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 formulæ 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,
e.g., H_{2}MgSO_{5}, Zn_{2}SO_{5}, etc. By replacing the third atom of O
we get S(HO)_{6} or H_{6}SOH_{6}; this corresponds to a class of salts,
gypsum, H_{4}CaSO_{6}, etc. These are admitted without dispute to be
atomic compounds. Are we to stop here? We may write the above compounds
thus: H_{2}SO_{4}, H_{2}SO_{4}H_{2}O, H_{2}SO_{4}2H_{2}O. If we measure
the heat evolved in the formation of the two latter compounds, it is,
for H_{2}SO_{4}+H_{2}O, 6.272; H_{2}SO_{4}+2H_{2}O, 3.092. But if we now
take the compound H_{2}SO_{4}+3H_{2}O we have heat evolved 1.744; so we
can have H_{2}SO_{4}4H_{2}O, etc. Where are we to draw the line between
atomic and molecular combination, and why? It comes to this: All
compounds which you can explain on your views of atomicity are atomic,
and all that you cannot thus explain are molecular. Similarly with
phosphates, arsenates, etc. In all these compounds it is impossible to
lay one's finger on any distinction as regards chemical behavior between
the compounds called atomic and those usually called molecular.
Two points remain to be mentioned: The first is the relationship between
alteration of adicity and two series (ous and ic) of compounds. Tin is
usually said to be dyad in stannous compounds and a tetrad in stannic
compounds, but in a compound like SnCl_{2}AmCl, is not tin really a
tetrad?
{Cl
{Cl
Sn {Cl
{NH_{4}
and yet it is a stannous compound, and gives a black precipitate with
H_{2}S; so that valency does not necessarily go with the series. The
second point is that an objection may be urged, as, for example, in
ammonium chloride (the lecturer stated above that here N was a pentad,
the addition of the chlorine having caused the N to assume the pentadic
character), it may be said, why should you not suppose that it is the
chlorine "which has altered its valency, and that the compound should be
written:
{H
{H
N { \
{H--Cl
{H/
There is something to be said for this view, but on the whole the
balance of the evidence is in favor of nitrogen being a pentad.
In conclusion the lecturer stated that his principal object was to
direct the attention of chemists, and especially of young chemists, to
the question: Is there or is there not any evidence derived from
the properties, the decompositions, or the relative stabilities of
substances to warrant us in believing that two classes of compounds
exist: one class in which there is interatomic connection alone, and
another in which the connection is molecular?
* * * * *