86 CELESTIAL SPECTROSCOPY. 
Let us turn aside for a moment from the nebule in the sky to the 
conclusions to which philosophers had been irresistibly led by a con- 
sideration of the features of the solar system. We have before us in 
the sun and planets obviously not a haphazard aggregation of bodies, 
but a system resting upon a multitude of relations pointing to a com- 
mon physical cause. From these considerations Kant and Laplace 
formulated the nebular hypothesis, resting it on gravitation alone, for 
at that time the science of the conservation of energy was practically 
unknown. These philosophers showed how, on the supposition that 
the space now occupied by the solar system was once filled by a vapor- 
ous mass, the formation of the sun and planets could be reasonably ac- 
counted tor. 
By a totally different method of reasoning, modern science traces 
the solar system backward step by step to a simlar state of things at 
the beginning. According to Helmholtz, the sun’s heat is maintained 
by the contraction of his mass, at the rate of about 220 feet a year. 
Whether at the present time the sun is getting hotter or colder we do 
not certainly know. We can reason back to the time the sun was suffi- 
ciently expanded to fill the whole space occupied by the solar system, 
and was reduced to a great glowing nebula. Though man’s life, the 
life of the race perhaps, is too short to give us direct evidence of any 
distinct stages of so august a process, still the probability is great that 
the nebular hypothesis, especially in the more precise form given to it 
by Roche, does represent broadly, notwithstanding some difficulties, 
the succession of events through which thesun and planets have passed. 
The nebular hypothesis of Laplace requires a rotating mass of fluid 
which at successive epochs became unstable from excess of motion, and 
left behind rings, or more probably perhaps lumps, of matter from the 
equatorial regions. 
The difficulties to which I have referred have suggested to some 
thinkers a different view of things, according to which it is not neces- 
sary to suppose that one part of the system gravitationally supports 
another. The whole may consist of a congeries of discrete bodies even 
if these bodies be the ultimate molecules of matter. The planets may 
have been formed by the gradual accretion of such discrete bodies. On 
the view that the material of the condensing solar system consisted of 
Separate particles or masses, we have no longer the fluid pressure 
which is an essential part of Laplace’s theory. Faye, in his theory of 
evolution from meteorites, has to throw over this fundamental idea of 
the nebular hypothesis, and he formulates instead a different succes- 
sion of events, in which the outer planets were formed last ; a theory 
which has difficulties of its own. 
Prof. George Darwin has recently shown, from an investigation of 
the mechanical conditions of a swarm of meteorites, that on certain 
assumptions a meteoric swarm might behave as a coarse gas, and in 
this way bring back the fluid pressure exercised by one part of the 
