386 M. MELLONI ON THE REFLECTION OF RADIANT HEAT. 
respect to glass, but also with respect to rock crystal, alum, fluate of 
lime, topaz, sulphate of barytes, &c., so that a thin plate of any of these 
substances, if very pure and well polished, will, when placed behind a 
thick plate of the same substance, always transmit 0-923 and lose 0-077. 
The same numbers will be found also when the thin plate is placed 
behind a thick plate of a different substance, provided the latter be less 
permeable to the direct rays of the source. Thus a thin plate of rock 
crystal transmits 0-923 of the radiation from thick glass ; and a thin plate 
of glass transmits the same proportion of the heat emerging from water 
oralum. The heat thus transmitted is so purified, that although it issues 
from a very thin layer, it is still capable of traversing considerable depths 
of glass or rock crystal without suffering any absorption. Hence it is 
that plates of glass or rock crystal measuring 7 or 8 millimetres in thick- 
ness will, when exposed to the rays emerging from a layer of water or 
alum measuring | or 2 millimetres in thickness, transmit 0°923, as is done 
by plates only half a millimetre thick. 
Concluding from all this that radiant heat undergoes a reflection of 
about four hundredths of the incident heat which falls perpendicularly on 
the surface of a diathermanous substance, we perceive at once the method 
that is to be pursued in order to determine the quantity of calorific rays 
reflected by athermanous bodies. We first observe the effect of the 
calorific transmission through a plate of rock salt when the radiation, 
emitted by a constant source, is perpendicular to its faces: the plate is 
afterwards inclined towards the incident rays. In the quantity of heat 
transmitted, there appears no sensible diminution so long as the inclina- 
tion does not exceed 25° or 30° around the normal. The reflection of the 
perpendicular rays is then sensibly equal to that of the rays forming an 
angle of between 60° and 65° with the reflecting plane. Now, this be- 
ing supposed, let us bring a bundle of radiant heat to fall on the well- 
polished surface of glass or rock crystal at an incidence of between 60° 
and 65°, and receive the reflected portion in the interior of the tube 
which envelopes the thermomultiplier. After having marked the calo- 
rifie force indicated by the galvanometer, let us repeat the same expe- 
riment on the polished surface of the athermanous body without making 
any change in the respective positions of the several parts of the appa- 
ratus." We shall thus obtain a second calorific force, differing from the 
first. The reflection of the athermanous body will evidently be 0:0393, 
which represents the value of the reflection at the surface of the rock 
crystal multiplied by the ratio of the observed forces. 
The following are the mean results of several comparisons made be- 
tween the quantities of heat reflected by rock crystal and yellow copper. 
Reflection of Reflection of — Ratiosof the — Product of the two numbers 
Rock Crystal. Yellow Copper. two reflections, (0°0393 and J1°3). 
3°15 35°63 11°3 04441. 
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