propagation  of  the  pulse,  which  spreads 
out  from  the  electron  as  centre.  When  tubes 
move,  a  magnetic  force  is  produced  at  right  angles 
both  to  their  length  and  to  their  direction  of 
motion  ;  and  thus,  in  the  thickness  of  the  pulse, 
a  magnetic  force  exists,  also  at  right  angles  to  the 
direction  of  propagation  of  the  pulse,  that  is,  in 
the  plane  of  the  advancing  wave-front,  and,  in 
that  plane,  at  right  angles  to  the  direction  of  the 
electric  force.  The  pulse  is  thus  an  electro- 
magnetic disturbance. 

Now,  if,  instead  of  imagining  the  moving 
electron  suddenly  brought  to  rest,  we  suppose 
that  it  is  reversed  in  its  path,  and  that  this 
reversal  occurs  periodically,  so  that  the  electron 
performs  simple  harmonic  vibrations,  we  get, 
instead  of  a  single  thin  pulse,  a  series  of  less 
abrupt  but  regularly  recurring  alternations  propa- 
gated out  from  the  corpuscle  as  centre.  Each 
Faraday's  tube  is  set  into  oscillation  at  its  inner 
end,  and  transverse  waves  travel  outwards  along 
it,  just  as  waves  travel  along  a  stretched  cord, 
when  one  end  is  oscillated  periodically  by  the 
hand.  The  distribution  of  electric  and  magnetic 
force  in  the  advancing  wave-front  is  exactly  the 
same  as  in  the  case  of  the  sudden  pulse  already 
studied :  we  get,  in  fact,  a  series  of  regular 
aethereal  waves,  in  which  there  are  electric  and 
magnetic  forces,  both  in  the  plane  of  the  wave- 


MATTER,  SPACE,  AND  TIME  229 

front,  and  at  right  angles  to  each  other  in  that 
plane.  But  such  an  arrangement  is  precisely 
that  required  to  explain  the  phenomena  of  light. 

In  the  simple  case  we  have  taken,  the  electron 
oscillates  backwards  and  forwards  in  a  straight 
path  :  the  vibrations  travel  as  tremors  along  the 
tubes  of  force  in  one  plane  only ;  the  resultant 
light  is  plane  polarized.  In  the  more  general 
case,  we  must  suppose  that  the  electron  oscillates 
in  a  circular,  or  elliptical  orbit,  and  the  tubes  of 
force  will  be  displaced  in  corresponding  motions  ; 
the  tremors  running  along  them  will  no  longer  be 
simple  to  and  fro  movements,  but  points  on  the 
tubes  will  describe  curved  paths.  These  paths 
continually  change  as  the  orbit  of  the  electron 
changes,  and  we  get  a  more  complete  model  of 
the  propagation  of  common,  non-polarized  light. 

Faraday's  tubes,  it  is  clear,  give  a  very 
powerful  and  convenient  method  of  studying  the 
phenomena  of  the  electro-magnetic  field,  and 
Thomson  has  used  them  in  such  ways  as  that 
just  suggested  to  elucidate  modern  problems. 
Indeed  it  is  almost  possible  that  electric  tubes 
of  force  may  represent  something  more  than  a 
useful  mathematical  fiction.  If  the  structure  of 
the  electric  field  be  discontinuous  in  reality,  as  our 
tube-picture  of  it  indicates ;  if  the  electric  and 
magnetic  effects  of  a  charge  of  electricity  are  in 
reality  exerted  throughout  the  surrounding  space 
by  means  of  discrete  tubes  of  force — vortex  fila- 
ments in  the  aether,  or  whatever  they  may  actually 
be — an  advancing  wave  of  light  must  be  discon- 
tinuous also.  Could  we  look  at  such  a  wave  from 
the  front,  and  magnify  it  millions  of  millions  of 
times,    we   should    see,    not   a   uniform   field   of 


230  PHYSICAL  SCIENCE 

illumination,  but  a  number  of  bright  specks 
scattered  over  a  dark  ground.  Each  tube  of 
force  would  convey  its  own  tremors,  and  these 
would  constitute  light,  but  between  them  would 
lie  undisturbed  seas  of  aether. 

Such  an  idea  about  the  nature  of  a  wave-front 
of  light  is  very  unexpected  and  surprising.  We 
are  inclined  at  once  to  relegate  our  tubes  of  force 
to  a  museum  of  conceptual  curiosities.  But  it  is 
a  remarkable  thing  that  certain  evidence  in  favour 
of  the  discontinuous  nature  of  a  wave-front  of 
light  really  does  exist.  It  is  impossible  to  examine 
the  luminous  effects  with  enough  magnification  to 
investigate  the  question,  but,  as  we  have  seen, 
ultra-violet  light,  and  still  more  effectively  Rontgen 
rays,  are  capable  of  ionizing  a  gas  through  which 
they  pass.  Here,  it  is  the  molecules  of  the  gas 
which  are  affected,  and,  in  examining  the  ionizing 
power  of  the  rays,  we  are  in  effect  using  on  them 
a  microscope  of  molecular  dimensions. 

If  the  wave-front  of  a  Rontgen  pulse  were 
continuous,  all  the  molecules  of  the  gas  would  be 
subject  to  the  same  disturbance.  But,  even  with 
the  strongest  ionizing  agency,  nothing  like  one 
molecule  in  a  million  is  found  to  be  affected. 
Thus,  if  the  wave-front  be  continuous,  we  must 
suppose  that  it  is  only  those  very  few  molecules 
which  are  in  some  peculiarly  receptive  state  that 
are  ionized.  The  stability  of  a  molecule  is 
greatly  affected  by  temperature,  and,  if  a  critical 
limit  of  stability  were  needed  for  a  molecule  to 
become  ionized  by  the  rays,  we  should  expect 
that  the  ionizing  power  would  increase  rapidly 
with  the  temperature.  Mr  M 'Clung  has  shown, 
however,   that    temperature   has    no   appreciable 


MATTER,  SPACE,  AND  TIME  231 

effect.  This  curious  result  indicates  that  the 
ionizing  action  is  independent  of  the  state  of 
stability  of  the  molecule,  and  prevents  us  from 
finding  in  this  way  an  explanation  of  the  small 
number  of  the  ionized  molecules  in  the  path  of 
the  rays. 

It  is  possible  that  some  other  rare  condition, 
unaffected  by  temperature,  may  be  the  necessary 
preliminary  to  ionization  by  incident  radiation  ; 
but  it  is  also  possible  that  the  explanation  of  the 
smallness  of  the  ionization  is  to  be  sought  in  the 
idea  that  the  advancing  wave  is  discontinuous, 
and  is  composed  of  a  number  of  parallel  tremors 
running  along  discrete  tubes  of  force.  The  tubes 
of  force  being  scattered  at  wide  intervals  through 
space,  comparatively  few  molecules  would  lie  in 
their  paths,  and  only  a  few  would  be  affected  by 
waves  running  along  the  tubes.  Matter  has  been 
analysed  into  discrete  particles  ;  electricity  has 
been  shown  to  be  made  up  of  indivisible  units  ; 
and  now  it  seems  possible  that  light  in  physical 
reality,  as  well  as  in  text-books  of  optics,  is 
composed  of  a  number  of  separate  rays.  Perhaps 
there  is  no  need  to  invent  a  continuous  sether — 
a  system  of  Faraday  tubes  radiating  from  electrons 
may  suffice. 

Moreover,  serious  difficulties  have  arisen  in 
the  interpretation  of  the  facts  of  radiation.  To 
meet  these  difficulties  we  have  been  forced  to 
regard  radiation  as  emitted  not  continuously  but 
in  discrete  units  or  quanta,  just  as  matter  is  not 
continuous  but  atomic. 

In  this  quantum  theory,  Thomson  finds  a  place 
for  Faraday's  tubes.  He  regards  the  electrons  in 
his  model  of  the  atom,  described  on  page  216,  as 


232  PHYSICAL  SCIENCE 

linked  to  the  nucleus  by  tubes  of  force.  A  dis- 
turbance may  throw  one  of  the  tubes  into  a  loop 
which  may  possibly  close  into  an  anchor-ring 
and  be  cast  out  into  space  attended  by  a  train  of 
electro-magnetic  waves,  as  a  quantum  of  radiation. 
We  shall  consider  the  quantum  theory  more  fully 
below.  It  forms  the  most  surprising  development 
of  modern  physical  research. 

From  the  time  of  Maxwell  onwards,  electro- 
magnetic considerations  have  formed  an  essential 
part  of  any  theory  of  the  aether.  It  is  certain 
that  luminous  and  electro-magnetic  radiations 
are  essentially  the  same  in  kind,  and  only  differ 
in  the  length  of  the  waves.  We,  of  course,  might 
have  ceased  to  try  to  represent  the  properties  of 
the  aether  by  means  of  any  imaginary  mechanical 
model,  and,  regarding  light  as  a  system  of  electro- 
magnetic waves,  have  pushed  the  inquiry  no 
further,  but,  besides  the  difficulty  of  explaining 
the  facts  of  radiation  at  least  two  considerations 
prevented  our  resting  content  with  a  mere  series 
of  electro-magnetic  equations  as  a  final  explana- 
tion. While  some  phenomena  maybe  co-ordinated 
successfully,  no  conception  is  thus  given  of  the 
natui^e  of  a  static  electric  charge,  or  of  an  ordinary 
electric  current,  and  there  seems,  on  this  mode 
of  representation,  no  means  of  attacking  the 
problem  of  the  nature  of  gravitation,  which,  it 
was  thought,  must  some  day  be  explained  in 
terms  of  the  universal  medium,  if  that  medium 
was  to  survive  as  a  permanent  conception  in 
physical  science. 

Attempts  were,  therefore,  often  made  to 
describe  ideal  models  which  should  represent  the 


MATTER,  SPACE,  AND  TIME  233 

properties  of  the  aether  by  familiar  mechanical  con- 
ceptions. But  it  was  realised  that,  even  if  such 
a  model  were  successfully  constructed,  it  would 
not  necessarily  represent  the  actual  structure  of 
the  aether  ;  that  was  not  its  object.  The  primary 
function  of  such  a  model  would  have  been  to  justify 
our  theory  of  the  aether  as  expressed  in  Maxwell's 
electro-magnetic  equations,  in  the  other  equations 
requisite  to  explain  electric  charges  and  currents, 
and,  if  possible,  to  suggest  an  explanation  of 
gravitation  also. 

And  so  a  tendency  arose  to  give  up  the  old 
elastic  solid  view  of  the  aether,  and  to  secure  the 
necessary  rigidity  in  another  way.  A  top  when 
spinning  possesses  rigidity  of  position.  It  main- 
tains its  vertical  position  against  the  effects  of 
its  weight,  and  any  displacement  from  the  vertical 
is  followed  by  definite  oscillations  around  the 
mean  position.  These  phenomena  can  best  be 
studied  in  the  gyroscope,  which  first  found  a 
practical  application  in  the  Whitehead  torpedo, 
where  a  direct  course  is  kept  by  the  tendency  of 
a  spinning  wheel  to  maintain  its  axis  of  rotation 
undeviated.  On  these  principles,  Lord  Kelvin 
and  others  described  a  gyrostatic  aether,  in  which 
the  rigidity  is  secured  by  the  motion  of  some  still 
more  primal  material.  The  aether  was  perhaps 
composed  of  a  number  of  interlacing  vortex 
filaments ;  its  structure  might  be  fibrous  like 
that  of  a  bundle  of  hay. 

Following  the  line  of  thought  Indicated  by 
Lord  Kelvin  with  his  conception  of  the  vortex 
atom,  we  conceived  matter  to  be  an  aethereal 
manifestation.  But  the  simple  vortex  ring  Itself 
soon  failed  to  meet  the  demands  made  upon  it. 


234  PHYSICAL  SCIENCE 

'*  The  fluid  vortex  atom,"  said  Larmor,  **  faith- 
fully represents  in  many  ways  the  permanence 
and  mobility  of  the  sub-atoms  of  matter  ;  but  it 
entirely  fails  to  include  an  electric  charge  as  part 
of  their  constitution.  According  to  any  aether 
theory,  static  electric  attraction  must  be  conveyed 
by  elastic  action  across  the  aether,  and  an  electric 
field  must  be  a  field  of  strain,  which  implies  elastic 
quality  in  the  aether  instead  of  complete  fluidity  : 
the  sub-atom  with  its  attendant  electric  charge 
must  therefore  be  in  whole  or  in  part  a  nucleus 
of  intrinsic  strain  in  the  aether,  a  place  at  which 
the  continuity  of  the  medium  has  been  broken 
and  cemented  together  again  (to  use  a  crude 
but  effective  image)  without  accurately  fitting  the 
parts,  so  that  there  is  a  residual  strain  all  round 
the  place." 

It  will  be  noted  that  any  such  theory,  by  which 
matter,  the  subject  of  experimental  mechanics,  is 
explained  as  an  aethereal  manifestation,  changes 
the  point  of  view  from  which  we  regard  mechanical 
models  of  the  aether  itself.  /Ether,  being  now 
regarded  as  a  sub-material  medium,  is  not  neces- 
sarily described  by  the  experimental  laws  to  which 
the  facts  of  ordinary  mechanics  conform.  In 
dealing  with  the  aether,  we  are  on  an  entirely 
different  plane,  and  have  no  right  to  assume 
that  a  mechanical  model  of  its  properties  is 
possible — strictly  speaking,  the  mere  statement 
in  mechanical  terms  of  the  problems  involved 
may  be  in  itself  misleading. 

However  that  may  be,  on  this  theory  the 
corpuscle  of  J.  J.  Thomson,  the  electron  of  Stoney, 
Larmor,  and  Lorentz,  was  represented  in  the 
aethereal  world  by  Larmor  s  conception  of  a  centre 


MATTER,  SPACE,  AND  TIME  235 

of  Intrinsic  strain.  Unlike  the  vortex  atom,  this 
strain-centre  is  not  a  part  of  the  medium  for  ever 
separated  from  the  rest ;  the  strain  alone  persists, 
the  part  of  the  aether  which  is  affected  by  it  con- 
stantly changes  as  the  sub-atom  is  moved.  The 
aether  is  stagnant,  and  the  sturdy  ghosts  which 
constitute  matter  float  to  and  fro  through  it  as 
waves  pass  over  the  surface  of  the  sea.  Such  a 
persistence  in  time  with  mobility  in  space  would 
be  impossible  for  a  strain-form  in  any  elastic  solid 
aether,  but  can  be  secured  by  a  rotational  aether 
of  the  type  described  by  Lord  Kelvin. 

According  to  this  view,  then,  an  electron  or 
unit  charge  of  electricity  is  a  centre  of  intrinsic 
strain,  probably  of  a  gyrostatic  type,  in  an  aether, 
which  is  also  the  medium  in  which  are  propagated 
the  waves  of  light  and  wireless  telegraphy.  More- 
over, the  electron  is  identical  with  the  sub-atom 
which  is  common  to  all  the  different  chemical 
elements,  and  forms  the  universal  basis  of  matter. 
Matter,  at  any  rate  in  its  relation  to  other  matter 
at  a  distance,  is  an  electrical  manifestation  ;  and 
electricity  is  a  state  of  intrinsic  strain  in  a  universal 
medium.  That  medium  is  prior  to  matter,  and 
therefore  not  necessarily  expressible  in  terms  of 
matter  ;  it  is  sub-natural  if  not  super-natural. 

To  reduce  all  physic  to  a  theory  of  the  aether 
as  described  above  is  a  bold  attempt  to  achieve 
uniformity.  Twenty  years  ago  it  seemed  almost 
on  the  threshold  of  success.  But  since  then  science 
has  developed  along  other  lines,  which  we  shall 
trace  on  future  pages.  For  these  developments 
it  has  proved  unnecessary  to  invoke  the  idea  of 
a  universal  aether.  For  the  time,  at  all  events, 
the  tendency   is    to   ignore   the  problem  of  the 


236  PHYSICAL  SCIENCE 

aether,  perhaps  partly  owing  to  the  great  diffi- 
culties of  interpretation  which  we  shall  describe 
presently. 

Nevertheless,  from  the  theory  of  radiation,  as 
well  as  from  Thomson's  experiments,  was  reached 
the  conception  of  an  electron  theory  of  matter. 
Within  a  few  years,  experimental  confirmations 
of  the  fundamental  conceptions  of  that  theory 
gave  it  a  firmer  position  than  could  have  been 
hoped  at  the  time  the  theory  was  formulated. 

The  property  of  mass,  the  most  fundamental 
property  of  matter  for  dynamical  science,  is 
explained  by  the  electron  theory  as  an  effect  of 
electricity  in  motion.  Forasmuch  as  a  moving 
charge  carries  its  lines  of  electric  force  with  it, 
it  possesses  something  analogous  to  inertia  in 
virtue  of  its  motion.  The  quantitative  value  of 
this  effect  has  been  calculated  by  Thomson, 
Heaviside,  and  Searle.  Definite  experimental 
evidence  was  first  given  by  Kaufmann,  who  found 
that  the  ratio  ejin  of  the  charge  to  the  mass  for 
the  corpuscles  ejected  by  radium  diminishes  as 
their  velocity  increases.  The  charge  is  almost 
certainly  constant,  and  thus  the  mass  must 
increase  with  the  velocity.  Theory  shows  that, 
for  a  slowly  moving  corpuscle,  the  electric  inertia 
outside  a  small  sphere  of  radius  a,  surrounding 
the  electrified  particle,  does  not  depend  on  the 
velocity,  and  is  measured  by  2^73^  where  e  is  the 
electric  charge  on  the  particle.  But  when  the 
velocity  of  light  is  approached,  this  electric  mass 
grows  very  rapidly  ;  and,  on  the  assumption  that 
the  whole  of  the  mass  is  electrical,  Thomson 
calculated  the  ratio  of  the  mass  of  a  corpuscle 
moving  with  different  speeds  to  the  mass  of  a 


MATTER,  SPACE,  AND  TIME 


237 


slowly  moving  corpuscle,  and  compared  these 
values  with  the  results  of  Kaufmann's  experi- 
ments. 


Velocity  of  Corpuscle 

in  Centimetres 

per  Second. 

Ratio  of  Mass  to  the  Mass  of  a 
slowl)^  moving  Corpuscle. 

Calculated. 

Observed. 

2-36  X  lO^^ 
2-48  X  iqIO 
2-59  X  iqI'^ 
272  X  10^0 
2.85  X  loio 

1-65 
1-83 
2-04 

2-43 
3-09 

1-5 
1-66 

2-0 
2-42 

3-1 

In  this  remarkable  manner  was  it  possible 
to  show  that  the  electrical  theory  of  mass  is  in 
accordance  with  these  striking  and  unexpected 
experimental  results.  Nevertheless,  as  we  shall 
see  later,  the  same  consequence  of  high  velocity 
in  changing  mass  may  be  shown  to  be  a  deduction 
from  the  theory  of  relativity,  without  invoking 
electric  conceptions. 

We  must  now  return  to  the  story  of  the 
modern  atom,  which  we  left  to  give  an  account 
of  the  facts  of  radiation  and  the  theory  of  a 
luminiferous  aether.  To  explain  all  the  properties 
with  which  we  know  the  chemical  atoms  to  be 
endowed,  and  more  especially  their  power  of  com- 
plex radiation,  a  theory  has  been  built  up  during 
recent  years,  chiefly  by  Thomson,  Rutherford,  and 
Bohr,  which  represents  an  atom  as  a  structure 
containing  one  or  more  electrons  in  orbital  motion 
round  a  centre.  It  would  be  difficult  to  explain 
such  results  if  the  electrons  were  crowded  to- 
gether ;  thus  it  seems  necessary  to  suppose  that 


238  PHYSICAL  SCIENCE 

the  electrons  occupy  an  exceedingly  small  fraction 
of  the  whole  volume  of  the  atom,  just  as  the 
planets  occupy  a  very  small  fraction  of  the  space 
comprised  within  their  orbits. 

The  mass  of  the  electron  being  electrical  in  its 
nature,  we  may  calculate  the  size  of  the  individual 
electrons  or  corpuscles  from  the  expression  2^73^ 
for  the  electrical  mass.  We  know  the  values  of 
e  and  of  elm,  and  from  these  results  we  calculate 
a  to  be  about  io~^^  centimetre.  According  to 
Thomson,  a  is  the  radius  of  a  sphere  outside 
which  the  momentum  of  the  electric  field  exists. 
It  seems  reasonable  to  identify  this  sphere  with 
the  effective  dimensions  of  the  electron  itself. 

We  have  already  seen  that,  in  a  substance  like 
water,  where  the  molecules  are  packed  fairly  closely, 
I  cubic  centimetre  contains  about  3  x  10^^  mole- 
cules, or,  let  us  say,  10^^  atoms.  Along  each  edge 
of  the  centimetre  cube  about  4X  10^  atoms  are 
ranged,  and  thus  we  may  take  the  effective  radius 
of  an  atom  to  be  about  5  x  io~"^  of  a  centimetre. 
Its  volume  would  be  about  io~^^  of  a  cubic  centi- 
metre, while  the  volume  of  an  electron,  according 
to  the  above  estimate  of  the  radius,  is  about 
4  X  10""^^  Thus,  while  the  diameter  of  an  electron 
is  less  than  the  hundred-thousandth  part  of  that 
of  an  atom,  the  volume  of  an  electron  is  only 
about  the  lo"^*^  part  of  that  of  an  atom,  and  their 
relative  sizes  might  be  compared  by  the  Illustration 
of  a  fly  roaming  about  Inside  a  cathedral. 

