46 M. Melloni on the Transmission of Heat through [Jan. 



any other diathermanous plate besides rock-salt consists. 

 In short, if we imagine the plate to be divided into several 

 equal slices, and if we examine by experiments, the pro- 

 portion of the quantity lost, to the quantity falling upon 

 these slices, we shall find that the loss thus calculated, 

 decreases rapidly with the distance from the surface, but 

 the decrease becomes less sensible, so that the loss ought to 

 take an invariable value when the rays arrive at a certain 

 depth. This is precisely what happens when ordinary light 

 enters a coloured medium, the loss of intensity being at first 

 great, and gradually diminishes. A third proof of the ana- 

 logy which exists between the action of diathermanous 

 bodies upon radiating heat, and the action of coloured 

 media upon light, is derived from successive transmission 

 through heterogeneous screens. The luminous rays which 

 proceed from a coloured plate pass abundantly through a 

 second plate similarly coloured, and undergo a great ab- 

 sorption, in proportion as the colour of the second plate is 

 more or less analogous to the colour of the first. 



There is only one diaphanous and colourless body, which 

 acts in the same manner on luminous and calorific rays. 

 All others allow every species of light to pass indistinctly, 

 but they absorb certain rays of heat, and transmit others. 

 Thus, we find in these bodies a true calorific colouring 

 power, which is invisible, and is called by the author, 

 diathermansie. 



The following table exhibits the relative diathermanous 

 power of different substances : — 



Rock-salt, clear, - - - - 92 

 Flint-glass, clear, - - - 67 

 Sulphuret of carbon, colourless, - 63 

 Chloride of sulphur, dark, - - 63 

 Rock-crystal - - - - 62 

 Rough topaz, clear-brown, - -57 

 Brazil topaz, clear, - - - 54 

 Crown-glass, clear, - - - 49 

 White agate, transparent, - - 35 

 Barytes spar - - - - 33 

 Oil of turpentine - - - - 31 

 Nut oil, yellow, - - - - 31 

 Oilofcolsa 30 



