SEPTEMBER 18, 1902] 
OAT CFL 
499 
more fully in the middle of the last century, through the brilliant 
work of Gerhardt, Wiliiamson, Laurent, Odling, Wurtz, and 
others, in the purely chemical field. Dalton gave us the con- 
ception of the molecule, though confused with that of the 
atom, as the unit of measure of chemical activity in place of the 
gravimetric unit ; the work of the chemists of the last mid- 
century gave us a fuller conception of the molecule, along with 
the notion of chemical change as being substitution in the mole- 
cule effected by what became known as double decomposition. 
Up to that time chemistry had been treated only as the science 
of compounding and decompounding or reducing. Sodium 
added to oxygen gives soda, sulphur added to oxygen gives sul- 
phuric anhydride, soda added to the anhydride gives sodium 
sulphate, ethylene added to chlorine gives dichlorethane, water 
subtracted from alcohol leaves ether, and so forth. All this is 
strictly true in a limited way, but then it is not chemistry ; and 
the addition precedes and does not constitute the chemical union. 
In the sodium sulphate we perceive no soda, no anhydride, no 
sodium, sulphur or oxygen. That is to say, there is evidence 
of the addition and subtraction of mass and some other such 
evidence ; but, for the rest, evidence of addition there is none. 
Were it otherwise there could be no chemistry. It is true that 
one of the great things accomplished by chemistry has been 
that of establishing the Jaw of the conservation of mass, with- 
out which to rely upon the chemist would be unable to carry on 
his experimental investigations. But that is only because, like 
the steady point to the seismologist, it is there unchangeable 
when all else is changing. Since it is the law of no change, it 
cannot serve to explain what is change. Far from being the 
science of the composition of substances, chemistry might be 
defined as being the science of the non-composition of sub- 
stances where that composition might have been looked for from 
the antecedents. If salt is verily a compound of sodium and 
chlorine, and can be broken up into these, why have the frag- 
ments not the marks on them of that whole of which they formed 
a part? It is true that 5850 parts of salt become 3545 parts of 
chlorine and 2305 parts of sodium, nothing being gained or 
lost in weight ; but to account for that there is no need of 
chemistry, a science which takes cognisance of the phenomena 
of change, and not of those of unchanged properties. The use 
of the word ‘‘composition” in chemistry cannot be discarded 
now, and all that is necessary to make it unobjectionable is to 
see that the term is always qualified by the prefix ‘‘ chemical ” 
when there is a possibility of mistake about its significance, and 
that that significance is carefully explained, if not defined and 
fully illustrated, before it is given over to the beginner. 
The facts of a chemical nature about common salt which 
cause the statement to be made that it is a chemical compound 
of chloride and sodium are such as these. Salt can be wholly 
changed into sodium and chlorine ; these substances brought 
together change into salt and nothing else; salt and sodium, 
each under conditions appropriate to it, change into the same 
substance, called also a sodium compound, such as sodium 
hydroxide ; salt and chlorine, each in its own way, change into 
the same chlorine compound, such as hydrochloric acid ; neither 
sodium nor chlorine, one apart from the other or the other’s 
chemical compounds, ever changes into salt; salt is, directly or 
indirectly, producible in the chemical interaction of a sodium 
compotind with a chlorine compound ; the properties of salt are 
much less like those of either sodium or chlorine than like those 
of some other substances; in sensible and other physical pro- 
perties the chemically compound substance, salt, is as simple as or 
simpler than either of the chemically simple substances, sodium 
and chlorine ; lastly, the laws of combining proportion by weight 
are obeyed in all the chemical changes in which salt takes part. 
With exclusive reference to such facts as these, the chemical 
composition of a substance will, I think, be found to be satis- 
factorily defined, as its having the power, capacity or property 
of being wholly producible from and wholly convertible into, 
directly or indirectly, those substances of which it is said to be 
composed. A simple substance differs from one that is com- 
pound only in not possessing the power of being by itself con- 
vertible into two others, or of being produced alone from any 
two others. Simple substances are not less varied or less com- 
plex in their physical properties than compound substances, 
while their chemical constitution is often more problematic than 
that of many which are compound. The term “simple,” 
therefore, is as misleading in the language of chemistry as 
“compound,” unless defined and qualified in use by the word 
‘© chemically.” 
