LECTURE X.

GUNPOWDER AND NITRO-GLYCERINE.

THERE is one further point in connection with the theory of combustion to which I wish to call your attention, at the outset of my lecture this evening. In the only cases of burning we have studied, the combustible unites with the oxygen of the atmosphere. It is possible, however, to have combustion without atmospheric air, the combustible obtaining the required oxygen from some associated substance. There are several substances in which a large amount of oxygen is so loosely combined, or, in other words, in which the oxygen-atoms are held in combination by such a feeble force, that they will furnish oxygen to the combustible as readily as the atmosphere, and in a vastly more concentrated form. Two of these substances are well known, nitre (potassic nitrate) and chlorate of potash (potassic chlorate). One ounce of this last salt-the quantity in this small crucible-contains enough oxygen to fill a large jar (1.7 gallon), and by simply heating the salt we should obtain that amount of oxygen gas. We have provided also one-third of an ounce of pulverized sugar, and we will now mix the two powders thoroughly together. Consider the conditions in this

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mixture: The sugar is a combustible substance, and every particle of this combustible is in contact with, or, I should rather say, in close proximity to, grains of chlorate of potassa, which contain sufficient oxygen to burn the whole. All is now quiescent, because both materials, being in the solid condition, their molecules are, as it were, imprisoned, and a certain degree of molecular activity is required to produce chemical change. This molecular activity we can readily excite by heat, but a more convenient, although less intelligible way, is to touch the mixture with a drop of sulphuric acid.

Here we have not merely a pretty firework, but an experiment which illustrates a very important phase of the phenomena of combustion, and one of immense practical value. I have chosen this particular example because you are familiar with both of the materials employed. You have seen that sugar contains a large amount of combustible carbon. You also know that potassic chlorate contains a large volume of oxygen, which can readily be driven off by heat; for you have seen me make oxygen from this very salt. You can, therefore, fully appreciate the conditions we had in our crucible at the beginning of the experiment, namely, a combustible with the oxygen required to burn it in close proximity. You will be prepared, then, to understand— 1. That the burning we have just witnessed does not differ from ordinary burning, except in the single point I have mentioned; that the combustible derives its oxygen from potassic chlorate, instead of from the air; and, 2. that it is possible to inclose in a confined space, as a gun-barrel or a bomb, all the conditions of combustion. In a word this experiment illustrates the simple theory of gunpowder.

What, then, is gunpowder? Essentially a mixture

of two substances-saltpetre and charcoal, with merely a small amount of sulphur added to facilitate the kindling of the charcoal. In the manufacture of this explosive agent, as is well known, the materials are first reduced to a very fine powder, and then intimately mixed together. Afterward, by great pressure, the mass is compacted to a firm, hard cake, which is subsequently broken up into grains of different sizes, adapted to various uses. Here we have some samples of these grains, varying from the size of a walnut to that of a millet-seed. These black grains, although they appear so homogeneous, are, in fact, a very intimate mixture of a combustible material (charcoal and a little sulphur) with a substance rich in oxygen (saltpetre), and, when we ignite the powder, the charcoal burns at the expense of the oxygen of the saltpetre. Two parallel experiments will make the whole matter clear.

In this jar we have about one gallon (100 grains) of pure oxygen, enough to combine with 37 grains of charcoal. This quantity of charcoal we will place in a copper spoon, and, having ignited the coal, we will plunge it into the jar of oxygen. We have at once a brilliant combustion, and a repetition of the experiment which you witnessed at the last lecture. We then learned that the process consists in the union of the oxygen with the carbon, and that each molecule of oxygen gas actually picks up an atom of carbon to form a molecule of carbonic dioxide. There are, therefore, just as many molecules in the jar at the close of the experiment as at the first, only they now consist of three atoms, instead of two; O-O has become O-C-O.

In the second jar is a cup containing a small quantity of gunpowder, and so arranged that the powder

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can be exploded by a voltaic battery. As the oxygenatoms required for the burning are lying in the cup side by side with the charcoal, we do not need the air in our experiment. Accordingly, we have connected the jar with an air-pump, so that we can exhaust the air. . . . The gauge of the pump now indicates that the greater part of the air has been removed. Notice further that, when we readmit a little air, the mercury column falls, and thus, as you see, this gauge will tell us when any gas enters the jar. . . . Having again completed the exhaustion, let us fire the powder. The powder has disappeared; but the gauge indicates that a large volume of gas has been formed.

A simple test will now show that the aëriform products in the two last experiments are identical. Here are two glasses, each filled with lime-water. To one we will add some of the gas from the first jar, pouring it in upon the lime-water, and to the other we will add some of the gas from the gunpowder, by pouring as before. On shaking the gas and liquid together, we. obtain in both cases the familiar milky turbidness which indicates the presence of carbonic dioxide. It is true that the carbonic dioxide from the gunpowder is not quite so pure as that found in the other jar, but this is an unessential matter.

Having seen that gunpowder, burnt in a vacuum, is quietly resolved into gas, we will next take an equal amount of powder and inclose it in a pasteboard case, which we call a cartridge, using the same arrangement for firing the powder as before. We make the connection, and off it goes!... There can be no occasion, I think, to seek far for the cause of the explosion. The chemical process must have been identical with that in our jar; but, while in the jar there was room for all the

gas-molecules formed in the burning, the small volume of the cartridge could not hold them, and they burst out, tearing away the paper walls in their course. The gas evolved would occupy, at the ordinary pressure of the air, about three hundred times the volume of the powder used, and, if confined in the space previously filled with the powder, would exert a pressure equal to about 300 x 14 = 4,200 lbs., or two tons, on a square-inch. The pressure obtained is really far greater than this, on account of the heat developed by the combustion. Moreover, as the powder burns rapidly, this pressure is suddenly applied, and has all the effect of an immensely heavy blow, which no strength of materials is sufficient to withstand. Of course, any chamber in which the powder is confined gives way at the weakest point. In the chamber of a gun the ball usually yields before the breech, and is hurled with violence from the mouth of the piece; but fearful accidents not unfrequently occur when, for any reason, the ball has been too tightly wedged, or when the metal of the breech is too weak.

You all know that a large amount of gas condensed into a small chamber must exert great pressure, and therefore you will undoubtedly regard the explanation I have given of the force exerted by gunpowder as satisfactory and sufficient. But, although this is the usual way of presenting the phenomena, I am anxious that you should view them in the light of our modern molecular theory, which gives to the imagination a far more vivid picture of the manner in which the power acts.

Begin with the black grains as they lie in the chamber of the gun behind the ball. You must remember that all the ingredients of the powder are in a solid

condition, and picture to your imagination the mole