Marvels of Scientific Invention. Thomas W. Corbin

The Working of a Refrigerating Machine 70 6. Hertz's Machine 155 7. Hertz "Detector" 156 8. 9. 10. Wireless Waves 158 11. A Wireless Antenna 164 12. Poulsen's Machine 166 13. 14. How Pictures are sent by Wire 177 15. Message received by Telewriter 189

MARVELS OF SCIENTIFIC

       INVENTION

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CHAPTER I

       Table of Contents

DIGGING WITH DYNAMITE

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      Most people are afraid of the word explosion and shudder with apprehension at the mention of dynamite. The latter, particularly, conjures up visions of anarchists, bombs, and all manner of wickedness. Yet the time seems to be coming when every farmer will regard explosives, of the general type known to the public as dynamite, as among his most trusty implements. It is so already in some places. In the United States explosives have been used for years, owing to the exertions of the Du Pont Powder Company, while Messrs Curtiss' and Harvey, and Messrs Nobels, the great explosive manufacturers, are busy introducing them in Great Britain.

      It will perhaps be interesting first of all to see what this terror-striking compound is. One essential feature is the harmless gas which constitutes the bulk of our atmosphere, nitrogen. Ordinarily one of the most lazy, inactive, inert of substances, this gas will, under certain circumstances, enter into combination with others, and when it does so it becomes in some cases the very reverse of its usual peaceful, lethargic self. It is as if it entered reluctantly into these compounds and so introduced an element of instability into them. It is like a dissatisfied partner in a business, ready to break up the whole combination on very slight provocation.

      And it must be remembered that an explosive is simply some chemical compound which can change suddenly into something else of much larger volume. Water, when boiled, increases to about 1600 times its own volume of steam, and if it were possible to bring about the change suddenly water would be a fairly powerful explosive. Coal burnt in a fire changes, with oxygen from the atmosphere, into carbonic acid gas, and the volume of that latter which is so produced is much more than that of the combined volumes of the oxygen and coal. When the burning takes place in a grate or furnace we see nothing at all like an explosion, for the simple reason that the change takes place gradually. That is necessarily so since the coal and oxygen are only in contact at the surface of the former. If, however, we grind the coal to a very fine powder and mix it well with air, then each fine particle is in contact with oxygen and can burn instantly. Hence coal-dust in air is an explosive. It used to be thought that colliery accidents were due entirely to the explosion of methane, a gas which is given off by the coal, but it has of recent years dawned upon people that it is the coal-dust in the mine which really does the damage. The explosion of methane stirs up the dust, which then explodes. The former is comparatively harmless, but it acts as the trigger or detonator which lets loose the force pent up in the innocent-looking coal-dust. Hence the greatest efforts in modern collieries are bent towards ridding the workings of dust or else damping it or in some other way preventing it from being stirred up into the dangerous state.

      So the essential feature of any explosive is oxygen and something which will burn with it. If it be a solid or liquid the oxygen must be a part of the combination or mixture, for it cannot get air from the surrounding atmosphere quickly enough to explode; and, moreover, it is generally necessary that explosives should work in a confined space away from all contact with air. So oxygen, of necessity, must be an integral part of the stuff itself. But when oxygen combines with anything it usually clings rather tenaciously to its place in the compound and is not easily disturbed quickly, and that is where the nitrogen seems to find its part. It supplies the disturbing element in what would otherwise be a harmonious combination, so that the oxygen and the burnable substances readily split up and form a new combination, with the nitrogen left out.

      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 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,