36 Dr. T. Godlewski on Actinium 



which they call ThX. The activity of this product decays 

 with the time according to an exponential law; i. e., the 

 equation of monomolecular chemical reaction, falling to half 

 value in about four days. At the same time, the thorium, 

 which by the removal of thorium X had been deprived of 

 about 75 per cent, of its activity, recovered its activity, the 

 recovery curve being complementary to the curve of decay of 

 ThX. The substance UrX, discovered earlier by Crookes, 

 acts in a manner analogous to ThX. The /3 activity of this 

 substance decayed according to an exponential law with the 

 time, falling to half value in twenty-two days. 



Rutherford and Soddy have explained these phenomena on 

 the supposition that the radioactive bodies are producing 

 fresh radioactive matter at a constant rate, and that the 

 activity of the matter so formed decreases according to an 

 exponential law with the time. The discovery of these phe- 

 nomena supplied the basis for the disintegration theory, which 

 supposes that the atom of a radioactive body breaks up through 

 a series of well-marked stages. The resulting products are 

 quite distinct bodies, though they escape detection by chemical 

 methods on account of the minute amount of the substance 

 under investigation. Their existence is proved first of all by 

 electrical measurements which allow us to make the quanti- 

 tative investigations of the rate of change of these products. 



On looking over the series of successive products arising 

 from different radioactive bodies, striking similarity between 

 the products of thorium and actinium is at once manifest. 

 Thorium produces ThX, ThX the emanation, this gives rise 

 to the active deposit which undergoes two further transfor- 

 mations, the first slow change being a rayless one, the other 

 emitting all kinds of rays. Actinium in like manner pro- 

 duces an emanation which is transformed into an active 

 deposit which undergoes two further changes, the first being 

 a slow rayless change and the other a rapid change. 



This analogy in the number and nature of the products 

 pointed to the possibility* that there existed between the 

 actinium and its emanation an intermediate product which 

 bore the same relation to actinium that ThX bears to thorium. 

 In a letter to ' Nature ' (26 Jan. 1905) I gave the prelimi- 

 nary results of the investigation which proved the existence 

 of this product. 



Taking into consideration the similarity of actinium and 

 thorium, I applied to the actinium the same method which 



* See Rutherford, Bakerian Lecture : "The Succession of Changes in 

 Radioactive Bodies," Phil. Trans. Royal Soc. ser. A, vol cciv. pp. 1 90 & 

 204. 



