182 PROCEEDINGS OF THE AJVIERICAN ACADEMY. 



only in the usual way, but also through the glass behind the mercury. 

 The freezing-points were taken both before and not long after the tran- 

 sition temperatures, and during the intervening time the instruments 

 were carefully protected from large temperature changes. 



The Temperature of the Transition from NazCrOi- lOffzO 

 to Na^CrOi ■ QH^O. 



When moist crystals of the dekahydrate are melted without the ad- 

 dition of the solid hexahydrate, the temperatures obtained are variable 

 and generally low. This is undoubtedly due to the fact that the water 

 of crystallization, together with the water adhering to the crystals, is 

 enough to dissolve the sodic chromate completely, thus preventing the 

 formation of solid hexahydrate. 



In order to avoid this difficulty, it was found best to heat about one 

 fifth of the moist hexahydrate in a quartz or platinum vessel to 60°, 

 thus driving off perhaps a third of the water of crystallization. The 

 cooled product yielded solid Na2Cr04 • 4H2O and its solution. This was 

 added to the remainder of the Na2Cr04 • lOHoO, after which the whole 

 mass was warmed to 28° C. in the test-tube in which the determination 

 was to be made. The proportion of Na2Cr04 • 4H2O to Na2Cr( \ • IOH2O 

 should be such that at this temperature the whole is in solution. The 

 solution was then cooled by dipping the test-tube into a beaker con- 

 taining water at 20°. When the temperature of the solution had 

 fallen to 25°, a crystal of Na2Cr04 • 6H2O was added ; it produced 

 copious crystallization. On further cooling to 19° or slightly lower 

 a crystal of dekahydrate was added, and still more heat was taken 

 from the mixture, with stirring, until the mixture of the two solid 

 phases and solution of sodium chromate was plastic. 



The temperature of the water bath was kept one or two tenths of a 

 degree higher than the equilibrium point (19.5°) and the room tem- 

 perature also was kept not far away. This latter condition was not very 

 essential, because with one of our thermometers the stem exposure was 

 6.5° and with the other only 0.5°. 



In a preliminary set of readings, corrected in the usual way for cali- 

 bration, for internal and external pressure, for error in the fundamental 

 interval, for the corrected ice-point, for the exposed column, and to the 

 hydrogen scale, the following values were found : — 



Preliminary determination by thermometer 15200 19.518° 

 " " " 15276 19.525° 



19.522° 



