210' Solidification of Gypsum. 



matter is in very great excess ; yet, no doubt, each case requires 

 a specific amount, in order to produce the maximum of sohdity. 

 When water alone is employed, after the first mixture, the paste 

 rarely exhibits any remarkable tendency to become hard ; but a 

 fresh application of one of the foregoing solutions never failed to de- 

 velop it promptly. 



There is also a marked difference as to the time required for the 

 operation ; solutions of carbonate and sulphate of potassa, if suffi- 

 ciently dilute, produce their effects so slowly as to admit of com- 

 plete incorporation, whereas Rochelle salt acts as soon as the pow- 

 der touches the fluid and all subsequent motion necessarily weakens 

 the cohesion. If crystals of Rochelle salt be triturated with raw gyp- 

 sum and water, and then brought in contact with the mixture, there 

 will be no apparent interval of time between contact and solidifica- 

 tion. This extreme rapidity effectually prevents incorporation by 

 the ordinary mode, and would induce one to imagine that Rochelle 

 salt does not possess 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 render 

 raw gypsum capable of solidification. Those of soda, as far as they 

 were examined, invariably produced a contrary effect, if we except 

 Rochelle salt, which, however, seems to operate by its potassa. Yet 

 it is remarkable that several neutral salts of the latter alkali, as the 

 nitrate and chlorate, did not occasion the slightest alteration. The 

 bi-carbonate of potassa invariably produced a brisk effervescence, 

 which considerably impaired, although it did not prevent, solidifica- 

 tion. The same disadvantage 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 re- 

 peated, with equal success, upon pure sulphate of lime obtained by 

 precipitation. 



The opinion that carbonate of lime facilitates or causes solidifica- 

 tion in the ordinary case, seems but little entitled to belief, when it 

 is considered that the heat, necessary for the burning of plaster, falls 



