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XXXI. On the Bate of Evolution of Heat by Pitchblende. 

 By Horace H. Poole *. 



Outline of A Jet hot/. 



rpHE following- method was adopted to directly measure 

 X the rate of evolution of heat by pitchblende. The 

 powdered pitchblende (Joachimsthal) is carefully dried and 

 placed in a spherical Dewar calorimeter. This is placed in 

 a large vessel of planed ice, and the difference of tempe- 

 rature between the surface layer of pitchblende in contact 

 with the bottom of the calorimeter and the ice is measured 

 by means of a sensitive thermo-couple. The pitchblende, 

 initially at air temperature, cools comparatively rapidly at 

 first, but eventually attains a temperature which remains 

 approximately constant. 



In this state the rate of generation of heat by the pitchblende 

 is equal to the rate of loss. 



To find this rate the calorimeter is tilled with water and 

 the rate of cooling noted. The cooling was in accordance 

 with Newton's law, so that the rate of loss of heat is pro- 

 portional to the difference of temperature between the inside 

 and the outside of the calorimeter. Now as it is the tempe- 

 rature of the surface layer of pitchblende that is measured, 

 the conductivity of the latter does not affect the result ; so 

 that the heat evolved per hour by the pitchblende equals the 

 constant temperature difference between the surface layer 

 and the ice multiplied by the thermal conductance of the 

 calorimeter (i. e. the number of calories which escape per hour 

 when the inside is 1° C. hotter than the outside). 



Calorimeter and Ice Vessel. 



The calorimeter employed is approximately spherical with 

 a neck about 8 cm. long and 1 cm. internal diameter. Its 

 capacity is about lu'O c.cs., and the walls are silvered in the 

 usual way. The calorimeter is supported on a wooden tripod 

 inside a cylindrical zinc vessel 12 inches high and 10 inches 

 diameter which is filled with planed ice. This is supported on 

 a wooden stand inside a larger galvanized iron vessel, also filled 

 with ice, which in turn is placed inside a barrel, the space 

 between the walls being filled with cork dust, and a bag 

 half full of the same being placed over the lid. The inner 



* Communicated bv the Author. 



