236 SCIENCE PROGRESS 



milligram, an order of magnitude corresponding to that in the 

 case of lithium. 



The measurement of minute quantities of gases probably 

 ranks next in accuracy. With great care, using small apparatus, 

 the presence of o'oi cubic centimetre of, say, carbon dioxide can 

 be detected with certainty. This volume weighs 0*02 milligram. 

 Using a micro-balance, quantities of not more than one or two 

 milligrams can be weighed with an accuracy of one part in a 

 thousand. But such quantities are too small for experimental 

 work of the nature described. Where changes of weight are 

 concerned, in which an ordinary balance must be employed, 

 the utmost degree of accuracy attainable is 0*02 milligram. 

 Where the substances or solutions must be put through several 

 chemical operations the degree of accuracy is considerably 

 lessened. 



The transformations of elements were detected first by radio- 

 active measurements ; the production of helium was proved by 

 spectroscopic evidence, on which also rests the case for the 

 transmutations dealt with in this section. The detection of 

 transmutation is therefore most likely in cases where the 

 spectrum of the product is easily recognisable. In order to 

 produce a measurable amount of a gas, or a definitely re- 

 cognisable change of weight, a relatively large amount of 

 transmutation must take place. 



Transformations occurring in nature through long periods 

 of time would lead to the accumulation of much larger amounts 

 of products, and these should therefore be much more easily 

 detected. 



Section III. — Chemical Evidences of Transformation 



It has been pointed out in Part I. (vol. ii. p. 531) that when 

 one radioactive element disintegrates into a second, and the life 

 of the latter is shorter than that of the first, then ultimately 

 a state of equilibrium is reached, in which the quantity of the 

 second element disintegrating in any given time is equal to the 

 amount formed in that time. Under such conditions, provided 

 the life of the parent element is great in comparison, so that its 

 amount is practically constant, then the amounts of the two in 

 any mineral containing them must be in a definite ratio. This 

 holds true even if intermediate products occur, provided that 

 their life is sufficiently short to permit the state of equilibrium 



