What is an Explosive?
If I were to ask each of you the question which I have put as the title of this chapter, I think that many of you would reply that an explosive is a thing that goes off with a bang when it catches fire. Some of you might add that certain chemicals will explode when they are struck a sharp blow. Most boys and girls have seen an explosion of some sort, either in real life or photographed by a cinematograph.
I have heard the story of an Irishman who said that he would much rather be killed by a collision than by an explosion. When Pat was asked for his reason, he said: "Well, you see, in a collision there you are, but in an explosion where are you?"
In war inventions we use explosives for two different purposes. One is to throw the bullets and shells at the enemy by means of guns. The other is to explode the shells when they reach the enemy. We use explosives also in bombs, or grenades, in torpedoes, and in mines, both on land and at sea. We shall have a talk about all these inventions a little later. Meantime we wish to see exactly why gunpowder and other explosives do explode.
Of course you know that we use explosives in peaceful industries as well as in warfare. You have heard the quarrymen blasting some great rocks, thus doing in a moment an amount of work which would have taken them a very long time to do with pickaxes and chisels. When you are travelling by train you sometimes see parts of the railroad which have been cut through solid rock, and you know that explosives were used to clear these passages.
Then as you walk along the street you occasionally hear a bang from a motor-car or motor-cycle, which reminds you that it is an explosion of petrol vapour and air, which propels these machines along. But none of these facts tell us just what an explosion really is. So I shall suppose that you put the question to me:
"What is an explosive?"
You may be surprised when I say that when a thing explodes it merely burns away very quickly. Surely there must be something more than that! Well, let us examine the matter.
If I were to ask you what happens when a thing burns away, many of you could explain what happens in the household fireplace. You could tell me that the combustion (or burning away) of the coal was due to the carbon of the coal joining hands with the oxygen of the air. There are other chemical combinations also which take place, but that is the principal fact. You know very well that the presence of air is necessary if the coal is to burn. You know how the blacksmith blows air through his burning coal by means of a huge pair of bellows. Our grandmothers used to keep a pair of bellows beside the fireplace, but nowadays, with grates and chimneys made on more scientific principles, we do not require bellows. I wish you to notice that there are two parties to the action of combustion; there is the coal and there is the air, or, to be more exact, we should say the carbon and the oxygen. If either of these two is absent there will be no combustion.
Take a look at an electric glow-lamp in which a little carbon thread is so white-hot that it sends out quite a big lot of light. Before the invention of the metallic filament lamps, we used these carbon filament lamps entirely. You know this little thread of carbon is kept white-hot by a current of electricity passing through it, and yet there is no combustion; the carbon thread does not burn away. Why? Because there is no air in the little globe; the lamp-maker has pumped out all the air, and then sealed up the globe.
Let's take another look at the coal fire. It is not burning very briskly, so we take the poker and break the coal, into smaller pieces. Why? Because the burning only takes place on the surface of the coal, where the air can reach it, and by breaking up the coal we allow the air to get through among it, and thus reach more surface. We shall see in a moment that this is where the sudden burning away which we call an explosion differs from an ordinary burning away. First of all we wish to see what happens when the coal burns.
If the fire has burnt briskly, there is nothing left but a few ashes. If I ask you where the coal has gone, you will tell me that it has gone up the chimney in the form of gases and smoke. The solid coal has been transformed into flimsy gases. If you could catch the gases and keep them you would find that they occupy a very much larger space than the coal did. Now let us watch what happens when some gunpowder burns.
I am not suggesting that you should make any experiment in this matter. We can make the experiment in imagination, and then we shall not get into any trouble. We make a little heap of gunpowder, and laying a small train of it to a safe distance, we set a light to the end of the train and we see the whole powder go off in a single puff. You say that it burnt away in a "jiffy," but why did it burn so very quickly? Because it did not depend upon getting oxygen from the air; it had a great deal of oxygen within itself. Therefore, instead of merely burning on the surface, it all burnt at the one time.
I have no doubt that many boys, and perhaps some girls, know that gunpowder is just a mixture of certain quantities of saltpetre, charcoal and sulphur. The saltpetre is in the form of white grains, not unlike common salt, but of coarser grain. It contains a great deal of oxygen, so if you were to see anyone making gunpowder, you would understand why they use so much saltpetre; they wish to have plenty of oxygen to ensure a good combustion.
You know what charcoal is like, and I need hardly tell you that it contains the carbon which you wish to unite with the oxygen of the saltpetre, and thus give combustion. But what about the sulphur? It is merely an assistant; the saltpetre and the charcoal are the two active parties in the combustion. The sulphur is added to make the gunpowder more easily fired, and this gives a more sudden burning away, and so our little heap of gunpowder was burnt away in a single puff, but no harm was done; there was no explosion. Why? Because when the gunpowder was transformed into a great quantity of gases they had plenty of room in the open air. Had we burnt the gunpowder in a box of any kind, the sudden arrival of all the gases in place of the gunpowder would have burst the box open; there would have been an explosion.
