The least thing upsets them, however. The carbon combines with oxygen into carbon dioxide, commonly called carbonic acid gas, the hydrogen and some more oxygen form steam, while the nitrogen is left out in the cold, so to speak. And the total volume of the gases so produced is about 6000 times that of the original liquid. It is easy to see that a substance which is liable suddenly to increase its volume by 6000 times is an explosive of no mean order.
But the fact that it is liable to make this change on a comparatively slight increase in temperature or after a concussion makes it too dangerous for practical use. It needs to be tamed down somewhat. This was first done by the famous Nobel, who mixed it with a fine earth known as kieselguhr, whereby its sensitiveness was much decreased. This mixture is dynamite.
It will be seen that the function of the "earth" is simply to act as an absorbent of the liquid nitro-glycerine, and several other things can be used for the same purpose. Moreover, there are now many explosives of the dynamite nature but differing from it in having an active instead of a passive absorbent, so that the decrease in sensitivity is accompanied by an increase in strength. For example, gelignite, which is being used for agricultural purposes in Great Britain, consists of nitro-glycerine mixed with nitro-cotton, wood-meal and saltpetre. The wood-meal acts as the absorbent instead of the kieselguhr, while the nitro-cotton is another kind of explosive and the saltpetre, one of the ingredients in the old gunpowder, provides the necessary oxygen for burning up the wood-meal. Nitro-cotton is made in much the same way as nitro-glycerine, except that cotton takes the place of the glycerine. Cotton is almost pure cellulose, another organic substance, like glycerine insomuch as it is composed of carbon and hydrogen, but, unlike it, containing also oxygen. Treated with nitric acid it also forms a combination of carbon, hydrogen, oxygen and nitrogen, which is called nitro-cotton, nitro-cellulose, or gun-cotton.
It may be asked, why, if these two substances are thus similar, need they be mixed? The answer is that although alike to a certain degree they are not exactly the same, and the modern manufacturer of explosives in his strife after perfection finds that for certain purposes one is the best, and for others another, while for others again a combination may excel any single one.
For some work another kind of explosive altogether is to be preferred. This is based upon chlorate of potash, a compound very rich in oxygen, which it is prepared to give up readily to burn any other suitable element which may be at hand. A well-known explosive of this class is that known as cheddite, since it was first made at a factory at Chedde, in Savoy.
For the sake of simplicity, however, I propose in the following descriptions to refer to all these explosives under the common term "dynamite," since that will probably convey to the general public an idea of their nature better than any other term or terms which I could choose.
So now we come to the great question, how can the modern farmer benefit by the use of high explosives such as these? The answer is, in many ways. Let us take the most obvious one first.
A farmer has been ploughing his land and growing his crops upon it for years. Perchance his forefathers have been doing the same for generations. Every year, for centuries possibly, a hard steel ploughshare has gone over that ground, turning over and over the top soil to a depth of six to eight inches. Each season the plants, whatever they may be, grow mainly in that top layer. They take the goodness or nourishment out of it and it eventually becomes more or less sterile. By properly rotating his crops he mitigates this to a certain extent, in addition to which he restores to the land some of its old nitrogenous constituents by the addition of manure. Yet, do what he will, this thin top layer is bound to become exhausted. And all the while a few inches lower down there is almost virgin soil which has scarcely been disturbed since the creation of the world.
Nay, more, that virgin soil, with all its plant food still in it, is not only doing little for its owner, it is positively doing him harm. For every time his plough goes over it it tends to ram it down flat; every time a man walks over it the result is the same; every horse that passes, everything that happens or has happened for centuries in that field, tends to make that soil just below the reach of the ploughshare a hard, impervious mass, through which only the roots of the most strongly growing plants can find a way, and which tends to make the soil above it wet in wet weather and dry in dry weather. Thus roots have to spread sideways instead of downwards; or, growing downwards with difficulty, each plant has to expend vital energy in forcing its roots through the hard ground which it might better employ in producing flowers or fruits. And there is no natural storage of water. A shower drenches the ground. In time it dries, through evaporation into the air, and then when the drought comes all is arid as the Sahara.
That hard subsoil is known by the term "hard-pan," and, as we have seen, it is produced more or less by all that goes on in the field. Even worse is the case—a very frequent one too—wherein there is a natural stratum of clay or equally dense waterproof material lying a few feet down.