fied by the senior writer of this article (41, 213, 1891), 

 and later (43, 17, 1892 et seq.) the results of many inves- 

 tigations on caesium and rubidium compounds, in which 

 the junior writer played an important part, carried out 

 in Sheffield Laboratory, were published in the Journal. 



The application of the spectroscope led to the discov- 

 ery of thallium in 1861 by Crookes of England, and to 

 that of indium in 1863 by Reich and Richter in Germany. 

 Both of these metals are extremely rare, but they are of 

 considerable theoretical interest. Thallium is particu- 

 larly remarkable in showing resemblances in its different 

 compounds to several groups of metals. 



The spectroscope was employed again in connection 

 with the discovery of gallium in 1875 by Boisbaudran. 

 It is in the same periodic group as thallium and indium, 

 and it has a remarkably low melting point, just above 

 ordinary room-temperature. It has been among the 

 rarest of the rare elements, but within two or three years 

 a source of it has been found in the United States in cer- 

 tain residues from the refining of commercial zinc. The 

 recent issues of the Journal (41, 351, 1916 ; 42, 389, 1916) 

 show that Browning and Uhler of Yale have availed 

 themselves of this new material in order to make import- 

 ant chemical and physical researches upon this metal. 



Germanium. The discovery of germanium in the min- 

 eral argyrodite in 1886 by "Winkler revealed a curious 

 metal which gives a white sulphide that may be easily 

 mistaken for sulphur and which is volatilized completely 

 when its hydrochloric acid solution is evaporated, so that 

 it is evasive in analytical operations. This element had 

 been predicted with much accuracy by Mendeleeff, and 

 it is rather closely related to tin. 



A few years after the discovery of germanium, Pen- 

 field published in the Journal (46, 107, 1893; 47, 451, 

 1894) some analyses of argyrodite, correcting the for- 

 mula given by Winkler to the mineral ; also he described 

 canfieldite, an analogous mineral from Bolivia, in which 

 a large part of the germanium was replaced by tin. 



The Rare Earths. Before the year 1818 two rare 

 earths, the oxides of yttrium and cerium, were known 

 in an impure condition. Since that time about fourteen 

 others have been discovered as associates of the first 



