Popular Science Monthly/Volume 21/October 1882/Explosions and Explosives
By ALLAN D. BROWN.
COMMANDER, UNITED STATES NAVY.
THE chief explosive mixture used in the arts of war and peace will probably for a long time continue to be that which we know by the name of gunpowder. It has been used so long that its origin (like that of the mariner's compass) is entirely lost in the misty atmosphere of the middle ages, if indeed it was not known before the Christian era. The ingredients spoken of by Roger Bacon in 1237, in his formula for "making thunder and lightning," are still in use, and in nearly the same proportion now as at that early date. The three solid substances which, when properly and intimately mixed, form gunpowder, are saltpeter, sulphur, and charcoal; the first being three fourths, or a little more, of the mixture, the two latter, in nearly equal but varying proportions, forming the remaining one fourth. The saltpeter contains a large amount of oxygen, which it gives up with considerable readiness upon the application of heat; this unites with the carbon contained in the charcoal, forming a large volume of carbonic acid gas; the potash of the saltpeter unites with the sulphur, while the nitrogen is set free, adding to the volume of the gases evolved. As these gases are given off with great rapidity, they are still further expanded by the action of the heat produced by the change of the solids into the gaseous form, until, under ordinary circumstances, they occupy a volume nearly three hundred times as great as that occupied by the powder itself. One can easily imagine that the expansion of a pound of powder to a size (if we may so say) so much greater than its original bulk would exert a tremendous pressure upon any object in which it was confined. If a small quantity of powder is freely exposed in the air and fired, the resulting explosive effect is small, as the gases produced so rapidly can readily push aside the air; but if it be confined in a large block of steel, in which is a cavity which will just receive it snugly, the resulting pressure is nearly ninety-three thousand pounds to the square inch.
From this illustration it will at once be understood that confinement, in a greater or less degree, is necessary, in order that the greatest amount of work may be accomplished by the explosion. Those explosives which give off their gases with intense rapidity require but little restraint, while the slower ones require more confinement. Among these less quick ones must be reckoned gunpowder, for, although the explosion seems to follow immediately upon the application of heat, yet an appreciable amount of time really elapses, in which the combustion goes on.
The ingredients of which gunpowder is composed are selected with the greatest possible care: the saltpeter is procured chiefly from India, being extracted from the soil by natural processes and then secured by the natives, for marketing. It is also largely prepared by artificial means, it being quite evident that it would not do for any country to depend upon a supply without its own borders in case of war. In this process, heaps of earth are mixed with decomposing organic matter—ashes, old mortar, and materials of like nature—the whole being wet with the liquor from stables sufficiently to keep it in a moist condition; the moistened heaps are worked over from time to time, and the air allowed free access. A chemical reaction takes place, and in time the crude saltpeter appears upon the exterior of the heap, whence it is removed and treated for the extraction of the pure material. Both the India and the artificial products are mixed with hot water, which takes up the salts, leaving the earthy matters behind; this liquid is then evaporated, the saltpeter crystallizing as a fine white powder; if the original be very crude, this product is again subjected to a similar process.
The charcoal requires more care in its preparation than either of the other ingredients, as upon its quality depends largely the violence of the action of the powder; the more nearly pure carbon it is, the better will be the result. It is made from some very light wood, such as the black alder or willow, as these contain much carbon, and but little ash: small pieces of these woods, stripped of their bark, are placed in a retort which is kept at a uniform heat; the vapors are allowed a free exit, and the roasting is kept up until the experienced eye of the workman warns him that it is time to withdraw the charge, lest it be over burned. The entire contents of the retort are removed at once, and covered in air-tight drums, where the charcoal is left to cool. Thus prepared, charcoal is quite a different material from that in ordinary use; it being of a bluish-black tinge, somewhat elastic and slightly resonant when struck lightly with the finger, with the appearance of the woody fiber clear and distinct.
The sulphur is prepared from its ore, by roasting the latter in a retort, the vapors given off being condensed, and the resulting liquid run into molds, and allowed to harden.
