204 FERGUSON'S LECTURES. 



LECT. ; a id of the effluvia from sulphureous, nitrous, acid, and 

 ^ alkaline bodies. In short, whatever may be called vo- 



latile, rises in the air to greater or less heights, accord- 

 ing to its specific gravity. 



Fermenta- When the effluvia which arise from acid and alkaline 

 tions. bodies, meet each other in the air, there will be a strong 

 conflict or fermentation between them ; which will some- 

 times be so great, as to produce a fire ; then if the ef- 

 fluvia be combustible, the fice will run from one part to 

 another, just as the inflammable matter happens to lie. 



Any one may be convinced of this, by mixing an acid 

 and an alkaline fluid together, as the spirit of nitre and 

 oil of cloves ; upon the doing of which, a sudden fer- 

 ment, with a fine flame, will arise ; and if the ingredients 

 be very pure and strong, there will be a sudden ex- 

 plosion. 



Thunder Whoever considers the effects of fermentation, can- 

 and light- not be at a loss to account for the dreadful effects of 

 thunder and lightning: for the effluvia of sulphureous 

 and nitrous bodies, and others that may rise into the 

 atmosphere, will ferment with each other, and take fire 

 very often of themselves ; sometimes by the assistance 

 of the sun's heat. 



If the inflammable matter be thin and light, it will rise 

 to the upper part of the atmosphere, Avhere it will flash 

 without doing any harm : but if it be dense, it will lie 

 near the surface of the earth, where taking fire, it will 

 explode with a surprising force ; and by its heat rarefy 

 and drive away the air, kill men and cattle, split trees, 

 walls, rocks, &c. and be accompanied with terrible claps 

 of thunder. 



The heat of lightning appears to be quite different 

 from that of other fires ; for it has been known to run 

 through wood, leather, cloth, &c. without hurting them, 

 while it has broken and melted iron, steel, silver, gold, 

 and other hard bodies. Thus it has melted or burned 

 asunder a sword, without hurting the scabbard ; and 



