UC-NRLF
B 4 ESS ait
1ICAL PHILOSOPHY
STANISLAO CANNIZZARO
1858)
alembic Clufr
No. 1
Hlernbtc Club IReprtnts IRo. 18
SKETCH OF A COURSE
OF
CHEMICAL PHILOSOPHY
BY
STANISLAO CANNIZZARO
(1858)
JEfcinburgb
THE ALEMBIC CLUB
Cbicago
THE UNIVERSITY OF CHICAGO PRESS
1911
PREFACE
THE value of the hypothesis of the Italian
physicist Avogadro* as a systematising prin-
ciple in chemistry was practically unrecognised for
forty years after its publication. It had been, it is
true, considered and in part applied by Dumas,
Gerhardt, and others, but the young Italian chemist
Cannizzaro was the first to show its consistent
applicability to the selection of atomic weights, and
to harmonise with it the results of other methods
directed towards the same end.
The eminence of Cannizzaro as a teacher is plain in
every page of the summary of his lecture course on
chemical philosophy which is here translated. The
facts are marshalled and their bearing explained with
absolute mastery of pedagogic method, and one is
impelled to the conclusion that Cannizzaro's students
of 1858 must have had clearer conceptions of chemical
theory than most of his scientific colleagues of a much
later date.
Permission to publish this translation was received
from the venerable chemist a few days before his death
on loth May 1910.
J. W.
Alembic Club Reprint, No. 4, p. 28.
LETTER OF
PROFESSOR STANISLAO CANNIZZARO
TO
PROFESSOR S. DE LUCA :
SKETCH OF A COURSE OF
CHEMICAL PHILOSOPHY
Given in the Royal University 0/ Genoa*
I BELIEVE that the progress of science made in
these last years has confirmed the hypothesis of
Avogadro, of Ampere, and of Dumas on the similar
constitution of substances in the gaseous state ; that
is, that equal volumes of these substances, whether
simple or compound, contain an equal number of
molecules : not however an equal number of atoms,
since the molecules of the different substances, or
those of the same substance in its different states,
may contain a different number of atoms, whether of
the same or of diverse nature.
In order to lead my students to the conviction which
I have reached myself, I wish to place them on the
same path as that by which I have arrived at it the
path, that is, of the historical examination of chemical
theories.
I commence, then, in the first lecture by showing
how, from the examination of the physical properties
* From 11 Nuovo Cimento, vol. vii. (1858), pp. 321-366.
2 Cannizzaro. [pp. 321-2
of gaseous bodies, and from the law of Gay-Lussac on
the volume relations between components and com-
pounds, there arose almost spontaneously the
hypothesis alluded to above, which was first of all
enunciated by Avogadro, and shortly afterwards by
Ampere. Analysing the conception of these two
physicists, I show that it contains nothing contra-
dictory to known facts, provided that we distinguish,
as they did, molecules from atoms ; provided that we
do not confuse the criteria by which the number and
the weight of the former are compared, with the
criteria which serve to deduce the weight of the
latter ; provided that, finally, we have not fixed in our
minds the prejudice that whilst the molecules of
compound substances may consist of different numbers
of atoms, the molecules of the various simple substances
must all contain either one atom, or at least an equal
number of atoms.
In the second lecture I set myself the task of
investigating the reasons why this hypothesis of
Avogadro and Ampere was not immediately accepted
by the majority of chemists. I therefore expound
rapidly the work and the ideas of those who examined
the relationships of the reacting quantities of substances
without concerning themselves with the volumes
which these substances occupy in the gaseous state ;
and I pause to explain the ideas of Berzelius, by the
influence of which the hypothesis above cited
appeared to chemists out of harmony with the
facts.
I examine the order of the ideas of Berzelius, and
show how on the one hand he developed and com-
pleted the dualistic theory of Lavoisier by his own
electro-chemical hypothesis, and how on the other
hand, influenced by the atomic theory of Dalton (which
PP. 322-3] Course of Chemical Philosophy. 3
had been confirmed by the experiments of Wollaston),
he applied this theory and took it for his guide in his
later researches, bringing it into agreement with the
dualistic electro-chemical theory, whilst at the same
time he extended the laws of Richter and tried to
harmonise them with the results of Proust. I bring
out clearly the reason why he was led to assume that
the atoms, whilst separate in simple bodies, should
unite to form the atoms of a compound of the first
order, and these in turn, uniting in simple propor-
tions, should form composite atoms of the second
order, and why (since he could not admit that when two
substances give a single compound, a molecule of the
one and a molegule of the other, instead of uniting
to form a single molecule, should change into two
molecules of the same nature) he could not accept the
hypothesis of Avogadro and of Ampere, which in
many cases leads to the conclusion just indicated.
I then show how Berzelius, being unable to escape
from his own dualistic ideas, and yet wishing to
explain the simple relations discovered by Gay-Lussac
between the volumes of gaseous compounds and their
gaseous components, was led to formulate a hypothesis
very different from that of Avogadro and of Ampere,
namely, that equal volumes of simple substances in the
gaseous state contain the same number of atoms,
which in combination unite intact ; how, later, the
vapour densities of many simple substances having
been determined, he had to restrict this hypothesis by
saying that only simple substances which are
permanent gases obey this law ; how, not believing
that composite atoms even of the same order
could be equidistant in the gaseous state under the
same conditions, he was led to suppose that in the
molecules of hydrochloric, hydriodic, and hydrobromic
4 Cannizzaro. [p. 323
acids, and in those of water and sulphuretted hydrogen,
there was contained the same quantity of hydrogen,
although the different behaviour of these compounds
confirmed the deductions from the hypothesis of
Avogadro and of Ampere.
I conclude this lecture by showing that we have
only to distinguish atoms from molecules in order
to reconcile all the experimental results known
to Berzelius, and have no need to assume any
difference in constitution between permanent and
coercible, or between simple and compound gases,
in contradiction to the physical properties of all
elastic fluids.
In the third lecture I pass in review the various
researches of physicists on gaseous bodies, and show
that all the new researches from Gay-Lussac to
Clausius confirm the hypothesis of Avogadro and of
Ampere that the distances between the molecules, so
long as they remain in the gaseous state, do not
depend on their nature, nor on their mass, nor on*
the number of atoms they contain, but only on their
temperature and on the pressure to which they are
subjected.
In the fourth lecture I pass under review the chemical
theories since Berzelius : I pause to examine how
Dumas, inclining to the idea of Ampere, had habituated
chemists who busied themselves with organic substances
to apply this idea in determining the molecular
weights of compounds ; and what were the reasons
which had stopped him half way in the application of
this theory. I then expound, in continuation of this r
two different methods the one due to Berzelius, the
other to Ampere and Dumas which were used to
determine formulae in inorganic and in organic
chemistry respectively until Laurent and Gerhardt
PP. 323-4] Course of Chemical Philosophy. 5
sought to bring both parts of the science into harmony.
I explain clearly how the discoveries made by Gerhardt,
Williamson, Hofmann, Wurtz, Berthelot, Frankland,
and others, on the constitution of organic compounds
confirm the hypothesis of Avogadro and Ampere, and
how that part of Gerhardt's theory which corresponds
best with the facts and best explains their connection,
is nothing but the extension of Ampere's theory,
that is, its complete application, already begun by
Dumas.
I draw attention, however, to the fact that Gerhardt
did not always consistently follow the theory which
had given him such fertile results ; since he assumed
that equal volumes of gaseous bodies contain the same
number of molecules, only in the majority of cases,
but not always.
I show how he was constrained by a prejudice, the
reverse of that of Berzelius, frequently to distort the
facts. Whilst Berzelius, on the one hand, did not
admit that the molecules of simple substances could
be divided in the act of combination, Gerhardt
supposes that all the molecules of simple substances
are divisible in chemical action. This prejudice
forces him to suppose that the molecule of mercury
and of all the metals consists of two atoms, like that
of hydrogen, and therefore that the compounds of all
the metals are of the same type as those of hydrogen.
This error even yet persists in the minds of chemists,
and has prevented them from discovering amongst the
metals the existence of biatomic radicals perfectly
analogous to those lately discovered by Wurtz in
organic chemistry.
From the historical examination of chemical theories,
as well as from physical researches, I draw the con-
clusion that to bring into harmony all the branches of
a 2
6 Cannizzaro. [pp. 324-5
chemistry we must have recourse to the complete
application of the theory of Avogadro and Ampere in
order to compare the weights and the numbers of
the molecules ; and I propose in the sequel to show
that the conclusions drawn from it are invariably in
accordance with all physical and chemical laws
hitherto discovered.
I begin in the fifth lecture by applying the
hypothesis of Avogadro and Ampere to determine the
weights of molecules even before their composition is
known.
On the basis of the hypothesis cited above, the
weights of the molecules are proportional to the
densities of the substances in the gaseous state. If we
wish the densities of vapours to express the weights of
the molecules, it is expedient to refer them all to the
density of a simple gas taken as unity, rather than
to the weight of a mixture of two gases such as
air.
Hydrogen being the lightest gas, we may take it as
the unit to which we refer the densities of other
gaseous bodies, which in such a case express the
weights of the molecules compared to the weight of
the molecule of hydrogen = i.
Since I prefer to take as common unit for the
weights of the molecules and for their fractions, the
weight of a half and not of a whole molecule of
hydrogen, I therefore refer the densities of the various
gaseous bodies to that of hydrogen = 2. If the
densities are referred to air=i, it is sufficient to
multiply by 14.438 to change them to those referred
to that of hydrogen = i ; and by 28*87 to r f er them
to the density of hydrogen = 2.
I write the two series of numbers, expressing these
weights in the following manner :
PP. 325-6] Course of Chemical Philosophy.
Names of Substances.
Densities or weights
of one volume, the
volume of Hydrogen
beins made = i,
i.e., weights of the
molecules referred to
the weight of a whole
molecule of Hydrogen
Densities referred to
that of Hydrogen
= 2, i.e., weights of
the molecules
referred to the weight
of half a molecule of
Hydrogen taken as
taken as unity.
unity.
Hydrogen
I
2
Oxygen, ordinary.
16
32
Oxygen, electrised
64
128
Sulphur below 1000
96
192
Sulphur* above 1000
32
64
Chlorine
35-5
71
Bromine
80
1 60
Arsenic
150
300
Mercury
100
200
Water .
