Mr Emmet ofi tlie Solidification (yfRaw Gypsum. 71 



time between contact and solidification. This extreme rapidity 

 effectually prevents incorporation by the ordinary mode, and 

 would induce one to imagine that Rochelle salt docs not pos- 

 sess the power ; for when the gypsum and solution are worked 

 together with a spatula, although the particles feel hard and 

 harsh, they readily crumble, and, by continuing the operation, 

 actually assume a semi-fluid condition. 



No other salts but those holding potassa were found to ren- 

 der raw gypsum capable of solidification. Those of soda, as 

 far as they were examined, invariably produced a contrary ef- 

 fect, if we except Rochelle salt, wbich, however, seems to ope- 

 rate by its potassa. Yet it is remarkable, that several neutral 

 salts of the latter alkali, as the nitrate and chlorate, did not oc- 

 casion the slightest alteration. The bi- carbonate of potassa inva- 

 riably produced a brisk effervescence, which considerably impair- 

 ed, although it did not prevent, solidification. The same dis- 

 advantage characterizes the action of super-sulphate of potassa, 

 whenever the mineral contains an admixture of carbonate of 

 lime, as was found to be the case with the specimen of gypsum 

 under examination. As the idea has been advanced that the 

 setting property of ordinary burnt plaster, depends upon the 

 presence of carbonate of lime, most of these experiments were 

 repeated, with equal success, upon pure sulphate of lime ob- 

 tained by precipitation. 



The opinion, that carbonate of lime facilitates or causes soli- 

 dification in the ordinary case, seems but little entitled to belief, 

 when it is considered that the heat, necessary for the burning of 

 plaster, falls far short of that required for bringing limestone to 

 its caustic state, or even to that half-calcined condition which 

 renders it capable of hardening under water ; but, whatever 

 may be its agency subsequent to the application of heat, the 

 operation must be totally different in the present case, since the 

 supersulphate of potassa completely decomposes all the carbo- 

 nate of lime in the gypsum. 



It is probable, as Gay-Lussac^has observed, in his examina- 

 tion of this singular property of burnt plaster *, that we should 

 refer the fact to an inherent property of the mineral ; yet I can- 



%Annales de Chimie et de Physique, torn. xi. p. 43(5. 



