CONDUCTION AND RADIATION. 539 



when the temperature of the surrounding space 

 increases in arithmetical progression ; whereas, ac- 

 cording to the Newtonian law, this quickness would 

 not have varied at all. Again, this variation being 

 left out of the account, it appeared that the quick- 

 ness of cooling, so far as it depends on the excess of 

 temperature of the hot body, increases as the terms 

 of a geometrical progression diminished by a con- 

 stant number, when the temperature of the hot body 

 increases in arithmetical progression. These two 

 laws, with the coefficients requisite for their appli- 

 cation to particular substances, fully determine the 

 conditions of cooling in a vacuum. 



Starting from this determination, MM. Dulong 

 and Petit proceeded to ascertain the effect of the 

 medium, in which the hot body is placed, upon its 

 rate of cooling; for this effect became a residual 

 phenomenon* \ when the cooling in the vacuum was 

 taken away. We shall not here follow this train of 

 research ; but we may briefly state, that they were 

 led to such laws as this ; that the rapidity of cool- 

 ing due to any gaseous medium in which the body 

 is placed, is the same, so long as the excess of the 

 body's temperature is the same, although the tem- 

 perature itself vary ; that the cooling power of a 

 gas varies with the elasticity, according to a deter- 

 mined law ; and other similar rules. 



In reference to the process of their induction, 

 it is worthy of notice, that they founded their rea- 



M Sec Phil. ln<t Science*. B. xm. c. 7. Sect. iv. 



