Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/388

 bardiglione. Plaster, which is the result of the calcination of gypsum, would then be bardiglione, which is far from being true. The distinctive property of gypsum, when changed to the state of plaster, is rapidly to absorb water, and, by that absorption, to acquire consistency, and even a certain degree of hardness. Bardiglione, neither before nor after calcination, has any action whatever on water; and if reduced to powder before it is mixed with it, its particles still retain their state of division.

It is certain, therefore, that bardiglione and plaster, though composed of the same principles, lime and sulphuric acid, and in the same proportions, are two substances of different natures. And as this difference cannot arise from the nature of the principles entering into combination, or from the manner in which they are proportioned to each other, it must necessarily arise from the mode of arrangement of the constituent molecules which form the integrand molecules. Directing our view to this point, we shall see that gypsum, in its transition to the state of plaster; having been deprived only of its water of composition, without the combination of sulphuric acid with the lime having been destroyed, each of the integrand molecules, which compose the mass of plaster, should be considered as a right trihedral prism, having scalene triangles for its base, perfectly similar to the integrand molecules of gypsum, and exactly the half of its primitive crystal; but having void spaces, within the solid, similar in shape to the molecule or molecules of water removed, the figure of which is yet unknown to us. Thus the component particles of lime and sulphuric acid are not in immediate contact in the integrand molecules of plaster, except in parts of their surface; while in bardiglione, on the contrary, these same component molecules are in that state of approximation which is adapted to their complete solidity.