CYPSU-M DEPOSITS OF NEW YORK 7I 



changed limestones Avhich aggregate many hundreds of feet in thick- 

 ness. This circumstance as well as the other facts regarding the 

 fauna of SaHna time becomes intelligible, however, when connected 

 with the vicissitudes that life must encounter in sea waters of fluc- 

 tuating salinity. 



PROPERTIES OF GYPSUM AND THEORY OF ITS TRANSFORMATION 



TO PLASTERS 



The composition and peculiar properties of gypsum have been 

 the subject of frequent study by chemists since the development of 

 exact methods of analysis. A brief review of the more important 

 investigations will serve to show the intricate nature of the prob- 

 lems encountered and assist their explanation in the light of recent 

 researches, so far as they may have been solved. 



We are indebted to Lavoisier for the first definite data on the 

 composition of gypsum.^ He dissolved the mineral in water and 

 found that its solubility was about one part by weight in 500 parts 

 water. From the solution he was able to crystallize the gypsum 

 out, and he therefore considered the mineral to be a chemical salt. 

 Furthermore he determined the nature of the acid and base, as 

 well as the presence of water of crystallization. By experiment it 

 was found that the cooking of gypsum produced no new compound 

 but simply drove off the water. In Lavoisier's opinion all of tlie 

 combined water disappeared in the process, though he seems to 

 have been familiar with the fact that commercial plaster of paris 

 contained a small amount of moisture ; consequently he was at loss 

 to understand why plaster heated to a higher temperature than cus- 

 tomary should be deprived of setting qualities. 



PayeUj, in 1830 found that gypsum heated at 80^ C. in a cur- 

 rent of dry air or 115° C. in a closed space "began to lose very 

 slowly a part of its water of crystallization. This drying proceeds 

 very rapidly as the temperature is raised, but beyond a certain 

 point (200° C.) an important modification takes place. The sul- 

 fate of lime hydrates with difficulty, and when heated at 300- 

 400° C. loses all power to take up water of crystallization." 



In 1840 Berthier^ showed that, contrary to the belief of Lavoisier 

 and others, calcined plaster contained from 3 to 8 per cent of water, 

 and his results were confimied later by Lanclrin. 



r- ' 



1 Acad, des Sci. Compt. Rend. Paris. 1765. 



»Chimie Industriolle 1830 and Pr(5cis dc Chimic Industiit'lle. Paris. 

 1 85 1, cd. 2. p. 301. 



3 Ann. des Mines, 1840, ser. 3, 19:655. 



* Ann. de Chimie et dc Phys. ser. 5, 3 1440 . 



