LIGHT. 93 



the rays of heat are found to extend, and to have their point of 

 maximum intensity considerably beyond the visible spectrum, 

 on the side of the red ray. Hence, although there are calorific 

 rays of all degrees of refrangibility, the great proportion of 

 them are even less refrangible than the least refrangible lumi- 

 nous rays. It is observed that the least refrangible rays are 

 absorbed in greatest proportion in passing through bodies 

 which are not highly diathermanous, such as crown-glass and 

 water. Hence prisms of these substances, allowing only the 

 more refrangible rays of heat to pass, give a spectrum which is 

 hottest in the red, or perhaps even in the yellow ray, and pos- 

 sesses little or no heat beyond the border of the red ray. The 

 inequality in refrangibility existing between the rays of heat 

 and of light is decisive of the fact, that they are peculiar rays, 

 that can be separated, although associated together in the sun- 

 beam. Indeed, Melloni finds that light from both solar and 

 terrestrial sources is divested of all heat by passing successively 

 through water, and a glass, coloured green by the oxide of cop- 

 per, being incapable as it issues from these media of affecting 

 the most delicate thermoscope. 



The light of the sun is capable of inducing certain chemical 

 changes which^do not depend either upon its luminous or calo- 

 rific rays, but upon the presence of what are called chemical 

 rays. Thus chlorine gas, under the influence of light, is capable 

 of decomposing water, combining with its hydrogen, and libe- 

 rating oxygen; and the chlorine in the freshly precipitated 

 chloride of silver has a similar effect ; but the oxygen in the last 

 case, instead of being set free, combines with the silver, and 

 causes the colour of the compound to change from white to 

 black. The moist chloride of silver is darkened more rapidly 

 by the violet than by the red rays of the spectrum ; but this 

 change is produced upon it even when carried a little way out of 

 the visible spectrum on the side of the violet ray. The rays 

 found in that situation are, therefore, more refrangible than any 

 other kind of rays in the spectrum. Their characteristic effect 

 is to promote those chemical decompositions in which oxygen is 

 withdrawn from water and other oxides, and hence they are 

 sometimes named de-oxidizing rays. These rays were likewise 

 supposed to communicate magnetism to steel needles exposed 

 to them but this opinion is no longer tenable. 