9
18
Hydrochloric Acid
18-25
3 6-$ot
Acetic Acid .
30
60
* This determination was made by Bineau, but I believe k requires con-
firmation.
t The numbers expressing the densities are approximate: we arrive at a
closer approximation by comparing them with those derived from chemical data,
and bringing the two into harmony.
Whoever wishes to refer the densities to hydrogen
= i and the weights of the molecules to the weight of
half a molecule of hydrogen, can say that the weights
of the molecules are all represented by the weight of
two volumes.
I myself, however, for simplicity of exposition,
prefer to refer the densities to that of hydrogen = 2,
and so the weights of the molecules are all represented
by the weight of one volume.
From the few examples contained in the table, I
show that the same substance in its different allotropic
states can have different molecular weights, without
concealing the fact that the experimental data on
which this conclusion is founded still require con-
firmation.
8 Cannizzaro. [p. 326
I assume that the study of the various compounds
has been begun by determining the weights of the
molecules, i.e., their densities in the gaseous state,
without enquiring if they are simple or compound.
I then come to the examination of the composition
of these molecules. If the substance is undecompos-
able, we are forced to admit that its molecule is
entirely made up by the weight of one and the
same kind of matter. If the body is composite, its
elementary analysis is made, and thus we discover the
constant relations between the weights of its
components : then the weight of the molecule is
divided into parts proportional to the numbers
expressing the relative weights of the components,
and thus we obtain the quantities of these components
contained in the molecule of the compound, referred
to the same unit as that to which we refer the
weights of all the molecules. By this method I have
constructed the following table :
[TABLE
p. 327] Course of Chemical Philosophy.
Name of Substance.
Weight of one
volume,
i.e., weight of
the molecule
referred to the
weight of half
a molecule of
Hydroj?en = i.
Component weights of one volume,
i.e., component weights of the
molecule, all referred to the weight
of half a molecule of Hydrogen
=x.
H \drogen
2
2 Hydrogen
Oxygen, ordinary
32
32 Oxygen
electrised
128
128
Sulphur below 1000 .
IQ2
192 Sulphur
above 1000 (?)
6 4
64
Phosphorus
Chlorine .
124
71
i 24 Phosphorus
71 Chlorine
Bromine .
1 60
160 Bromine
Iodine
254
254 Iodine
Nitrogen .
28
28 Nitrogen
Arsenic
300
300 Arsenic
Mercury .
200
JOG Mercury
Hydrochloric Acid .
36-5
35-5 Chlorine I Hydrogen
Hydrobromic Acid
81
80 Bromine I
Hydriodic Acid
Water .
128
18
127 Iodine I
1 6 Oxygen 2
Ammonia .
17
14 Nitrogen 3
Arseniuretted Hyd. .
78
75 Arsenic 3
Phosphuretted Hyd. .
Calomel .
35
235-5
32 Phosphorus 3
35-5 Chlorine 200 Mercury
Corrosive Sublimate .
271
71 200
Arsenic Trichloride .
181-5
106-5 ii 75 Arsenic
Protochloride of Phos-
phorus .
138-5
106-5 ,, 32 Phosphorus
Perchloride of Iron .
325
213 112 Iron
Protoxide of Nitrogen
44
16 Oxygen 28 Nitrogen
Binoxide of Nitrogen
30
'6 14
Carbonic Oxide
28
16 T2 Carbon
Acid .
44
32 ,, 12
Ethylene .
28
4 Hydrogen 24
Propylene
42
6 36
Acetic Acid, hydrated
60
f 4
32 Oxygen
( 24 Carbon
f 6 Hydrogen
anhydrous .
102
48 Oxygen
I 48 Carbon
j' 6 Hydrogen
Alcohol .
4 6
1 6 Oxygen
1 24 Carbon
f 10 Hydrogen
Ether
74
1 6 Oxygen
1 48 Carbon
10 Cannizzaro. [p. 328
All the numbers contained in the preceding table
are comparable amongst themselves, being referred to
the same unit. And to fix this well in the minds of
my pupils, I have recourse to a very simple artifice :
I say to them, namely, " Suppose it to be shown that
the half molecule of hydrogen weighs a millionth of a
milligram, then all the numbers of the preceding table
become concrete numbers, expressing in millionths of
a milligram the concrete weights of the molecules and
of their components : the same thing would follow if
the common unit had any other concrete value," and
so I lead them to gain a clear conception of the
comparability of these numbers, whatever be the
concrete value of the common unit.
Once this artifice has served its purpose, I hasten to
destroy it by explaining how it is not possible in
reality to know the concrete value of this unit ; but
the clear ideas remain in the minds of my pupils
whatever may be their degree of mathematical know-
ledge. I proceed pretty much as engineers do when
they destroy the wooden scaffolding which has served
them to construct their bridges, as soon as these can
support themselves. But I fear that you will say, " Is
it worth the trouble and the waste of time and ink to
tell me of this very common artifice ? " I am, however,
constrained to tell you that I have paused to do so
because I have become attached to this pedagogic
expedient, having had such great success with it
amongst my pupils, and thus I recommend it to all
those who, like myself, must teach chemistry to
youths not well accustomed to the comparison of
quantities.
Once my students have become familar with the
importance of the numbers as they are exhibited in
the preceding table, it is easy to lead them to discover
PP. 328-9] Course of Chemical Philosophy. 11
the law which results from their comparison.
"Compare," I say to them, "the various quantities
of the same element contained in the molecule of the
free substance and in those of all its different com-
pounds, and you will not be able to escape the
following law : The different quantities of the same
element contained in different molecules are all whole
multiples of one and the same quantity, which, always
being entire, has the right to be called an atom.' 1 ' 1
Thus :-
One molecule of free hydrogen . contains 2 of hydrogen 2 x
of hydrochloric acid . I = ix
of hydrobromic acid ,, T ,, = I x
of hydriodic acid ,, I ,, = ix
,, of hydrocyanic acid . ,, I ,, = I x
of water ,, 2 ,, -- 2 x
of sulphuretted hy-
drogen . . ,, 2 ,, =2x1
,, of formic acid . . ., 2 ,, = 2x
,, of ammonia ,, 3 ,, = 3x
of gaseous phosphur-
etted hydrogen . ,, 3 ,, = 3 x
,, of acetic acid . . ,,4 M = 4 x
,, ofethylene . . ,,4 >* = 4 X
,, of alcohol . . ,,6 ,, 6 x
of ether . . . ,, 10 ,, =iox
Thus all the various weights of hydrogen contained
in the different molecules are integral multiples of the
weight contained in the molecule of hydrochloric
acid, which justifies our having taken it as common
unit of the weights of the atoms and of the molecules.
The atom of hydrogen is contained twice in the
molecule of free hydrogen.
In the same way it is shown that the various
quantities of chlorine existing in different molecules
are all whole multiples of the quantity contained in
the molecule of hydrochloric acid, that is, of 35.5 ; and
12 Cannizzaro. [pp. 329-30
that the quantities of oxygen existing in the different
molecules are all whole multiples of the quantity
contained in the molecule of water, that is, of 16,
which quantity is half of that contained in the
molecule of free oxygen, and an eighth part of that
contained in the molecule of electrised oxygen (ozone).
Thus :
One molecule of free oxygen . contains 32 of oxygen = 2x16
of ozone . . ,, 128 ,, = 8 x 16
of water . . 16 ,, = i x 16
of ether . . . 16 ^.-1x16
of acetic acid . 32 _- 2x16
etc. etc.
One molecule of free chlorine . contains 71 of chlorine = 2 x 35-5
,, of hydrochloric acid ,, 35-5 ,, = 1x35.5
,, of corrosive sublimate ,,71 ,, = 2x35-5
,, of chloride of arsenic ,, 106-5 ,, = 3 x 35-5
,, of chloride of tin . ,, 142 ,, =4x35-5
etc. etc.
In a similar way may be found the smallest quantity
of each element which enters as a whole into the
molecules which contain it, and to which may be
given with reason the name of atom. In order, then,
to find the atomic weight of each element, it is neces-
sary first of all to know the weights of all or of the
greater part of the molecules in which it is contained
and their composition.
If it should appear to any one that this method of
finding the weights of the molecules is too hypothetical,
then let him compare the composition of equal volumes
of substances in the gaseous state under the same
conditions. He will not be able to escape the follow-
ing law : The various quantities of the same element
contained in equal volumes either of the free element or
of its compounds are all whole multiples of one and the
same quantity ; that is, each element has a special
PP. 330-1] Course of Chemical Philosophy. 13
numerical value by means of which and of integral co-
efficients the composition by weight of equal volumes
of the different substances in which it is contained
may be expressed. Now, since all chemical reactions
take place between equal volumes, or integral multiples
of them, it is possible to express all chemical reactions
by means of the same numerical values and integral
coefficients. The law enunciated in the form just
indicated is a direct deduction from the facts : but
who is not led to assume from this same law that the
weights of equal volumes represent the molecular
weights, although other proofs are wanting ? I thus
prefer to substitute in the expression of the law the
word molecule instead of volume. This is advan-
tageous for teaching, because, when the vapour
densities cannot be determined, recourse is had to
other means for deducing the weights of the molecules
of compounds. The whole substance of my course
consists in this : to prove the exactness of these latter
methods by showing that they lead to the same results
as the vapour density when both kinds of method can
be adopted at the same time for determining molecular
weights.
