Of all the harmless things in the world one would think that that sweet, sticky fluid, glycerine, which most of us have used at one time or another to lubricate a sore throat, was the most harmless. As it stands in its bottle upon the domestic medicine shelf, who would suspect that it is the basis of such a thing as dynamite?
Such is the case, however, for glycerine on being brought into contact with a mixture of sulphuric and nitric acids gives birth to nitro-glycerine, an explosive of such sensitivity, of such a furious, violent nature, that it is never allowed to remain long in its primitive condition, but is as quickly as possible changed into something less excitable.
Glycerine is one of those organic compounds which is obtained from once-living matter. Arising as a by-product in the manufacture of soap, it consists, as do so many of the organic substances, of carbon and hydrogen, the atoms of which are peculiarly arranged
to form the glycerine molecule. To this the nitric acid adds oxygen and nitrogen, the sulphuric acid simply standing by, as it were,
and removing the surplus water which arises during the process. So while glycerine is carbon and hydrogen, nitro-glycerine is carbon, hydrogen, nitrogen and oxygen. In this state they form a compact liquid, which occupies little space.
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[12] 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, contain-
ing 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
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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[13] 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[14] 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.
Beyond the reach of any plough, this hard stratum can be broken up by the use of dynamite. The usual method is to drive holes in the ground about fifteen to twenty feet apart and about three or four feet deep, right into the heart of the hard layer. At the bottom of each hole is placed a cartridge of dynamite with a fuse and a detonator. This latter is a small tube containing a small quantity of explosive which, unlike the dynamite, can be easily fired, and initiates the detonation of the cartridge.
When these miniature earthquakes have taken place all over a field a very different state of things prevails. The "hard-pan" has been broken. The explosive used for such a purpose has a sudden shattering power, whereby it pulverises the ground in its vicinity rather than making a great upheaval at the surface. The sudden shock makes cracks and fissures in all directions, through which roots can easily make their way. Moreover, it permits air to find an entrance, thereby aerating the soil in such a way as to increase its fertility. The heat, or else the chemical products of the explosion, seem to destroy the fungus germs in the ground. Finally a natural storage of water is set up. Heavy rain, instead of drenching the upper soil, simply moistens it nicely, while the surplus water descends into the