On  the  planetary  theory  of  the  atom,  the 
moving  electric  charges  produce  a  magnetic  field, 
just  as  does  a  current  flowing  round  the  coils  of 
a  galvanometer.  Thus,  conversely,  an  impressed 
magnetic  force  should  modify  the  movement  of 


MATTER,  SPACE,  AND  TIME  239 

the  electrons,  and  affect  their  radiation,  which 
depends  on  the  rate  of  acceleration  of  their  motion. 
The  theory  of  this  effect  was  considered  by  Lorentz 
and  Larmor,  who  predicted  the  subdivision  of  the 
spectral  lines,  afterwards  experimentally  discovered 
by  Zeeman. 

The  connection  of  the  electron  theory  with 
the  phenomena  of  radio-activity  has  already  been 
considered  in  the  aspects  which  were  first  appreci- 
ated. The  conception  of  an  atom  as  a  system  of 
electrons  in  rapid  orbital  motion  naturally  suggests 
its  occasional  disintegration ;  indeed  the  possibility 
of  such  disintegration  had  been  treated  as  a 
difficulty  of  the  theory  by  Larmor  before  the 
discovery  of  radio-activity  directly  indicated  its 
occurrence.  But  we  now  know  much  more  about 
the  modus  operandi  of  atomic  disintegration,  and 
have  discovered  that  the  changes  concerned  in 
radio-activity  have  not  to  do  with  planetary 
electrons,  but  with  a  much  more  deep-seated 
and  essential  part  of  the  atom,  its  nucleus. 

The  first  step  in  the  formulation  of  the  modern 
theory  of  the  atom  was  the  discovery  of  the 
electron,  the  negative  electric  unit.  The  second 
step  was  the  recognition  of  a  positive  nucleus. 
This  step  was  taken  by  Rutherford. 

The  a  particles,  as  we  have  seen  on  page  1 93,  are 
helium  atoms  projected  with  high  velocities.  Their 
mass  is  four  times  that  of  a  hydrogen  atom,  that 
is,  4  X  1850  or  7400  times  the  mass  of  an  electron. 
The  forces  exerted  on  a  particles  by  electrons  would 
be  much  too  small  to  affect  their  paths  appreci- 
ably. And  most  a  rays,  it  is  true,  pursue  a  straight 
path  through  matter.     Yet  here  and  there  one  is 


240  PHYSICAL  SCIENCE 

found  to  be  hurled  aside  as  by  a  mighty  force. 
Rutherford  examined  this  scattering  of  the  a 
particles,  and  found  that  it  would  be  accurately 
explained  if  we  suppose  that  the  atoms  of  the  gas 
through  which  they  passed  were  formed  of  elec- 
trons revolving  about  a  central,  very  minute,  but 
relatively  massive  nucleus  with  a  positive  charge, 
which  repelled  the  flying  positively  electrified 
a  particles  as  they  passed,  in  the  orthodox 
manner  according  to  the  inverse  square  of  the 
distance,  and  thus  swung  them  aside  in  hyperbolic 
orbits. 

Since  the  electrons  are  of  negligible  mass  com- 
pared with  the  nucleus,  the  mass  of  the  nucleus  is 
very  nearly  the  atomic  weight  of  the  atom.  Thus 
the  mass  of  the  uranium  nucleus  is  about  238  times 
that  of  the  nucleus  of  hydrogen.  The  size  may 
be  estimated  by  measuring  the  amount  of  scatter- 
ing of  a  particles  by  different  atoms  at  large  angles. 
A  heavy  atom  seems  to  have  a  nucleus  with  a 
radius  not  greater  than  6  x  lO""^^  centimetres  and 
those  of  light  atoms  would  be  yet  smaller. 

The  electric  charge  on  the  nucleus  may  also  be 
estimated  by  a  study  of  the  same  scattering  effect. 
It  seems  to  increase  with  the  place  of  an  atom  in 
the  periodic  table.  But  this  fundamental  result  was 
first  discovered,  and  has  been  most  satisfactorily 
demonstrated  by  another  line  of  research. 

As  we  saw  on  page  1 39,  the  X-rays  discovered 
by  Rontgen  have  been  proved  to  consist  of  waves 
similar  in  kind  to  those  of  light,  but  of  very  much 
shorter  wave-length  and  greater  frequency  of 
vibration.  Their  wave-lengths  may  be  measured 
by  analysing  them  by  a  crystal,  the  layers  of  mole- 
cules in  which  act  towards  X-rays  as  the  parallel 


MATTER,  SPACE,  AND  TIME  241 

scratches  on  a  diffraction  grating  act  towards 
light,  separating  the  waves,  and  spreading  them 
out  into  a  spectrum. 

Now  X-rays  are  produced  by  the  impact  of 
cathode  rays  on  solid  obstacles,  and  it  is  found 
that  the  wave-lengths  of  the  X-rays  so  produced 
depend  on  the  nature  of  the  target  exposed  to  the 
cathode  rays.  Generally  there  is  a  certain  amount 
of  diffuse  radiation,  mixed  with  rays  of  definite 
frequency  and  wave-length  corresponding  to  the 
line  spectra  of  visible  light. 

The  spectra  from  these  characteristic  X-rays 
were  examined  in  191 3  and  19 14  by  H.  G.  J. 
Moseley,  who  directed  the  rays  from  different 
elements,  when  used  as  cathode  ray  targets, 
successively  on  to  the  surface  of  a  large  crystal 
of  potassium  ferrocyanide  to  serve  as  a  grating. 
The  resulting  line  spectra  when  photographed 
and  measured  showed  a  surprising  regularity. 
Similar  groups  of  lines  are  found  in  the  spectra- 
of  different  elements,  and  Moseley  discovered  that 
the  square  roots  of  the  frequency  of  vibration  of 
the  chief  lines  in  each  X-ray  spectrum  increased 
regularly  by  a  constant  amount  as  he  passed  from 
element  to  element  in  the  periodic  table.  By 
adjusting  the  constants,  this  constant  difference 
can  be  made  equal  to  unity,  and  Moseley  was  thus 
able  to  assign  to  each  element  an  atomic  number, 
representing  its  true  place  in  the  periodic  table 
which  begins  with  hydrogen  =  i. 

On  the  nuclear  theory  of  the  atom,  the 
frequencies  of  vibration  must  depend  on  the 
electric  charge  on  the  nucleus,  and  Moseley 
concluded  that  the  atomic  number  also  repre- 
sented the  number  of  electric  units  in  the  charge 

R 


242  PHYSICAL  SCIENCE 

on  the  nucleus.  This  conclusion  is  supported  by 
the  estimation  of  nuclear  charges  obtained  from 
the  scattering  of  a  rays  and  is  now  fundamental 
in  modern  physics. 

Moseley's  atomic  number  is  a  constant  for 
each  element  more  important  even  than  the  atomic 
weight.  It  gives  a  new  base  for  the  periodic 
table,  founded  on  known  physical  principles 
instead  of  on  mere  empirical  observation.  By 
its  means,  we  can  obtain  values  for  the  most 
important  property  of  an  atom,  its  nuclear  electric 
charge,  and  thus  start  on  new  investigations  into 
the  fascinating  problem  of  atomic  structure. 

The  fact  that  a  rays  are  flights  of  helium 
atoms,  shows  that  the  atomic  break-down  which 
accompanies  radio-activity  is  an  affair  of  the 
nucleus,  and  not  a  mere  ejection  of  some  of  the 
outer  planetary  electrons.  It  shows,  too,  that 
nuclei  are  constructed  partly  at  all  events  of  helium. 
Yet  the  mass  of  the  helium  atom  is  almost  exactly 
four  times  that  of  the  hydrogen  atom — a  fact  which 
can  hardly  be  mere  coincidence. 

If  we  make  the  simplest  assumption,  and 
regard  the  lightest  atom,  that  of  hydrogen,  as 
made  up  of  a  positive  nucleus  with  one  revolving 
negative  electron,  we  shall  find  it  in  accordance 
with  all  the  evidence  given  above  or  following 
below.  The  electron  is  the  fundamental  negative 
unit,  and  the  hydrogen  nucleus,  or  proton,  is 
the  corresponding  ultimate  positive  unit.  From 
these  two  opposite  units,  all  matter  seems  to  be 
made  up. 

Helium,  with  atomic  number  2  and  atomic 
weight  4,  must  have  2  units  of  charge  on  its 
nucleus,    and    therefore,   in   its    neutral    form,    2 


MATTER,  SPACE,  AND  TIME  243 

attendant  planetary  electrons.  Its  nucleus  must, 
therefore,  be  made  up  of  4  hydrogen  nuclei  or 
protons  bound  together  by  2  nuclear  negative 
electrons.  Since  this  nucleus  holds  togfether 
during  its  violent  projection  as  an  a  ray  particle, 
its  structure  must  be  very  stable — no  power  on 
earth  seems  able  to  break  it  up  once  it  is  formed. 
As  a  secondary  unit,  it  enters  into  the  making 
of  other  more  complex  nuclei.  With  atoms  the 
atomic  weight  of  which  is  divisible  by  4,  there  is 
no  reason  to  suppose  that  any  more  fundamental 
building  materials  than  helium  nuclei  bound 
together  with  electrons  are  used.  But  atoms 
such  as  nitrogen,  atomic  weight  14,  or  aluminium 
27,  cannot  be  so  constructed.  As  Rutherford  has 
found  by  experiment  (see  page  201)  they  contain 
also  hydrogen  nuclei.  Nitrogen,  we  may  suppose, 
is  made  of  3  helium  nuclei,  3x4=12,  and 
2  hydrogen  nuclei  each  weighing  i,  that  is, 
14  in  all.  Atoms  are  probably  built  up,  as  far 
as  may  be,  of  helium  ;  the  odd  corners  are  filled 
in  with  hydrogen,  and  the  whole  bound  together 
with  the  necessary  electric  mortar  of  negative 
electrons.  Round  this  complex  nucleus,  which 
has  an  excess  positive  charge  indicated  by  the 
atomic  number,  electrons  revolve  as  planets  round 
the  sun,  the  number  of  negative  electrons  being 
equal  to  the  net  positive  charge  on  the  nucleus. 
Thus  an  atom  of  uranium,  atomic  number  92  and 
weight  238,  would  be  composed  of  59  helium 
and  2  hydrogen  nuclei,  bound  together  with 
146  negative  electrons  into  a  central  mass,  round 
which  92  electrons  revolve. 

These  attendant  electrons  must  in  some  way 
give  rise  to  electro-magnetic  radiation,  and,  since 


244  PHYSICAL  SCIENCE 

we  find  two  such  differing  types  of  radiation  as 
heat  and  light  on  the  one  hand  and  X-rays  on 
the  other,  we  may  well  guess  that  the  many 
planetary  electrons  revolve  in  different  rings, 
X-rays  coming  from  the  Inner,  and  heat  and 
light  from  the  outer  rings.  This  guess  has 
been  abundantly  supported  by  evidence  In  more 
recent  research. 

Hitherto,  wonderful  as  are  the  results 
described,  they  Involve  no  breach  with  the  old 
and  well-tried  principles  of  Newtonian  dynamics. 
The  paths  of  a  particles,  deflected  by  atoms  of 
a  gas,  show  the  law  of  inverse  squares,  and  the 
atomic  corpuscles  whirl  round  in  their  orbits  as 
the  planets  round  the  sun.  But,  if  we  push  our 
analysis  further,  we  find  that  we  are  forced  to 
assumptions  which  are  not  in  accord  with  this 
familiar  scheme  of  science.  We  are  brought  to 
contemplate  conditions  which  we  cannot  explain 
on  any  known  principles,  conditions  which,  in  the 
present  state  of  knowledge,  seem  not  only  In- 
explicable but  inconceivable  to  our  minds.  It 
may  be  that  future  years  will  see  these  difficulties 
resolved  by  human  insight  as  so  many  others 
have  been.  But  we  must  not  overlook  the  possi- 
bility that  the  orderliness  we  perceive  in  nature 
may  be  merely  the  rediscovery  of  conventions  we 
have  ourselves  inserted  when  framing  the  problems 
to  be  Investigated.  We  choose  mass  and  energy 
as  convenient  fundamental  physical  quantities. 
But,  all  unconsciously,  this  choice  is  made 
because  mass  and  energy  happen  to  remain 
constant  throughout  a  series  of  physical  and 
chemical    changes — and    then   triumphantly    we 


MATTER,  SPACE,  AND  TIME  245 

rediscover  the  persistence  of  matter  and  the 
conservation  of  energy.  As  Professor  Eddington 
disturbingly  suggests,  every  law  of  nature  which 
seems  to  us  rational  may  be  a  concealed  con- 
vention which  we  have  ourselves  unconsciously 
inserted.  Hence  an  unavoidable  conclusion  which 
yet  seems  to  us  irrational  may  be  the  sign  of 
transcendent  importance — the  sign  of  a  real  law 
of  nature  at  last.  If  so,  we  seem  almost  brought 
back  to  Tertullian's  credo  qttid  impossibile. 

The  new  outlook  on  physics  was  first  suggested 
to  Planck  by  the  facts  of  radiation.  If  the  aether 
be  continuous,  all  radiant  energy  must  pass  from 
matter  to  aether,  just  as  the  energy  of  floating 
bodies  set  in  vibration  passes  into  the  surround- 
ing water.  If  we  are  to  hold  any  mechanical 
view  of  the  aether,  we  must  therefore  consider 
that  it  too  possesses  a  structure,  though  probably 
much  finer  than  that  of  matter.  Even  if  we  take 
it  as  of  the  same  order  of  fineness  as  that  of 
matter,  mathematical  calculation  shows  that  most 
of  the  energy  radiated  by  matter  should  be 
concentrated  in  the  short  wave-lengths  of  the 
ultra-violet  light.  But  observation  shows  that 
in  a  continuous  spectrum,  such  as  that  of  the 
sun,  the  maximum  heat  effect,  which  measures 
the  total  energy,  is  in  the  infra-red,  that  is,  in 
those  waves  too  long  to  affect  our  eyes  instead 
of  in  those  too  short. 

To  meet  this  difficulty  Planck,  in  1901, 
supposed  that  radiation  was  emitted  and  absorbed 
by  matter  not  continuously  but  in  small  indi- 
visible units.  To  calculate  the  rate  of  emission 
of  energy  then  becomes  an  exercise  in  the  theory 
of  probability — how  many  units  are  likely  to  be 


246  PHYSICAL  SCIENCE 

absorbed  or  emitted  in  a  given  case.  With  quite 
reasonable  assumptions,  this  statistical  method 
accounts  for  the  facts  it  was  framed  to  meet. 

But  most  theories  can  do  as  much  as  that. 
The  real  test  comes  when  a  theory  is  extended 
to  cover  other  facts  which  were  not  in  mind 
during  its  inception.  Hence  the  evidence  for 
the  quantum  theory  was  much  strengthened  when 
Einstein,  in  1907,  applied  it  successfully  to  explain 
the  fact  that  the  specific  heats  of  certain  solid 
elements  like  carbon,  and  other  elements  at  low 
temperatures,  were  not  constant,  as  classical 
physics  required,  but  varied  with  temperature. 
If  energy  be  absorbed  not  in  infinitely  small 
quantities  but  by  finite  units,  we  can  explain  this 
result,  for  when  temperature  is  low  and  heat 
units  scarce,  some  atoms  will  possess  no  units 
at  all,  and  thus  the  total  content  of  energy,  and 
therefore  the  specific  heat,  is  small.  A  mathe- 
matical investigation  shows  that  the  expected 
change  of  specific  heat  with  rising  temperature 
is  in  accurate  accordance  with  observation. 

To  fit  in  with  the  numerical  results,  the 
unit  of  energy  e  must  be  equal  to  /iv,  where  v  is 
the  frequency  of  vibration  and  /i  a  constant  called 
Planck's  constant,  which  has  the  value  6.5  x  io~^^ 
erg-seconds.  It  will  be  seen  that  the  size  of  the 
units  of  energy  depends  on  the  frequency  of 
vibration,  and  is  larger  when  that  frequency  is 
great,  as  in  violet  and  ultra-violet  light,  than 
when  the  frequency  is  small  and  the  wave-length 
large,  as  in  red  light  or   invisible  radiant  heat. 

The    real    constant    is    Planck's  quantity  /i  =  -, 

V 

which   is    not    energy,    but    energy   divided    by 


MATTER,  SPACE,  AND  TIME  247 

frequency.  Since  frequency  is  a  number  of 
vibrations  in  a  given  time,  it  follows  that  Planck's 
constant  is  energy  multiplied  by  time.  This 
quantity  is  of  fundamental  importance  in  modern 
physics  and  is  called  action. 

A  unit  of  energy  multiplied  by  the  time  during 
which  it  is  applied  is  called  a  unit  of  action,  and 
Planck's  constant  h  is  the  natural,  real  unit  of 
action,  just  as  the  electron  is  the  natural  real 
unit  of  electric  charge  or  of  mass. 

We  are  now  ready  to  take  up  again  our  story 
of  the  development  of  the  modern  theory  of  the 
atom  where  we  dropped  it  on  page  244.  We  had 
then  reached  the  conception  of  a  central  nucleus, 
made  up  of  a  conglomerate  of  positively  electrified 
helium  and  in  some  cases  hydrogen  nuclei  cemented 
together  with  negative  electrons,  and  surrounded 
at  great  comparative  distances  by  attendant  rings 
of  other  revolving  planetary  electrons.  As  the 
simplest  assumption,  we  supposed  that  the 
hydrogen  atom,  the  lightest  known  to  us,  was 
a  central  positive  unit  particle  or  proton,  with 
one  planetary  electron. 

Now  the  spectrum  of  hydrogen  is  of  con- 
siderable complexity.  It  is  not  continuous, 
not  a  uniform  band  of  coloured  light  like  the 
rainbow,  but  it  consists  of  sharp  lines  many  in 
number. 

An  electron  revolving  round  a  nucleus  would, 
as  we  saw  on  page  149,  on  any  classical  electro- 
dynamic  theory,  emit  electro-magnetic  radiation. 
It  must  thus  lose  energy,  fall  nearer  the  nucleus, 
and  swing  round  it  with  steadily  increasing 
velocity.     A   collection   of  atoms   of  hydrogen, 


248  PHYSICAL  SCIENCE 

then,  should  emit  radiations  of  all  frequencies  of 
vibration,  that  is,  should  give  a  continuous 
spectrum. 

Hence  we  see  that  the  existence  of  line 
spectra,  not  from  hydrogen  merely  but  from 
many  other  elements,  again  leads  us  to  contem- 
plate the  difficulties  of  Newtonian  dynamics 
applied  to  electro-magnetic  atoms,  and  once  more 
brings  us  to  some  form  of  quantum  theory. 

The  application  of  these  conceptions  to  the 
problems  of  atomic  structure  was  first  made  by 
Niels  Bohr  of  Copenhagen,  then  working  in 
Rutherford's  laboratory  at  Manchester. 

Certain  regularities  in  the  complex  spectrum 
of  hydrogen  become  apparent  if  we  consider  not 
the  wave-lengths  of  its  luminous  lines  but  the 
number  of  waves  in  a  centimetre — a  quantity 
which  may  be  called  the  vibration  number.  It 
is  found  that  the  vibration  numbers  of  all  the 
lines  may  be  expressed  as  the  difference  between 
two  terms.  There  is  first  a  fundamental  term, 
called  Rydberg's  constant,  after  its  discoverer  ; 
its  number  is  about  109700  waves  per  centimetre. 
Other  terms  are  obtained  from  it  by  dividing  it 
by  four  (2  x  2),  nine  (3  x  3),  sixteen  (4  x  4),  and 
so  on.  If  we  subtract  these  terms  from  R, 
Rydberg's  constant,  we  get  vibration  numbers, 

R  5  "DO 

R  —  —  =  ^R,  R =-R,  etc.,  and  these  numbers 

4      4  9      9.. 

correspond  to  hydrogen  lines  in  the  ultra-violet. 

If  we  begin  with  the  first  derived  term,  that  is 

one-fourth  of  109700  or  27425,  and  subtract  the 

higher  derived  terms   from    it,  we   get   another 

.         ,          R     R      (9-4)R        5p     . 
series  ot  numbers,  — =  ^— — ^ —  =  ^K,  etc., 

4      9  36  36 


MATTER,  SPACE,  AND  TIME  249 

corresponding  to  the  visible  lines  of  hydrogen 
known  as  Balmer's  series.  Another  group, 
obtained  from  one-ninth  of  109700,  was  found  in 
the  infra-red  by  Paschen. 

These  relations  were  discovered  by  making 
experiments,  and  then  guessing  at  arithmetical 
rules  till  one  was  found  to  fit  the  facts.  They  are 
purely  empirical.  But  Bohr  saw  how  to  explain 
them  all  by  applying  Planck's  quantum  theory  to 
the  atom. 

