2M 



the other ; is it less reasonable to attribute the former pheno- 

 mena to the special susceptibility of the molocules, than to 

 ascribe the latter to the special tension of the reciprocating 

 strings ? It is quite true that incandescent bodies in the solid 

 or fluid state emit rays constituting a continuous, not an 

 interrupted, spectrum. This is no doubt due to the inter- 

 ference of aggregation with the motion to which the molecules 

 are most prone ; for it has been observed that the bright lines 

 in the spectrum become more sharply defined by attenuation 

 of the emitting vapours or gas, and that they become broader 

 and less defined by its condensation. 



43. It has appeared, from the investigations of Messrs. 

 Huggins and Lockyer, that the periodic time of vibrations 

 emitted by incandescent hydrogen in the vicinity of the sun is 

 sometimes slightly modified by the proper motion of the emitting 

 gas ; in this case some portion of the bright line will be slightly 

 deflected towards the violet or red end of the spectrum, 

 accordingly as the wave-length is diminished or increased by 

 the proper motion of the gas; occasionally deflections in both ' 

 directions simultaneously have been observed, showing the 

 existence of a solar cyclone. A precisely analogous acoustical 

 phenomenon may be demonstrated by placing a free reed at one 

 end of a long hollow rod, and a small pair of bellows at the 

 other end : if the rod be briskly waved to and fro while the 

 sound of the reed continues, its pitch appears to be sharpened 

 to those whom it approaches, and flattened to those from whom 

 it is receding. It follows from these facts, as an irresistible 

 conclusion, that the molecules of ordinary matter are suscep- 

 tible of the vibrations both of light and heat, and are there- 

 fore equally capable of transmitting them ; and if so, the 

 hypothesis of the necessity of interstitial ether becomes abso- 

 lutely groundless. It may be asked how, if ether be admitted 

 to occupy infinite space, it can be imagined to be excluded 

 from the spaces occupied by ordinary matter; to this the 

 writer would reply, by means of a very simple hypothesis, 

 which he ventured to put forward in the introduction to the 

 last edition of his '^Elements of Natural Philosophy" — 

 namely, that ether (like its liquid namesake with water) is 

 immiscible with ordinary gaseous matter, and therefore floats 

 above the attenuated confines of the atmosphere; it would 

 thus be not less capable of fulfilling its beneficent mission of 

 supplying organic life with the indispensable energies of light 

 and heat; for, as no limit can be assigned to the possible 

 amount of molecular displacement in a medium so attenuated 

 as ether must necessarily be, an amount of energy is con- 

 ceivable in its molecules which would be sufficient to impart 



