82 HlSTOltY OF CHEMISTRY [LECTT. V.

also, would then necessarily fall into two classes ; or in other
words, it would be necessary to assume differences in con-
stitution in a series of substances whose behaviour is similar in
every respect. He thinks, further, that he is justified in con-
eluding from the laws of combination, that chlorine is not an
element. I do not enter more minutely into the matter since
his arguments had little effect. They came too late. Gay-
Lussac's investigation of hydrocyanic acid in the same year58
indisputably proves the acid nature of this compound, and the
fact that it does not contain oxygen ; and hence even Ber-
zelius cannot maintain Lavoisier's definition of the acids and
of the acidifying principle.

Some other cause which might furnish the acid character
observed in certain substances was now sought for. The
conception of an acid seemed so definite at that time, and the
substances included in this class were so distinctly separated
from all other substances, that it was necessary to enquire into
the cause which occasioned this difference. Besides, it cannot
be denied that Lavoisier, and even the chemists of the begin-
ning of the nineteenth century were still influenced, in a certain
respect, by the ideas of the Greek philosophers. Just as the
latter ascribed general properties to the presence of a common
constituent and identified the particular property to a certain
extent with a particular constituent—just as they explained
combustibility, for example, by the presence of a fire-material—
so Lavoisier an d his adherents believed that in oxygen they had
discovered the acidifying principle.

In a similar manner we find Davy, after he was satisfied
that hydrochloric acid contains hydrogen and chlorine only,
stating the view that the chlorine is the acidifying principle in
it, and the hydrogen its basis or radical.59 At a later date
Gay-Lussac60 introduces the name '" hydracids " for the acids
free from oxygen, and places hydrochloric acid, hydrocyanic
acid, sulphuretted hydrogen, and hydriodic acid in this class.

60

58 Ann. Chim. 95, 136. 59 Phil. Trans. 1810, 231 ; A.C.R. 9, 21.
Ann. Chim. 91, 148 ; 95, 162.