The law above enunciated, called by me the law of
atoms, contains in itself that of multiple proportions
and that of simple relations between the volumes ;
which I demonstrate amply in my lecture. After this
I easily succeed in explaining how, expressing by
symbols the different atomic weights of the various
elements, it is possible to express by means of formulae
the composition of their molecules and of those of
their compounds, and I pause a little to make my
pupils familiar with the passage from gaseous volume
to molecule, the first directly expressing the fact and
the second interpreting it. Above all, I study to
14 Cannizzaro. [pp. 331-2
implant in their minds thoroughly the difference
between molecule and atom. It is possible indeed to
know the atomic weight of an element without know-
ing its molecular weight ; this is seen in the case of
carbon. A great number of the compounds of this
substance being volatile, the weights of the molecules
and their composition may be compared, and it is
seen that the quantities of carbon which they contain
are all integral multiples of 12, which quantity is
thus the atom of carbon and expressed by the symbol
C ; but since we cannot determine the vapour density
of free carbon we have no means of knowing the
weight of its molecule, and thus we cannot know how
many times the atom is contained in it. Analogy
does not in any way help us, because we observe that
the molecules of the most closely analogous substances
(such as sulphur and oxygen), and even the molecules
of the same substance in its allotropic states, are
composed of different numbers of atoms. We have
no means of predicting the vapour density of carbon ;
the only thing that we can say is that it will be either
12 or an integral multiple of 12 (in my system of
numbers). The number which is given in different
treatises on chemistry as the theoretical density of
carbon is quite arbitrary, and a useless datum in
chemical calculations ; it is useless for calculating and
verifying the weights of the molecules of the various
compounds of carbon, because the weight of the
molecule of free carbon may be ignored if we know
the weights of the molecules of all its compounds ; it
is useless for determining the weight of the atom of
carbon, because this is deduced by comparing the
composition of a certain number of molecules contain-
ing carbon, and the knowledge of the weight of the
molecule of this last would scarcely add a datum more
PP. 332-3] Course of Chemical Philosophy.
15
to those which are already sufficient for the solution of
the problem. Any one will easily convince himself of
this by placing in the following manner the numbers
expressing the molecular weights derived from the
densities and the weights of the components contained
in them :
Weights
Names of Compounds
of Carbon.
of the
molecules
referred
to the
atom of
Weights of the components
of the molecules referred to
the weight of the atom of
Hydrogen taken as unity.
Formulsej
making
H= i
C=I2
O = i6
Hvdrogen.
S = 3 2
Carbonic Oxide
28
12 Carbon 1 6 Oxygen
CO
Acid
44
12 32
CO 2
Sulphide of Carbon
Marsh Gas .
76
16
12 64 Sulphur
12 4 Hydrogen
CS 2
CH 4
Ethylene
28
24 4
C 2 H 4
Propylene .
42
36 6
C 3 H 6
Ether .
74
(48 10 -1
\ 1 6 Oxygen /
C 4 H*>
etc.
etc.
etc.
etc.
In the list of molecules containing carbon there
might be placed also that of free carbon if the weight
of it were known ; but this would not have any greater
utility than what we would derive by writing in the
list one more compound of carbon ; that is, it would
do nothing but verify once more that the quantity of
carbon contained in any molecule, whether of the
element itself or of its compounds, is 12 or n x 12 = C w ,
n being an integral number.
I then discuss whether it is better to express the
composition of the molecules of compounds as a
function of the molecules of the components, or if, on
the other hand, it is better, as I commenced by doing,
to express the composition of both in terms of those
constant quantities which always enter by whole
numbers into both, that is, by means of the atoms.
16
Cannizzaro.
[PP. 333-4
Thus, for example, is it better to indicate in the
formula that one molecule of hydrochloric acid contains
the weight of half a molecule of hydrogen and half a
molecule of chlorine, or that it contains an atom of
one and an atom of the other, pointing out at the
same time that the molecules of both of these sub-
stances consist of two atoms ?
Should we adopt the formulae made with symbols
indicating the molecules of the elements, then many
coefficients of these symbols would be fractional, and
the formula of a compound would indicate directly the
ratio of the volumes occupied by the components and
by the compounds in the gaseous state. This was
proposed by Dumas in his classical memoir, Sur quelques
points de la Theorie atomique (Ann ales de Chimie et de
Physique, torn. 33, 1826).
To discuss the question proposed, I give to the
molecules of the elements symbols of a different kind
from those employed to represent the atoms, and in
this way I compare the formulae made with the two
kinds of symbols.
tx
Symbols of the
Symbols of
2
molecules of
the atoms of
y, f
Atoms or Molecules.
the Elements
and formulae
the Elements
and formulae
ll
made with
made with
w'E
these symbols.
these symbols
O ~
& ~
Atom of Hydrogen .
m
= H
I
Molecule of Hydrogen
m
-- H 2 =-.
2
Atom of Oxygen
i =--*&
^ O
16
Molecule of ordinary Oxygen
Molecule of electrised Oxygen
"
= O 2 =
32
(Ozone)
<&z
= o 8 =
128
Atom of Sulphur
*=*
= s
32
Molecule of Sulphur above 1000
(Bineau) ....
b
= s 2 =
64
Molecule of Sulphur below 1000
&*
= s 6
192
Water .
ii^-i&*4
= H 2 O =
18
Sulphuretted Hydrogen
3H$i = fe$a
= H 2 S =
34
PP. 334-5] Course of Chemical Philosophy. 17
These few examples are sufficient to demonstrate
the inconveniences associated with the formulae
indicating the composition of compound molecules
as a function of the entire component molecules,
which may be summed up as follows :
i. It is not possible to determine the weight of the
molecules of many elements the density of which
in the gaseous state cannot be ascertained.
2. If it is true that oxygen and sulphur have dif-
ferent densities in their different allotropic states, that
is, if they have different molecular weights, then their
compounds would have two or more formulas according
as the quantities of their components were referred to
the molecules of one or the other allotropic state.
3. The molecules of analogous substances (such as
sulphur and oxygen) being composed of different
numbers of atoms, the formulas of analogous com-
pounds would be dissimilar. If we indicate, instead,
the composition of the molecules by means of the
atoms, it is seen that analogous compounds contain in
their molecules an equal number of atoms.
It is true that when we employ in the formulas the
symbols expressing the weights of the molecules, z>.,
of equal volumes, the relationship between the volumes
of the components and those of the compounds follows
directly ; but this relationship is also indicated in the
formulas expressing the number of atoms ; it is suffi-
cient to bear in mind that the atom represented by a
symbol is either the entire molecule of the free
substance or a fraction of it, that is, it is sufficient to
know the atomic formula of the free molecule. Thus, to
take an example, it is sufficient to know that the atom
of oxygen, O, is one-half of the molecule of ordinary
oxygen and an eighth part of the molecule of electrised
oxygen to know that the weight of the atom of
B
18 Canniszaro. [PP. 335-6
oxygen is represented by J volume of free oxygen and
\ of electrised oxygen. In short, it is easy to accustom
students to consider the weights of the atoms as being
represented either by a whole volume or by a fraction
of a volume, according as the atom is equal to the
whole molecule or to a fraction of it. In this system
of formulae, those which represent the weights and the
composition of the molecules, whether of elements
or of compounds, represent the weights and the
composition of equal gaseous volumes under the same
conditions. The atom of each element is represented
by that quantity of it which constantly enters as a
whole into equal volumes of the free substance or of
its compounds ; it may be either the entire quantity
contained in one volume of the free substance or a
simple sub-multiple of this quantity.
This foundation of the atomic theory having been
laid, I begin in the following lecture the sixth to
examine the constitution of the molecules of the
chlorides, bromides, and iodides. Since the greater
part of these are volatile, and since we know their
densities in the gaseous state, there cannot remain any
doubt as to the approximate weights of the molecules,,
and so of the quantities of chlorine, bromine, and
iodine contained in them. These quantities being
always integral multiples of the weights of chlorine,
bromine, and iodine contained in hydrochloric, hydro-
bromic, and hydriodic acids, i.e., of the weights of the
half molecules, there can remain no doubt as to the
atomic weights of these substances, and thus as to the
number of atoms existing in the molecules of their com-
pounds, whose weights and composition are known.
A difficulty sometimes appears in deciding whether
the quantity of the other element combined with one
atom of these halogens is i, 2, 3, or ;/ atoms in the
P. 336] Course of Chemical Philosophy.
19
molecule ; to decide this, it is necessary to compare
the composition of all the other molecules containing
the same element and find out the weight of this
element which constantly enters as a whole. When
we cannot determine the vapour densities of the other
compounds of the element whose atomic weight we
wish to determine, it is necessary then to have recourse
to other criteria to know the weights of their molecules
and to deduce the weight of the atom of the element.
What I am to expound in the sequel serves to teach
my pupils the method of employing these other
criteria to verify or to determine atomic weights and
the composition of molecules. I begin by making
them study the following table of some chlorides,
bromides, and iodides whose vapour densities are
known ; I write their formulae, certain of justifying
later the value assigned to the atomic weights of some
elements existing in the compounds indicated. I do
not omit to draw their attention once more to the
atomic weights of hydrogen, chlorine, bromine, and
iodine being all equal to the weights of half a molecule,
and represented by the weight of half a volume, which
I indicate in the following table :
Symbol.
Weight.
Weight of the atom of Hydrogen or half a mole-
cule represented by the weight of \ volume .
Weight of the atom of Chlorine or half a mole-
cule represented by the weight of \ volume .
Weight of the atom of Bromine or half a mole-
cule represented by the weight of | volume .
Weight of the atom of Iodine or half a mole-
cule represented by the weight of \ volume .
H
Cl
Br
I
I
35-5
80
127
These data being given, there follows the table of
some compounds of the halogens :
20
Cannizzaro.
[p. 337
13 c o> a" 1 - <" <u .2
> Is i^oolgooool
ffl ffi g g<^c^H^^<^cJ
.2
^ fcfl-M >
ill-
w <n C >L O .
&a^|
O "X. oj
-i 0^-3
"Si
J4{lflpjj!l|l
ffi
M r^vo oo a\ rt- N O
OO
% a
a
i'l
'
.
PQ O
-'
S fes
o ewto
PP. 337-8] Course of Chemical Philosophy. 21
I stop to examine the composition of the molecules
of the two chlorides and the two iodides of mercury.
There can remain no doubt that the protochloride
contains in its molecule the same quantity of chlorine
as hydrochloric acid, that the bichloride contains
twice as much, and that the quantity of mercury
contained in the molecules of both is the same. The
supposition made by some chemists that the quantities
of chlorine contained in the two molecules are equal,
and on the other hand that the quantities of mercury
are different, is supported by no valid reason. The
vapour densities of the two chlorides having been
determined, and it having been observed that equal
volumes of them contain the same quantity of
mercury, and that the quantity of chlorine contained
in one volume of the vapour of calomel is equal to
that contained in the same volume of hydrochloric
acid gas under the same conditions, whilst the quantity
of chlorine contained in one volume of corrosive
sublimate is twice that contained in an equal volume
of calomel or of hydrochloric acid gas, the relative
molecular composition of the two chlorides cannot be
doubtful. The same may be said of the two iodides.
Does the constant quantity of mercury existing in
the molecules of these compounds, and represented by
the number 200, correspond to one or more atoms ?