Bohr  pointed  out  that,  if  ''action "  is  absorbed 
only  in  units,  of  all  conceivable  orbits  in  which 
the  hydrogen  electron  might  revolve,  only  a  certain 
limited  number  would  be  possible.  In  the  smallest 
orbit,  the  action  would  be  one  unit  or  h^  in  the 
next  orbit  2/^,  and  so  on.  Mathematical  investiga- 
tion shows  that  the  energy  of  motion  in  the  second 
orbit  is  a  quarter  that  in  the  first,  in  the  third 
orbit  one-ninth,  and  in  the  fourth  one-sixteenth. 
As  an  electron  falls  in  from  an  outer  to  an 
inner  orbit,  it  loses  energy  of  position  and  gains 
energy  of  motion.  It  may  be  shown  that  the  total 
loss  of  energy  is  equal  to  the  gain  in  energy  of 
motion.  Hence,  if  e  be  the  energy  of  motion  in 
the  first  or  smallest  orbit,  it  follows  that,  in  passing 
from  the  second  orbit  to  the  first,  the  loss  of  energy 

is  -  e,  in  passing  from  the  third  to  the  second, 

8  T       T  e 

-  e,  and  from  the  third  to  the  first,  -  —  or  -^  e. 

9  ,  '  4     9       36 

It  will  be  seen  that  this  series  of  numbers  Is  the 
same  as  that  found  experimentally  in  the  vibration 
numbers  of  the  hydrogen  spectrum. 

On   this  evidence    Bohr  founded    his    theory 
of  the  hydrogen  atom.     He  supposes   that  the 


250  PHYSICAL  SCIENCE 

hydrogen  electron  has  four  possible  stable  orbits, 
corresponding  to  successive  units  of  action.  Here 
we  leave  Newtonian  dynamics.  A  planet  can 
revolve  round  the  sun  in  any  one  of  an  infinite 
number  of  orbits,  the  actual  path  being  adjusted 
to  its  velocity.  But  an  electron  can  only  move  in 
one  of  a  few  paths,  each  of  which  corresponds  to 
an  integral  number  of  units  of  action.  If  it  leaves 
one  such  path,  it  must  jump  instantaneously  to 
another,  apparently  without  passing  over  the 
intervening  space.  Perhaps  there  is  no  inter- 
vening space  :  perhaps  space,  perhaps  even  time, 
is  discontinuous.     But  that  is  another  story. 

When  the  electron  leaps  from  one  stable  path 
to  another  it  radiates  energy  hv^  the  action  of 
which  is  h,  and  the  frequency  of  vibration  v.  The 
energies  lost  in  the  changes  described  above  are 

-  e,  -  e,  — ^  6,  etc.  Hence,  smce  k  is  constant,  the 
4   '  9    '  i6 

frequencies  v^,  v^^  v^,  etc.,  must  be  in  the   ratios 

-,  -,  ^,  etc.,  and  we  get  the  known  series  of  lines 
4'  9'  1 6         '  ^ 

in  the  hydrogen  spectrum.  It  is  possible,  further- 
more, to  calculate  the  numerical  value  of  the 
fundamental  term  corresponding  to  Rydberg's 
constant,  and  to  reach  the  amazing  result  that  it 
agrees  with  the  figure  on  page  248,  as  obtained  by 
observation.  Even  more  complex  phenomena 
of  the  hydrogen  spectrum  are  fully  explained  by 
Bohr's  theory  as  developed  by  Sommerfeld,  and 
it  is  impossible  to  doubt  that  we  are  on  the  right 
road.  Hydrogen  atoms  must  be  something  like 
Bohr's  picture  of  them.  Heavier  atoms  with 
more  planetary  electrons  give  problems  beyond 
the  present  power  of  mathematical  analysis.     But 


MATTER,  SPACE,  AND  TIME  251 

what  progress  can  be  made  on  the  general  lines 
of  Bohr's  theory  is  all  consistent  with  observed 
facts. 

Here,  then,  in  the  quantum  theory  and  Bohr's 
application  of  it  to  atomic  structure,  we  have  quite 
a  new  departure  in  science.  No  explanation  can 
be  given  at  present  on  the  principles  of  classical 
dynamics  of  the  existence  of  an  indivisible 
quantum  of  action,  or  of  its  consequence  the 
restriction  of  atomic  planetary  electrons  to  a  few 
definite  orbits.  The  quantum  seems  a  brute  fact, 
which  we  must  accept,  but  cannot,  yet  at  any  rate, 
explain.  But  this,  as  Eddington  holds,  may  be  a 
sign  of  its  real  importance.  It  is  certain  that  we 
ourselves  have  not  read  it  into  the  story  of  Nature. 

We  must  now  pass  to  the  consideration  of 
the  theory  of  relativity,  which  has  shared  with 
the  theory  of  quanta  and  the  allied  problem  of 
atomic  structure  the  chief  attention  of  mathe- 
matical physicists  during  recent  years. 

The  idea  of  relativity  arose  from  the  results 
of  various  measurements  of  the  velocity  of  light, 
and  the  discordant  indications  they  gave  of  the 
relation  between  the  earth  and  the  hypothetical 
aether  of  space. 

Astronomical  observations  suggest  that  the 
earth  moves  through  a  quiescent  aether,  the  aether 
streaming  through  the  moving  atoms  of  matter 
as  wind  through  a  grove  of  trees.  Moreover, 
Lodge  found  that  the  velocity  of  light  between 
two  parallel  steel  discs,  whirled  round  their  axis 
at  great  speed,  was  the  same  whether  the  light 
passed  in  the  same  or  the  opposite  direction  to 
the  movement  of  the  plates.     In  this  case,  the 


252  PHYSICAL  SCIENCE 

steel  seems  to  exert  no  drag  on  the  aether  just 
outside  it. 

On  the  other  hand,  a  classical  experiment  made 
in  1887  by  the  American  physicists  Michelson  and 
Morley,  indicated  a  contrary  result.  If  the  aether 
is  quiescent,  as  the  earth  moves  through  space 
there  is  relative  motion  between  it  and  the  aether, 
and  a  stream  of  aether  must  pass  through  a 
laboratory. 

Now  a  swimmer  can  pass  across  a  rapid  river 
and  back  again  quicker  than  he  can  swim  an 
equal  distance  up  and  down  stream.  Hence  a 
ray  of  light  should  take  longer  to  pass  along  the 
aether  stream  and  back  from  a  mirror,  than  if  it 
travelled  at  right  angles  to  its  first  path  and 
were  reflected  back  from  across  the  stream. 

When  Michelson  and  Morley  carried  out  this 
experiment,  they  found  to  everyone's  surprise  that 
no  difference  could  be  detected. 

Of  course  they  could  not  know  beforehand 
in  which  direction  the  aether  was  moving  through 
their  laboratory,  but  by  rotating  the  apparatus 
into  another  position,  and  trying  the  experiment 
at  different  seasons  of  the  year,  they  got  over 
this  difficulty.  But  the  two  rays,  at  right  angles 
to  each  other,  one  going  across  and  one  along 
the  stream,  always  arrived  at  the  observation 
post  together — the  race  was  always  a  dead-heat, 
however  the  apparatus  was  placed  and  whatever 
were  the  time  of  year. 

This  experiment  has  been  repeated  in  more 
recent  years  to  a  higher  order  of  accuracy  with 
the  same  result.  It  clearly  indicates  that  the 
measured  velocity  of  light  is  the  same  when 
travelling   with  and  against    the  motion   of  the 


MATTER,  SPACE,  AND  TIME  253 

earth.  It  suggests  that  the  aether  is  carried 
along  with  the  earth,  a  conclusion  apparently 
inconsistent  with  the  astronomical  evidence. 

The  first  successful  attempt  to  explain  this 
discrepancy  was  made  by  Fitzgerald,  Lorentz,  and 
Larmor.  If  the  atoms  of  matter  are  electrical  in 
nature,  or  held  together  by  electrical  forces,  it  is 
possible  that  matter  may  contract  when  it  is 
moving  through  the  electro-magnetic  medium  or 
aether.  A  very  small  contraction  when  matter 
is  moving  through  aether  would  suffice  to  explain 
the  facts,  and  the  contraction  could  never  be 
observed  directly,  for  all  scales  used  to  measure 
it  would  undergo  the  same  proportionate  change. 
It  is  probable  that  this  explanation  represents 
one  aspect  of  the  truth.  However  this  may  be, 
the  velocity  of  light,  as  experimentally  measured 
by  an  observer,  has  always  been  found  to  be 
3  X  10^^  centimetres,  or  186,000  miles  a  second. 

Impressed  by  this  experimental  result,  Einstein, 
in  1905,  accepted  the  constancy  of  the  measured 
velocity  of  light  as  an  ultimate  fact  of  Nature, 
and  was  hence  led  to  see  that  real  experimental 
space  and  time  are  always  relative  to  some 
observer,  and  that  the  ideas  of  absolute  space  and 
time  are  mere  figments  of  the  imagination. 

By  length  or  distance  we  always  mean  a 
quantity  arrived  at  by  measurements  made  with 
material  or  optical  appliances  by  some  particular 
observer.  Thus,  we  see  that  the  length  of  a  rod 
is  not  an  absolute  property  of  the  rod,  but  is  a 
relation  between  the  rod  and  the  observer.  If 
they  are  moving  together,  the  length  seems  in- 
variable, but,  if  the  rod  is  moving  past  the 
observer,  it  suffers  the  contraction  suggested  by 


254  PHYSICAL  SCIENCE 

Fitzgerald.  The  time-scale  undergoes  a  corre- 
sponding change.  A  clock,  to  an  observer 
moving  with  it,  goes  steadily,  but,  if  it  were 
carried  past  the  observer  at  great  speed,  it  would 
seem  to  slow  down. 

But,  while  both  space  and  time  separately 
are  relative  to  the  observer,  Minkowski,  in  1908, 
pointed  out  that  there  is  a  combined  space-time 
which  is  absolute  :  a  space-time  of  such  a  nature 
that  the  velocity  of  light  is  a  true  natural 
constant.  Hence,  instead  of  the  familiar  three 
dimensions  of  space,  length,  breadth,  and  height, 
with  time  as  a  completely  independent  quantity, 
we  must  add  time  as  another  dimension,  and 
picture  the  universe  in  terms  of  an  inseparable 
space-time  involving  four  dimensions.  Any  given 
particle  is  moving  through  both  space  and  time. 
The  distance  it  moves  depends  on  the  observer,  and 
the  time  it  takes  to  traverse  that  distance  depends 
on  the  observer,  but  its  track  through  four  dimen- 
sional space-time,  what  is  called  the  *' interval" 
between  its  first  state  and  its  last,  does  not  depend 
on  the  observer,  but  is  the  same  for  all  observers 
— a  true  characteristic  quantity  for  the  particle. 

Since  clocks  are  timed  by  pendulums  or  heavy 
balance  wheels  which  possess  mass,  the  slowing 
down  of  time  suggests  that  mass,  like  length 
and  time,  changes  with  the  observer,  and  that 
the  mass  of  a  fast  moving  body  becomes  greater 
to  an  observer  at  rest.  The  amount  of  this 
increase  may  be  calculated  from  Minkowski's 
space-time  ''interval"  which  we  have  just 
described.  The  velocity  of  a  body  may  be 
written  as  z^  =  Ijt,  where  /  is  the  length  described 
in  a  time  t.     But,  if  instead  of  t  we  write   the 


MATTER,  SPACE,  AND  TIME  255 

''interval"  s,  we  get  a  new  kind  of  velocity  //s. 
Similarly  momentum,  which  In  the  old  mechanics 
is  Mv  or  M  .  //^,  mass  multiplied  by  velocity,  may 
be  modified  into  a  new  kind  of  momentum,  m .  //s. 
Here  m  is  constant.  But  in  physics  it  is,  for  the 
present  at  all  events,  more  convenient  to  keep 
to  the  old  definition  of  momentum  as  mass  and 
velocity.     We  then  have — 

/         /    /      ,,./ 

m  -  =  m  -  .  -  =  M  -  J 
s  s     i  ^ 

where  in.  tjs  is  a  modified  mass,  M,  identical  with 
our  old  mass,  no  longer  constant  but  dependent 
on  the  motion  through  the  observer's  space  and 
time.  It  is  easy  to  show  mathematically  that, 
if  momentum  is  conserved, 

M  = 


where  c  is  the  constant  velocity  of  light. 

This  is  the  same  law  of  Increase  that  was 
calculated  on  the  electro-magnetic  theory  by 
Thomson,  and  verified  by  the  experiments  of 
Kaufmann  and  others  on  /3  particles  (page  237). 
We  now  see  that,  while  consistent  with,  It  does 
not  necessarily  verify,  the  electrical  theory  of 
matter,  since  it  follows  also  from  the  general 
theory  of  relativity.  If  the  observer  moved  with 
the  ^  particles,  their  mass  as  measured  would 
of  course  remain  constant.  The  change  Is  a 
consequence  of  the  relative  motion. 

The  last  equation  can  be  put  in  the  form — 

M  =  —. -^  /       v\-h 


4-5) 


.,„,,--, 


256  PHYSICAL  SCIENCE 

Since  the  velocity  z^  of  a  moving  body  is  usually 
small  compared  with  the  velocity  c  of  light,  this 
result  gives — 

M  =  ;;^l  I  +  — ^\  =  m  ■\-  2 

That  is,  the  effective  inertial  mass  of  a  moving 
body  is  its  mass  at  rest  plus  its  kinetic  energy 

-  mv^  divided  by  the  square  of  the  velocity  of 

liofht.  It  seems  that  mass  is  of  the  nature  of 
energy  or  energy  of  the  nature  of  mass  :  matter 
and  energy  may  be  identified. 

From  this  result  it  seems  reasonable  to 
suppose  that  a  region  filled  with  any  form  of 
energy,  even  for  instance  light  or  radiant  heat, 
would  possess  inertia  equal  to  the  energy  contents 
divided  by  the  square  of  the  velocity  of  light. 
It  does  not  immediately  follow  that  the  energy 
would  be  subject  to  gravitation.  It  may  possibly 
be  that  the  equivalence  between  mass  and  weight, 
proved  experimentally  by  Galileo  and  Newton, 
applies  to  m  in  the  equation,  to  the  mass  at  rest, 
and  not  to  M  which  contains  also  the  kinetic 
energy.  The  problem  of  gravitation  needs  further 
consideration. 

The  principle  of  relativity  was  first  applied 
to  the  phenomena  of  gravitation  by  Einstein,  in 
191 1.  He  pointed  out  that  it  was  impossible 
by  any  experiment  to  distinguish  a  gravitational 
force  from  the  force  experienced  by  an  observer 
who  is  accelerated,  that  is,  whose  motion  is 
changing.  For  instance,  when  a  lift  starts  to 
rise,  the  occupants  feel  all  the  effects  of  a  sudden 


MATTER,  SPACE,  AND  TIME  257 

though  temporary  increase  of  weight ;  indeed,  a 
mass  hung  from  a  spring  balance  would  weigh 
heavier  till  the  upward  speed  of  the  lift  became 
uniform.  Einstein  assumed  that  this  principle 
of  equivalence  held  not  only  for  mechanical  but 
also  for  electrical  effects,  including  light. 

The  application  of  these  ideas  involved  great 
mathematical  difficulties,  and  Einstein  did  not 
publish  a  full  account  of  his  researches  till  191 5. 
It  then  appeared  that  many  of  the  theories  of 
the  older  physics,  including  Newton's  law  of 
gravitation,  might  be  replaced  by  new  explana- 
tions of  the  phenomena. 

Mathematical  analysis  shows  that  the  space 
and  space-time  of  Einstein  and  Minkowski  have 
certain  peculiarities.  At  places  they  are  impene- 
trable, and  there  we  may  fairly  suppose  to  exist 
what  we  call  particles  of  matter.  Near  these 
places  the  equations  show  that  space  and  space- 
time  are  subject  to  what  in  a  line  or  a  surface  we 
call  curvature.  How  three  dimensional  space  and 
four  dimensional  space-time  can  be  curved,  we  must 
imagine  as  best  we  may.  The  wonders  of  nature 
are  not  necessarily  comprehensible  to  our  minds. 

The  curvature  of  space  and  space-time  may 
perhaps  best  be  left  in  the  decent  obscurity  of 
mathematical  equations.  The  equations  show 
that  the  natural  path  of  a  particle  of  matter 
traversing  a  region  near  a  massive  body  is  not 
the  straight  line  passed  over  with  uniform  speed 
contemplated  by  Newton's  First  Law  of  Motion, 
but  a  path  in  space-time  that  bends  towards  the 
mass  in  space,  and  in  time  moves  faster  the  nearer 
it  passes  to  the  matter — the  path,  in  fact,  of  a 
planet  swinging  round  the  sun. 

s 


258  PHYSICAL  SCIENCE 

Thus  the  effect  of  the  sun,  which  for  two 
hundred  years  and  more  has  been  referred  to  an 
attractive  force  between  the  sun  and  the  planet, 
can  be  explained  as  due  to  a  curvature  in  space, 
which  makes  the  natural  path  of  the  planet  when 
undisturbed  an  ellipse  instead  of  a  straight  line. 
If  a  body  be  falling  freely,  and  so  following  its 
natural  path,  it  feels  no  sensation  of  force.  If 
it  be  prevented  from  falling  by  a  chair  or  the 
platform  of  a  weighing  machine,  it  is  turned 
from  its  natural  path  and  shows  the  phenomenon 
we  call  weight,  which  may  be  regarded  as  due  to 
the  upward  acceleration  impressed  on  the  body 
by  the  bombardment  of  the  molecules  of  the  chair 
or  the  platform. 

The  mathematical  theory  shows  that  all  the 
long  known  facts  of  gravitation  can  be  deduced 
equally  well  from  Newton's  theory  or  Einstein's 
principle.  Yet  three  phenomena  have  been  found 
in  which,  when  great  accuracy  of  observation  is 
reached,  differences  should  appear  and  crucial 
experiments  be  possible. 

(i)  A  minute  divergence  of  the  planet  Mercury 
from  its  Newtonian  path — a  divergence  only 
amounting  to  43  seconds  of  arc  in  a  century — 
was  at  once  explained  by  Einstein. 

(2)  Both  on  Newton's  theory  and  on  Einstein's, 
the  path  of  a  ray  of  light,  passing  near  a  massive 
body  like  the  sun,  should  be  bent  towards  the 
body,  but  Newton's  deflection  is  one  half  that 
of  Einstein.  The  only  way  of  observing  this 
deflection  is  to  measure  accurately  the  apparent 
position  of  the  image  of  a  star  very  near  the  sun 
on  a  photographic  plate  exposed  during  an  eclipse. 
This  was  done  in  two  places,  Sobral  in  Brazil  and 


MATTER,  SPACE,  AND  TIME  259 

the  Island  of  Principe  in  the  Gulf  of  Guinea,  on 
29th  May  1 9 19.  The  result  was  in  accordance 
with  Einstein's  prediction. 

(3)  On  the  principle  of  relativity,  the  electrons 
in  a  gravitational  field  should  vibrate  more  slowly 
than  the  normal.  Hence  the  lines  in  the  spectrum 
of  the  sun,  where  gravity  is  strong,  should  be 
slightly  shifted  towards  the  red.  For  long  this 
effect  was  looked  for  in  vain,  but  lately  evidence 
in  its  favour  has  been  obtained. 

The  balance  of  experimental  evidence,  then, 
leans  decidedly  towards  Einstein's  interpretation 
of  nature,  or  possibly  some  modification  of  it  such 
as  that  put  forward  by  Professor  A.  N.  Whitehead, 
and  we  must  learn  to  regard  as  merely  relative, 
many  concepts  we  had  come  to  accept  as 
absolute. 

Doubtless  this  new  outlook  has  not  only  a 
physical  but  also  a  philosophical  importance. 
Space  and  time  no  longer  exist  independently  of 
each  other  and  of  events  which  happen.  Nature 
is  a  complex  continuum,  in  which  matter,  space, 
and  time  are  all  inextricably  involved.  The 
separation  is  no  more  than  a  question  of  human 
convenience  like  the  separation  of  science  into 
physics,  chemistry,  and  biology.  Many  of  the 
familiar  concepts  in  which,  first  by  the  ordered 
common  sense  of  successive  generations  of  men, 
and  then  by  the  more  subtle  analysis  of  science, 
we  had  come  to  express  our  mental  picture  of  the 
world,  have  been  proved  to  be  merely  relative  to 
ourselves.  The  length  of  a  rod,  the  time  beaten 
out  by  a  pendulum,  the  mass  of  a  chemical  atom, 
so  constant  and  absolute  to  our  fathers,  are  now 
seen  to  be  relations   between   us  and   the  body 


26o  PHYSICAL  SCIENCE 

observed,  no  more  essentially  constant  than  the 
value  of  gold  to  a  mariner  when  in  New  York  at 
one  time  and  on  a  desert  island  at  another. 

