It may well happen, however, that among the hypotheses which have up to the present time been framed, none will be found to entirely satisfy all the conditions of the problem. New discoveries in physics and chemistry have suggested fresh explanations of volcanic phenomena in the past, and may continue to do so in the future; and the true theory of volcanic action, when it is at last discovered, may combine many of the principles which now seem to be peculiar to different hypotheses. Let us, in the first place, enquire what are the facts which must be accounted for in any theory of volcanic action. We have already been led to the conclusion that the phenomena exhibited by volcanoes were entirely produced by the escape of imprisoned water and other gases from masses of incandescent and fluid rock. Our subsequent examination of the problem confirmed the conclusion that in all cases of volcanic outburst we have molten rock-materials from which water and other gases issue with greater or less violence. The two great facts to be accounted for, then, in any attempted explanation of volcanic phenomena, are the existence of this high temperature at certain points within the earth's crust, and the presence of great quantities of water and gas, imprisoned in the rocks. We shall perhaps simplify the enquiry if we examine these two questions separately, and, in the first place, review those hypotheses which have been suggested to account for high temperatures in the subterranean regions, and, in the second place, examine those which seek to explain the presence of large quantities of imprisoned water and gases. [Sidenote: INCREASE OF TEMPERATURE WITH DEPTH.] That a high temperature exists in the earth's crust at some depth from the surface is a £act which does not admit of any doubt. Every shaft sunk for mining operations, and every deep boring made for the purpose of obtaining water, proves that a more or less regular increase of temperature takes place as we penetrate downwards. The average rate of this increase of temperature has been estimated to be about 1° Fahrenheit for every 50 or 60 feet of depth. Now if it be assumed that this regular increase of temperature continues to great depths, a simple calculation proves that at a depth of 9,000 feet a temperature of 212° Fahrenheit will be found--one sufficient to boil water at the earth's surface--while at a depth of 28 miles the temperature will be high enough to melt cast-iron, and at 34 miles to fuse platinum.