In eight-and-twenty cases there have occurred but the three exceptions presented by carburet of sulphur, chloride of sulphur, and protochloride of phosphorus, in which the transmissions did not change when the liquid was substituted for glass. I found it therefore impossible to decide at first whether these three substances acted in the same manner as the others; for if they had acted even in a contrary way, provided their least transmission were equal to 30°, the result obtained would be the same. But in all probability these three anomalies are merely apparent; for the chloride of sulphur, the carburet of sulphur, and the protochloride of phosphorus being in a high degree permeable to radiant heat, the same thing will happen in respect to these three liquids inclosed in glass vessels that happens when very pure fluate of lime is substituted for them; that is to say, the transmissions of the system retain their proper values, though the fluate of lime itself be subject to the general law.
Thus the radiant heat from different sources is absorbed in greater or less proportions while it is passing through diaphanous bodies (solid or liquid); but while it is passing through the same body the absorption constantly increases as the temperature of the source decreases.
It happens quite otherwise to the luminous rays. Let us look through a plate of glass at the most vivid flame or at any other phosphorescent substance. If the plate is very pure, its interposition will produce no sensible effect, and the images will retain all the relations of intensity which they had when viewed directly. The pale phosphoric gleam therefore suffers in the interior of the glass screen the same absorption as the strong light of the flame does.
The bodies on which I have made my experiments have been taken indiscriminately from the three kingdoms of nature: some crystallized, others amorphous; some solid, others liquid; some natural, and others artificial: yet they all act in a similar order relatively to the rays of the different sources of caloric. Does not this constancy in their manner of acting, notwithstanding such great differences in their physical and chemical constitutions, indicate that this law of decrement belongs to the very nature of the heat? We should not however infer from this that there are not bodies which afford a passage equally free to calorific rays of every kind. For we see by the table that a flake of rock salt,
most opake bodies become diaphanous when they are sufficiently attenuated), so, in order to judge of the calorific transmissions through different bodies, we must take the greatest possible care not to employ excessively thin plates, or at least, if we are compelled by particular circumstances to use such, the substances compared should be perfectly equal in thickness; for in that state of tenuity the least difference of thickness might disturb the order of permeability and cause us to attribute a greater calorific transparency to substances possessing this property in an inferior degree. This is probably the cause of the mistake into which those have fallen who have fancied that they could prove by their experiments that water is more diathermanous than glass.
