132 CHEMICAL DISCOVERY AND INVENTION 



of zinc is 9-2, and of arsenic 18, therefore that of our metal should 

 be near to 12. This is also evident from the fact that the volume 

 of aluminium is 11, and that of indium 14, and our metal is 

 situated in the III group between aluminium and indium. If 

 its volume is 11-5 and its atomic weight be about 69, then its 

 density will be nearly 5-9. The fact of zinc being more volatile 

 than magnesium gives reason for thinking that the metal in 

 question will be more volatile than aluminium, and therefore 

 for expecting its discovery by the aid of the spectroscope, etc." 



In 1875 Lecoq de Boisbaudran discovered, by means of the 

 spectroscope, a new metal from the zinc blende in the Pyrenees, 

 which he named gallium. It was found to yield an oxide R 2 3 

 and an alum, that is a double sulphate with ammonium and 

 potassium which crystallised in regular octahedrons. Its density 

 was found to be 5*9, and its atomic weight 69*8. The metal was 

 found to possess many of the properties of aluminium, being, 

 however, much more fusible, just as zinc is more fusible than the 

 next metal above it, namely magnesium. 



In a similar way the properties of ekasilicon, foreseen by 

 MendeleefE in 1871, were recognised in the metal Germanium, 

 discovered by Clemens Winkler in 1886 in the peculiar silver 

 ore argyrodite. This element stands in Group IV of Mendeleeff's 

 scheme, immediately below titanium, which follows silicon. The 

 missing but expected element was described on the basis of a 

 consideration of the properties of the known elements, silicon, 

 zinc, tin, and arsenic, which in the table are placed at nearly 

 equal distances from the vacant place. 



It was expected to have an atomic weight nearly 72 with a 

 higher oxide Es0 2 , and a lower oxide EsO, haloid compounds of 

 the type EsCl 4 which would boil at about 90. Its sulphide 

 EsS 2 would resemble tin sulphide SnS 2 , and probably dissolve 

 in ammonium sulphide and so forth. Germanium has an atomic 

 weight 72-5, the metal melts at about 900 C. It forms a dioxide 

 Ge0 2 . 



If germanium or its sulphide is heated in a stream of chlorine 

 gas it forms a volatile liquid GeCl 4 , which boils at 86, and is 

 decomposed by water after the manner of stannic chloride. In 

 fact the new element was found to possess just the properties 

 to be expected of an element occupying a position intermediate 

 between those of silicon and tin. 



A similar correspondence was found to exist between Mende- 



