178 PHYSICS OF THE ELECTRON 



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 compon- 

 ents of the atom or of an expulsion of an a or /? 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 ordinary 

 matter may also be undergoing slow "rayless changes," for such 

 changes could not have been detected in the radio-elements, unless 

 its succeeding products emitted 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 in- 

 dicated 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 to compare and arrange the chemical 

 properties of these products and to see if 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 electro- 

 chemical series. The products which change most rapidly are present 

 in the least quantity in radium and pitchblende. Only the slower 

 changing products like the radium emanation and radium D and E 

 exist in sufficient quantities to be examined by the balance. It is 

 possible that the products radium A, B, and C may be obtained in 

 sufficient quantity to obtain their spectrum. 



VIII. Connection between the a Particles and Helium 



The discovery of Ramsay and Soddy that helium was produced by 

 the radium emanation was one of the greatest interest and importance, 

 and confirmed in a striking manner the disintegration theory of radio- 



