Duane — Heat Generated by Radio-active Substances. 265 



calorimeter, and the abscissas of the points,/ are the instants at 

 which the polonium was raised again. The numbers written 

 near the lines cd and de are the electric currents in amperes 

 which were flowing through the iron-nickel thermo-couple dur- 

 ing the corresponding intervals of time. 



Between the points b and e I was searching for the proper 

 value of the current to counterbalance the heat effect, and 

 between the points e and/" I was reducing the current to zero. 

 The last experiment was not as good as the others, because the 

 natural drift of the bubble was large and changed a little dur- 

 ing the experiment. 



I compared the ionization due to the polonium when spread 

 out in a thin layer on a disk of platinum (b e) with that due to 

 a thin layer of radium. The results of the experiments appear 

 in Table 2. 



Table 2. 













Heat 











"Weight of 



effect 











EaBr- 2 that 



of this 



Date of 



Current that 



Heat pro- 



Ionization 



has same 



quantity 



experiment 



compensated 



duced cal- 



current due 



activity as 



of radium 



1909 



heat effect. 



orie per 



to polonium 



polonium 



calorie 





Ampere 



hour 





g- 



per hour 



May 4 



•00143 



•012 



1-3 X10" 



1 -75 



•0110 



June 4 



•oouo 



•009 



1-19+10- 



•66 



•0095 



June 25 



•00100 



•008 



•99X10" 



7 -57 



•0084 



It is evident that the generation of heat and the ionization 

 current due to the polonium decrease with the time. The 

 ionization current decreases at a rate indicating decay to half 

 value in 136 days, which is very close to the value previously 

 found by other experimenters. The heat effect decays a trifle 

 faster than this, but the differences are not greater than one 

 would expect considering the magnitude of the quantities of 

 heat evolved. It follows from this that the heat effect was 

 certainly due to the polonium. 



On account of the difficulty of obtaining saturation in 

 measuring the ionization of the radium and of the polonium, 

 the method of comparing the activities of the two substances 

 must be regarded as approximate only. Remembering this, 

 it appears that the heat generated by the polonium is very 

 close to that generated by the quantity of radium that would 

 produce the same ionization as the polonium. 



I have made a number of experiments on phosphorescent 

 salts to see if they generate heat when in the phosphorescent 

 state. Every time I examined such a salt one or two hours 

 after it had been withdrawn from the light of the sun (or of an 

 ultra-violet ray arc lamp), I found a small but measurable 

 generation of heat. Twenty-four hours later not the slightest 

 effect could be detected in the majority of cases, but a few 

 times I observed a small generation of heat and on withdraw- 



Am. Jour. Sci. 

 . 19 



-Fourth Series, Vol. XXXI, No. 184.— April, 1911. 



