506 SPECIAL METHODS OF DISCOVERY. 



with aqua-regia ; and Descotils, by further examining this 

 powder, found that it Contained a metal which imparted 

 a red colour to the ammoniacal precipitate. Vauquelin, 

 by treating the powder with alkali and heat, found a 

 metallic oxide, which he considered to be the same as that 

 discovered by Descotils ; but Tennant, in 1804, finally 

 showed that the powder really contained two metals, viz., 

 osmium and iridium. 1 ' Mitscherlich himself found, in 

 the scoriae of the mines of Sweden and Germany, artifi- 

 cial minerals having the same composition and the same 

 crystalline form with natural minerals : as silicates of iron, 

 lime, and magnesia, agreeing with peridote ; bisilicate of 

 iron, lime, and magnesia, agreeing with pyroxene ; red 

 oxide of copper ; oxide of zinc ; protoxide of iron (fer 

 oxydule) ; sulphurets of iron, zinc, lead, arseniuret of 

 nickel; black mica. These were accidental results of 

 fusion.' 2 



Investigation of the concentrated residues of large 

 manufacturing operations often yield new discoveries, be- 

 cause substances which exist only in very minute proportions 

 in the crude or native materials of a manufacture frequently 

 become concentrated to so great an extent by the processes 

 employed that they become conspicuous. The concen- 

 trated residues of Courtois's manufacture of saltpetre so 

 acted upon the vessels he employed that he was induced 

 to investigate the circumstance, and thus discovered 

 iodine. Balard, also, by analysing the concentrated mother- 

 liquor of sea-water, was led to the discovery of bromine. 

 By analysing the residues of the vitriol works of Fahlun, 

 Berzelius discovered selenium. The concentration of thal- 

 lium in the process of burning sulphur and sulphides in 



1 See Thomson, History of Chemistry, vol. ii. p. 234. 



2 Whewell, Philosophy of the Inductive Sciences, vol. i. p. 510. 



