INTRODUCTION. 7 



arranged, forms a series with pretty equal intervals 

 between the successive atomic weights; at several 

 points however considerable gaps occur. From this 

 irregularity Mendelejeff concluded that there must 

 exist other elements — till then unknown — and he 

 ventured to predict that the gaps in his series would 

 gradually be filled up by these missing links, of which 

 he even predicted the atomic weights. 



In many cases the gaps have really been thus filled, 

 and this of course gave strong support to the idea that 

 elements are not such independent primary substances, 

 as one had previously supposed them to be. 



Yet, decisive proof for a genetic connexion between 

 the several elements lacked until the discovery of the 

 radioactive elements by Monsieur et Madame Curie, de- 

 monstrating the direct transf ormability of the elements. 



We now know that a number of elements, among 

 which Ionium, Radium, Polonium and very probably 

 lead also, can be directly derived from Uranium by a 

 process of splitting off Helium, with the atomic weight 

 4, and electrons, whose apparent mass is but '/mm part 

 of a hydrogen-atom. 



Unfortunately, it is as yet impossible to excert even 

 the slightest influence on the process of radioactivity, 

 and it is especially unfortunate ^that we cannot reverse 

 the process and thus, by adding Helium and Electrons 

 to lead f. i., build up Uranium. 



But the fact that other elements e. g. Potassium and 

 Rubidium also, though in a much slighter degree, show 

 signs of radioactivity, justifies the supposition that 

 Helium takes part in the building up of all elements, 



