and of the Analysis of the Solar Atmosphere. 259 



this conclusion cannot be considered to be thus proved, but 

 must be taken as an hypothesis needing a stricter demonstration, 

 as well as a greater precision in the terms employed. Stewart 

 himself does not consider that his proposition is thus rigidly 

 demonstrated; for immediately after the enunciation of it he 

 says, "a more rigid demonstration may be given thus:" and 

 then he proceeds to more abstruse considerations which are 

 intended to give such a rigid demonstration, and in which the 

 meaning he attaches to the expressions absorption and radiation 

 are more nearly defined. These considerations, however, are 

 not sufficiently general or sufficiently precise to attain the re- 

 quired end ; so that, after all, Stewart's proposition remains an 

 hypothesis to which some probability is attached. 



Stewart finds from his experiments that partially diather- 

 manous plates radiate the more heat the thicker they are, and 

 hence he concludes, with perfect right, that the radiation proceeds 

 from the interior of bodies as well as from the surface. The 

 question then occurs, " Are we to suppose each particle of each 

 substance to have at a given temperature an independent radia- 

 tion of its own, equal, of course, in all directions ? " " A priori," 

 he continues, "this is the most probable supposition; and it 

 seems likewise to be conformable to experiment." The principle 

 expressed in these words is the test of the truth of the proposi- 

 tion, according to the proof which Stewart endeavours to give. 

 He says, " The question arises, Is the law of an equal and inde- 

 pendent radiation of each particle of a body theoretically con- 

 sistent with equilibrium of temperature ? That is, suppose we 

 have any irregularly-shaped enclosure walled round with a variety 

 of substances, and each particle of each substance radiating into 

 the enclosure, — from the sides of which it is reflected many 

 times backwards and forwards before it is finally absorbed, — this 

 being the case, will the law of equal and independent radiation, 

 and those of reflexion and refraction so fit with one another, 

 that every particle of the walls of the enclosure shall absorb 

 precisely as much heat as it radiates ? It will be endeavoured 

 to show that these laws are so adapted to each other." 



By employing the law " of equal and independent radiation" 

 and the laws of reflexion and refraction, Stewart forms the equa- 

 tion expressing the proposition which has to be proved con- 

 cerning the equality of absorption and radiation for heat of 

 every kind. It appears that this equation contains no contra- 

 diction, but expresses a possible property of the internal radia- 

 tion in a body. He argues from this that the proposition con- 

 cerning the equality of absorption and radiation for every kind of 

 ray must hold good. This is evidently a false conclusion. The 



S2 



