RADIANT HEAT. 303 



4.) The effect of the surface on radiation is beautifully exemplified 

 in the laws which regulate the formation of dew as developed by Dr. 

 Wells.— (Essay on Dew, 1814. See also Dufay, Mem., Paris, 1736, 

 p. 352; and Harvey on Dew, Quarterly Journ. of Science, No. 33; 

 Edinb. Journ. of Science, i, 161.) 



5.) Dr. Ritchie (Edinb. Phil. Journ., xxiii, 15) explains his theory 

 of the mode in which the radiating' power of surfaces is increased by 

 making them rough, or furrowing, &c. He contends that it is not 

 owing to the increase of surface, but to the quantity of heat reflected 

 by the sides of the furrows. 



He adopts the hypothesis of material caloric, and that its mole- 

 cules are mutually repulsive. 



The effect of surface is an essential distinction between radiation 

 and conduction by air ; the latter being shown by Dulong and Petit 

 to be absolutely independent of the nature of the surface. — (Annals 

 of Phil, xiii, 322.) 



d.) Effect of surface on the absorption of heat from non-luminous hot bodies. 



1.) De Saussnre and Pictet, with the apparatus before described, 

 found that the thermometer rose in two minutes: 



Plain 4|° Fahrenheit. 



Blackened 3| 



2.) By the same apparatus before described, Professor Leslie found 

 that on coating the bulb of the thermometer with the different sub- 

 stances, the absorptive power was very nearly in the same proportion 

 as the radiative; and by making the same modifications in the surface 

 of the reflector, he found that reflective power is inversely as the ra- 

 diative or absorptive. — (Inquiry, pp. 19, 81, 98.) He also gives a very 

 precise set of experiments on the effect of coatings of jelly of increasing 

 thicknesses. — (p. 100.) 



3.) Dr. Ritchie has devised a very elegant mode of showing that the 

 absorptive power of surfaces is precisely proportional to their radiating 

 power. — (Royal Inst. Journ., vol. v., p. 305.) 



The instrument consists of a large differential thermometer, whose 

 bulbs are chambers of considerable size, presenting large and equal 

 plane surfaces on the sides which are towards each other; of these 

 one is plain or polished, the other coated. Midway between them is 

 placed a canister having equal plane surfaces facing each of the former 

 respectively, and one polished, the other coated with the same pig- 

 ment as before; this canister is filled with hot water, and is capable 

 of turning on a vertical axis; thus the coated surface of the canister 

 can be turned to the coated bulb or to the polished; in the former 

 case a great effect is produced on the coated bulb, and a very small 

 effect on the plain; in the second case the better radiating surface is 

 directed to the worse absorptive one, and the worse radiating to the 

 more absorptive, and the liquid in the tube remains perfectly stationary; 

 the exact equality, therefore, of the absorptive and radiating powers 

 is established. The whole is on a large scale, and can be exhibited 

 to a class. 



