The sharpening of bits of a form other than that of the chisel is done by means of “swages.” The tempering is effected in the way already described in reference to hand drills. As in the latter case, the degree of temper must be suited to the hardness of the rocks to be penetrated. Generally the straw colour will be found to be the best degree. It is a remarkable fact that the wear of the cutting edge of a machine drill is, for a given length of boring, five or six times less than that of a hand drill. Steel of the best quality should always be used.

As in the case of hand boring, each successive length of drill must diminish slightly in the width of its cutting edge; a diminution of about ¹⁄₃₂ inch may be considered sufficient. Care should, however, be taken to ensure the proper dimensions being given to the edge, and it will be found advantageous to have at hand an accurate gauge through which the tool may be passed previously to its being fixed to the machine. It is important that the tool be truly “centred,” that is, the centres of the edge of the bit, of the shank, and of the piston rod, should be perfectly coincident.

Rock-Drill Supports.

—A machine rock-drill may satisfy every requirement, and yet, by reason of the defective character of the support to which it is attached, it may be unsuitable to the work required of it. Hence it becomes desirable to carefully study the design and construction of a drill support, and to consider the requirements which it is needful to fulfil. Assuming the necessity for a high degree of strength and rigidity in the support, a primary condition is that it shall allow the machine to be readily adjusted to any angle, so that the holes may be bored in the direction and with the inclination required. When this requirement is not fulfilled, the machine is placed, in this respect, at a great disadvantage with hand labour. If a machine drill were not capable of boring in any position and in any direction, hand labour would have to be employed in conjunction with it, and such incompleteness in the work of a machine would constitute a serious objection to its adoption.

Besides allowing of the desired adjustment of the machine, the support must be itself adjustable to uneven ground. The bottom of a shaft which is being sunk, or the sides, roof, and floor of a heading which is being driven, present great irregularities of surface, and, as the support must of necessity in most cases be fixed to these, it is obvious that its design and construction must be such as will allow of its ready adjustment to these irregularities. The means by which the adjustment is effected should be few and simple, for simplicity of parts is important in the support as well as in the machine, and for the same reasons. A large proportion of the time during which a machine drill is in use is occupied in shifting it from one position or one situation to another; this time reduces, in a proportionate degree, the superiority of machine over hand labour, in respect of rapidity of execution, and it is evidently desirable that it should be shortened as far as possible. Hence the necessity for the employment of means of adjustment which shall be few in number, rapid in action, and of easy management.

For reasons similar to the foregoing, the drill support must be of small dimensions, and sufficiently light to allow of its being easily portable. The limited space in which rock drills are used renders this condition, as in the case of the machine itself, a very important one. It must be borne in mind that, after every blast, the dislodged rock has to be removed, and rapidity of execution requires that the operations of removal should be carried on without hindrance. A drill support that occupies a large proportion of the free space in a shaft or a heading is thus a cause of inconvenience and a source of serious delay. Moreover, as it has to be continually removed from one situation to another, it should be of sufficiently light weight to allow of its being lifted or run along without difficulty. In underground workings, manual power is generally the only power available, and therefore it is desirable that both the machine and its support should be of such weight that each may be lifted by one man. Of course, when any endeavour is made to reduce the weight of the support, the necessity for great strength and rigidity must be kept in view.

In spacious headings, such as are driven in railway tunnel work, supports of a special kind may be used. In these situations, the conditions of work are different from those which exist in mines. The space is less limited, the heading is commenced at surface, and the floor is laid with a tramway and sidings. In such a case, the support may consist of a more massive structure mounted upon wheels to run upon the rails. This support will carry several machines, and to remove it out of the way when occasion requires, it will be run back on to a siding; but for ordinary mining purposes, such a support is suitable.

The Stretcher Bar.

—The simplest kind of support is the “stretcher bar.” This consists essentially of a bar so constructed that it may be lengthened or shortened at pleasure, by means of a screw. It is fixed in position by screwing the ends into firm contact with the sides, or with the roof and the floor, of a heading. The machine is fixed to this bar by means of a clamp, which, when loosened, slides along the bar, and allows the drill to be placed in the required position, and to be directed at the required angle. The bar illustrated in Fig. 26, Plate V., is that which is used with the Darlington drill; in it, lightness and rigidity are combined in the highest possible degree by the adoption of the hollow section. The mode of setting the bar in a heading is shown in the drawing; the end claws are set against pieces of wood on the floor and the roof, and are tightened by turning the screw with a common bar.

