26 POPULAR SCIENCE MONTHLY. 



the period of change of thorium A is 55 minutes or 11 hours. In 

 order to settle the point, it is necessary to find some means of separa- 

 ting the products thorium A and B from each other. In the case of 

 thorium, this is done by electrolyzing a solution of thorium. Pegram 

 obtained an active product which decayed according to an exponential 

 law with the time falling to half value in a little less than one hour. 

 This result shows that the radiating product thorium B has the 

 shorter period. In a similar way, by recourse to electrolysis, it has 

 been found that the change actinium B has a period of 1.5 minutes. 

 In the case of radium, P. Curie and Danne utilized the difference 

 in volatility of radium B and C in order to fix the period of the 

 changes. 



It is very remarkable that the third successive product of radium, 

 thorium and actinium should not give out rays. It seems probable 

 that these rayless changes are not of so violent a character as the 

 other changes, and consist either of a rearrangement of the com- 

 ponents of the atom or of an expulsion of an a or /3 particle with so 

 slow a velocity that it fails to ionize the gas. The appearance of 

 such changes in radioactive matter suggests the possibility that ordi- 

 nary matter may also be undergoing slow ' rayless changes/ for such 

 changes can not be detected in the radio-elements unless the succeeding 

 products emit rays. 



It is seen that the changes occurring in radium, thorium and 

 actinium are of a very analogous character and indicate that each of 

 these bodies has a very similar atomic constitution. 



While there can be no doubt that numerous kinds of radioactive 

 matter with distinct chemical and physical properties are produced 

 in the radio-elements, it is very difficult to obtain direct evidence in 

 some cases that the products are successive and not simultaneous. 

 This is the case for products which have either a very slow or very 

 rapid rate of change compared with the other product. For example,, 

 it is difficult to show directly that radium B is the product of radium 

 A and not the direct product of the emanation. In the same way, 

 there is no direct evidence that radium C is the parent of radium D. 

 At the same time, the successive nature of these products is indicated 

 by indirect evidence. 



There can be little doubt that each of the radioactive products 

 is a distinct chemical substance and possesses some distinguishing 

 physical or chemical properties. There still remains a large amount 

 of chemical work to be done in comparing and arranging the chemical 

 properties of these products and in determining whether the successive 

 products follow any definite law of variation. The electrolytic method 

 can in many cases be used to find the position of the product in the 

 electrochemical series. The products which change most rapidly are 



