68 M. MELLONI ON THE IMMEDIATE TRANSMISSION 
screens, but is abundantly transmitted by all the diaphanous colourless 
plates. It suffers no appreciable 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 which 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 éwo thermoscopes, 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 
each time on the black surface is represented by 100. 
Absorbent 
Radiant heat from a Locatelli lamp, power of the faces 
(direct, or transmitted through several screens). | —~_____ 
black. white. 
ea as and sab EMP SE Bead ire, NACo Sac ae ae ee 
Rays direct from the lamp......sses-eseeeeeereeeeeenes 100 =| =80°5 
Rays transmitted through rock salt ........0+++++++++ _ 80°5 
ee TSIEN BR Soonasad scares tns _ 42:9 
od lye Nae — glass, colourless ......... -- 54:2 
a bright red......... — 60°6 
SS eepired faerie: — 778 
ee De Velo wa. sens — 55°5 
Se "deep yellow’ !-. .2 — 63°6 
Se bright green ...... — 67:4 
Seep Sheen erence — 70°5 
eS — pright blue sti... —_— 61-0 
a deep blue ......... — 66:9 
ese bright violet ...... — | 676 
eee wee eee deep wioletay essa — | 767 
ee _ or ae opake black ...... _ ; 
Thus the interposition of the rock salt has no influence on the ratio of the quan- 
tities of heat absorbed by the 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 isdeeper. 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 however 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 immediately 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. 
