The proof is so clear and self-evident when once one's attention is
directed to it, that, though I intend to develop it more fully on
another occasion, I feel that it is better to publish an outline of it
at once.
Thomsen has found that the heat of neutralization of the soluble bases
of the alkalies and alkaline earths with sulphuric acid has a mean
value of 31.150 c. within very narrow limits. When hydrochloric or
nitric acid is employed, the value is 27.640 c., also within very
narrow limits. Now, this agreement of the six bases in their behavior
with sulphuric acid, much more of the seven bases with both HNO_{3}
and HCl, is so close that it cannot be regarded as accidental, but, in
the words of Meyer, the heat of formation of a salt in aqueous
solution is a quantity made up of two parts, one a constant for the
base, the other for the acid. But of the twenty salts thus formed,
some are anhydrous in the solid state, others have water of
crystallization, up to ten molecules in the case of Na_{2}SO_{4}. If
water of crystallization exists in solution, it will be necessary to
suppose that this agreement is accidental, which is absurd, as a
glance at the probabilities will show. Thomsen himself expressly
states that he regards the dissolved state as one in which the
conditions are comparable for all substances; this would be impossible
if water of crystallization were present.
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