86 THERMOGRAPHY. 



of a " heat-colouration." That is, tlie heat-rays are 

 supposed to possess properties like luminous colour 

 although invisible ; and, consequently, that a blue sur- 

 face has a strong affinity for the blue heat-rays, a red 

 surface for the red ones, and so on through the scale. 

 The ingenuity of this hypothesis has procured it much 

 attention ; but now, when the Newtonian hypothesis of 

 the refrangibility of light is nearly overturned, we must 

 not, upon mere analogy, rush to the conclusion that the 

 rays of heat have different orders of refrangibility, which 

 Melloni's hypothesis requires.* 



Can anything be more calculated to impress the mind 

 with the consciousness of the high perfection of natural 

 phenomena, than the fact, that the colour of a body 

 should powerfully influence the transmission of a prin- 

 ciple which is diffused through all nature, and also deter- 

 mine the rate with which it is to pass off from its surface. 

 Some recent experiments have brought us acquainted 

 with other facts connected with these heat-radiations, 

 and the power of heat, as influenced by the calorific 

 rays, to produce molecular changes in bodies, which bear 

 most importantly on our subject. 



If we throw upon a plate of polished metal a prismatic 

 spectrum (deprived, as nearly as possible, of its chemical 

 power, by being passed through a deep yellow solution 

 which possesses this property in a very remarkable 

 manner, as will be explained when we come to the exa- 

 mination of the chemical action of the sun's rays) it 

 Avill be found, if we afterwards expose the plate to the 



* This paper of Melloni's will be found in the Bibliotheque Uni- 

 verselle de Geneve, for 1843. The conclusions are highly ingenious, 

 but they rest entirely on the analogy supposed to be discovered be- 

 tween the relations of heat, like light, to the coloured rays of the 

 spectrum. This, it must be remembered, is not the case, since 

 even Sir William Herschel showed that red light might exist with 

 only a minimum of calorific power, notwithstanding the fact, that 

 the maximum heat-ray of the spectrum coincides with the red rays. 



