SECT. XXVII. TRANSMISSION OF HEAT. 261 



rays is also proved by another experiment, which will be more 

 easily understood from the analogy of light. Red light, emanat- 

 ing from red glass, will pass in abundance through another piece 

 of red glass, but it will be absorbed by green glass ; green rays will 

 more readily pass through a green medium than through one of 

 any other colour. This holds with regard to all colours ; so in 

 heat. Rays of heat of the same intensity, which have passed 

 through different substances, are transmitted in different quan- 

 tities by the same piece of alum, and are sometimes stopped 

 altogether ; showing that rays which emanate from different sub- 

 stances possess different qualities. It appears that a bright flame 

 furnishes rays of heat of all kinds, in the same manner as it 

 gives light of all colours ; and, as coloured media transmit some 

 coloured rays and absorb the rest, so bodies transmit some rays of 

 heat and exclude the others. Rock-salt alone resembles colour- 

 less transparent media in transmitting all kinds of heat, even 

 that of the hand, just as they transmit white light, consisting of 

 rays of all colours. Radiant heat is unequally refracted by a 

 prism of rock-salt like light, and the rays of heat thus dispersed 

 are found to possess properties analogous to the rays of the 

 coloured spectrum. 



The property of transmitting the calorific rays diminishes to a 

 certain degree with the thickness of the body they have to 

 traverse, but not so much as might be expected. A piece of 

 very transparent alum transmitted three or four times less 

 radiant heat from the flame of a lamp than a piece of nearly 

 opaque quartz about a hundred times as thick. However, the 

 influence of thickness upon the phenomena of transmission in- 

 creases with the decrease of temperature in the origin of the rays, 

 and becomes very great when that temperature is low. This is 

 a circumstance intimately connected with the law established by 

 M. de Laroche ; for M. Melloni observed that the difference 

 between the quantities of heat transmitted by the same plate of 

 glass, exposed successively to several sources of heat, diminished 

 with the thinness of the plate, and vanished altogether at a 

 certain limit ; and that a film of mica transmitted the same 

 quantity of heat, whether it was exposed to incandescent pla- 

 tinum or to a mass of iron heated to 360. 



Coloured glasses transmit rays of light of certain degrees of 

 refrangibility, and absorb those of other degrees. For example, 



