234 
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 
