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ROYAL INSTITUTION OF GREAT BRITAIN.
Weekly Evening Meeting, March 21, 1862.
A Lecture, by F. A. Abel, Esq., F.R.S.,
on Some of the Causes, Effects, and Military
Applications of Explosions.
The full lecture can be read later in this post, but here are the interesting parts on explosives -
In a case of feeble chemical action, such as that of an acid upon a weak base, heat is developed, but slowly and to comparatively a slight extent, because of the very gradual nature of the combination. Thus, if we dissolve this oxide of zinc, which is a weak base, in acid, the heat developed will only gradually melt the very fusible material with which this vessel is coated. This is an example of feeble chemical action. If we proceed a step further, taking, for example, such a substance as this oxide of phosphorus, which possesses great chemical affinity for water, and we allow it to combine with a small quantity of that liquid, we observe that it will do so much more energetically than the oxide of zinc did with the acid, producing almost an explosive result. We have a proportion of the water suddenly converted into vapour by the heat developed, which is sufficiently intense to ignite a highly combustible material, as we find if we allow guncotton to come into contact with the oxide of phosphorus which we are combining with water. If two very active elementary bodies, such as bromine on the one hand, and potassium on the other, are brought together, we find that a still more violently rapid combination takes place. No substance existing naturally in the form of a vapour or gas is produced by their combination; it is simply here the intense heat, suddenly generated by bringing these two substances together, which suffices to produce a powerfully explosive result, by instantaneously generating a quantity of vapour. And, lastly, if we take a mixture of two gases, such as hydrogen and oxygen, or hydrogen and chlorine, and confine them as we have in this glass vessel, we know that the mere momentary contact of some portion of the mixture with a body raised to a high temperature, or the passage of an electric spark through it, produces instantaneous combination throughout the whole volume of gas. We will fire these mixed gases in this vessel by the passage of the electric spark through them. A violent explosion takes place, and the vessel is shattered into innumerable minute fragments, in consequence of the enormous force suddenly exerted by the intensely heated product of combination of the gases.
These are one or two instances in which combination produces explosion. We can produce a much greater variety of examples in which explosion is the result of the instantaneous or very rapid decomposition of a chemical compound. We are acquainted with several classes of compounds remarkably unstable in their character, these are to be found particularly among the bodies which we term organic; there are, however, a few inorganic compounds which are also remarkable for their instability; 6uch are the combinations of hydrogen and nitrogen, with chlorine, bromine, and iodine. This, for instance, is the iodide of nitrogen, with which you are well acquainted: observe its great tendency to explode. [A small quantity was exploded by being touched with a piece of folded paper.] If it is sufficiently dry, a very slight touch will cause the explosion. It is not quite dry, or I might show that the explosion may also be produced simply by allowing the substance to fall upon water; but I have sufficiently attained my object, which was to show you the highly explosive nature of this substance. The combinations of mercury and of silver with carbon, nitrogen, and oxygen, which we know as the fulminates, are remarkably explosive in their character. When these fulminates are perfectly dry, a very slight blow or a very small amount of friction is sufficient to bring about their decomposition. Thus, here is a small quantity of fulminate of mercury; you will observe that a very slight application of heat to this is sufficient to cause it to undergo decomposition. It inflames with a dull sort of sound, which could, of course, be rendered more violent if the particles were confined. Here is some fulminate of silver, which is much more explosive in its character. We will take a much smaller portion of this than of the other fulminate, and place it upon the copper, and submit it to the action of the heat. [This was done ] You see it explodes much more readily and violently, and we perforate the copper instantly; while in the case of the mercury compound, the copper was hardly indented. As I have said, the explosive characters are exhibited by various organic compounds—bodies not of natural occurrence, but produced from non-explosive organic substances by the action of an acid remarkable for the amount of oxygen which it contains, and for the tendency which it has to impart that oxygen to other substances—nitric acid. This nitric acid, as many of you know, may be made to produce changes in organic substances, resulting in the oxidation of a proportion of hydrogen-atoms in the organic structure, and their removal in the form of water; a corresponding proportion of the partially de-oxidised acid (nitrous acid) passing into the space created by the abstraction of the hydrogen from the group; and thus we can produce, for example, from cotton, from cellulose, or lignine, highly explosive substances. Here is one of these bodies—the well-known gun-cotton [holding it to the spirit-lamp and so firing it.] If we confine a small quantity of it in a little vessel of this description (a small glass globe), and fire it, which we can readily do by applying this hot iron to the exterior, we shatter the vessel; so that you see that the substance which appeared but feeble in its explosive power when burnt in the open air, may readily be made to produce powerful effects. There is also an explosive substance produced by a similar action of nitric acid upon the sweet principle of manna, beet-root, parsnips, or onions, known as mannite. Again, we are enabled by the mere contact of nitric acid, in its most concentrated form, at a low temperature, with glycerine, to produce a substance of a highly explosive character known as nitroglycerine or glonoine. It is only necessary to moisten a small portion of filtering-paper with a little of this substance, and then to expose this to a moderately violent blow, in order to show its explosive nature. [A piece of filtering-paper moistened with the compound of glycerine and acid was placed on an anvil and struck with a hammer.]
