RS ee > 
OF RADIANT HEAT THROUGH DIFFERENT BODIES. 53 
It is also known from the experiments of Delaroche and others that 
the radiant heat which has traversed a plate of glass and suffered a cer- 
tain loss will in passing through a second plate sustain a second loss pro- 
portionally less than the first. In the same manner does the ineident 
white light in passing through the first layer of a coloured substance be- 
come considerably weaker, while the emergent coloured light passes al- 
most without suffering any diminution of intensity. 
By exposing a given plate of a diaphanous substance successively to 
equal quantities of calorific rays from different sources we have seen 
their transmissions vary with the temperature of the source, that is to 
say, with the nature of the rays emitted. We have seen moreover that 
the differences between one transmission and another decrease in pro- 
portion as the plates employed are thinner, until within a certain limit 
of tenuity they vanish or have a tendency to vanish altogether. All 
these effects are observable in the differently coloured lights transmitted 
through a coloured medium; for if the medium be red the quantities of 
light transmitted will be greater in proportion to the greater number of 
red rays contained in each radiation. The other rays will be absorbed 
in a greater or less degree. But the quantities of light transmitted ap- 
proach more nearly to an equality in proportion as the plate to be passed 
through is thinner. In short, the coloured media become more faint as 
their mass is reduced, and when sufficiently attenuated retain no sensible 
tint whatsoever, in other words, they become permeable to luminous rays 
of all colours. 
We have several times remarked the striking differences exhibited 
in the calorific transmissions of diaphanous substances. But this cu- 
rious fact, which constitutes, as it were, the basis of our inquiries, ceases 
to surprise us as soon as we feel convinced that bodies which are trans- 
parent and colourless act upon heat in a manner similar to that in which 
coloured media act upon light. For, as upon the intensity of the co- 
lour depends the degree of transparency, that is, the number of lumi- 
nous rays that.pass through the coloured substances, in like manner upon 
this species of invisible calorific tint which diaphanous bodies possess 
will depend whether a greater or a less quantity of heat be transmitted*. 
* Seeing that in respect to all the substances given in the table, the rock salt 
excepted, the order of decrement is similar though the sources of heat are dif- 
ferent, one might be inclined at first to infer that they belong to the same species 
of partially diathermanous bodies, that is, that they may be compared with co- 
loured media. But that such a conclusion is not legitimate will be shown by 
one example: let a be the species of rays transmitted by the medium A, 6 that 
species which is transmitted by the medium B, andc the rays intercepted by the 
same media. Let us suppose a calorific source that will give 30 a, 30 J, and 
40 c; it is clear that the two media A and B will intercept 70 parts of the hundred 
and transmit 30. However, the rays emerging from A will be different from 
those which emerge from B, If we suppose a second source of heat sucly as 
will give 20 a, 20 6, and 60 ¢, we shall have 80 as the quantity intercepted and 
