the Cause and Nature of Radioactivity. 371 



with tiie source and maintenance of the energy dissipated by 

 radioactive substances. Radioactivity is shown to be accom- 

 panied by chemical changes in which new types of matter 

 are being continuously produced. These reaction products 

 are at first radioactive, the activity diminishing regularly 

 from the moment of formation. Their continuous production 

 maintains the radioactivity of the matter producing them at 

 a definite equilibrium-value. The conclusion is drawn that 

 these chemical changes must be sub- atomic in character. 



The present researches had as their starting-point the facts 

 that had come to light with regard to thorium radioactivity 

 (Rutherford, Phil. Mag. 1900, vol. xlix. pp. 1 k 161). Besides 

 being radioactive in the same sense as the uranium com- 

 pounds, the compounds of thorium continuously emit into 

 the surrounding atmosphere a gas which possesses the 

 property of temporary radioactivity. This " emanation/ - ' as 

 it has been named, is the source of rays, which ionize gases 

 and darken the photographic film *. 



The most striking property of the thorium emanation is 

 its power of exciting radioactivity on all surfaces with which 

 it comes into contact. A substance after being exposed for 

 some time in the presence of the emanation behaves as if it 

 were covered with an invisible layer of an intensely active 

 material. If the thoria is exposed in a strong electric field, 

 the excited radioactivity is entirely confined to the negatively 

 charged surface. In this way it is possible to concentrate 

 the excited radioactivity on a very small area. The excited 

 radioactivity can be removed by rubbing or by the action of 

 acids, as, for example, sulphuric, hydrochloric, and hydro- 

 fluoric acids. If the acids be then evaporated, the radio- 

 activity remains on the dish. 



The emanating power of thorium compounds is independent 

 of the surrounding atmosphere, and the excited activity it 

 produces is independent of the nature of the substance on 

 which it is manifested. These properties made it appear that 

 both phenomena were caused by minute quantities of special 

 kinds of matter in the radioactive state, produced by the 

 thorium compound. 



The next consideration in regard to these examples of 

 radioactivity, is that the activity in each case diminishes 

 regularly with the lapse of time, the intensity of radiation at 

 each instant being proportional to the amount of energy 

 remaining to be radiated. For the emanation a period of 



* If thorium oxide be exposed to a white heat its power of giving an 

 emanation is to a large extent destroyed. Thoria that has been so 

 treated is referred to throughout as " de-emanated." 