The observation that in these compounds the same
quantity of mercury is combined with one or two
atoms of chlorine or of iodine, would itself incline us
to believe that this quantity is that which enters
always as a whole into all the molecules containing
mercury, namely, the atom ; whence Hg = 200.
To verify this, it would be necessary to compare the
various quantities of mercury contained in all the
molecules of its compounds whose weights and
22
Cannizzaro.
"PP. 338-9
composition are known with certainty. Few other
compounds of mercury besides those indicated above
lend themselves to this ; still there are some in
organic chemistry the formulae of which express well
the molecular composition ; in these formulae we
always find Hg 2 = 200, chemists having made Hg= TOO
andH=i. This is a confirmation that the atom of
mercury is 200 and not 100, no compound of mercury
existing whose molecule contains less than this
quantity of it. For verification I refer to the law
of the specific heats of elements and of compounds.
I call the quantity of heat consumed by the atoms
or the molecules the product of their weights into
their specific heats. I compare the heat consumed by
the atom of mercury with that consumed by the
atoms of iodine and of bromine in the same physical
state, and find them almost equal, which confirms the
accuracy of the relation between the atomic weight of
mercury and that of each of the two halogens, and
thus also, indirectly, between the atomic weight of
mercury and that of hydrogen, whose specific heats
cannot be directly compared.
Thus we have
Name of
Substance.
Atomic
weight.
Specific heat,
z.., heat required
to heat unit
weight i.
Products of specific
heats by atomic
weights, i.e., heat
required to heat the
atom i.
Solid Bromine .
Iodine
Solid Mercury .
80
127
200
0-08432
0-05412
0-03241
6-74560
6-87324
6-48200
The same thing is shown by comparing the specific
heats of the different compounds of mercury.
Woestyn and Gamier have shown that the state
of combination does not notably change the calorific
PP. 339-40] Course of Chemical Philosophy. 23
capacity of the atoms ; and since this is almost equal
in the various elements, the molecules would require,
to heat them 1, quantities of heat proportional to the
number of atoms which they contain. If Hg = 2OO,
that is, if the formulae of the two chlorides and
iodides of mercury are HgCl, Hgl, HgCl 2 , Hgl 2 , it will
be necessary that the molecules of the first pair should
consume twice as much heat as each separate atom,
and those of the second pair three times as much ;
and this is so in fact, as may be seen in the following
table :
Formulae
of the
Weights of
Specific
heats of
Specific
heats of
Number
of atoms
Specific
heats of
compounds
of
molecules
unit
weight
the
molecules
in the
molecules
each atom
X
Mercury.
P-
= c.
=pxc.
= .
n
HgCl .
235-5
O-O52O5
12-257745
2
6-128872
Hgl .
327
0-03949
12-91323
2
6-45661
HgCl* .
271
0-06889
18-66919
3
6-22306
Hgl 2 .
454
0-04197
19-05438
3
6-35146
Thus the weight 200 of mercury, whether as an
element or in its compounds, requires to heat it i the
same quantity of heat as 127 of iodine, 80 of bromine,
and almost certainly as 35.5 of chlorine and I of
hydrogen, if it were possible to compare these two
last substances in the same physical state as that in
which the specific heats of the above-named substances
have been compared.
But the atoms of hydrogen, iodine, and bromine are
half their respective molecules : thus it is natural to
ask if the weight 200 of mercury also corresponds to
half a molecule of free mercury. It is sufficient to
look at the table of numbers expressing the molecular
weights to perceive that if 2 is the molecular weight
of hydrogen, the weight of the molecule of mercury is
24 Cannizzaro. [pp. 340-1
200, i.e. } equal to the weight of the atom. In other
words, one volume of vapour, whether of protochloride
or protoiodide, whether of bichloride or of biniodide,
contains an equal volume of mercury vapour ; so that
each molecule of these compounds contains an entire
molecule of mercury, which, entering as a whole into
all the molecules, is the atom of this substance. This
is confirmed by observing that the complete molecule
of mercury requires for heating it i, the same quantity
of heat as half a molecule of iodine, or half a molecule
of bromine. It appears to me, then, that I can sustain
that what enters into chemical actions is the half
molecule of hydrogen and the whole molecule of
mercury : both of these quantities are indivisible, at
least in the sphere of chemical actions actually known.
You will perceive that with this last expression I avoid
the question if it is possible to divide this quantity
further. I do not fail to apprise you that all those
who faithfully applied the theory of Avogadro and
of Ampere, have arrived at this same result. First
Dumas and afterwards Gaudin showed that the
molecule of mercury, differing from that of hydrogen,
always entered as a whole into compounds. On this
account Gaudin called the molecule of mercury mon-
atomic, and that of hydrogen biatomic. However, I
wish to avoid the use of these adjectives in this special
sense, because to-day they are employed as you know
in a very different sense, that is, to indicate the
different capacity for saturation of the radicals.
The formulae of the two chlorides of mercury having
been demonstrated, I next compare them with that of
hydrochloric acid. The atomic formulae indicate that
the constitution of the protochloride is similar to
that of hydrochloric acid, if we consider the number
of atoms existing in the molecules of the two ; if,
PP. 341-2] Course of Chemical Philosophy.
25
however, we compare the quantities of the components
with those which exist in their free molecules, then a
difference is perceived. To make this evident I bring
the atomic formulae of the various molecules under
examination into comparison with the formulae made
with the symbols expressing the weights of the entire
molecules, placing them in the manner which you see
below :
Symbols of the
u
molecules of the
Symbols of
5 *
elements and
the atoms
&
formulae of their
of the
'Z'u
compounds made
elements,
lit
with these symbols,
and formulae
&c
i.e., symbols and
of their
oj ^
formulae represent-
compounds
sl
ing the weights of
made
equal volumes in
with these
"B |
the gaseous
symbols.
!z
state.
Atom of Hydrogen .
m
H =
I
Molecule of Hydrogen
$1
H 2 =
2
Atom of Chlorine
l&
Cl =
35-5
Molecule of Chlorine
cr
Cl 2 -
7i
Atom of Bromine
i3fi =
Br =r
80
Molecule of Bromine
45 1
Br 2 =
160
Atom of Iodine
ji
I =
127
Molecule of Iodine .
I
I 2
254
Atom of Mercury
3^3
Hg =
200
Molecule of Mercury
,, Hydrochloric Acid
ft* i =
H| =
HC1 =
200
36-5
,, Hydrobroruic Acid
i^ij^ti =
HBr =
8(
,, Hydriodic Acid
p^iji"
HI =
128
Mol. of protochloride of Mercury
fflQ(.[l -
HgCl =
235-5
, , pr otobromide of Mercury
,, protoiodide of Mercury .
,, bichloride of Mercury .
%*!? =
HgBr -
Hgl -
HgCl 2 =
280
327
271
,, bibromide of Mercury .
Insist
HgBr 2 -
360
,, biniodide of Mercury
*" =
454
The comparison of these formulae confirms still
more the preference which we must give to the
atomic formulae, which indicate also clearly the
26
Cannizzaro.
[P. 342
relations between the gaseous bodies. It is sufficient
to recall that whilst the atoms of chlorine, bromine,
iodine, and hydrogen are represented by the weight
of | volume, the atom of mercury is represented by
the weight of a whole volume.
I then come to the examination of the two chlorides
of copper. The analogy with those of mercury forces
us to admit that they have a similar atomic consti-
tution, but we cannot verify this directly by determin-
ing and comparing the weights and the compositions
of the molecules, as we do not know the vapour
densities of these two compounds.
The specific heats of free copper and of its com-
pounds confirm the atomic constitution of the two
chlorides of copper deduced from the analogy with
those of mercury. Indeed the composition of the
two chlorides leads us to conclude that if they have
the formulae CuCl, CuCl 2 , the atomic weight of copper
indicated by Cu is equal to 63, which may be seen
from the following proportions :
Ratio between the
components expressed
by numbers whose
sum = 100.
Ratio between the
components
expressed by
atomic weights.
Protochloride of Copper .
Bichloride of Copper
36-04 : 63-96
Chlorine. Copper.
52-98 : 47-02
Chlorine. Copper.
35-5 : 63
Cl. Cu.
71 : 63
CR Cu.
Now 63 multiplied by the specific heat of copper
gives a product practically equal to that given by the
atomic weight of iodine or of mercury into their
respective specific heats. Thus :
63 x 0-09515 = 6
Atomic weight Specific heat
of copper. of copper.
pp. 342-3] Course of Chemical Philosophy. 27
The same quantity of heat is required to heat the
weight of 63 of copper in its compounds through i.
Thus :
Formulas
of the
compounds
Weights of
their
molecules
Specific
heats of
unit weights
Specific
heats of the
molecules
Number of
atoms in the
molecules
Specific
heat of
each atom
of Copper.
/
= c.
=pxc.
= n.
= - -.
n
CuCl .
98-5
0-13817
I3-6I9595
2
6-809797
Cul .
I 9
0-06869
I4-05II
2
7-0255
After this comes the question, whether this quantity
of copper which enters as a whole into the compounds,
the calorific capacity of the atoms being maintained,
is an entire molecule or a sub-multiple of it. The
analogy of the compounds of copper with those of
mercury would make us inclined to believe that the
atom of copper is a complete molecule. But having
no other proof to confirm this, I prefer to declare that
there is no means of knowing the molecular weight of
free copper until the vapour density of this substance
can be determined.
I then go on to examine the constitution of the
chlorides, bromides, and iodides of potassium, sodium,
lithium, and silver. Each of these metals makes with
each of the halogens only one well characterised and
definite compound ; of none of these compounds is the
vapour density known ; we are therefore in want of
the direct means of discovering if in their molecules
there are one, two, or more atoms of the halogens.
But their analogies with the protochloride of mercury,
HgCl, and with the protochloride of copper, CuCl,
and the specific heats of the free metals and of their
compounds make us assume that in the molecules of
each of these compounds there is one atom of metal
28
Cannizzaro.
[PP. 343-4
and one of halogen. According to this supposition, the
atomic weight of potassium K = 39, that of sodium
Na = 23, that of silver Ag=io8. These numbers
multiplied by the respective specific heats give the
same product as the atomic weights of the substances
previously examined.
Name of Substance.
Atomic weight
=P>
Specific heats of
unit weight = 5.
