OF RADIANT HEAT THROUGH DIFFERENT BODIES. 51 
to differently coloured lights? Lights of the same tint as the glass pass 
abundantly, the rest are almost totally intercepted. 
These analogies lead us therefore to consider the radiations from dif- 
ferent sources of heat as not being of the same nature. This seems in- 
deed sufficiently established by the mere fact that the calorific trans- 
mission of glass, Iceland spar, or any other diathermanous body varies 
with the temperature of the radiating source. 
Thus boiling water, copper heated to 390°, incandescent platina, and 
the flame of oil will be to us the sources of a heat that is more or less 
coloured, that is to say, sources each of which gives out a greater quan- 
tity of calorific rays of a certain quality; but the flame will furnish ca- 
lorie rays of every kind as it furnishes light of all colours. 
We shall distinguish bodies into diathermanous and athermanous*. 
The diathermanous we shall subdivide into universal and partial. The 
first of these subdivisions, which is analogous to colourless media, will 
contain but one substance, namely, rock salt; the second, which cor- 
responds with the coloured media, will contain all the bodies comprised 
in our table, in addition to diaphanous liquids and diaphanous substances 
in general. 
As to the class of athermanous bodies I had supposed at first that 
every substance which completely intercepted light intercepted the whole 
of the radiant heat also. This is found to be the fact in the greatest 
number of cases. But subsequent experiments have shown me that flakes 
of black mica and black glass, though they completely intercept the most 
intense solar light, yet exhibit very strongly marked calorific transmis- 
sions. The following are the results: 
Transmissions 
out of 100 rays issuing from 
¥ TR 
a incan- | copper | copper 
Locatelli] descent at 
lamp. | platina. | 390°. 
Black glass(1™"in thickness)} 26 25 12 
Ditto (2™™ § ditto } 16 15°5 8 
Black mica(O™™6 ditto 29 28 13 
Ditto (0™™9 ditto ) | 20 | 20 9 
* Athermanous, in contradistinction to diathermanous, evidently signifies the 
absence of the power of transmitting heat. I adopt this term merely for con- 
venience, without attaching to it a definite meaning; for, as there is no body 
_ which, if reduced to an extremely thin plate, may not become in some degree’ 
_ transparent, I think also that some rays of heat may pass through all substances 
in a state of great tenuity. 
EQ 
