In treating of the subject of explosion I shall deal only with those explosive bodies which are in general use as projectile or blasting agents and whose explosive properties result from the re-arranging of their constituent atoms.
To learn the nature of the changes which take place during an explosion it is necessary that we should know the composition of the explosive body and of the products of the explosions. The apparatus used in determining these facts is so elaborate and the analytical processes occupy so long a time that it is impossible for me to undertake them before you but I have brought together these facts in the diagram before you which is as follows:
- Gunpowder 2KNO3 + 3C + S=3 CO2 + N2 + K2S
- Gun Cotton 2 C6H7 (NO2)3 O5 = 3CO2 + 9CO + 3 N2 + 7 H2O
- Tri-nitro-glycerine 2C3H5 (NO2)3 O3 =6CO2 + 5H2O + N2O + 2N2
- Detonating gas 2 H2 + O2 =2H2O
- Combustion of carbon in oxygen C + O2 = CO2
As gunpowder is a mechanical mixture the relative proportions of the nitre, charcoal and sulphur in the mixture may of course vary to almost any extent, but the above formula closely represents the composition of the U. S. Regulation powder. There has been considerable discussion as to the composition of gun cotton, but the formula given in the diagram is the one which is accepted by Abel in his paper (Researches on Gun Cotton, Phil. Trans, ,Vol. 156, pg. 269), and which will I believe find general acceptance among chemists. The formula given for tri-nitro-glycerine is undisputed.
When, however, we come to consider the products of the explosion we find great difficulty in arriving at any exact knowledge of their composition. If we alter the conditions of temperature or pressure under which a chemical change takes place we alter materially the character of the resulting products and hence it is obvious to any one that the substances that will be produced when an explosive is burned under great pressure in a gun will differ very much from those which will be formed when it is burned in an open tube or only under such slight pressure as obtains generally in laboratory experiments. Especially is this so with gunpowder, for from a complex mixture much more complex products must result. We have but to examine the experimental results obtained by Bunsen & Schischkoff, by Karolyi and several others, but especially those obtained by Noble and Abel in their elaborate and exact Researches on Fired Gunpowder, (Phil, Trans, Vol. 165, pg. 49,) to learn how very complex and variable these products are. So complex are they that it is well-nigh, if not quite, impossible to formulate them.
Granting then that the formula for the products given in the diagram do not ever represent what takes place in practice, they do represent what would result if certain theoretical conditions obtained, and these conditions have been secured for gun-cotton and nitro-glycerine, and they answer our purpose in showing the principal and most efficient substances that are present in the products of every explosion of the bodies we are studying.
Accepting then these formulae as representing approximately what takes place, let us inspect them and our attention is called to the fact that in every case we have carbon and oxygen present in the explosive and a compound of carbon and oxygen (CO2) resulting from each explosion. These are the substances which we employ in our ordinary processes of combustion and the products of the processes are similar compounds of carbon and oxygen. Let us expose one of our explosive agents freely in the open air and ignite it, and it will exhibit all the phenomena of the combustion of charcoal, only more markedly. Hence it is probable that from a study of the phenomena of combustion under such conditions that we may be able to control their operation, we may gain some insight into the cause of the explosion and the source of the energy of these explosive bodies.
I will perform the well known experiment of burning charcoal in oxygen. You now observe how much more readily the charcoal burns and how much more brightly it glows than when burned as usual in the open air. The change that is going on is identical with what takes place when the charcoal is burned in a grate, but whereas in this last case the oxygen comes to it diluted with four parts of inert nitrogen, in the experiment we are witnessing the charcoal is in contact with oxygen only. The chemical change is represented in reaction (5) on the diagram.
As we witness this experiment this question naturally arises; what is the cause of this manifestation of energy? The experiment shows that when ignited carbon is brought in contact with oxygen combustion takes place, but how is the development of heat and light accounted for?
