SUM r.xxio.N OK (IIVM1KS I\ KADIOACII\ I. BODIES. 211 



therefore equal to Id 7 gramme-calories. Now the heat emitted during the combination 

 of 1 cub. centim. of hydrogen and oxygen to form water is 2 gramme-calories. Thus 

 the emanation gives out during its changes 5 X 10* times as much energy as the 

 combination of an equal volume of hydrogen and oxygen to form water. The energy 

 emitted from a vessel containing the radium emanation is almost equally divided 

 between the emanation and the products radium A and ra Hum C. Each of these 

 products gives out a rays. It is probable that the ' rayless ' product radium B gives 

 out flu- less heat than the other products. 



There seems to be little doubt that the energy emitted from radium is about equally 

 divided between the products which break up with the expulsion of a particles, 

 i.e., 25 per cent, of the total heat emission is supplied in each case by the breaking up 

 of radium, the emanation, radium A and radium C. The energy radiated is, in 

 all probability, mainly derived from the kinetic energy of the expelled a particles. 

 Since the a particles expelled from the products of uranium, thorium, and actinium 

 are projected with about the same velocity as from radium, it necessarily follows that 

 each atom of the radioactive products which breaks up with the expulsion of a particle 

 gives out about an equal quantity of energy. This amount of energy is about 

 6 X 10~ B erg for each atom at each stage of its disintegration. 



Since there is the same number of changes in thorium as in radium, the heating 

 effect of thorium will be proportional to its activity, i.e., will be only about 5 X I0~ 7 

 of that from an equal quantity of radium. 



Since the discovery of the actual production of helium from the radium emanation 

 by RAMSAY and SODDY, there has been a tendency to assume that helium is the final 

 transformation product of radium. There is no evidence in support of such a 

 conclusion, for, as we have already seen, the radium atom goes on through a further 

 series of slow changes after the first rapid changes have taken place during which the 

 helium makes its appearance. In addition, the evidence supports the view that one 

 a particle is expelled from each atom at each stage of its disintegration, excepting 

 }x>88ibly the rayless change. The expulsion of four a particles, of mass alxnit that of 

 the helium atom, still leaves a heavy atom behind. 1 have previously pointed out 

 that the particles, in all probability, consist of helium atoms expelled at the 

 successive stages of the disintegration. This conclusion js supported by measurements 

 of the mass of the a particle, and by the observations of the rate of production of 

 helium by the radium emanation made by RAMSAY and SODDY. 



The similarity of the a particles from the different radio-elements indicates that 

 they consist of expelled particles of the same kind. On this view, helium should be 

 produced by each of the radio-elements. The presence of helium in minerals such as 

 thorium, for example, in monazite sand, and the Ceylon mineral described by RAMSAY, 

 suggests that helium is a product of thorium as well as of radium. 



Taking the view that the n. particles are projected helium atoms, we must regard 

 the atoms of the radio-elements as compounds of some known or unknown substance 



2 E 2 