Having procured the materials of proper fineness and in the desired proportions, the sulphur and charcoal are placed in a revolving cylinder with cylindrical rollers inside, by the action of which they are broken up into small pieces. These are then transferred to a similar cylinder containing bronze or zinc balls, in which they become very highly pulverized. When this is fully accomplished, the saltpeter is mixed with them, and the whole mass placed in the incorporating mill, being kept moist enough to be like dough, but still not too wet, as that would interfere with its proper mixture. This mill consists of two heavy iron wheels, revolving at the extremities of an horizontal axis, the whole being revolved about a vertical axis in the center of a cast-iron bed, surrounded with wooden sides. As this upright axis revolves, the wheels move about their own axes, having at the same time a forward motion, which causes the powder to be both mixed and pressed at the same time; in this manner, the mixture is rendered much more intimate than by the old method of stamping, in which the ingredients were placed in huge wooden mortars, and subjected to the action of heavy pestles of the same material. When sufficiently mixed, the mill-cake, as it is then called, is allowed to dry; after which it is taken to the breaking-down machine, where, on passing between heavy wooden rollers, it is broken into small fragments. The next step is to subject it to the action of the press; this is an horizontal wooden trough in which are placed (about an inch and a half apart) accurately fitting sheets of hard rubber; the crushed powder is shoveled into the trough, and a powerful hydraulic press applied; a screw-press would be dangerous, as particles of powder might get into the thread of the screw, and be fired by the friction. When removed from the press, the powder is in slabs of close texture, not unlike slate in appearance. These slabs go next to the granulator, in which they pass through a series of rollers, separated by sieves; as the broken powder passes from one set of rollers to the succeeding one, the sieves are kept in constant agitation, the pieces which pass through them falling to the bottom of the machine, where they are collected. Each sort is then taken to a rotating reel of wire gauze, in which, as it revolves, the dust is removed. If the powder is to be glazed, the clean grains are placed in a slowly revolving drum, with a very small quantity of plumbago, or black-lead; if glazing is not required, as is the case with some sorts of powders, the same process is gone through, the black-lead being omitted; in this manner, the grains are rounded off, and rendered smooth.
The final step is the drying, which is done in a steam-heated house, the powder being spread upon shelves for this purpose. The finished product is placed in oak kegs fastened with copper hoops, and care is always enjoined to use no iron tool in opening them. These various operations are conducted in buildings situated at a distance from each other sufficiently great to prevent the explosion of one causing that of another; they are generally placed along the banks of some stream from which the requisite power for operating the machinery can readily be obtained.
The manner in which the powder burns is greatly affected by variations in its manufacture: the greater the size of the grains, the more slowly does the burning take place, as the combustion goes on upon the surface, particle by particle; the shape of the grain regulates the amount of space taken up by the charge, as also in some degree the amount of surface exposed to the action of the flame; the density (depending upon the amount of pressure applied to the press-cake) also affects the rapidity of the burning; the lower the density, the more quickly does the combustion take place.
A desire to reduce the strain upon the walls of the modern guns has led to many experimental trials of various sizes and shapes of grain; the principal credit for these ideas is unquestionably due to an American ordnance-officer, Rodman. Much care has been taken in this regard, and we now see much higher velocities given to projectiles than heretofore, with, at the same time, less strain upon the gun.
There are various other mixtures which are explosive in their character, but their use is prevented by various considerations, chiefly by the fact of their greater sensitiveness to friction or percussion, and their consequent greater danger, or by their corroding effect upon the metal of which the gun is constructed. A powder which might prove to be useful where a stronger explosive than ordinary gunpowder is desired is known as Abel's picric powder; this is a mixture of picrate of ammonia and saltpeter, prepared in an ordinary powder-mill by the processes just detailed. Picric acid (by means of which the picrate of ammonia is obtained) is derived from the action of strong nitric acid upon carbolic acid. This powder requires confinement to develop its force, is not exploded by friction, and, as it absorbs no moisture by exposure to the air, can be used and stored like gunpowder; for the ordinary uses of gunnery, however, it can never supersede its elder brother.
