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 solidity. 
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 sufli- 
ciently dilute, produce their effects so slowly as to admit of com- 
te 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 beiho Apparent interval of time between. contact and solidifica- 
is 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 bard 
and harsh, they readily crumble, and by continuing the operate 
actually assume a semi-fluid condition. i 
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, 
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 obrained by. 
precipitation. : 
The opinion that carbonate of lime facilitates or causes “slidifica- 
tion in the ordinary case, seems but little entitled to belief, when it 
is considered that the heat, necessary for the Scie os kopet ae 
