CONDUCTION AND RADIATION. 151 



Newton's law will materially affect the mathematical calculations on 

 the subject, which were made to depend on that law both by Fourier. 

 Laplace, and Poisson. Probably, however, the general features of the 

 results will be the same as on the old supposition. M. Libri, an 

 Italian mathematician, has undertaken one of the problems of this 

 kind, that of the arrnil, with Dulong and Petit' s law for his basis, in a 

 Memoir read to the Institute of France in 1825, and since published 

 at Florence. 21 



Sect. G. Other Laws of Phenomena with respect to R.. citation. 



THE laws of radiation as depending upon the surface of radiating 

 bodies, and as affecting screens of various kinds interposed between 

 the hot body and the thermometer, were examined by several inquirers. 

 I shall not attempt to give an account of the latter course of research, 

 and of the different laws which luminous and non-luminous heat have 

 been found to follow in reference to bodies, whether transparent or 

 opaque, which intercept them. But there are two or three laws of 

 the phenomena, depending upon the effects of the surfaces of bodies, 

 which are important. 



1. In the first place, the powers of bodies to emit and to absorb 

 heat, as far as depends upon their surface, appear to be in the same 

 proportion. If we blacken the surface of a canister of hot water, it 

 radiates heat more copiously ; and in the same measure, it is more 

 readily heated by radiation. 



2. In the next place, as the radiative pow r er increases, the power of 

 reflection diminishes, and the contrarv. A brio-lit metal vessel reflects 



*' O 



much heat ; on this very account it does not emit much ; and hence 

 a hot fluid which such a vessel contains, remains hot longer than it 

 does in an unpolished case. 



3. The heat is emitted from every point of the surface of a hot 

 body in all directions; but by no means in all directions with equal 

 intensity. The intensity of the heating ray is as the sine of the angle 

 which it makes with the surface. 



The last law is entirely, the two former in a great measure, due to 

 the researches of Leslie, whose Experimental Inquiry into the Nature 

 and Propagation of Heat, published in 1804, contains a great number 

 !>f curious and striking results and speculations. The laws now just 



Mhn. deMalh. ct de Phys. 1829. 



