TRANSACTIONS OF THE SECTIONS. 49 
yaluable meaning. Take, for instance, the formula of acetic acid, H,C—CO—OH, 
or 3CH,, 2C00,, 3H,0, 4U,; it is this last term which indicates the coupled cha- 
racter of the compound. If we look upon acetic acid as a compound of carbon, 
it is a coupled compound because all the equivalents of carbon in it do not be- 
long to the same atom, and the two atoms of carbon are directly united together, 
and replacement of the equivalents united to one of these atoms does not very 
greatly affect the function or chemical character of the equivalents united to the 
other. 
I have perhaps spent too much of your time upon these historical questions. Let 
us now shortly consider what is the present state of our knowledge as to chemical 
constitution. This I have already defined as the order in which the constituents 
are united in the compound. We may indeed use metaphorical language, and speak 
of the relative position of atoms, perhaps deluding ourselves into the notion that 
such language is more than metaphorical ; but the phenomena of combination and 
decomposition, although we cannot doubt that they depend solely upon the relative 
position and dynamical relations of the atoms, are not alone sufficient to prove even 
that atoms exist. Our knowledge of the intimate structure of matter comes from 
another souree—from the study of the properties rather than of the changes of sub- 
stances, and of the transformations of energy which accompany the Lanarntatere 
of matter. 
This is strictly a branch of Chemistry: the aim of chemistry is to connect the 
properties of substances and the changes they undergo with their composition, 
taking this word in its widest sense; and we must not allow our friends in 
Section A to cut owr science in two and appropriate the half of it. We all frankly 
admit that Chemistry is a branch of Physics; but it isso as a whole—no section of 
it is more purely physical than all the rest. To accept a narrower definition 
of Chemistry is to reduce ourselves to the position which the collector occupies 
among naturalists; it is to admit that it is our business to provide part of 
the materials out of which a science in which we have no share may be constructed 
by others. Butwe need not fear that this so-called physical side of Chemistry will 
ever be divorced from the study of chemical change. The names of Faraday and 
Graham among those who have left us, of Andrews among those who are still at 
work, are sufficient proof of this; and a study of their researches will conclusively 
show that great results can be looked for in this direction only from a physicist 
who is also a chemist. 
There are three special directions in which such investigations have already 
influenced chemical theory :—Ist. Electrolysis, which has confirmed the equivalent 
as a chemical unit, has proved that equivalents unite in pairs, thus forming the 
basis of the electro-chemical theory, and has shown us how to estimate the amount 
of energy involved in the union of a given pair of equivalents. 2nd. Vapowr- 
density, from which Avogadro inferred the law of molecular volumes (since proved 
by Clerk-Maxwell), which has given us the molecule as a chemical unit, and 
formed the basis of the Unitary theory. 38rd. Specific heat, from which Dulong 
and Petit inferred their empirical law, which gives us the most satisfactory 
physical definition of the atom as a chemical unit. 
‘We naturally turn to the future, and try to guess whence the next great revolution 
willcome. For although periods of quiet have their use, as affording time for filling 
up the blank schedules furnished by the last speculative change, such periods have 
seldom been long, and each has been shorter than its predecessor. 
But it is impossible to make a certain forecast: looking back, we see a logical 
sequence in the history of chemical speculation; and no doubt the next step 
will appear, after it has been taken, to follow as naturally from the present 
position. One thing we can distinctly see—we are struggling towards a theory of 
Chemistry. Such a theory we do not possess. What we are sometimes pleased 
to dignify with that name is a collection of generalizations of various degrees of 
imperfection. We cannot attain to a real theory of Chemistry until we are able to 
connect the science by some hypothesis with the general theory of Dynamics. No 
attempt of this kind has hitherto been made ; and it is difficult to see how any such 
attempt can be made until we know something in reference to the absolute size, 
mass, and shape of molecules and atoms, the position of the atoms in the molecule, 