The simple stretcher bar is frequently used in narrow drivings and in shafts of small diameter. But a more satisfactory support in drivings is afforded by a bar suitably mounted upon a carriage designed to run upon rails. The carriage consists simply of a trolly, to the fore part of which the bar is fixed usually by some kind of hinge-joint. It is obvious that the details of the construction of this support may be varied greatly, and numerous designs have been introduced and adopted. In Figs. 27 and 28, Plate VI., is shown a support of this character designed by J. Darlington. A single vertical bar is carried on the fore part of the trolly, and fixed, by the usual means, against the centre of the roof. This vertical bar carries an arm, which is capable of turning upon it, as upon a centre, and of sliding up and down it. This arm carries the drill. The central bar having been fixed in position, the arm is slid up to the highest position required, and fixed against the side of the heading. A row of holes are then bored from this arm. When these are completed, the arm is lowered the requisite distance, and another row of holes are bored. This is continued until all the holes are bored over one-half the face. The arm is then swung round, and fixed against the other side of the heading, and the holes are bored over that half the face in like manner. In this way, one-half the heading is kept clear to allow the operations of removing the dislodged rock to be carried on at the same time. If desired, two arms may be used. This arrangement gives undoubtedly great facilities for working the drill, and leaves the heading comparatively unencumbered.

In shaft sinking, the same support, slightly modified, is used without the trolly. The arrangement adopted in this case is shown in Fig. 29, Plate VII. The central bar is held firmly in its position by a cross stretcher bar set against the sides of the shaft. The arms are made to revolve upon this bar to allow the holes to be bored in the positions required. When all the holes have been bored, the support, with the machines, is hauled up, by means of a chain attached to the central bar, out of the way of the blast. With this support, the time of fixing, raising, and lowering is reduced to a minimum; while the facility with which the machines may be slid along and fixed to the arm, and the positions of the latter changed, allows the boring to be carried on rapidly.

For open work, as in quarrying, where the stretcher bar cannot be used, the tripod stand is adopted.

The Dubois-François Carriage.

—The support commonly used in France and in Belgium consists of a kind of carriage carrying bars upon which the drills are set. This carriage is used in drivings of all kinds; but it is particularly suitable for tunnelling. It has been adopted, with but slight modification, in the St. Gothard tunnel, and in several other important works of the like character.

A modification of the carriage is shown in Figs. 30 and 31. Being designed for ordinary mining operations, it carries but two machines; but it will be readily perceived that, by increasing the number of vertical screws, the same support may be made to carry a larger number. It consists essentially of a vertical frame of flat bar iron a b c d, 8 feet in length, and 4 feet 9 inches in height above the rails, the hinder portion of which rests upon a cast-iron plate e f g h, carried upon two wheels; on this are fixed the two uprights l, l′, which, being bound to the upper part by a transverse bar m m′, form a framing to serve as a support to the two vertical screws p′, q′. The front framing is formed of two longitudinals b c and b′ c′ and the uprights a, a′, and the vertical screws p, q, which are connected to the upper part by the single piece a d. This framing is supported below upon a small trolly with four wheels, connected to the two longitudinals of the framing by a pivot bolt n of T form, the bar of the T being inserted into the elongated openings o cut through the middle of the curved portion of the longitudinals. The cast-iron plate behind, the use of which is only to give stability to the carriage, carries above it, by means of the two curved pieces h, h′, a wrought-iron plate V, upon which the small tools needed for repairs are kept. Two screws, s, s′, carried by lugs cast upon the back of the plate, serve, by turning them down upon the rails, to fix the carriage, the latter being slightly lifted by the screws.

Each machine is supported at two points. Behind, the point of support is given by a cast-iron bracket t, having a projecting eye which enters between the two cheeks formed at the back end of the machine by the continuations of the two longitudinals of the framing. A pin bolt, carried by the machine, allows the latter to be fixed to the bracket, while leaving sufficient freedom of motion to allow of its being directed at the required angle. This bracket, shown in plan in Fig. 33, is supported by a kind of nut, Fig. 32, having two handles whereby it may be easily turned. By raising or lowering this, the hinder support of the drill may be brought to the requisite height. To prevent it turning upon the screw, a pin is passed through the hole o, which pin forms a stop for the handles aforementioned. The rotation of the bracket itself is rendered impossible by the form of the vertical screw upon which it is set, as shown in Fig. 33. In front, the support is a fork, the shank of which slides along in the piece U, Figs. 30 and 31. This support, which is not screwed on the inside, rests upon a nut of the same form as that already described, and the same means are employed to prevent rotation as in the case of the hinder supports.

