THEORY OF VOLCANOES. 99 



2. Pressure on a general interior liquid from any cause at any place 

 would, by the law of hydrostatics, be transmitted equally to every part 

 of the crust, which would, therefore, yield at the weakest point, wher- 

 ever that may be, even though it be on the opposite side of the globe ; 

 but the force of volcanic eruption is evidently just beneath the volcano. 



3. Volcanoes belonging to the same group, and therefore near to- 

 gether, often erupt independently, as if each had its own reservoir of 

 liquid matter. The pressure of these two objections has driven many 

 to the admission of a sort of honey -combed remains of the interior 

 liquid inclosed in the solid crust, and now isolated both from the inte- 

 rior liquid and from each other. 



4. There is a limit to the descent of water into the interior of the 

 earth; gravity urges it downward, but the interior heat drives it 

 back. The limit, therefore, will be where these two balance each 

 other, i. e., where the elastic force of steam is equal to the superincum- 

 bent column of water. We will call this point the limit of volcanic 

 waters. It is evident that when water was first condensed on the cool- 

 ing earth, this limit was at the surface : water could not penetrate at 

 all. As the earth cooled, this limit became deeper and deeper ; and, if 

 the earth became perfectly cool to the center, there is little doubt that 

 the whole of the water on the earth would be absorbed. This is per- 

 haps the case with the moon now. 



Now, it seems probable that at the limit of volcanic water the in- 

 terior heat of the earth, increasing at the rate of 1° for every fifty feet, 

 would be far short of the temperature necessary for igneous fusion of 

 rocks. Again, the elastic force necessary to sustain the superincum- 

 bent 2vater would by no means be sufficient to break up the crust of 

 the earth, or raise melted lava to the surface. 



But we will not pursue this subject, as it is too complex to be yet J / 

 solved by science. We rely, therefore, on the first three objections. 



Chemical Theory. — Whether or not the earth consist of solid crust 

 covering an interior liquid, it almost certainly consists of an oxidized 

 crust covering an unoxidized interior. Now, the oxidizing agents are 

 water and air, and therefore the limit of the oxidized crust is the limit 

 of volcanic water. Therefore, the oxidizing agent and the unoxidized 

 material are in close proximity, and the former ever encroaching on the 

 latter, and therefore liable at any moment to set up chemical action, 

 the intensity of which would vary with the nature of the material. If 

 the action be intense, heat may be formed sufficient to fuse the rocks 

 and to develop elastic force necessary to produce eruption. 



In this general form, the chemical theory seems plausible, but many 

 have attempted to give it more definiteness, and to explain the special 

 forms of oxidization which cause volcanoes. The most celebrated of 

 these definite forms is that of Sir Humphry Davy, who attributed it to 



