Radio-activity 571 
speed is greater, approaching in some cases within five per cent. that 
of light, the mass is very much less. The 8-rays 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 y type of radiation was also detected. 
More penetrating even than f-rays, the y-rays have never been 
deflected by any magnetic or electric force yet applied. Like 
Rontgen rays, it is probable that y-rays 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 different types of radiation involves a 
continual drain of energy from the radio-active body. When M. and 
Mme Curie had prepared as much as a gramme 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 gramme-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 without apparent 
diminution the wonderful 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 the 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 
