116 NATURE OF THE RADIATIONS [CH. 
power and marked action in causing phosphorescence in many 
substances. The a rays were, in comparison, very little studied, 
and their importance was not generally recognized. It will, how- 
ever, be shown that the a rays play a far more important part 
in radio-active processes than the 8 rays, and that the greater 
portion of the energy emitted in the form of ionizing radiations 
is due to them. 
81. The nature of the a rays. The nature of the a rays 
was difficult to determine, for a magnetic field sufficient to cause 
considerable deviation of the 8 rays produced no appreciable effect 
on the « rays. It was suggested by several observers that they 
were, in reality, secondary rays set up by the @ or cathode rays in 
the active matter from which they were produced. Such a view, 
however, failed to explain the radio-activity of polonium, which 
gave out a rays only. Later work also showed that the matter, 
which gave rise to the @ rays from uranium, could be chemically 
separated from the uranium, while the intensity of the a rays was 
unaffected. These and other results show that the a and @ rays 
are produced quite independently of one another. The view that 
they are an easily absorbed type of Réntgen rays fails to explam 
a characteristic property of the a rays, viz. that the absorption of 
the rays in a given thickness of matter, determined by the elec- 
trical method, mcreases with the thickness of matter previously 
traversed. It does not seem probable, that such an effect could 
be produced by a radiation like X rays, but the result is to be 
expected if the rays consist of projected bodies, which fail to 
ionize the gas when their velocity is reduced to below a certain 
value. From observations of the relative ionization produced in 
gases by the a and @ rays, Strutt? suggested in 1901 that the « 
rays might consist of positively charged bodies projected with 
great velocity. Sir Wiliam Crookes?, in 1902, advanced the same 
hypothesis. From a study of the a rays of polonium Mme Curie’ 
in 1900 suggested the probability that these rays consisted of 
bodies, projected with great velocity, which lost their energy by 
passing through matter. 
1 Phil. Trans. p. 507, 1901. 
2 Proc. Roy. Soc. 1902. Chem. News, 85, p. 109, 1902. 
3 C. R. 130, p. 76, 1900. 