As has already been said, gunpowder burns, rapidly it is true, but nevertheless the action is a true combustion. We now come to the consideration of a new class of explosives, which do not burn, but resolve themselves into their constituent gases immediately upon the application of the force which suffices to bring about their disintegration. These are not mixtures like gunpowder, but definite chemical compounds in which the carbon, oxygen, and hydrogen are held a little way apart, as it were, by the nitrogen, but ready to rush together at the first opportunity; the explosion of one particle means generally the contemporaneous explosion, or detonation, of the whole mass: with such bodies confinement to any great extent is not necessary.
Pre-eminent among these detonating substances for its use in mining and engineering operations of a like character stands the compound known as nitro-glycerine. This is manufactured in greatly increasing quantities in many places, both in this country and in Europe, and its use for the purposes above mentioned is becoming more and more general. Its transportation in the liquid state being excessively dangerous, it was for a long time but little used; but, it having been ascertained that it can be mixed with other materials and carried with comparative safety, its field of usefulness has become greatly enlarged. It is formed by the action of very strong nitric acid upon glycerine at a low temperature, the resulting product being removed and washed free of its impurities.
Glycerine is a by-product of the stearine-candle manufacture, being separated from the stearic acid contained in animal fat upon the application of superheated steam. It is subsequently redistilled until its impurities are removed, in which condition it is proper to be used in the manufacture of nitro-glycerine. Unfortunately, however, many manufacturers do not restrain themselves to a pure quality of the glycerine, but, using inferior ones (which are, of course, cheaper), make a nitro-glycerine which is of a much more dangerous character than should ever be used.
The process of manufacture is as follows: The nitric acid is mixed with twice the quantity of sulphuric acid, and both then mixed with the glycerine; the nitric acid acts upon the glycerine, leaving a quantity of water free; were the sulphuric acid not present, this water would dilute the nitric acid, but, as the sulphuric acid has a greater affinity for water, it takes it up, leaving the other of its normal strength. To proceed to the details: Around a brick chimney is placed a wooden trough, containing large earthenware pitchers in which is the acid mixture, the trough being filled with ice. On a shelf above this trough are placed bottles holding glycerine, communicating each with a pitcher below, by means of a small rubber tube, so arranged as to permit the passage of the liquid in a fine stream. The contents of the pitchers are kept in a constant state of agitation by a stream of cold dry air forced through them; as the reaction between the nitric acid and the glycerine goes on, great heat is evolved, accompanied by nitrous fumes extremely unpleasant and unhealthy to inhale; these fumes are drawn into the chimney through an overhanging hood, by means of the draught created by a furnace-fire at the bottom. Great care is necessary that the temperature be not allowed to rise above 48° Fahr., as there would then be danger of the newly formed nitroglycerine taking fire and exploding; constant attention must be paid to this point, each pitcher being tried with a thermometer at short intervals. The proportion of materials is about two pounds of glycerine to twenty pounds of the acid, and at the expiration of the process the glycerine and nearly all the nitric acid have disappeared, forming nearly four pounds of their compound; the sulphuric acid, diluted as before mentioned, also remains. The nitro-glycerine is now partly in solution and partly suspended in the acid mixture; the contents of the pitchers are poured through a strainer into a vat of water, which is kept in agitation by a stream of compressed air: after all the pitchers are emptied, the air is shut off and the mixture comes to rest, when the nitro-glycerine settles at the bottom of the vat, and the acid water is then drawn off. It is next taken in small quantities at a time, and carefully washed a number of times, until all the acid is washed out, and only the pure nitro-glycerine remains; in this state it is thin, oily, creamy white, and opaque, but, on being placed in jars and allowed to stand, it soon becomes transparent. It is now ready for use as an explosive agent; it has a sweet, aromatic, pungent taste, and possesses the very peculiar property of causing an extremely violent headache when placed in a small quantity upon the tongue, or any other portion of the skin, particularly upon the wrist. It has long been employed by homœopathic practitioners as a remedy in certain kinds of headaches. In those who work much with it, the tendency to headache is generally overcome, though not always, It freezes at about 40° Fahr., becoming a white, half-crystallized mass, which must be melted by the application of water at a temperature of about 100° Fahr. If perfectly pure—that is, if the washing has been so complete as to remove all traces of the acid—it can be kept for an indefinite period of time; and, while many cases of spontaneous decomposition have occurred in impure specimens, there has never been known such an instance, where the proper care has been given to all the details of the manufacture.
