REPORT ON RADIANT IIEAT, 
207 
e.) Effect of screens on heat from nonluminous hot bodies. 
1.) Pictet found a difference in the interceptive effect, accord¬ 
ing as the plain or the silvered side of a glass screen was towards 
the source of heat. 
Towards Hot Body. 
Glass. 
Amalgam. 
Amalgam, blackened. 
Amalgam removed,—glass blackened 
Ratio of Effects on 
Thermometer. 
. 0 
. 35 
QQ 
«••••• <L//V 
.ISO 
(Essai , Sj'c., p. 72.) 
2. ) He tried to refract simple heat, without effect. 
Sir W. Herschel tried with a lens, and supposed it effected : 
this has been refuted by Sir D. Brewster. ( Vide infra; Phil. 
Trans. 1800, Part II. No. 15. Exp. 19, 20.) 
3. ) Prof. Leslie’s experiments on screens are perhaps the most 
valuable portion of his inquiry. 
He found the effect of a screen increase rapidly with its di¬ 
stance from the source (p. 28), and less so with its thickness 
(p. 38). 
Different substances appear to have a different interceptive 
power; but this upon examination appears always to be depen¬ 
dent on their conducting power, and the absorptive nature of 
their surf ace jointly. 
The most decisive experiment on this point was that made 
with two panes of glass, each having one side coated with tin- 
foil : according as the plain or coated sides were placed in the 
contact, the compound screen had a greater or less apparent 
interceptive power ; that is, a greater or a less power of absorb¬ 
ing and subsequently radiating the heat. Again, either might 
be used separately, or the two at an interval, (p. 35.) 
4. ) Prevost concluded that a certain portion of heat is directly 
transmitted through transparent screens, by employing move- 
able screens which continually presented a fresh surface, so that 
it was supposed all communication of heat and conveyance by 
way of secondary radiation would be prevented. 
But it must be considered that it is impossible to prevent 
entirely any portion of a screen in the most rapid motion from 
acquiring heat:—no such experiments therefore can be strictly 
conclusive. 
Dr. Ritchie tried experiments with the same view, by means 
of a film of liquid adhering to threads stretched across a frame 