Section III.—Appliances for Firing Blasting Charges.

In the foregoing sections, the machines and tools used in rock boring have been treated of. It now remains to describe those which are employed in firing the charges after they have been placed in the bore-holes. In this direction, too, great progress has been made in recent times. With the introduction of new explosive agents, arose the necessity for improved means of firing them. Attention being thus directed to the subject, its requirements were investigated and its conditions observed, the outcome being some important modifications of the old appliances and the introduction of others altogether new. Some of the improvements effected are scarcely less remarkable than the substitution of machine for hand boring.

The means by which the charge of explosive matter placed in the bore-hole is fired constitute a very important part of the set of appliances used in blasting. The conditions which any such means must fulfil are: (1) that it shall fire the charge with certainty; (2) that it shall allow the person whose duty it is to explode the charge to be at a safe distance away when the explosion takes place; (3) that it shall be practically suitable, and applicable to all situations; and (4) that it shall be obtainable at a low cost. To fulfil the second and most essential of these conditions, the means must be either slow in operation, or capable of being acted upon at a distance. The only known means possessing the latter quality is electricity. The application of electricity to this purpose is of recent date, and in consequence of the great advantages which it offers, its use is rapidly extending. The other means in common use are those which are slow in operation, and which allow thereby sufficient time to elapse between their ignition and the explosion of the charge for a person to retire to a safe distance. These means consist generally of a train of gunpowder so placed that the ignition of the particles must necessarily be gradual and slow. The old, and in some parts still employed, mode of constructing this train was as follows: An iron rod of small diameter and terminating in a point, called a “pricker,” was inserted into the charge and left in the bore-hole while the tamping was being rammed down. When this operation was completed, the pricker was withdrawn, leaving a hole through the tamping down to the charge. Into this hole, a straw, rush, quill, or some other like hollow substance filled with gunpowder, was inserted. A piece of slow-match was then attached to the upper end of this train, and lighted.

The combustion of the powder confined in the straw fired the charge, the time allowed by the slow burning of the match being sufficient to enable the man who ignited it to retire to a place of safety. This method of forming the train does not, however, satisfy all the conditions mentioned above. It is not readily applicable to all situations. Moreover, the use of the iron pricker may be a source of danger; the friction of this instrument against silicious substances in the sides of the bore-hole or in the tamping has in some instances occasioned accidental explosions. This danger is, however, very greatly lessened by the employment of copper or phosphor-bronze instead of iron for the prickers. But the method is defective in some other respects. With many kinds of tamping, there is a difficulty in keeping the hole open after the pricker is withdrawn till the straw can be inserted. When the holes are inclined upwards, besides this difficulty, another is occasioned by the liability of the powder constituting the train to run out on being ignited. And in wet situations, special provision has to be made to protect the trains. Moreover, the manufacture of these trains by the workmen is always a source of danger. Many of these defects in the system may, however, be removed by the employment of properly constructed trains. One of these trains or “squibs” is shown full size in Fig. 28.

Safety Fuse.

—Many of the defects pertaining to the system were removed by the introduction of the fuse invented by W. Bickford, and known as “safety fuse.” The merits of this fuse, which is shown full size in Fig. 29, are such as to render it one of the most perfect of the slow-action means that have yet been devised. The train of gunpowder is retained in this fuse, but the details of its arrangement are changed so as to fairly satisfy the conditions previously laid down as necessary. It consists of a flexible cord composed of a central core of fine gunpowder, surrounded by hempen yarns twisted up into a tube, and called the countering. An outer casing is made of different materials, according to the circumstances under which it is intended to be used. A central touch thread, or in some cases two threads, passes through the core of gunpowder. This fuse, which in external appearance resembles a piece of plain cord, is tolerably certain in its action; it may be used with equal facility in holes bored in any direction; it is capable of resisting considerable pressure without injury; it may be used without special means of protection in wet ground; and it may be transported from place to place without risk of damage.

In the safety fuse, the conditions of slow burning are fully satisfied, and certainty is in some measure provided for by the touch thread through the centre of the core. As the combustion of the core leaves, in the small space occupied by it, a carbonaceous residue, there is little or no passage left through the tamping by which the gases of the exploding charge may escape, as in the case of the squibs. Hence results an economy of force. Another advantage offered by the safety fuse is, that it may be made to carry the fire into the centre of the bursting charge if it be desired to produce rapid ignition. This fuse can be also very conveniently used for firing charges of compounds other than gunpowder, by fixing a detonating charge at the end of it, and dropping the latter into the charge of the compound. This means is usually adopted in firing the nitro-glycerine compounds, the detonating charge in such cases being generally contained within a metallic cap. In using this fuse, a sufficient length is cut off to reach from the charge to a distance of about an inch, or farther if necessary, beyond the mouth of the hole. One end is then untwisted to a height of about a quarter of an inch, and placed to that depth in the charge. The fuse being placed against the side of the bore-hole with the other end projecting beyond it, the tamping is put in, and the projecting end of the fuse slightly untwisted. The match may then be applied directly to this part. The rate of burning is about two and a half feet a minute.

