430 THE PRINCIPLES OF SCIENCE. [CHAP. 



upon flames almost at the same time as Talbot, and 

 Herschel unequivocally enounced the principle of spec- 

 trum analysis. 1 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 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 confi- 

 dence of all previous experimenters in the use of the 

 prism. Some references concerning the history of this 

 curious point are given below. 2 



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 small depth below it. 

 But they happened to experiment upon surfaces covered 

 by coats of varnish, which is highly athermanous or 

 opaque to heat. Had they properly varied the character 

 of the surface, using a highly di athermanous substance like 

 rock salt, they would have obtained very different results. 3 



One of the most extraordinary instances of an erroneous 

 opinion due to overlooking interfering agents is that con- 

 cerning the increase of rainfall near to the earth's surface. 

 More than a century ago it was observed that rain-gauges 

 placed upon church steeples, house tops, and other elevated 

 places, gave considerably less rain than if they were on the 

 ground, and it has been recently shown that the variatior- 

 is most rapid in the close neighbourhood of the ground.* 

 All kinds of theories have been started to explain this 

 phenomenon ; but I have shown 5 that it is simply due to 



1 Encyclopaedia Metropolitana, art. Light, 524 ; Herschel's 

 Familiar Lectures, p. 266. 



8 Talbot, Philosophical Magazine, 3rd Series, vol. ix. p. I (1836) ; 

 Brewster, Transactions of the Eoyal Society of Edinburgh [1823], 

 vol. ix. pp.433, 455 ; Swan, ibid. [1856] vol. xxi. p. 411 ; Philosophical 

 Magazine, 4th Series, vol. xx. p. 173 [Sept. 1860] ; Koscoe, Spectrum 

 Analysis, Lecture III. 



3 Balfour Stewart, Elementary Treatise on Heat, p. 192. 



4 British Association, Liverpool, 1870. Report on Rainfall, p. 176. 



5 Philosophical Magazine. Dec. 1861. 4th Series, vol. xxii p. 421. 



