THE NEBULAR HYPOTHESIS: ITS PRESENT CONDITION. 21 
into the origin of the solar system which it holds out, and ready 
to seize at once on the important discovery of true gaseous 
nebulae by Mr. Huggins as direct confirmation of the hypothe- 
sis. There can be no question that the proved existence of 
immense masses of gaseous matter does place the theory upon 
a much firmer footing than before ; at least, the speculations of 
Sir William Herschel are now established, notwithstanding that 
the superstructure raised thereon by Kant and Laplace is little, 
if, indeed, as we shall see further, at all supported by it. 
For this important advance in our knowledge we are in- 
debted to the spectroscope. When all other means had failed 
to give us faithful indications of the existence of remote gaseous 
bodies, this invaluable instrument at once decided without 
question that, of the nebulae unresolved by the largest and most 
powerful telescopes, a considerable proportion are simply me- 
chanical mixtures of three gases, two of which are well known to 
us, namely, nitrogen and hydrogen ; and the third, although 
unknown or unidentified, is probably of somewhat similar 
character — that is, an elementary, non-liquefiable gas. It is 
not improbable that it may even be a terrestrial element, for 
our knowledge of the variety of spectra obtainable from the 
same substance, under different circumstances, is still too defi- 
cient to enable us to speak positively on this point. But the 
spectroscope is capable of telling us still more regarding the 
nature of the light analysed by it, and it is here that the evi- 
dence it gives is unfavourable to the hypothesis of Laplace. 
The density of the gas from which the light emanates produces 
an effect upon the spectrum, and is measured by the breadth of 
the lines composing it. Now, the nebular hypothesis requires 
as a necessary corollary — and it has accordingly always been ad- 
mitted as such — that nebulae of every degree of condensation 
should be found in the heavens, and the variations of brilliancy 
of these bodies has therefore been pointed out as evidence of 
variation of density. The width of the spectral lines, however, 
provides us with a much more certain, delicate, and reliable 
test. From the observations of Dr. Huggins, it would appear 
that the bright lines in the nebular spectra present no appre- 
ciable difference of thickness in all those cases in which it has 
been possible to use a very narrow slit. The lines have inva- 
riably been found to be exceedingly fine ; and hence we are fur- 
nished with distinct proof that the gases so examined are not 
only of equal or nearly equal density, but that they exist in a 
state of very low tension ; facts, as I have already stated, which 
are fatal to the hypothesis. It is, of course, possible that simi- 
larly as the numerous lines of the spectra of nitrogen and hy- 
drogen are reduced to two or three in the nebular spectrum, in 
consequence of the faintness of the light operated upon, so may 
