150 



The cbymical hypothesis havmg I'eceived this much negative evideuce 

 iu its favour, let us examine iuto the positive evidences that support it. 



In so doing we must necessarily be brief, as it would be impossible 

 in a short paper to treat the subject with the detail a strict proof 

 demands. 



First, all the phenomena we have noticed, i. e. the expulsion of 

 boiling water, of steam, of mud, of carbonic, muriatic, and sulphuric 

 acids, the presence of electric sparks, all tend to show that volcanic 

 eruptions are attended by occurrences on a large scale similar to those 

 that take place iu the laboratory during cbymical decompositions on a 

 small scale. 



Second, that the detonations which are incompatible with the idea of 

 a flow through a vent of some central fluid fire are quite compatible 

 with the liypothesis that they arise from the explosion of gases. 

 Hydrogen and oxygen coming together and being ignited, will, by their 

 explosive power, produce an earthquake shock ; and, by their united 

 product being water, there will be a reaction, and return to the original 

 condition. In this we can see an explanation of the back-stroke of the 

 earthquake wave. 



The development of carburettcd hydrogen would have a similar 

 residt. The presence of water would increase the bulk of the material ; 

 but as the mass gets hotter, the water, which in the first instance 

 probably began the cbymical change, would be converted into steam, 

 and its pressure gradually increasing would at last burst the boiler 

 with a terrific explosion. As the steam gets free vent the explosion 

 would cease, just as we find that it does. 



Again, we find evidence of the action being near the surface, and of 

 its being greatly increased by rain. We find, from the accumulated 

 observations of the past, that a drying up of the wells, i. e., a large 

 absorption of water in the lower strata, is a common precursor of 

 volcanic eruptions. We find that most active volcanoes are situated 

 near large accumulations of water. We find that they all abound 

 in sulphur and other elementary substances, which we know will 

 unite together in the laboratory wdth the extiication of light, heat, and 

 inflammable and explosive gas. We know that those unions are 

 favoured by moisture. We know that the thermal springs in the 

 volcanic districts contain commonly some of the combinations of 

 sulphur. 



We have seen pheuomena elsewhere of a similar nature to those we 

 are seeking to explain. Thus, for example, on the west coast of 

 Ireland, we rtiad of a cliff' having fallen which contained a large quan- 

 tity of sulphuret of iron. This exposed a new surface to the waves : 



