RADIANT HEAT. 311 



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 radia- 

 tion from the outer surface of the bulb. 



Dr. Ritchie has also maintained the same conclusions in his paper 

 before referred to, {Phil. Trans., 1S27, Part II, p. 142,) by varying 

 the distance of the screen, which he found to produce no sensible dif- 

 ference in the effect, though with screens of moderate thickness it 

 diminishes rapidly with the distance, according to Leslie's experi- 

 ments. 



Division II. 



TERRESTRIAL LUMINOUS HOT BODIES. 



a.) Nature of radiation. 



The earliest observers noticed differences between this case and 

 that of heat from non-luminous 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, c&c.) 



Cavallo (Phil. Trans., 1780,) found a blackened thermometer affect- 

 ed by the light of a lamp. 



Leslie (Inquiry, p. 448,) found a fire affect his photometer; also 

 candles, &c, (p. 447,) — a distinction pointed out between this and 

 the solar rays, (p. 83, 54.) 



The light from putrescent substances does not appear to be accom- 

 panied with any appreciable degree of heat, according to Dr. Hulme. 

 (Thomson's Ghem , i, 414, 4th edit.) But the effect, if any, must be 

 so small that we cannot positively assert there is none. 



The same remark may apply to many other very faint lights. 



b.) Reflection of heat. 



1.) Mariotte collected the heat of a fire in the focus of a reflector. 

 — (Mem. Acad, of Sciences, 1G82.) 



Lambert, with burning charcoal in the focus of conjugate reflectors, 

 found a combustible body kindled in the other focus. — (Lambert, 

 Pyrometrie ; Saussure, Voyage, iv, 119.) 



Scheele (On Air and Fire, p. 67, 71,) observes that a glass mirror, 

 though it reflects the light of a fire, does not reflect the heat, (it is 

 not stated by what means the heat was estimated,) but the mirror 

 becomes heated. A polished metallic mirror reflected both the light 

 and heat, and did not become much heated itself; if blackened it was 

 soon hot. 



Pictet extended the experiments with conjugate reflectors to this 

 case, by placing a candle in one focus. The thermometer rose nearly 

 10° in six minutes. — (Essais de Phys., p. G3.) 



