274 SECOND REPORT— 1832. 
outside half of one being coated with black , the liquid sinks 
from that side, 
Hence he infers that the coating has here stopped the heat, 
which otherwise radiates freely through the very thin glass. 
He varied the experiment by using portions of glass blown 
thin as screens over an aperture: when blackened in a flame 
or coated with silver-leaf, they intercepted heat; when trans¬ 
parent, not. That this was not from increase of thickness, was 
shown by using three thicknesses transparent, then removing 
the middle one, and blackening the inner surface of the others. 
He explains the subject by the theory of material caloric and 
mutual repulsion of its particles. 
The same author in another paper (Ann. of Phil. 2nd Series, 
xii. 123,) gives a variation of the experiment: the hot body is 
placed between two large and very thin bulbs; one of the he¬ 
mispheres of one bulb, formed by a plane passing through the 
centres of both, is coated with China ink; as are also two of 
the alternate quarters of the other, formed by a plane cutting 
the former at right angles. 
A greater effect is produced on this second bulb. 
This is an argument against the effect being due to greater 
radiation, from the outer surface of the bulb. 
Dr. Ritchie has also maintained the same conclusions in his 
paper before referred to, (Phil. Trans. 1827, Part II. p. 142,) by 
varying the distance of the screen, which he found to produce 
no sensible difference in the effect; though with screens of 
moderate thickness it diminishes rapidly with the distance, ac¬ 
cording to Leslie’s experiments. 
\ 
Division II. 
Terrestrial luminous hot bodies. 
a.) Nature of radiation. 
The earliest observers noticed differences between this case 
and that of heat from nonluminous bodies. 
The heat from flame, &c. at least in part, passes through 
air, &c., without heating it. 
Scheele observed this with a fire, and that currents of air 
did not change the direction of the rays. ( Treatise on Air and 
Fire , Sfc.) 
Cavallo (Phil. Trans. 1780,) found a blackened thermometer 
affected by the light of a lamp, 
Leslie (Inquiry, p. 448,) found a fire affect his phbtometer ; 
