Fa al 
OF RADIANT HEAT THROUGH DIFFERENT BODIES. Dy 
fact, everything leads us to believe that the equal layers which succeed 
one another in a diaphanous medium act in the same manner on the 
rays of light which come in succession to pass through them, and that 
they consequently absorb or reflect a quantity of light proportional 
to the intensity of the incident rays; that is to say, that the loss sus- 
tained by the luminous radiation at every layer of equal thickness is 
constant. In the case under consideration, the invariable decrement of 
the light at each of the layers into which we suppose the screen divided 
is found to be none at all, or extremely feeble, because of the perfect 
transparency of the glass; and yet the caloric rays undergo in their 
successive passages an absorption, the sum of which is equal to about 
the half of their whole value ; and the losses at each layer, instead of 
being constant, as happens to those sustained by the luminous rays, are 
found to differ enormously from one another, being in the proportion of 
the numbers 381, 71, 31, and 16. 
The resistance of diaphanous media to the immediate transmission of 
the rays of heat is therefore of a nature altogether different from that 
which is presented by the same media to the propagation of light. 
Whatever be the cause of this singular difference, it is highly im- 
portant to determine with certainty whether it takes place at great di- 
stances from the surface at which the rays enter ; and this may be done 
by repeating the experiments on layers of glass much thicker than those 
which we have been using. 
With this view I took several pieces of the glass of Saint-Gobain, and 
caused them to be recast. This operation was not completely success- 
ful. The matter either formed itself into layers that were too thin, or 
was slightly striated. From among the thick pieces I selected that which 
was the purest. It was six inches in length. I divided it into three parts, 
of one, two, and three inches in thickness. The defects being uniformly 
distributed over all the points of the mass might probably enough alter 
the quantity of the caloric rays that would have passed. through a per- 
fectly pure mass of the same matter and thickness ; but it is clear that 
they could have no influence on the nature of the progression of the 
losses which these rays might undergo in passing from one layer to an- 
other. 
The following are the results obtained by exposing these screens to 
the ordinary radiation of 30°: 
Thickness of the screens 
Aig HS Galvanometric deviations. 
in millimetres. de 
°o 
27 177105 
54 13°458 
81 - 10°702 
_ By a calculation exactly similar to that already made we find that, 
_ 
