A coal fire, as we all know, burns slowly, for the simple reason that it is only at the surface of the lumps that carbon and oxygen are in contact. If we grind up the coal into a fine powder and then blow it into a cloud, so that every tiny particle is surrounded with air, a spark will cause an explosion. That is how these terrible explosions in coal-pits are caused.
This is sometimes seen on a small scale when one shakes the empty fire-shovel after putting coal on the fire to get rid of the fine dust adhering to it and to save making a mess in the fender. That little cloud of fine dust will often burst into flame like a mild explosion.
We see from this that to make an explosion we require fuel, just as we do to make a fire: but we need that it shall be very intimately mixed with oxygen, so that all of it can burn up in practically a single instant. Now in gunpowder we get these conditions fulfilled. We have the carbon in the shape of charcoal, we also have some sulphur which likewise burns readily, and we have saltpetre which contains oxygen.
Thus, you see, we do not need to go to the air for the oxygen, for the gunpowder possesses it already, locked up in the saltpetre. Moreover, we can see now why it is so important for all the materials to be ground up very fine, for it is only by so doing that we can ensure that every particle of charcoal or sulphur shall have particles of saltpetre close by ready to furnish oxygen at a moment's notice.
Another thing to be observed, for it lets us into the great key to the manufacture of nearly all explosives, is the scientific name of saltpetre. It is "nitrate of potassium," and all substances whose names begin with "nitr-" contain nitrogen: while the termination "ate" signifies the presence of oxygen. We need the oxygen to make the explosion but we do not need the nitrogen, yet the latter has to be present for without it the oxygen would be too slow in getting to work.
Nitrogen is one of the strangest substances on earth. Extremely lazy itself, it has the knack of hustling its companions, particularly oxygen, and making them work with tremendous fury. Whenever we get the lazy gas nitrogen to enter into a combination with other things we may confidently look for extraordinary activity of some sort.
So when we put a light to a quantity of gunpowder we set up those conditions under which the carbon and oxygen can combine, and at the same moment our lazy friend the nitrogen turns out his partner oxygen from the nitrate in which they were till then combined and a sudden burning is the result. The solid gunpowder is suddenly changed into a volume of hot gas 2500 times as great. That is to say, one cubic inch of gunpowder changes suddenly into 2500 cubic inches of gas. That sudden expansion to 2500 times its volume is what we term an explosion. If it takes place in an enclosed space so that the gas formed wants to expand but cannot, the result is a pressure of about forty tons per square inch.
If that enclosed space were the interior of a gun, that force of forty tons per square inch would be available for driving out the projectile.