Safety fuse is sold in coils of 24 feet in length. The price varies according to the quality, and the degree of protection afforded to the train.

Electric Fuses.

—The employment of electricity to fire the charge in blasting rock offers numerous and great advantages. The most important, perhaps, is the greatly increased effect of the explosions when the charges are fired simultaneously. But another advantage, of no small moment, lies in the security from accident which this means of firing gives. When electricity is used, not only may the charge be fired at the moment desired, after the workmen have retired to a place of safety, but the danger due to a misfire is altogether avoided. Further, the facility afforded by electricity for firing charges under water is a feature in this agent of very great practical importance. It would therefore seem, when all these advantages are taken into account, that electricity is destined to become of general application to blasting purposes in this country, as it is already in Germany and in America.

An electric fuse consists of a charge of an explosive compound suitably placed in the circuit of an electric current, which compound is of a character to be acted upon by the current in a manner and in a degree sufficient to produce explosion. The mode in which the current is made to act depends upon the nature of the source of the electricity. That which is generated by a machine is of high tension, but small in quantity; while that which is generated by a battery is, on the contrary, of low tension, but is large in quantity. Electricity of high tension is capable of leaping across a narrow break in the circuit, and advantage is taken of this property to place in the break an explosive compound sufficiently sensitive to be decomposed by the passage of the current. The electricity generated in a battery, though incapable of leaping across a break in the circuit, is in sufficient quantity to develop a high degree of heat. Advantage is taken of this property to fire an explosive compound by reducing the sectional area of the wire composing a portion of the circuit at a certain point, and surrounding this wire with the compound. It is obvious that any explosive compound may be fired in this way; but for the purpose of increasing the efficiency of the battery, preference is given to those compounds which ignite at a low temperature. Hence it will be observed that there are two kinds of electric fuses, namely, those which may be fired by means of a machine, and which are called “tension” fuses, and those which require a battery, and which are known as “quantity” fuses.

In the tension, or machine fuses, the circuit is interrupted within the fuse case, and the priming, as before remarked, is interposed in the break; the current, in leaping across the interval, passes through the priming. In the quantity or battery fuses, the reduction of the sectional area is effected by severing the conducting wire within the fuse case, and again joining the severed ends of the wire by soldering to them a short piece of very fine wire. Platinum wire, on account of its high resistance and low specific heat, is usually employed for this purpose. The priming composition is placed around this fine wire, which is heated to redness by the current as soon as the circuit is closed.

The advantages of high tension lie chiefly in the convenient form and ready action of the machines employed to excite the electricity. Being of small dimensions and weight, simple in construction, and not liable to get quickly out of order, these sources of electricity are particularly suitable for use in mining operations, especially when these operations are entrusted, as they usually are, to men of no scientific knowledge.

Another advantage of high tension is the small effect of line resistance upon the current, a consequence of which is that mines may be fired at long distances from the machine, and through iron wire of very small section. A disadvantage of high tension is the necessity for a perfect insulation of the wires.

When electricity of low tension is employed, the insulation of the wires needs not to be perfect, so that leakages arising from injury to the coating of the wire are not of great importance. In many cases, bare wires may be used. Other advantages of low tension are the ability to test the fuse at any moment by means of a weak current, and an almost absolute certainty of action. For this reason, it is usually preferred for torpedoes and important submarine work. On the other hand, the copper wires used must be of comparatively large section, and the influence of line resistance is so considerable that only a small number of shots can be fired simultaneously when the distance is great.

In Fig. 30 is shown an external view of an electric tension fuse. It consists of a metal cap containing a detonating composition, upon the top of which is placed the priming to be ignited by the electric spark. The ends of two insulated wires project into this priming, which is fired by the passage of the spark from one of these wires to the other. The insulated wires are sufficiently long to reach a few inches beyond the bore-hole.

