DISCOVERIES BY EXAMINING RESIDUES OF MANUFACTURES. 507 



the manufacture of oil of vitriol enabled Crookes to discover 

 that metal in the flue-dust deposited during that process. 



Professor Bunsen, also, by concentrating a great bulk 

 of the water of a mineral spring at Durckheim, was led to 

 the discovery of two new alkali-metals, viz., caesium and 

 rubidium. The more soluble substances in the brine of 

 salt-works become immensely concentrated by continual 

 boiling and removal of the less soluble salts. During a long 

 series of years of evaporation, in this way, and by further 

 similar treatment of the brine of Stoke-Prior salt-works, 

 and reduction of it to a very minute amount less than one 

 thousand-millionth part of its original bulk I have been 

 enabled, with the further aid of the spectroscope, to dis- 

 cover the presence of several alkaline metals (but no new 

 ones) not previously known to be contained in it ; and by 

 washing with water large bulks of crude substances, such 

 as chalk, sand, fire-clay, &c., and concentrating the liquid 

 in a similar manner, I have found minute quantities of 

 salts of lithium. 



j. By examining the ashes of rare plants and ani- 

 mals. Special opening for discovery lies in this direc- 

 tion, because we know that each different species of plant 

 possesses special powers of assimilation, which enable it to 

 select particular ingredients ; this is particularly seen in 

 the ability of sea-weeds to appropriate iodine and bromine. 

 Similar remarks may be made with regard to the secretions 

 and ashes of animals, which notably contain phosphorus ; 

 it was the distillation of dried urine which led to the dis- 

 covery of that element, and the analysis of the ashes of 

 bones which further led to its abundant production. 1 



1 The earth of bones had been considered as a peculiar simple 

 earth ; but Gahn ascertained by analysis that it was a compound of 

 phosphoric acid and lime.' Thomson, History of Chemistry, vol. ii. 

 p. 243. 