The doctrine of the conservation of energy teaches us that whenever motion is arrested the energy of the moving mass is not lost, but is converted into some other form of energy. When we bore a piece of iron, friction arrests the motion of the bit, but the mechanical energy of the bit is converted into heat and the bit becomes warm. When a ball is fired against a target which it cannot penetrate, its motion of translation is arrested but the mechanical energy of the mass is converted into molecular energy and the ball becomes heated. If it is moving with a high velocity at the time of impact its temperature may become so high that the shot will glow and be fused. So if we place a nail upon an anvil and strike it with a hammer we are able to heat the nail by the conversion of the mechanical energy of the falling hammer into the energy of heat, and the greater the energy of the blows which fall upon the nail, and the greater the number of the blows which fall in a given time, the higher will be the temperature of the nail. Since the mass of the nail cannot be moved its molecules are set in motion thus producing the phenomena of heat, and the more rapid these molecular motions are, and the greater the distance over which the molecules move the more intense will be this manifestation of energy and the higher wall the temperature produced be.
We see from these examples that when the molecules of a body are set in rapid motion heat is produced, and that when the motion of a body is arrested it communicates its motion to the body against which it strikes and causes either the body struck or its molecules to move. Keeping these principles in mind let us return to the combustion of the charcoal in the jar of oxygen. When we introduced the charcoal into the jar it was heated up to such a point that it could combine with the oxygen.
As the oxygen was in a gaseous state its molecules had great freedom of motion and hence could come in sufficiently close contact with the molecules on the surface of the charcoal, to be influenced by the force of chemical attraction. Urged on by the force of chemical attraction the atoms in these molecules rushed against each other in order to unite and as these motions were arrested by impact heat was produced. As these sub-divisions of matter were very small the energy of any one was insignificant but they were myriad in number, and as they were moving with a very high velocity the sum of their energies was very great and was sufficient to produce the intense heat and light emitted by the charcoal.
This explanation is, of course, largely hypothetical, but if it be at all a true one, it follows that if we can increase the number of blows delivered in a given time against a mass of charcoal we shall be able to increase the velocity of the motion of the molecules of the charcoal and hence raise their temperature, and consequently the manifestation of energy will be more intense. The most obvious way to do this is by increasing the surface of the charcoal and this can easily be done by powdering it. When we now throw this glowing powdered charcoal into the oxygen gas we have a vivid combustion which is much more brilliant and much more intense than in the first experiment.
In the ordinary process of combustion we observe that the different substances used ignite at different temperatures. Thus phosphorus we know is heated up to the point of ignition by simple friction, sulphur burns at a higher temperature, soft wood still higher, charcoal next and then hard coal, and we make use of all these, in the order given, in building fires, for we cover a piece of dry, soft wood with sulphur and tip this with phosphorus. Then by friction the phosphorus is ignited, this fires the sulphur, this heats the wood so it burns, then the paper or shavings are lighted, next the charcoal and finally the anthracite. We can show this difference in the ignition point by placing a piece of phosphorus one of sulphur and one of wood upon an iron plate and putting a lamp beneath. In a short time the phosphorus takes fire, after some time the sulphur, and the wood does not burn at all.
Since phosphorus burns so readily we may expect it to unite very readily when exposed to pure oxygen. When the experiment is performed you see how rapidly the union takes place and how brilliant the experiment is. If we subdivide the phosphorus or increase its surface it combines even more readily with oxygen, so readily indeed that it will take fire in the open air without the aid of friction, simply by the heat generated by the condensation of the oxygen in the interstices between the particles. This experiment is performed by dissolving the phosphorus in bisulphide of carbon. A piece of bibulous paper is moistened with the solution and exposed to the air. The bisulphide evaporates and leaves the phosphorus distributed in a finely divided thin layer, quite over the surface of the paper. The oxygen unites with phosphorus and owing to the large surface of the phosphorus this union goes on very rapidly and the heat generated raises the uncombined phosphorus to its point of ignition and it takes fire.
