THE PERIODIC SYSTEM 281 



Applications of the Periodic System. The system has 

 found application chiefly in four ways : 



1. In the prediction of new elements. Mendelejeff (1871) drew 

 attention to the blank then existing between calcium (40) and 

 titanium (48). He predicted that an element to fit this place 

 would have an atomic weight 44 and would be trivalent. From 

 the nature of the surrounding elements, he very cleverly deduced 

 many of the physical and chemical properties of the unknown 

 element and of its compounds. In 1879 Nilson discovered scan- 

 dium (44), and its behavior corresponded closely with that pre- 

 dicted. Mendelejeff described accurately two other elements, like- 

 wise unknown at the time. In 1875 Lecoque de Boisbaudran found 

 gallium, and in 1888 Winkler discovered germanium, and these 

 blanks were filled. 



2. By enabling us to decide on the correct values for the atomic 

 weights of some elements, when the equivalent weights have been 

 measured, but no volatile compound is known (cf. pp. 77 and 86). 

 Thus, the equivalent weight of indium was 38 and, as the element 

 was supposed to be bivalent, it received the atomic weight 76. It 

 was quite out of place near arsenic (75), however, being decidedly 

 a metallic element. As a trivalent element with the atomic weight 

 115, it fell between cadmium and tin. Later work fully justified 

 the change. More recently, when radium was discovered, it was 

 found to have the equivalent weight 113 and to resemble barium. 

 Consequently we assume that, like barium, it is bivalent, and 

 assign it a vacant place under this element, in the last series. 



3. By suggesting problems for investigation. The periodic 

 system has been of constant service in the course of inorganic 

 research, and has often furnished the original stimulus to such 

 work as well. 



For example, the atomic weights of the platinum metals at first 

 placed them in the order, Ir (197), Pt (198), Os (199), although 

 the resemblance of osmium to iron and ruthenium would have 

 led us to expect that this element should come first. For similar 



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