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little cloud in the distance no bigger than a man's hand, which in ages yet to come, will develop into large clouds, sending down refreshing showers, that shall spread intelligence and life and knowledge over the whole surface of the earth.

I see my time is up; I can only say therefore, in conclusion, that if I apprehend rightly the essential objects of these lectures, they may be appropriately summed up as those of mutual improvement and the investigation of truth, the development of the seeds of knowledge and the detection of falsehood, the emancipation of the mind from the fetters of ignorance, and the cultivation of a true humanity by social gatherings and intellectual discourse.

GUN COTTON.

A LECTURE, Delivered in the Hulme Town Hall, Manchester, on
Wednesday, November 19th, 1873.

By F. A. ABEL, ESQ., F.R.S.

EFORE I say a word to you about gun cotton, I should like to call your attention, for a few moments, to a substance with which it has, during the last forty years, attempted to stand in rivalry, namely, gunpowder; because I believe that if I can in a few words make you understand the nature of gunpowder, that of gun cotton will be much clearer to you afterwards. Now, gunpowder consists of three substances very intimately mixed together-the substance charcoal, made by charring wood; sulphur, a substance found in its elementary or native condition, and also extracted from several minerals; and saltpetre, a natural product originating in the gradual decomposition of a variety of vegetable and animal substances. Now, when gunpowder is subjected to the action of heat, or contact with flame, with a spark, or red-hot body of any kind, the following results are produced both charcoal and sulphur are very readily oxidisable, or eager to burn at any time. We know how readily a piece of wood kindles when it is heated to a sufficient degree in air-we know how much more readily a piece of sulphur burns; both of them combining with a certain portion of the air which we call oxygen; and, therefore, we call these two bodies "combustible" bodies-charcoal, occurring in wood, and known to the chemist as carbon; sulphur, a substance extracted from various minerals.

These two bodies burn very readily if we heat them in air : they burn still more readily if we heat them in oxygen, or if brought into contact with some substance which is ready to give

its oxygen to them; and this oxygen is derived, in the case of gunpowder, from the substance saltpetre. Saltpetre is a body which contains a large quantity of oxygen. It holds this oxygen in what we chemists call a feeble state of combination; that is to say, it is ready to give this oxygen to other substances greedy of it. And, therefore, if we heat sulphur or carbon (charcoal) in contact with saltpetre, it gives up to these bodies the oxygen which they require to burn, and then they burn rapidly and are converted in the course of that burning into gaseous substances. In passing from the state of solid to the state of gases, they assume many times their original volume; and at the moment at which they pass from the solid to the gaseous state a quantity of heat is generated by the chemical action that takes place between these substances. The result of all this is that the original solid mixture becomes expanded to very many times its volume, and this transformation occurs with great rapidity.

The almost sudden conversion of a solid, occupying little space. into gas occupying a great space, and which is, moreover, highly expanded by the heat developed during the transformation, gives rise to the development of a large amount of force, which has the power of overcoming great obstacles, or impediments to the violent expansion of the substance. The sudden or very rapid transformation of a solid or liquid into gas or vapour is generally productive of noise, and is attended by some more or less violent demonstration of force. It is called an explosion; and any substance which is susceptible of undergoing such transformation suddenly or very rapidly on the application of heat, or other disturbing cause, is called explosive. Therefore we call gunpowder an explosive substance.

Now the oxidising or burning component of gunpowdersaltpetre contains a metal possessed of some very interesting properties, into which I cannot now enter; and this metal holds in combination with it a substance consisting of nitrogen combined with a large quantity of oxygen, both of which are gases existing in the air, as you may remember. Oxygen is ready to escape in the form of gas from any compound in which it is contained in a solid condition, combined, but not very strongly, with other substances. Thus, if we heat this saltpetre sufficiently, we can make it give up its oxygen without the assistance of the sulphur or the charcoal. If we continue to heat it very strongly we can also liberate the nitrogen from it. But if we heat it only moderately, when mixed with some substance which has a very great liking for the metal it contains, such as this poweriul acid,

