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