ON THE S0URCE3 AND EFFECTS OF HEAT. 637 



face is only one sixth of that which the air receives. Mr. Leshe has also 

 found that the same surfaces which emit heat the most freely, are also the 

 readiest to receive it from the radiation of other bodies. 



The solar heat radiates freely through air, glass, water, ice, and many 

 other transparent mediums, without producing any sensible effect on their 

 temperatures, and even when it is concentrated by the effect of a burning 

 mirror, it scarcely affects the air through which it passes, and other transpa- 

 rent mediums but little. But the heat of a fire warms a piece of common 

 glass very rapidly, and its further progress is almost entirely interrupted by 

 the glass, although probably a certain portion still continues to accompany 

 the light in all cases. Hence a sci-een of glass is sometimes practically con- 

 venient for allowing us the sight of a fire, and protecting us at the same 

 time from its too great heat. Mr. Lambert showed that culinary heat was 

 much more strongly reflected by mirrors of metal than of glass, although lit- 

 tle diff'erence was observable in the quantity of light, and he very justly at- 

 tributed this difference to the interception of a part of the heat by the glass, 

 which operated with respect to it like an opaque substance, although it trans- 

 mitted the light with freedom. Opacjue substances in general appear to be 

 wholly impervious to radiating heat of all kinds; but Dr. Herschel has found 

 that dark red glass, which transmits a very small portion of light only, suffers 

 some kinds of radiant heat to pass through it with very little interruption. 



In other respects, radiating heat is subject, in all cases, to the optical laws 

 which govern the reflection and refraction of light. Dr. Hoffmann appears 

 to have been the first that collected the invisible heat of a stove into a focus 

 by the reflection of one or more concave mirrors. Buff'on, Saussure, Pictet, 

 and Mr. King, made afterwards similar experiments on the heat of a plate of 

 iron and of a vessel of boiling water. Mr. Pictet, as well as Hoff^mann, em- 

 ployed two mirrors facing each other; and by means of this arrangement the 

 experiment may be performed when the thermometer is placed at a considera- 

 ble distance from the heated body. 



The temperature of the air, not being affected by the radiation of heat, is 

 probably in all respects indifferent to its emission iir this manner; and as the 

 raj's of light cross each other freely in all possible directions, so it appears 



VOL. I. 4 k 