Much  of  our  old  scheme  of  science  has  been 
put  into  nature  by  our  own  minds,  and  then  redis- 
covered. Possibly  that  is  why  nature  has  seemed 
to  us  to  be  rational.  We  are  beginning  to  fear 
that  things  too  easily  rationalised  are  but  the 
delusive  image  of  ourselves  seen  in  nature's  mirror. 
The  real  nature  may  have  but  little  in  common 
with  that  looking-glass  world.  Yet  one  quantity 
stands  out,  at  present  incomprehensible,  with  all 
the  signs  of  a  real  natural  constant — the  unit  of 
action  in  Planck's  great  quantum  theory. 


CHAPTER  IX 

ASTRO-PHYSICS 

For  who  so  list  into  the  heavens  looke, 
And  search  the  courses  of  the  rowling  spheares, 
Shall  find  that  from  the  point  where  first  they  tooke 
Their  setting  forth,  in  these  few  thousand  yeares 
They  all  are  wandred  much  ;  that  plaine  appeares  : 

•  •••••• 

Ne  is  that  same  great  glorious  lampe  of  light, 
That  doth  enlumine  all  these  lesser  fyres. 
In  better  case,  ne  keepes  his  course  more  right, 
But  is  miscaried  with  the  other  Spheres  : 
For  since  the  terme  of  fourteene  hundred  yeres, 
That  learned  Ptolomas  his  hight  did  take, 
He  is  declyned  from  that  marke  of  theirs 
Nigh  thirtie  minutes  to  the  Southern  lake  ; 
That  makes  me  feare  in  time  he  will  us  quite  forsake. 
— Spenser,  The  Faerie  Queene^  Book  V. 

The  origins  of  the  ancient  science  of  astronomy 
are  lost  in  the  mists  of  the  past.  Unlike  some  of 
the  subjects  we  have  discussed  in  this  volume, 
its  phenomena  are  familiar  to  the  most  unobservant 
of  mankind,  and  some  of  these  phenomena,  in  the 
apparently  unfailing  regularity  of  their  manifesta- 
tion, have  served  as  measurers  of  time  and  fore- 
warners  of  seasons  during  immemorial  ages. 

The  recognition  of  the  possibility  of  slow 
change  in  this  regularity,  and  the  attempt  to  detect 
such  change  by  careful  observation,  are  also  an 
old  story,  while  unusual  manifestations,  such  as 
comets  and  eclipses,  were,  till  comparatively  recent 

261 


262  PHYSICAL  SCIENCE 

times,  regarded  with  fear  and  consternation,  and 
considered  as  direct  signs  of  Divine  wrath. 

Yet  the  oldest  of  the  sciences  is  also,  in  some 
respects,  if  not  the  newest,  at  any  rate  among  the 
youngest  of  the  fraternity;  for  in  its  recent  growth, 
its  spirit  of  adventure,  its  capacity  of  immediate 
development,  it  shows  all  the  characteristics  of 
sturdy  youth. 

In  the  history  of  the  different  branches  of 
physical  science,  it  is  constantly  found  that 
periods  of  great  activity  and  advancing  know- 
ledge alternate  with  periods  when,  owing  to  the 
exhaustion  of  the  possibilities  of  the  apparatus 
available  or  of  the  methods  of  research  employed, 
progress  seems  almost  to  cease. 

Seventy  years  ago  astronomy  appeared  to  be 
sinking  into  one  of  these  periods  of  comparative 
stagnation.  The  power  of  the  telescope  seemed 
almost  to  have  reached  a  limit,  for  although 
improved  and  larger  instruments  were  being 
produced  continually,  the  revelations  they  made 
were  apparently  unworthy  of  the  knowledge  and 
skill  lavished  on  their  manufacture.  It  was  not 
more  elaborate  instruments,  but  new  methods  of 
research  that  were  wanting. 

But  even  while  the  older  astronomy  was  flag- 
ging, the  new  method  had  appeared,  and  was  only 
waiting  for  development  in  its  apparatus  to  carry 
forward  the  torch  of  learning  into  untrodden 
paths,  and  even  to  rival  the  discoveries  of  Adams 
and  Leverrier,  who  had  stirred  so  profoundly  the 
imagination  of  their  generation. 

The  new  science  of  astro-physics  dates  from 
the  application  of  the  spectroscope  to  astronomical 


ASTRO-PHYSICS  263 

problems.  The  spectroscope  itself  illustrates  the 
progressive  triumph  of  modern  science,  for  it  is 
the  work  neither  of  one  man  nor  of  one  century. 
Its  principles  have  been  developed  gradually  and 
its  construction  elaborated  throughout  a  couple 
of  hundred  years.  Newton  was  the  first  to 
analyse  the  light  of  the  sun  by  a  prism,  to  study 
the  spectrum  thus  obtained,  and  to  show  that  it 
consists  of  rays  of  every  colour,  which,  when 
blended  together  in  the  eye,  produce  the  sensa- 
tion of  white  light.  In  the  year  1802,  Wollaston 
noticed  that  the  spectrum  of  the  sun's  light  was 
crossed  by  a  number  of  fine  dark  lines,  and, 
shortly  afterwards,  the  relative  positions  of  these 
lines  were  mapped  carefully  by  Fraunhofer, 
whose  name  the  lines  have  borne  since  that 
time. 

The  next  great  advance  was  made  by  the 
chemists  Bunsen  and  Kirchhoff,  who  repeated 
and  amplified,  in  the  year  i860,  an  almost  for- 
gotten experiment  of  Foucault,  though  the 
principles  which  underlie  their  discovery  had 
previously  been  understood  by  Sir  George 
Stokes.  Any  vibrating  system — a  child's  swing, 
for  example — is  set  into  violent  oscillation  if 
impulses  are  given  to  it  exactly  timed  to  coincide 
with  its  own  proper  period  of  vibration.  Just  as 
the  particular  piano  wires  which  are  tuned  to  a 
particular  note  will  be  set  in  vibration  when  that 
note  is  sounded  in  their  neighbourhood,  so  the 
molecules  or  atoms  of  a  gas  will  be  set  in 
vibration  by  waves  of  light  which  possess  a 
period  of  oscillation  corresponding  with  their 
own.  A  complex  wave  of  light,  then,  passing 
through  a  collection  of  such  molecules  or  atoms, 


264  PHYSICAL  SCIENCE 

will  have  those  constituent  waves  absorbed 
which  are  tuned  to  the  characteristic  periods  of 
the  absorbing  systems.  Substances,  that  is  to 
say,  absorb  the  particular  kinds  of  radiation 
which  they  would  themselves  emit  when  hot. 

Applying  these  principles  to  the  Fraunhofer 
lines,  Stokes  held  that  when  coincidences  existed 
between  their  positions  and  those  of  the  bright 
lines  obtained  by  examining  with  a  prism  the 
light  of  incandescent  vapours,  the  coincidence 
was  to  be  interpreted  by  the  supposition  that 
similar  vapours  were  present  in  the  atmosphere 
of  the  sun,  and  absorbed  the  light  coming  from 
the  hotter  regions  below  them. 

In  i860  Bunsen  and  Kirchhoff,  without  know- 
ing that  Foucault  had  anticipated  them  in  1849, 
devised  and  carried  out  an  experiment  on  the 
artificial  production  of  Fraunhofer  lines.  They 
passed  the  light  of  an  electric  arc,  which  gave 
a  perfectly  continuous  spectrum  with  no  such 
lines  as  those  in  the  solar  light,  through  the 
vapour  of  sodium  volatilised  in  the  comparatively 
cool  region  of  a  spirit  lamp  flame.  They  had  the 
joy  of  seeing  a  black  absorption  line,  coincident 
with  the  bright  line  given  by  hot  sodium  vapour, 
crossing  the  continuous  spectrum  of  the  arc,  just 
as  the  black  line,  called  by  Fraunhofer  the  line  D, 
crosses  the  spectrum  of  the  sun.  The  possibility 
of  determining  the  chemical  constitution  of  the 
heavenly  bodies  had  opened  before  the  eyes 
of  man. 

Hitherto  the  sun  had  been  studied  chiefly 
in  relation  to  the  earth  and  the  general  solar 
system,  while  little  else  was  known  about  the 
stars   than  their  apparent  relative  positions  on 


ASTRO-PHYSICS  265 

a  hypothetical  celestial  sphere.  Their  composi- 
tion and  physical  condition  were  held  to  be 
outside  the  range  of  any  definite  scientific  investi- 
gation ;  subjects,  perhaps,  better  fitted  to  the 
romancer  than  to  the  serious  student.  But  with 
the  advent  of  the  spectroscope,  sun  and  stars, 
in  a  new  aspect,  re-entered  the  realm  of  exact 
knowledge,  and  began  to  give  up  the  secrets  of 
their  composition  and  state. 

Many  of  the  chemical  elements  known  on 
the  earth  were  detected  in  the  sun,  while  dark 
lines,  not  corresponding  with  the  spectrum  of 
any  terrestrial  substance,  suggested  the  existence 
of  solar  elements  hitherto  unrecognised  by  the 
chemist.  The  spectra  of  the  stars  were  found 
to  vary,  some  showing  the  presence  of  hydrogen 
only,  while  others  indicated  the  existence  of 
constitutions  more  nearly  approaching  that  of 
our  sun. 

The  structure  of  the  nebulae,  those  vast, 
vague  sources  of  luminosity,  had  long  been  a 
matter  of  speculation.  Were  they  clusters  of 
innumerable  stars,  so  minute  and  so  distant 
that  the  most  powerful  telescopes  could  not 
resolve  them,  or  were  they,  indeed,  as  their 
name  indicated,  foregatherings  of  cloud -like, 
light-giving  vapours  ?  The  question  was  settled 
as  soon  as  the  spectroscope  was  turned  towards 
their  light.  A  continuous  gradation  in  properties 
was  found  between  stars  and  nebulae.  Some 
nebulae  gave  continuous  spectra,  indicating  high 
density  and  pressure  at  the  source  of  radiation, 
others  gave  bright  lines  on  a  dark  background — 
the  spectra,  not  of  dense  suns  surrounded  by 
cooler  atmospheres,  but   of  masses   of  glowing 


266  PHYSICAL  SCIENCE 

vapour  of  great  tenuity — the  beginnings,  perhaps, 
of  suns  and  worlds  yet  to  be. 

Then  came  a  pause  in  the  progess  of  this 
new  branch  of  knowledge.  The  spectroscope 
alone  seemed  to  have  told  all  it  could  to  the 
human  eye.  A  more  sensitive  instrument  was 
needed  to  receive  its  messages,  to  intensify  them, 
and  to  interpret  them  to  the  senses  of  mankind. 
It  was  not  till  photography  was  employed  to 
record  the  results  of  spectrum  analysis  that  the 
full  power  of  the  spectroscope  was  understood. 
Although  previous  attempts  had  been  made  by 
means  of  inferior  processes  to  photograph  the 
spectra  of  the  sun  and  stars,  the  great  success 
of  the  method  dates  from  the  application  of  the 
dry  gelatine  process  by  Sir  William  Huggins  in 
1876. 

The  photographic  method  has  many  advan- 
tages over  direct  visual  observation.  The  sen- 
sitive plate  can  be  exposed  for  a  considerable 
length  of  time,  and  the  effect  of  the  light  on  it 
is  cumulative.  Excessively  feeble  light  will,  by 
prolonged  action,  produce  a  sensible  impression 
on  the  photographic  plate  when  it  would  be 
quite  insensible  to  the  eye,  which  has  none  of 
this  power  of  gradually  storing  and  intensifying 
its  impressions.  Again,  the  photograph  will 
record  ultra-violet  radiation  to  which  the  nerves 
of  the  eye  do  not  respond,  and,  in  this  way, 
it  has  revealed  many  invisible  lines.  Finally, 
the  photograph  forms  a  permanent  record,  to 
which  reference  can  be  made  at  any  future 
time,  and  permits  measurements,  more  accurate 
than  those  made  by  direct  visual  observation, 
to  be  obtained  at  leisure  in  the  laboratory  many 


ASTRO-PHYSICS  267 

hours  or  days  after  the  exposure.  In  several 
observatories,  systematic  records  are  kept  of 
the  state  of  the  sky  from  night  to  night,  and, 
more  than  once,  when  a  new  star  has  been 
detected,  its  previous  history  has  been  unfolded 
by  reference  to  photographic  plates  exposed 
before  the  existence  of  the  new  star  was  sus- 
pected. 

Two  methods  of  obtaining  spectra  are  known 
to  the  physicist,  the  instruments  used  being 
respectively  the  prism  and  the  grating.  The 
grating  consist  of  a  number  of  equidistant 
parallel  scratches  ruled  on  a  reflecting  surface 
of  polished  metal  or  on  a  transparent  surface  of 
glass.  The  scratches  are  very  close  together, 
many  thousands  of  them  being  included  in  the 
space  of  an  inch.  When  a  wave  of  light  falls 
on  a  metallic  grating,  the  scratches  refuse  to 
reflect  the  light.  The  distances  between  the 
scratches  are  comparable  with  the  minute  wave- 
lengths of  light,  and  thus  different  waves  are 
differently  treated  by  the  grating.  The  com- 
ponent rays  of  a  complex  beam  of  light  are 
separated  from  each  other,  and,  if  the  source 
of  light  be  a  narrow  slit,  a  number  of  parallel 
images  are  formed,  and  a  spectrum  is  obtained. 
The  deviation  of  any  particular  wave,  such  as 
the  yellow  sodium  ray,  will  depend  on  the  wave- 
length of  the  light,  and,  for  the  same  grating, 
will  depend  on  this  wave-length  alone.  The 
spectral  lines  obtained  will  therefore  have  posi- 
tions simply  depending  on  the  wave-length  or 
*  period  of  vibration  of  the  corresponding  rays 
of  light ;  in  this  differing  from  the  similar  lines 
given  by  the   prism,  which   depend   in  position 


268  PHYSICAL  SCIENCE 

on  the  qualities  of  the  glass  as  well  as  on  the 
periodic  times  of  vibration  of  the  different  rays. 

The  sharpness  of  definition  of  a  spectrum 
taken  from  a  grating  depends  on  the  accuracy 
with  which  the  scratches  are  ruled,  and  thus 
the  perfection  of  the  grating  depends  on  our 
power  of  moving  the  scratching  tool  through 
exactly  equal  intervals  between  two  scratches. 
To  control  the  movement  a  perfect  screw  is 
required,  and  to  Professor  Rowland's  improve- 
ment in  the  manufacture  of  screws  in  1882,  and 
to  his  idea  of  using  them  to  rule  gratings  on 
concave  metallic  surfaces,  is  directly  due  the 
possibility  of  making  adequate  use  of  the 
resources  of  photography  in  the  province  of 
solar  and  stellar  spectrum  analysis.  The  arts 
and  the  sciences  are  closely  related  ;  an  advance 
in  one  of  them  often  leads  to  a  corresponding 
advance  in  the  other,  and  it  is  not  always 
science  that  leads  the  way. 

The  concave  grating  banished  the  need  for  a 
lens  to  focus  the  rays  after  diffraction,  and  an 
image  of  the  spectrum  could  now  be  obtained 
from  the  grating  alone.  Glass  is  opaque  to  much 
of  the  ultra-violet  radiation,  in  which  sunlight,  at 
any  rate,  is  very  rich.  Prismatic  spectra  and 
spectra  taken  with  plane  gratings  and  lenses  do 
not  show  the  ultra-violet  lines.  But,  by  the  use 
of  a  concave  grating  and  a  reflecting  telescope, 
the  presence  of  glass  becomes  unnecessary,  and 
investigation  can  be  prolonged  into  the  ultra- 
violet region  till  the  increasing  absorption  of 
the  earth's  atmosphere  for  waves  of  shorter 
and  shorter  wave-length  prevents  the  rays  from 
reaching  the  surface  of  the  ground. 


ASTRO-PHYSICS  269 

Glass  is  opaque  to  the  infra-red  radiation  also, 
and  here  again  the  advantages  of  the  concave 
grating  are  manifest.  The  infra-red  spectrum 
was  examined,  chiefly  by  Professor  S.  P.  Langley 
of  Washington,  through  the  heating  effects  of  its 
constituent  rays.  Professor  Langley  used  an 
instrument  called  the  bolometer,  in  which  the 
heating  effects  of  different  parts  of  the  spectrum, 
and  consequently  the  position  of  the  dark  lines, 
are  determined  by  measuring  the  change  in 
electric  resistance  of  a  very  thin  strip  of 
platinum  exposed  to  the  radiation.  This  form 
of  platinum  thermometer  is  extremely  sensitive, 
and  the  spectrum  of  the  sun  has  been  mapped 
far  below  the  limits  within  which  the  eye  re- 
sponds to  the  stimulus  of  light.  And,  in  more 
recent  years,  the  invention  of  new  processes  has 
carried  photographic  methods  beyond  the  range 
of  the  eye  at  this  end  of  the  spectrum  also. 

Perhaps  the  most  striking  and  interesting 
results  given  by  the  combination  of  camera 
and  spectroscope  are  those  obtained  by  the 
determination  of  the  change  in  the  refrangi- 
bility  of  light  produced  by  relative  motion  of 
approach  or  retrocession  of  the  source  of  light 
and  the  receiving  station.  Let  us  imagine  that 
waves  are  proceeding  from  some  source  which 
remains  at  rest.  A  certain  number  of  waves 
reach  an  observer  in  one  second.  If,  however, 
the  observer  is  approaching  the  source,  it  is 
evident  that,  as  he  is  going  to  meet  the  waves, 
a  greater  number  of  them  will  reach  him  in  one 
second  than  when  he  was  at  rest.  Similarly,  if 
the   observer  move  away   from    the  source,  the 


270  PHYSICAL  SCIENCE 

number  of  waves  which  reach  him  in  a  given 
time  will  be  less  than  before.  The  same  effects 
will  be  produced  if  the  observer  be  stationary 
and  the  source  of  light  move.  Doppler's 
principle,  as  this  change  in  periodic  time  is 
called,  is  well  illustrated  in  the  case  of  sound. 
Here  the  frequency  of  wave  impulse  on  the  ear 
determines  the  pitch  of  the  note  heard,  and  it 
is  easy  to  detect  a  distinct  flattening  by  a  semi- 
tone or  more,  as  the  whistling  engine  of  an 
express  train  passes  the  observer.  The  source  of 
the  waves  of  sound  still  vibrates  with  the  same 
frequency,  the  change  is  only  in  the  number  of 
impulses  reaching  the  observer  per  second. 

The  frequency  with  which  waves  of  light  are 
received  by  the  optic  nerve  determines  the  colour 
perceived  by  the  brain,  and  also  the  amount  of 
refraction  in  passing  through  a  prism.  Thus 
the  colour  of  a  ray  of  a  single  definite  wave- 
length, as  well  as  its  position  in  the  prismatic 
spectrum,  will  be  different  from  the  normal  value 
when  the  source  of  light  and  the  observer  are 
moving  relatively  to  each  other.  An  approach 
will  result  in  a  shifting  towards  the  blue  end  of 
the  spectrum  owing  to  the  increase  in  frequency  ; 
a  recession  will  involve  a  reddening  of  the  light, 
or  a  movement  of  the  spectral  lines  towards  the 
red  end  of  the  spectrum.  Owing  to  the  great 
velocity  of  light,  the  change  will  relatively  be 
much  less  than  in  the  case  of  sound.  Light 
travels  about  186,000  miles  in  one  second,  and, 
great  though  the  speeds  of  the  stars  may  be, 
they  fall  far  short  of  such  tremendous  values. 
A  velocity  of  eighteen  miles  a  second,  for 
example,  the  velocity  of  the  earth  in  her  orbit. 


ASTRO-PHYSICS  271 

is  but  the  ten-thousandth  part  of  the  velocity 
of  light.  This  velocity  of  approach,  then, 
would  involve  a  change  of  the  ten-thousandth 
part  in  the  period  of  vibration  of  the  incident 
light.  The  whole  visible  spectrum,  from  the 
red  to  the  violet  of  the  rainbow,  includes  a  range 
of  frequencies  of  about  an  octave,  that  is,  the 
period  of  vibration  of  the  extreme  red  is  about 
double  that  of  the  extreme  violet.  A  velocity 
equal  to  that  of  the  earth,  then,  would  involve 
a  change  in  position  of  a  spectral  line  of  about 
the  five-thousandth  part  of  the  total  length  of 
the  spectrum.  Many  stars  are  approaching  or 
receding  from  the  earth  at  velocities  higher  than 
that  which  we  have  taken  as  an  example,  but 
still  the  changes  in  position  to  be  measured 
are  very  small,  and  refined  methods  and  great 
experimental  skill  are  needed  for  accurate  results. 

The  problem  of  determining  the  movement 
of  a  star  travelling  along  the  straight  line  joining 
it  to  the  observer  would,  before  this  principle 
was  discovered,  have  seemed  one  of  the  most 
hopeless  problems  which  a  cynical  scientific 
sceptic  could  propose  for  solution  to  the 
physicist.  Yet  such  problems  are  now  solved 
daily,  or  rather  nightly ;  solved,  indeed,  much 
more  readily  than  they  could  be  if  the  star  were 
moving  across  the  line  of  sight.  In  the  latter 
case,  even  if  a  knowledge  of  the  distance  makes 
the  determination  possible,  prolonged  observa- 
tions are  needed,  extending  over  months  or 
years,  till  the  movement  becomes  apparent  at 
the  distance  of  the  earth.  Many  stars  are  so 
distant  that   no  such  cross  movement  could  be 


272  PHYSICAL  SCIENCE 

detected  in  any  reasonable  time.  If,  however, 
the  star  is  moving  towards  or  away  from  the 
earth,  the  spectroscope  is  turned  towards  it,  and 
in  the  short  time  required  to  fix  a  photographic 
impression,  develop  and  print  the  plate  and 
measure  the  lines  upon  it,  the  velocity  of  the 
star  can  be  determined. 

