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incliued to think, must have been attended with a coiTespouding 

 depression of the earth's general surface, or of the part immediately in 

 connection with the vent. But so far from finding depression the rule, 

 we find that elevation of land precedes, accompanies, and succeeds 

 volcanic action ; and that, when depression does take place, it takes 

 place slowly, and at a considerable distance of time. 



The phenomena suggests to us the idea, not of a fountain, but of the 

 overboiling of some pot containing a dense material, when air separating 

 the particles, gives the appearance of a vast increase in size and 

 quantity. 



That the comparison between the lava stream and an overboiling pot 

 is not simply a flight of fancy may readily be seen by any one who 

 considers the phenomena presented by both. 



Let us fill a large pot, or a long tube, with rosin, or some other 

 dense material capable of undergoing gaseous decomposition by heat ; 

 we wUl apply our heat suddenly; the result will be that a layer of the 

 material is first melted, the upper portion of which imparts its heat to 

 the cool part above, while the lower is converted into gaseous vapour, 

 and thus being increased immensely and suddenly in size, drives up the 

 whole mass and makes its escape. Here we have in petto the pheno- 

 menon of the earthquake which precedes for a long time the eruption. 

 The same process continues until the whole mass is heated thoroughly, 

 and in a state of ebullition. Gaseous products still continue to be 

 formed, and explode when they reach the surface ; others, retained by 

 the cloggy matter, are diffused through the whole, making it porous as 

 bread, — a phenomenon repeated on a large scale by the rise of the coast 

 of Misenum, prior to the eruption of Monte Nuova. As the heat 

 continues or increases, ebullition goes on more furiously ; the material 

 is decomposed into its gaseous elements with greater intensity ; they 

 detonate more loudly as they are nearer the surface ; the mass, swelled 

 to greater dimensions by the increasing size of the bubbles enclosed 

 within, and by the diminished pressure to which they are subjected, at 

 length boils over, and continues to do so as long as there is material in 

 the pot and heat in the fire. When we examine the rosin that has 

 boiled over we find it equally porous with lava and pumice ; and thus 

 it is we see that the real is not equal to the apparent increase of bulk. 

 By and by the fluid cools ; the heated gases diminish in volume ; the 

 whole material contracts, and the substance remaining in the pot is 

 seen diminished in size. This is nothing moi-e than analogous to the 

 depression of soil which takes place after the eruption of a volcano has 

 passed, and the heated matter has had time to cool, which it does 

 slowly. 



