Herschel, and Talbot, from anticipating by thirty years the discoveries of Bunsen and Kirchhoff. As I pointed out,16 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.17 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 his attention. 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 suggested 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.18 Brewster and Herschel both experimented upon flames almost at the same time as Talbot, and Herschel unequivocally enounced the principle of spectrum analysis.19 Nevertheless Brewster, after numerous experiments attended with great trouble and disappointment, found that yellow light might be obtained from the combustion of almost any substance. It was not until 1856 that Swan discovered that an almost infinitesimal 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 condition 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.20
In the science of radiant heat, early inquirers were led to the conclusion that radiation proceeded only from the surface of a solid, or from a very