Specific heats of
the atoms p X c.
Solid Bromine
80
0-08432
6-74560
Iodine
127
0-054I2
6-87324
Solid Mercury
200
003241
6-48200
Copper
63
0-095IS
6
Potassium .
39
0-169556
6-612684
Sodium
23
0-2934
6-7482
Silver
108
O-O57OI
6-15708
Besides this, the specific heats of the chlorides,
bromides, and iodides of these metals confirm the
view that their molecules contain the same number
of atoms of the two components. Thus :
Formulae and
Names of the
compounds.
Weights
of their
molecules
=/
Specific
heats of
unit weight
Specific
heats of the
molecules
=pxc.
No. of
atoms
in the
mole-
cules n
Specific
heat of
each atom
=*.
KC1
74-5
0-17295
12-884775
2
6-442387
Chi. of Potassium.
NaCl
58-5
0-21401
12-519585
2
6-259792
Chi. of Sodium.
AgCl
H3-5
0-09109
13-071415
2
6-535707
Chi. of Silver.
KBr
119
0-11321
13-47318
2
6-73659
Brom. of Potassium
NaBr
103
0-13842
14-25726
2
7-12863
Brom. of Sodium.
AgBr .
188
0-07391
13-89508
2
6-94754
Brom. of Silver.
KI .
166
0-08191
13.59706
2
6-79853
lod. of Potassium.
Nal .
ItO
0.08684
13-0260
2
6-5130
Iodide of Sodium.
Agl. .
235
0-06159
14-47365
2
7-23682
Iodide of Silver.
p. 345] Course of Chemical Philosophy. 29
Are the atoms of potassium, sodium, lithium, and
silver equal to \ molecule, like that of hydrogen, or
equal to a whole molecule, like that of mercury ? As
the vapour densities of these elements are wanting, we
cannot answer the question directly ; I will give you
later some reasons which incline me to believe that the
molecules of these elements, like that of hydrogen, are
composed of two atoms.
Gold makes with each of the halogens two com-
pounds. I show that the first chloride is analogous to
calomel, i.e., that it has AuCl as it formula. The
atomic weight of gold deduced from the composition
of the protochloride to which this formula is given
corresponds to the law of specific heats, as may be
seen from what follows :
196-32 x 0-03244 = 6-3696208
Au Specific heat
of Gold.
I show in the sequel that the first or only chlorides
of the following metals have a constitution similar to
the bichloride of mercury and of that of copper, that
is, for each atom of metal they contain two atoms of
chlorine.
Not knowing the density in the gaseous state of
these lower or only chlorides, we cannot show directly
the quantity of chlorine existing in their molecules,
yet the specific heats of these free metals and of
their compounds show what I have said above. I
write the quantities of these different elements
combined with the weight of two atoms of chlo-
rine in the lower or only chlorides, and confirm in
these quantities the properties of the other atoms ;
I write the formulae of the lower chlorides, bro-
mides, and iodides all as MCI 2 , and verify that they
30
Cannizzdro.
[PP. 345-6
correspond to the laws of specific heats of compound
substances.
Names of
Substances.
Symbols and
weights of the
atoms.
Specific heats
of
unit weight.
Specific heats
of the atoms.
Iodine
I = 127
0-054I2
6-87324
Solid Mercury
Hg = 200
0-03241
6-48200
Copper .
Cu = 63
0-09515
6
Zinc
Zn = 66
0-09555
6-30630
Lead .
Pb = 207
0-03I4
6-4998
Iron
Fe = 56
0-II379
6-37224
Manganese
Mn = 55
0-1181
6-4955
Tin
Sn = 117-6
0-05623
6-612648
Platinum
Pt = 197
0-03243
6-38871
Calcium .
Ca = 40
Magnesium
Mg = 24
Barium .
Ba - 137
Formulae
of the
compounds.
Weights
of their
molecules
=P-
Specific
heats of
unit weight
c.
Specific
heats of the
molecules
=px.c.
No. of
atoms in
the
molecules
= n.
Specific
heat of
each atom
_fXC
n
HgCl 2
271
0-06889
18-66919
3
6-22306
ZnCl 2
134
0-13618
18-65666
3
6-21888
SnCl 2
ifeS-6
0-10161
19-163646
3
6-387882
MnCl 2
126
0-14255
I7-96I30
3
5-987IO
PbCl 2
278
0-06641
18-46198
3
6-15399
MgCl 3
95
0-1946
18-4870
3
6-1623
CaCl 2
in
0-1642
l8'2262
3
6-0754
BaCl 2
208
0-08957
18-63056
3
6-2IOI8
Hgl 2
Pbl a
454
461
0-04197
0-04267
19-05438
19-67087
3
3
6-35^6
6-55695
Some of the metals indicated above make other
compounds with chlorine, bromine, and iodine, whose
molecular weights may be determined and composi-
tions compared ; in such cases the values found for the
atomic weights are confirmed. Thus, for example, a
P. 347] Course of Chemical Philosophy. 31
molecule of perchloride of tin weighs 259.6, and
contains 117.6 of tin ( = Sn) and 142 of chlorine
( = Cl 4 ). A molecule of perchloride of iron weighs 325,
and contains 112 of iron ( = Fe 2 ) and 213 of chlorine
For zinc there are some volatile compounds which
confirm the atomic weight fixed by me. Chemists
believing chloride of zinc to be of the same type as
hydrochloric acid, made the atom of zinc Zn = 33, that
is half of that adopted by me ; having then prepared
some compounds of zinc with the alcohol radicals,
they were astonished that, expressing the composition
by formulae corresponding to gaseous volumes equal
to those of other well-known compounds, it was
necessary to express the quantity of zinc contained in
the molecule by Zn 2 . This is a necessary consequence
of the quantity of zinc represented by other chemists
by Zn 2 being only a single atom, which is equivalent
in its saturation capacity to two atoms of hydrogen.
Since in the sequel of my lectures I return to this
argument, you will therefore find it spoken of later
in this abstract.
Are the atoms of all these metals equal to their
molecules or to a simple sub-multiple of them ? I gave
you above the reasons which make me think it prob-
able that the molecules of these metals are similar to
that of mercury ; but I warn you now that I do not
believe my reasons to be of such value as to lead to
that certainty which their vapour densities would give
us if we only knew them.
Reviewing what I show in the lecture of which I
have given you an abstract, we find it amounts to the
following : Not all the lower chlorides corresponding
to the oxide with one atom of oxygen have the same
constitution ; some of them contain a single atom
32 Cannizzaro. [PP. 347-8
of chlorine, others two, as may be seen in the follow-
ing list :
HC1 HgCl CuCl KC1 NaCl LiCl AgCl AuCl
Hydro- Proto- Proto- Chloride Chloride Chloride Chloride Prolo-
chloric chloride chloride of of of of chloride
acid. of of potassium, sodium, lithium. silver. of gold,
mercury, copper.
HgCl 2 CuCl 2 ZnCl 2 PbCl 2 CaCl 2 SnCl 2 PtCl 2 etc. etc.
Bichloride Bichloride Chloride Chloride Chloride Proto- Proto-
of of of of of chloride chloride of
mercury, copper. zinc. lead. calcium, of tin. platinum.
Regnault, having determined the specific heats of the
metals and of many of their compounds, had observed
that it was necessary to modify the atomic weights
attributed to them, namely, to divide by 2 those of
potassium, sodium, and silver, leaving the others
unaltered ; or, vice versa, to multiply these latter by
2, leaving unaltered those of potassium, sodium, silver,
and hydrogen. From this he drew the conclusion
that the chlorides of potassium, sodium, and silver,
are analogous to calomel (protochloride of mercury)
and to protochloride of copper : on the other hand,
that those of zinc, lead, calcium, etc., etc., are
analogous to corrosive sublimate and to bichloride
of copper ; but he supposed that the molecules of
calomel and of the analogous chlorides all contained
2 atoms of metal and 2 of chlorine, whilst the
molecules of corrosive sublimate and the other ana-
logous chlorides contained i atom of metal and 2 of
chlorine. Here follows the list of the formulae pro-
posed by Regnault.
H 2 C1 2 Hg 2 Cl 2 Cu 2 Cl 2 K 2 C1 2 Na 2 Cl 2 Li 2 Cl 2 Ag 2 Cl 2 Au 2 CI 2
Hydro- Proto- Proto- Chloride Chloride Chloride Chloride Proto-
chloric chloride of chloride of of of of chloride
acid. mercury, of copper, potassium, sodium, lithium, silver, of gold.
HgCl 2 CuCl 2 ZnCl 2 PbCl 2 CaCl 2 etc. etc.
Bichloride Bichloride Chloride Chloride Chloride
of of of zinc. of lead. of
mercury, copper. calcium.
PP. 348-9] Course of Chemical Philosophy. 33
In truth, using the data for specific heats alone, it
is not possible to decide whether the molecules of the
chlorides written in the first horizontal line are MCI
or M' 2 C1 2 ; the only thing that can be said is that they
contain the same number of atoms of metal and of
chlorine. But knowing the densities in the gaseous
state of hydrochloric acid and of the two chlorides of
mercury, and thus the weights of their molecules, we
can compare their composition and decide the ques-
tion ; and I have already explained to you how I
show to my pupils that the molecules of the two
chlorides of mercury contain the same weight of
mercury, and that the molecule of one of them con-
tains the same quantity of chlorine as hydrochloric
acid, i.e., J molecule of free chlorine, whilst the
molecule of the other chloride contains twice as much.
This shows with certainty that the two formulae
Hg 2 Cl 2 , HgCl 2 are inexact, because they indicate that
in the molecules of the two chlorides there is the
same quantity of chlorine and different quantities of
mercury, which is precisely the opposite of what is
shown by the vapour densities. The formulae pro-
posed by me harmonise the results furnished by the
specific heats and by the gaseous densities.
Now I wish to direct your attention to an incon-
sistency of Gerhardt. From the theory of Avogadro,
Ampere, and Dumas, that is, from the comparison of
the gaseous densities as representing the molecular
weights, Gerhardt drew arguments in support of the
view that the atoms of hydrogen, of chlorine, and of
oxygen are half molecules ; that the molecule of water
contains twice as much hydrogen as that of hydrochloric
acid ; that in the molecule of ether there is twice as
much of the radical ethyl as in that of alcohol ; and that
to form one molecule of anhydrous monobasic acid two
C
34 Cannizzaro. [pp. 349-50
molecules of hydrated acid must come together : and
yet Gerhardt did not extend to the whole of chemistry
the theory of Ampere, but arbitrarily, in opposition to
its precepts, assumed that the molecules of chloride
of potassium, of bichloride of mercury, in fact of all the
chlorides corresponding to the protoxides, had the
same atomic constitution as hydrochloric acid, and
that the atoms of all the metals were, like that of
hydrogen, a simple sub-multiple of the molecule.
