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 certain loss will in passing through a second plate sustain a second loss proportionally less than the first. In the same manner does the incident white light in passing through the first layer of a coloured substance become considerably weaker, while the emergent coloured light parses almost 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 proportion 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 approach 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 curious 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 transparent 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 colour depends the degree of transparency, that is, the number of luminous 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[1].
- ↑ 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 different, 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 coloured 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, b that species which is transmitted by the medium B, and c the rays intercepted by the same media. Let us suppose a calorific source that will give 30 a, 30 b, 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 such as will give 20 a, 20 b, and 60 c, we shall have 80 as the quantity intercepted and
