72 



NEW YORK STATE MUSEUM 



It remained for Le Chatelieri to make the first accurate observa- 

 tions in relation to the changes involved by the calcination of gyp- 

 sum. He noted that calcined plaster contained some 7 per cent 

 w^ater, as shov^n by his own experiments and by analyses made in 

 L'Ecole des Fonts et Ghausses. " M. Debray has demonstrated," 

 says Le Chatelier, '' that different hydrates of the same salt are 

 characterized by different tensions of dissociation, greater as the 

 proportion of water is greater. This results in the fact that the 



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F\G. 6 Temperature gradient f;r the decomposition of gypsum. (After Davis) 



temperature of decomposition of the different hydrates, under 

 given pressure will not be the same. In studying from this vi( 

 the decomposition of gypsum, I have discovered that it took place 

 in two very distinct periods of time." 



To confirm this conclusion he placed two grams of powdered^ 

 gypsum in a glass tube, which he heated gradually in a paraffii 

 bath, recording by a thermometer the temperature every five min- 

 utes. Using the time as abcissa and the temperature as ordinates' 

 he constructed a curve. This curve did not rise regularly but 

 contained two horizontal breaks in its regularity. The tempera-^ 

 ture after rising rapidly to 110° rose more slowly from 110° to 



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1 Acad. des. vSci, Compt. Rend. 96, 1668. 1883. 