That was what took place in the old-fashioned cannons; the gunpowder was suddenly burnt away, and the gases had to escape as best they could. If they could have taken plenty of time they might have escaped gradually through the touch-hole, but they had to get away at once or else they would have burst the gun. Their easiest way of escape was through the open muzzle of the gun, and the only thing in their way was the solid iron cannon-ball. The sudden rush of the gases drove the cannon-ball out of the cannon with great energy, and off it flew towards the enemy.
Suppose for a moment that you are away in the Wild West of America, and that you have a quantity of gunpowder stored in the hut in which you live, as you require the explosive for some peaceful operations. When you have occasion to cook your food in the hut, you begin to wish that the gunpowder was not present. You wish that you had kept the different ingredients separate from one another, and then there would have been no fear of an explosion. You get so nervous about this gunpowder that you determine to separate the parties that are so willing to unite with a big bang when any flame reaches them. You have learnt some chemistry at school, and you know that the saltpetre will dissolve in water, and that the charcoal will not dissolve. You are not worrying about the sulphur as it is not one of the active agents in the explosion, and so you boil the gunpowder in water, and when the saltpetre has dissolved you pour the whole contents of the pot into a large sheet of blotting-paper, which you hold over an empty vessel. Only some clear liquid gets through, and you know very well that what is caught by the blotting-paper is a mixture of charcoal and sulphur; the saltpetre having dissolved in the water has been carried with it through the blotter into the vessel beneath. When this clear liquid cools you see little white crystals, and you know these to be saltpetre. Your gunpowder is now quite safe, and you may make up the mixture again when required.
I have made up this little imaginary story about your being away in a Wild West hut with gunpowder, not merely to amuse you, but to try and impress you with the fact that it requires two active contracting substances to make an explosion, otherwise when I come to speak of gun-cotton, someone might ask if cotton is explosive.
Gunpowder is a very, very old invention; indeed we cannot trace its origin; it was certainly known before the time of Christ, and some suppose that it even existed in the time of Moses. Be that as it may, what concerns us at present is that gunpowder had the whole field to itself for a long time. It had no rival until the invention of gun-cotton in the nineteenth century, less than one hundred years ago.
No one is to ask if cotton is an explosive, for I have surely made it clear that no one single substance is explosive; it requires two different substances to unite before there can be an explosion. Neither saltpetre nor charcoal are of themselves explosive, but you have to be careful when they are mixed together in the form of gunpowder.
You know that cotton grows on a little grass-like plant, but you may not know that this cotton which Nature produces is composed chiefly of a substance which we call cellulose. It is this cellulose which is one of the active agents in gun-cotton. I have no doubt that some of you can guess that the other active agent is to be oxygen, and that we must get the oxygen into close touch with the cellulose in the cotton. This is done by steeping the cotton well in a strong solution of nitric and sulphuric acids. Any boy or girl who has learnt a little chemistry will be able to tell me which of these two acids is going to play the active part. Indeed some quick-thinking girl or boy who knows no chemistry may be able to recognise that the sulphuric acid is quite apparently related to the sulphur which you will remember was put into the gunpowder merely to hasten the explosion. And those who know that nitre is another name for saltpetre will recognise that there is some close family connection between nitric acid and saltpetre. From this you will be able to see that the nitric acid (like the saltpetre) provides the necessary oxygen.
After the cotton-wool has been well steeped in the acids, it is washed and dried, and is then a much more violent explosive than gunpowder. We describe gun-cotton as a high explosive, while we speak of gunpowder as a low explosive. Gun-cotton was looked upon at first as being too dangerous a substance to risk making; several factories attempting to make it were blown to pieces. Later on better ways and means were found of making it, but even to-day great care has to be taken when pressing it, as will be seen in the photograph facing page 88. Here we see a workman protected by a rope screen while he is operating a press for the gun-cotton.
If I were to ask you to name some high explosives, I think the first that would occur to you would be "dynamite," and you may be interested to know how it was invented. It was found that cotton was not the only substance which could be rendered explosive when treated with nitric acid. Glycerine and nitric acid go to make a very high explosive called "nitro-glycerine." Being a liquid, it is not convenient to handle, and it was soon found to be a most dangerous explosive. It was then that Alfred Nobel tried mixing nitro-glycerine with a porous earth, which absorbed the liquid and produced that solid explosive substance which we call dynamite. Dynamite is too energetic an explosive to use in guns; it would burst the gun before the projectile had time to escape from the barrel.
Many boys and girls know that the explosives which we use in modern guns are called "smokeless powders." This descriptive name requires no explanation, but some of you may be curious to know what these smokeless powders are. For instance, what is the British "cordite"?
It is cordite which throws the shells from our great naval guns as well as the bullets from our rifles. It is a mixture of gun-cotton and nitro-glycerine, but these two high explosives would produce too sudden and violent an explosion. How can we slow down their action? We have to add some substance which is not explosive, and which will prevent these high explosives burning too quickly. It is for this reason that we add some vaseline in the manufacture of the cordite; we are able to give the projectile a good long push off, without over-straining the gun.
In the succeeding chapter we shall see the part played by the high explosives.