Nature and Art, April 1, 1867. 
THE FOUNDATIONS OF THE EARTH. 
99 
its kindred planets ever having been a mass of 
vapour, we may state that if all the matter com- 
posing the whole solar system were so spread out 
and diffused as to he equally distributed over an area 
as large as the orbit of the planet Neptune, the 
expanded matter would be so light and thin as to 
exceed in rarity atmospheric air ; its condition in 
such a state has been compared to the infinitesimal 
density of what is called a vacuum in the receiver 
of an air-pump. A ton weight of matter that 
would fill the space of a cubic mile must be as thin 
as a gas ; but if that ton of matter could be con- 
densed into the space of a cubic foot, it would be a 
material denser than iron. 
When we consider the fact that the nebular 
hypothesis of Laplace satisfies nearly, if not 
actually, all the conditions that we observe in the 
solar system ; when, too, we regard the character 
of its exponent, his stupendous mathematical 
achievements, and the improbability of his pro- 
posing a mere fantastic scheme without satisfying 
himself of its validity on all points, we shall have 
no difficulty in appreciating the assertion of the 
illustrious Arago, that the ideas of Laplace upon 
the constitution of the solar system “ are those 
only that by their grandeur, their coherence, and 
their mathematical character can be truly con- 
sidered as forming a physical cosmogony.”* 
But sceptics had a good reason for disbelieving 
this theory. It depended solely upon the existence 
of this nebulous matter, of which the only evidence 
was the visibility of patches of nebulous light 
scattered about the heavens ; and many of those 
were undoubtedly proved, upon scrutiny by power- 
ful telescopes, to be nothing but remote clusters of 
thickly aggregated stars, so remote and so closely 
clustered that they were visible to the dimmer 
vision of less powerful instruments only by their 
united lustre. Why should not all the so-called 
nebulae be clusters, which future and yet more 
powerful instruments would resolve into their 
component stars 1 The question was one fraught 
with interest to speculative astronomers. One of 
the brightest nebulae in the skies, that in the 
sword-liandle of Orion, although its brightness 
indicated a proximity that would have allowed its 
component stars to be seen with comparative 
facility, nevertheless baffled all attempts to resolve 
it, until at length the stupendous telescope of the 
Earl of Rosse was completed and turned upon it. 
Then, as was thought, it succumbed, for the Earl of 
Rosse declared, upon the authority of his obser- 
vations, the greater part of the nebulae to abound 
with stars, and to exhibit the characteristics of 
resolvability strongly marked. With the supposed 
resolution of this nebula the last stronghold of the 
nebular hypothesis was thought to have been over- 
thrown. 
Thus the matter stood twenty years ago, and 
thus it might have remained till now, had not one 
# It is a noteworthy fact that the French Bureau des 
Longitudes has significantly expressed its opinion concerning 
Laplace’s views by republishing his original notes, in the 
last volume of its Annuaire ~~ that for the present year. 
of astronomy’s sister sciences stepped in within the 
past few years, we might even say months, to lend 
her aid to the solution of the mystery. This 
science is optics, and that branch of it to which we 
are about to allude is the newly-found means of 
discovering the chemical constitution of celestial 
bodies by analysis of the light they emit. Every 
one knows that when a beam of light passes 
through a prism of glass and falls upon a wall, it is 
formed into a beautiful luminous band tinged with 
all the colours of the rainbow ; this luminous band 
being known as the prismatic spectrum. But it 
may not be known that this spectrum is not the 
same for every sort of light ; that a different one 
is produced, according as the light emanates from 
one luminous source or another. For instance, a 
solid body in combustion will give one species of 
spectrum, while a flame of a particular gas will 
give one of a totally different class, and the light 
emitted from a metallic substance in a state of 
fusion another. So that if an astronomer applies 
a prism to the eyepiece of his telescope while 
observing any celestial body, he can tell something 
of what the chemical constitution of the body may 
be. The sun has been found to be a solid body, in 
a state of incandescence, surrounded by the in- 
tensely heated vapours of a variety of chemical 
substances. The fixed stars have been found to 
resemble the sun generally, but with slight vari- 
ations attributable to a difference of some of the 
elements composing them. Now, if the nebulae 
were clusters of stars, it would be found that their 
light would yield spectra analogous to those of 
the stars ; but, on the contrary, many of them 
yield spectra which leave no doubt whatever 
that they are composed of immense masses of some 
gaseous or vapoury matter. The observations of 
this class are exceedingly difficult and delicate, and 
the subject is as yet comparatively in its infancy ; 
but all that has been done, as yet, goes to support 
the nebular hypothesis, by at least proving that all 
the nebulse are not remote clusters of stars. 
But, thanks to the scientific achievements of the 
past quarter of a century, we have yet another link 
between the facts that we observe, and the theory 
by which we would explain them. That the 
foundations of our earth were laid under the 
action of a fervent heat, is a fact of which the 
igneous rocks that form those foundations yield 
abundant testimony : and whence was such a heat 
derived 1 This question carries us on to another, 
for we are led to inquire, what is heat 1 A few 
years ago we should have been told that it was a 
subtle fluid pervading the inter-atomic spaces of 
matter : now we learn that it is only one of the 
many phenomena of motion. The “mechanical 
theory of heat,” — “ the great philosophical doctrine 
of the present era of science,” as it has been justly 
termed, teaches us that heat is nothing more than a 
species of motion amongst the atoms or molecules 
of bodies. Arrest the motion of a cannon ball by 
placing a target in its path ; and what is the 
consequence ? The ball is raised to a fiery heat 
by the concussion. We rub our hands briskly 
h 2 
