22 
POPULAR SCIENCE REVIEW. 
the width of the lines he diminished from a like cause ; but with 
this I have nothing to do. It is my object merely to show that 
at present, so far as the spectroscope has afforded us increased 
knowledge of the state of these bodies, it is fatal to the theory, 
and it remains for those who uphold it in its integrity to estab- 
lish by experiment that the spectrum of a dense gas, when very 
faint, not only is reduced to a single line, but that that line 
itself is narrow when the slit is narrow, as in the case of rarefied 
gases. Unless this proposition can be established, there remains 
no alternative but to reject the hypothesis, as an inviting but 
fallacious guide to the explanation of the origin of the solar 
system, and to look for some new theory, or for some modifica- 
tion of the old one, for the solution of those problems which it 
would otherwise afford. 
There are still further reasons for believing that the nebular 
hypothesis in its old form is not altogether trustworthy, and, 
though of less weight, may help to turn the scale, at the same 
time that they prepare the ground for an altered conception of 
it, free from these objections. It has been necessary hitherto 
to assume that the nebulous matter existed originally at a great 
heat, without suggesting, it seems to me, any sufficient force 
by which this high temperature was reached. This is, at least, 
wanting in completeness, especially as there is at hand, as I 
hope to show, the means by which the matter may have been 
thus raised in temperature. But a still more fatal objection 
would appear to be, that the gases which have been identified 
in the nebulae do not seem to be, in themselves, adequate to 
form a system such as our own, unless by the addition of foreign 
matter from without. Probably no advocate of Mr. Lockyer’s 
theory of the disassociation of the material molecules into their 
primary or truly elementary components by enormous heat, 
will go so far as to imagine that the two known gaseous consti- 
tuents of the nebulae, together with one other unknown sub- 
stance, is all that is essential to form a globe such as our own 
sun, especially when it has been proved that the actual mate- 
rials required are known to exist in the immediate neighbour- 
hood, and appear to have no other use in the economy of the 
universe than that of forming with the nebulas suns and systems 
such as our own. 
It is, perhaps, scarcely necessary to point out that it is to the 
cometary system that I allude, as capable of supplying the 
necessary material from without, as well as of causing the enor- 
mous evolution of heat of which I have spoken ; but there are 
one or two misconceptions which must be cleared away before 
the mind is prepared to admit the possibility of such a circum- 
stance. These misconceptions have reference, firstly, to the 
distance of the nebulae from our own system, and hence to 
