diathermancy of these bodies therefore approximates nearly to that of the alum. The glass, the rock crystal, and the Iceland spar have evidently 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 manner 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 diathermancy 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 1mm.85.) |
Transmissions out of 100 rays of heat. | |
| Glass, | white | 40 |
| — | red (deep) | 33 |
| — | orange | 29 |
| — | yellow (brilliant) | 22 |
| — | green (apple) | 25 |
| — | green (mineral) | 23 |
| — | blue | 21 |
| — | indigo | 12 |
| — | violet (deep) | 34 |
| — | black (opake) | 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 investigate this point by means of experiments similar to the preceding, in which we have taken equal quantities of heat issuing from different
