H_{2}C/
\
O
/
OC\
H_{3}C/

Or condensed--

H_{5}C_{2} \
}O
H_{3}C_{2}O/

Or H_{5}C_{2}O.C_{2}H_{3}O, or H_{5}C_{2}.C_{2}H_{3}O_{2}. Now each
of these two latter formulae is a partial formula, each represents a
one-sided view; it is justifiable if you use both, but unfair if you use
only one.

We now come to the question as to the existence or non-existence of
two distinct classes of compounds, one in which the atoms are combined
directly or indirectly with each other, and the other in which a group
of atoms is combined as an integer with some other group of atoms,
without any atomic connection by so-called molecular combination. These
two modes of combination are essentially distinct. The question is not
one of degree. Are there any facts to support this theory that one set
of compounds is formed in one way, another in a different way? Take the
case of the sulphates: Starting with SO_{3}, we can replace one atom
of O by HO_{2}, and obtain SO_{2}(HO)_{2} or H_{2}SO_{4}; replacing a
second atom, we get SO(HO)_{4} or H_{4}SO_{5}, glacial sulphuric acid, a
perfectly definite body corresponding to a definite class of sulphates,