230 OUTLINES OF NATURAL PHILOSOPHY. 



321. The heat propagated by radiation from 

 different bodies varies with the nature of their 

 external surfaces, the quantity that flows in a gi- 

 ven time from a body with a polished surface, 

 being much less than would flow from the same 

 body with a rough surface. 



This was discovered by Professor LESLIE, and proved 

 by a variety of curious experiments. Experimental 

 Inquiry into the Nature and Propagation of Heat, 

 p. 17., &c. 



When the fluid contained in a vessel, is intended to 

 retain its heat long, the vessel should be of metal, 

 and it surface smooth and bright. 



If the fluid is required to cool as fast as possible, the 

 surface should be rough covered with paper, char- 

 coal, &c. D ALTON'S New System of Chemical Phi- 

 losophy, Part i. p. 116. 



322. If two portions of the same fluid, of which 

 the masses are M and M', and the temperatures 

 t and /', be mixed together, the temperature of the 



mixture will be 



M + M' 



This formula is investigated, on the supposition that 

 no part of the heat is lost by the mixture. 



On making the experiment, the results are not found 

 to agree exactly with those deduced from the for- 

 mula. The expansions of mercury, therefore, do 



not 