Sometimes the fuse is attached to the end of a stick, and the wires are affixed to the latter in the manner shown in Fig. 31. The rigidity of the stick allows the fuse to be readily pushed into the bore-hole. When the ground is not very wet, bare wires are, for cheapness, used, and the stick is in that case covered with oiled paper, or some other substance capable of resisting moisture. These “blasting sticks,” as they are called, are extensively used in Germany. When heavy tamping is employed, the stick is not suitable, by reason of the space which it occupies in the bore-hole.

A mode of insulating the wires, less expensive than the guttapercha shown in Fig. 30, is illustrated in Fig. 32. In this case, the wires are cemented between strips of paper, and the whole is dipped into some resinous substance to protect it from water. These “ribbon” wires may be used in ground that is not very wet. They occupy little or no space in the bore-hole, and therefore are suitable for use with tamping.

To connect the fuses with the machine or the battery, two sets of wires are required when a single shot is fired, and three sets may be needed when two or more shots are fired simultaneously. Of these several sets of wires, the first consists of those which are attached to the fuses, and which, by reason of their being placed in the shot-hole, are called the “shot-hole wires.” Two shot-hole wires must be attached to each fuse, and they must be of such a length that, when the fuse has been placed in its proper position in the charge, the ends may project a few inches from the hole. These wires must also be “insulated,” that is, covered with a substance capable of preventing the escape of electricity.

The second set of wires consists of those which are employed to connect the charges one with another, and which, for this reason, are called “connecting wires.” In connecting the charges in single circuit, the end of one of the shot-hole wires of the first charge is left free, and the other wire is connected, by means of a piece of this connecting wire, to one of the shot-hole wires in the second hole; the other wire in this second hole is then connected, in the same manner, to one of the wires in the third hole; and so on till the last hole is reached, one shot-hole wire of which is left free, as in the first. Whenever the connecting wires can be kept from touching the rock, and also from coming into contact one with another—and in most cases this may be done—bare wire may be used, the cost of which is very little. But when this condition cannot be complied with, and, of course, when blasting in water, the connecting wires, like the shot-hole wires, must be insulated. When guttapercha shot-hole wires are used, it is best to have them sufficiently long to allow the ends projecting from one hole to reach those projecting from the next hole. This renders connecting wire unnecessary, and moreover saves one joint for each shot.

Cables.

—The third set of wires required consists of those used to connect the charges with the machine or the battery. These wires, which are called the “cables,” consist each of three or more strands of copper wire well insulated with guttapercha, or better, indiarubber, the coating of these materials being protected from injury by a sheathing of tape or of galvanized iron wire underlaid with hemp. Two cables are needed to complete the circuit; the one which is attached to the positive pole of the machine, that is, the pole through which the electric current passes out, is distinguished as the “leading cable,” and the other, which is attached to the negative pole, that is, the pole through which the current returns to the machine, is described as the return cable. Sometimes both the leading and the return cables are contained within one covering. When a cable having a metallic sheathing is used, the sheathing may be made to serve as a return cable, care being taken to make good metallic contact with the wires that connect the sheathing to the fuses and to the terminal of the machine. The best kind of unprotected cable consists of a three-strand tinned copper wire, each 0·035 inch in diameter, insulated with three layers of indiarubber to 0·22 inch diameter, and taped with indiarubber-saturated cotton to 0·24 inch diameter, as shown in Fig. 33. The best protected cable consists of a similar strand of copper wire, covered with guttapercha and tarred jute, and sheathed with fifteen galvanized iron wires of 0·08 inch diameter each, to a total diameter of 0·48 inch, as shown in Fig. 34.

Detonators.

—The new explosives of the nitro-cotton and nitro-glycerine class cannot be effectively fired by means of safety or other fuse alone. To bring about their instantaneous decomposition, it is necessary to produce in their midst the explosion of some other substance. The force of this initial explosion causes the charge of gun-cotton, or dynamite, as the case may be, to detonate. It has been found that the explosion of the fulminate of mercury brings about this result most effectively and with the greatest certainty; and this substance is therefore generally used for the purpose. The charge of fulminate is contained in a copper capsule about a quarter of an inch in diameter, and from 1 inch to 1¹⁄₄ inch in length. These caps, with their charge of fulminate, which are now well known to users of the nitro-compounds, are called “detonators.” It is of the highest importance that these detonators should contain a sufficiently strong charge to produce detonation, for if too weak, not only is the whole force of the explosive not developed, but a large quantity of noxious gas is generated. Gun-cotton requires a much stronger charge of fulminate than dynamite.

