72 Mr Emmet on the Solidification of Raw Gypsum, 



not but think the foregoing experiment abundantly proves that 

 it does not always depend 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 solutions, added partiality, af- 

 fect the composition of the gypsum ; yet I have satisfied myself 

 that the alteration is neither uniform nor essential to the result, 

 although it is extremely difficult to ascribe the solidification, 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 cannot 

 effect the composition of sulphate of lime, and although the 

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

 ture enumerated^ it does not seem to form any permanent 

 combination with the gypsum, since the latter, in two experi- 

 ments, was found to lose one-twelfth of its weight by the mixture 

 of the substances, and subsequent washing with warm-water. 

 The only uniformity observable in all the saline solutions ca- 

 pable of producing solidification, 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 pulverised 

 gypsum, and saturate it by the solution of carbonate of potassa, 

 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 ap- 

 pears but little diminished 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 gypsum, and thus 

 tend to promote that solidification which is so very characteristic 

 of it in the burnt state. 



The experiment which first exhibited the solidifying property 

 of raw gypsum, was well calculated to give the impression that 

 chemical decomposition was necessary for the result. I wished 

 to determine how far fresh precipitated carbonate of lime was 

 capable of improving gypsum, (intending subsequently to burn 

 the mixture). With this view, pulverised raw gypsum was 