I have already explained to you the reasons which
show the contrary.
After having demonstrated the constitution of the
chlorides corresponding to the oxides containing one
atom of oxygen, I postpone the study of the other
chlorides to another lecture, and now define what I
mean by capacity for saturation of the various metallic
radicals.
If we compare the constitution of the two kinds of
chlorides, we observe that one atom of metal is now
combined with one atom of chlorine, now with two \
1 express this by saying that in the first case the atom
of metal is equivalent to I of hydrogen, in the
second case to 2. Thus, for example, the atom of
mercury, as it is in calomel, is equivalent to i of
hydrogen, whereas in corrosive sublimate it is equiva-
lent to 2 ; the atoms of potassium, sodium, and silver
are equivalent to i of hydrogen : the atoms of zinc,
lead, magnesium, calcium, etc., to 2. Now it is seen
from the study of all chemical actions that the number
of atoms of the various substances which combine
with one and the same quantity of chlorine combine
also with one and the same quantity of oxygen, of
sulphur,, or of any other substance, and vice versa.
Thus, for example, if the same quantity of chlorine
which combines with a single atom of zinc, or lead,
P. 350] Course of Chemical Philosophy. 35
or calcium combines with 2 atoms of hydrogen, of
potassium, or of sodium, then the same quantity of
oxygen or of any other substance which combines
with a single atom of the first will combine with two
of the second. This shows that the property pos-
sessed by the first atoms of being equivalent to 2 of
the second depends on some cause inherent either in
their own nature or in the state in which they are
placed before combining. We express this constant
equivalence by saying that each atom of the first has
a saturation capacity twice that of each of the second.
These expressions are not new to science, and we now
only extend them from compounds of the second order
to those of the first order.
For the same reasons given by chemists when they
say that phosphoric acid assumes various saturation
capacities without changing in composition, it may
also be said that the atom of mercury and that of
copper assume different saturation capacities accord-
ing as they are found in the protochlorides or in the
bichlorides. Thus, I express the fact that the atoms
of these two metals being equivalent to I atom of
hydrogen in the protochlorides, tend, in double decom-
positions, to take the place of a single atom of hydrogen,
whilst in the bichlorides they tend to take the place
of 2 atoms of hydrogen. For the same reason that
we say there are three different modifications of
phosphoric acid combined with various bases, we may
also say that there are two different modifications
of the same radical mercury or copper. I call the
radicals of the protochlorides and of the corresponding
salts, mercurous and cuprous ; those of the bichlorides
and of the corresponding salts are called mercuric and
cupric radicals.
To express the various saturation capacities of the
36 Cannizzaro. [pp. 350-1
different radicals, I compare them to that of hydrogen
or of the halogens, according as they are electro-
positive or electro-negative. An atom of hydrogen
is saturated by one of a halogen, and vice versa. I
express this by saying that the first is a monatomic
electro-positive radical, and the second a monatomic
electro - negative radical : thus, potassium, sodium,
lithium, silver, and the mercurous and cuprous radicals
are monatomic electro-positive radicals. The biatomic
radicals are those which, not being divisible, are
equivalent to 2 of hydrogen or to 2 of chlorine ;
among the electro - positive radicals there are the
metallic radicals of the mercuric and cupric salts, of
the salts of zinc, lead, magnesium, calcium, etc., and
amongst the electro-negative we have oxygen, sulphur,
selenium, and tellurium, i.e., the amphidic substances.
There are, besides, radicals which are equivalent to
three or more atoms of hydrogen or of chlorine, but I
postpone the study of these until later.
Before finishing the lecture I take care to make clear
that the law of equivalents must be considered as a
law distinct from the law of atoms.
The latter in fact only says that the quantities of the
same element contained in different molecules must
be integral multiples of one and the same quantity,
but it does not predict, for example, that an atom of
zinc is equivalent to 2 of hydrogen not only in its
compounds with chlorine, but in all other compounds
in which they may replace each other. These con-
stant relations between the numbers of atoms of
various substances which displace one another, what-
ever may be the nature and the number of the other
components, is a law which restricts the number of
possible combinations, and sums up with greater
definiteness all the cases of double decomposition.
pp. 351-2] Course of Chemical Philosophy. 37
I occupy the whole of the seventh lecture in study-
ing some monatomic and biatomic radicals, namely,
cyanogen and the alcohol radicals.
I have already told you the method which I faith-
fully follow for ascertaining the weights and numbers
of the molecules of the various substances whose
vapour densities can be determined. This method,
applied to all the substances which contain alcohol
radicals, permits us r so to speak, to follow the path
from one molecule to another. To discover the
saturation capacity of a radical, it is expedient to
begin with the examination of a molecule in which
it is combined with a monatomic radical : thus for
electro-negative radicals I begin by examining the
compounds with hydrogen or with any other mon-
atomic electro-positive radical ; and conversely, for
the electro-positive radicals, I examine their com-
pounds with chlorine, bromine, and iodine. Those
electro-negative radicals which form a molecule with
a single atom of hydrogen are monatomic ; those
which combine with 2 of hydrogen are biatomic, and
so on. Conversely, the electro-positive radicals are
monatomic if they combine with a single atom of
halogen, biatomic if they combine with 2.
With these rules I establish
i. That cyanogen, CN, is a monatomic electro-
negative radical, and that the molecule of free cyanogen
contains twice the quantity of carbon and nitrogen
contained in the molecule of the monocyanides ; and
that in this way cyanogen, CN, behaves in all respects
like an atom of chlorine, Cl ;
2. That cacodyle, C 2 H 6 As, methyl, CH 3 , ethyl, C 2 H 5 r
and the other homologous and isologous radicals, are,
like the atom of hydrogen, monatomic, and like it
cannot form a molecule alone, but must associate
38 Cannizzaro. [PP. 352-3
themselves with another monatomic radical, simple
or compound, whether of the same or of a different
kind ;
3. That ethylene, C 2 H 4 , propylene, C 3 H 6 , are bi-
atomic radicals analogous to the radicals of mercuric
and cupric salts, and to those of the salts of zinc, lead,
calcium, magnesium, etc. ; and that these radicals, like
the atom of mercury, can form a molecule by them-
selves.
The analogy between the mercuric salts and those
of ethylene and propylene has not been noted, so far
as I know, by any other chemist. All that I have
expounded previously shows it with such clearness
that it appears useless to stop and discuss it with you
at length. In fact, just as I volume of the vapour
of mercury, combining with an equal volume of
chlorine, makes I volume of vapour of mercury
bichloride, so i volume of ethylene combined with
an equal volume of chlorine makes a single volume of
vapour of chloride of ethylene (oil of Dutch chemists).
If the formula of this last is C 2 H 4 C1 2 , that of bichlo-
ride of mercury should be HgCl 2 ; and if this is the
formula of the bichloride of mercury, the chlorides of
zinc, lead, calcium, etc., must also be MCI 2 ; that is,
the atoms of all these metals are, like ethylene and
propylene, biatomic radicals. Observing that all the
electro - positive monatomic radicals which can be
weighed free in the gaseous state, behave like hy-
drogen, that is, cannot of themselves form molecules,
it appears to me very probable that a capacity of satu-
ration equal to that of hydrogen in atoms, or groups
which can act as their substitutes, constantly coincides
with the fact of their not being able to exist in the
isolated state. This is the reason why, until there is
proof to the contrary, I believe that the molecules of
PP. 353] Course of Chemical Philosophy. 39
potassium, sodium, lithium, and silver in the free state
are formed of two atoms, that is, are represented by the
formulae K 2 , Na 2 , Li 2 , Ag 2 .
Conversely, observing that if the atom of mercury
(which tends to form a biatomic rather than a mon-
atomic radical) like ethylene and propylene can exist
in the free state, forming a distinct molecule by itself,
it appears to me probable that the atoms of zinc,
lead, and calcium should be endowed also with this
property, that is, that the molecules of these metals
should consist of a single atom. If this correspond-
ence between the number of atoms contained in the
molecule and the capacity of the saturation of the
atom, or of the group which takes its place, is verified,
we may sum up as follows : the metallic radicals whose
molecules enter as a whole into compounds are biatomic,
those whose atom is half a molecule are monatomic.
You already perceive the importance of this correlation,
which forces us to conclude that one molecule of
mercury (in mercuric salts), or of zinc, or ethylene, or
propylene, etc., is equivalent to a molecule of hydrogen,
of potassium, or of silver ; thus the former as well as
the latter combines with an entire molecule of chlorine,
yet with this important difference that the former, not
being capable of division, forms a single molecule with
two atoms of chlorine, whilst the latter, being divisible,
makes with the two atoms of chlorine two distinct
molecules. But before drawing a general conclusion
of such importance, it is necessary to demonstrate
somewhat better the accuracy of the data on which
it is founded.
In the eighth lecture I begin to compare the mode
of behaviour in some reactions of monatomic and
biatomic radicals. The compound radicals indicated
in the preceding lecture, since they form volatile com-
40 Cannizzaro. [PP. 353-4
pounds, frequently afford the means of explaining
by analogy what holds good for metallic compounds,
the molecular weights of which cannot often be
determined directly, since few of them are vola-
tile. This is the great benefit which the study
of organic chemistry has rendered to chemistry in
general.
In the use of formulae I adhere to the following
rules, which I state before representing by means of
equations the various types of reaction :
i. I use the coefficients of the symbols in the
position of the exponents only when I wish to express
that the number of atoms indicated is contained in
one and the same molecule ; in other cases I place
the coefficient before the symbols. Thus, when I wish
to indicate two atoms of free hydrogen as they are
contained in a single molecule, I write H 2 . If, however,
I wish to indicate four atoms as they are contained in
two molecules, I do not write H 4 but 2H 2 ; for the
same reason I indicate n atoms of free mercury by the
formula ?/Hg.