In the electric fuses illustrated, the metal case shown is the detonator, the fuse being placed inside above the fulminate. When safety fuse is used, the end is cut off clean and inserted into the cap, which is then pressed tightly upon the fuse by means of a pair of nippers, as shown in Fig. 35. When water tamping is used, and when, with ordinary tamping, the hole is very wet, a little white-lead or grease must be put round the edge of the cap as a protection. The electric fuses are always made waterproof; consequently, they are ready for use under all circumstances. When the safety fuse burns down into the cap, or when, in the other case, the priming of the electric fuse is fired, the fulminate explodes and causes the detonation of the charge in which it is placed.

Firing Machines and Batteries.

—The electrical machines used for firing tension fuses are of two kinds. In one kind, the electricity is excited by friction, and stored in a condenser to be afterwards discharged by suitable means provided for the purpose. In the other kind, the electricity is excited by the motion of an armature before the poles of a magnet. The former kind are called “frictional electric” exploders; the latter kind are known as “magneto-electric” exploders. When a magneto-electric machine contains an electro-magnet instead of a permanent magnet, it is described as a “dynamo-electric exploder.”

Frictional machines act very well as exploders so long as they are kept in a proper state. But as they are injuriously affected by a moist atmosphere, and weaken rapidly with use by reason of the wearing away of the rubbers, it is necessary to take care that they be in good electrical condition before using them for firing. Unless this care be taken, the quantity of electricity excited by a given number of revolutions of the plate will be very variable, and vexatious failures will ensue. If, however, the proper precautions be observed, very certain and satisfactory results may be obtained. In Germany and in America, frictional exploders are generally used.

Magneto-electric machines possess the very valuable quality of constancy. They are unaffected, in any appreciable degree, by atmospheric changes, and they are not subject to wear. These qualities are of inestimable worth in an exploder used for ordinary blasting operations. Moreover, as they give electricity of a lower tension than the frictional machines, defects of insulation are less important. Of these machines, only the dynamo variety are suitable for industrial blasting. It is of primary importance that an exploder should possess great power. The mistake of using weak machines has done more than anything else to hinder the adoption of electrical firing in this country.

The machine most used in Germany is Bornhardt’s frictional exploder, shown in Fig. 36. This machine is contained in a wooden case 20 inches in length, 7 inches in breadth, and 14 inches in depth, outside measurement. The weight is about 20 lb.

To fire the charges by means of this exploder, the leading wire is attached to the upper terminal B, and the return to the lower terminal C, the other ends of these wires being connected to the fuses. The handle is then turned briskly from fifteen to thirty times, according to the number of the fuses and the state of the machine, to excite the electricity. The knob A is then pressed suddenly in, and the discharge takes place. To ascertain the condition of the machine, a scale of fifteen brass-headed nails is provided on the outside, which scale may be put in communication with the poles B and C by means of brass chains, as shown in the drawing. If after twelve or fourteen turns, the spark leaps the scale when the knob is pushed in, the machine is in a sufficiently good working condition. To give security to the men engaged, the handle is designed to be taken off when the machine is not in actual use; and the end of the machine into which the cable wires are led is made to close with a lid and lock, the key of which should be always in the possession of the man in charge of the firing operations.

In America, there are two frictional exploders in common use. One, shown in Fig. 37, is the invention of H. Julian Smith. The apparatus is enclosed in a wooden case about 1 foot square and 6 inches in depth. The handle is on the top of the case, and is turned horizontally. This handle is removable, as in Bornhardt’s machine. The cable wires having been attached to the terminals, the handle is turned forward a certain number of times to excite the electricity, and then turned a quarter of a revolution backward to discharge the condenser and to fire the blast. By this device, the necessity for a second aperture of communication with the inside is avoided, an important point in frictional machines, which are so readily affected by moisture. The aperture through which the axis of the plate passes, upon which axis the handle is fixed, is tightly closed by a stuffing-box. A leathern strap on one end of the case allows the machine to be easily carried. The weight of this exploder is under 10 lb.

The other exploder used is that designed by G. Mowbray. This machine, which is shown in Fig. 38, is contained in a wooden barrel-shaped case, and is known as the “powder-keg” exploder, the form and dimensions of the case being those of a powder-keg. The action is similar to that of the machine last described. The cable wires having been attached to the terminals at one end of the keg, the handle at the other end is turned forward to excite the electricity, and the condenser is discharged by making a quarter turn backward, as in Smith’s machine. The handle is in this case also removable. The weight of the powder-keg exploder is about 26 lb.

Both of these machines are very extensively used, and good results are obtained from them. They stand well in a damp atmosphere, and do not quickly get out of order from the wearing of the rubbers. They are also, especially the former, very easily portable.

