268 Dua >ie — Heat Generated hy Radio-active Substances. 



In the first series of experiments the phosphorescence was 



produced for the most part by the a-rays of the radium. "We 



know that each a-particle that strikes the phosphorescent zinc 



sulphide produces enough light to affect the eye, and it follows 



from the experiments described above that the energy of this 



light is no larger than the energy of the a-particle. The s-mallest 



velocity of an a-particle that has been measured and at the 



cm 

 same time detected by its scintillation is 5X10 8 - - , and the 



sec. 



kinetic energy of the a-particle at this velocity is 8xl0 -7 erg. 



This energy is about that required to raise of a mil-le- 



bj 1 100 



gram of a millimeter. The energy necessary to pro- 



b 1000 bJ J I 



duce the sensation of sight is less than the above quantity, 

 since only a part of the total light energy enters the eye, and 

 since probably the whole energy of the a-particle is not trans- 

 formed into luminous energy. 



The heat generated by one gram of pure radium can be 

 calculated from the data of Table 3. It is for tube A 110 and 

 for tube B 108 calories per hour. The difference between these 

 two numbers is not greater than the errors of experiment. 



The heat effect of one gram of radium calculated from 

 the data of Table 4 is 117 calories per hour, a value considerably 

 larger than the preceding values. This difference cannot be 

 explained by errors of experiment. It is probably due to the 

 fact that the radium employed in the second series of experi- 

 ments is several years older than that employed in the first 

 series, and contains, therefore, more of the disintegration 

 products of the radium, especially polonium, which generate 

 heat. 



I have made a number of attempts to measure the heat pro- 

 duced by the rays from radium at a distance from their source. 

 In the first" experiments a thermopile, a bolometer, and a 

 radiometer were tried, but none of these instruments gave 

 satisfactory results. A modified form of differential gas ther- 

 mometer gave positive indications of a heating effect, but the 

 only instrument that proved satisfactory was the differential 

 calorimeter described in the present paper. I hope shortly to 

 publish some details of these experiments, but will state here 

 simply that the problem is somewhat different from that 

 of measuring the energy of ordinary radiations (at least as 

 far as the penetrating radium rays is concerned), because a 

 relatively large amount of matter is required to stop these 

 penetrating rays, and the heat is generated throughout the 

 mass of this matter. 



