/ 



326 Rate of Evolution of Heat by Pitchblende, 



From the known volume of calorimeter we obtain ?• = 3*4 cms. 

 nearly. Using the mean value found for the heat evolution 

 per gram of pitchblende we find # = 5*7 x 10~ 8 . The con- 

 ductivity of powdered pitchblende is probably not less than 

 that of dry sand, about 9 X 10 ~ 4 . Putting in these values we 

 obtain 6 = 1'2 x 10~ 4 of a degree or about 0*15 scale-division. 

 Hence, even if the junction were at the centre of the 

 pitchblende, the error would be very small. 



Conclusion. 



The three experiments for which the temperature charts 

 are given seem much the most reliable, as in the earlier 

 experiments the importance of having the powder thoroughly 

 dry had not been realized, nor were the experiments continued 

 long enough to ensure that heating caused by initial dampness 

 and oxygen had ceased. The same remark applies to the 

 experiment in which damp air was intentionally introduced. 

 It will be observed that of the three charts No. 1 is much 

 more regular than Nos. 2 and 3. 



From chart No. 1, Heat evolution 7-1 X 10~ 5 calorie per hour per grin. 

 „ No. 2, „ „ 5-45x10-5 „ „ „ 



„ „ No. 3, „ „ 5-85x10-5 ., 



We may take as the mean of these three values that each 

 gram of pitchblende evolves 6*1 x 10 -5 calorie per hour. 



This result is surprisingly high. An analysis of a sample 

 of the pitchblende used indicated the presence of 64 per cent, 

 of uranium. According to Boltwood each gram of uranium 

 in equilibrium is associated with 34xl0~ 7 of a gram of 

 radium. The accepted value for the rate of evolution of heat 

 by the uranium and products in equilibrium with one gram 

 of radium is 5*6 x 10 ~ 2 calorie per second. According to 

 these figures each gram of pitchblende should only evolve 

 4*4 x 10~ 5 calorie per hour. However, the figure 5'6xl0~ 2 

 calorie per second for total heat associated with each gram 

 of radium is based on the assumption that one gram of 

 radium in equilibrium with its immediate products evolves 

 110 calories per hour; the latest determination of this quantity 

 by von Schweidler and Hess [Le Radium, Feb. 1909) gives 

 118 calories per hour, which would somewhat reduce the 

 discrepancy. 



It is hoped to repeat the experiment in a slightly modified 

 form, using a solid block of pitchblende. 



In conclusion, I wish to express my gratitude to Dr. Joly, at 

 whose suggestion the work was carried out, for his kindness 

 and valuable advice during the progress of the research. 



Physical Laboratory, Trinity College, Dublin, 

 October, 1909. 



