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Full text of "Sketch of a course of chemical philosophy"

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. 




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OLIVER AND BOYD 

EDINBURGH 



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