Chap. 47-] Probable Cwje of Volcanic Fires. 

 are natural combinations of iron and fulphur, are 

 known to exift in different parts of the earth ; the 

 only difficulty which attends this explanation of the 

 origin of volcanoes, as well as of earthquakes, is, that 

 the prefence of air is in general necefiary for the pro- 

 duction of actual flame. It is well known, however, 

 that martial pyrites, when moiftened, acquire heat; 

 and if we fuppofe the heated pyrites to have been in 

 contact with black wad and petroleum, we may fup- 

 pofe the flame to arife, as we fee it produced by art, 

 from the deficcation of the former fubftance, and its 

 mixture with mineral oil. Many minerals, when 

 heated, afford vital air, a very fmall quantity of which 

 is fufficient to produce flame ; this flame, once pro- 

 duced, may be fupported from other ores, and the 

 combuftion be maintained by the prefence of bitumi- 

 nous fchiftus, bitumen, and coal. Marl, fchiitus, horn- 

 ftone, Ihoerl, with a further addition of iron, are the 



quantities of the materials are not requifite to make the experiment 

 fucceed, provided there DC a due proportion of water : half a 

 pound of fteel filings, half a pound of flowers of brimllone, and 

 fourteen ounces of water, will, when well mixed, acquire heat 

 enough to make the mafs take fire. 



That heat and fire mould, be generated from the fpontaneou? 

 actions of minerals upon each other, is a phenomenon by no means 

 fingular in nature, how difficult foever it may be to account for if. 

 The heat of putrefcent dunghills, of the fermenting juices of ve- 

 getables, and, above all, the fpontaneous firing of hay not pro- 

 perly dried, are obvious proofs that vegetables poffefs this property 

 as well as minerals. In both vegetables and minerals, a definite 

 quantity of moifture is requifite to enable them to commence that 

 inteftine motion of their parts, which is neceflary for the produc- 

 tion of fire. Iron and fulphur would remain mixed together for 

 ages without taking fire, if they were either kept perte&ly free 

 /rom moifture, or drenched with too much water; and vegetables 

 in like manner, which are quite dry, or exceedingly wet, are inca- 

 pable of" taking fire wlulft they continue in that Hate. 



Y 2 true 