I wish to call your attention to but one other instance of this kind, viz. the union of hydrogen with oxygen. Hydrogen is as you see by the diagram one of the constituents of both gun cotton and nitro-glycerine and a compound of hydrogen and oxygen is found in the products of the explosion. As they are both gases I will cause them to unite by igniting them at this jet. Since they are both gases we should expect the mixture to be very intimate and the energy developed in a given time to be greater than if one was a solid or liquid. That the energy developed is very great is shown by placing a file in the flame when we see that it is rapidly burnt, or by turning it against a piece of lime when we see that the lime becomes so highly heated as to emit an intense white light. That the gases are intimately mixed is shown by blowing soap bubbles with a mixture of the gases and touching these bubbles with a lighted taper. They combine instantly throughout their whole volume and an explosion is the result. The chemical change is represented in reaction (4) on the diagram.
We have seen that in all these cases intense energy is developed and I have explained that this is due to the impact of the atoms against each other and have compared them to the impact of a shot against a target or a hammer against a nail resting on an anvil. But you may ask how do these atoms acquire their energy of motion? We have seen that the shot acquires its energy from the energy of the explosive with which the gun was charged, and we have seen that the hammer acquires its energy from the arm of the man that wields it, whence then is the source of the energy of these atoms? My answer is that their energy is due to the force of chemical attraction and their energy is acquired by being separated from each other just as a weight acquires its energy by being raised from the earth against the attraction of gravitation. Take the case of the union of the carbon with oxygen. The product is a compound of the two, known as carbonic acid gas. This exists in the air and when it comes in contact with the leaves of plants in the presence of the sun's rays it is decomposed; the carbon is stored up in the plant and oxygen is given off to the air. To effect this decomposition just as much energy is employed as is developed in the union of this carbon with the same amount of oxygen and this energy came from the sun. And we may consider that the heat and light emitted by the piece of carbon we burnt to-night was the heat and light of the sun, for the separation of this carbon and oxygen was effected by the sun's agency. Just as a weight in falling will do as much and no more work than was required to raise it, so the atoms of our elements in combining will generate as much and no more energy than is required to tear them apart. But we have seen that more energy is developed when oxygen combines with hydrogen than when it combines with carbon. Chemical affinity is in this respect unlike the attraction of gravitation, for, whereas in the latter the attractive force is proportional to the masses attracted, in chemical affinity the attraction varies with the kind of matter between which the attraction takes place. Therefore since the attraction between hydrogen and oxygen is greater than between carbon and oxygen more energy is required to separate the first two, and when they combine more energy is developed. Hence, when comparing two explosive bodies which are under the same conditions and in the same state, that one will develop the greatest energy which contains the largest amount of hydrogen together with sufficient oxygen to completely burn it, provided also there is sufficient oxygen to unite completely with the other combustible substances present.
Let us now turn our attention to some of the means by which combustion may be induced. We have seen that carbon burns by union with oxygen and that this union goes on most readily when the oxygen is pure and when the contact between the two is quite intimate. I have here the substance from which our oxygen was produced, potassic chlorate. We have simply to heat it and it gives off its oxygen readily. If we now powder some of this and mix it thoroughly with powdered charcoal and heat them on a glowing metal plate, we see that the charcoal burns very readily. Why? Because the potassic chlorate furnishes it with a supply of pure oxygen. Let us make another mixture of the same but adding to it some powdered sulphur. When this is heated, we observe that it takes fire at a lower temperature and burns more rapidly. Why? Because the sulphur has a lower point of ignition, as shown in a former experiment, than the other substances in the mixture and when it ignites it rapidly heats the others to their point of ignition and fires them. We have in this mixture a type of the usual explosive mixtures. Compare it with the gunpowder on the diagram and we see that it differs from the gunpowder only in containing potassic chlorate in the place of potassic nitrate. But the potassic nitrate serves the same purpose in the gunpowder as the potassic chlorate in our mixture, it yields oxygen when heated though not so readily or at so low a temperature as the potassic chlorate does. With such a mixture as this we can produce a combustion without requiring the presence of the oxygen of the air, and hence we can burn such a mixture in a confined space such as the chamber of a gun or a hole in a rock. Indeed some of these mixtures will burn when submerged under water and when in direct contact with the water. All that we have to do is to liberate the oxygen from its state of combination by suitable means and the combustion is effected at once. This may be shown as follows. A few crystals of potassic chlorate are placed in this glass which contains water, some small pieces of phosphorus are then dropped upon the potassic chlorate and finally strong sulphuric acid is added to it. By the union of a part of the acid with the water sufficient heat is developed to heat the phosphorus to its point of ignition, and by the action of the rest of the acid the potassic chlorate is decomposed and compounds of chlorine and oxygen are formed which readily yield their oxygen to the phosphorus and the vivid combustion which you witness ensues.
