Plaster completely burned at moderate temperature, CaSO4, probably amorphous. Anhydrite and dead-burned plaster, CaSO4, crystalline. Selenitic deposit from boilers, 2 CaSO4 + H2O, or CaSO4 + 1/2 H2O, containing 6.2 per cent. of water. The circumstance that the hot calcium sulphate can crystallize with 1/4 its normal amount of water indicates that for this proportion of water it has a greater attraction than for the other 3/4. Having a similar bearing is the fact that when burned at lower temperatures, gypsum only loses the last portions of water with extreme slowness. Now, if it be the case that anhydrous calcium sulphate has a greater attraction for the first half molecule of water, then the operation of hydration will proceed very rapidly at first, more slowly afterward. Many such cases are known, e.g., that of copper sulphate. Conversely, if only 3/4 of the water of hydration be expelled during the baking of gypsum, the material obtained should hydrate itself more slowly. For our present purpose it will be convenient to recalculate the numbers given by Landrin (_vide supra_) so as to make the calcium sulphate and water add up to 100. This treatment of the numbers gives a mean result for the six analyses of 7.68 per cent. of water, the amounts not varying by more than 1 per cent. It will be seen that the dehydration has never passed the composition corresponding to 2 CaSO4 + H2O; indeed, the material approximates more nearly to the composition 3 CaSO4 + H2O. It appears probable, therefore, that in the successful preparation of plaster the whole, or nearly the whole, of the gypsum is changed, but that this change does not result in the production of CaSO4, or of a mixture of CaSO4 and CaSO4 + 2 H2O, but of a lower hydrate of calcium sulphate. In the case of the analyses, given by Landrin, of fine plaster for potteries, the percentages of water (8.14 and 8.08) correspond closely to that of a hydrate, 3 CaSO4 + 2 H2O, which would contain 8.1 per cent. of water. Some surprise may have been excited by the fact that the well known method of revivifying hydrated calcium sulphate has recently formed the subject of a patent (Eng. pat., No. 15,406). The method described in the specification consists in reducing the materials (waste moulds, etc.) to small lumps, and baking between the temperatures of 95 deg. and 300 deg.. It is mentioned that the whole of the water must not be expelled. This is no doubt correct, but it must be effected by regulating the _time_ of baking, since by prolonging the operation all the water of crystallization can be expelled far below 300 deg.. To secure even baking the mass is kept stirred by mechanical stirrers, a necessary precaution, since the operation is to be carried out in an ordinary kiln. The process is stopped when a portion of the plaster is found to set in the required time, a method of regulation which will probably be found to work well in practice.--_Chem. Trade Jour._