
Dec. 8, 1870] 
NATURE 
109 


QUERIES RESPECTING 4: THER 
p= following speculations first appeared in the pages 
of the Exgineer :— 
When light and caloric were supposed to have a 
material existence, the hypothesis df the universal exist- 
ence of a highly elastic medium was unnecessary, since | 
matter might with the utmost freedom be projected 
through vacuous space; but as light and heat are now | 
generally admitted to consist not of transmitted matter, | 
but of transmitted vibratory motion (and why may not | 
electricity, so freely interchangeable with the former, be 
admitted into the same category ?), the necessity of the 
existence of a transmitting medium, pervading infinite 
space, becomes at once apparent; and this medium, 
hitherto not Cognisable to our senses, has been termed 
ether. 
But it has been further assumed that <ether is alone ca- 
pable of transmitting the extremely rapid vibration of 
light and heat, and that it must therefore necessarily per- 
vade or permeate all kinds of sensible matter. The 
questions proposed to be raised in this communication 
are the necessity of this interstitial hypothesis, and the 
probable capability of ordinary matter to transmit the 
vibrationis of light arid heat. 
It is now generally admitted that when a body becomes | 
heated, its own particles, and not those of the supposed 
interstitial zther. are thrown into a state of vibratory 
‘notion, the amount of heat corresponding probably to the 
amplitude of the vibrations ; hence a certain amount of | 
‘energy has been communicated to those particles, and, 
at all events, in the case of -celestial mediations, the 
molecules of «ther must previously have possessed the 
‘energy or vis viva which they have communicated. 
Hence zxther, being susceptible of vzs viva, has recently 
been admitted to be ponderable, but this admission is not 
a necessary consequence, for although the idea of the 
existing enerzy is associated with that of weight, in conse- 
quence of the constant energy acquired by gravitation 
having been taken as the measure or unit of energy, how- 
ever acquired, there is no necessary connection between 
them. Suppose, for example, a flea were placed on an 
‘orbilating planet of the size of a pumpkin, while its mus- 
‘cular energy would remain undiminished, its weight would 
be infinitesimal, and the first leap would obviously plunge 
it into infinite Space, to perform subsequently, perhaps, 
‘an independent orbit. 
_. Further, it has been shown frém the inyestigations of 
Mr. Norman Lockyer, to whom the progress of solar physics 
is so largely indebted, that incandescent gases are ca- 
pable of initiating vibrations of definite periods, which 
are, moreover, occasionally accelerated or retarded by 
the proper motion of the emitting gas. What reason can 
there then be for doubting that gaseous matter is capable 
of transmitting heat waves, and if so, of likewise trans- 
mitting the waves of light, since the two are so intimately 
connected by the identical phenomena of reflection, re- 
fraction, and polarisation? May not in fact, in some 
instance, the perceptions of light and heat be but different 
sensuous impressions produced by the same vibrations ? 
The only basis on which the interstitial-eether hypo- 
thesis 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, 
namely, sonorous, with which we are acquainted, are 
almost immeasurably slower than those of light and heat, 
the one being numbered by at most a few thousands, the 
other by hundreds of millions of millions in one second 
of time. 
But it must be borne in mind that sonorous vibra- 
tions are always longitudinal, in the production of which 
repulsive forces are alone concerned; whilst, on the 
contrary, light and heat vibrations are necessarily trans- 
Verse, and the production of these is solely due to attractive 

forces. Now these respective forces obey very different 
laws, for whilst attractive forces obey generally, and pro- 
bably universally, the laws of the inverse square of the 
distance, molecular repulsion must obviously, at all 
events in gaseous matter, obey the laws of the inverse 
cube of the distance ; therefore, from the rate of trans- 
mission of longitudinal vibrations, nothing can be pre- 
dicted respecting the rate of transmission of transverse 
waves. It has been asserted that molecular repulsion is 
a dynamical resultant effect, 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. Now, in the denser forms 
of matter, namely, the solid and liquid, it appears that 
the wave-lengths of created transverse vibrations are 
indefinitely modified, probably 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 forma 
continuous spectrum, the same matter in the form of incan- 
descent gas will emit only a few sets of waves of definite 
and invariable lengths; 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 de- 
fined by condensation. Moreover, as regards density, the 
absorption bands in the spectrum appear to obey the same 
lawas 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 tenacity; that is, 
as the results of molecular attraction are less and less 
interfered with by those of repulsion. The well-known 
peculiar incapacity of any given translucent 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 appropriate the wave motion that has 
been induced by some intervening medium by similar 
molecules, seems further to argue that ordinary matter is 
capable of assuming vibrations having the extreme rapi- 
dity of those of light andheat. And that there exists no 
valid ground for a distinction between light and heat in 
this respect is further confirmed by the experiments of 
Prof. B. Stewart, who has shown that the emission of light 
by incandescent bodies closely corresponds with their 
absorptive power (whether selective or otherwise) when 
not incandescent ; and, further, that even in the decompo- 
sition of light into true polarised beams by the tourmaline 
it emits, when incandescent, the ray that is otherwise 
absorbed. ; 
Can there, then, be any valid reason for doubting 
the ability of ordinary matter to transmit those trans- 
verse vibrations, which it is obviously capable of either 
absorbing or emitting: and if so, what ground is there 
for the hypothesis that the transmission of light and heat 
waves necessitates the presence of imperceptible zther 
in the interstices of perceptible matter ? 
If the existence of cether in infinite space, essential to 
the undulatory theory, be admitted, it may be asked, how 
is it possible to conceive its exclusion from any portion 
of space? A very simple hypothesis propounded by the 
writer in the introduction to the last edition of his “ Ele- 
ments of Physics” will meet this difficulty, namely, that 
zether (like its fluid namesake with water) is zzzscible 
with known gaseous matter. This, it must be admitted, 
is sheer hypothesis; but if true, it must ever remain so, 
as being beyond the reach of human ken. But of this 
we may rest assured, that if it be not wanted in and 
around éveh our corporeal frames, it is not there; the 
contrary supposition would be inconsistent with the infi- 
nite wisdom of the Creator of the universe. 
CHARLES BROOKE 