If now we again return to our diagram we shall see that what is true of the potassic chlorate mixture and of the gunpowder is true of all the others, they contain sufficient oxygen to combine with the carbon or hydrogen which they contain and to form oxygen compounds of these substances, hence they too will burn when confined if we only employ some means to effect their decomposition. The energy developed by the combustion of the carbon and hydrogen under these circumstances will be even greater than when burned to the same degree in pure oxygen gas because the elements are in a nascent state. We mean by the nascent state the condition of the atoms when just liberated from a state of combination. They then possess their greatest energy of chemical separation, and they exert the entire power of chemical attraction which they possess. For we believe that when we brought the hydrogen and oxygen together in their free state the atoms of the hydrogen were united with a certain force with each other and so likewise the atoms of the oxygen were united with one another and before the hydrogen and oxygen atoms could combine a certain amount of energy was employed in separating these elementary atoms from each other, but at the moment when the hydrogen and oxygen are separated from the compounds of which they form a part the atoms exist independently and are in their freest state and are very ready to enter into combination. Hence when they unite the energy developed is the greatest which is possible in the formation of the new compound.
The violence of the explosion of one of these substances depends upon various circumstances. The most important of these are the manner in which the decomposition is effected, the degree of confinement, the readiness with which decomposition takes place, and the character of the product.
The most common way of effecting the explosion of a substance is by flame or a spark, or by contact with a heated surface. Heat is in all these cases the agent which causes the decomposition and liberates the atoms. Another method is in vogue of employing some chemical substance which will decompose the compound in the mixture which contains the oxygen. Sufficient heat is thus generated to raise the phosphorus, carbon or other substance of a similar nature in the mixture to its point of ignition and at the same time the oxygen is liberated in such a form that it may combine with them. We produced combustion in this way when we burned phosphorus under water. In the Harvey torpedo the fuse consists of a mixture of potassic chlorate and sugar (C12H22O11). A bottle containing strong sulphuric acid is placed in contact with this mixture. By a suitable contrivance when any object such as a ship, comes in contact with the torpedo the bottle is broken and the acid runs out and decomposes the potassic chlorate, thus liberating its oxygen which then combines with the carbon of the sugar and an explosion results. I have here such a mixture and we see the result of adding the sulphuric acid.
Still another method of producing explosion is by a blow. The percussion cap, which is charged with mercuric fulminate, is exploded in this way. I have here some crystals of potassic chlorate and I will put with them a small piece of phosphorus, wrap them in paper and place them on the anvil. When the paper is struck we have a sharp explosion. The agent which caused this becomes evident. It is again heat, heat caused by the impact of the hammer on the anvil, heat resulting from the conversion of the mechanical energy of they arm into molecular energy. Nitro-glycerine and gun cotton may be exploded in this way.
In the case of the explosion of the priming of a percussion cap still another cause may operate; for we find that if we touch dry mercuric fulminate in the slightest it will explode, and the same is true of argentic fulminate and nitrogen iodide. This may be due to the heat generated by friction.
When the bodies which are decomposed by a blow or by friction explode, the explosion of the whole is apparently simultaneous. Such complete and instantaneous explosions are called detonations.
