CONTRIBUTIONS FROM THE CHEMICAL LABORATORY OF 



HARVARD COLLEGE. 



NOTE ON THE RATE OF DEHYDRATION OF CRYS- 

 TALLIZED SALTS. 



By Theodore William Richards. 



Presented May 12, 1897. 



In the course of the investigation of the atomic weights of barium and 

 strontium, a few observations were made upon the rate of dehydration 

 of several of the salts of these metals. These observations supplement 

 the work of Lescoeur,* Roozeboom,t and others, concerning the vapor 

 tension of water contained in crystallized salts. It was found that the 

 rate of the withdrawal of water by means of a constant desiccating agent 

 varied very greatly with different substances, as well as with different 

 hydrates of the same substance. Baric chloride, for example, exposed 

 to a dry atmosphere at first effloresces with great rapidity, and then, 

 after it has lost one of its molecules of water of crystallization, con- 

 tinues to be dehydrated at a much slower rate. This is nothing more 

 than was to be expected, for the rate of dehydration must be approxi- 

 mately proportional to the tension of the aqueous vapor in the salt, pro- 

 vided that the aqueous vapor is equably removed from the surrounding 

 atmosphere. It is obvious, therefore, that successive weighings of any 

 material which is gradually efflorescing may furnish a useful clue to the 

 relative vapor tensions of the successive molecules of crystal water con- 

 tained in that substance. 



As an example of the definiteness of the change of rate, one of several 

 concordant experiments upon baric cloride may be cited. 1.2223 grams 

 of the finely powdered crystallized salt were weighed out in a small 

 crucible ; and with this substance was mixed a small amount of pui'e 



* Ann. Chim. Phys., (G.), XIX. .5.33, XXL 511, (7.). II 78. 



+ Coraptes Rendus, CX. 134 ; Zeit. Phys. Ch., H. 449 ; also IV. 31, etc. etc. 



