71 INTRODUCTION. 



Dr. Davy's " In the concluding lecture. Dr. Davy stated, that the 

 .ervations. em j ss j on o f j ava f rom volcanoes was one of the principal 

 operations by which nature supplied the waste of rocks, and 

 the destruction of the land noticed in his former lectures. 

 The agency of volcanoes in the production of islands, and the 

 increase of continents, is more extensive, than those who 

 reside at a distance from their influence are disposed to admit. 

 Proofs of this may be traced in the islands and shores of the 

 Mediterranean, in the continent of America, and in Asia, 

 and in other parts of the globe. Nearly the whole of Sicily, 

 and the southern parts of Italy and France, offer evidence of 

 their volcanic origin ; and Rome, which has by ancient writers 

 been proudly styled the " Eternal City," is built on the crater 

 of an extinct volcano. The phenomena attending the erup- 

 tion of volcanoes were described from Hamilton, Dolomieu, 

 Spalanzani, and others, who had been present during the 

 eruptions of Etna and Vesuvius. 



" The convulsion of the solid ground, the lofty columns of 

 flame, smoke, and vapour, the tremendous explosions, the 

 torrents of rain, and the thunder and lightning, which ac- 

 company the eruption of lava, all indicate that the immediate 

 cause is the expansion of steam and hydrogen gas, which in- 

 flames when in contact with the atmosphere. The doctrine 

 of a central fire was unsupported by proof or analogy : did 

 such a fire exist its effects must be felt at the surface, even 

 if it had to pass through the most imperfect conductors of 

 heat. 



" To ascertain the cause which produced the expansion of 

 vapour, and the other phenomena of volcanoes, we must 

 examine the products of these august operations of Natural 

 Chemistry. If we observe a fire at a distance, and are able 

 to collect its products, we may thence determine the nature 

 of the substances which have been in a state of combustion. 

 The products of volcanoes are hydrogen gas, vapour, and 

 lava, of which lava is a compound of the earths, the alkalies, 

 and the oxyd of iron, In his former lectures, Dr. Davy pb- 



