EFFECTS OF PRESSURE ON NITRATES. 593 



great that at 239° the transition pressure is below 500 kgm. Failure 

 to find the transition means either that the transition point of the 

 pure substance is appreciably higher than 219°, or else that II will 

 support considerable superheating with respect to I. A special run 

 ruled out the possibility that the transition from II to I is of the ice 

 type and had been overlooked for that reason. I also made search 

 for new modifications at room temperature to 12300 kgm. without 

 success. 



During the first run I made measurements of the time rate of transi- 

 tion and of the breadth of the band of indifference. The breadth of 

 the band varied from 100 to 200 kgm. The transition velocity is 

 unique in that the velocity with falling pressure is less than with 

 rising pressure. This may be an effect of the free HNO3. 



The transition temperature II to III is given as 161° by Wallerant^ 

 and 161°. 4 by Gossner.'^ Both of these values are lower than the 

 value found above, 164.4°. It is significant that when I repeated the 

 run, after the substance had become impure, I found a lower value 

 than 164.4°, about 163°. It is therefore plausible to suppose that the 

 lower values of Wallerant and Gossner may have been due to impurity. 

 No previous measurements of the change of volume or of the heat of 

 transition have been made. 



Thallium Nitrate. This salt was prepared by dissolving metallic 

 Thallium in warmed dilute nitric acid, and by repeatedly crystallizing 

 from aqueous solution. Preparation from the element was necessary 

 because the nitrate is not kept in stock by any of the chemical supply 

 houses. After each crystallization the crystals were washed; the end 

 product was entirely free from acid reaction. The purification of 

 this substance is particularly easy because of the great difference of 

 solubility in hot and cold water; water at 100° dissolves about 100 

 times as much water at 0°. Only enough of the salt could be made 

 for one filling of the apparatus. I used for this all the metallic Thal- 

 lium available at the time in this country, and none was to be pro- 

 cured from abroad. The salt was hammered cold into an open 

 perforated steel shell, and pressure transmitted directly to it by kero- 

 sene. 



With this single specimen a large number of readings were made. 

 Measurements of this substance are of unusual difficulty because of 

 the very small changes of volume and the large amount of super- 

 pressure required to force the reaction to run. At least one unsuccess- 



7 B. Gossner, ZS. Kryst. 38, 110-168 (1903). 