Abel (in a paper which will be referred to again) speaks of these detonations as follows, "The readiness and certainty with which gunpowder, gun cotton, and nitro-glycerine and other explosive substances may be detonated through the agency of a blow from a hammer or falling body are regulated by several circumstances; they are in direct proportion to the weight of the falling body, to the height of its fall, or the force with which it is impelled downwards, to the velocity of its motion, to the mass and rigidity or hardness of the support or anvil upon which the body falls; to the quantity and mechanical condition of the explosive agent struck and to the ready explosibility of the latter. Thus a sharp blow from a small hammer upon an iron surface will detonate gunpowder with very much greater certainty than the simple fall of a heavy hammer or a comparatively weak blow from the latter. It is very difficult by repeated blows, applied at very brief intervals, to ignite gun-cotton, if placed upon a support of wood or lead, both of which materials yield to the blow, the force set in operation by that blow being transferred through the explosive agent and absorbed in work done upon the material composing the support, If, however, the latter be of iron, which does not yield permanently to the blow of the hammer, the detonation of these substances is readily accomplished. If the quantity of the explosive agent employed be so considerable as to form a thick layer between the hammer and support, the force applied appears to be to so great an extent absorbed in the motion imparted to the particles of the compressible mass, that its explosion is not readily accomplished; and if the material be in a loose or porous condition (as e.g. in a state of powder or loose wool) much work has to be accomplished in moving particles of the mass through a comparatively considerable space and a second or third blow is therefore required to determine explosion.
These circumstances would appear to afford support for a belief that the detonation of an explosive material through the agency of a blow is the result of the development of heat sufficient to establish energetic chemical change, by the expenditure of force in the compression of the material or by the friction of the particles against each other, consequent upon a motion being momentarily imparted to them. It is conceivable that, from either of these causes, sufficient heat may be accumulated, with almost instantaneous rapidity, in some portions of the mass struck, to develop sudden chemical change."
A peculiarity of the explosion of such substances is that by detonating one portion of the substances the explosion may be communicated to another portion which is near to it. To avoid igniting the second portion by ignited particles of the first being projected against it, it may be separated in the experiment by a glass plate. Since there is apparently no source of heat present in this case to cause the decomposition of the body, what is the agent which produces the explosion of the second portion? Abel (Phil. Trans. Vol. 159, 489) investigated very thoroughly the subject of detonation and obtained some very interesting results. He showed that not only would a detonating body cause the detonation of another mass of the same body but that it would cause also the detonation of other explosive bodies. For instance, by detonating mercuric fulminate in contact with gun cotton or nitro-glycerine these bodies were also readily detonated. Only a small quantity of the fulminate was required, .32 of a gram (5 grains) when confined in a sheet metal cap and placed in direct contact with the nitro-glycerine or compressed gun-cotton being sufficient to cause the detonation of the latter. He found that a mass of nitro-glycerine by its explosion would cause the explosion of another mass of nitroglycerine even though both were immersed in water. His experiment further showed that a peculiar kind of detonation was required in order to cause the detonation of an explosive. For instance, while the detonation of gun cotton would cause the detonation of nitro-glycerine in close proximity to it, the detonation of nitro-glycerine would not cause the detonation of gun-cotton. This shows that this property of causing detonation does not depend alone upon the violence of the detonation, for we all know that nitro-glycerine is much more powerful than gun-cotton. Again argentic fulminate which explodes more violently and sharper than mercuric fulminate is no more efficient in producing the detonation of nitro-glycerine or gun cotton than mercuric fulminate, and nitrogen iodide and nitrogen chloride which are the most violent explosives we possess are very much less efficient in causing detonation than mercuric fulminate. In the course of his investigations Abel was led to the conclusion "that a particular explosion or detonation may possess a power of determining at the instant of its occurrence similar violent explosions in distinct masses of the same material, or in contiguous explosive bodies of other kinds, which power is independent of or auxiliary to, the direct operation of mechanical force developed by that explosion; that as a particular musical vibration will establish synchronous vibrations in particular bodies while it will not affect others, and as a chemical change may be wrought in a body by its interception of only particular waves of light, so some kind of explosions or powerful vibratory impulses may exert a disturbing influence over the chemical equilibrium of certain bodies, resulting in their sudden disintegration which other explosions, that develop equal or greater mechanical force, are powerless to exercise.
