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A variety of naturally occurring forms of calci~un sulphate dihydrate producc substantially identical dityerential thermograms under conditions of ~~n i f o r m heating rate and particle size. 'These thermograms closely resemble those for synthetic dihydrate and 8-hemihydrate, showing four endother~nic el'fects and one esotherrilic effect below 500" C. Two of these endotherlnic effects, a t about 170" and 300" C, respectively, which have not bcen reported previously, were found to be easily masked by changes in heating rate or sample concentration. Resolution of the several effects \\,as improved by ~~s i n g differeiit heating rates a t different stages of the thermogram. Thcrnmograms of the a-hemihydrate were similar, except that the endothermic effect a t 300' C was not evident and the exothermic effect occurred a t a much lo\\rer temperature than for the dihydrate. No conspicuous differences were found in the temperatures corresponding to the v a r i o~~s endothermic and cxotherrnic effects which might be correlated with the general dehydration behavior of the particular material. The endothermic effect a t about 170" C appears to be associated with part of the hemihydrate to solubleanhydrite transition, possibly arising during the removal of the last traces of water.In the differential thermograms reported for many natural and sy~lthetic gypsunls of widely different origin (1-7), two large endother~nic effects occurring below 200" C have been attributed to the formation of the hemihydrate a~l d its subsequent deco~npositioil to soluble anhydrite. The exact temperatures a t which these effects appear depend on a variety of experilnental conditions, the resolution of the two endotherms being improved a t slow rates of heating (2). In the region of 400" C an exothernlic effect, not found when natural anhydrite is heated from room temperature, has been attributed to the conversion I t has been recorded that the above exothermic effect appears a t different temperatures, depending upon the origin of the gypsum sample (2, 3), and in the gypsum industry it is known that natural gypsums of different origin produce hemihydrates of widely differing properties. Hence an examination of several natural gypsums, as well as the pure hydrates of calcium sulphate, was undertalcen by differential thernlal analysis to investigate the possibility that some correlation might exist between both the endothermic and exothermic effect temperatures and the dehydration behavior of the particular gypsum. The natural gypsums selected represented several mineralogical forlns and attention was focused on thermal effects appearing below 500" C. EXPERIMENTALThe instrument used contained a horizontal furnace, wound with 15-gauge Kanthal A wire, having a maximum operating temperature of 1300" C. KO attempt was made to control the atmosphere within this furnace except in cases where a sample was cooled in a stream of dry nitrogen, after determination of a thermogram, prior to determination of a second thermogram on the same sample.'i...

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