436 PEOCEEDINGS OF THE AMERICAN ACADEMY. 



is clear that the newly made sodic sulphate is identical with that pre- 

 pared in 1898; and since the most serious deviation to be observed 

 above is seen in the two determinations of the purest sample (No. IV) 

 it is evident that the slight irregularities are to be ascribed rather to the 

 difficulties of thermometry than to any fault of the preparations. 



The preliminary values with this one thermometer having thus proved 

 that the salt made by different methods gives identical results within the 

 limit of error of the experiment, readings were taken with the more 

 accurate thermometers. 



In the final work, all the precautions mentioned by Guillaume to be 

 observed in using thermometers were adhered to as closely as possible.* 



Inasmuch as it was not convenient to make large quantities of pure 

 ice where it was desired to make only one reading at a time, we preferred 

 to make our ice readings in a tube similar to that used for the sodic 

 sulphate ; but the tube was always well filled with the pure ice and the 

 requisite volume of purest cold water and then surrounded with a large 

 jar of ordinary ice. The thermometer, in a vertical position, was read 

 with an accurate Geneva cathetometer. 



The purest sodic sulphate, as obtained by the methods given above, 

 was used. It was found that a slight efflorescence was amply sufficient 

 to supply the anhydrous phase of the sodic sulphate. The average 

 specific gravity of the inverting mixture was found to be about 1.5. 



Besides the barometric pressure, the depth of immersion of the ther- 

 mometer in the solution was always noted in order to correct for external 

 pressure. 



The complete record of an observation is given below in order to 

 show the manner of applying the corrections : f — 



No. 15, March 7, 1901. 



* Guillaume, Traite pratique de la Thermome'trie de Precision (1889). 

 t See " Notice sur les Thermometres a mercure," Paris, Gauthier-Villars et 

 Fils, 1896. This paper accompanies tlie thermometers. 