NO. 1716, VOL. 66 ] 
The ground really occupied by chemical composition in 
theoretical chemistry is now greatly limited; for with the full 
acceptance of the idea of the molecule and of the atom as a 
derivative of it, its place has been taken by chemical constitution 
to an extent hardly realised. The useful and_ practically 
necessary expression of the results of the quantitative analysis 
of a new substance gravimetrically is all that can strictly 
receive the name of its chemical composition. When the 
term is applied more widely it is used for what are really the 
simpler forms of chemical constitution. It was otherwise 
before the conception of the molecule had become current and 
the atom had become a derived function of the molecule. 
Chemical composition as expressed by Dalton in atoms is 
indeed that and nothing else. Carbonic anhydride is com- 
posed, according to him, of two atoms of oxygen to one of 
carbon, as against carbonic oxide, which is composed of one; 
marsh gas of two atoms of hydrogen to one of carbon, as 
against olehant gas composed of one. But then it was only 
numerical necessity which led him to adopt such a mode of 
expressing the facts. The same necessity, it is true, affects us 
also.in the matter of carbon dioxide, of water and of ammonia, 
but how little it does so is shown by the many cases in which 
the empirical or simple composition is expressed in multiples. 
The atomic chemical composition of ethylene is two of hydrogen 
to one of carbon, and that of benzene one of hydrogen to one of 
carbon. When we say, as we always do, that the one 
substance is ‘‘composed” of four atoms of hydrogen to two of 
carbon, and the other of six of hydrogen to six of carbon, we 
give what is information concerning the constitution of these 
substances. Call it the composition of the molecule as we may, 
it is evident that by composition we can here mean only con- 
stitution. As with polymerism, so with isomerism, and in a 
more marked way. Mercurous sulphate and mercuric oxy- 
sulphite, quite distinct salts, have yet the same composition. 
In the great reformation wrovght by the chemists to whom 
I have referred, but by Gerhardt in particular, the new light set 
up in chemistry was the notion of what came to be called 
““double decomposition” in chemical change. The phrase is 
not, perhaps, happily constructed, but it has the merit of 
needing some explanation of its meaning before it can be under- 
stood, and troubles, therefore, through a too simple apprehen- 
sion of the sense of the word ‘‘ composition” are hardly to be 
feared. Its introduction into chemistry marked the ascendency 
of the idea of the molecule as the factor in chemical change 
whose interactions with other molecules were to be considered, 
instead of those additions which, as chemical phenomena, never 
take place. It led also to new conceptions of the nature of the 
atom and the compound radical as being the quantitative and 
qualitative expressions of the powers possessed by substances to 
change into others, and to the conception of the valency of 
atoms and radicals as expressing the nature of the connection of 
successive chemical changes. The zeal with which it was 
attempted to force all chemical changes into the form of double 
decomposition interfered, perhaps, with the full recognition of 
its importance; but the fact remains that, with hardly an 
exception, all that is stated concerning the nature of those 
chemical changes in which two or three substances become one, 
or one becomes two or more, is based upon notions derived from 
the study of double decomposition. 
The fundamental value of double decomposition consists in 
its displaying threads running through chemical transformations 
which can be followed up. - When two substances change into 
two others, and only then, there can be found, in most cases, 
relations of resemblance, both physicil and chemical, between 
the before and after of a chemical change. Instead of the 
striking unlikenesses shown by the substances formed by quasi- 
addition to those from which they are formed, there are here 
met with the similarities of the outcoming to the interacting 
substances, and the similarities between the products of different 
interactions in which the acting substances are similar. 
Chemists had been for very long familiar with acids, bases, salts, 
without becoming deeply impressed with the significance of the 
resemblances which these class-names imply, and also with the 
facts that acids beget acids, bases bases, and salts salts, or in 
more general terms, that substances in interaction produce 
others like them, and that differences between the products and 
the agents in one change are distinctly repeated in a similar 
change in which other substances are concerned, points now 
given expression to by such terms as ‘‘ chemical constitution,” 
“homologous” and ‘‘ analogous series,” ‘* Kopp’s law,” &e. 