2. Sometimes I repeat in the same formula more
than once the same symbol to indicate some difference
between one part and another of the same element.
Thus I write acetic acid C 2 H 3 HO 2 , to indicate that one
of the four atoms of hydrogen contained in the mole-
cule is in a state different from the other three, it
alone being replaceable by metals. Occasionally I
write the same symbol several times to indicate several
atoms of the same element, only to place better in
relief what occurs in some reactions.
3. For this last reason I often write the various
atoms of the same component or the residues of
various equal molecules in vertical lines. Thus, for
example, I indicate the molecule of bichloride of
PP. 354-5] Course of Chemical Philosophy. 41
f Cl
mercury, HgCl 2 , as follows: Hg \ p^; the mole-
cule of acetate of mercury, C 4 H 6 HgO 4 , as follows :
f C 2 H 3 O 2
Hg -j r2fj3Q2 1 to indicate that the two atoms of
chlorine or the two residues of acetic acid come
from two distinct molecules of hydrochloric acid and
of hydrated acetic acid.
4.' I indicate by the symbol R u \ any monatomic
metallic radical whether simple or compound ; and
with the symbol Rji any biatomic metallic radical.
If in the same formula or in the same equation I
wish to indicate in general two or more monatomic
radicals, the one different from the other, I add to the
symbol the small letters , b, c, etc., thus R^, R^
indicates a single molecule formed of two different
monatomic radicals ; such are the so-called mixed
radicals.
The molecules of the monatomic metallic radicals
are represented by the formula (R,i) 2 ; those of the
biatomic radicals by the same symbol as for the
radical existing in its compounds, since it is the char-
acter of these radicals to have the molecule formed of
a single atom or of a single group which takes its
place. You understand that in speaking of metallic
radicals I include all those which can replace metals
in saline compounds.
5. Since all compounds containing in their mole-
cule a single atom of hydrogen replaceable by metals
behave similarly when they act on metals or on their
compounds, it is convenient to adopt a general formula,
and I shall use the following. In HX, X indicates all
that there is in the molecule except metallic hydro-
gen ; thus, for example, in the case of acetic acid,
X = C 2 H 3 O 2 , these being the components which to-
42 Cannizzaro. [PP. 355-6
gether with H make up the molecule of hydrated
acetic acid. Since there are compounds, also called
acids, whose molecules contain two atoms of hydrogen
replaceable by metals, and since owing to this last
fact they behave in a similar manner towards mole-
cules containing metals, I adopt for them the general
formula H 2 Y, indicating by Y all that there is in the
molecules except the two atoms of hydrogen. I
hasten to mention that when I indicate by X and by Y
the things which in the molecules of acids are combined
with H and H 2 , I do not intend to affirm that X and
H, or Y and H 2 , are detached within the molecule
as its two immediate components ; but without touch-
ing the question of the disposition of the atoms within
the molecule of acids, I only wish to indicate distinctly
the part which is not changed in the transformation
of the acid into its corresponding salts.
Before treating and discussing the various reactions,
I remind my pupils once more that all the formulae
used by me correspond to equal gaseous volumes, the
theory of Avogadro and Ampere being constantly the
guiding thread which leads me in the study of chemical
reactions.
This done, I now give very rapidly an abstract of
what I explain in this lecture concerning some re-
actions of the monatomic and biatomic radicals. I
always write the reaction of the molecule containing
a monatomic radical alongside a corresponding one
of a molecule containing a biatomic radical, in order
that the comparison may be easier.
P. 356] Course of Chemical Philosophy.
43
Of the Monatomic Metallic Radicals
with the Ha'ogens.
DIRECT COMBINATION
A
* H 2 + Cl 2 = 2 HC1
i molecule i molecule 2 molecules
of of of hydro-
hydrogen, chlorine. chloric acid.
K 2 + Ci 2 = 2 KC1
i molecule i molecule 2 molecules
of of of chloride of
potassium. chlorine. potassium.
t(CH 3 ) 2 + Cl 2 = 2 CH 3 C1
i molecule i molecule 2 molecules
of of of chloride of
methyl. chlorine. methyl.
Cl 2 =
Apparent direct combination, in
reality molecular double decomposi-
tion, in virtue of which two molecules
of different kinds give two of the same
kind.
Of the Biatomic Metallic Radicals
with the Halogens.
Hg + Cl 2 = HgCl 2
i molecule i molecule i molecule
of of of bichloride
mercury. chlorine. of mercury.
Zn + Cl 2 = ZnCl-
i molecule i molecule i molecule
of zinc. of of chloride
chlorine. of zinc.
C 2 H 4
Cl 2 = C 2 H 4 C1 2
i molecule i molecule i molecule
of of of chloride of
ethylene. chlorine. ethylene.
+ Cl 2 = R m Cl 2
True direct combination or union of
two different entire molecules into a
single molecule.
* The direct combination of hydrogen and chlorine is expressed
by some as H + CI = HC1; in the equations used by me I always
employ molecules.
t It appears that in practice this direct combination succeeds
with difficulty, the chlorine having an action on the hydrogen of the
radical ; it has been indicated merely for comparison with that of
ethylene.
From what precedes it may be observed that a com-
plete molecule of chlorine, and thus of any halogen,
always reacts with a complete molecule of a metallic
radical ; if the latter is monatomic it makes two
molecules, if it is biatomic it forms only one.
44
[P. 357
'S c "S-
N
oo "H oo * . -H^
OO g
1
'.a'
' ' ' w*C.I ^** ' aj-H.S bJO ""3
. a 8
B
o
N || N N || N ffi |||
"b? ^
i
.
o
+ + ^"
u
(4
|
1
T3
j={ > j| & JJ
o 1
-s
si/
a
-c M j:
1
_o
II II II
II
"rt
"So o^ *S
S
C/3
S
--||^ 00||S HH^II^
o o .s
O
o
HH ,_, _!> o "G 5?.$? %>'- ffiffi^^y
"ol 1
o
E
" ^ O 13 ^ ^ ^ 'o rS ^ "o >,
*"^
o
2^ e-g e^
1
.2
p
55
PQ
+ -f +
+ "5
c
*s
oT
o
(U 0)
c
H
Q
"3 d "5 o "^ b
C o5 C C S c bfl aji 3
o
i
M
CQ
N 'O'N SJ"o'N ffi'o "
Go S 1 ^ E |
^ '5
^3
3
8-
C/3
p
^t^ 'c.o
i _^ - i; o E ^_ _ i <u o H- i <S *o
X
u
g
nother.
gglli gffllll' <^l^l
lit lt rt 13"
, N N C n
"3 "3 1
i
c
a
a
H
tt-i
<0 0) ^J
(M <5
1
p
1
ffl 1-sl ^ 1-s^ ffl I'si 5
"J 1
c/>
PH
1 | 1^ 1 |
v ^ 0)
.a
1
u u u
II
"o o 'o "o
|
a
o
d
c
ii|p Sllll 5S l|i
5 S
"* 8 ~ 9 o
s|> 1-g H^
Is
c
c
,
*t" ~r "T~
Vo
| | | g | j:
2-> '^
g J 3
! 1 ! 1 < ! r
fe 5
P. 358] Course of Chemical Philosophy. 45
From what is written in this table it is seen that
two molecules of hydrochloric acid or of another
analogous monochloride always react with a single
molecule of metallic radical ; if this is monatomic, they
change into two molecules of monochloride, if it is
biatomic into a single molecule of bichloride. The
cause of the last difference consists in this : that
the molecule of the monatomic radical is divisible
into two ; that of the biatomic radical, not being
capable of division, collects into a single molecule
the residues of two molecules of monochloride or
monoiodide.
The biatomic radicals behave similarly to the acids
containing i atom of monatomic metallic radicals
(H, Ag, K) ; collecting into a single molecule the
residues of two molecules of acids or of salts, as may
be seen in the following comparative table.
[TABLE
46
[P. 359
(N (M
66 Jb
gg ** ^
XX
C -a
N N a u o
*N OQ 3 QJ JU
PH
<3 <3 ^ g .E
bo |||
- s
g
c g'S
^^""^ S rt
DC
N M
IS!
rt
C
o
rS
1
ffi I'sl
SB 1*S
Q
sa
s |-
E >,
^-
H
1
II
II
ii
Pi
tfl
H
1
PS
66 111
M ' O
OO g-oS
n n v o
X X
<2
1
<< |||
66 -3|.|
- 1 s - a
Cd Di
H
X
s
N ^
ffiffi 6 ce
B
E
+
+
+
o
^
.5
4i
42
I
C aj .E
-a
H
rt
N "o N
M " N
'7v
O
E-o
"0
K
S
w
M
o
33
B
Pi
9% ill
^23
99 J'og
<M (M J> rt -
oo l^-g
X X
08 oS
-s -s
05 05
WW g-5|
jg jl 1 M
H
I
o
c
+
+
+
H
rt
g
_OJ
OJ
<M
gj jigi
"3
& 1*1
_Q
|
Pi"
15
E >,
||
E J
||
II
1
1
o
"rt
<D
%%|P
^^ ^ ^ 'O .^
ffiK If!
(N (N 4) -5 -
OO o^
^ ^
' rt S ' rt S
Pd Pd
1
^' s
ffiffi e-^s
rt
+
+
+
~ E
42
rt
5j I's'i
rf g^ E
*Z. "5 c; 5
73
-1
V.
! I
E S
H
P. 360] Course of Chemical Philosophy. 47
These examples are sufficient to show that the
compounds containing a monatomic metallic radical
behave like the monochlorides : two molecules of
these react with a single molecule of metallic radical,
changing into two molecules if the latter is mon-
atomic, into a single molecule if it is biatomic. We
can prove more easily that the biatomic metallic
radicals bind in a single molecule the residues X of
two molecules R^X, by comparing the double decom-
positions or mutual substitutions of the chlorides of
the monatomic and biatomic radicals with the com-
pound R^X.
I write in the following table some examples of
these double decompositions.
[TABLE
48
Cannizzaro.
[P. 361
:3S
55^0
S b
OO 'o
CM Cl g 3
00 |s
DC? "
+ w-
o
+ -^
XX
?i N IN
d OO >s O
'^ TOCO fljO. C^
CO C/5 n- T3 V^ V/ ./) *W >/ ^-/ *t5 I
^^^aj'^ J^coojO. rsco ^O ' \> K^
llUt IIP ttJk-33
ff "s
ffi -
ffi
OO
O
- a
33 o .
