The lecturer then referred to an important difference in the adicity of
chlorine and oxygen. Chlorine can combine with methyl or ethyl singly.
Oxygen can combine with both and hold them together in one molecule. The
recognition of this fundamental difference between chlorine and oxygen,
this formation of double oxides as opposed to single chlorides, marks an
epoch in scientific chemistry.

The lecturer then considered the subject of chemical formulae; it is the
bounden duty of every formula to express clearly the number of atoms of
each kind of elementary matter which enters into the constitution of the
molecule of the substance. A formula may do much more than this. If we
attempt to express too much by a complex formula we may veil the number
of atoms contained in it. This difficulty may be avoided by using two
formulae, a synoptic formula giving the number of atoms present, and a
complex formula perhaps covering half a page, giving the constitution
of the molecule. But between the purely synoptic formula and the very
elaborate formula there are others--contracted formulae--which labor
under the disadvantage, as a rule, of being one-sided, and so create a
false impression as to the nature of the substance. Thus, for instance,
to take the formula of sulphuric acid, H_{2}SO_{4}. This suggests that
all the oxygen is united to the S; (HO)_{2}SO_{2} suggests that two
atoms of hydroxyl exist in the molecule; then, again, we might write the
formula HSO_{2}OH, or H_{2}OSO_{3}. All of these are justifiable, and
each might be useful to explain certain reactions of sulphuric acid, but
to use one only creates a false impression. The only plan is to use them
variously and capriciously, according to the reaction to be explained.
Again, ethyl acetate may be written--

H_{3}C\