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 may be 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- 
ee on it does not seem to form any permanent combination with 
{hégypsum, 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 solidification, is the 
“hecessity of the presence of potassa, and the ed with which the 
takes place seems greatly opposed to the supposition that 
the wedi 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 career As plain water does not answer, 
until after the evaporation of the fluid, it seems more probable that 
a saline solutions exert a kind of repulsion towards the heres: of 
eicediligpapnonss 
tnattbacnbietelliedsas 
* Annales de Chimie et de Physique, tom. xl. p. 436. 