There are other methods of manufacture, differing, however, only in the details, the principle of course being the same.
When pure, nitro-glycerine is not very sensitive to friction, or even to moderate percussion: if a small quantity be placed on an anvil and struck with a hammer, that portion which is touched explodes sharply, but so quickly as to drive away the other particles; if, however, it were even slightly confined, so that none could escape, it would all explode or detonate. It must be fired by a fuse containing fulminate of mercury (the compound used in percussion-caps), not being either readily or certainly fired by gunpowder, the shock of the latter not being sufficiently quick or sharp to detonate the nitro-glycerine. It is highly probable that in this case, as in that of other high explosives, the vibrations set up by the fulminate (which is not stronger than gunpowder) are of just such a character as to find an answering chord, so to speak, in the explosive, so that the desired effect is produced. This would seem to be a correct theory, for it is not always the most powerful explosive which most readily causes the explosion of another body. For instance, although nitro-glycerine is much more powerful than fulminate of mercury, yet seventy grains of it will not explode gun-cotton, while fifteen grains of the weaker fulminate will readily do so. The fuse generally used, then, for firing nitro-glycerine, is composed of from fifteen to twenty-five grains of fulminate, and this quantity is sufficient to detonate a large mass as well as a small one.
If flame be applied to nitro-glycerine it will not explode, but burn with comparative sluggishness. When frozen it is very difficult and uncertain of firing. If the material be perfectly pure, it forms, upon detonation, a volume of gases nearly thirteen hundred times as great as that of the original liquid; these gases are also further expanded, by the heat developed, to a theoretical (though not practical) volume ten thousand times as great as that of the charge. Practically speaking, the forces exerted by gunpowder and nitro-glycerine are in the proportion of one to eight.
The great objection to nitro-glycerine, in its liquid state, is the difficulty of its transportation; it is liable to leak from the packages in which it is contained, and there have been several occasions on which disastrous accidents have taken place owing to this circumstance. The explosion of a large case on board of a steamer in Aspinwall some twelve or fourteen years ago, and, about the same time, of a box in an express-office in New York, caused great precautions to be taken with regard to it, and also very great fear of it on the part of all transportation companies. Fortunately, it has been found that it can be carried from place to place by mixing it with some absorbent substance, which takes up a large quantity of it; it is just as powerful in this state, the presence of the absorbent having no deleterious effect. This mixture is called dynamite, or giant-powder; it is made by mixing nitro-glycerine with a siliceous earth, in the proportion of three to one by weight. This earth is a fine white powder, composed of the skeletons of microscopic animals; it is found in Hanover and also in New Hampshire—that coming from the latter locality being the finer, and therefore most used in this country. The dynamite formed by this mixture is not unlike moist brown sugar in appearance; care must be taken not to put too much nitro-glycerine in it, as there must not be such a quantity as would cause exudation. Its properties as an explosive, are, of course, those of the nitro-glycerine alone; but it can be much more readily handled, and there is less danger from either percussion or friction. It has been dropped from a height upon rocks, heavy weights have been allowed to fall upon it, and other experiments of a like nature have been made to show how readily it can bear transportation and hard knocks. This safety, however, presupposes a pure nitro-glycerine; and whenever an accident occurs it may safely be laid to the impurity of the explosive, and not to anything necessarily consequent upon the use of dynamite. Under the action of cold, dynamite freezes at 40° Fahr. in a hard, compact mass, in which condition it is very difficult to explode. Sometimes, however, it freezes in a loose and powdery state, and there is then no difficulty in causing its explosion with the ordinary fulminate-fuse. Generally speaking, however, it is best with this, as with nitro-glycerine, to thaw it out before attempting to use it. Instances have been known of careless men attempting to do this with a red-hot poker, with consequences, naturally, of a disastrous character. It will admit of being slightly moistened without injury, and hence can be used when gunpowder could not be. As a military explosive for mines and torpedoes, as also for the breaking up and destruction of guns, it has proved itself useful; and it has been used as a bursting-charge for shells, though this is considered dangerous.
