184 PROCEEDINGS OF THE AMERICAN ACADEMY. 



range of error of each. The value of 19.525°, the result of the final 

 determination above, may be accepted for the present as the most 

 l)robable figure. ^^ 



TJie Temperature of the Transition from Na ^Cr i. 10 H^O 

 to Na^CrOiAH^O. 



This mixture of hydrates is somewhat more difficult to prepare than 

 the mixture of deka- and hexahydrates. From a study of the diagram 

 on page 178 it appears that the tetrahydrate is metastable with respect 

 to the hexahydrate between the temperatures 19.53° and 25.9 ±. The 

 difficulty is to exclude the hexahydrate during the cooling of the tetra- 

 hydrate and solution from 26° to 19.99°, the transition point of the 

 deka- to the tetrahydrate. ^^ Even after the dekahydrate has appeared, 

 and has in part relieved the supersaturation, care must be taken to ex- 

 clude the intermediate hydrate. At 19.99° both dekahydrate and 

 tetrahydrate are metastable with respect to the hexahydrate. If to a 

 mixture of the two former a crystal of the latter be added, a depression 

 and irregularity in the temperature occurs, and the temperature 

 gradually falls to that of the lower transition point. ^^ 



The mixture of hydrates is prepared (within the test-tube in which 

 the transition temperature is to be determined) by a method closely 

 resembling that described for preparing the mixture of dekahydrate and 

 hexahydrate. To the dekahydrate is added such a quantity of tetra- 

 hydrate that nearly all the solid will be in solution at 28°. By heat- 

 ing the mixture to this temperature, one may feel certain that none of 

 the hexahydrate is present. The tube is then closed by a clean stop- 

 per through which the handle of a stirrer passes. The stopper serves 

 to protect the contents from accidental inoculation by hexahydrate. 

 When the solution has been cooled to about 19°, it is inoculated with 

 some freshly prepared dekahydrate, which must be free from hexahy- 

 drate. Further gradual subtraction of heat, with constant stirring, 

 gives a mixture of the desired plastic consistency. 



The material used in the preliminary determination of this transition 

 point was the same as that used in the preliminary determination of 



2" Over one third of an atmosphere's pressure is necessary to alter this 

 temperature by as much as 0.001°. (SeeTammann, "Krys. and Schmelzen," 

 262 (Leipzig, 190.3).) 



2^ Gernez's supposed disintegration of Na2Cr04.4H20 in the presence of the 

 IOH2O salt (Compt. Rend., 149, 77 (1910), would not affect this point, if, 

 as Gernez reasonably supposes, this disintegration is due simply to the for- 

 mation of the dekahydrate from included water within the salt, and not to 

 the formation of a hydrate with less than 4H2O. 



