OF RADIAXT HEAT THROUGH DIFFERENT BODIES. 49 



In eight-and-twentj' cases there have occurred but the three excep- 

 tions presented by carburet of sulphur, cliloride 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 chloi'ide 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 hajipens 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 veiy pure, its interposition will produce no 

 sensible eflfect, and the images will retain all the relations of intensity 

 which they had when viewed directlj'. 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 exiieriments 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 verj' 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 pi-o- 

 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 

 tliat water is more diathermanous than glass. 

 ^'oI,. I Part I. i-; 



