EXPERIMENT. 39 



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, how- 

 ever different the light they yielded in other respects, 

 were identical as regards the production of yellow light. 

 Talbot knew that the salts of soda all 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 the sodium must be. He suggested 

 water as the most likely source of the yellow light, be- 

 cause of its usual presence ; but even substances which 

 were apparently devoid of water gave the very same 

 yellow light*. Brewster and Herschel both experimented 

 upon flames almost at the same time as Talbot, and 

 Herschel unequivocally enounced the principle of spec- 

 trum analysis". Nevertheless Brewster, after numerous 

 experiments attended with great trouble and disappoint- 

 ment, found that yellow light might be obtained from 

 the combustion of almost any substance. It was not until 

 1856 that Professor W. Swan discovered that an almost in- 

 finitesimal quantity of sodium chloride, say a millionth 

 part of a grain, was sufficient to tinge a flame of a bright 

 yellow colour. The universal diffusion of the salts of 

 sodium, joined to this unique light-producing power, was 

 thus shown to be the unsuspected circumstance which had 

 destroyed the confidence of all previous experimenters in 

 the use of the prism. Some references concerning the 

 history of this curious point are given below x . 



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



11 'Encyclopaedia Metropolitana/ article Light, 524; Herschel's 

 ' Familiar Lectures,' p. 266. 



x Talbot, * Philosophical Magazine/ 3rd Series, vol. ix. p. i (1836); 

 Brewster, 'Transactions of the Royal Society of Edinburgh/ [1823] vol. 



