The Romance of Chemistry 727 



is concerned to minerals containing the two radio-elements just 

 mentioned. The final product of the metamorphosis appears to 

 be lead, and in this way a radio-active mineral may be utilised as 

 a "geological clock." Professor Soddy explains this : 



In a uranium mineral each 1 per cent, of lead in terms of the 

 quantity of uranium signifies a lapse of a period of 80,000,000 

 years. Errors, of course, are possible, if lead should have 

 been an original constituent of the mineral, but these are 

 minimised by taking a large number of different minerals. 

 On the other hand, every cubic centimetre by volume of 

 helium per gram of uranium in a uranium mineral signifies 

 9,000,000 years, and as here helium, being a gas that forms 

 no compounds, cannot have been initially present, and as, 

 moreover, some will have escaped the age of the mineral by 

 this method is a minimum, whereas the age by the lead con- 

 tent may be too high. The Carboniferous rocks tested by 

 this new method appear to have an age of some 350,000,000 

 and the oldest Archaean rocks of over 1,500,000,000 years. 



The difficult subject of radio-activity has been referred to in 

 other chapters, but it cannot be more than touched on in a work 

 of this scope. Enough has been said, however, to show that the 

 doctrine of the unchangeableness of the chemical elements re- 

 quires revision. We have already quoted from Professor Soddy, 

 well known as one of the brilliant pioneers in the study of radio- 

 activity, and we select a summing up sentence : 



The radio-elements are in course of spontaneous transmu- 

 tation into other elements, and the process proceeds through 

 a long succession of more or less unstable intermediate 

 elements, until the final stable is reached. 



In the domain of things this is the nearest analogue to evolution 

 in the realm of organisms. 



4 

 Chemistry of the Living Creature 



Of the fourscore and more eiements, no fewer than twenty- 

 nine are known to occur in living creatures; but twelve of these 



