534 SCIENCE PROGRESS 



phenomena were easily explained by the hypothesis that 

 thorium A, the first product, volatilised below 700 and was 

 inactive — i.e. gave no radiations ; the thorium B, already pro- 

 duced from it, remained on the wire unvolatilised at 700 , 

 and a fresh amount was slowly produced on the lead cylinder. 

 Further work has slightly modified this result, thorium A 

 being found to have a very slight radiation, and there being 

 two later products, thorium B and C, all three having an 

 extremely short-life period. The final product is inactive. 

 So far as present knowledge indicates, the whole series is : 



Thorium -> mesothorium -> radiothorium -> thorium X -» 

 thorium emanation -> thorium A -> thorium B -> thorium C 

 -> inactive product. 



Most of the above work has been accomplished by what 

 may be described as the direct method of work. In addition 

 an indirect method, based on chemical means, is frequently 

 employed. It will be discussed more particularly in the sub- 

 sequent parts of this paper. An example will be mentioned 

 here. 



The final product of the uranium-radium disintegrations is 

 supposed to be lead, another product being helium. Boltwood 

 has analysed a great number of uranium minerals ; they all 

 contain lead. He finds that the ratio, amount of uranium to 

 amount of lead, varies with the age of the mineral, the older 

 rocks giving a higher percentage of lead. This agrees with 

 the theory that lead is slowly produced from the uranium 

 and radium. Strutt has analysed thorium minerals for helium ; 

 he finds evidence of some definite relationship between the 

 quantities of thorium and helium which they contain. 



Section 2. — Nature and Properties of the Radiations from 



Radioactive Elements 



When an electric current is passed through a highly 

 exhausted vacuum tube, three kinds of rays are produced in it. 

 If the cathode is perforated, canal rays — so named by Wien — 

 are projected with enormous velocity in the direction opposite to 

 that in which the current is travelling ; they consist of particles, 

 each at least as heavy as a hydrogen atom, and always carry a 

 positive charge of electricity. Cathode rays, consisting of nega- 

 tive particles of electricity, convey the current from cathode to 



