62 M. MELLONI ON THE IMMEDIATE TRANSMISSION 
diathermancy of these bodies therefore approximates nearly to that of 
the alum. The glass, the rock crystal, and the Iceland spar have evi- 
dently a different diathermancy, for the rays which pass through them 
are less transmissible by the invariable screen. The same may be said 
of borax, adularia, and carbonate of ammonia. As to the heat emerging 
from rock salt (limpid or dull) it acts in a manner similar to that in which 
the unobstructed light of the lamp would. The reason is evident, since 
the salt, acting equally on the different species of calorific rays, must 
transmit them all without reflecting their relative properties in any man- 
ner whatsoever. 
These facts then completely confirm the conclusions which we had 
drawn from the preceding experiments: namely, that, 1st, flame sends 
forth rays of several kinds; 2nd, that diaphanous colourless bodies, with 
the exception of rock salt, act so as to extinguish certain caloric rays 
and allow others to pass, just as coloured media act in respect to light. 
Here a very interesting question is naturally suggested. If the dia- 
thermancy or quality which constitutes the tint of a medium relatively 
to the radiant caloric is invisible, what part then do colours act in the 
transmission of heat? 
When the quantity of radiant heat that passes through coloured glass 
is measured, it is always found to be less than that which passes through 
white glass of the same thickness. The difference indeed is sometimes 
considerable, though having no apparent relation to the prismatic order 
or intensity of the colour. We have already remarked this in the first 
memoir, and the truth of the remark will be readily admitted by any 
one who casts an eye over the following little table. 
Screens of glass exposed to the 
radiation of a Locatelli lamp. 
(Common thickness 17°85.) 
Transmissions 
out of 100 rays of heat. 
GIBBS AWE. CPSP. Se 40 
— red (deep) 0. iiscccececscccscseneees 33 
Anm je OTANGS 4. doch ts edomisiganidemasucses 29 ° 
— yellow (brilliant) .................. 22 
—~ | green (Apple) x: nystoen vk cezebaeae 25 
— green (mineral)..................... 23 
Se PRUE’ ss. ereresan tt star stone overt ss 21 
vai inate!) Apaeeabatlite 2At ie Hoe. 12 
— ° violet (deep) !2..ccccetued.vens.Jdeeee 34 
— black (opake).........cccseeseeeecees 17 
It is therefore not to be doubted that an absorption of caloric is caused 
by the colouring matter. But is the power of absorption elective like the 
action of the invisible calorific tints in colourless diaphanous bodies, or 
does it affect all sorts of rays indiscriminately? We are about to inves- 
tigate this point by means of experiments similar to the preceding, in 
which we have taken equal quantities of heat issuing from different 
