Solidification of Gypsum. 211 



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 maybe its agency, subse- 

 quent to the application of heat, the operation must be totally differ- 

 ent in the present case, since the super-sulphate of potassa com- 

 pletely decomposes all the carbonate of lime in the gypsum. 



It is probable, as Gay-Lussac has observed, in his examination of 

 this singular property of burnt plaster,* that we should refer the fact 

 to an inherent property of the mineral ; yet I cannot but think the 

 foregoing experiment abundantly proves that it does not always de- 

 pend upon the simple union with water, and subsequent aggregation 

 of the saturated particles, as seems to be the fact with burnt plaster. 

 These cases may not, indeed, be parallel, as some of the saline solu- 

 tions, added, partially affect the composition of the gypsum ; yet I 

 have satisfied myself that the alteration is neither uniform nor essen- 

 tial to the result, although it is extremely difficult to ascribe the so- 

 lidification, in the foregoing instances, to the proper cause. Both 

 potassa and its carbonate are extremely deliquescent, and do not, 

 therefore, act by rapidity of crystallization ; sulphate of potassa can- 

 not affect the composition of sulphate of lime, and, although the 

 former salt may possibly be formed in all the cases of mixture enu- 

 merated, it does not seem to form any permanent combination with 

 the gypsum, since the latter, in two experiments, was found to lose 

 one twelfth of its weight by the mixture of the substances and sub- 

 sequent washing with warm water. The only uniformity observable, 

 in all the saline solutions capable of producing sohdification, is the 

 necessity of the presence of potassa, and the rapidity with which the 

 operation takes place seems greatly opposed to the supposition that 

 the result depends upon double decomposition. If we take the pul- 

 verized gypsum and saturate it by the solution of carbonate of po- 

 tassa, all subsequent chemical action, from the same substances, 

 should be prevented, and yet, when the solidified mass, thus formed, 

 is worked up again with a fresh portion of the same saline solution, 

 it sets with equal facility. This property appears but little dimin- 

 ished by three or four repetitions. As plain water does not answer, 

 until after the evaporation of the fluid, it seems more probable that 

 the saline solutions exert a kind of repulsion towards the particles of 



* Annates de Chimie et de Physique, torn. xl. p. 436. 



