46 REPORT—187 4. 
The questions as to chemical constitution raised about forty years ago by Dumas 
and the new French school, in opposition to Berzelius, may now be said to be prac- 
tically settled. The great majority of chemists are agreed as to what is to he 
understood by chemical constitution, and also as to the nature and amount of evi- 
dence required in order to determine the constitution of a substance. How has this 
agreement been produced? Some historical writers seem to wish us to believe that 
it is the result of the triumph of the ideas of Dumas, Gerhardt, and Laurent, and 
the defeat of the dualistic radical theory of Berzelius; that the arguments of Ber- 
zelius and his followers were only useful as giving occasion for a more full and 
convincing proof of the unitary substitution theory than would otherwise have been 
called for; that, in fact, the adherents of Dualism played the part (not untrequently 
supposed to be that of the conservative party in polities) of checking and criticising 
the successive developments of truth, and thus allowing them time to ripen. 
In opposition to the view thus broadly stated, I would place another, and for the 
sake of contrast shall state it also in perhaps too broad a form. That the two 
theories, the dualistic radical theory and the unitary substitution theory, were both 
true and both imperfect, that they underwent gradual development, scarcely in- 
fluenced by each other, until they have come to be almost identical in reference to 
points where they at one time seemed most opposed. 
I have said that the development of the one theory was scarcely influenced by 
that of the other. Of course the facts discovered by both parties were common 
property, and the development of both theories depended upon the discovery of these 
facts; but the explanations of facts and the reasoning from them given by each party 
seemed to the other scarcely worthy of serious consideration and were treated as 
matter of ridicule. And the habit of mind created by this mode of viewing the 
opposed theory has rendered it difficult for those who were engaged in the contro- 
versy on either side to see how nearly the two theories have now come to coinci- 
dence. Their language still remains different; but as the facts are the same for both, 
it is not difficult for a neutral critic to translate from the one to the other; and if 
we do so we shall see that there is much real agreement between the two modes of 
representing chemical ideas, historically derived, the one from Berzelius, the other 
from Dumas, Laurent, and Gerhardt. 
In both, chemical constitution is regarded as the order in which the constituents 
are wuited in the compound; and the same fundamental notion is indicated in the one 
by reference to proximate constituents, in the other by the concatenation of atoms. 
To show that thisis so, and that the fundamental notion can be arrived at from the 
dualistic as well as from the unitary starting-point, I shall cite an illustrative case. 
Kvery student of chemical history will remember the view of the constitution of tri- 
chloracetic acid propounded by Berzelius, and afterwards supplemented by a similar 
view of the constitution of acetic acid and an explanation of the likeness of some 
of the properties of these two substances. This has sometimes been spoken of as a 
subterfuge of a not very creditable kind, by means of which Berzelius apparently 
saved his consistency while really yielding to the arguments of his opponents. But 
if, instead of looking at it in the light of the substitution controversy, we consider it 
in itself as a contribution to speculative chemistry, we at once recognize in it a 
statement, in Berzelian language, of the views we now hold as to the constitution of 
these acids. The view was that acetic acid is a compound of oxalic acid and methyl, 
trichloracetic acid a compound of oxalic acid and the sesquichloride of carbon. 
They differ considerably from each other, because the “ copulze ” (methyl and sesqui- 
chloride of carbon respectively) are different; but their resemblance is strongly 
marked because they contain the same active constituent, oxalic acid; and most of 
the prominent characters of the substances depend upon it, and not upon the copula. 
Let us first free this statement from what we may call archaisms of language. It 
will then assume something like the following form :—The carbon in acetic acid is 
equally divided between two proximate constituents, one of which is an oxide, the 
other a hydride of carbon. ‘'Trichloracetic acid similarly contains an oxide and a 
chloride of carbon, between which the carbon is equally divided. The oxide is the 
same in both acids, and is that oxide which occurs in oxalic acid. The hydride 
and the chloride have the composition of the substances, the formule of which are 
C,H, and C, Cl, respectively. Oxalic acid undergoes chemical change much more 