Here is an explosive substance belonging to this class,of very recent origin, a member of a most interesting family, one of the derivatives of that remarkable substance, aniline, to which we are indebted for those beautiful new colours, Mauve and Magenta, of whose history we shortly expect an interesting account from Dr. Hofmann. It is curious that this body, aniline, which has become of such importance in connection with arts and manufactures, should also exhibit what may be called "warlike tendencies." By the action of nitrous acid at a low temperature upon aniline, the explosive substance to which 1 refer is produced. This singular body rejoices, I am happy to say, in the comparatively simple name of nitrate of diazobenzol; compared with the names which are possessed by some of the members of this family, this is certainly not a hard one. [A small quantity was exploded on copper foil, which was shattered.] You see that it appears quite equal in its explosive power to fulminate of silver. I cannot help devoting a few moments to a comparison of its explosive properties with those of fulminate of silver. The silver compound explodes with a slight touch given by a hard substance; whereas this new compound can actually be rubbed between hard surfaces without exploding ; at least, I have found it so on frequent occasions, although I have finally exploded it in that way. Some time is necessary, however, for producing, by friction, the requisite heat for its decomposition. The slightest touch, you observe, explodes fulminate of silver, and it does not admit of being rubbed. The nitrate of diazobenzol, when compared with the fulminate in this way, appears to be the less explosive substance; and yet, when directly exposed to heat, it is certainly the most sensitive of the two. If we expose the fulminate of silver in this little tin tube to the heat of boiling water, I think we shall find that it is not affected. [The experiment was performed with the result anticipated.] But if I heat, in the same manner, a little of this fulminate of aniline—if I may use the term— (although there are one or two compounds, also derivatives of aniline, still more recently discovered by Dr. Hofmann, which may claim the title,) if I similarly heat this nitrate of diazobenzol, you will find that it will undergo decomposition as soon as the tube has reached the temperature of the water. There we have a very violent explosion; the tube is shattered, and has been thrown out of the water-bath by the force of the explosion. The fulminate of silver has not exploded, but we had better get rid of it. I shall be able to explode it by exposing it to this source of heat [the flame of the spirit lamp], for it explodes at more than double the temperature of boiling water, or about the temperature of melting tin, so that as soon as the tube begins to melt, the fulminate of silver will explode. [The explosion shortly ensued.] We see, therefore, that this fulminate of silver appears far less sensitive, when exposed direct to heat, and more sensitive when submitted to friction, than the derivative of aniline.