I venture to offer the following as being the most satisfactory explanation of the remarkable differences pointed out. The vibrations produced by a particular explosion, if synchronous with those which would result from the explosion of a neighboring substance which is in a state of high chemical tension, will, by their tendency to develop those vibrations, either determine the explosion of that substance, or at any rate greatly aid the disturbing effect of mechanical force suddenly applied, while, in the case of another explosion which produces vibrations of a different character, the mechanical force applied by its agency has to operate with little or no aid; greater force, or a more powerful detonation must, therefore, be applied in the latter instance, if the explosion of the same substance is to be accomplished."
That vibrations will induce the decomposition of chemical compounds whose atoms are in a state of unstable equilibrium is a well recognized fact in science, and numerous instances can be cited where advantage is taken of this fact to induce chemical change. The effects of the vibrations which produce heat are too well known to need illustration here. The most marked instances of the action of the vibrations which produce light are the decomposition of the silver salts on the photographic plate, and the decomposition of carbonic acid gas in cells of leaves. The vibrations which produce electricity will cause the decomposition of chemical compounds, as is seen in the process of electroplating, etc. Why, then, should not the vibrations which produce sound be also capable of inducing chemical change?
The condition of a compound whose atoms are in a state of unstable equilibrium, is probably somewhat similar to that of a Prince Rupert's drop, in which the molecules are in a state of unstable equilibrium. Separate but the smallest bit from the end of the drop, and the whole drop flies to pieces, as you see. So with a chemical compound, if by means of vibrations we can increase the amplitude of the vibration of any of its atoms, so that the force of chemical attraction is overcome, the molecule breaks up with violence; and in explosive bodies this violence is increased by the character of the compounds which are formed by the re-arrangement of the atoms.
Abel's theory was examined experimentally by Champion and Pellet (Compt. Rendus, lxxv. 110). They took a tube seven meters long, made in two lengths, and joined by a paper band. Small quantities of nitrogen iodide were placed in each end, and when one was exploded it immediately caused the explosion of the iodide at the other end. But if the paper band connecting the two lengths was removed, this result was not produced. By a suitable apparatus it was shown that the effect produced was not due to the action of a puff of air, but to vibrations of the air such as are caused by a sounding body. When they attached nitrogen iodide to the strings of a double-bass, and bowed the strings, the iodide exploded when placed on the string giving the highest note, but not when on the two lower strings. The lowest number of vibrations which would cause explosion was found to be thirty per second. Similar results were obtained with other musical instruments.
A further set of experiments was made to determine the difference between the vibratory motion excited by various detonants, and thus to account for the difference in their power of causing, by means of the intervening air, the explosion of other detonants placed at a distance. A series of sensitive flames was arranged corresponding with the complete scale of g major and 0.03 grm. of mercuric fulminate and, nitrogen iodide were exploded near them. The nitrogen iodide produced no effect; but the mercuric fulminate excited the flames a, c, e, f and g. This showed that the vibrations excited by the two explosives were very different; and also that the vibrations excited by the mercuric fulminate act on flames belonging to some notes of the scale, to the exclusion of others. On exploding these bodies nearer the flames than in the former experiment, while the nitrogen iodide excited only flames corresponding with the higher notes of the scale, the mercuric fulminate affected all of them. On exploding 20 grms. of nitrogen iodide near the flames, it excited all of them. In these experiments it was observed that acute sounds predominate in explosions.
Abel (Proc. Roy. Soc. xxii. 160), continued these investigations on the transmission of detonation by means of tubes—using explosive agents which were less highly susceptible; such as gun-cotton, dynamite, etc., and the results tended to confirm his theory.
The second condition governing the violence of an explosion, to which I have referred, is the degree of confinement. Abel, in the paper cited, (Phil. Trans., vol. 159, 4S9), shows this by numerous experiments. In these he found that .32 grains of mercuric fulminate, when confined in a sheet metal cap, will do the work as a detonator which it requires 2 grm. of the unconfined mercuric fulminate to do.
