London, February 2, 1882.

The lecturer said that it had been found useful to occasionally bring
forward various points of chemical doctrine, on which there were
differences of opinion, to be discussed by the society. On this occasion
he wished not so much to demonstrate certain conclusions, or to make a
declaration of his opinions, as to invite discussion and a thoughtful
consideration of questions of importance to chemists. Originally three
questions were proposed: First, Is there any satisfactory evidence
deducible of the existence of two distinct forms of chemical combination
(atomic and molecular)? Second, Is the determination of the vapor
density of a body alone sufficient to determine the weight of the
chemical molecule? Third, In the case of an element forming two or more
distinct series of compounds, e.g., ferrous and ferric salts, is the
transition from one series to another necessarily connected with the
addition or subtraction of an even number of hydrogenoid atoms? He
would, however, limit himself to the first of these questions; at the
same time the three questions were so closely associated with one
another that in discussing the first it was difficult to know where to
begin. The answer to this question (Is there any satisfactory
evidence deducible of the existence of two distinct forms of chemical
combination?) depends materially on the view we take of the property
called in text-books valency or atomicity; and before discussing the
question it is important to have a clear idea of what these words
valency and atomicity really mean. It is necessary, too, to start with
some propositions which must be taken for granted. These propositions
are: First, that in all chemical changes, those kinds of matter which we
commonly call elementary, do not suffer decomposition. Second, That the
atomic weights of the elements as received are correct, i.e., that they
do really express with great exactitude the relative weights of the