CELESTIAL SPECTROSCOPY, 87 
system on thé other, which is required by Laplace’s theory. One chief 
assumption consists in supposing that such inelastic bodies as meteoric 
stones might attain the effective elasticity of a high order which is 
necessary to the theory through the sudden volatilization of a part of 
their mass at an encounter, by which what is virtually a violent explo- 
sive is introduced between the two colliding stones. Prof. Darwin is 
careful to point oat that it must necessarily be obscure as to how a 
sinall mass of solid matter can take up a very largé amount of energy 
i a small fraction of a second. 
Any direct indications from the heavens themselves, however slight, 
are of so great value that I should, perhaps, in this connection call at- 
tention to a recent remarkable photograph, by Mr. Roberts, of the 
great nebula in Andromeda. On this plate we seem to have presented 
to us some stage of cosinical evolution on a gigantic scale. The photo- 
graph shows a sort of whirlpool disturbance of the luminous matter 
which is distributed in a plane inclined to the line of sight, in which a 
series of rings of bright matter separated by dark spaces, greatly fore- 
shortened by perspective, surround a large, undefined central mass. 
We are ignorant of the parallax of this nebula, but there can be little 
doubt that we are looking upona system very remote, and therefore of 
a magnitude great beyond our power of adequate comprehension. The 
matter of this nebula, in whatever state it may be, appears to be dis- 
tributed, as in so many other nebulie, in rings or spiral streams, and to 
suggest a stage in a succession of evolutional events not inconsistent 
with that which the nebular hypothesis requires. To liken this object 
more directly to any particular stage in the formation of the solar sys- 
tem would be “to compare things great with small,” and might be in- 
deed to introduce a false analogy; but, on the other hand, we should 
err through an excess of caution if we did not accept the remarkable 
features brought to light by this photograph as a presumptive indica- 
tion of a progress of events in cosmical history following broadly upon 
the lines of Laplace’s theory. 
The old view of the original matter of the nebulw, that it consisted 
of a “fiery mist,” 
a tumultuous cloud 
Instinct with fire and niter. 
fell at once with the rise of the science of thermodynamics. In 1854 
Helmholtz showed that the supposition of an original fiery condition 
of the nebulous stuff was unnecessary, since in the mutual gravitation 
of widely separated matter we have a store of potential energy sufii- 
cient to generate the high temperature of the sun and stars. We can 
scarcely go wrong in attributing the light of the nebulz to the conver- 
sion of the gravitational energy of shrinkage into molecular motion. 
The idea that the light of comets and of nebula may be due to a suc- 
cession of ignited flashes of gas from the encounters of meteoric stones 
