268 MODEKN SCIENCE READER 



to be directly observable. To this rule there is, we admit, 

 one exception. The degradation of uranium into radium 

 has become perceptible even within the brief span of recent 

 experimental inquiry. 



For the rest there is room and to spare for speculation. 

 The metals are very curiously interrelated, both in their 

 qualities and in their distribution. Some occur in almost 

 inseparable companionship. Among these cognate couples 

 are silver and lead. In Mr. Donald Murray's words: "A 

 lead mine is a silver mine, and a silver mine is a lead mine 

 all the world over, and yet the chemical attraction between 

 silver and lead is slight, and the two metals are not suffi- 

 ciently common to concur by chance. ' n The inference was 

 irresistible, and has been reached by others, that silver is a 

 disintegration product of lead. And it is interesting to 

 remember that lead, until superseded by mercury, was 

 accounted in alchemistic theory the " mother of metals." 

 Now the persuasion is gaining ground that the supplies of 

 the various elements existing in the earth are regulated by 

 the proportion between their rates of development and 

 dissolution. Elemental distribution does not show the 

 extreme inequalities which would stamp it as the outcome 

 of chance. The approximate constancy in the quantities 

 present in all quarters of the globe of such rare metals as 

 gold, platinum, thallium, indium, gallium, and so on, 

 appears to intimate the working of a genetic law. It sug- 

 gests that they are, in Professor Soddy's phrase, at once 

 offspring and parent elements ; 2 that fhey are derived from 

 substances more highly elaborated 5 that they give rise, as 

 they in turn spontaneously decompose, to others less com- 

 plex, the relative speed of these ineffably slow alterations 

 determining the amount of each product found in the earth 

 at a given time. This remarkable hypothesis may be veri- 

 fied, according to Professor Soddy's anticipation, by the 

 discovery of occluded helium in antique gold. 



Thus physical science in the twentieth century has been 

 1 Nature, vol. Ixxiii, page 125. 2 Hid, page 151. 