Besides dynamite, there are several other mixtures of nitro-glycerine, in which the inactive siliceous earth is replaced by some active substance, such as charcoal, saw-dust, wood-fiber treated with acid, chlorate of potash, or even gunpowder. It is extremely doubtful whether anything is added to the explosive effect of the nitro-glycerine by their presence, as its own explosion is so rapid as to gain nothing from the slower combustion of these substances. Experiments with the pressure-gauge tend to show the correctness of this theory. Of all these combinations, that called cellulose dynamite is the best; it is a combination of the wood-pulp, so much used for the manufacture of paper, treated with nitric acid and nitro-glycerine; it possesses the excellent quality of being able to absorb a considerable quantity of water without injury; for this reason it may prove itself a commercial rival to dynamite pure and simple.
Large quantities of dynamite are manufactured in this country for use in mining and engineering operations; among others, the Hoosac Tunnel and the works at Hallett's Point in New York harbor undoubtedly owed much to the powers of nitro-glycerine in hastening their completion. The effect of a confined charge upon rock is to pulverize the portion near the blast-hole, the action being so sudden, quick, and intense; it is, therefore, found better not to tamp the hole, a saving both of time and labor, allowing the gases a greater surface upon which to act. Ordinarily the dynamite is contained in paraffined paper cartridges, and is fired with a fulminate-fuse.
Gun-cotton is formed by the action of nitric acid on cotton—a portion of the hydrogen being displaced in the cotton, just as it is in the glycerine by the active constitution of the nitric acid. The essential features of the process are the same as those of the manufacture of nitro-glycerine; that is to say, a mixture of strong nitric and sulphuric acids is made, the cotton exposed to its action, and the excess of acid removed from the cotton by careful washing; the sulphuric acid plays the same part, namely, that of taking up the water formed, and so keeping the nitric acid at its full strength. As in the former case, only perfect purity will insure safety; the presence of acid in the gun-cotton will ultimately cause decomposition and explosion. Many accidents have taken place since the introduction of gun-cotton to public notice some thirty-five years ago; but, as in the case of nitro-glycerine, all of them may be ascribed to imperfect washing, reference being had, of course, to instances of what may be called spontaneous combustion. With the improved methods of to-day, however, and the exercise of ordinary care, gun-cotton can be rendered perfectly stable and safe, far safer for transportation than nitro-glycerine in any of its forms; when wet it can not be readily exploded, and hence it is generally carried in that state, and either dried for use or else exploded by the use of a dry primer of the same material. It possesses, however, the disadvantage of rapidly absorbing moisture, and hence it is extremely difficult to keep primers dry unless they are prepared with great care. In a military point of view this is no great objection, but commercially speaking it is, for the expensive water-proof cases would perhaps make its use financially impracticable.
