56 Scientific Proceedings, Royal Dublin Society. 



Another cause would have been the reduction of gas-pressure 

 within the plaster on cooling, while the experiment was being 

 prepared. Complete equalization of this by the flow of water 

 would be almost indefinitely postponed, owing to the resistance 

 offered by the plaster of Paris. 



But the principal cause of these pressure-differences resides in 

 a property of plaster of Paris, which seems, up to the present, to 

 have been overlooked, and the possibility of which was suggested 

 to me by Dr. Joly. 



On setting, even when an excess of water is present, plaster of 

 Paris does not take up the full amount of water it is capable of, 

 but continues steadily to absorb water long after setting is com- 

 plete. Boiling plaster of Paris immediately after setting (as in 

 Copeland's experiment) does not appear to satisfy its avidity for 

 water, but even seems to increase its powers of absorption. Whether 

 this absorption is due to slow hydration of the calcium sulphate, or 

 to the formation of interstices in it during crystallization, I have 

 not determined. The former view is favoured by the fact that 

 boiling the plaster seems to increase the effect ; and it is known 

 that plaster of Paris begins to lose water at comparatively low 

 temperature, viz. 70° C. 1 



In order to show how easily this absorption of water might 

 have produced the results which Copeland has observed, I shall 

 quote a few of my own experiments : — 



Exp. i. — 100 grm. of plaster of Paris was well mixed with 80 

 c.c. of water in a wide-necked bottle of about 200 c.c. capacity. 

 As the plaster set, a layer of free water lay over its surface. 

 When set, the surface of plaster exposed to the water was 4'8 

 cm. diameter. The bottle was now transferred into a vessel 

 of boiling water, which was kept in ebullition for two hours. 

 The water was then allowed to cool ; and the bottle lay in 

 it for twenty-four hours. A rubber bung, with two perforations, 

 was then fitted tightly into the neck of the bottle. Through the 



1 Mendeleef : The Principles of Chemistry, vol. i., p. 611. "Gypsum loses 1J 

 and 2 equivalents of water at a moderate temperature." (Note.) "According to le 

 Chatelier (1888), l\ H 3 is lost at 120° ; that is, H 2 0, 2CaS0 4 is formed, hut at 194° 

 all the water is expelled. According to Shenstone and Cundall (1888), gypsum begins 

 to lose water at 70° in dry air. The semi-hydrated compound H 2 0, 2CaSC>4 is also 

 formed when gypsum is heated with water in a closed vessel at 150° (Hoppe- 

 Seyler)." 



