74 



CEMENTS, LIMES, AND PLASTERS. 



"By this method the floor-gypsum, when heated for ten hours at 

 400, showed an unmistakable loss of binding capacity: 



"While the determination of the influence of temperature was pos- 

 sible both by dilatometric methods and by weighing, the determina- 

 tion of the influence of crystalline structure was rendered impossible 

 in both cases by the fact that we had to deal with an uneven surface. 

 Nevertheless the orienting experiments showed that the half-hydrate 

 crystalline form, if retained, exerts an influence in that it materially 

 retards dead-burning; in other words, an amorphous gypsum, by burn- 

 ing, loses the setting capacity much more easily than a half-hydrate 

 of well-developed crystalline structure. 



"To determine this fact more accurately three forms of gypsum 

 were used: 



" 1. Well-developed half-hydrate obtained by nitric acid. 



"2. Alabaster gypsum of commerce, with little crystalline develop- 

 ment. 



"3. Gypsum obtained by treating a stucco gypsum with an excess 

 of water, leading to a development of exceedingly fine needles, which 

 were very apt to lose water and in such case show nothing of the half- 

 hydrate form. 



" These samples were heated for about five minutes on a gentle-red 

 glow in the platinum crucible and then sealed up with a slight excess 

 of water in a test-tube and left to harden. After about twenty-four 

 hours, sample No. 1 had already become much harder and showed gyp- 

 sum crystals under the microscope; the other two samples had remained 

 perfectly soft and showed no change under the microscope. After three 

 days the first sample had been completely hardened and entirely trans- 

 formed into gypsum crystals. The alabaster gypsum is still soft, but 