The details of the usual process of its manufacture are as follows: cotton-waste is picked and cleaned, then dried at a high temperature. After cooling, in quantities of about a pound, it is immersed in the strong acid mixture contained in a trough surrounded by cold water; after a short exposure it is removed, and the acid pressed from it as far as practicable. After another immersion of twenty-four hours, it is placed in a centrifugal strainer, by the rapid revolutions of which nearly all the acid is expelled; it is then washed in a large amount of water, and again placed in the strainer. When the acid-water no longer remains, the gun-cotton is placed in the pulper, an oblong tub full of water, in which revolves a wheel having strips of steel upon its rim, similar strips projecting from the bottom of the tub. As the wheel revolves, the floating pieces are drawn between the steel strips and thus reduced to a fine pulp. This is again washed for a long time by mechanical means, after which it is taken to the press, where nearly all the water is expelled from it, the final pressure applied being about fifteen thousand pounds to the square inch. When taken from the press, it is in the shape of a disk or cylinder, of a close texture, easily broken when dry, and capable of being cut in a manner not unlike pasteboard. When a small quantity is ignited by a flame, it burns quite rapidly, but quietly, if dry; if wet, it is consumed very slowly. If a large quantity is ignited, there may be sufficient confinement of the inner portions by the outer shell to cause an explosion. If dry loose gun-cotton be ignited, it burns with extreme rapidity, like a flash, but without any violence; in fact, a wisp placed on a small pile of gunpowder and fired will not ordinarily cause its explosion. In order to have a complete explosion of the dry material, fulminate of mercury must be used, and this is accordingly employed in the primers made for the explosion of that which is wet. Two pounds of the dry, detonated by twenty-five grains of fulminate, will cause the detonation of five hundred pounds of the wet. In this wet state it must be carefully guarded from a low temperature, as the expansion of the water in freezing will tend to break up the disks.
Many experiments have been made with a view to its adoption in gunnery practice, but it is not at all likely that either it or any of the high explosives will ever displace gunpowder; their action is so violent and sudden that, before the projectile has time to take up its motion in the bore of the gun, the walls yield and the piece is burst. Its use for military purposes must therefore be confined to mines and torpedoes, as in the case of dynamite, or as a bursting-charge for shells, for which purpose experiments show that it is most admirably adapted. It is used in Europe for torpedo purposes, and is carried for that use by war-vessels of the English and other navies.
The explosive gun-cotton is not the only kind made; another sort (in which a less amount of the hydrogen of the cotton is displaced) is used for making collodion, largely employed by photographers. This variety, called collodion gun-cotton, combined with nitro-glycerine, forms a new explosive* agent called gum-dynamite, or explosive gelatine. Singular to relate, the ordinary gun-cotton used for explosive purposes will not enter into this combination, and hence probably the late discovery of the fact that it requires the collodion variety to do so. This is finely shredded, generally by hand, and placed in small quantities at a time in the nitro-glycerine, which is kept at a temperature of 80 Fahr. by means of a water-bath, the whole being constantly stirred with a wooden spatula; the proportion of materials is seven per cent by weight of the gun-cotton to ninety-three per cent of the nitro-glycerine. The latter dissolves the former, and the result is an elastic, gelatinous, semi-transparent mass, which is easily cut or torn apart, and shows no trace whatever of nitro-glycerine on handling. Its explosive properties are unaffected by contact with water, and in this respect it is the most useful of all the high explosives for military purposes. With the change in the physical condition of the two components comes also a change in the ease of explosion; these two bodies, each of itself highly explosive, form when united one which is quite the reverse. When unconfined, a primer of fifty grains of fulminate will cause the explosion of but a very small portion of a charge, the rest being torn in pieces; if, however, it be strongly confined, so that the blow of the fulminate exerts its whole force, which is propagated through the gelatine, it then explodes with a violence as great as that of nitro-glycerine, if not somewhat greater. This latter point has not been fully determined, but the probabilities are that the expansion of the constituents of the gelatine is more complete and is accompanied with more heat than is the case with nitro-glycerine alone. The gelatine freezes at 40° Fahr., and in this state is fired with no difficulty whatever, being in this respect much superior to dynamite. When subjected to a pressure of two hundred and fifty pounds to the square inch, no nitro-glycerine is separated; the union between the two constituents seems to be complete and definite. If subjected to the action of flame, it takes fire less readily than dynamite, but burns very much like it, with perhaps a greater strength of flame, as if urged by a bellows. When heated to 100° it softens, but does not become at all greasy, and there is no exudation of nitro-glycerine. Explosion by the application of heat takes place at about 420°; but it is found that by the addition of a small amount of camphor, say four per cent, it will bear an increased heat of 100° before explosion. Experiments made with the gelatine thus camphorated show that the camphor exercises no deleterious effect upon the strength of the material, while rendering it less like jelly, and more like that form of confection known as fig-paste. Six per cent of camphor may be added without harm, but any greater quantity materially diminishes the explosive effect. Portions of this gelatine, both pure and camphorated, have been subjected to a constant heat of 100° for more than six weeks, and no exudation of the dangerous nitro-glycerine has been observed. It will not explode under circumstances which ordinarily render certain the detonation of either nitro-glycerine or dynamite—that is to say,' a quantity of the gelatine will resist the shock of the detonation of another quantity placed within a very few feet of it; if very near, it may take fire and burn, but detonation will not ensue unless the two masses are almost in actual contact, and even then it will not always occur. It further possesses the property of permitting the impact of a ball from a gun without exploding, while both dynamite and gun-cotton may be readily detonated by a blow of this kind. All these tests tend to show that it possesses in a high degree the elements desired in the ideal high explosive for military purposes, if not for commercial use.
