202 RADIAXT HEAT. SECT. XXVII. 



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 colouring 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 transmit- 

 ting the least refrangible calorific rays, and stopping those that 

 are most refrangible. It has therefore the same elective action 

 for heat that coloured 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 refrangible rays with the greatest 

 facility. 



The heat which has already passed through green or opaque 

 black glass will not pass through alum, whilst that which has 

 been transmitted through glasses of other colours traverses it 

 readily. 



By reversing the experiment, and exposing different substances 

 to heat that had already passed through alum, M. Melloni found 

 that the heat emerging from alum is almost totally intercepted 

 by opaque substances, and is abundantly transmitted by all such 

 as are transparent and colourless, and that it suffers no appreciable 

 loss when the thickness of the plate is varied within certain 

 limits. The properties of the heat therefore which issues from 

 alum nearly approach to those of light and solar heat. 



Radiant heat in traversing various media is not only rendered 

 more or less capable of being transmitted a second time, but, 

 according to the experiments of Professor Powell, it becomes 

 more or less susceptible of being absorbed in different quantities 

 by black or white surfaces. 



M. Melloni has proved that solar heat contains rays which are 

 affected by different substances in the same way as if the heat 

 proceeded from a terrestrial source ; whence he concludes that 

 the difference observed between the 'transmission of terrestrial 

 and solar heat arises from the circumstance of solar heat con- 

 taining all kinds of heat, whilst in other sources some of the 

 kinds are wanting. 



Radiant heat, from sources of any temperature whatever, is 



