xix.] EXPEEIMENT. 429 



acid and alkali were still further diminished ; and having 

 thus obtained a clue to the cause, he completed the ex- 

 clusion 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 case of the detection of a 

 condition previously unsuspected. 1 



It is 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 pointed out, 2 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. 3 But Melvill 

 was struck in his examination of coloured flames by the 

 extraordinary predominance of homogeneous yellow light, 

 which was due to some circumstance escaping Ms atten- 

 tion. Wollaston and Fraunhofer were equally struck by 

 the prominence of the yellow line in the spectrum of 

 nearly every kind of light. Talbot expressly recommended 

 the use of the prism for detecting the presence of substances 

 by what we now call spectrum analysis, but he found that 

 all substances, however different the light they yielded in 

 other respects, were identical as regards the production of 

 yellow light. Talbot knew that the salts of soda gave this 

 coloured light, but in spite of Davy's previous difficulties 

 with salt in electrolysis, it did not occur to him to assert 

 that where the light is, there sodium must be. He sug- 

 gested water as the most likely source of the yellow light, 

 because of its frequent presence; but even substances 

 which were apparently devoid of water gave the same 

 yellow light. 4 Brewster and Herschel both experimented 



1 Philosophical Transactions [1826], vol. cxvi. pp. 388, 389. Works 

 of Sir Humphry Davy, vol. v. pp. i 12. 



2 National Review, July, 1861, p. 13. 



3 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 Papers in Philosophical Trans- 

 actions [1785], vol. Ixxv. p. 190. 



* Edinburgh Journal of Science, vol. v. p. 79. 



