180 PROCEEDINGS OF THE AMERICAN ACADEMY. 



The relative composition of crystals and mother liquor was evidently 

 worthy of study. A solution of sodium chromate containing a consid- 

 erable amount of sodium sulphate was cooled to 10° to crystallize. The 

 crystals were drained in a centrifuge, washed with a little cold water, 

 and again centrifuged. 7.05 grams of this solution contained 0.432 

 gram of sodium sulphate and 1.707 grams of the chromate, whereas 

 2.25 grams of the hydrated crystals contained 0.425 gram of the sul- 

 phate and 0.515 gram of the chromate. The solubility of the chromate 

 at 10° is roughly speaking 50 parts in 100 of water and of the sulphate 

 9 parts in 100 of water. The distribution of the dissolved substances 

 between their solution and mixed crystals is thus seen to be in the 

 same sense as their respective solubilities at the temperature of crys- 

 tallization. The sulphate decidedly tends towards the crystals, the 

 chromate towards the mother liquors. 



As has been elsewhere pointed out, Salkowski was the first to observe 

 that the curve of solubility of the hexahydrate cuts those of the deka- 

 and tetrahydrates at about 19.5° and 26° respectively. Mylius and 

 Funk ^^ earlier observed that the transition from the dekahydrate to the 

 anhydride is accomplished through the intermediate tetrahydrate, which 

 is stable between 20° and 65". Salkowski,^^ commenting upon their 

 work in view of his later discovery of the hexahydrate, says that this 

 statement must be extended to include the hexahydrate for its few de- 

 •grees of stability. The statement of Salkowski must now be extended 

 to include the fact that the transition from dekahydrate to the anhy- 

 dride may be through the hexa- and tetrahydrates, or in the absence 

 of the hexahydrate, this transition may be through the tetrahydrate 

 alone, as originally stated by Mylius and Funk. 



We have confirmed the statement of Salkowski that the temperature 

 of the transition from the hexahydrate to the tetrahydrate is in the 

 neighborhood of 26°, but owing to lack of time we have been unable to 

 determine this point with exactness. 



The exact determination of the two lower transition points, for use 

 in precise thermometry, will now be described. 



The Apparatus. 



The apparatus employed was like that employed in determining the 

 transition temperature of sodium sulphate.^^ It was essentially similar 



" Abhandlung der Physik. Techn. Reichsanstalt, III, 449 (1900). 



^■f Loc. cit. ; this fact had also been observed but not published by Richards 

 and Churchill before that time. 



" Richards and Wells, Zeit. phya. Chem., 43, 465 (1903) ; These 

 Proceedings, 38, 43 (1902). 



