IDEAL COLORATION OF HEAT. 73 



take place ; showing, thus, that heat alone cannot cause the digestion of plants. More- 

 over, as is well known to chemists, carbonic acid gas may be passed through a tube 

 that is white hot without giving the most remote appearances of decomposition. 



268. The beautiful experiments of MELLONI have proved, however, that rays of heat 

 emitted by bodies at different temperatures vary in their constitution. At a low tem- 

 perature, such as that of the human hand, the caloric emitted is of a high refrangibility, 

 and possesses invisible violet coloration. As the heat rises, rays of a lower refrangi- 

 bility are sent forth, so that if we examine the character of the rays coming from a se- 

 ries of bodies, the temperatures of which are successively higher and higher, as the 

 hand, a vessel of boiling water, a red-hot iron, a gas flame, the refrangibilities become 

 lower and lower ; the radiant heat possessing a calorific tint, which successively de- 

 scends through the colours of the spectrum, the violet, indigo, blue, green, yellow, &c. 

 From the sun, the temperature of which, therefore, must be excessively high, radiant 

 heat is emitted which occupies a region in the spectrum corresponding to the red rays, 

 and even below that is found beyond the region where red light has ceased to be visi- 

 ble. In the sunbeam, therefore, rays of heat of every refrangibility and every colour 

 are found ; they occupy a space commencing with a region beyond the extreme violet, 

 and, descending through the whole length of the spectrum, are found beneath its lowest 

 extremity. 



269. This is a phenomenon analogous to what we witness under similar circum- 

 stances in the case of light. When a lamp, the wick of which is placed very low, is 

 first lighted, it burns with a violet-coloured flame, giving forth little heat, and possess- 

 ing small illuminating power. By-and-by, as the combustion goes on, the colour passes 

 through various shades of indigo, and presently becomes of a purer blue. If the wick 

 is now elevated, and air more abundantly supplied, the light increases in brilliancy ; 

 and, if seen through a prism, all the colours begin to be perceptible from the violet to 

 the yellow and orange. Lastly, if fed with oxygen gas, or consumed in one of the im- 

 proved burners, the light assumes a beautiful whiteness, and, if dispersed by a prism, 

 exhibits all the colours of the spectrum. It is to the presence of these that its white- 

 ness is due, for white light contains all the coloured rays. 



270. In these beautiful and perfect analogies, which may thus be traced between the 

 phenomena of light and heat, there are some points which require to be considered in 

 the case we have before us. As common observation assures us, rays of light of dif- 

 ferent refrangibilities excite in our eyes specific sensations ; the most refrangible ray 

 affects us with that sensation which produces in the mind the idea of a violet colour ; 

 the middle refrangible ray, a yellow ; the lesser refrangible ray, a red. And now these 

 impressions, thus passing along the optic nerve to the brain, originate in specific changes 

 which are happening to the constitution of the retina, for this delicate expansion must, 

 in the nature of things, be acted upon under the influence of the light, in order to give 

 rise to a mental sensation. The different rays of light, each one for itself, operates in 

 its own way and produces its proper result. 



271. Considerations like these would, therefore, lead us to suppose that rays of heat 

 of different invisible colours ought to have the property of producing specific changes. 



K 