The machine commonly used in England is the dynamo-electric exploder of the Messrs. Siemens. This machine, which is the best of its kind yet introduced for blasting purposes, is not more than half the size of Bornhardt’s frictional exploder; but it greatly exceeds the latter in weight, that of Siemens’ being about 55 lb. The apparatus, which is contained within the casing shown in Fig. 39, consists of an ordinary Siemens’ armature, which is made, by turning the handle, to revolve between the poles of an electro-magnet. The coils of the electro-magnet are in circuit with the wire of the armature; the residual magnetism of the electro-magnet cores excites, at first, weak currents; these pass into the coils, thereby increasing the magnetism of the cores, and inducing still stronger currents in the armature wire, to the limit of magnetic saturation of the iron cores of the electro-magnets. By the automatic action of the machine, this powerful current is, at every second turn of the handle, sent into the cables leading to the fuses.

To fire this machine, the handle is turned gently till a click is heard from the inside, indicating that the handle is in the right position to start from. The cable wires are then attached to the terminals, and the handle is turned quickly, but steadily. At the completion of the second revolution, the current is sent off into line, as it is termed, that is, the current passes out through the cables and the fuses. As in the case of the frictional machines, the handle is, for safety, made removable. This exploder is practically unaffected by moisture, and it is not liable to get out of order from wear.

Induction coils have been used to fire tension fuses; but it is surprising that they have not been more extensively applied to that purpose. A coil designed for the work required of it is a very effective instrument. If constructed to give a spark not exceeding three inches in length, with comparatively thick wire for quantity, it makes a very powerful exploder. An objection to its use is the necessity for a battery. But a few bichromate of potash cells, provided with spiral springs to hold the zincs out of the liquid, and designed to be set in action by simply pressing down the zincs, give but little trouble, so that the objection is not a serious one. The writer has used an induction exploder in ordinary mining operations without experiencing any difficulty or inconvenience. It is cheap, easily portable, and constant in its action.

Batteries are used to fire what are known as “quantity” or “low tension” fuses. Any cells may be applied to this purpose; but they are not all equally suitable. A firing battery should require but little attention, and should remain in working order for a long time. These conditions are satisfactorily fulfilled by only two cells, namely, the Léclanché and the Bichromate of Potash. The latter is the more powerful, and generally the more suitable. The Léclanché is much used in this country for firing purposes, under the form known as the “Silvertown Firing Battery.” This battery consists of a rectangular teak box, containing ten cells. Two, or more, of these may be joined up together when great power is required. In France, the battery used generally for firing is the Bichromate. This battery is much more powerful than the Léclanché, and as no action goes on when the zincs are lifted out of the liquid, it is equally durable. It is moreover much cheaper. At the suggestion of the writer, Mr. Apps, of the Strand, London, has constructed a bichromate firing battery of very great power. It is contained in a box of smaller dimension than the 10-cell Silvertown. The firing is effected by simply lowering the zincs, which rise again automatically out of the liquid, so that there is no danger of the battery exhausting itself by continuous action in case of neglect. Externally, this battery, like the Silvertown, appears a simple rectangular box, so that no illustration is needed. With either of these, the usual objections urged against the employment of batteries, on the ground of the trouble involved in keeping them in order, and their liability to be injured by ignorant or careless handling, do not apply, or at least apply in only a very unimportant degree.

To guard against misfires, the machine or the battery used should be constructed to give a very powerful current. If this precaution be observed, and the number of fuses in circuit be limited to one-half that which the machine is capable of firing with a fair degree of certainty, perfectly satisfactory results may be obtained. The employment of weak machines and batteries leads inevitably to failure. In the minds of those who have hitherto tried electrical blasting in this country, there seems to be no notion of any relation existing between the work to be done and the force employed to do it. The electrical exploder is regarded as a sort of magic box that needs only to be set in action to produce any required result. Whenever failure ensues, the cause is unhesitatingly attributed to the fuses.

CHAPTER II.
EXPLOSIVE AGENTS USED IN BLASTING ROCKS.

Section I.—Phenomena accompanying an Explosion.

Nature of an Explosion.

—The combination of oxygen with other substances for which it has affinity is called generally “oxidation.” The result of this combination is a new substance, and the process of change is accompanied by the liberation of heat. The quantity of heat set free when two substances combine chemically is constant, that is, it is the same under all conditions. If the change takes place within a short space of time, the heat becomes sensible; but if the change proceeds very slowly, the heat cannot be felt. The same quantity, however, is liberated in both cases. Thus, though the quantity of heat set free by a chemical combination is under all conditions the same, the degree or intensity of the heat is determined by the rapidity with which the change is effected.