Another  application  of  the  same  principle  has 
enabled  us  to  demonstrate  directly  the  rotation 
of  the  sun  on  its  axis,  and  to  separate  those 
absorption  lines  in  the  spectrum  of  the  sun's 
light  which  are  due  to  the  effect  of  the  earth's 
atmosphere  from  the  lines  of  true  solar  origin. 
One  limb  of  the  sun  is,  at  any  moment,  approach- 
ing the  earth,  while  the  opposite  limb  is  in  like 
manner  receding.  By  pointing  a  spectroscope 
first  at  one  limb  and  then  at  the  other,  a  shift 
of  the  spectral  lines  is  seen  ;  and,  from  the  amount 
of  the  displacement,  the  velocity  of  movement  of 
the  glowing  gases  which  produce  the  lines  of 
absorption  can  be  calculated.  Lines  which  are 
not  shifted  by  this  operation  are  clearly  not  of 
solar  origin,  and  are  consequently  to  be  referred 
to  absorption  by  the  atmosphere  of  the  earth. 

Other  problems  in  solar  physics  have  been 
solved  by  the  same  method.  The  existence  of 
sun-spots  has  long  been  known ;  they  were, 
indeed,  familiar  to  the  Chinese  in  very  early 
times,  and,  in  the  middle  of  the  nineteenth 
century,  their  periodic  increase  and  decrease  in 
a  cycle  of  ten  or  eleven  years  was  noted  by 
Western  observers,  and  a  coincident  period  of 
terrestrial  magnetic  phenomena  was  established. 
The  structure  and  properties  of  sun-spots  were 
then    seen   to  possess    more    than  a  local    solar 


Fig.  37. — C  Line  in  the  Spectrum  of  a  Sun-Spot 

(^Professor  Hale). 


[To  face  page  273. 


ASTRO-PHYSICS  273 

interest,  and  their  importance  with  regard  to 
terrestrial  meteorology  became  manifest.  It 
has  long  been  held  that  sun-spots  were  the  seat 
of  movements  of  gases  on  a  gigantic  scale  in 
the  solar  atmosphere,  and  direct  evidence  of 
such  storms  is  supplied  by  the  spectroscope. 
Professor  Hale  gives  a  drawing  of  the  spectrum 
of  a  sun-spot  in  the  neighbourhood  of  the  C  line. 
This  drawing  is  reproduced  in  Fig.  '^'j.  The 
slit  of  the  spectroscope  was  directed  to  the  sun's 
disc  so  as  to  include  the  area  covered  by  the 
spot.  The  figure  shows  a  small  part  of  the 
spectrum,  which  extends  from  left  to  right  across 
the  paper.  The  faint  horizontal  dark  line  shows 
the  effect  of  the  sun-spot,  from  which  much  less 
light  proceeds  than  from  the  rest  of  the  sun's 
surface.  Several  faint  Fraunhofer's  lines  cross 
the  diagram  vertically,  and  it  will  be  seen  that 
these  lines  are  still  dark  lines  in  the  sun-spot 
region.  The  sun-spot  itself,  then,  must  be  the 
source  of  continuous  radiation,  from  which 
definite  rays  are  abstracted  by  cooler  gases  in 
higher  regions,  the  process  being  identical  with 
that  going  on  in  other  parts  of  the  sun.  The 
heavy  dark  line  crossing  the  figure  from  top  to 
bottom  is  the  C  line  to  which  reference  has  been 
made.  It  is  much  stronger  and  darker  than 
any  of  the  other  lines  shown.  In  the  neighbour- 
hood of  the  sun-spot,  like  the  fainter  lines,  it  still 
shows  dark,  but  in  its  centre  is  a  bright  patch 
or  reversal  of  the  line.  This  intense  luminosity 
indicates  that,  superposed  on  the  layer  of  gas 
which  absorbs  the  light,  is  a  mass  so  hot  that 
its  radiation  is  even  greater  than  the  normal 
radiation  from  the  sun's  surface.     The  curious 


274  PHYSICAL  SCIENCE 

hook-like  appendage  to  the  line,  which  begins 
as  a  fine  point  in  the  middle  of  the  sun-spot 
absorption,  and  ends  above  by  fusing  with  the 
C  line,  tells  of  an  extraordinary  outrush  of  cool 
hydrogen  coming  from  the  centre  of  the  sun- 
spot  area,  and  travelling  outwards  with  a  radial 
velocity  of  about  one  hundred  and  twenty  miles 
a  second.  In  its  outward  course  it  passes  away 
from  the  sun-spot  area,  and  finally  comes  to  rest 
at  a  distance  of  thirty  to  forty  thousand  miles 
from  its  point  of  origin.  Its  absorption  then  of 
course  coincides  in  spectral  position  with  the 
normal  C  line. 

Similar  work,  carried  on  in  several  observa- 
tories, has  thrown  much  light  on  the  movements 
of  the  prominences,  which  come  into  view  at 
the  edge  of  the  sun's  disc,  and  seem  to  be 
connected  intimately  with  the  spots.  These 
enormous  masses  of  glowing  gas  produce 
bright  line  spectra,  and  the  displacement  of  the 
lines  gives  the  movement  in  one  plane,  while 
direct  visual  observation  gives  that  in  a  plane 
at  right  angles  to  the  first.  Thus  the  motion 
of  the  prominences  can  be  specified  completely. 
Their  velocities  are  often  as  high  as  two  or  three 
hundred  miles  a  second. 

It  seems  unlikely  that  such  high  velocities  can 
be  the  result  of  differences  of  gaseous  pressure 
and  the  convection  currents  thus  engendered. 
They  are  more  probably  to  be  explained  by 
the  local  action  of  some  explosive  source  of 
energy,  by  which  matter  is  projected  with  great 
violence. 

The    application    of    Doppler's    principle    to 


i 


ASTRO-PHYSICS  275 

stellar  movement  has  led  to  other  results  quite 
as  remarkable  as  those  already  described.  Our 
sun  is  a  single  system,  but  many  of  his  fellows 
among  the  stars  are  accompanied  by  partners  ; 
the  two  existing  in  more  or  less  close  conjunc- 
tion, and  showing  all  the  signs  of  a  common 
origin.  Some  of  these  double  stars  can  be 
examined  by  telescopic  means,  but  the  majority 
of  them  lie  too  close  together  to  be  separated 
thus.  Often,  too,  one  of  the  pair  is  not  luminous, 
and  therefore  would  never  be  visible.  In  this 
class  are  probably  to  be  placed  variable  stars, 
such  as  the  star  known  as  Algol  or  /3  Persei,  the 
light  from  which  undergoes  periodical  fluctuations 
in  intensity.  The  light  keeps  constant  for  the 
greater  part  of  the  cycle,  and  then  diminishes  for 
a  short  time  before  again  rising  to  its  normal 
value.  This  behaviour  was  long  suspected  to 
be  due  to  the  partial  concealment  of  the  star  by 
a  dark  companion  or  satellite,  and  the  surmise 
was  confirmed  by  the  spectroscope,  which  shows 
that  the  star  is  always  receding  from  us  before 
the  loss  of  light  and  approaching  after  it.  This 
result  is  exactly  what  we  should  expect  on  the 
eclipse  theory,  the  dark  companion  being  so 
nearly  of  the  same  size  as  the  visible  Algol  that 
the  joint  motion  is  similar  to  that  of  two  partners 
waltzing  round  each  other  rather  than  like  the 
revolution  of  a  small  satellite  round  a  large 
central  body,  which  remains  nearly  stationary. 
In  other  cases,  such  as  that  of  (3  Lyrae,  the 
intensity  of  the  light  is  never  constant,  but 
undergoes  continual  variation,  accompanied  by 
complicated  changes  in  the  spectrum.  An 
explanation  has    been  given    by   imagining   two 


2^6  PHYSICAL  SCIENCE 

ellipsoidal  luminous  bodies,  which  revolve  round 
each  other  very  near  together,  and  send  to  the 
earth  more  light  when  they  lie  side  by  side  than 
when  one  lies  behind  the  other  and  to  a  certain 
extent  obscures  it. 

It  is  evident  that  the  double  nature  of  such 
systems  can  be  demonstrated  by  variations  in 
luminosity  only  in  the  few  cases  where  the 
motion  is  in  such  a  plane  that  one  of  the 
partners  is  periodically  interposed  between  the 
other  and  the  earth.  Only  a  limited  number 
of  Algol  variables  are  known.  When  this  eclipse 
does  not  happen,  the  dark  companion  could  never 
be  detected  without  the  aid  of  the  spectroscope. 
By  continuous  records  of  the  spectra  of  many 
stars,  however,  periodic  changes  in  the  lines 
have  been  observed,  and  the  times  of  the  orbital 
movements  determined.  Binary  systems  have 
been  discovered  with  periods  varying  from  a 
few  hours  to  many  years.  In  some  cases  the 
spectral  changes  merely  consist  in  periodic  shift- 
ings  of  the  lines.  Here  we  probably  have  a 
luminous  body  and  a  dark  companion  revolving 
round  their  common  centre  of  gravity.  In  other 
cases,  a  periodic  doubling  of  the  lines  indicates 
two  bodies,  both  luminous,  but  too  near  together 
and  too  far  from  us  to  be  separated  by  the 
telescope.  The  number  of  both  classes  seems 
to  be  considerable,  and  our  visible  universe  must 
be  studded  pretty  closely  with  dark  stars,  the 
existence  of  which  is  only  to  be  detected 
when  they  are  associated  with  some  luminous 
companion. 

Triple  and  multiple  stars  are  also  known. 
For  instance,  the   Pole  Star  is  a  spectroscopic 


ASTRO-PHYSICS  277 

binary  with  a  period  of  four  days,  which  revolves 
in  a  period  of  some  twenty  years  round  a  third 
star  invisible  to  us. 

Passing  from  these  questions  to  the  problems 
of  our  own  planetary  system,  we  find  the  same 
principle  applied  to  the  examination  of  Saturn's 
rings.  These  remarkable  structures,  which  in 
the  telescope  look  like  rings  of  continuous  matter 
encircling  the  planet,  were  long  a  puzzle  to  the 
astronomer.  Theory  indicates  that  such  rotating 
rings  of  continuous  matter,  whether  solid  or 
liquid,  would  be  unstable,  and  would  break  up 
under  the  forces  which  must  necessarily  exist. 
The  alternative  hypothesis,  that  the  rings  consist 
of  a  swarm  of  tiny  meteorites,  each  revolving 
round  the  planet  in  its  own  separate  orbit,  was 
elaborated  mathematically  by  Clerk  Maxwell, 
but  no  confirmation  of  this  view  was  obtained 
till  Keeler  examined  the  rings  with  the  spectro- 
scope. He  found  that  the  inner  parts  of  the 
rings  revolved  faster  than  the  outer  parts,  in 
accordance  with  the  requirements  of  the  meteoritic 
hypothesis.  If,  on  the  other  hand,  the  ring 
were  solid,  the  outermost  parts  would  possess 
the  highest  velocity,  on  the  same  principle  by 
which  the  circumference  of  a  fiy-wheel  moves 
faster  than  its  inward  parts. 

While  the  knowledge  of  sun  and  stars 
delivered  to  us  by  spectrum  analysis  has  been 
both  extensive  and  striking,  the  interpretation 
of  spectral  phenomena  has  proved  a  much  more 
complicated  problem  than  was  anticipated  when 
Bunsen    and    Kirchhoff's    great    discovery   first 


2;8  PHYSICAL  SCIENCE 

placed  the  new  method  In  the  hands  of  investi- 
gators.    The    Hnes    of    the    spectrum,    whether 
bright  or  dark,  were  thought  at  first  to  be  fixed 
and    constant    in    position — that    is,   the    modes 
of  vibration  of  the  atoms  from  which  the  light 
proceeded    were  imagined   to    be   unaffected    by 
any    external     circumstances.       This     supposed 
simplicity  has   been   shown    to   be  illusory.     As 
we    have    seen,    movement    of   the    source    and 
observer,  although   it   may  not  alter  the  atomic 
vibrations,  affects  the  number  of  them  received 
in  any  time,  and  thus  changes  the  refrangibility 
of  the  light  they  emit  as  it  is   received   by  the 
observer.      But    other    variations,    more    funda- 
mental in  their  origin,  are  known.     Laboratory 
experiments  have  shown  that  the  spectral  lines 
alter  their  character  with  changes  in  the  physical 
conditions  of  the  experiments.     It  was  thought 
that  luminous   gases  evolved  only  bright,  sharp 
lines.     It  is   now  found  that   the  lines    may  be 
broadened  and  softened   by  an   increase   in   the 
pressure  or  the  density  of  the  gas,  while,  in  some 
cases,  a   simultaneous  shift   in   position   may  be 
produced.     An   intense  magnetic   field  has  been 
shown    by   Zeeman    to    result    in    separation   of 
single   lines    into   two    or   more   components,  in 
this    fulfilling    the    predictions    of   the    electro- 
magnetic   theory  of   light,  which    suggests   that 
some  such  connection  is  probable.     The  spectra 
of  elements   have  long  been  known   to  depend 
on    the    temperature,    the    spectrum    of   the    arc 
discharge  often  being  different  from  that  obtained 
by    the    use    of    a    discontinuous    spark,    while 
neither    correspond    with    the    spectrum    of    the 
incandescent  vapour  existing  in  the  flame  of  a 


ASTRO-PHYSICS  279 

gas-burner.  More  recent  experiments  have  shown 
that  traces  of  impurities  may  modify  the  spectrum 
considerably,  while,  in  some  cases,  the  presence 
of  one  substance  will  completely  mask  the 
spectrum  of  another. 

Again,  when  an  atom  is  ionized,  that  is  given 
an  electric  charge,  some  of  its  spectral  lines 
are  often  found  to  become  much  more  intense. 
These  ** enhanced"  lines  are  very  important  in 
the  interpretation  of  solar  and  stellar  spectra. 

The  possibilities  introduced  by  all  these 
effects  naturally  complicate  the  interpretation  of 
solar  and  stellar  spectra.  On  the  other  hand, 
the  very  complications  greatly  increase  the  interest 
of  the  luminous  messages,  and  the  investigation 
of  the  connection  between  the  external  conditions 
and  the  nature  of  the  spectra  in  the  physical 
laboratory  opens  an  almost  limitless  field  to 
profitable  research.  Co-operation  between  the 
laboratory  and  the  observatory  doubtless  will 
elucidate  gradually  the  fascinating  problems  of 
the  nature  of  the  celestial  bodies. 

The  spectra  of  various  substances  differ 
widely  in  complexity.  Some  consist  of  a  few 
lines,  some  of  very  many.  Iron,  for  instance, 
emits  light  of  at  least  two  thousand  different 
wave-lengths.  Of  recent  years,  as  explained  on 
page  248,  order  has  been  introduced  into  our 
knowledge  of  complex  spectra  by  the  discovery 
that  fairly  simple  relations  hold  between  the 
wave-lengths,  or  rather  the  number  of  vibrations 
in  a  centimetre.  Simple  formulae  have  been 
devised,  which  in  a  general  way  express  the 
connection  between  the  vibration  number  of  one 
fundamental  line  and  its  companions,  somewhat 


28o  PHYSICAL  SCIENCE 

as  can  be  expressed  the  connection  between  a 
musical  note  and  its  overtones.  Two  or  even 
three  series  of  Hnes  may  exist,  and  two  or  three 
modifications  of  the  formulae  are  then  needed  to 
co-ordinate  their  vibration  numbers.  That  such 
distinctions  possess  a  physical  significance  was 
shown  by  experiments  of  Lenard,  who  found 
that  the  three  series  of  sodium  and  lithium  lines 
are  separated  in  the  flame  of  the  electric  arc, 
the  outer  shell  of  flame  giving  only  the  funda- 
mental series,  while,  in  the  physical  conditions 
appertaining  to  the  inner  flame,  the  second  and 
third  series  become  dominant. 

We  have  already  dealt  in  Chapter  VIII.  with 
the  importance  of  these  series  of  spectral  lines 
in  the  problem  of  atomic  structure,  and  shown 
how  they  enabled  Bohr  to  explain  the  emission 
of  radiation  in  finite  units  as  required  by  Planck's 
quantum  theory. 

Some  most  interesting  work  relating  to  the 
sun  was  carried  out  by  means  of  Professor  Hale's 
method  of  photographing  the  sun  itself  and  its 
prominences  by  the  light  corresponding  with  one 
definite  spectral  line.  Two  of  the  commonest 
elements  present  in  the  sun  are  hydrogen  and 
calcium,  and  these  elements  are  marked  by  the 
strong  lines  h  and  k  respectively.  The  resultant 
photographs,  then,  show  the  distribution  of 
hydrogen  or  calcium  throughout  the  region  in- 
vestigated. The  spectra  of  the  prominences  at 
the  edge  of  the  sun's  disc  consist  of  bright  lines, 
while  some  of  the  dark  absorption  lines  of  the 
light  from  the  surface  of  the  sun  possess  bright 
centres,  like  those  shown  in  Fig.   '^'],  indicating 


h4 

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[To  face  page  281. 


ASTRO-PHYSICS  281 

the  existence  of  masses  of  luminous  vapour  lying 
above  the  reversing  layer.  These  bright  central 
lines  give  sufficient  light  for  the  purpose  we  are 
now  considering,  and  the  resulting  photographs 
show  the  distribution  of  glowing  clouds  of  vapour 
in  the  higher  regions  of  the  solar  atmosphere. 
Even  the  dark  absorption  lines  are  only  dark 
by  comparison  with  the  brighter  background, 
and  thus  new  photographs  can  be  taken  with 
the  darker  sides  of  these  reversed  lines.  The 
light  then  used  comes  from  a  deeper  layer  in  the 
solar  atmosphere,  and  as  many  as  three  calcium 
photographs  have  been  taken  in  this  way  from 
a  single  line,  showing  the  distribution  of  calcium 
at  three  different  levels  in  the  sun's  envelope. 

The  method  by  which  Professor  Hale  obtains 
these  wonderful  results  consists  in  the  employ- 
ment of  a  spectro-helioscope  possessing  two  slits. 
The  solar  light  is  focussed  into  an  image  by  the 
telescope,  passed  through  one  of  these  slits,  and 
thrown  on  to  a  prism  or  grating.  The  spectrum 
thus  produced  shows  the  usual  lines,  and  the 
second  slit  is  fixed  so  as  to  coincide  with  the 
line  by  the  light  of  which  the  sun  is  to  be 
photographed.  The  light  coming  through  the 
second  slit  is  thus  monochromatic  light — simple 
light  of  the  particular  kind  desired.  The  first 
slit  is  made  to  travel  slowly  over  the  disc  of 
the  sun,  and  the  second  slit,  by  appropriate 
movements,  is  kept  constantly  in  position  to  allow 
the  particular  line  to  fall  upon  it.  In  this  way 
a  complete  picture  of  the  calcium  or  hydrogen 
flames  above  the  surface  of  the  sun  can  be 
obtained. 

One  of  the  striking  features  of  the  photo- 


282  PHYSICAL  SCIENCE 

graphs  taken  by  this  method  consists  in  the 
well-marked  differences  in  the  distribution  of 
hydrogen  and  calcium.  The  faculae  and  promi- 
nences, which  stud  the  solar  disc,  contain  floating 
clouds  of  hydrogen,  and  other  clouds  of  calcium, 
but  these  clouds  are  often  separate  from  each 
other,  and  possess  distinctive  forms  which  are 
well  shown  in  Figs.  38  and  39,  and  can  at  once 
be  recognised  by  an  accustomed  observer  as  due 
to  hydrogen  or  calcium  respectively.  Prominent 
objects  on  the  sun,  such  as  spots,  often  show 
clearly  only  in  one  of  these  two  kinds  of  light, 
when  they  are  faintly  seen  or  are  quite  invisible 
by  the  other  elemental  ray.  Vast  clouds  of 
calcium  seem  to  arise  from  the  neio'hbourhood 
of  sun  spots,  obscuring  the  calcium  light  coming 
from  the  regions  below,  while  at  the  same  time 
the  hydrogen  light  from  those  regions  is  able  to 
make  good  its  escape. 

