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In this paper Graham began to distinguish what is now called water of crystallization from water of constitution, a distinction pointed to by some of Davy's researches, but a distinction which has remained too much a mere matter of nomenclature since the days of Graham.

The temperature at which the water is expelled is higher than is usually the case with zeolites; none is given off below 200�, and only about half at 250�; this is slowly reabsorbed again from moist air, and is therefore regarded as water of crystallization, the remainder being water of constitution.

There is also a large amount of water combined molecularly with many of the minerals of rocks, in which form it is called water of constitution.