PRESENT PROBLEMS IN RADIOACTIVITY. 23 



The radioactive products derived from each radio-element together 

 with the type of radiation emitted during their disintegration, are 

 shown graphically in Fig. 1. 



The radiations from actinium have not been so far examined 

 sufficiently closely to determine the character of the radiation emitted 

 by each product. There is some evidence that a product, actinium 

 X, exists in actinium corresponding to Th X in thorium.* It has 

 not, however, been very closely examined. 



The question of nomenclature for the successive products is im- 

 portant. The names Ur X, Th X have been retained and also the 

 term emanation. The emanation from the three radio-elements in 

 each case gives rise to a non-volatile type of matter which is deposited 

 on the surface of the bodies. The matter initially deposited from the 

 radium emanation is called radium A. Eadium A changes into B 

 and B into C, and so on. A similar nomenclature is applied to the 

 further products of the emanation of thorium and actinium. This 

 notation is simple and elastic and is very useful in mathematical dis- 

 cussion of the theory of successive changes. In the following table 

 a list of the products is given, together with the nature of the 

 radiation and the most marked chemical and physical properties of 

 each product. The time T for each of the products to be half trans- 

 formed is also added. 



The changes which occur in the active deposits from the emana- 

 tion of radium, thorium and actinium have been difficult to determine 

 on account of their complexity. For example, in the case of radium, 

 the active deposit obtained as a result of a long exposure to the emana- 

 tion contains quantities of radium A, B and C. The changes 

 occurring in the active deposit of radium have been determined by 

 P. Curie, Danne and the writer. The value of T for the three 

 successive changes is 3, 21 and 28 minutes, respectively. Radium A 

 gives only a rays, B gives out no rays at all, while C gives out a, (3 

 and y rays. These results have been deduced by the comparison of 

 the change of activity with time, with the mathematical theory of 

 successive changes. The variation of the activity with time depends 

 upon whether the activity is measured by the a, /? or y rays. The 

 complicated curves are very completely explained on the hypothesis of 

 three successive changes of the character already mentioned. 



The activity of a vessel in which the radium emanation has been 

 stored for some time rapidly falls to a very small fraction after the 

 emanation is withdrawn. However, there always remains a slight 

 residual activity. The writer has recently examined the activity in 



* Godlewski {Nature, Jan. 19, 1905) has recently separated actinium X. 

 It is similar in chemical properties to thorium X and loses half of its activity 

 in about 10 davs. 



