18 REPORT—1905. 
is the celebrated Nebular Hypothesis, first suggested by the German 
philosopher Kant, and later restated independently and in better form 
by the French mathematician Laplace. 
Laplace traced the origin of the solar system to a nebula or cloud of 
rarefied gas congregated round a central condensation which was ulti- 
mately to form the sun. The whole was slowly rotating about an axis 
through its centre, and, under the combined influences of rotation and of 
the mutual attraction of the gas, it assumed a globular form, slightly 
flattened at the poles. The justifiability of this supposition is confirmed 
by the observations of astronomers, for they find in the heavens many 
nebule, while the spectroscope proves that their light at any rate is 
derived from gas. The primeval globular nebula is undoubtedly a stable 
or persistent figure, and thus Laplace’s hypothesis conforms to the general 
laws which I have attempted to lay down. 
The nebula must have gradually cooled by radiation into space, and as 
it did so the gas must necessarily have lost some of its spring or elasticity. 
This loss of power of resistance then permitted the gas to crowd more 
closely towards the central condensation, so that the nebula contracted. 
The contraction led to two results, both inevitable according to the laws 
of mechanics: first, the central condensation became hotter; and, 
secondly, the speed of its rotation became faster. The accelerated rota- 
tion led to an increase in the amount of polar flattening, and the nebula 
at length assumed the form of a lens, or of a disk thicker in the middle 
than at the edges. Assuming the existence of the primitive nebula, the 
hypothesis may be accepted thus far as practically certain. 
From this point, however, doubt and difficulty enter into the argu- 
ment. It is supposed that the nebula became so much flattened that it 
could not subsist as a continuous aggregation of gas, and a ring of matter 
detached itself from the equatorial regions. The central portions of the 
nebula, when relieved of the excrescence, resumed the more rounded 
shape formerly possessed by the whole. As the cooling continued the 
central portion in its turn became excessively flattened through the influ- 
ence of its increased rotation ; another equatorial ring then detached 
itself, and the whole process was repeated as before. In this way the 
whole nebula was fissured into a number of rings surrounding the central 
condensation, whose temperature must by then have reached incandescence. 
Each ring then aggregated itself round some nucleus which hap- 
pened to exist in its circumference, and so formed a subordinate nebula, 
Passing through a series of transformations, like its parent, this nebula 
was finally replaced by a planet with attendant satellites. 
The whole process forms a majestic picture of the history of our 
system. But the mechanical conditions of a rotating nebula are too com- 
plex to admit, as yet, of complete mathematical treatment ; and thus, in 
discussing this theory, the physicist is compelled in great measure to adopt 
the qualitative methods of the biologist, rather than the quantitative 
ones which he would prefer. 
The telescope seems to confirm the general correctness of Laplace’s 
