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XXXII. On the Dynamical Theory of Radiation. 

 By Arthur Schuster, Ph.D.] F.B.S.* 



IF we attempt to draw up a summary of our knowledge on 

 the radiation of heat and light, we shall find that it has 

 been derived from two distinct lines of investigation. The 

 first of these, originated by Prevost of Geneva, has been fol- 

 lowed up by Balfour Stewart and Kirchhoff. It is founded 

 on the experimental fact that bodies placed in an enclosure of 

 uniform temperature will ultimately assume that tempera- 

 ture, and on a law called, after its inventor, Prevost' s law. 

 The characteristic distinction of this line of investigation, as 

 opposed to the second one, presently to be noticed, consists in 

 the fact that it makes no assumption whatever on the nature 

 of radiation. Whether the constitution of the body be mole- 

 cular or not, it claims to found some important laws, laws 

 which have a wide practical application, on experimental facts 

 simply, and one or two assumptions against which no serious 

 objections have as yet been urged. The second line of inves- 

 tigation to which I have referred starts from the assumption 

 of the existence of atoms and molecules. It tries to form 

 ideas, more or less hypothetical as they are more or less 

 distinct, on the nature of these molecules, and endeavours to 

 fit the observed phenomena of nature into the results of an 

 almost pure deductive reasoning. The first method may be 

 considered more satisfactory and philosophical ; but the second 

 is more productive, and any further advances which the science 

 of radiation may be expected to make must be derived from 

 the dynamical considerations on which it is founded. At 

 present it may be useful to compare the results already 

 obtained with observed facts, and to point out where a further 

 examination of the assumptions which had to be made seems 

 necessary. 



In the first place, it is to be remarked that nearly all the 

 investigations confine themselves to the consideration of an 

 enclosure of uniform temperature — that is, to the state of the 

 body in which it neither gains nor loses heat. It is hardly 

 necessary to remark that no observations can be carried on in 

 such an enclosure. If, at the ordinary atmospheric tempera- 

 ture, we shut all windows and doors so as to keep out all radia- 

 tions which might directly or indirectly reach us from our 

 luminary, observations on radiation and absorption will be 

 prevented by our inability to see. But even if we should 

 raise the temperature of the room and every thing it contains 



* Communicated by the Author, having been read at the York Meet- 

 ing of the British Association. 



