around it, that the whole solar system is formed of the same kinds of matter, and is subject to the same general physical laws. These conclusions further support the supposition that the earth and other planets have been formed by the aggregation of matter once diffused in space around the sun; that the first consequence of this aggregation was to develop intense heat in the consolidating masses; that the heat thus generated in the terrestrial sphere was subsequently lost by radiation; and that the surface at length cooled and became a solid crust, inclosing a nucleus of much higher temperature. The heat of the interior of the globe increases about 1° Fahr. for every fifty or sixty feet of depth below the surface. The surface appears to have now reached a temperature which is virtually fixed, the gain of heat from the sun being just compensated by the loss from radiation into surrounding space. As the exterior gradually cooled, contractions necessarily ensued, producing change of form and dimensions; and to these, acting in combination with gravity, are, no doubt, largely due the great irregularities of the earth's surface. The strains set up by these forces must have continued to cause movements for a vastly prolonged period, and are doubtless still in action. But the irregularities of the surface constitute only a small part of the effects of internal heat on the earth, and mineralogy is the branch of science to which reference must be made for a knowledge of the many simple and compound substances that have issued, under the operation of chemical forces, from the vast laboratory contained within the cooling crust of the once incandescent globe.
During the passage of the globe to its present state many wonderful changes must have taken place. The ocean, after its condensation from a gaseous state into that of liquid, must have long continued in a state of ebullition, or bordering on it, surrounded by an atmosphere densely charged with watery vapor. Apart, however, from the movements in the solid crust of the earth caused by its gradual cooling and contraction, its early higher temperature hardly enters directly into any of the considerations that arise in connection with its present climate; and it must remain doubtful how long and to what extent those conditions of climate which interest us most, as having occurred during the period in which the existence of life is indicated, have been affected by such early higher temperature.
In the absence of any direct means of ascertaining the condition of the earth's interior, aid has been sought from mathematical science, by which it has been established that the thickness of the solid outer shell of the earth must be considerable; and that if the interior is in a fluid state at all, which is very doubful, it must be covered by a great thickness (probably not less than several hundred miles) of solid, comparatively unyield-
