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.
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