gypsu:m deposits of new york 73 



120°, stood stationary a long time at 128° and then went on up- 

 ward more rapidly between 130° and 140° ; a second stop analo- 

 gous to the first but less important took place at 163°. From 

 the results of this experiment W. A. Davis^ has plotted the fol- 

 lowing curve [fig. 6]. 



Le Chatelier then says, '' These two halts in the rise of the 

 thermometer were brought about by the absorption of heat which 

 accompanied the elimination of the water. They indicate the ex- 

 istence of two hydrates having different temperatures of decompo- 

 sition. To determine the composition of the intermediate hydrate, 

 I heated 10 grams of gypsum at a temperature of 155° which from 

 the above figures is intermediate between the decomposition tem- 

 peratures of the two hydrates. The loss of weight was as follows : 



TIME GRAMS 

 Hours Minutes 



.... 15 .66 



30 1-36 



45 152 



I 1 .56 



I 15 1-56 



"The loss of weigbt at 155° tends then to a well defined limit 

 of 1.56 grams which corresponds exactly to 1.5 molecules H^O 

 for I molecule of CaS04.'' This leaves us a material with a for- 

 mula of CaS04.3'^?H20 identical with " half hydrate '' noted by 

 Johnston^ as found in the form of scale in a steam boiler heated 

 to 121° C. and by Hoppe Seyler^ as formed by gypsum in presence 

 of water at 140-60° C. 



The same sample was then heated to 200° C. with the following 

 results : 



TIME LOSS OF WEIGHT 



/a Hours Minutes Grams 



1-56 



15 1-78 



.... 30 1 .98 



45 1-98 



I 2 .08 



This loss of 2.08 grams corresponds to two molecules of water to 

 one of CaSO^, that is, at 200° C. the dehydration is complete. 



In summing up his results Le Qiatelier says : " These experi- 

 ments show that there exists at least one inferior hydrate of cal- 

 cium sulfate having the formula CaS04.>2H20 and that it 



' Soc. Chem. Ind. Jour. 1907. 26:728. 



2 Phil. Mag. 1838. 



' Pogg. Ann. 1866. i27:i6r. 



