68 



M. MELLONI ON THE IMMEDIATE TRANSMISSION^ 



screens, but is abundantly transmitted by all the diaphanous colourless 

 plates. It suffers no appi'eciable loss when the thickness of the plates 

 is varied within certain limits. Its properties of transmission therefore 

 bear a close resemblance to those of light and solar heat. 



Let us now direct our attention to the rays Avhich issue from the last 

 two screens. The opake bodies transmit nearly the half of them ; the 



circumstance which makes it easier to compare the results than it is found to be 

 when we are obliged to have recourse to two tliermoscopes, which seldom or never 

 possess the same degree of sensibility. 



I shall now give the ratios derived from this process applied to direct heat, 

 and to heat transmitted through several screens. The calorific effect produced 

 f^ach time on the black surface is represented by 100. 



Radiant heat from a Locatelli lamp, 

 (direct, or transmitted through several screens). 



Rays direct from the lamp 



Kays transmitted through rock salt 



alum 



glass, colourless .... 



' bright red ... . 



deep red .... 



bright yellow. 



deep yellow . 



■ bright green . 



■ deep green . 



bright blue . 



. deep blue 



. bright violet . 



. deep violet . 



. — opake black . 



Thus the intei-position of the rock salt has no influence on the ratio of the quan- 

 tities of heat absorbed by tlie two surfaces ; but the alum affects it so strongly 

 that the heat which has traversed a plate of this substance is much less capable 

 than the direct heat is of being absorbed by the white surface. Colourless glass 

 acts in a similar manner though with somewhat less energy. As to coloured 

 glasses, their action is more feeble in proportion as their tint is less vivid. In 

 short the greatest decrease in the absorption of the white surface is produced by 

 the interposition of a yellow glass, and the least by the interposition of the red 

 and the violet, and, as to each pair of plates of the same tint, the less effect is 

 invariably derived from that in which the tint is deeper. This decrease of action 

 which takes place in the vitreous matter in proportion as its transparency is di- 

 minished by addition of colouring substances more and more sombre, continues 

 even when "the glass loses its transparency altogether; for the plate of opake 

 black glass is that which produces the least difference of absorption between the 

 black and the white surfaces. It is liowever an exceedingly curious fact that the 

 rays of heat in their passage through the black glass become more absorbable by 

 the white surface than the rays issuing inunediately from the lamp, so that the 

 interposition of the black glass has on the direct heat an effect contrary to that 

 produced on it by the interposition of the white glass. 



