THE OLD AND NEW CHEMISTRY. 
35 
The molecular theory teaches us to regard the molecule of 
a substance as a structure, as a definite whole. The old 
chemistry looked on chemical compounds as made up of two 
or more parts ; it tended to the view that, when two sub- 
stances unite, these substances are contained as such in the 
product of the combination. The old chemistry was always 
more or less a dualistic chemistry. The new chemistry rests 
upon a unitary basis. When two substances unite, the product 
is regarded as one new whole ; the properties of the combining 
bodies are lost in those of the compound. Not that the com- 
pound is completely homogeneous ; it is regarded as a structure, 
but yet a structure in which no one part is complete without 
every other, and in which the general properties of the whole 
are dependent upon the mutual action of all the parts. The 
new chemistry is, in this view of it, entirely in keeping with 
the molecular theory. 
The introduction of the principle of substitution into chemistry 
led to the adoption of a unitary notation ; such a notation finds a 
very probable explanation in terms of the molecular theory. 
The old equivalent notation has never been able to accommodate 
itself to the requirements of the unitary system ; it does not 
find its fullest explanation in, but is rather opposed to, the idea 
of a. molecular structure. 
The molecular theory of matter is adopted by the great 
majority of physicists as a good and workable scientific hy- 
pothesis ; the physicists find that this theory enables them to 
group together and to explain a greater number of facts than 
can be brought within the grasp of any other theory concerning 
the structure of matter which has yet been propounded. And is 
it not of the utmost importance that the fabric of chemical 
science should be reared from a foundation upon which rests also 
the whole building of natural philosophy ? 
Chemical facts, as well as physical facts, can be explained 
most fully in terms of the molecular theory : the new chemistry 
recognizes this; the old chemistry passes it by. Chemistry 
tends more and more to become a branch of dynamical science. 
To reintroduce the old equivalent system of notation would, it 
appears to me, be to render more distant the arrival of the 
time when it shall be possible to treat the science of the 
dynamics of atoms as a branch of the larger science of the 
dynamics of molecules. 
But, nevertheless, the old system has some points in its 
favour ; it has had — it still has — many great names identified 
with it. If it be really better than the new, it will again become 
paramount. What the investigators of Nature seek is not 
systems, but truth. Why, then, should we quarrel with one 
another ? 
