12 M‘CLeLLanp— The Energy of Secondary Radiation. 
We can conclude from these experiments that when 6 rays of radium fall 
on a metal plate no easily-absorbed secondary radiation—similar, say, to a rays— 
is produced. By this conclusion we merely mean that no such radiation is 
produced comparable in intensity to the incident B rays. We know from the 
work of various observers that substances, in general, spontaneously emit an 
easily-absorbed radiation of very weak intensity. Our apparatus is not designed 
to detect such a very weak radiation, but merely to make certain that the 
penetrating secondary radiation studied in the previous papers, and which will 
be shown to be comparable in intensity to the incident B rays, is the only 
secondary radiation that we need consider when estimating the total energy of 
all the secondary effects. 
It is perhaps worth mentioning that the method used in the above experiments 
was altered by keeping the pressure of the gas between the plates constant 
during an experiment, and having the lower plate so mounted that it could 
be gradually screwed away from the upper plate. For small distances between 
the plates, the saturation-current might be expected to be proportional to the 
distance, provided there is no easily-absorbed radiation present. This was not 
found to be the case, and the experiment seemed at first to indicate the presence of 
easily-absorbed radiation. ‘The method, however, fails for the following reason, 
and it affords an example of the errors that may arise from neglecting effects due 
to secondary radiation:—The penetrating radiation between the plates is not 
merely a cylindrical pencil perpendicular to the plates, and cannot be made so; 
there are secondary rays travelling at all inclinations to the plates, so that, when 
the distance between the plates is increased, the path-lengths of all rays are 
not increased in the same ratio; and the saturation-current is therefore not 
proportional to the distance. This method of showing that a radiation is 
composed of two parts, one more easily absorbed than the other, is often 
employed; but for the reason stated it is not reliable when any secondary 
radiation is produced. 
DETERMINATION OF THE RATIO OF THE ENERGY OF THE SECONDARY RADIATION TO 
THAT OF THE PrRiMARY RADIATION PRODUCING IT. 
Having shown by the work in the previous papers, and the preliminary 
experiments in the present, that for all practical purposes the secondary radiation 
consists entirely of 8 rays very similar in every way to the incident 6 rays that 
produce them, we may therefore assume that the energy of the two radiations 
may be compared by measuring their ionising powers under similar conditions. | 
It was only necessary to make this comparison for one substance, as the relative 
intensities of radiation from a large number of elements had been previously 
determined ; the substance used was lead. | 
