38 THE PRINCIPLES OF SCIENCE. 



that electricity must have the power of generating acids 

 and alkalis, and one chemist thought he had discovered a 

 new substance called electric acid. But Davy proceeded 

 to a systematic investigation of the circumstances, by 

 varying the conditions. Changing the glass vessel for 

 one of agate or gold, he found that far less alkali was 

 produced ; excluding impurities by the use of very care- 

 fully distilled water, he found that the quantities of acid 

 and alkali were still further diminished ; and having thus 

 obtained a clue to the cause he completed the exclusion of 

 impurities by avoiding contact with his fingers, and by 

 placing the apparatus under an exhausted receiver, no 

 acid or alkali being then detected. It would be difficult 

 to meet with a more elegant or successful case of the 

 detection of a condition previously unsuspected <i. 



It is highly remarkable that the presence of common 

 salt in the air, proved to exist by Davy, nevertheless 

 continued a stumbling-block in the science of spectrum 

 analysis, and probably prevented men, such as Brewster, 

 Herschel, and Talbot, from anticipating by thirty years 

 the discoveries of Bunsen and Kirchhoff. As I have else- 

 where pointed out r , the utility of the spectrum was 

 known in the middle of the last century to Thomas 

 Melvill, a talented Scotch physicist, who died at the early 

 age of 27 years 8 . But Melvill was struck in his examina- 

 tion of various coloured flames by the extraordinary pre- 

 dominance of homogeneous yellow light, which was due to 

 some circumstance escaping his attention. Wollaston and 



q 'Philosophical Transactions/ [1826] vol. cxvi. pp. 388, 389. Works 

 of Sir Humphry Davy, vol. v. pp. 1-12. 



r 'National Review,' July, 1861, p. 13. 



8 His published works are contained in ' The Edinburgh Physical and 

 Literary Essays/ vol. ii. p. 34 ; ' Philosophical Transactions/ [1753] vol. 

 xlviii. p. 261; see also Morgan's Paper in 'Philosophical Transactions/ 

 [1785] vol. Ixxv. p. 190. 



