_MALLET’S THEORY OF VOLCANOES._ There are few subjects less satisfactorily treated in scientific treatises than that which Humboldt calls the Reaction of the Earth’s Interior. We find, not merely in the configuration of the earth’s crust, but in actual and very remarkable phenomena, evidence of subterranean forces of great activity; and the problems suggested seem in no sense impracticable: yet no theory of the earth’s volcanic energy has yet gained general acceptance. While the astronomer tells us of the constitution of orbs millions of times further away than our own sun, the geologist has hitherto been unable to give an account of the forces which agitate the crust of the orb on which we live. The theory put forward respecting volcanic energy, however, by the eminent seismologist Mallet, promises not merely to take the place of all others, but to gain a degree of acceptance which has not been accorded to any theory previously enunciated. It is, in principle, exceedingly simple, though many of the details (into which I do not propose to enter) involve questions of considerable difficulty. Let us, in the first place, consider briefly the various explanations which had been already advanced. There was first the chemical theory of volcanic energy, the favourite theory of Sir Humphry Davy. It is possible to produce on a small scale nearly all the phenomena due to subterranean activity, by simply bringing together certain substances, and leaving them to undergo the chemical changes due to their association. As a familiar instance of explosive action thus occasioned, we need only mention the results experienced when any one unfamiliar with the methods of treating lime endeavours over hastily to “slake” or “slack” it with water. Indeed, one of the strong points of the chemical theory consisted in the circumstance that volcanoes only occur where water can reach the subterranean regions—or, as Mallet expresses it, that “without water there is no volcano.” But the theory is disposed of by the fact, now generally admitted, that the chemical energies of our earth’s materials were almost wholly exhausted before the surface was consolidated.