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 

 extremel}^ 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 w^hat 

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

 an indirect method, based on chemical means, is frequentl}' 

 emplo3'ed. 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 anal3'sed 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 slowl}^ 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 the}^ 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 



