ON HEAT. 271 



Mr. Leslie foufld, that when a screen of tinfoil or even gold- professor Les- 

 leaf, which is 600 times thinner than the tinfoil, was inter- lie's Experi- 

 posed between the thermomeler and the most powerful radi- '" 

 eating surface of the heated veSe!, the eft'ect on the thermo- 

 meter was completely intercepted. But that a pane of glass 

 only intercepted four fifths of the caloric, while a sheet of paper 

 did not intercept so much ; and, in order to do away the sup- 

 position, that the effect produced on the thermometer in the 

 experiments with the glass and paper, was owing to part ot 

 the radiant caloric passing through their substance, he ob- 

 serves, that this effect was only produced when the screens 

 were placed about two inches from the heated surface, and 

 that when about a foot from the tin vessel, the rise in the ther- 

 mometer was not one thirteenth of what it was in the first po- 

 sition. Hence he concludes, that the calorific influence is 

 completely arrested, and that the screen, by this, acquires heat, 

 and, in its turn, displays the same energy as if it had formed the 

 surface of a new canister of the corresponding temperature. 



Now, Sir, in repeating these experiments, I observed the Facts and ob- 

 same results, but was led to somewhat different conclusions, ^^""^^^'""^j to 

 1 conceive that the screens ot glass and paper do not entirely diant cak>ris 

 arrest the radiant caloric, but that they allow part of it to pass, ^^^^9^ througb 

 and I do so for this reason. When the screen was placed two 

 inches from the heated surface, I observed it acquired heat 

 not only from the rays which it had arrested, but also by com' 

 munication ; when I placed it about a foot from the canister, 

 it had not its temperature varied at all, and therefore I con- 

 ceive the effect upon the thermometer was produced zfholli/ by 

 the rays of caloric which passed through its substance. 



In the first instance, the glass screen received caloric, not Heat transmit- 

 only by radiation, but still more by communication from the ted through bo- 

 heated surface; so that its temperature was raised, and it be- filei-Tn^'^"' 

 came capable of radicating in its turn; of course, the rays tuic being 

 from the canister which passed through the screen, assisted ^^*^'^'■'• 

 by those from tha screen itself, produced a greater effect on the 

 thermometer. But, in the second case, the screen received no 

 caloric byco7/z7?2zmzc«/zo/?, its temperilurewas notraisea, there- 

 fore it could not, as Mr. Leslie would have it, " display any 

 energy in causing a fluctuation, or partial swell, in ih^ mass 

 of air, 90 as to transport the heat. I was anxious to ascertain 

 whether or not the pane of glass, when placed about a foot 



distant, 