Most  of  the  dark  lines  of  the  solar  spectrum 
are  probably  due  to  elements  known  on  the  earth, 
some  imperfect  coincidences  being  attributed 
to  the  difference  in  physical  conditions,  which, 
as  we  now  know,  affect  the  character  of  the 
spectral  lines.  The  bright  lines  of  the  outer 
luminous  layer  or  chromosphere,  and  of  its 
attendant  prominences,  were  first  detected  during 
eclipses,  though  with  modern  instruments  they  ' 
can  always  be  seen  at  the  edge  of  the  sun's  disc. 
A  brilliant  unknown  line  in  the  yellow  was  in 
1868  referred  by  Sir  Norman  Lockyer  to  a 
new  element,  to  which  was  given  the  name  of 
helium.  In  1895  Sir  William  Ramsay  detected 
the  same  spectrum  by  passing  an  electric  spark 


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I 


ASTRO-PHYSICS  283 

through  the  gases  evolved  from  a  specimen  of 
the  mineral  cleveite,  and  by  this  means  isolated 
the  gas  helium,  thus  showing  that  the  element, 
first  discovered  in  the  sun,  was  present  also  upon 
the  earth.  The  complete  spectrum  of  helium 
contains  two  sets  of  lines,  one  in  the  yellow  and 
one  in  the  green.  In  the  laboratory  these  two 
sets  are  usually  found  together,  though,  by 
manipulating  an  electric  discharge  in  helium, 
separation  may  be  effected.  In  the  light  of  the 
sun,  too,  the  yellow  line  is  sometimes  found  with- 
out the  green.  Other  separations  of  the  same 
kind  between  the  constituents  of  the  spectra  of 
certain  elements  have  also  been  observed,  and 
have  sometimes  suggested  the  idea  of  atomic 
dissociation.  Other  explanations,  however,  seem, 
on  the  whole,  more  probable.  Professor  J.  J. 
Thomson  has  shown  that,  when  an  electric 
discharge  passes  through  rarified  hydrogen,  the 
red  line  becomes  more  intense  near  the  positive 
and  the  green  line  near  the  negative  electrode. 
This  observation  indicated  a  separation  of 
hydrogen  molecules  into  positive  and  negative 
parts  giving  different  spectra.  Taken  in  con- 
junction with  more  recent  work  on  the  enhance- 
ment of  lines  by  ionization,  it  is  very  suggestive 
in  relation  to  solar  and  stellar  physics. 

During  total  eclipse,  a  vast  radiance  sur- 
rounding the  sun,  known  as  the  corona,  springs 
into  view.  Spectroscopic  examination  shows 
that  hydrogen,  helium,  and  calcium,  the  main 
constituents  of  the  chromosphere,  are  absent  in 
the  corona.  The  principal  part  of  the  light 
seems  to  be  due  to  a  brilliant  green  line,  not 
produced    by   any    terrestrial    substance.       The 


284  PHYSICAL  SCIENCE 

hypothetical  element  emitting  this  light  has  been 
named  coronium. 

Although  recent  research  has  not  yet  led 
to  a  completely  satisfactory  conception  of  the 
general  condition  of  the  sun  as  a  physical 
system,  substantial  progress  in  knowledge  has 
nevertheless  been  made.  The  gigantic  output 
of  heat  would  be  impossible  for  any  solid  globe, 
even  if  surrounded  by  a  gaseous  envelope.  The 
external  shell  would  cool  too  rapidly,  unless  a 
process  of  convection  replaced  the  cooling  gases 
on  the  surface  by  hotter  ones  from  below.  The 
temperature  of  the  sun  is  above  the  critical 
points  of  most,  at  any  rate,  of  known  substances, 
and  thus,  although  the  pressures  may  be  very 
high,  liquids  or  solids  are  probably  non-existent, 
except  perhaps  as  clouds  in  the  upper  regions  of 
the  atmosphere.  The  best  estimates  of  the 
temperature  of  the  radiating  part  of  the  sun, 
based  on  the  amount  of  solar  heat  received  by 
the  earth,  corrected  for  absorption,  agree  in 
indicating  a  temperature  of  about  6000^  C. 

A  fairly  general  consensus  of  opinion  had 
been  reached  to  the  effect  that  the  source  of  the 
energy  required  for  the  sun's  continual  output  of 
heat  was  to  be  sought  in  the  mutual  gravitating 
condensation  of  his  parts.  A  mass  of  gravitating 
gas  may  become  actually  hotter  by  radiation. 
As  it  loses  heat,  its  parts  approach,  and  the 
whole  mass  contracts.  Two  bodies  attracting 
each  other  will,  by  their  collision,  set  free  energy 
which  appears  as  heat,  and  the  mutual  approach 
of  the  gravitating  parts  is  an  eftect  of  the  same 
kind.     The  heat   thus  developed  may   be  more 


ASTRO-PHYSICS  285 

than  enough  to  compensate  for  that  lost  by 
radiation.  This  reasoning  was  appHed  to  the 
sun,  and  estimates  of  the  sun's  Hfe  as  a  useful 
radiating  system  were  made  by  Lord  Kelvin  and 
others.  But  the  past  history  of  the  sun  was, 
on  these  calculations,  far  too  short  to  admit  of 
the  periods  required  by  the  geologist  and  the 
biologist  for  the  formation  of  the  earth's  crust 
and  the  evolution  of  species  thereon. 

It  was  thought  at  first  that  the  phenomena 
of  radio-activity  would  throw  new  light  on  this 
problem.  If  but  two  or  three  parts  in  a  million 
of  the  sun's  mass  consist  of  radium,  the  present 
rate  of  heat  emission  would  be  maintained.  The 
prevalence  of  helium  suggests  the  occurrence  of 
radio-active  processes,  during  which,  as  we 
know,  helium  may  be  formed.  But  the  balance 
of  evidence  seems  against  the  idea  that  enough 
radio-active  substance  exists  in  the  sun  to  explain 
his  output  of  heat. 

Indeed,  no  explanation  yet  suggested  by 
known  processes  is  adequate.  We  are  forced 
to  believe  that  some  change,  never  seen  in  our 
laboratories,  must  be  going  on  inside  the  sun. 
With  the  glimpses  we  have  now  had  into  the 
wonders  of  atomic  structure  such  an  idea  need 
not  surprise  us.  The  temperature  within  the  sun 
must  be  far  higher  than  the  mere  6000°  of  the 
radiating  layer,  and  may  be  of  the  order  of  a 
million  degrees.  At  such  temperatures  phe- 
nomena quite  unknown  to  us  may  appear — the 
disintegration  of  atoms  stable  on  earth,  or 
possibly  the  direct  conversion  of  electronic 
matter  into  radiant  energy  by  collision.  This 
is  perhaps  only  vain  speculation,  put  forward  to 


286  PHYSICAL  SCIENCE 

hide  our  ignorance,  but  it  is  certain  that  many 
changes  quite  beyond  our  range  of  terrestrial 
experiment  may  be  going  on  in  the  flaming 
furnaces  of  the  sun  and  stars. 

The  problem  of  the  probable  age  of  the  earth 
is  also  surrounded  with  difficulty.  The  tempera- 
ture of  the  earth  rises  as  we  pass  underground, 
and,  from  the  present  temperature  gradient, 
Lord  Kelvin  had  calculated  that  about  one 
hundred  million  years  ago  the  earth  was  a 
molten  mass.  Although  from  the  nature  of  the 
assumptions  made  in  this  calculation,  little 
weight  could  be  attached  to  the  exact  result 
obtained,  the  estimated  age  of  the  earth,  as  the 
home  of  organic  life,  was  again  too  short  for  the 
requirements  of  geology  and  biology.  But  it  is 
now  known  that  radio-active  matter  in  small 
quantities  is  very  widely  distributed  throughout 
the  earth  and  its  atmosphere.  Clay,  for  instance, 
yields  a  radio-active  emanation  in  appreciable 
quantities,  and  potassium  is  slightly  radio-active. 
Rutherford  calculated  that,  if  all  the  substance 
of  the  earth  were  as  active  as  clay,  the  present 
distribution  of  temperature  might  be  maintained 
by  this  cause  alone.  Such  activity  is  unlikely, 
but  the  result  shows,  at  all  events,  that  the 
observed  temperature  gradient  is  not  a  safe 
guide  when  used  as  the  sole  means  of  estimating 
the  age  of  the  habitable  globe. 

The  great  advance  in  knowledge,  gained  by 
the  study  of  the  conduction  of  electricity  through 
gases  and  the  phenomena  of  radiation  and  radio- 
activity, cannot  fail  to  exert  a  powerful  influence 
on  the  future  of  astro-physics,  and,  in  particular, 


ASTRO-PHYSICS  287 

on  our  conceptions  of  the  nature  of  solar  pro- 
cesses. The  leak  of  electricity  from  hot  bodies, 
studied  in  the  physical  laboratory,  shows  that 
corpuscles  or  electrons  must  be  emitted  in 
enormous  quantities  by  the  substance  of  the 
sun  and  hot  stars.  The  likelihood  of  the 
presence  of  radio-active  matter,  too,  and  of  the 
ejection  of  other  corpuscles,  with  the  transcendent 
velocities  impressed  on  them  by  a  radio-active 
origin,  must  not  be  forgotten.  Although  the 
corpuscles,  before  they  reached  the  surface  of 
the  earth,  would  be  absorbed  by  its  atmosphere — 
equivalent  as  that  atmosphere  is  to  a  thickness 
of  thirty  inches  of  mercury — they  might  produce 
striking  phenomena  in  the  regions  of  the  upper 
air.  Perhaps  on  these  lines  is  to  be  explained 
the  appearance  of  the  Aurora  Borealis  and 
kindred  manifestations,  while  the  luminosity  of 
the  solar  corona  may  well  have  an  electric 
origin. 

One  important  application  of  photography 
to  astronomy  consists  in  the  better  estimation 
of  stellar  distances.  Even  the  nearest  fixed  star 
is  so  far  away  from  us  that  accurate  measure- 
ment is  difficult,  while  some  stars  are  so  in- 
conceivably remote  that  all  ordinary  methods 
fail. 

Nearer  objects  seem  to  move  past  more 
distant  ones  as  we  look  at  them  from  the  window 
of  a  train  ;  and,  if  some  stars  are  nearer  to  the 
earth  than  others,  they  also  should  seem  to 
move  in  one  direction  as  the  earth  moves  in 
the  other.  We  should  therefore  expect  to  see 
the  nearer  stars  shift  over  a  background  of  more 


288  PHYSICAL  SCIENCE 

distant  stars  as  the  earth  travels  in  its  orbit 
round  the  sun,  and  observation  confirms  this 
prediction.  The  first  successful  measurements 
were  made  telescopically  in  1838,  but  much 
more  accurate  results  can  now  be  obtained  by 
photography.  If  three  photographs  be  taken  of 
Sirius,  for  instance,  at  intervals  of  six  months, 
and  the  distance  of  the  image  from  those  of 
three  surrounding  distant  stars  be  measured  with 
a  micrometer,  it  will  be  found  that  Sirius  moves. 
The  small  distance  between  his  first  position 
and  that  he  occupies  a  year  later  gives  his  own 
proper  motion  compared  with  the  earth  and  the 
very  distant  stars  taken  as  ''  fixed."  This 
motion  is  1.32  seconds  of  arc  in  the  year.  Half 
this  distance  gives  the  position  Sirius  would 
occupy  at  the  intermediate  six  months'  interval 
if  viewed  from  the  same  spot  in  space.  The 
angle,  0.38  second,  between  this  position  and 
that  actually  observed  is  called  the  parallax ;  it 
shows  the  effect  of  changing  the  point  of 
observation  to  the  opposite  side  of  the  earth's 
orbit,  that  is  by  185  million  miles.  From  this 
it  is  clear  that  the  distance  of  Sirius  from  the 
earth  may  be  calculated.  It  proves  to  be  about 
50  million  million  miles,  a  distance  it  would  take 
light  8.6  years  to  travel  at  its  speed  of  186,000 
miles  a  second. 

Beyond  the  range  within  which  parallax  is 
appreciable,  stellar  distances  can  only  be  esti- 
mated by  indirect  means.  As  examples  we  may 
cite  the  following  methods. 

The  mean  distance  of  a  connected  group 
of  stars  of  given  type  may  be  estimated  from 
their  average  magnitude,  that  is,  their  apparent 


ASTRO-PHYSICS  289 

brightness,  for  on  the  average  the  fainter  a  star 
is  the  greater  is  its  distance. 

Certain  stars  show  variations  in  the  intensity 
of  their  light,  with  short  periods  ranging  from 
a  day  to  one  or  two  months.  A  definite  relation 
has  been  discovered  between  this  period  and  the 
absolute  brightness  of  the  star  in  cases  where 
the  distances  are  known.  Extending  the  same 
relation  to  a  variable  star  at  an  unknown  distance, 
an  observation  of  the  period  gives  its  absolute 
brightness,  which,  compared  with  the  apparent 
brightness,  indicates  the  distance. 

Globular  clusters  of  stars  of  one  type  are 
found  to  possess  approximately  equal  dimensions, 
so  that  the  apparent  diameter  of  the  cluster 
enables  an  observer  to  make  a  guess  at  its 
distance. 

The  average  velocity  of  the  arms  of  spiral 
nebulae  in  the  line  of  sight  as  measured  spectro- 
scopicallyis  several  hundred  kilometres  per  second. 
This  gives  a  rough  value  for  the  distance  of 
those  nebulae  which  show  an  angular  movement 
of  the  arms  across  the  line  of  sight,  and  a  lower 
limit  for  the  distance  of  those  too  far  away  for 
those  movements  to  be  measurable. 

We  can  now  appreciate  the  methods  by 
means  of  which  the  dimensions  of  our  stellar 
system  have  been  estimated,  and  may  pass  to 
consider  some  of  the  results. 

The  mean  diameter  of  the  earth's  orbit  is 
185.6  million  miles.  That  of  the  orbit  of 
Neptune,  the  outermost  known  planet,  is  about 
5600  million  miles,  and  this  may  be  taken  as 
the  size  of  our  solar  system.  Light  would  take 
S.^6  hours  to  travel  across  it. 

u 


290  PHYSICAL  SCIENCE 

Around  our  system  lies  a  great  abyss  of 
space.  The  nearest  known  star — a  faint  speck 
called  Proxima  Centauri — is  24  million  million 
miles  or  4.1  light  years  away,  more  than  four 
thousand  times  the  stretch  of  Neptune's  orbit. 
Then  come  three  other  stars  before  we  reach 
Sirius  at  8.6  light  years. 

A  good  eye  unaided  may  see  upwards  of 
5000  stars,  while  a  large  modern  telescope  (100- 
inch  reflector)  reveals  a  number  estimated  at 
100  million.  The  number  does  not  increase  in 
proportion  to  the  power  of  the  telescope ;  hence 
we  may  conclude  that  our  stellar  universe  is  not 
infinite.  The  total  number  of  stars  is  thought 
to  be  somewhere  about  1 500  million. 

Some  of  this  colossal  number  of  stars  are 
perhaps  twenty  thousand  times  as  far  away  as 
Sirius,  at  a  distance  of  some  170,000  light  years. 
As  we  probe  these  appalling  depths,  we  find 
gigantic  spiral  and  spheroidal  nebulae,  and 
globular  star-clusters.  One  of  these  clusters 
is  distant  from  us  about  200,000  light  years, 
while  another  is  so  remote  that  the  light  by 
which  we  see  it  probably  started  a  million 
years  ago. 

The  milky  way  which  stretches  across  the 
sky  shows  that  the  apparent  distribution  of 
stars  is  not  uniform ;  the  milky  way  contains 
more  than  we  see  in  other  directions.  .  The 
stellar  system  seems  to  be  roughly  circular  in 
one  plane  and  flattened  like  a  double  convex 
lens  with  a  diameter  of  at  least  300,000  light 
years.  Our  sun  lies  somewhat  to  the  north 
of  the  median  plane,  and  about  60,000  light 
years   from    its    centre.     When    looking   at    the 


ASTRO-PHYSICS  291 

milky  way,  we  are  looking  towards  the  rim  of 
the  lens,  and  therefore,  owing  to  the  greater 
depth,  see  more  stars. 

The  appearance  of  temporary  stars  is  a 
phenomenon  which  has  been  observed  repeatedly 
in  historical  times.  Hipparchus,  Tycho  Brahe, 
and  Kepler,  for  instance,  have  recorded  such 
manifestations.  But  the  first  case  critically 
examined  by  modern  photographic  methods  was 
that  of  Nova  Aurigae,  a  star  discovered  in 
February  1892,  the  origin  and  growth  of  which 
were  traced  by  subsequent  examinations  of 
photographs  taken  in  the  previous  December  and 
January,  and  preserved  as  part  of  the  systematic 
photographic  log-book  of  the  heavens  now  kept 
by  astronomers.  For  three  months  the  star's 
brightness  lasted  and  then  rapidly  it  decreased, 
till  at  the  end  of  April  the  Nova  was  barely 
visible  in  the  great  refracting  telescope  of  the 
Lick  Observatory.  Soon  afterwards,  however, 
a  faint  nebula  appeared  in  its  place,  with  a  quite 
different  kind  of  spectrum. 

More  completely  studied  were  the  striking 
phenomena  of  the  second  Nova  Persei,  first 
sighted  at  Edinburgh  in  February  1901.  Its 
rise  and  decline  were  followed  in  many  places, 
particularly  by  Father  Sidgreaves  at  Stony- 
hurst,  and  by  Professor  Campbell  at  the  Lick 
Observatory.  It  attained  its  maximum  bright- 
ness about  a  day  and  a  half  after  its  detection, 
and  then  grew  fainter  in  a  fluctuating  manner  for 
about  ten  days.  Finally,  a  nebula  was  seen  to 
develop,  which  increased  in  visible  dimensions 
at  a  prodigious   rate — so  fast,  indeed,  that   the 


292  PHYSICAL  SCIENCE 

most  probable  explanation  supposes  that  the 
nebula  was  pre-existent  but  non-luminous,  and 
was  made  visible  by  the  flood  of  light  released 
by  the  star.  That  light  was  reflected  as  it 
spread  outwards  from  the  centre  in  ever-widening 
spheres,  and  illuminated  the  scattered  wisps  of 
attenuated  matter  it  encountered  on  its  way 
through  space.  Calculating  from  this  assumption, 
it  is  obviously  possible  to  deduce  the  distance 
of  the  star,  which  proves  to  be  such  that  light 
would  take  about  three  hundred  years  to  reach 
our  eyes.  It  would  follow  that  the  phenomena 
we  studied  in  the  last  days  of  Queen  Victoria 
represented  changes  that  were  occurring  in  the 
depths  of  space  while  Queen  Elizabeth  occupied 
the  throne  of  England. 

When  examined  spectroscopically,  the  light 
of  all  the  temporary  stars  yet  investigated  shows 
one  remarkable  property.  Bright  lines,  displaced 
towards  the  red,  are  accompanied  by  dark  lines 
of  similar  origin  displaced  towards  the  violet. 
Doppler's  principle  would  indicate  that  the  source 
of  these  double  lines  was  a  double  star,  the  bright 
lines  coming  from  a  gaseous  system  emitting  a 
line  spectrum,  and  the  dark  lines  from  a  partner 
star  in  which  absorption  was  predominant.  But 
the  difficulties  of  such  a  view  seem  insuperable. 
The  requisite  velocities  are  of  the  order  of  many 
hundreds  of  miles  a  second,  and  no  sign  of 
periodicity  or  even  diminution  appears  in  their 
values.  At  one  time  it  was  thought  that  the 
temporary  blaze  of  light  might  be  due  to  the 
shock  of  collision  of  two  stars  meeting  in  space  ; 
but  the  doubling  of  the  spectral  lines  indicates 
a  common  constitution  unlikely  inv^iably  to  be 


ASTRO-PHYSICS  293 

possessed  by  disconnected  systems  flying  through 
space  from  distant  sources.  On  the  other  hand, 
the  opposite  velocities,  constant  in  amount,  show 
that  the  two  stars  cannot  be  two  members  of 
the  same  group,  colliding  with  each  other  as  an 
effect  of  ill-directed  mutual  gravitation,  which 
would  lead  to  a  decrease  in  velocity  as  the  stars, 
after  collision,  receded  from  each  other.  The 
theory  of  collision  has  perforce  been  abandoned. 
No  satisfactory  hypothesis  has  yet  been  proposed 
in  its  place.  Perhaps  the  one  least  open  to 
objection  is  that  which  regards  the  luminosity 
as  due  to  the  passage  of  a  star,  possibly  a  dark 
one  sometimes  double,  through  the  scattered 
matter  constituting  a  nebula,  in  much  the  same 
way  as  a  shooting  star  shines  only  during  its 
transit  through  the  earth's  atmosphere. 

Many  years  ago  Clerk  Maxwell  showed 
theoretically  that  a  stream  of  light,  incident  on 
a  body,  should  produce  a  pressure  in  the  direction 
of  the  advancing  rays.  Maxwell  deduced  the 
effect  from  the  electro-magnetic  theory  of  light, 
but  it  has  since  been  shown  by  Larmor  to  be 
necessary  on  almost  any  wave  theory.  The 
undulations  must  possess  energy,  and,  therefore, 
momentum.  An  absorbing  body  is  gaining 
momentum,  and  therefore  experiences  a  pressure 
in  the  direction  of  the  incident  beam.  A  reflect- 
ing body  reflects  the  same  momentum  back  again, 
and  therefore  is  acted  on  by  a  double  pressure. 
This  result  was  first  confirmed  experimentally  by 
Professor  Lebedef,  of  Moscow.  The  difficulties 
to  be  overcome  are  best  appreciated  by  the 
statement  that  when   bright  sunlight  falls  on  a 


294  .         PHYSICAL  SCIENCE 

reflecting  surface,  the  pressure  to  be  detected 
amounts  to  less  than  a  milligram  per  square 
metre.  For  an  absorbing  surface  such  as  lamp- 
black, the  pressure  is  half  as  great  as  for  a 
reflector,  and  it  is  the  difference  between  these 
two  effects  that  M.  Lebedef  has  detected,  the 
results  of  unequal  heating  and  of  molecular 
recoil  being  successfully  eliminated.  By  another 
method  the  same  pressure  was  also  demonstrated 
by  Nichols  and  Hull. 

