OF RADIANT HEAT THROUGH DIFFERENT BODIES. 49 
In eight-and-twenty cases there have occurred but the three excep- 
tions presented by carburet of sulphur, chloride of sulphur, and proto- 
chloride 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 protochlo- 
ride 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 ealorific 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 pro- 
 perty 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. 
Vor. L—Parr I. E 
