Radio-activity 571 



speed is gi-eater, approaching in some eases within five per cent, that 

 of light, the mass is very much less. The /3-ray3 must be streams of 

 particles, identical with those of cathode rays, possessing the minute 

 mass of J. J. Thomson's corpuscle, some eight-hundredth part of that 

 of a hydrogen atom. A third or 7 type of radiation was also detected. 

 More penetrating even than /S-rays, the 7-ray8 have never been 

 deflected by any magnetic or electric force yet applied. Like 

 Rbntgen rays, it is probable that 7-ray8 are wave-pulses in the 

 luminiferous aether, though the possibility of explaining them as 

 flights of non-electrified particles is before the minds of some 

 physicists. 



Still another kind of radiation has been discovered more recently 

 by Thomson, who has found that in high vacua, rays become apparent 

 which are absorbed at once by air at any ordinary pressure. 



The emission of all these difterent types of radiation involves a 

 continual drain of energy fi-om the radio-active body. When I\I. and 

 Mme Curie had prepared as much as a gi-amme of radium chloride, 

 the energy of the radiation became apparent as an evolution of heat. 

 The radium salt itself, and the case containing it, absorbed the major 

 part of the radiation, and were thus maintained at a temperature 

 measureably higher than that of the surroundings. The rate of 

 thermal evolution was such that it appeared that one gramme of 

 pure radium must emit about 100 gTamme-calories of heat in an hour. 

 This observation, naturally as it follows from the phenomena pre- 

 viously discovered, first called attention to the question of the source 

 of the energy which maintains indefinitely and ^^Ithout apparent 

 diminution the Monderful stream of radiation proceeding from a 

 radio-active substance. In the solution of this problem lies the 

 point of the present essay. 



In order to appreciate the evidence which bears on the question 

 we must now describe two other series of phenomena. 



It is a remarkable fact that the intensity of the radiation from a 

 radio-active body is independent of the external conditions of tem- 

 perature, pressure, etc. which modify so profoundly almost all other 

 physical and chemical processes. Exposure to the extreme cold of 

 liquid air, or to the great heat of a furnace, leaves the radio-activity 

 of a substance unchanged, apparent exceptions to this statement 

 having been traced to secondary causes. 



Then, it is found that radio-activity is always accompanied by some 

 chemical change; a new substance always appears as tlie parent 

 substance emits these radiations. Thus by chemical reactions it is 

 possible to separate from uranium and thorium minute quantities 

 of radio-active materials to which the names of uranium-X and 



