I 



TRANSACTIONS OF THE SECTIONS. 37 



municated. Hence aether, being susceptible of vis viva, has recently been admitted 

 to be 2}onderable ; but tliis admission is not a necessaiy consequence ; for although 

 the idea of existing energy is associated witli that of iveight, in consequence of the 

 constant energy acquired by gravitation having been taken as the measure or unit 

 of energy however acquired, there is no necessary connexion between them. Sup- 

 pose, for example, that a flea were placed on an orbitating planet of the size of a 

 pumpkin ; while its muscular energy would remain undiminished, its weight would 

 be infinitesimal, and the first leap woidd obviously plimge it into infinite space, to 

 perform subsequently perhaps <an independent orbit. 



The only basis on which the interstitiul-ather hypothesis rests is the assumed 

 incapacity of ordinary matter, whether in the solid, liquid, or gaseous state, to 

 transmit the vibrations of light and heat, because the only vibrations hithei-to 

 recognized, namely those of sound, are almost immeasurably slower than those 

 of light and heat ; the one being numbered by at most a few thousands, the other 

 \)y hundreds of millions of millions in one second of time. ]3ut it must be borne in 

 mind that sonorous vibrations are always longitudinal, in the production of which 

 repulsive forces are alone concerned ; whilst, on the contrary, light- and heat-vibra- 

 tions are necessarily transverse, and tlie production of these is solely due to attrac- 

 tive forces. Now these respective forces obey very different laws; for whilst 

 attractive forces obey generally, and probably universally, the law of the inverse 

 square of the distance, molecular repulsion must obviously, at all events in gaseous 

 matter, obey the law of the inverse cube of the distance ; therefore from the rate 

 of transmission of longitudinal vibrations nothing can be predicated respecting the 

 rate of transmission of transverse waves. It has been asserted that molecular re- 

 pidsion is a dynamical resultant eftect, and therefore incapable of expression by a 

 statical law ; but it is very doubtful whether molecular attraction is not equally 

 a dynamical sequence, and therefore not a whit more entitled to claim a statical 

 law than the former. 



It has been shown from the investigations of jNIr. Norman Lockyer that incan- 

 descent_ gases existing in the -Nicinity of the sun are capable of initiutin;/ vibrations 

 of definite periods, which are moreover occasionally accelerated or retarded by tlio 

 proper motion of the emitting gas, just as sound-waves have been sliown by Savart 

 to be accelerated or retarded, and the sound consequently raised or lowered in 

 pitch, by the proper motion of the body producing the vibrations. "Wliat reason 

 can there then be for doubting that gaseous matter is capable of transmitting heat- 

 waves, and, if so, of likewise transmitting the waves of light, since the two are so 

 intimately connected by the identical phenomena of reflexion, refraction, and 



Eolarizatiou ? may not, in fact, in some instances the perceptions of light and heat 

 e but different sensuous impressions produced by the same vibrations ? 

 Now in the denser forms of matter, namely the solid and liquid, it appears that 

 the wave-lengths of excited transverse vibrations are indefinitely modified, pro- 

 bably by the more energetic action of repulsive forces ; for whilst any given kind 

 of matter in the solid or fluid state is found, when incandescent, to emit light- and 

 heat-waves of all lengths, and so to fonn a continuous spectrum, the same matter 

 in the form of incandescent gas will emit only a few sets of waves of definite and 

 invariable lengths, forming an interrupted spectrum of bright lines ; and moreover 

 some of these wave-lengths are frequently found to bear very simple numerical ratios 

 to each other. And even in gaseous matter it has been observed that the bright 

 lines in the spectrum become narrower and more sharply defined by rarefaction, 

 and, on the contrary, broader and less defined by condensation. Moreover, as 

 regards the density of the absorbing medium, the absorption-bands in the spectrum 

 appear to obey the same law as the bright lines. In other words, every kind of 

 matter appears to be capable of emitting or absorbing its own peculiar waves, 

 according to its tenuity, that is, as the results of molecular attraction are less and 

 less interfered with by those of repulsion. The well-known pecidiar incapacity of any 

 given transcalent substance to transmit the heat-rays emitted by a heated portion 

 of the same substance, or, in other words, the ability of the molecules to freely ap- 

 propriate the wave-motion that has been induced in some intervening medium by 

 similar molecules, seems further to argue that ordinaiy matter is capable of assu- 

 ming vibrations having the extreme rapidity of those of light and heat ; and that 



