194 SOLAE RADIATION. 



control ; but in order to exercise such control, it is necessary 

 to understand correctly the theory of what is called eadiation. 



This cannot be better explained than in the words of DanieU. " The 

 power of emitting heat in straight lines in every direction, indepen- 

 dently of contact, may be regarded as a property common to all matter ; 

 but differing in degree in different kinds of matter. Co-existing with 

 it, in the same degrees, may be regarded the power of absorbing heat 

 so emitted from other bodies. Polished metals and the fibres of vege- 

 tables may be considered as placed at the two extremities of the scale 

 upon which these properties in different substances may be measured. 

 If a body be so situated that it may receive just as much radiant heat 

 as itself projects, its temperature remains the same ; if the surrounding 

 bodies emit heat of greater intensity than the same body, its tempera- 

 ture rises, till the quantity which it receives exactly balances its 

 expenditure, at which point it again becomes stationary ; and if the 

 power of radiation be exerted under circumstances which prevent a 

 return, the temperature of the body declines. Thus, if a ther- 

 mometer be placed in the focus of a concave metallic mirror, and 

 turned towards any clear portion of ijie sky, at any period of the 

 day, it wilL faU many degrees below the temperature of another 

 thermometer placed near it, out of the mirror; the power of radi- 

 ation is exerted in both thermometers, but to the first all return of 

 radiant heat is cut off, while the other receives as much from the 

 surrounding bodies, as itself projects. This interchange amongst bodies 

 takes place in transparent media as well as in vaouo ; but in the former 

 case, the effect is modified by the equalising power of the medium. 

 Any portion of the surface of the globe which is fully turned towards 

 the sun receives more radiant heat than it projects, and becomes heated ; 

 but when, by the revolution of the axis, this portion is turned from the 

 source of heat, the radiation into space still continues, and, being un- 

 compensated, the temperature declines. In consequence of the different 

 degrees in which different bodies possess this power of radiation, two 

 contiguous portions of the system of the earth wiU. become of different 

 temperatures ; and, if on a clear night we place a thermometer upon a 

 grass-plat, and another upon a gravel walk or the bare soil, we shall 

 find the temperature of the former many degrees below that of the 

 latter. The fibrous texture of the grass is favourable to the emission of 

 the heat, but the dense surface of the gravel seems to retain and fix it. 

 But this unequal effect will only be perceived when the atmosphere is 

 unclouded, and a free passage is open into space ; for even a light mist 

 will arrest the radiant matter in its course, and return as much to the 

 radiating body as it emits. The intervention of more substantial 

 obstacles will of course equally prevent the result, and the balance of 

 temperature will not be disturbed in any substance which is not placed 



