112 PROGRESS OF SCIENCE IN THE CENTURY. 



boron (at. wt. 44), eka-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, 

 Avith 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 have 

 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 Greeks imagined one primeval substance 

 developing into all the different kinds of matter; 



* Ladenburg, 1900, p. 313. 

 tMendelejeff, Principles of Chemistry, Vol, II., trans., 

 p. 34. 



