RADIANT HEAT. 351 



form of radiant energy known as heat only differs from the other 

 form called visible light in certain respects of degree rather than 

 of kind, and more especially in the particular quality known as 

 refrangibility (Expt. 349). 



Exactly the same remarks apply to that other form of radiant 

 energy known as actinism (Chapter XXV.), or radiant chemical 

 energy ; the essential differences between the three being that an 

 intensely heated source of radiant action (such as the electric arc 

 or lime light amongst artificial sources, and the sun amongst 

 natural ones) emits rays of different degrees of refrangibility ; those 

 of the highest refrangibility do not produce any effect on the 

 ordinary human eye, and are consequently invisible ; but they 

 possess the power of setting up certain chemical changes to a much 

 greater degree than rays of less refrangibility, and also when made 

 to illuminate fluorescent bodies (Chapter XXV.) they enable these 

 bodies to emit a visible form of light. Those of medium refran- 

 gibility are perceptible to the human eye, and produce the sensa- 

 tion of different kinds of colour, according to the degree of 

 refrangibility, the tint termed red being due to less refrangible rays 

 than those giving the colour green, which, again, is developed by rays 

 less refrangible than those to which the sensations of blue and violet 

 colours are due. Those rays the refrangibility of which is mostly 

 too low to render them perceptible to the human eye are mainly 

 such as affect the nerves of heat sensation and ordinary thenno- 

 metric apparatus ; so that it is quite possible to take a beam of 

 bright sunlight or electric arc light, and by means of screens of 

 suitable materials or other devices to absorb or remove one or 

 other of the three kinds of rays, and obtain either a beam of 

 invisible heat rays capable of producing large calorific effects, or a 

 beam of visible light possessing but little heating power or chemi- 

 cal activity, or a beam of rays barely visible to the eye and almost 

 destitute of heating power, but powerfully photographic, and 

 capable of setting up a high degree of fluorescence. It must not, 

 however, be supposed that the luminous rays are entirely devoid 

 of heating action or photographic power, that the chemical rays 

 are incapable of producing calorific effects, or that the invisible rays 

 of low refrangibility are unable to effect chemical changes, for this 

 is not at all the case; thus photographs of the invisible less refran- 

 gible part of the spectrum can be taken in virtue of the chemical 

 changes capable of being set up by this class of rays, although it 

 is true that they are much less active than more refrangible rays 

 in this respect ; and similarly, perceptible calorific effects are pro- 

 ducible by means of the ultra-violet rays when sufficiently delicate 

 means of observation are employed. Again, when rays of particular 



