55o POPULAR SCIENCE MONTHLY 



cient 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 emana- 

 tion penetrates, the break-up of its particles generates heat and the tem- 

 perature rises in proportion to the amount of emanation which under- 

 goes 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. 



