372 RADIANT HEAT. 



creases, is more raised the less it is diathermanous to the rays trans- 

 mitted to it." 



In Section III, "On the Property of Radiating Heat in Bodies," 

 the author examines various cases in which the state of the surface, 

 as in cast and rolled lead, smooth and scratched more or less closely, 

 was observed as to its influence on the radiation, when the plates 

 were kept heated by boiling water. — (Table VIII.) 



Similar experiments with copper (Table IX) confirmed Melloni's 

 conclusion, that the action is purely mechanical. 



In another set of experiments (Table X) the increase of radiation 

 with increased thickness of coating confirms the conclusion of Rum- 

 ford and Melloni, that radiation commences from a certain depth be- 

 low the surface. 



The next set of experiments was directed to answer the inquiry, 

 " Does the radiating power of one and the same body vary according 

 as it is heated to a given degree by rays from different sources of 

 heat?" The answer was distinctly in the negative, the sources of 

 heat being a lamp and a hot cylinder, and the body heated and then 

 radiating, being successively paper coated with carmine, and with 

 lamp-black on the absorbing side, and lastly on both sides; also a 

 plate of charcoal, and carmine spread upon wire gauze (Table XI;) 

 and again, using carmine blackened next the thermoscope, and plain; 

 and black paper coated with lamp-black next the thermoscope, and 

 plain.— (Table XII.) 



The author thus arrives at the conclusion, "under those circum- 

 stances in which the same bodies exhibit an unequal absorptive 

 power their radiating power is one and the same; and those differ- 

 ences which have hitherto been observed when they are not heated 

 to the same extent are, therefore, pure functions of the former and 

 independent of the latter." 



The conclusion, if we understand it rightly, would appear capable 

 of being more easily stated, and, indeed, rendered at once obvious, 

 from the distinctions at first pointed out; whence it is evident that 

 the rays of Species I (as before described) from the cylinder, and 

 those of Species II from the lamp, will necessarily act very unequally,, 

 according to the texture and color of the surface. 



But when a body has received the radiation, from whatever source, 

 and converted it into heat of temperature, as in these cases, to an 

 equalized degree, it will necessarily radiate it again in an equal de- 

 gree with the same surface, from whatever species of rays it was 

 originally obtained. 



Section IV is headed "Comparison of the Heat radiated from differ- 

 ent bodies within a certain range of Temperature." 



The author commences by remarking that "all former observations 

 upon radiation have only related to the quantities of heat emitted by 

 different substances at certain temperatures." The object of the 

 present investigation is to ascertain whether there are any qualitative 

 differences; or, as the author expresses it, "whether the heat which 

 radiates from certain bodies at one and the same temperature, or 

 within certain limits of temperature, is of a different kind, according 



