Page:Popular Science Monthly Volume 68.djvu/554

548 quantity and intensity to furnish heat enough to meet the wants of the vulcanologist.

Let us now look for a moment at the presumable details of the process. At a depth of two or three miles in the earth let us assume that radium is in process of being generated. It starts at once upon that process of transformation of which one stage is the production of the so-called emanation, which is a gas of very high density and great penetrating power and diffusibility. We know that the upper strata and soils everywhere contain it, and no reason appears why the same should not be the case with the rock beneath. Wherever the emanation penetrates, the break-up of its particles generates heat and the temperature rises in proportion to the amount of emanation which undergoes transformation in a given time, and falls in proportion to the rate at which it is conducted away. So long as the gain of heat exceeds the loss, so long will the temperature rise until it becomes sufficient to melt the rocks.

All volcanic lavas contain water and those whose reservoirs are near the surface contain a large amount of it. Those which have a deeper origin contain a smaller amount of it. The deeper lavas are hotter and are erupted with less violence and in greater mass than the shallow ones, and the reason is obvious.