them, but does not act, in any degree, the essence of the substance containing it; this is water of crystallization in the true sense of that term. But it happens not unfrequently that water enters as an essential part or principle into the formation of their integrand molecule, in which case it is not water of crystallization but water of composition, and ought so so be denominated. It is obvious, that the one does not necessarily exclude the other, for they are totally unconnected with each other. It is obvious too, that we cannot consider the water, which a substance may contain, as making one of its component principles, unless there he solid reasons for such a conclusion. For instance, in the case in question, the combination of lime with sulphuric acid gives rise to rectangular tetrahedral prismatic molecules with square or rectangular bases; while water, uniting with the same principles, combined in the same proportion, gives rise to molecules in right tetrahedal prisms, with rhomboidal bases, having angles of 118° 4′ and 66° 56′; it is very evident, that this water, by its union, has totally changed the form of the integrant molecule, which it could only do by combining with it. This that will be rendered still more evident by what follows.
Calcined gypsum then, from what has been shewn, most have lost its. water of composition. This loss must he owing to the molecules of water having a much greater affinity for caloric in motion, or heat, than it has for the other component molecules of gypsum
If the gypsum had lost in calcination only its water of crystallization, or, what comes to the same thing, tint which was regularly interposed between its molecules, there would remain a simple combination of lime and sulphuric acid, in the proportion of 32 parts of lime and 46 of acid in every 78 parts, or 40 of lime and 60 of acid in 100: a combination precisely the same with that which constitutes
