RICHARDS, — DEHYDRATION OF CRYSTALLIZED SALTS. 



25 



Rate of Dehydration op Baric Clokide. 



0.14 

 0.13 

 0.12 

 0.11 

 0.10 

 0.09 

 0.08 

 0.07 

 0.06 

 0.05 

 0.04 

 0.03 

 0.02 

 0.01 

 0. 



40 60 



80 



100 120 140 160 180 200 220 



The sharp angle occurs exactly at the point corresponding to BaCLHoO. 



A number of other salts also were experimented upon, — those with 

 little water of crystallization usually behaving in the same general man- 

 ner as baric chloride, while those with much water of crystallization 

 (such as sodic carbonate) gave less angular curves, in which the various 

 hydrates were only slightly, if at all, perceptible. It is well known that 

 all the hydrates are not necessarily formed during such a dehydration. 

 In order to obtain a sharp change with any salt, it is manifest that one 

 must use a desiccating agent which does not remove the water much 

 faster from the surface than it can be supplied by diffusiou from below. 

 Hence from such a substance as sodic carbonate one obtains more satisfac- 

 tory results by using diluted sulphuric acid instead of phosphoric oxide in 

 the desiccator ; by no means a new idea. For any given case, the aqueous 

 vapor tension of the desiccating agent should be not much less than that 

 of the salt during the removal of the water corresponding to the second 

 limb of the desired curve. 



It is convenient to have at hand for this purpose a table giving the 

 approximate strength of sulphuric acid necessary to obtain a given tension 

 of aqueous vapor at a given temperature. The data for such a table may 

 be found in the work of Regnault,* and Lunge and Isler.f 



* Ann. Cliim. Pliys., (3.), XV. 179. Landolt und P.orii.'^tein, Tiibellen, 1894, p. G5. 

 t Z. Angew. Cliem., 1890, p. 129. Landolt und Burnstein, p. 196. 



