Under these circumstances, geologists have been led to enquire whether there are any means by which we can conceive of such an amount of heat, as would be competent to produce volcanic outbursts, being locally developed at certain points within the earth's crust. Recent discoveries in physical science which have shown the close relation to one another of different kinds of force, and their mutual convertibility, have at least suggested the possibility of the existence of causes by which such high temperatures within certain portions of the earth's crust may be originated. [Sidenote: DAVY'S CHEMICAL THEORY.] When, at the commencement of the present century, Sir Humphry Davy discovered the remarkable metals of the alkalies and alkaline earths, and at the same time demonstrated the striking phenomena which are exhibited if these metals be permitted to unite with oxygen, he at once perceived that if such metals existed in an uncombined condition within the earth's crust, the access of water and air to the mass might give rise to the development of such an amount of heat, as would be competent to produce volcanic phenomena at the surface. It is true that at a later date Davy recognised the chemical theory of volcanoes as being beset with considerable difficulties, and was disposed to abandon it altogether. It was argued, with considerable show of reason, that if the heat at volcanic centres were produced by the access of water to metallic substances, great quantities of hydrogen would necessarily be evolved, and this gas ought to be found in prodigious quantities among the emanations of volcanoes. The fact that such enormous quantities of hydrogen gas are not emitted from volcanic vents has been held by many authors to be fatal to the chemical theory of volcanoes. But the later researches of Graham and others have made known facts which go far towards supplying an answer to the objections raised against the chemical theory of volcanoes. Various solids and liquids have been shown to possess the power of absorbing many times their volume of certain gases. Among the gases thus absorbed in large quantities by solids and liquids, hydrogen is very conspicuous. In some cases gases are absorbed by metals or other solids in a state of fusion, and yielded up again by them as they cool. It is a very remarkable circumstance that some meteorites are found to have absorbed large quantities of hydrogen gas, and this is given off when they are heated in vacuo. Thus it has been demonstrated that certain meteorites have contained as much as forty seven times their own volume of hydrogen gas.