HYPOTHESIS OF LA PLACE. 87 
sign the causes of their activity, and state the modes of their ac- 
tion, if we suppose the interior of the earth to be an intensely heated 
and fluid mass. It is probable that if so constituted, a portion of 
the melted matter will from time to time escape in consequence 
of some change it undergoes. 
46. The facts and arguments of the two preceding sections, 
directed; those of the 44th, to the proof of the more elevated tem- 
perature of the surface of the globe in the most ancient times; 
those of the 45th to the preseni condition of its interior mass, 
furnish a good example of the difference that generally obtains 
in the character of the evidence that is offered in the two great 
branches of geological science. In positive geology the proof is 
direct and simple. The thermometer indicates a constantly in- 
creasing temperature as we descend into the earth. That we 
may be qualified to appreciate the force and certainty of the evi- 
dence on which- the conclusions of theoretical geology are found- 
ed, we must have studied the laws that regulate the organization 
of living plants and animals, and also of those races which are ex- 
tinct and have left their remains behind them, and of the distri- 
bution of both through the different climates of the globe : we 
must also by long continued and often repeated examinations, 
have become acquainted with the appearances they present, 
whether at remote points or where they come into contact with 
each other. To the mind thus furnished and prepared for these 
investigations, the conclusions at which geologists have arrived in 
the two branches of the science, will appear to be of nearly, if not 
quite, equal safety and certainty. In the present instance, the 
two, forming essential parts of the same theory, lend each other 
a mutual support. 
Astronomy has within the present century proposed to ascend to 
a still earlier epoch in the earth's history ; when the solar sys- 
tem of which it is a part, was a hot and luminous vapour, re- 
sembling in appearance when viewed from a distance, the nebu- 
las that are still observed in some parts of the heavens. Under 
this attenuated form, the matter of our system is conjectured to 
have extended beyond the. orbit of the planet Uranus, and to have 
revolved upon an axis from west to east. As it parted with its heat 
by radiation, it would be condensed, and every particle describ- 
ing a smaller circle, that the amount of motion might remain 
the same, the velocity must be continually accelerated. The cen- 
trifugal force is supposed to have been so much increased, that 
at distant intervals, the matter of the planets separated from the 
principal mass, each assuming a globular form, revolving on its 
axis, and circulating at that distance from the centre of gravity of 
the system at which the separation took place, and some in the 
progress of their condensation, affordingother or secondary plan- 
ets in their turn. By means of this hypothesis, some remarka- 
ble phenomena of the solar system are easily explained. 
