244 MELLONl's EXPERIMENTS. [SECT. xxv. 



the other three sources. Out of 39 different substances, 34 

 were pervious to the calorific rays from hot water, 14 ex- 

 cluded those from, the hot copper, and 4 did not transmit 

 those from the platina. 



Thus it appears that heat proceeding from these four 

 sources is of different kinds : this difference in the nature of 

 the calorific rays is also proved by another experiment, which 

 will be more easily understood from the analogy of light. 

 Red light, emanating 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 caloric 

 of the same intensity, which have passed through different 

 substances, are transmitted indifferent quantities by the same 

 piece of alum, and are sometimes stopped altogether ; showing 

 that rays which emanate from different substances 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 caloric and exclude the others. Rock-salt alone 

 resembles colourless transparent media in transmitting all 

 kinds of caloric, even the heat of the hand, just as they 

 transmit white light, consisting of rays of all colours. 



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 increases 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 con- 

 nected with the law established by M. de Laroche ; for M. 



