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the molecule, granting that groups of atoms do exist in the 
molecular building, does the fact that in a certain reaction 
certain atoms are withdrawn as a group, prove that these 
existed in the form of the same group in the original molecule ? 
In other words, do our structural formulae express the relative 
collocation of atoms within the molecule while the molecule is 
unacted on by extraneous force, or do they merely roughly 
represent the condition of things when the molecule is in a 
state of strain, because of the stress between its parts and those 
of another molecule, or molecules, brought within its sphere of 
action ? Here is another question which can only be answered 
after much experimental evidence has been accumulated. Now 
these questions, I make bold to say, are the direct outcome of the 
dualism of Berzelius, modified by the unitary chemistry of 
Dumas and his followers. 
If we glance back on the development of the two theories, 
the course of which I have endeavoured to outline, we find that 
both began with a purely qualitative study of reactions, but 
that it was only when to this had been added the careful use of 
weights and measures, that any solid advance became possible. 
Further, we find that the older theory was founded chiefly on a 
study of reactions, whilst that which was broached after the 
time of Lavoisier was founded most largely on a study of 
composition. With the Phlogisteans function was of para- 
mount importance ; with the Dualists composition was all. 
The modern theories, which have been developed from these, 
have attempted, with varying success, to combine both con- 
siderations. And if we examine the latest advances of theo- 
retical chemistry we still find it at work on these two lines of 
advance. The composition of chemical compounds is studied 
by the majority of chemists ; but the general laws of action of 
chemical force itself have of late received most important 
elucidation. 
Again, if we look to the ‘ lines of advance along which 
dynamical science is working its way to undermine, at least, 
the outworks of chemistry/ we can distinguish two, essentially 
the same, lines as were used by the two classes, whose theories 
I have dealt with in this paper. ‘ One is conducted by the help 
of the hypothesis that bodies consist of molecules in motion, 
and it seeks to determine the structure of the molecules and the 
nature of their motion from the phenomena of portions of 
matter of sensible size. The other line of advance, that of 
Thermodynamics, makes no hypothesis about the ultimate 
structure of bodies, but deduces relations among observed 
phenomena by means of two general principles, the conser- 
vation of energy, and its tendency towards diffusion.’ (Clerk 
Maxwell. South Kensington Science Conferences , 1876, p. 145.) 
