186 PROCEEDINGS OF THE AMERICAN ACADEMY. 



It is interesting to note that the difference between the two transi- 

 tion temperatures is given as almost exactly the same quantity by the 

 final readings of the two thermometers. 



Difference by thermometer 15200 = 0.462° 

 " " 15276 = 0.461° 



This of course gives a means of calibrating the Beckmann ther- 

 mometer, which gave 0.459° for the interval — evidently the marks 

 were a trifle too far apart on the latter instrument. 



The close parallelism of the results with the two Baudin thermome- 

 ters seems to indicate that the most probable cause of error in such 

 work is not due to mistake in observing either of the transition temper- 

 atures, but rather to the difficulty of determining the ice-point. There 

 can be no question that either of the sodium chromate equilibrium- 

 temperatures is a far better means of fixing the scale of a thermometer 

 to be used near room temperature than is the freezing point of water. 



The Temperature of the Transition from NazCrO^ ■ QH^O 

 to Na^CrO^ ■ 4.H^0. 



The material used in this determination was sodium chromate ob- 

 tained from the mother liquors of previous preparations, purified by 

 recrystallization as dekahydrate until the temperature of transition 

 from the dekahydrate to the hexahydrate as shown by the Beckmann 

 thermometer was 0.580. A supersaturated solution of this pure salt 

 at 25° was inoculated with hexahydrate and further cooled with stirring 

 to 18°, the dekahydrate being scrupulously excluded. The plentiful 

 crystals of hexahydrate were then centrifuged to remove the excess of 

 mother liquor, and in order to make certain that none of the dekahy- 

 drate was present, the crystals were allowed to stand in a bath at 25° 

 for an hour. No change occurred in the appearance of the material. 

 Just before taking the transition temperature a few crystals of the 

 tetrahydrate were added. 



Whether on account of the interference of other unknown hydrates, or 

 of the comparatively small heat of this transition, or of the unavoidable 

 haste in which the experiments had to be conducted, much less constant 

 results were obtained in this case than in the preceding. It appeared 

 that the equilibrium temperature of the hexa- and tetrahydrates was 

 not far from 25.90° on the hydrogen scale ; but this figure is subject to 

 further revision. 



This point is worthy of further investigation, for it is exceedingly 



