RADIANT HEAT. 299 



a.) Radiation (or communication of heat to sensible distances) is distinct 

 from its conveyance by conduction through the air; since, 



1.) It takes place perpendicularly downwards: 

 2.) Only in elastic media. 



The relative cooling in different media is seen in the following- ex- 

 periments. — (Rumford' s Essays, ii, 425; Torricelli ; Murray's Chem., 

 i, 328.) 



Thermometer cooled from 212° to 32° Fahrenheit : 



In Vacuo... in 10 min. 5 sec. 



Air 7 3 



Water.. 1 5 



Mercury.. 36 



Dulong and Petit, in their elaborate researches on the cooling of 

 bodies, have investigated the law of cooling in the most perfect 

 vacuum they could form : but they admit that there was always a 

 minute portion of air present. The radiation, therefore, of heat in 

 an absolute vacuum is by no means conclusively established. — (See 

 Annals of Phil., vol. xiii, p. 241.) 



3.) Professor Leslie ascertained that the effect from a mass of 

 given size is nearly proportional to the angle which it subtends at 

 the thermometer ; and that the heat suffers little or no diminution in 

 its passage through the air. 



The radiation is most copious in the direction perpendicular to a 

 plane surface of the hot mass, and is proportional to the sine of its 

 inclination to the direction of the thermometer. — (Inquiry into the 

 Nature and Propagation of Heat, p. 51, &c.) 



For the same position the effect is proportional to the excess of 

 temperature of the hot body above that of the air. 



4.) Pictet made an attempt to estimate the velocity with which 

 heat radiates, by means of concave reflectors at sixty -nine feet dis- 

 tance. The effect on the focal thermometer was absolutely instanta- 

 neous. — (Essais de Phys.) 



b.) Reflection of simple heat from non-luminous hot bodies. 



1.) The general principles are established by Professor Leslie. — 

 (Inquiry, pp. 14, 51.) 



2.) He shows that the quantity of heat reflected is proportional to 

 the sine of incidence on a plane surface. 



3.) It is affected by the polish of the surface. — (Leslie, Inquiry, pp. 

 81, 20, 98, 10G.) 



4.) The most exact experiments are those made with conjugate con- 

 cave reflectors ; a ball of iron below luminosity in one focus, a ther- 

 mometer in the other : a glass of boiling water may be substituted 

 for the iron ball. In either case a great effect is produced in the 

 opposite focus, though little out of it. — (Saussurc. Voyages, t. iv, p. 

 120; Sir W. Herschel, Phil. Trans., 1803, p. 305.) 



