210 MELLONI'S EXPERIMENTS. SCT. XXV. 



rainishes 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 veiy transparent alum trans- 

 mitted 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 influ- 

 ence 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 

 connected with the law established by M. de Laroche ; 

 for M. Melloni observed that the difference between 

 the quantities of caloric 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 caloric, whether it 

 was exposed to incandescent platina or to a mass of iron 

 heated to 360. 



Colored glasses transmit rays of light of certain 

 degrees of refrangibility, and absorb those of other 

 degrees. For example, red glass absorbs the more 

 refrangible rays, and transmits the red, which are the 

 least refrangible. On the contrary, violet glass absorbs 

 the least refrangible, and transmits the violet, which 

 are the most refrangible. Now M. Melloni has found, 

 that although the coloring matter of glass diminishes its 

 power of transmitting heat, yet red, orange, yellow, 

 blue, violet, and white glass transmit calorific rays of all 

 degrees of refrangibility. Whereas green glass possesses 

 the peculiar property of transmitting the least refrangi- 

 ble calorific rays, and stopping those that are most re- 

 frangible. It has therefore the same elective action 

 for heat that colored glass has for light, and its action 

 on heat is analogous to that of red glass on light. Alum 

 and sulphate of lime are exactly opposed to green glass 

 in their action on heat, by transmitting the most re- 

 frangible rays with the greatest facility. 



The heat which has already passed through green or 

 opaque black glass will not pass through alum, while 

 that which has been transmitted through , glasses of 

 other colors traverses it readily. 