Having glanced at the nature of explosive chemical compounds, we will pass on to explosions which result from the reaction upon each other of substances in a state of mechanical mixture,—not chemically combined. We know that if we mix together some substances which differ widely in their chemical properties, we may, under favourable circumstances, bring about the ignition or explosion of the mixture. We have a number of substances which may be applied to the preparation of such mixtures. For instance, here are, on the one hand, substances all rich in oxygen, oxides of manganese and lead, and combinations of metals with nitric acid or the corresponding acid, chloric acid. These substances have a tendency to part with their oxygen, and, therefore, bodies which, on the other hand, are easily oxidisable, such as sulphur or phosphorus (which we can employ with comparative safety in the amorphous state), or the sulphides of arsenic and of antimony, when brought into a state . of mixture with these sources of oxygen, will produce materials endowed with explosive properties. Thus, for example, we have here a mixture of binoxide of lead with sulphur, which ignites most readily. We will just apply heat to this for a very short time, and we shall nave chemical action established. You see the mixture burns violently, the heat produced has actually broken the tube. Here we have a compound of a high oxide of manganese. We mix this together with iron in a very fine state of division. It is enabled to oxidise that iron very rapidly on the application of heat, and produces an effect bordering upon an explosion in its character. Here we have the particles of iron burning beautifully in the oxygen imparted to them from this oxide of manganese. Then, again, if we take the same substance, together with sulphur, and mix them in this mortar, we can do so in safety, but, on applying a small amount of pressure, the mixture will ignite. You see how beautifully it burns when I have mixed the substances with a sufficient degree of intimacy. And if we take these compounds, many of which are familiar to you, nitric and chloric acids, in combination with various metals, such as potassium, sodium, or barium, we impart to them explosive characters by simply mingling them with an oxidisable substance such as resin or sulphur. We have there chlorate of baryta mixed with a small quantity of resin. Observe how violently it burns, with a bright green-coloured flame. Here are mixtures of chlorate of potash with charcoal,— and in effecting their ignition, I wish to direct your attention to the circumstance that the two mixtures have different degrees of rapidity of burning, as this affords me an opportunity of pointing out to you the very great influence which may be exerted upon the properties of an explosive mixture, quite independent of the chemical properties of the substance, by the degree of intimacy with which its constituents are mixed together. The more perfect the mixture, the more rapid and complete will be the transformation suffered by the components, and, consequently, the more powerful will be the explosion. I need hardly observe that the characters of these mixtures are regulated also by the chemical and physical properties of their constituents; for instance, some bodies are more readily oxidisable than others, and partly, sometimes, because they are more easily convertible into vapour. A comparison of the facility with which sulphur and charcoal are oxidised, will furnish an illustration of this. Sulphur, as you know, is melted and vaporised at a comparatively low temperature, while charcoal is not at all similarly affected; and, therefore, though the latter possesses a most powerful tendency to combine with, oxygen, sulphur may be readily oxidised or inflamed at a very much lower temperature. I bring forward this example because it bears particularly upon the subject of gunpowder, in which the high degree of inflammability of sulphur is turned to account to promote the very rapid oxidation of the charcoal. I have here, in an atmosphere of oxygen, the two substances, carbon and sulphur, in a fine state of division. I shall place over the mouth of this vessel which confines the oxygen, a piece of heated iron gauze, which will not be hot enough to ignite the charcoal when I puff it upwards so as to bring it into contact with the metal. But while that charcoal still remains suspended in the oxygen, I will throw up some sulphur, which will inflame directly it reaches the iron, and will ignite the particles of the charcoal, combustion being thus brought about almost instantaneously throughout the mixture of oxygen and carbon. [The oxygen was contained in a loosely-covered glass jar, into the lower end of which two tubes were fixed communicating with the receptacles containing the charcoal and the sulphur. These two substances were respectively blown, at an interval of about two seconds, to the top of the jar, and so thrown into contact with the hot piece of iron. No change was apparent when the charcoal was blown up, but immediately upon the arrival of the sulphur to the top, combustion ensued throughout the whole mass, producing for an instant a very dazzling body of light.]
The substances which furnish the oxygen in explosive mixtures exhibit equally important differences in their properties, which also assist us in regulating the characters of explosive mixtures. For the purpose of illustrating this, I beg to direct your attention to a very simple experiment with the two well-known substances, nitrate of potash and chlorate of potash. Here are mixtures of these two bodies with amorphous phosphorus (which we employ as a substance very easily oxidised, although its use requires very great care, as it is liable to oxidation in such mixtures on a very slight impulse). This is the mixture with nitrate of potash; you see, it simply burns brilliantly when heat is applied. And here is the phosphorus mixed with chlorate of potash, which ignites immediately with a violent explosion, and the copper on which it rested is shattered.
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