518 SPECIAL METHODS OF DISCOVERY. 



opinion by transmitting and refracting the rays of each 

 colour separately.' l 



Whilst Columbus, in the year 1492, was searching 

 for a new continent, he' discovered the variation of the 

 magnetic needle. Faraday, in the year 1831, searching 

 for a continuous electro-dynamic inductive action, found 

 a momentary one. 2 BerzeliuS; in 1817, searching for tel- 

 lurium, discovered selenium. Crookes, in 1861, searching 

 for selenium, discovered thallium. And a company, re- 

 cently searching for coal, by boring in the sub-wealden 

 formation at Netherfield, near Battle, discovered beds of 

 gypsum. 



Smithson Tennant, while endeavouring to make an 

 alloy of lead with the powder which remains after treating 

 crude platinum with aqua regia, observed remarkable pro- 

 perties in this powder, and found that it contained a new 

 metal. 'In 1791 he made his celebrated analysis of car- 

 bonic acid, which fully confirmed the opinion previously 

 stated by Lavoisier respecting the constituents of this 

 substance. His mode was to pass phosphorus through 

 red-hot carbonate of lime. The phosphorus was acidified, 

 and carbon deposited. It was during this experiment 

 that he discovered phosphide of calcium.' 3 



The author of this treatise also, whilst searching for 

 thermic changes by electrolysing a solution of double 

 cyanide of mercury and potassium with mercury elec- 

 trodes, suddenly heard a faint sound, and then observed 

 the surface of the mercury in motion ; and, by further 

 research, was led to the discovery of electrolytic vibrations 

 and sounds, the motion and sound being due to rapid 

 alternate formation and destruction of films upon the 



1 Whewell, History of the Inductive Sciences, vol. ii. 3rd edit. p. 282. 



2 See p. 522. 



8 Thomson, History of Chemistry, vol. ii. p. 234. 



