102 CORRELATION OF PHYSICAL FORCES. 



than with blue light, while the chemical effects are the 

 inverse. 



There are other apparently more direct agencies of light 

 in producing electricity and magnetism, such as those observed 

 by Morichini and others, as well as its effects upon crystallisa- 

 tion; but these results have hitherto been of so indefinite a 

 character, that they can only be regarded as presenting fields 

 for experiment, and not as proving the relations of light to 

 the other forces. 



Light would seem directly to produce heat in the pheno- 

 mena of what is termed absorption of light : in some of these 

 we find that heat is developed in proportion to the disappear- 

 ance of light. It was thought, and Franklin's experiment of 

 different coloured cloths on snow exposed to the sun seemed 

 to prove it, that heat was produced in a direct ratio to the 

 absorption of light, but Dr. Tyndall has shown that this 

 result does not depend on the colour, but on the chemical 

 and physical character of the substance exposed, and that the 

 heating effect is due mainly to non-luminous radiation. The 

 heating powers of different colours are by no means in exact 

 proportion to the intensity of their light as affecting the visual 

 organs. Thus, red light when produced by refraction from 

 a prism of glass, produces greater heating effect than 

 yellow light in the phenomena of absorption, as has been 

 observed by Sir W. Herschel. The red rays appear to pro- 

 duce a dynamic effect greater than any of the others ; thus 

 they penetrate water to a greater depth than the other 

 colours ; but, according to Dr. Seebeck, we get a further ano- 

 maly, viz. that when light is refracted by a prism of water 

 the yellow rays produce the greater heating effect. The 

 subject, therefore, requires much more experiment before we 

 can ascertain the rationale of the action of the forces of light 

 and heat in this class of phenomena. 



In a former edition of this essay I suggested the following 

 experiment on this subject : Let a beam of light be passed 

 through two plates of tourmaline, or similar substance, and 

 the temperature of the second plate, or that on which the light 

 last impinges, be examined by a delicate thermoscope, first 

 when it is in a position to transmit the polarised beam coming 



