316 RADIO-ACTIVE PROCESSES [CH. 
librium, the activity of the second product is always found to vary 
at the same rate as that of the parent product. For example, the 
amount of the emanation produced by Th X is always proportional 
to the activity of the Th X, and decays at the same rate, we. it 
falls to half value in about four days. In the same way, the activity 
of the emanation X, produced by the radium emanation, after some 
hours have elapsed for conditions to become steady, is always found 
to be proportional to the activity of the emanation. In other 
words, the activity of the emanation X decays according to the 
same law, and at the same rate, as the radium emanation which 
produces it, z.e. to half value in a little less than four days. 
The rapid rate of heat emission of radium is connected with 
the radio-activity of that element. More than two-thirds of the 
heat emission of radium is due to the radium emanation and the 
secondary products to which the emanation gives rise. The heat 
emission seems to be for the most part connected with the emission 
of @ rays. 
The total energy which would be given out by a given quantity 
of radium is of quite a different order of magnitude to that ob- 
served in ordinary chemical reactions. 
198. Theories of radio-activity. A brief review will now 
be given of the working hypotheses which have served as a guide 
to the investigators in the field of radio-activity. These working 
theories have in many cases been modified or extended with the 
growth of experimental knowledge. . 
The early experiments of Mme Curie had indicated that radio- 
activity was an atomic and not a molecular phenomenon. This 
was still further substantiated by later work, and the detection and 
isolation of radium from pitchblende was a brilliant verification of 
the truth of this hypothesis. 
The discovery that the 8 rays of the radio-elements were 
similar to the cathode rays produced in a vacuum tube was an 
important advance, and has formed a basis of several subsequent 
theories. J. Perrin’, in 1901, following the views of J. J. Thomson 
and others, suggested that the atoms of bodies consisted of parts 
and might be likened to a miniature planetary system. In the 
1 Revue Scientifique, April 13, 1901. 
