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 1 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 a-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. 



