OF RADIANT HEAT THUOUGH DlI'liiRENT BODIES. 53 



It is also known from the experiments of Delaroche and others that 

 the radiant heat which has traversed a plate of glass and suffered a cer- 

 tain loss will in passing through a second plate sustain a second loss pro- 

 portionally less than the first. In the same manner does the incident 

 white light in passing through the first layer of a coloured substance be- 

 come considerably weaker, while the emergent coloured light parses al- 

 most without suffering any diminution of intensity. 



By exposing a given plate of a diaphanous substance successively to 

 equal quantities of calorific rays from different sources we have seen 

 their transmissions vaiy with the temperature of the source, that is to 

 say, with the nature of the rays emitted. We have seen moreover that 

 the differences between one transmission and another decrease in pro- 

 portion as the plates employed are thinner, until within a certain limit 

 of tenuity they vanish or have a tendency to vanish altogether. All 

 these effects are observable in the differently coloured lights transmitted 

 through a coloured medium ; for if the medium be red the quantities of 

 light transmitted will be greater in proportion to the greater number of 

 red rays contained in each radiation. The other rays will be absorbed 

 in a greater or less degree. But the quantities of light transmitted ap- 

 proach more nearly to an equality in proportion as the plate to be passed 

 through is thinner. In short, the coloured media become more faint as 

 their mass is reduced, and when sufficiently attenuated retain no sensible 

 tint whatsoever, in other words, they become permeable to luminous rays 

 of all colours. 



We have several times remarked the striking differences exhibited 

 in the calorific transmissions of diaphanous substances. But this cu- 

 rious fact, which constitutes, as it were, the basis of our inquiries, ceases 

 to surprise us as soon as we feel convinced that bodies Avhicli are trans- 

 parent and colourless act upon heat in a manner similar to that in which 

 coloured media act upon light. For, as upon the intensity of the co- 

 lour depends the degree of transparency, that is, the number of lumi- 

 nous rays that^jass through the coloured substances, in like manner upon 

 this species of invisible calorific tint which diaphanous bodies possess 

 will depend whether a greater or a less quantity of heat be transmitted*. 



• Seeing that in respect to all the substances given in the table, the rock salt 

 excepted, the order of decrement is similar though the sources of heat are dif- 

 ferent, one might be inclined at first to infer that they belong to the same species 

 of partially diathermanous bodies, that is, that they may be compared with co- 

 loured media. But that such a conclusion is not legitimate will be shown by 

 • one example : let a be the species of rays transmitted by the medium A, h that 

 species which is transmitted by the medium B, and c the rays intercepted by the 

 same media. Let us suppose a calorific source that will give 30 a, 30 h, and 

 40 c; it is clear that the two media A and B will intercept 70 parts of the hundred 

 and transmit 30. However, the rays emerging from A will be different from 

 those which emerge from B. If we suppose a second source of heat such as 

 will give 20 a, 20 b, and GO v, wc sliall have 80 as the quantity intercepted and 



