INDEPENDENCE OF LIGHT AND HEAT. 



49 



rock-salt, which is equally permeable to them all It therefore constitutes the true 

 glass of radiant heat, and glass and water and other diaphanous bodies act towards heat 

 as coloured media do to light ; they possess for it an invisible coloration. 



169. When, therefore, we attempt to investigate the distribution of heat in the solar 

 spectrum by the aid of a colourless flint glass prism, we are doing the same thing as 

 if we were attempting to study its coloured spaces by employing a prism of blue or 

 green glass. Such an instrument would, of course, wholly disturb the proper constitu- 

 tion of the spectrum, absorbing some rays and letting others pass ; and as it acts towards 

 light so does flint glass act towards heat, because it possesses an invisible coloration 

 for that principle. But rock-salt, which is transparent equally to all these rays, will 

 not produce such an effect, and hence, in all investigations connected with radiant heat, 

 prisms of that mineral ought to be employed. When this is done, it is found that the 

 maximum of temperature in this normal spectrum exists in the dark space, not in con- 

 tact with the red extremity, as Sir W. HERSCHEL supposed, but wholly detached from 

 the colours, at a mean distance equal to that which exists in the contrary direction 

 between the red and the yellow. If the rays which form this normal spectrum are 

 made to pass through a plate of flint glass of sufficient thickness, the maximum ap- 

 proaches towards the red region; if one of ordinary glass, it passes into the red ; if water 

 or alcohol be used, it enters the beginning of the yellow. But, by reason of the limpid- 

 ity of these different media, the colours undergo no sensible alteration, and the maxi- 

 mum remains always invariably fixed at the beginning of the yellow. 



170. Thus, the inferior bands of the spectrum may preserve the same ratio of lumi- 

 nous intensities, and lose the relations which exist in their temperatures. The calorific 

 elements do not follow the lot of their corresponding luminous elements. Therefore, 

 light and heat are two different agents, or two modifications essentially distinct of one 

 common agent. (MELLONI, Comptes Rendus, Jan., 1844, p. 39, &c.) 



171. To these beautiful results of MELLONI, Sir J. HERSCHEL (Phil. Trans., 1840, p. 

 52) has added other very remarkable ones, obtained by a different process. If we 

 take a slip of thin writing paper, and, having blackened one side of it by exposure to 

 the smoke of a candle, adjust it so as to receive the solar spectrum, and with a flat 

 brush equal in breadth to the paper dipped in rectified spirits of wine, wash over the 

 white surface until the paper is completely saturated, and looks of a uniform black 

 colour, " a whitish spot begins to appear below the extreme red end of the luminous 

 spectrum, which increases rapidly in breadth until it equals the breadth of the luminous 

 spectrum, and even somewhat surpasses it, and in length till it forms a long appendage 

 exterior to the spectrum, and extends, moreover, within it, till it reaches up to, and be- 

 yond the centre of the yellow ray. In this state, and just as the general drying of the 

 paper begins by whitening the whole surface to confuse the appearances, a second sudden 

 and copious wash of alcohol from above downward must be applied, without disturbing 

 the spectrum, or in any way shaking the apparatus. The superfluous alcohol will have 

 hardly run off", when the phenomena of the thermic spectrum begin to appear in all 

 their characters ; at first faintly, and, as it were, sketched in by a dimness and dulness 

 of t^ie otherwise shining and reflective surface of the wetted paper ; but this is speedily 



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