Owing  to  this  pressure,  two  bodies  radiating 
towards  each  other  will  experience  a  mutual 
repulsion,  which,  for  small  particles,  may  over- 
come the  gravitational  attraction.  Even  the 
attraction  of  the  sun  on  a  body  may  be 
neutralised  if  the  body  is  of  minute  size,  for  the 
radiation  effect  depends  on  the  area  of  surface, 
while  the  weight  depends  on  the  volume.  As 
the  size  is  diminished,  the  area  decreases  less 
rapidly  than  the  volume,  and,  for  microscopic 
particles  less  than  o.oooi  millimetre  in  diameter, 
the  radiative  repulsion  of  the  sun  becomes  greater 
than  the  gravitational  attraction.  An  interesting 
application  of  this  principle  has  explained  the 
curious  phenomena  of  comets'  tails,  which  have 
long  puzzled  the  ingenuity  of  astronomers.  If, 
as  is  probable,  a  comet  consists  of  a  collection 
of  meteorites,  varying  in  size  from  small  worlds 
to  microscopic  particles,  on  approaching  the  sun 
the  large  masses  will  follow  the  parabolic  path 
ABC  (Fig.  40),  indicated  by  the  ordinary 
gravitational  theory.  Particles  of  the  particular 
size  at  which  the  radiative  force  just  balances 
that  due  to  gravity  will  pursue  a  path,  ADE, 
in  an  undeviated  course,  for  both  the  forces  vary 


Fig.  40. — Diagram  to  explain  the  Phenomena  of 

Comets'  Tails. 


r  Tr,    f/rnn    1-,,-,/y/,    OQA 


ASTRO-PHYSICS  295 

inversely  as  the  square  of  the  distance,  and  will 
thus  balance  each  other  at  all  distances.  Particles 
intermediate  in  size  will  follow  intermediate  paths, 
AF,  AG,  AH,  etc.,  while  the  dust  which  suffers 
a  resultant  repulsion  will  fly  away  outside  the 
path  ADE.  As  the  comet  swings  round  the 
sun,  the  tail  becomes  expanded  into  the  fan-like 
form  commonly  observed.  The  head  of  the 
comet  goes  on  its  way  into  the  depths  of  space, 
having  lost  some  of  the  smaller  constituents  of 
its  tail,  which  are  scattered  throughout  inter- 
planetary regions. 

Not  only  does  the  radiation  from  the  sun 
cause  a  repulsion  of  small  objects,  but  their 
radiation  to  each  other  will,  as  Professor  Poynting 
has  shown  from  the  theory,  lead  to  a  mutual 
repulsion  when  the  bodies  are  placed  in  a  region 
of  space  where  the  effective  temperature  is  lower 
than  their  own.  Two  meteorites  at  ordinary 
temperatures,  say  at  300^  on  the  absolute  scale, 
will  in  cold  space  repel  each  other  with  a  force 
equal  to  their  mutual  gravitative  attraction  when 
their  radii  are  about  3.4  centimetres,  and,  in  the 
case  of  smaller  bodies,  the  repulsion  will  overcome 
the  gravitative  effect.  In  this  case,  when  the 
gravitational  force  is  that  between  bodies  of 
small  mass,  instead  of  that  between  some  small 
body  and  the  gigantic  sun,  a  resultant  repulsion 
is  reached  at  much  larger  dimensions  than  those 
of  the  case  formerly  considered.  It  is  evident 
that  a  swarm  of  meteorites  of  the  riofht  size 
might  continue  to  revolve  round  a  planet  or 
sun  without  mutual  forces  and  independently 
of  each  other.     It  is   possible  that   this    result 


296  PHYSICAL  SCIENCE 

has  some  bearing  on  the  problem  of  Saturn's 
rings. 

A  curious  conclusion  may  be  drawn  from 
the  theory  of  the  radiation-force  between  small 
bodies.  Unless  the  temperatures  are  the  same, 
the  force  on  one  need  not  necessarily  be  equal 
to  the  force  on  the  other  :  action  and  reaction 
it  seems  are  not  equal  and  opposite.  The  incon- 
sistency is,  of  course,  prevented  if  ,we  remember 
that  the  momentum  of  the  radiation  must  also 
be  taken  into  account.  In  reality  each  body  is 
emitting  a  stream  of  momentum  which  exists 
for  a  while  in  the  medium.  In  the  interaction 
between  radiation  and  either  body,  Newton's 
laws  may  still  hold.  Constantly  the  energy  and 
momentum  of  radiation  seem  to  be  exchanged 
with  those  of  matter,  and  to  be  just  as  much 
physical  realities. 

If  we  neglect  this  last  effect,  there  is  no 
reason,  in  the  case  considered,  why  action  and 
reaction  should  be  equal  and  opposite.  It  is 
even  possible  to  imagine  the  gravitation-pull  and 
the  radiation-push  so  adjusted  that  the  accelera- 
tions become  equal  but  in  the  same  direction. 
The  hotter  body  will  then  chase  the  colder  body 
through  space  with  constantly  increasing  velocity. 
A  limit  will,  however,  eventually  be  reached,  for, 
owing  to  the  Doppler  principle,  the  waves  in 
front  of  a  moving  body  are  crowded  up,  and 
those  behind  it  lengthened  out.  The  radiation- 
pressure  in  front  is  thus  increased,  and  that 
behind  diminished,  so  that  the  net  result 
is  a  retardation  which  tends  to  check  the 
motion.  In  the  case  of  meteorites  small  but 
yet  large  enough  for  the  gravitative  pull  to  be 


i 


ASTRO-PHYSICS  297 

predominant,  which  are  revolving  round  large 
bodies  in  orbits  with  high  speeds,  this  retarda- 
tion becomes  important,  and  will  eventually  cause 
the  meteorites  to  gravitate  towards  the  centre. 
In  this  way  it  is  possible  that  the  sun  may  clear 
the  neighbouring  space  of  meteoritic  dust,  which 
would  otherwise  move  round  him  in  permanent 
orbits  ;  and  the  earth  would  draw  back  to  herself 
any  particles  shot  out  by  volcanic  eruptions, 
such  as  that  of  Krakatoa,  when  the  velocities 
impressed  may  have  been  great  enough  to  carry 
them  beyond  the  atmosphere,  and  in  the  right 
direction  to  set  them  moving  as  satellites. 

The  theory  of  radiation  also  enables  us  to 
solve  many  other  interesting  problems  connected 
with  the  solar  system.  By  means  of  a  thermo- 
dynamic proof  it  has  been  shown  that  the  total 
radiation  from  a  source  should  vary  as  the  fourth 
power  of  the  absolute  temperature  T,  that  is, 
as  T  *.  By  experimental  investigation  it  is 
possible  to  establish  a  numerical  relation,  and,  if 
R  be  the  energy  radiated  per  square  centimetre 
per  second  by  a  full  radiator  such  as  lamp- 
black, the  constant  k  in  the  theoretical  equation 
R  =  kT  ^  has  been  found  by  Kurlbaum  to  be 
about  5.32   io~^   erg.^ 

Now  we  can  calculate  the  total  energy 
radiated  from  the  sun  per  second  by  measuring 
the  amount  received  at  the  surface  of  the  earth, 
and  estimating  the  amount  lost  by  reflection 
and  absorption  by  the  atmosphere.  These  con- 
siderations lead  directly  to  the  effective  tempera- 

^  The  erg  is  the  French  unit  of  work  or  energy.  About  an  erg 
of  work  is  done  when  the  thousandth  part  of  a  gram  is  raised 
through  one  centimetre. 


298  PHYSICAL  SCIENCE 

ture  of  the  radiating  layer  of  the  sun,  which  is 
thus  estimated  to  be  from  6200  to  7000^ 
absolute.  Professor  Poynting  prefers  the  lower 
value,  which  means  about  6000  C. 

A  small  body,  isolated  in  space,  will,  when  a 
steady  state  is  reached,  radiate  as  much  heat 
as  it  absorbs.  If  it  be  shielded  from  the  sun,  it 
will  attain  a  temperature  which  may  be  con- 
sidered to  be  the  effective  temperature  of  space. 
From  estimates  of  the  amount  of  heat  received 
from  the  stars,  as  compared  with  that  received 
from  the  sun,  Poynting  calculates  the  effective 
temperature  of  space  to  be  10'  absolute,  or 
263''  C.  below  the  freezing-point  of  water. 

Similar  principles  give  a  basis  for  a  determina- 
tion of  the  temperatures  of  planets  at  any  given 
distance  from  the  sun.  Assuming  that  all  the 
heat  absorbed  is  eventually  radiated  out  again, 
and  that  about  one-tenth  of  the  incident  heat  is 
reflected,  and  making  certain  simplifying  assump- 
tions, the  mean  temperature  of  the  surface  of 
the  earth  is  calculated  as  290'  absolute,  or  17°  C. 
The  average  temperature  of  the  earth's  surface 
is  known  to  be  about  60' F.,  or  16C.  The 
calculation  is  made  on  the  assumption  that  the 
effective  temperature  of  the  sun  is  6200^  absolute, 
and  its  concordance  with  observation  is  the  ground 
given  by  Poynting  for  preferring  that  value  for 
the  solar  temperature. 

This  success  in  calculating  the  effective 
temperature  of  the  earth  lends  weight  to  the 
values  given  by  the  same  method  for  the 
temperatures  of  the  other  planets.  Mercury 
and  Venus,  with  orbits  inside  that  of  the 
earth,  possess  temperatures   of  194'   and  69°  C. 


ASTRO-PHYSICS  299 

respectively,  while  the  outer  planets,  Mars  and 
Neptune,  fall  as  low  as  —38"  and  —221°.  If 
there  are,  indeed,  inhabitants  on  Mars,  it  seems 
that,  according  to  terrestrial  ideas,  they  must 
lead  a  very  chilly  existence. 

We  may  now  collect  the  various  threads  of 
thought  we  have  followed,  and  weave  them  into 
a  picture  of  the  physical  universe  and  its 
history. 

Our  stellar  system  seems  to  be  a  flattened 
lens-shaped  galaxy  of  some  1500  million  stars 
and  nebulae,  about  300,000  light  years  across 
its  diameter.  Though  the  stars  vary  greatly 
to  us  in  brightness,  that  variation  is  chiefly  an 
effect  of  distance  or  temperature,  and  the  absolute 
masses  of  most  stars  range  from  about  the  same 
dimensions  as  those  of  our  sun  to  some  twenty 
times  larger. 

Interspersed  with  the  stars,  or  perhaps  in 
some  cases  beyond  our  stellar  system,  are 
nebulae,  some  irregular  clouds  of  light,  others 
of  regular  lens-like  form,  and  again  others  with 
spiral  arms,  like  an  instantaneous  photograph 
of  a  "Catharine's  wheel"  firework. 

Laplace  suggested  that  our  sun  and  planets 
were  formed  from  a  nebula,  and,  in  the  three 
kinds  of  nebulae  mentioned,  modern  science  sees 
the  development  of  star-worlds  in  the  making. 
Mathematical  analysis  shows  that  a  mass  of 
nebulous  matter  of  the  size  of  our  puny  solar 
system  would  not  develop  as  Laplace  thought ; 
its  power  of  gravitation  would  not  be  enough, 
and  its  gaseous  matter  would  diffuse  into  space 
and  not  condense.     But  the  regular  and  spiral 


300  PHYSICAL  SCIENCE 

nebulae  are  on  a  far  vaster  scale — possibly  a 
million  times  as  great  as  the  dimensions  of 
Neptune's  orbit.  On  this  scale,  gravitation 
would  overcome  the  diffusive  effect  of  gaseous 
pressure,  and  detached  masses  might  become 
stars. 

Mr  Jeans  has  investigated  mathematically  the 
history  of  a  mass  of  gravitating  gas.  It  would, 
of  course,  form  a  sphere  when  at  rest ;  but  if, 
in  the  changes  and  chances  of  its  nebulous  life, 
it  be  set  in  rotation,  the  sphere  will  broaden  out 
round  the  equator  and  flatten  at  the  poles. 
While  it  is  contracting  under  gravity,  the 
angular  momentum  must  keep  constant,  and 
therefore  the  speed  of  rotation  must  increase. 
The  shape  gets  flatter  and  flatter,  till  the  nebula 
resembles  a  double  convex  lens.  And  such, 
indeed,  is  the  form  of  some  of  the  nebulae  :  one 
such  is  shown  in  Fig.  41. 

At  still  greater  speed,  the  lens-nebula  must 
break  up  at  the  edge,  and  detach  isolated  globes 
of  matter.  This  is  clearly  the  meaning  of  the 
many  spiral  nebulae,  one  of  which  is  shown  in 
Fig.  42.  They  are  casting  forth  future  stellar 
worlds.  Each  isolated  globe  is  shown  by  calcula- 
tion to  be  about  equal  in  mass  to  that  of  the 
average  star,  and  large  enough  to  develop  into 
a  system  such  as  our  sun  and  his  attendant 
planets. 

Measurements  of  the  actual  movement  of  the 
arms  of  spiral  nebulae  indicate  that  they  are 
indeed  matter  flying  out  from  the  nucleus.  The 
arms  in  Fig.  42,  for  example,  are  moving  fast 
enough  to  complete  a  revolution  in  45,000 
years. 


Fig.  41.  — Regular  Shaped  Nebula  (X.G.C.  5866)  with  Band  of 

Dark  Matter  on  Equator. 


Fig.  42. — Spiral  Nebula  in  Ursa  Major  (M.  ioi). 

Reproduced  by  kind  permission  from  an  article  on  "  The  Origin  of  the  Solar 
System,"  by  Dr  J.  H.  Jeans  in  Nature,  1st  March  1924. 

[To/acepagie  300 


V* 


ASTRO-PHYSICS  301 

The  mathematical  possibilities  of  development 
of  these  isolated  masses,  shows  that  they  will  not 
repeat  the  story  of  their  parent.  They  are  smaller 
beings,  and  instead  of  giving  birth  to  a  million 
new  stars,  or  even  a  modest  solar  system,  if  left 
alone  they  will  become  covered  with  a  gaseous 
atmosphere,  or,  if  rotating  rapidly  enough,  break 
up  into  two  partners  that  spend  their  lives  waltz- 
ing round  each  other,  and  are  represented  in 
nature  by  the  countless  host  of  binary  stars. 

No  analogue  of  our  solar  system  has  been 
seen  in  the  sky.  Indeed,  if  one  exists,  it  would 
be  too  small  for  the  planets  to  be  detected  at  the 
distance  of  even  the  nearest  star.  We  have  no 
model  then  by  which  its  history  may  be  illus- 
trated, and  can  but  turn  to  unconfirmed  mathe- 
matical speculation. 

Jeans  has  shown  that  the  facts  may  be 
explained  by  the  influence  of  a  foreign  body  at 
an  early  stage  of  solar  evolution.  When  the  sun 
was  a  tenuous  mass  of  nebulous  substance,  lately 
cast  forth  into  space  from  the  arms  of  some 
primordial  spiral  nebula,  it  may  have  passed  near 
one  of  its  brother  stars  or  some  other  wandering 
body,  which  raised  a  tidal  wave  on  its  glowing 
surface.  If  the  body  came  within  a  certain  range, 
the  tide  would  not  subside  as  the  body  passed, 
but  would  surge  upward,  till  finally  the  crest  of 
the  wave  would  fly  off  as  a  long  streamer  into 
space.  This,  being  much  denser  than  a  mass 
detached  by  centrifugal  action,  might  be  held 
together  by  gravity,  and  would  itself  break  up 
into  masses  which  may  have  formed  our  family 
of  planets. 

Leaving  this  speculative  account  of  the  origin 


302  PHYSICAL  SCIENCE 

of  our  own  little,  probably  abnormal,  system,  let 
us  now  look  at  the  more  usual  life  of  a  star. 

We  have  seen  that  a  gaseous  nebula  will 
spin  faster  as  it  contracts  under  its  own  gravita- 
tion, but  other  changes  will  also  occur.  It 
radiates  heat,  but,  owing  to  the  fall  of  its  outer 
layers  towards  the  centre,  more  heat  is  developed 
from  this  loss  of  mechanical  energy,  and  the 
nebula  or  star  grows  hotter.  As  the  tempera- 
ture rises,  radiation  pressure  reinforces  gaseous 
pressure,  and  these  two  causes  oppose  gravitation. 
All  the  time  the  star  is  growing  denser,  and  the 
possibility  of  further  shrinkage,  and  therefore  of 
heat  development,  less.  Hence  a  maximum 
temperature  must  be  reached,  after  which  the 
star,  having  passed  middle  age,  gets  slowly  older 
and  colder. 

These  mathematical  predictions  are  well  sup- 
ported by  astronomical  evidence.  The  earliest 
classification  of  stars  was  made  on  a  scale  of 
apparent  brightness.  Hipparchus  chose  about 
twenty  of  the  brightest  stars  as  of  the  first 
magnitude,  and  classed  the  faintest  stars  he 
could  see  as  of  the  sixth  magnitude.  In  the 
modern  form  of  this  grouping,  a  star  of  one 
magnitude  gives  2.5  times  as  much  light  as  one 
of  the  next  lower  magnitude,  and  a  difference 
from  the  first  magnitude  to  the  sixth  corresponds 
to  a  ratio  in  brightness  of  a  hundred  to  one. 

When  photography  was  applied  to  this 
problem,  a  new  scale  of  brightness  was  obtained, 
for  the  ordinary  plate  is  more  sensitive  than  the 
eye  to  blue  light  and  less  sensitive  to  red. 
Hence  the  number  found  by  subtracting  the 
visual   magnitude    from    the   photographic   is   a 


ASTRO-PHYSICS  303 

criterion  of  the  colour  of  the  star  and  is  called 
its  colour  index. 

The  next  method  of  grouping  is  by  means 
of  stellar  spectra.  This  was  first  done  by  Father 
Secchi,  who  found  that  the  spectra  could  be 
grouped  in  four  broad  classes,  agreeing  closely 
with  a  classification  according  to  colour,  from 
white  to  dark  red. 

This  grouping  has  been  superseded  by  a 
great  catalogue  of  about  a  quarter  of  a  million 
stellar  spectra  made  at  Harvard  Observatory. 
The  spectra  are  found  to  fall  in  a  continuous 
series,  the  various  main  groups  being  denoted 
by  the  letters  O  B  A  F  G  K  M  N  R,  and  each 
of  these  groups  being  subdivided.  The  spectra 
range  from  a  faint  continuous  background  with 
bright  lines  of  class  O,  through  bright  spectra  with 
helium  and  hydrogen  dark  lines,  and  then  through 
lines  of  metals  such  as  calcium  to  the  complex 
spectra  of  type  G,  which  includes  that  of  our  sun. 
In  type  K  the  hydrogen  lines  get  fainter  and  the 
blue  end  of  the  spectrum  becomes  less  intense  ; 
then  in  groups  M  and  N  are  seen  absorption 
bands  due  to  titanium  oxide  and  carbon 
compounds.  These  latter  stars  are  red  in 
colour. 

Now  as  we  heat  a  body,  it  first  glows  with 
a  deep  red  light  and  then  becomes  yellow  and 
finally  white  hot.  The  spectrum  shows  that, 
in  accordance  with  this  common  observation, 
for  a  black  body  which  is  a  perfect  radiator, 
the  wave-lengths  which  give  the  maximum  energy 
of  radiation  are  shorter  the  higher  be  the 
temperature.  Hence,  by  measuring  the  distribu- 
tion of  intensity  in  the  spectrum  of  a  star,  the 


304  PHYSICAL  SCIENCE 

effective  temperature  may  be  estimated.  It 
proves  to  range  from  about  25,000°  C.  for  the 
hottest  stars  of  type  O,  to  about  2300°  C.  for 
stars  classed  as  R.  These  figures,  of  course, 
refer  to  the  radiating  layer  towards  the  outside 
of  the  star ;  within,  the  temperature  must  be 
much  higher,  mounting  perhaps  to  some  millions 
of  degrees. 

The  spectrum  of  a  star  must  not  be  expected 
to  show  lines  corresponding  to  all  the  elements 
which  that  star  contains.  Experiment  in  our 
laboratories,  as  we  have  seen,  shows  that  electric 
ionization  greatly  increases  the  intensity  of  the 
spectral  lines  of  the  element  ionized,  and  it 
will  be  chiefly  these  ** enhanced"  lines  that 
mark  a  stellar  spectrum.  Ionization  depends 
on  temperature  as  well  as  on  the  nature  of 
the  elements  present — another  reason  why  a 
classification  by  spectra  is  also  a  classification 
by  temperature. 

If  the  distance  of  a  star  be  known,  the 
apparent  magnitude  may  be  used  to  calculate 
the  absolute  magnitude,  that  is,  the  brightness 
the  star  would  show  if  removed  to  a  standard 
distance. 

