Radio- Active Matter in ordinary substances. 397 



intensity of the radiation depends on the ratio of the emission 

 to the absorption of the substance, and as long as this ratio 

 remains the same, the radiation will be unaltered ; thus for 

 example, since the radiation and absorption of a gas are each 

 proportional to the number of molecules, the ratio of the two 

 will be independent of the pressure, and thus the stream of 

 radiation through a gas due to its own radiation will be inde- 

 pendent of the pressure of the gas. If the absorption for all 

 substances bore the same proportion to the radiation, the flow 

 of ionization would be the same for all substances; in this case 

 there would be no screening, for it is easy to see that the screen 

 would emit as much radiation as it would absorb. We see that 

 for a substance to act as a screen I /\ must be less for the screen 

 than for the substance it displaces ; if I /\ is greater for the screen 

 than for the substance the screen will increase the ionization. 

 For a substance to be efficient as a screen the absorption must 

 be abnormal in comparison with the radiation. Now we have no 

 direct information as to the amount of radiation in different 

 substances, but we know that whereas in general the absorption 

 of substances for Becquerel radiation is proportional to the density, 

 the heavier metals absorb considerably more than this law suggests; 

 thus if radiation is proportional to density the substances which 

 obey Lenard's law would not act as screens, it is only substances 

 such as lead, iron, platinum, &c. which give abnormal absorption that 

 would be effective. This view is confirmed by the experiments 

 described above, which show that lead as a screen is enormously 

 more effective in comparison with such substances as sand or 

 water than its density warrants. 



Radiation of the kind we are contemplating involves a con- 

 tinual transformation of the internal energy of the atom into 

 heat; for taking the case of air as an example, the radiation 

 absorbed by the air is not wholly used in producing radiation of 

 the same kind, part of it is spent in ionizing the gas, and is, on 

 the recombination of the ions, converted into heat; the mechanical 

 equivalent of the heat developed by the recombination of the ions 

 in a cubic centimetre of air at atmospheric pressure and tempera- 

 ture is between 1 and 10 ergs per century, and this will be a 

 measure of the amount of internal atomic energy lost by a cubic 

 centimetre of air in the same time. 



Just as Rontgen radiation in its passage through matter gives 

 rise to more absorbable secondary radiation, and this again to 

 still more absorbable tertiary, so the radiation streaming through 

 bodies will not be all of one type, the primary will give rise to 

 secondary, the secondary to tertiary, and so on, and it is I think 

 probable that in small vessels part of the ionization in the vessel is 

 due to the secondary and tertiary radiation proceeding from its walls. 



