still however retaining a slighter cohesion between its particles than existed between those of the gypsum previous to its calcination, and without receiving any degree of transparency. In the transition of Plaster to this state of solidity, the gypsum has hitherto been said to resume its water of crystallization; hut what is this water of crystallization? what idea. ought this expression to convey to the mind?
If, in order to settle my opinion on the subject, I consult those who have written on chemistry, or on mineralogy, the majority conceive, that this water is that which a large quantity of salts and earthy substances retain when they crystallise; and that this water is necessary, in these cases, to the crystallization of the substances, but constitutes no part of their essence. But how can the water, absorbed by the plaster, which is evidently very different from water of crystallization, be necessary to the process? What part does the water in crystallized gypsum act in the crystallization of it? a satisfactory answer to these two questions, is, I conceive, requisite in all the cases in which they occur.
Some of these authors think, that, in several instances, the water enters into their composition as an essential ingredient; and upon this subject I refer to a passage in the first volume of the Mineralogy of M. Brongniart, page 96; as also to a very judicious doubt on pressed by the Ahbé Haÿ, in his Mineralogy, vol. IV. p. 351. I confess, that I had long ago adopted this opinion. But is all the water, that may be included in these mineral substances, to be so considered? Certainly not. In many of them the water is foreign to their substance, and has entered merely in consequence of the attraction (to which I have given the name of attraction by approximation) more or less powerful, exerted upon it by their integrand molecules; in this case it is only imbibed and interposed between