known as oil of vitriol, we then obtain from this saltpetre, not oxygen and nitrogen separately, but a combination of them with water- —a fluid called nitric acid, which used to be called by old chemists aquafortis, or strong water, on account of its highly corrosive properties. Now nitric acid behaves very much in the same way towards sulphur and charcoal as the original saltpetre does. If we drop a piece of saltpetre upon red-hot charcoal it begins to deflagrate; that is, the charcoal begins to be burned very violently by the oxygen contained in the saltpetre. If we allow a drop of this nitric acid to fall upon red-hot charcoal, the action is just the same as if we allowed saltpetre to fall upon it. Nitric acid acts upon substances which are easily burned or oxidised even more readily than saltpetre. It does not require the assistance of heat to such an extent to develope its action, and to cause it to burn up the sulphur and charcoal. This nitric acid is manufactured by putting saltpetre and oil of vitriol into a retort together and applying heat, when this most valuable chemical agent distils over as a pale yellow fuming liquid. It is one of the most useful agents in manufacturing and scientific chemistry. I hope now I have made you understand generally what gunpowder is, and what nitric acid is, and, if so, I believe you will have little difficulty in understanding what gun cotton is. I will, therefore, proceed with my history of this remarkable substance.

About forty years ago some new substances were first produced in France which excited considerable curiosity amongst chemists. They were obtained by acting, with the highly corrosive liquid which we obtain from saltpetre, upon the well-known substance, starch, and upon cotton fabrics, such as muslin or calico, and even paper. When cold nitric acid was added to starch this substance was dissolved, and, upon adding water, the starch appeared to be separated again, but it had no longer the properties of the original substance-it had become endowed with explosive properties. When muslin or paper were dipped into nitric acid, allowed to remain for a very short time in that liquid, and then taken out and washed, though they appeared to be unaltered in character, except that they had become somewhat tender, there was a most remarkable alteration in their chemical properties, for, instead of being simply inflammable, that is, instead of burning quietly when set fire to, they burned very rapidly after this treatment, and almost with explosive violence. I have here specimens of paper and of calico which have been submitted to this kind of treatment, and you perceive how very quickly and brilliantly they burn. [The lecturer ignited the paper and muslin, which were instantly

consumed. These and numerous subsequent experiments were very successful, and were much applauded.]

The discovery of these substances was very interesting, because they were really the foundation of the discovery of gun cotton. About fourteen years afterwards a German chemist, Schönbein, found that if common cotton wool was submitted for a short time to the action of cold very strong nitric acid, its weight was increased about 80 per cent; that is, 100 parts of the original cotton, after having been steeped in acid and washed and dried, furnished about 180 parts of what still appeared to be cotton wool. But this substance, when thus treated, was found to have the properties of the paper and muslin products which have been described, only to a more marked extent. Here is a portion of finely-carded cotton wool which has been treated in this way. As you perceive, it burns almost instantaneously. We observe no smoke; and, if we light a piece upon a perfectly clean plate, we shall find that there is nothing left-that the whole goes off in the form of gas and vapour. Gunpowder, when it is set fire to, leaves a considerable black residuum, which consists of certain solid substances formed besides the gases, which are generated as I have described. You have noticed how rapidly this gun cotton burns. It burns so very rapidly that, if I am successful in the experiment-which I never fail in when performing it by myself I may even wrap some grains of gunpowder in gun cotton, which, when set fire to, will burn, leaving the gunpowder unburnt, simply because the gun cotton burns so rapidly that there is no time for the flame produced to set fire to the mixture of charcoal, sulphur, and saltpetre. [Experiment.]

Now, this gun cotton ignites at a very much lower heat than gunpowder, which will not inflame at a heat lower than that at which sulphur begins to burn, about 560 degrees on Fahrenheit's scale, and will then only ignite gradually. Gun cotton will ignite at a temperature of about 300 degrees Fahrenheit, which is still a high temperature, though very low as compared with that at which gunpowder inflames. I will heat a small quantity of this gun cotton in a tube, which I have lightly closed with a cork. You see in how very short a time the gun cotton ignites, with a slight explosion, accompanied by a sudden flash of flame, which I daresay many of you did not see, as you were not prepared for it.

There is another peculiarity about gun cotton when compared with gunpowder, namely, that it is comparatively readily ignited by a blow. I say comparatively, because, though

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