X
"
-ss
o ||
hjo a) n
< o :
9 So .
ffi
X
ffl -fo-S
P. 362] Course of Chemical Philosophy. 49
All the reactions indicated in this table may be
summed up as follows : Whatever is combined with
one atom of hydrogen or any other equivalent radical
= (X) replaces one atom of chlorine, and conversely is
replaceable by the latter ; if an indivisible radical in
the double decompositions is found combined in a
single molecule with two atoms of chlorine, it will,
if the chlorine is exchanged for X, remain combined
in a single molecule with 2X.
That ethylene is combined with two atoms of
chlorine in choride of ethylene, and that the acetate
of ethylene contains in one molecule twice C 2 H 3 O 2 ,
is shown by the comparison of the gaseous densities
of these substances. From the vapour density and
from the specific heats, it is further demonstrated
that the molecule of corrosive sublimate, like that of
chloride of ethylene, contains two atoms of chlorine.
Hence the mercuric salts are constituted in a similar
manner to those of ethylene, whilst the salts of potas-
sium, sodium, and silver are formed like those of
ethyl.
Having proved, then, as I think I have already
sufficiently indicated, that the lower or only chlorides
of iron, manganese, zinc, magnesium, calcium, barium,
etc., are constituted like corrosive sublimate, that is,
have the formula MCI 2 , there can remain no further
doubt that the salts which are obtained by means of
these chlorides and of the monobasic acids, or of their
salts, are all similar to those of ethylene, propylene,
etc. These important conclusions may be summed
up as follows :
i. Amongst the salts of monobasic acids only those
of hydrogen, potassium, sodium, lithium, silver, to-
gether with mercurous and cuprous salts, are similar
to those of methyl and ethyl, that is, to compounds
D
50
Cannizzaro.
[PP. 362-3
of the alcohols containing a monatomic radical ; all
the other salts, of the so-called protoxides, are similar
to those of ethylene and propylene, that is, to the
compound ethers of the alcohols with biatomic
radicals.
2. A single molecule of the first is not sufficient to
form the anhydrous acid and the metallic oxide ; two
molecules instead are required ; but a single molecule
of the second contains the components of the molecule
of the anhydrous acid and of that of the protoxide.
This becomes clear by bringing the following equa-
tions into comparison :
A|c 2 H 3 O 2 :
Agj
+ C 4 H 6 O 3
Her/ HcrO
Si f^2U3O2 &
+ C<H03
2 molecules of
acetate of
silver.
i molecule
of oxide
of silver.
i molecule
of
anhydrous
acetic
acid.
i molecule of i molecule
mercuric of oxide
acetate. of
mercury.
i molecule
of
anhydrous
acetic
acid.
C 2 H 5 ',C 2 H 3 O 2 "C 2 H 5 ) (
D+CWO 3
C.H'{gHg = OTTO
+ C 4 H 6 O 3
2 molecules of
acetate of
ethyl.
i molecule
of oxide
of ethyl.
i molecule
of
anhydrous
acetic
acid.
i molecule of i molecule i molecule
acetate of of oxide of of
ethylene. ethylene. anhydrous
acetic
acid.
R ^ X =
R'/ 4
-(2X-0)
Rmjx = RmO +
(2X-0)
The mercuric salts and the salts of zinc, etc., being
similar to those of ethylene, it is probable that salts
of this type exist containing the residues of two
different monobasic acids. I indicate by what re-
actions they might be generated :
PP. 363-4] Course of Chemical Philosophy.
51
H ( Cl
. AgC 2 H 3 O 2
f AgC 2 H 3 O 2 :
AgCl
AgCl +
H {CTK?
i molecule of
bichloride of
mercury.
2 molecules of
acetate of
silver.
2 molecules of
chloride of
silver.
i molecule of
acetate of
mercury.
g {8!
, AgC 2 H 3 2
f AgC 7 H 5 O 2 :
A g r! +
AgCl
rc 2 H 3 o 2
Hg \C 7 H 5 O 2
i molecule of
bichloride of
mercury.
i molecule of
acetate and i of
benzoate of silver.
2 molecules of
chloride of
silver.
i molecule of
benzacetate of
mercury.
<?H{g
, AgC 2 H 3 2
f AgC 7 H 5 2 :
AgCl
AgCl
C 2 H 4 | C7H5()2
i molecule of
chloride of
ethylene.
i molecule of
acetate and i of
benzoate of silver.
2 molecules of
chloride of
silver.
i molecule of
benzacetate of
ethylene.
Just as acetates are produced from anhydrous acetic
acid and the oxides of biatomic metallic radicals, so
from anhydrous benzacetic acid the benzacetates will
be formed, as I indicate in the following equation :
C 4 H 6 3
= R"C 4 H 6 O 4 - Rm
C 9 fTO 3 + RO =
Having already proved that zinc is a biatomic
radical, and that in consequence its atomic weight
should be doubled, I stop to examine the reactions
and the mode of formation of zinc ethyl, zinc methyl,
etc. I show you by means of equations the method
by which I interpret these reactions.
The vapour densities demonstrate the accuracy
of the following formube corresponding to equal
volumes : C 2 H 5 C1 (chloride of ethyl) C 2 H 5 , H
(hydride of ethyl) C 2 H 5 , C 2 H 5 (free ethyl) C 2 H 5 , CH 3
(methyl ethyl), Zn(C 2 H 5 ) 2 = Zn 15 ( zinc
52 Canmzzaro. [PP. 364-5
C 2 H 5 C1 -
f H 2
C 2 H 5 , H +
HC1
C 2 H5Cl
C 2 H 5 C1
f Zn
(C 2 H 5 ) 2 +
Zn {ci
C 2 H 5 C1
C 2 H 5 C1
-t- 2Zn =
C 2 H 5 / Zn +
Zn {ci
C 2 H 5 C1
C 2 H 5 C1
C 2 H 5 \ 7
t- c 2 H*j Zn :
2(C 2 H 5 ) 2 +
Zn {ci
CH 3 C1
f Zn
C 2 H 5 , CH 3 +
Zn {cl
C 2 H 5 C1
CH 3 C1
f 2Zn =
CH*} Zn +
7 fCl
Zn {ci
No one has yet demonstrated, as far as I know, the
existence of the type of compound indicated in the
last equation. But it being proved from the density
of zinc ethyl vapour, and from its specific heat, that
the complete molecule of zinc ethyl contains a single
atom of zinc combined with two ethyl radicals, that
is, with the molecule of the free radical, no one can
deny that there will be prepared compounds contain-
ing a single atom of zinc combined with two different
monatomic radicals. It may also be predicted that
ethylene and propylene will form compounds in whose
molecules an atom of zinc is combined with the
biatomic radical.
I will give you later an account of some of my
experiments directed to show the existence of the
compounds just mentioned.
After having spoken of the mode of behaviour of
the compounds containing monatomic or biatomic
radicals with regard to monobasic acids, I examine
the mode of behaviour with regard to those com-
pounds which contain in each molecule two atoms of
hydrogen, or, as they are called, the bibasic acids, to
which I have given the general formula H 2 Y.
To predict the reactions, it is sufficient to bear in
mind what follows ;
PP. 365-6] Course of Chemical Philosophy.
53
i. The two atoms of hydrogen are united in a
single molecule by the forces of all the other com-
ponents which together we call Y, hence what
is equivalent to H 2 can enter into a single molecule
with Y.
2. What is combined with H 2 is equivalent to two
atoms of chlorine Cl 2 ; hence in double decomposition
H 2 Y will act either on a single molecule of a bi-
chloride ( = RC1 2 ) or on two molecules of a mono-
chloride ; what is combined with two atoms of
chlorine, whether in one or in two molecules,
will combine with Y ; and H 2 combining with Cl 2
will always form two molecules of hydrochloric
acid.
The examples of double decomposition which follow
clearly show what I have just indicated.
DOUBLE DECOMPOSITIONS OF HYDRATHD SULPHURIC ACID, H 2 SO 4 ,
With the Monochlorides Rlld.
With the Bichlorides
+Na2s 4
+H ' 2S 4= HC1
Nail
Ag 2 S0 4 = + C 2 H 4 SO 4
In connection with this point I compare the formulae
of the oxy-salts proposed by me with those of Ber-
zelius and of Gerhardt, and discuss the causes of the
54 Cannizzaro. [p. 366
differences and of the coincidences, which may be
summed up as follows :
i. All the formulae given by Berzelius to the oxy-
salts of the biatomic metallic radicals are the same as
those proposed by me, whether the acid is monobasic
or bibasic ; all these oxy-salts contain in each mole-
cule the elements of a complete molecule of oxide
and of a complete molecule of anhydrous acid.
2. There correspond also to the formulae proposed
by me all those of Berzelius for sulphates and analogous
salts, if we introduce the modification by Regnault,
i.e., if we consider the quantity of metal contained in
the molecules of potassic, argentic, mercurous, and
cuprous sulphates equal to 2 atoms, and those on the
other hand of metal contained in the molecules of
mercuric, cupric, plumbic, zincic, calcic, baric, etc.,
sulphates, equal to a single atom.
3. The formulae proposed by me for the oxy-salts
of potassium, sodium, silver, hydrogen, methyl, and all
the other analogous monatomic radicals with a mono-
basic acid, are equal to half the formulae proposed by
Berzelius and modified by Regnault, i.e., each mole-
cule of them contains the components of half a
molecule of anhydrous acid and half a molecule of
metallic oxide.
4. The formulae of Gerhardt coincide with those
proposed by me only for the salts of potassium,
sodium, silver, hydrogen, methyl, and all the other
monatomic radicals, but not for those of zinc, lead,
calcium, barium, and the other metallic protoxides ;
Gerhardt having wished to consider all the metals
analogous to hydrogen, which I have shown to be
erroneous.
In the succeeding lectures I speak of the oxides
with monatomic and biatomic radicals, afterwards I treat
P. 366] Course of Chemical Philosophy.
55
of the other classes of polyatomic radicals, examining
comparatively the chlorides and the oxides ; lastly, I
discuss the constitution of acids and of salts, returning
with new proofs to demonstrate what I have just
indicated.
But of all this I will give you an abstract in another
letter.
GENOA, \*th March 1858.
PR1NTKU BY
OLIVER AND BOYD
EDINBURGH
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