70 Mr Emmet on the Solidificatian of Raw Gypsum. 



far as I am aware, it has not been supposed that the raw or na- 

 tural production is capable of exhibiting the same property. 

 The following experiments, although resulting from an inquiry 

 not confessedly connected with the subject of the present com- 

 munication, and therefore not, perhaps, carried so far as they 

 might have been with advantage, are considered of sufficient 

 importance to receive a distinct notice. They satisfactorily 

 show, that native gypsum may be rendered capable of perfect 

 solidification, without having undergone the operation of burn- 

 ing, and may perhaps contribute to illustrate or render more 

 available the setting property of this valuable natural production. 



Raw gypsum, finely pulverized, is capable of undergoing 

 immediate and perfect solidification, when mixed with certain 

 solutions of the alkali potassa. Among those that answer best, 

 may be enumerated caustic potassa, carbonate and bi-carbonate, 

 sulphate and supersulphate, silicate and double tartrate or 

 Rochelle salt. 



In all these cases, the process may be easily rendered more 

 expeditious than when burnt plaster alone is employed, and the 

 resulting solid, after having been properly dried, does not seem 

 to differ essentially from that usually obtained, except in com- 

 position. There does not appear to be any exact point of sa- 

 turation ; for the solid masses, when broken up and worked with 

 fresh portions of the solutions, constantly recover their tendency 

 to set, even when the saline matter is in very great excess ; yet, 

 no doubt, each case requires a specific amount, in order to pro- 

 duce 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 develope it promptly. 



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

 the operation ; solutions of carbonate and sulphate of potassa, if 

 sufficiently dilute, produce their effects so slowly, as to admit of 

 complete incorporation, whereas Rochelle salt acts as soon as 

 the powder touches the fluid, and all subsequent motion neces- 

 sarily weakens the cohesion. If crystals of Rochelle salt be tri- 

 turated with raw gypsum and water, and then brought in con- 

 tact with the mixture, there will be no apparent interval of 



