RADIANT HEAT. 383 



Density of JEther. 



Another speculation,* closely connected with, the former and the 

 general subject of radiation, has been pursued by the same author, 

 on the probable density which can be assigned to the luminiferous and 

 caloriferous aether. 



This speculation is founded, like the former, on the data furnished 

 by Pouillet' s Researches on the Solar Radiation, and Joule' s theory 

 of the mechanical energy equivalent to the effect of heat produced. 

 The calculation turns on the assumption that the velocity of vibration 

 can only be a small fraction (probably not one-fiftieth) of the velocity 

 of the propagation of waves, and from the velocity of vibration we may 

 calcuate the density, or conversely. 



Hence the author conceives that we may assign a limit, and that a 

 cubic foot of luminiferous (ether, at the distance of the earth from the sun 



cannot contain less than f 15(jo v ini? I of one pound of matter. 



With regard to the results of Professor Thomson, especially when the 

 novel character of some- of the reasonings is taken into account, some 

 difference of opinion may reasonably be expected. There are certainly 

 many considerations involved which might suggest important topics of 

 discussion. On these it is not my purpose to enter. I will merely re- 

 mark, that in all these investigations the essential point is the expendi- 

 ture of mechanical energy in producing vibrations, of whatever kind. 

 The whole question then assumes a more strictly mechanical aspect. 

 The sure indication that this entire branch of science is in a state of 

 approximation at least towards that stage which characterizes the 

 perfection of any branch of physical knowledge, when all its varied 

 phenomena shall be shown to be susceptible of analysis up to simple 

 combinations of the elementary laws of force and motion. 



I would merely add that, in speaking of the effect of the evolution 

 of heat, there is nothing in Professor Thomson's conclusion which 

 restricts them to any one species of heat. The essential point is the 

 production of vibrations: and his results are thus in entire accordance 

 with the theory which refers all kinds of heating effect to the stop- 

 page or absorption of rays; in other words, the extinction or destruc- 

 tion of the vibratory motions, constituting rays of different wave- 

 lengths, some of which are also within those limits, and belong to that 

 part of the scale,- which renders them capable of affecting our eyes 

 with the sensation of vision; which (as already remarked) considera- 

 tions on all hands seem now tending to show is the most probable 

 hypothesis on the subject. 



Eadiation of Heat from the Zodiacal Light and from tlue Comet of 1843. 



Some interesting observations on these points are given by M. 

 Matthiessen in the " Comptes Rendus," April, 1843, (vol. xvi, p. 687.) 



* Professor W. Thomson " On the Possible Density of the Luminiferous Medium," &e.' 

 Transactions of tha Royal Society of Edinburgh, vol. xxi, Part I. 