When  stars  in  the  different  spectral  types 
OBAFGKMNRare  examined  for  absolute 
magnitude,  a  remarkable  result  becomes  apparent. 
While  the  brightness  of  the  very  hot  stars  in 
class  B  is  of  the  same  order  throughout,  ranging 
only  from  about  40  to  1600  times  the  brightness 
of  our  sun,  the  cooler  stars  such  as  those  of  type 
M,  fall  into  two  well-marked  groups,  one  with 
luminosities  approaching  those  of  the  hottest 
stars,  and    the  other  with    a    brightness  of  the 


ASTRO-PHYSICS  305 

order  if  only  the  one  ten-thousandth  part  of  that 
of  the  others. 

Professor  H.  N.  Russell,  who  discovered 
these  two  types,  calls  them  ** giant  stars"  and 
"dwarf  stars"  respectively.  They  illustrate  in 
a  marvellous  way  the  mathematical  theory  of 
stellar  evolution.  Beginning  as  a  diffuse  nebulous 
mass,  our  new-born  star,  as  we  saw  on  page  302, 
grows  hotter  by  contraction,  and  passes  up  the 
scale  of  spectral  types  from  R  through  M,  and 
if  it  be  large  enough,  reaches  the  class  B  or 
even  O.  All  through  these  ages,  it  is  radiating 
energy  fiercely,  and  shining  afar.  It  is  a  "giant" 
star.  But  a  maximum  temperature  is  reached, 
perhaps  in  the  types  A  or  B,  and  thereafter,  the 
density  having  already  become  great,  the  heat 
gained  by  further  contraction  is  less  than  that 
lost  by  radiation.  The  temperature  of  the  outer 
radiating  layers  drops  back  through  its  old  range, 
and  so  the  light  of  the  star  also  passes  back 
along  the  series  of  spectral  types  from  B  or  A 
towards  M,  N,  and  R,  though  certain  differ- 
ences between  ascending  and  descending  spectra 
have  been  recognised.  But  now  the  star  is 
no  longer  inwardly  a  gigantic  mass  of  turbulent 
vapour  ever  growing  hotter,  but  a  much  smaller, 
denser  body,  with  a  colder,  calmer  future  before 
it.  The  star  has  become  a  "  dwarf."  As  it 
declines  in  vigour,  its  light  becomes  redder,  like 
that  of  a  cooling  iron  bar,  and  finally  it  vanishes 
out  of  sight,  to  make  its  existence  known  to  us, 
if  at  all,  by  passing  periodically  as  a  dark  body 
round  a  still  luminous  partner. 

When  any  branch  of  learning  first  finds  itself 

X 


3o6 


PHYSICAL  SCIENCE 


in  a  position  to  use  the  methods  and  accumu- 
lated experience  of  another  science,  a  period  of 
striking  discoveries  may  confidently  be  antici- 
pated. Thus  it  was  that  Newton  applied  to 
the  phenomena  of  the  heavens  the  mechanical 
knowledge  of  previous  ages,  and  his  law  of 
gravity  revealed  a  harmony  of  the  spheres. 
When  it  was  found  that  the  generalisations  of 
thermodynamics  and  of  electrical  science  could 
be  used  in  chemical  problems,  a  new  world 
opened  before  the  investigator.  So  it  is  with 
the  transfer  of  physical  methods  and  data  to  the 
problems  of  astro-physics.  The  first-fruits  of 
this  harvest  of  knowledge  have  already  proved 
of  momentous  import,  and  in  the  combination  of 
physics  and  astronomy  the  present  labourers 
and  those  that  come  after  them  may  hope  to 
find  one  of  the  most  fertile  unions  in  the  whole 
realm  of  Natural  Philosophy. 


INDEX 


a  Rays,  169,  170,  173,  174,  188, 

191,  194,  200,  239 
Aberration  of  light,  223 
Actinium,  166 
Action,  2,  40,  247 
Active  deposits,  177,  188 
Adams,  262 
^ther,    154,    160,   220    et  seq.^ 

232,  234,  245,  252 
^Ethereal  strain,  234 
Aitken,  132 
Algol,  275,  276 
Aluminium,  202 
Andrews,  43  ;  another  Andrews, 

89 
Antimony,  78,  91 
Argon,  55,  56 
Arrhenius,  3,  105,  124,  158 
Aston,  F.  W.,  156,  197 
Astro-physics,    10,    158,   261   et 

seq. 
Atom,     the     individual,     193 ; 
structure     of,     214,     237 ; 
nucleus  of,  216,  240,  247 
Atomic  disintegration,  185,  186, 
191,  195,  199,  200,  201,  239 
nucleus,  216,  240,  247 
numbers,  196,  241 
structure,  147  etseq.^  211,  214, 

219,  237,  22,9  et  seq. 
theory,  3,  25,  102,   104,   108, 
193,  204  et  seq. 
Aurora  borealis,  158,  287 

807 


Rays,  169,  170,  174,  195,  221, 

255 
Bacon,  Lord,  13 

Balmer,  249 

Barium,  connection  with  radium 

167 

Becquerel,  165,  182 

Beilby,  Sir  G.  T.,  90 

Bemmelen,  Van,  117 

Bemont,  167 

Benzene  ring,  142 

Bohr,  N.,  2,  249,  251,  280 

Bolometer,  269 

Bolt  wood,  188 

Boron,  202 

Boscovich,  218 

Bragg,  Sir  Wm.  and  W.  L.,  139 

Broad,  C.  D.,  8,  12 

Broadcasting,  226 

Bronzes,  82 

Buchanan,  J.  G.,  y2> 

Bunsen,  263,  264 

Burton,  120 

Cailletet,  45,  46 

Calcium  light  from  sun,  2S0 

Campbell,  291 

Cathode  rays,  136,   142  et  seq., 

213 
Cause  and  effect,  28 
Cavendish,  56 
Chemical  combination,  78,  104, 

114,  206,  212,  217 


3o8 


INDEX 


Clausius,  58 

Clocks,  254 

Cloud  formation,  132 

Coagulation,  117,  119,  181 

Colloids,  115  et  seq. 

Comets'  tails,  294 

Condensation  nuclei,  132 

Conduction  of  electricity 
through  gases,  2,  4,  9,  125 
et  seq.  ;  through  liquids, 
see  electrolysis ;  through 
solids,  158 

Continuous  waves,  226 

Cooke,  179 

Copper,  75,  78,  82 

Corona,  283 

Coronium,  284 

Corpuscles,  see  electrons 

Corpuscular  theory  of  light,  220 

Crookes,  SirWm,,  136,  148,  172, 
182,  205 

Cryohydrates,  71 

Crystal  structure,  141 

Curie,  M.  et  Mme.,  5,  165,  166, 
167,  169,  191 

Dalton,  206,  212 
Democritus,  148,  217,  218 
Dewar,  Sir  J.,  8,  51,  52,  60,  62, 

172,  J91 
Diamond,  141 

Diffusion,  116,  131,  204,  209 
Dissociation,    ionic,    106,    112, 

113,  118 
Doppler's  principle,  269  et  seq.^ 

292,  296 
Double  stars,  275 
Dust  nuclei,  132 
Dwarf  stars,  305 
Dyeing,  123 

Earth,  age  of,  286 
Eddington,  A.  S.,  245,  251 


Einstein,  2,  3,  246,  253,  256,  258 
Elasticity  of  the  sether,  222 
Electric  charge,  nature  of,  232, 

234 
deflection,  142,  145 

inertia,  150 

Electrical  conductivity  of 
metals,  61 

Electrolysis,  3,  94,  107  et  seq. 

Electromagnetic  waves,  9,  170, 
221  et  seq. 

Electrons,  4,  9,  39,  131,  147  et 
seq.,  213,  215,  217,  227,  234 

Electroscopes  and  electro- 
meters, 126 

Elster,  155 

ejjH,  I43»  192 

Emanations,   radio-active,   173, 

175,  ^77,  198 
Energetics,   5,   24,  80,  98,   102, 

104 
Enhanced  lines,  279,  304 
Entropy,  37 
Equilibrium,  i,  5,  8,  68  et  seq., 

80  et  seq.,  206 
Eutectic  alloy,  76,  78 
Evaporation,  42,  44,  52 

Faraday,  3,  43,  107,  152,  153, 
160,  162,  227 

Fitzgerald,  G.  F.,  253 

Fluorescence  and  phosphor- 
escence, 136,  165,  172 

Fluorine,  202 

Force,  24 

Foucault,  263 

Frankland,  57 

Fraunhofer,  263 

Freezing-point  curves,  fig.  6,  p. 
75  ;  fig-  7,  p.  78  ;  fig.  8,  p. 
79;  fig-  9,  p.  81  ;  fig.  10, 
p.  83  ;  fig.  iS,  p.  89 

Fresnel,  221 


INDEX 


309 


Fusion  and  solidification,  8,  41, 
68 


7  Rays,  169,  170,  174 
Galileo,  26,  30 

Gases,  conduction  of  electricity 
through,  4,  8,  9,  125  et  seq. 
Gay  Lussac,  47 
Geiger,  192,  206 
Geitel,  156 
Gelation,  117 
Giant  stars,  305 

Gibbs,  Willard,  i,  7,  80,  98,  124 
Giesel,  177 
Graham,  115,  124 
Grating,  267 

Gravitation,  nature  of,  232 
Gravity,  256 
Guthrie,  71 
Gyroscope,  233 


Ice,  structure  of,  71,  73 
Induction  and  deduction,  31 
Internal  work  of  gases,  46  et  seq. 
Interval,  254 
Introduction,  i 

Ionic  charge,  107  et  seq.,   134, 
142  ^/  seq. 
dissociation,    106,     112,    113, 

118 
theory,    3,   8,   105,    106,    112, 

118,  126,   158 
velocities,  108  et  seq. 
Ionium,  188 
Ionization,  279,  304  ;  of  gases, 

125  et  seq.,  171,  230 
Iron,  87,  279 
Isotopes,  156,  197 

Jeans,  300,  301 
Joule,  48,  49 


Hale,  G.  E.,  273,  280,  281 
Hardy,  W.  B.,  117,  120,  181 
Harvard  Observatory,  303 
Heaviside,  236 
Helium,  57,  60,  91,  282  ;  atom, 

242  ;     liquid,    61  ;     nuclei, 

202,  242. 
Helmholtz,  Von,  6,  58,  98,  108, 

162,  219 
Hertz,  224 

Heycock,  C.  T.,  74,  82,  83,  87 
Hipparchus,  291 
Hittorf,  3,  108,  no 
Huggins,  Sir  Wm.  and  Lady, 

172,  266 
Hull,  294 

Huygens,  24,  25,  221 
Hydrogen,    atom,    242 ;    light 

from  sun,  280 ;  nuclei,  201, 

202,  242 
Hypnotism. 


Kahlenberg,  115 
Kaufmann,  150,  236,  255 
Keeler,  277 
Kelvin,  Lord,  5,  48,  49,  58,  210 

219,  233,  285 
Kepler,  30,  291 
Kirchhoff,  263,  264 
Kohlrausch,  3,  108,  no 

Langevin,  129 
Langley,  S.  P.,  269 
Laplace,  18,  299 
Larmor,  Sir  J.,  4,   7,  98,    149, 
213,  218,  234,  239,  253,  293 
Laue,  139 
Laws  of  Nature,  26,  29,  30,  32, 

34,  245 
Leak    of   electricity   from    hot 

surfaces,  156  et  seq.,  287 
Lebedef,  293 
Le  Chatelier,  78,  89 


310 


INDEX 


Lenard,  147,  280 
Leverrier,  262 
Lewis,  G.  N.,  no 
Light,    corpuscular    theory   of, 
220  ;  velocity  of,  252,  254, 

255 
Lindeman,  65 

Linder,  117 

Lines  of  force,  151,  160,  227  et 

seq. 
Liquefaction  of  gases,  8,  41  et 

seq. 
Lockyer,  Sir  N.,  57,  282 
Lodge,  Sir  O.,  108,  no,  251 
Lorentz,  4,    149,  213,  218,  234, 

239,  253 
Low  temperature   research,  41 

et  seq. 
Lucretius,  217 
Lyr^,  /3,  275 

Mach,  7,  18 

M'Clung,  R.  K.,  230 

M'Lennan,  179 

Magnetic  deflection,  142,  145, 
169 

Magnets,  equilibrium  of  float- 
ing, 214 

Mars,  temperature  of  the  planet, 
299 

Mass,  22,  25,  35,  143  etseq.,  234, 
255  ;  conservation  of,  23  ; 
variation  of,  with  velocity, 
23,  38  ;  and  weight,  25 

Masson,  Orme,  no 

Mathematics,  31 

Matter,  see  mass, 
and  energy,  17,  256 

Maxwell,  Clerk,  6,  80,  153,  160, 
162,  223,  224,  277,  293 

Mayer,  214,  215 

Mechanics,  7,  16 

Mendeleefl",  212,  216 


Mercuiy,     the     planet,     258 ; 
temperature  of  the  planet, 

294 
Metallic  conduction,  \iZ  et  seq. 
Metals,  structure  of,  71,  84,  90 
Metaphysics,    12,    14,   22,    151, 

210 
Michelson,  252 
Microscopic  study  of  metals,  8, 

69,  72,  84,  89 
Milikan,  144,  193 
Milky  way,  290 
Miller,  L.,  no 
Minkowski,  3,  254,  257 
Molecular  structure, dimensions 

of,  207  et  seq. 
theory,  3,  102,  104,  204  et  seq. 
Momentum,   255  ;  conservation 

of,  37;  of  the  aether,  153 
Morley,  252 
Moseley,  H.  G.  J.,  241,  242 

Nebula,  265,  299,  300  ;  spiral, 

289 
Nebular  hypothesis,  299 
Neptune,   temperature    of   the 

planet,  299 
Neville,  F.  H.,  74,  82,  83,  87 
Newton,  Sir  Isaac,  frontispiece, 

3,  24,  25,  218,  220,  221,  257, 

258,  263 
NichoUs,  294 
Nitrogen,  202,  243 
Nova,  291  et  seq. 
Nuclei,  helium,  202  ;  hydrogen, 

201,  202,  242 

Oden,  n9 
Ohm's  law,  112,  128 
Olszewski,  51 
Onnes,  K.,  57,  61,  65 
Osmond,  89 


INDEX 


311 


Osmotic  pressure,  96  etseq,^  105 
Ostwald,  104 


Paschen,  249 

Pedigree  of  radium  family,  196 

Periodic  law,  212  ;  table,  216 

Perrin,  142 

Persei,  /S,  275 

Pfeffer,  94,  96,  97 

Phases,  8,  88 

Philosophical  basis  of  physical 

science,  7,  11  et  seq. 
Phosphorescence     and      fluor- 
escence, 136,  165,  172 
Phosphorescence  at   low  tem- 
perature, 65 
Phosphorus,  202 
Photographs  applied   to  astro- 
physics, 266,  280 
Physiology,  16,  93,  116,  181 
Pictet,  45,  46 
Picton,  117 
Pitch-blende,  166 
Planck,  2,  10,  105,  245,  246,  247, 
260 

Platinum  thermometer,  62,  74, 
269 

Polarization  of  light,  222,  229 
Polish,  90 

Polonium,  166,  195 

Porous  plug  experiment,  48 

Positive  rays,  155 

Potassium,  radio-activity  of,  180 

Poynting,  J.  H.,  295,  298 

Pressure  of  radiation,  293  etseq. 

Principe,  259 

Proton,  242 

Prout,  213 

Psycho-physics,  14,  17,  34 

Quantum  Theory,  2,  10,  40, 
231,245,251,260,280 


Radiation,  213,  223  et  seq., 
237  et  seq.,  265,  293,  297  ; 
stellar,  303 

Radio-active  deposits,  174,  179, 
188 

Radio-activity,  5,  9,  164  et 
seq.',  analysis  by  means  of, 
167 

decay    of,    175  ;    curves,   fig. 
32,  p.  178;  fig.  33,  p.   183; 
H'  34,  p.  189 
energy  of,  165,  186,  197 
of   ordinary    materials,    180, 
199,    286;     of    the    earth 
and  atmosphere,  179,  180 
Radium,  idd etseq.,  189  ;  atomic 
weight  of,  194  ;  life  of,  194; 
pedigree  of,  196 
Ramsay,  Sir  Wm.,  55,  57,  191, 

282 
Rankine,  58 
Rayleigh,   the    late    Lord,   55 

56 
Rayleigh,  Lord,  170,  179 

Regenerative  process  of  lique- 
faction, 50 

Relativity,  151,  253,  256; 
principle  of,  10,  17,  21,  28, 
38 

Resonance,  263 

Reversal  of  spectral  lines,  264, 
273 

Richardson,  O.  W.,  156 

Roberts-Austen,  Sir  W.  C,  82, 
205 

Rock  salt,  structure  of,  141 

Rontgen,  137,  240 

Rontgen  rays,  125,  137  etseq. 

Roozeboom,  80,  81,  88 

Rotation  of  sun,  272 

Rowland,  268 

Royds,  194 

Russell,  Hon.  Bertrand,  8 


312 


INDEX 


Russell,  H.  N.,  305 

Rutherford,  Sir  E.,  2,  5,  9,  164, 
168,  1735  175)  182,  191,  192, 
194,  200,  202,  206,  240,  248, 
286  ;  portrait  of,  164 

Rydberg,  248 

Salt  Solutions,  69,  71,  73» 

105,  109 
Saturation  current,  128 
Saturn's  rings,  277,  296 
Schultze,  117 
Searle,  G.  F.  C,  236 
Sea-water,  freezing  of,  73 
Secchi,  Father,  303 
Semi-permeable  membranes,  95 
Sidgreaves,  Father,  291 
Silver,  75 
Sirius,  288 
Sobral,  258 
Soddy,   F.,    156,   176,  182,  191, 

196 
Sodium,  202 
Soil,  the,  123 
Solar  radiation,  273 
Solar  system,  289,  301 
Solid  solutions,  80,  83,  87 
Solution,  problems  of,  8,  93  et 

seq. 
Sommerfeld,  250 
Sorbite,  89 
Sorby,  89 

Space  and  time,  3,  17,  21 
Space-time,  254,  259 
Specific  heats,  246 
Spectro  helioscope,  281 
Spectroscope,  262  et  seq.^  277  et 

seq. 
Speculum  metal,  91 
Star  clusters,  289 
Stars,    classification     of,    302  ; 

distance  of,  287,  290  ;  giant 

and  dwarf,  305  ;  magnitude 


of,    302,    304  ;   number   of, 

290  ;  spectra  of,  303  ;  tem- 
perature of,  304  \  temporary, 

291  et  seq.  ;  variable,  289 
Stead,  J.  E.  88,  89 

Steel,  B.  D.,  1 10 

Stellar  system,  290,  299 

Stokes,  Sir  G.  G.,  134,  151,  263 

Stoney,  J.,  4,  234 

Strutt,    Hon.    R.    J.,   see    Lord 

Rayleigh 
Sugar  solutions,  96,  98,  105 
Sun  spots,  273,  282 
Sun,   age   of,   285  ;   energy   of, 

284,  297  ;   temperature   of, 

284,  297 
Surface    tension,    91,  92,    121, 

123,  207 

Telegraphy,  158,  224 
Telephony,  158,  224 
Telescope,  262 
Temperature  of  space,  298 
Thermionic  valve,  225 
Thermodynamics,  i,  2,  5,  25,  80, 

98,  102  et  seq.j  206 
Thermos  flask,  54 
Thomson,  Sir  J.  J.,  i,  3,  65,  125, 

I3i>  i33j  134,  142,  143)  146 
et  seq.^  153  ^^  seq.^  21^  et 
seq.,  229,  231,  234,  283 
Thorium  and  Thorium-A',  182, 

183 
Thought-transference,  34 

Tin,  82 

Townsend  J.  S.,  129,  134,  146 

Tubes   of  force,  152,  153,  160, 

12']  et  seq. 

Tycho-Brahe,  291 

Undulatory      theory      of 
light,  221 


I 

i 


INDEX 


313 


Units,  physical,  20,  35 
Uranium,  165,  168,  182,  243 

Vacuum  vessels,  fig.  i,  p.  54 
Valency,  117,  217 
Valve,  thermionic,  225 
Van't  Hoff,  97,  98,  105,  124 
Velocity,  of  light,  252,  254,  255  ; 

of  stars,  271  et  seq. 
Venus,     temperature     of    the 

planet,  298 
Viscosity  of  gases,  209 
Vortex  rings,  219,  233 

Wald,  F.J  104 
Waves,  continuous,  226 
Weight,  22,  26 
Weyprecht,  'Ji 


Whetham,    C.     Dampier,    109, 

no,   118 
Whitehead,  A.  N.,  8,  259 
Wilson,  C.  T.  R.,  132,  133,  144, 

147,  179,  206 
Wilson,    H.   A.,    131,  144,  155, 

157 
V/ireless  telegraphy,  158,  224 

Wireless  telephony,  158,  224 

Wollaston,  263 

X-COMPOUNDS,  183,  188 
X-Ray  spectra,  141 
X-rays,  240 

Young,  221 

Zeeman,  239,  278 
Zeleny,  129 


iji