114 ANNUAL REPORT OF NEW- YORK 



It is liowever the fact that the radiating and absorptive power of 

 a body for heat are absolutely equal. That body which absorbs 

 heat most readily, radiates it also most readily, and vice versa. 

 It must be understood however, that bodies may differ in their 

 power of absorbing or radiating heat of different degrees of inten- 

 sity. Lampblack absorbs and radiates heat of all intensities in 

 the same degree. White-lead absorbs heat of low intensity (such 

 as radiates from a vessel filled with boiling water) as fully as 

 lampblack, but of the intense heat of a lamp it absorbs only 

 about one-half as much. Snow seems to resemble white-lead in 

 this respect. If a black cloth or black paper be spread on the 

 surface of snow, upon which the sun is shining, it will melt much 

 faster under the cloth than elsewhere, and this too if the cloth 

 be not in contact with, but suspended above the snow. In our 

 latitude every one has had opportunity to observe that snow 

 thaws most rapidly when covered by or lying on black earth. 

 The reason is that snow absorbs heat of low intensity with 

 greatest facility. The heat of the sun is converted from a high 

 to a low intensity, by being absorbed and then radiated by the 

 black material. But it is not color that determines this difference 

 of absorptive power, for indigo and prussian blue though of 

 nearly the same color, have very different absorptive powers. So 

 far, however, as our observations extend, it appears that dark- 

 colored soils usually absorb heat more rapidly, and that the sun's 

 rays have least effect on light colored soils. This topic will be 

 recurred to. 



3. Reflection. — Bodies exposed to radiant heat may reflect it to 

 a great extent. This is the case with polished metals, while glass 

 is a poor reflector. Reflection is oj>posed to absorption. 



4. Transmission. — Radiant heat may also be transmitted through 

 bodies precisely in the way that light is. Rock salt transmits 92 

 per cent, of the heat that falls upon it; alum allows only 12 per 

 cent, to pass, while blue-vitriol intercepts radiant heat totally; 

 it is so to speak, opake to heat. On the other hand black 

 glass, which is opake to light, allows considerable heat to 

 pass through it. This kind of transmission is instantaneous and 

 /nust be distinguished from 



6. Conduction. — This is a slower process, and consists in the 

 passage of heat from particle to particle of a solid substance. 



