VOL. XIX.] PHILOSOPHICAL TRANSACTIONS. 197 



separate, for the better kindling of it. So that if we suppose in the air, a conve- 

 nient mixture of nitrous and sulphureous vapours, and those by accident to take 

 fire ; such explosion may well follow, as in the firing of gunpowder. And being 

 once kindled, it will run on from place to place, as the vapour leads it, as in a 

 train of gunpowder. 



This explosion, if high in the air, and far from us, will do no mischief, or 

 not considerable ; like a parcel of gunpowder fired in the open air ; but if near 

 us, or among us, it may kill men or cattle, tear trees, fire gunpowder, break 

 houses, or the like, as gunpowder would do in the like circumstances. 



Now this distance may be estimated by the interval of time between seeing 

 the flash of lightning and hearing the noise of the thunder. For though in 

 their generation they be simultaneous, yet, light moving faster than sound, 

 they come to us successively. I have observed that, commonly, the noise is 

 about 7 or 8 seconds after the flash, that is, about half a quarter of a minute ; 

 but sometimes much sooner, in a second or two, or less, and almost immedi- 

 ately on the flash. And at such times, the explosion must needs be very near 

 us, or even among us. And, in such cases, I have more than once presaged 

 the mischievous consequences, which happened accordingly. 



Now, that there is in lightning a sulphureous vapour, is manifest from the 

 smell which attends it, especially when mischief is done, and even when there 

 is none, from the lightning itself, which is more or less discernible. And a 

 sultry heat in the air is commonly a forerunner of lightning, soon after to 

 ensue. And that there is also a nitrous vapour with it, we may reasonably 

 judge, because we know not of any body so liable to a sudden and violent ex- 

 plosion. 



Now as to the kindling of these materials, in order to such explosion, che- 

 mists observe, that a mixture of sulphur, filings of steel, and a little water, 

 will not only cause a great effervescence, but will of itself break out into an 

 actual flame. So that there wants only some chalybeate or vitriolic vapour, or 

 something equivalent, to produce the whole effect, there being no want of 

 aqueous matter in the clouds. And there is no doubt, but that among the 

 various effluvia from the earth, there may be copious supplies of matter for such 

 mixtures. And it is known that hay, if laid up too green, will not only heat, 

 but take fire of itself. 



The same account may also be given of ^tna, and other burning mountains, 

 where the admixture of steel and sulphur may give a flame, which is often 

 attended with prodigious explosions and earthquakes, from the great quantities 

 of nitre, as in springing a mine. 



This may also suggest something as to the generation of hail, which is very 



