90 THE EVOLUTION OF MATTER 



as if it had never been interrupted, and goes on 

 till the last element, uranium, is reached. This was 

 a veritable cryptogram challenging interpretation, 

 and although far from deciphered the first step in 

 the finding of the key has now been taken. The 

 Periodic Law is Nature as it is, not as we would 

 have it, or as we would have made it, if the making 

 of it had been ours. There are some curious minor 

 exceptions even in its very arbitrary regularities. 

 At first, also, gaps had to be left for missing elements 

 to satisfy the scheme, and so the existence of 

 elements not yet discovered, and even their very 

 properties, were predicted, and in the majority of 

 cases these predictions have been verified by the 

 subsequent discovery of the missing members. 



With regard to the very simplest constituents, 

 into which the material universe has been resolved, 

 there is thus a veritable tangle of complex relation- 

 ships in contrast to that craving for simplicity, 

 symmetry, and order which the mind is always 

 attempting to satisfy in its interpretations of the 

 external world. 



In 1896 one of the elements, uranium, the last on 

 the list, was discovered by Becquerel in Paris to 

 possess a new property. It was described as 

 radioactive to signify that it was continually and 

 spontaneously emitting a new kind of radiation, 

 analogous in its chief characteristics to the X-rays of 

 Rontgen, discovered the year previously. M. and 

 Mme. Curie then showed that thorium, the element 

 next to uranium in atomic weight, possessed a similar 

 property, but, with the doubtful exception of two 

 others, potassium and rubidium, none of the other 

 elements then known show the least evidence of 

 radioactivity. Going back to the natural minerals in 

 which uranium occurs, such as pitchblende, M. and 

 Mme. Curie discovered therein several intensely 



