246 Dr Charles Daubeny on the 



position of sulphuretted-hydrogen disengaged from the same 

 source ; and conclude, therefore, that it is not unfrequently 

 evolved from the orifices of volcanoes, although, for the most 

 part, prevented from inflaming by the large intermixture of 

 aqueous vapour which usually accompanies it. 



But without entering at this advanced period of the even- 

 ing into a general discussion of the question, I will merely 

 point out to you, how completely this theory squares with 

 the manner, in which I have shewn the several products of 

 volcanic action to be successively produced from the consti- 

 tuents of granite. 



We have seen that these changes of form and structure 

 have been produced by the addition of lime, magnesia, potass, 

 soda, oxide of iron, water, &c., to the mica, quartz, and fel- 

 spar, present in the original material. 



Now it is evident that, besides the water, only one of these 

 bodies, namely, the soda, could have been supplied in suffi- 

 cient quantities by the sea ; the access of which to the focus 

 of the volcano there are so many reasons for supposing an 

 immediately exciting cause of the operations we witness. Is 

 it not, therefore, reasonable to suppose the other constituents 

 to have existed in their unoxidized state below, and thus to 

 have contributed, by their subsequent oxidation, to the pro- 

 duction of the high temperature, as well as to the generation 

 of those inflammable gases which arise during the process ? 



Again, it is not an unimportant circumstance to remark, 

 that the iron found in lavas and in trap is usually magnetic, 

 or partly in the state of protoxide ; whilst in granite it exists 

 wholly as a peroxide. May not this partial change from per- 

 oxide to protoxide be brought about by the action of the 

 hydrogen disengaged, and does not the presence of protoxide 

 of iron sufficiently explain, why none of the more oxidizable 

 metals are even found in lava, except saturated with oxygen % * 



* This is alleged by Dr John Davy, as a circumstance which operated on his 

 brother's mind in inducing him, towards the close of his life, to abandon the 

 chemical theory. I cannot, however, agree with him in thinking, that the pre- 

 sence of potassium, sodium, or even calcium, amongst the ejections of a volcano, 

 ought to be expected according to the conditions of this hypothesis. 