So much difficulty was encountered in the first attempts at the construction of a suitable primer for its explosion, that it seemed doubtful whether it would ever be a practicable material, as it was thought that nitro-glycerine must be used to accomplish the desired result. Subsequent experiments conducted in this country have shown, however, that a dry gun-cotton fuse with a fulminate cap containing twenty-five grains will fire the gelatine with ease and certainty, even when unconfined. The problem so long confronting the manufacturer of explosives would seem to be nearly solved: the requisites of great power in small compass, of permanency when subjected to tropical heat, of ease of firing when but slightly confined, of safety from the explosion of neighboring masses of the same or on being struck by a projectile, and of not being affected injuriously by water, all seem to be fulfilled by this agent in a manner more complete than by any other.
If it should be found that a long-continued exposure to heat tends to produce decomposition, as may prove to be the case, greater care in the preparation of the materials from which it is manufactured will probably overcome this difficulty, and it will then bid fair to supersede gun-cotton for very many purposes, if it does not altogether take its place.
Constant allusion has been made to the use of fulminate of mercury as an agent for the firing of other explosives. It is prepared by dissolving mercury in nitric acid, and then mixing this solution with alcohol, in a vessel placed in a hot-water bath. Dense white fumes soon arise from the agitated liquid, until finally, the disturbance having subsided, the bottom of the vessel is found covered with a gray powder, which is afterward thoroughly washed. This gray powder is the fulminate used in the caps and cartridges familiar to sportsmen, as well as in the primers for cannon and the fuses for the explosion of a quantity of gunpowder or other explosives. Being harmless when wet, it is usually kept and handled in that condition. Generally speaking, electricity is the agent by means of which the fulminate is ignited; the fuse for this purpose is ordinarily constructed as follows: A brass or copper cylinder, about half an inch in diameter, closed at one end, is partially filled with the desired quantity of the wet fulminate; when this has become thoroughly dry, a wooden plug closing the entrance is inserted; in this plug are two holes, through each of which passes an insulated copper wire with bared ends, which project a short distance above the surface of the plug and are connected by a very small wire composed of an alloy of platinum and silver; around this wire, or bridge, as it is called, is twisted a small wisp of dry guncotton, which, when the plug is in place, comes in contact with the fulminate. After the insertion of the plug, the whole fuse is dipped in some water-proof composition and thoroughly dried. In use, the wires are connected with other wires leading from a galvanic battery or an electrical machine; when the current is caused to pass through these wires it reaches the bridge, where meeting with greater resistance to its progress, it raises the platinum wire to a heat sufficient to ignite the gun-cotton wisp, which in turn ignites the fulminate. It will be seen that in all cases it is absolutely necessary to keep the ultimate explosive dry, as even those high explosives which are not themselves affected by water require the use of perfectly dry primers. The orders of Cromwell must still be obeyed—to "trust in God, and keep your powder [or primers] dry."