PRESinENT S ADDUESS. V. 



the sea which, invading' ihc land, sweep all before thoni, and 

 these, iu the recent case, were the most destructive of all. 



We have here obviously very different conditions and 

 relations from those which bring about the tolerably regular 

 and gentle puffings of Stromboli, the almost constant, quiet out- 

 pouring of lava from the great mountains of the Sandwich 

 Islands, or even the usual paroxysms of Etna and Vesuvius. 

 The lava in the interior of the earth must be at a high tempera- 

 ture, and such large quantities of water must find their way at 

 intervals to it, that no gradual relief of pressure by the escape 

 of iluid lava is sufficient, but the whole superincumbent mass, 

 at any rate in the weak place — the volcano — is blown away at 

 once. It should be noted that the rapid access of water would 

 be facilitated by the melted matter coming nearer to the surface 

 than is usually the case, and this would also give less power of 

 resisting the sudden pressure of steam. In a paper on the ashes 

 from Krakatoa, the Abbe Kenard draws attention to the fact 

 that the numerous cavities of the glass and minerals of which 

 it is made up contain no water, but only gas or air ;* and this, 

 notwithstanding the essential part that steam evidently plays 

 in their eruption. He also points out that the fragments of the 

 dust are by no means such as would result from trituration of 

 a mass already solid to any great extent, but rather has the 

 appearance of being produced by the sudden relief of pressure 

 from a mass of molten rock containing, intimately mixed with 

 it, highly compressed gases. To use a homely illustration, — 

 such a mass as is produced in the manufacture of the so-called 

 ■aerated bread, where the dough has been made with water, 

 highly charged with carbonic acid. On running this out of the 

 cylinder in which it has been prepared, the escape of the gas 

 gives the bread its spongy textm'e. Carried a step further, it is 

 easy to conceive it as actually bursting the mass into fragments. 



Among the lavas which have been found in Sumatra, Java, 

 and the various volcanic islands of those seas, the group called 



* But the glass of which the ashes are mainly composed seems to 

 coutaiu water. {See next note.) 



