RICHARDS. — TRANSITION TEMPERATURES OF SODIUM CHROMATE. 187 



convenient, taken together with the other points given by this same 

 salt, as a means of calibrating a Beckmann thermometer. The interval 

 between the dekahydrate-tetrahydrate point (19.987) and the higher 

 transition temperature is almost 6°, the usual length of a Beckmann 

 thermometer. By adding a degree to the scale, which could easily be 

 done in future, the lowest transition temperature of sodium chromate 

 also could be brought within the scope of the instrument. To be able 

 to verify a thermometer over its whole length, and at the same time to 

 evaluate accurately the interval 0.462 on one part of the scale, all with 

 the single purified salt, is indeed a highly desirable circumstance. 



Of course the determination of the several points as given in this 

 paper must be looked upon as merely preliminary, because time per- 

 mitted the study of these temperatures on only two thermometers. 

 The average of the readings of many thermometers should ultimately 

 be taken. Nevertheless we believe that the averages given in this 

 paper are sufficiently exact to be a great boon to those desiring to verify 

 thermometers about the temperature of the room. 



It is a pleasure to acknowledge the usefulness of a grant made many 

 years ago by the Rumford Fund, of the American Academy of Arts 

 and Sciences, for this purpose. Much of the material used in the 

 investigation was purchased with its aid. 



Summary, 



1. The great advantage of sodium chromate as a means of verifying 

 thermometers at three fixed temperatures in the neighborhood of 20° 

 is emphasized. 



2. A method for the easy preparation of pure sodium chromate has 

 been elaborated. 



3. The existence of three hydrates of sodium chromate; namely, the 

 deka-, hexa- and tetrahydrates has been confirmed. 



4. The accurate estimation of the point of conversion of each of 

 these hydrates into each of the other two has been shown to be easily 

 feasible. 



5. The addition of heteromorphous substances lowers the transition 

 points as usual. 



6. The addition of sodium sulphate (which gives an isomorphous 

 dekahydrate) raises the temperature of the dekahydrate-hexahydrate 

 e(iuilibrium. This is perhaps the first observation of this type of 

 phenomenon. 



7. The temperature of the dekahydrate-hexahydrate transition is 

 found to be approximately 19.525°, on the international hydrogen 

 scale. 



