OF PX EL -VAT 1C*. 205 



money in a man's pocket without hurting his clothes : 

 the reason of this seems to be, that the particles of that 

 fire are so fine, as to pass through soft loose bodies with- 

 out dissolving them ; whilst they spend their whole 

 force upon the hard ones. 



It is remarkable, that knives and forks which have 

 been struck with lightning have a very strong magneti- 

 cal virtue for several years after ; and I have heard 

 that lightning striking upon the mariner's compass, will 

 sometimes turn it round ; and often make it stand the con- 

 trary way, or with the north pole towards the south.*' 



Much of the same kind with lightning, are those ex- Fire _ 

 plosions, called fulminating or Jire-damps, which some- damps. 

 times happen in mines ; and are occasioned by sulphu- 

 reous and nitrous, or rather oleaginous particles, rising 

 from the mine, and mixing with the air, where they will 

 take fire by the lights which the workmen are obliged 

 to make use of. The fire being kindled will run from 

 one part of the mine to another, like a train of gun- 

 powder, as the combustible matter happens to lie. And 

 as the elasticity of the air is increased by heat, that in 

 the mine will consequently swell very much, and so, for 

 want of room, will explode with a greater or less degree 

 of force, according to the density of the combustible 

 vapours. It is sometimes so strong as to blow up the 

 mine; and at other times so weak, that when it has 

 taken fire at the flame of a candle, it is easily blown 

 out. 



Air that will take fire at the flame of a candle may be 

 produced thus. Having exhausted a receiver of the air- 



Note 57. The power of communicating magnetism to ferruginous 

 bodies by the agency of electricity has long been known ; bnt the most 

 remarkable effects yet exhibited were produced by a powerful galvanic 

 apparatus, constructed under the direction of W. H. Pepys,Esq. In the 

 experiments performed with this apparatus, a compass-needle, placed 

 more than ten feet from the galvanic coil, was sensibly deflected from 

 its ordinary direction, and polarity was communicated to bars of steel, 

 which thus became permanent magnets 



