164 RATE OF EMISSION OF ENERGY [CH. 
store of latent energy, which can only be manifested when the 
atom breaks up. For some reason, the atomic system becomes 
unstable, and an @ particle, of mass about twice that of the hydro- 
gen atom, escapes, carrying with it its energy of motion. Since 
the a particles would be practically absorbed in a thickness of 
radium of less than ‘001 cm., the greater proportion of the a 
particles, expelled from a mass of radium, would be stopped in the 
radium itself and their energy of motion would be manifested in 
the form of heat. The radium would thus be heated by its own 
bombardment above the temperature of the surrounding air. The 
suggestion that the heat emission of radium was connected with 
the expulsion of the « rays was first given by Sir Oliver Lodge! 
The energy of the expelled a particles does not account for the 
whole emission of heat by radium. It is evident that the violent 
expulsion of a part of the atom must result in intense electrical 
disturbances in the atom. At the same time, the residual parts of 
the disintegrated atom rearrange themselves to form a permanently 
or temporarily stable system. During this process also, energy is 
emitted, which is manifested in the form of heat in the radium 
itself. 
It has already been calculated (section 104) that the emission 
of energy in the form of @ particles, probably corresponds to about 
50 gram-calories per hour for one gram of radium. The observed 
heat emission of radium, under conditions when the a rays are 
nearly all absorbed in the radium itself, is 100 gram-calories per 
hour per gram. On account of the uncertainty attaching to the 
estimate of the energy of the a rays, it is not possible to deduce 
with accuracy how much of the total energy emitted is due to 
them. The evidence, taken as a whole, pots to the conclusion 
that a considerable fraction of the total emission of energy is due 
to the kinetic energy of the a rays. 
Runge and Precht (oc. cit.) determined the heat emission of 
radium by means of a thermometer, (1) when the radium was in a 
thin tube, and (2) when it was surrounded by a lead screen several 
millimetres in thickness. Within the limit of accuracy of the 
1 Nature, April 2, 1903. 