The readiness with which a body may be decomposed depends upon various circumstances. If we apply fire to gun-cotton in a mass, as it is a poor conductor, heat enough may be stored up in one portion of the cotton to cause its explosion, while with gun powder, on the contrary, the heat is conducted through the mass, and the heating body is cooled down; hence the temperature of the igniting body must be greater for gun powder than for gun-cotton. I can show this in the following way: In this cup I place some gunpowder. A fine platinum wire is passed through it, and it is heated by the current from a battery. Now the wire begins to glow; but where it is in contact with the powder it is cooled down on account of the heat being conducted away. Let me now drop in a piece of gun-cotton, and it at once ignites and fires the powder.
We have learned what the general character of the constituents of our explosives is, and we have seen how they can be exploded and by what means the violence of the explosion may be controlled. The next point to be considered is the manner in which the energy developed by the explosion is communicated to the projectile or to the body which is to be moved. Our first experiments taught us that when carbon and hydrogen unite with oxygen the products are gaseous. The nitrogen in these explosives is also liberated in the gaseous state, either as free nitrogen or combined with oxygen. It is a distinguishing property of gases that they exert pressure in every direction and this property is explained by the theory that the molecules, of which they are composed, are constantly moving at a high velocity and that the pressure which the gas exerts is due to the impact of these molecules against the walls of the chamber in which they are held. We daily meet with numerous examples of the effect of this molecular motion and we can show that the gases formed by the combustion of the powder exerts a similar pressure, by the aid of a simple apparatus similar to that used for showing the pressure exerted by steam. A mercury gauge is fitted to the top of a stout iron cylinder. Through another opening in the top a gram of powder is passed and then the opening is closed tightly by a plug. The wires from a battery pass through this plug and connect with a fine platinum wire passing through the powder. The apparatus is now exhausted and you will observe the level of the mercury in the gauge. Now we connect the battery and fire the powder and we see the mercury rapidly rising in the gauge thus proving that pressure is exerted by the gaseous products. If any part of this vessel were movable of course it would be forced aside, for the molecules would move in the direction of the least resistance, and if they came in contact with a movable body they would impart their motion to this body just as one billiard ball imparts motion to another. When the volume of a gas is constant the pressure is proportional to the density, therefore the greater the quantity of gas developed by the explosion of the same quantities of different explosives, in the same space, the greater the explosive effects.
There is another property of gases which comes into play here also and that is the property of augmenting their pressure when heated, if they are confined. The heat increases the velocity of the molecules and consequently a greater number of blows is delivered upon the same surface during the same time when the gas is heated than when it is at the lower temperature. Hence we find that not only does the force developed by an explosive depend upon its condition, i.e. whether a mechanical mixture or chemical compound, but also upon the number, kind, and arrangement of the atoms in the explosive and the character of the products of the explosion. From the observations already made we should conclude that of two explosives that one would be the most powerful in which the atoms, which are united in the product, are most widely separated from each other and in which the combustion is most complete, for under these circumstances the greater amount of heat would be developed and consequently the products would exert the greater pressure.
I hoped to be able to develop this part of the subject much more fully but I find that I have already occupied more of your time than I am entitled to and I must rest here.
To sum up, then, we learn that an explosion is an instance of chemical decomposition wherein the constituent atoms of a solid or liquid substance re-arrange themselves and yield products which are gaseous and which occupy a very much larger volume than the explosive substance; that all useful explosives are mechanical mixtures or chemical compounds containing carbon, hydrogen and oxygen, and that they owe their energy to the fact that by the union of the carbon and the hydrogen with the oxygen, gaseous products are formed which exert pressure and are capable of doing work, and to the fact also that by this union of the elements the potential energy of chemical separation becomes kinetic and, heat being developed, it very much increases the energy of the resulting gases. We learn also that the work which an explosive is capable of doing depends upon the kind of atoms composing it, upon the intimacy of their contact, upon the ease with which the decomposition can be effected, and upon the manner in which it is induced.