RICHARDS. DEHYDRATION OF CRYSTALLIZED SALTS, 27 



does not lose an essential amount of water at 22° in an atmosphere con- 

 taining aqueous vapor at a tension of 4 millimeters ; while the aqueous 

 vapor tension of its saturated solution is more than 15 millimeters.* 



It is almost needless to call attention to the fact, recognized as a 

 matter of course by any one who has especially studied the subject of 

 water of crystallization, that the temperatures of dehydration given in 

 text-books are often wholly indeterminate and almost meaningless, since 

 the tensions of the aqueous vapor in the surrounding atmosphere are 

 frequently not given. Much of our literature upon this subject needs 

 a systematic revision. 



* W. D. Bancroft has recently pubhshed statements bearing upon this subject, 

 some of wiiich are founded upon slight experimental evidence, under the title 

 " Solids and Vapors." (The Physical Review, Ithaca, N. Y., Vol. III. p. 401, and 

 The Journal of Physical Chemistry, Ithaca, N. Y., Vol. I. p. 345.) 



Cambridge, Mass., May, 1897. 



