The Theory of Transmutation 573 
gression, the loss per second being proportional to the mass of 
substance still left at the moment, and independent of its state of 
concentration or dilution. This type of reaction is well known in 
chemistry to mark a mono-molecular change, where each molecule 
is dissociated or altered in structure independently. If two or more 
molecules were concerned simultaneously, the rate of reaction would 
depend on the nearness of the molecules to each other, that is, to 
the concentration of the material. (4) The amount of energy liberated 
by the change of a given mass of material far transcends the amount 
set free by any known ordinary chemical action. The activity of 
radium decays so slowly that it would not sink to half its initial 
value in less than some two thousand years, and yet one gramme of 
radium emits about 100 calories of heat during each hour of its 
existence. 
The energy of radio-activity is due to chemical change, but clearly 
to no chemical change hitherto familiar to science. It is an atomic 
property, characteristic of a given element, and the atoms undergo 
the change individually, not by means of interaction among each 
other. The conclusion is irresistible that we are dealing with a 
fundamental change in the structure of the individual atoms, which, 
one by one, are dissociating into simpler parts. We are watching the 
disintegration of the “atoms” of the chemist, hitherto believed in- 
destructible and eternal, and measuring the liberation of some of the 
long-suspected store of internal atomic energy. We have stumbled 
on the transmutation dreamed by the alchemist, and discovered the 
process of a veritable evolution of matter. 
The transmutation theory of radio-activity was formulated by 
Rutherford! and Soddy in 1903. By its light, all recent work on the 
subject has been guided ; it has stood the supreme test of a hypo- 
thesis, and shown power to suggest new investigations and to co- 
ordinate and explain them, when carried out. We have summarised 
the evidence which led to the conception of the theory; we have now 
to consider the progress which has been made in tracing the successive 
disintegration of radio-active atoms. 
Soon after the statement of the transmutation theory, a striking 
verification of one of its consequences appeared. The measurement 
of the magnetic and electric deflection of the -rays suggested to 
Rutherford the idea that the stream of projectiles of which they 
consisted was a flight of helium atoms. Ramsay and Soddy, confining 
a minute bubble of radium emanation in a fine glass tube, were able 
to watch the development of the helium spectrum as, day by day, the 
1 Rutherford, Radio-activity (2nd edit.), Cambridge, 1905, p. 307. 
