112 PROGRESS OF SCIENCE IN THE CENTURY. 



boron (at. wt. 44), cka-aluminium (at. wt 68), and 

 eka-silicon (at. wt. 72). Since that time, these three 

 elements have been discovered, and they have been 

 found to possess, approximately, the properties pre- 

 dicted by Mendelejeff. They are: scandium, discov- 

 ered by Nilson, with atomic weight 44.1; gallium, 

 discovered by Lecoq de Boisbaudran, with atomic 

 weight 70; and germanium, discovered by Winkler, 

 with atomic weight 72." * 



To sum up: 



" The periodic law has not only embraced the mu- 

 tual relations of the elements and expressed their 

 analogy, but has also to a certain extent subjected 

 to law the doctrine of the types of the compounds 

 formed by the elements; has enabled us to see a regu- 

 larity in the variation of all chemical and physical 

 properties of elements and compounds, and has ren- 

 dered it possible to foretell the properties of ele- 

 ments and compounds yet uninvestigated by exper- 

 imental means; it therefore prepares the ground for 

 the building up of atomic and molecular me- 

 chanics" f 



Inorganic Evolution. An alluring, but perhaps il- 

 lusory, idea has occurred to many chemists who nave 

 pondered over the relations of the elements to one 

 another, the idea that chemically analogous ele- 

 ments may be related in a real, i.e., genetic, sense, 

 or that they may be derivatives of a common stock. 

 The historians of chemistry have shown that this is 

 an ancient and frequently recurrent idea. Some of 

 the early Q reeks imagined one primeval substance 

 developing into all the different kinds of matter; 



Ladenburg, 1900, p. 813. 



t Mendelejeff, Principle! of Chemittry, Vol. II., tram* 

 p. 84. 