When oxidation is sufficiently rapid to cause a sensible degree of heat, the process is described as “combustion.” The oxidation of a lump of coke in the furnace, for example, is effected within a short space of time, and, as the quantity of heat liberated by the oxidation of that weight of carbon is great, a high degree results. And it is well known and obvious that as combustion is quickened, or, in other words, as the time of change is shortened, the intensity of the heat is proportionally increased. So in the case of common illuminating gas, the oxidation of the hydrogen is rapidly effected, and, consequently, a high degree of heat ensues.

When oxidation takes place within a space of time so short as to be inappreciable to the senses, the process is described as “explosion.” The combustion of a charge of gunpowder, for example, proceeds with such rapidity that no interval can be perceived to intervene between the commencement and the termination of the process. Oxidation is in this case, therefore, correctly described as an explosion; but the combustion of a train of gunpowder, or of a piece of quick-match, though exceedingly rapid, yet, as it extends over an appreciable space of time, is not to be so described. By analogy, the sudden change of state which takes place when water is “flashed” into steam, is called an explosion. It may be remarked here that the application of this expression to the bursting of a steam boiler is an abuse of language; as well may we speak of an “explosion” of rock.

From a consideration of the facts stated in the foregoing paragraphs, it will be observed that oxidation by explosion gives the maximum intensity of heat.

Measure of Heat, and specific Heat.

—It is known that if a certain quantity of heat will raise the temperature of a body one degree, twice that quantity will raise its temperature two degrees, three times the quantity, three degrees, and so on. Thus we may obtain a measure of heat by which to determine, either the temperature to which a given quantity of heat is capable of raising a given body, or the quantity of heat which is contained in a given body at a given temperature. The quantity of heat requisite to produce a change of one degree in temperature is different for different bodies, but is practically constant for the same body, and this quantity is called the “specific heat” of the body. The standard which has been adopted whereby to measure the specific heat of bodies is that of water, the unit being the quantity of heat required to raise the temperature of 1 lb. of water through 1° Fahr., say from 32° to 33°. The quantity of heat required to produce this change of temperature in 1 lb. of water is called the “unit of heat,” or the “thermal unit.” Having determined the specific heat of water, that of air may in like manner be ascertained, and expressed in terms of the former. It has been proved by experiments that if air be heated at constant pressure through 1° Fahr., the quantity of heat absorbed is 0·2375 thermal units, whatever the pressure or the temperature of the air may be. Similarly it has been shown that the specific heat of air at constant volume is, in thermal units, 0·1687; that is, if the air be confined so that no expansion can take place, 0·1687 of a thermal unit will be required to increase its temperature one degree.

Heat liberated by an Explosion.

—In the oxidation of carbon, one atom of oxygen may enter into combination with one atom of that substance; the resulting body is a gas known as “carbonic oxide.” As the weight of carbon is to that of oxygen as 12 is to 16, 1 lb. of the former substance will require for its oxidation 1¹⁄₃ lb. of the latter; and since the two enter into combination, the product, carbonic oxide, will weigh 1 + 1¹⁄₃ = 2¹⁄₃ lb. The combining of one atom of oxygen with one of carbon throughout this quantity, that is, 1¹⁄₃ lb. of oxygen, with 1 lb. of carbon, generates 10,100 units of heat. Of this quantity, 5700 units are absorbed in changing the carbon from the solid into the gaseous state, and 4400 are set free. The quantity of heat liberated, namely, the 4400 units, will be expended in raising the temperature of the gas from 32° Fahr., which we will assume to be that of the carbon and the oxygen previous to combustion, to a much higher degree, the value of which may be easily determined. The 4400 units would raise 1 lb. of water from 32° to 32 + 4400 = 4432°; and as the specific heat of carbonic oxide is 0·17 when there is no increase of volume, the same quantity of heat will raise 1 lb. of that gas from 32° to 32 + 4400 0·17 = 25,914°. But in the case under consideration, we have 2¹⁄₃ lb. of the gas, the resulting temperature of which will be 25,9142¹⁄₃ = 9718°.

In the oxidation of carbonic oxide, one atom of oxygen combines with one atom of the gaseous carbon; the resulting body is a gas known as “carbonic acid.” Since 2¹⁄₃ lb. of carbonic oxide contains 1 lb. of carbon, that quantity of the oxide will require 1¹⁄₃ lb. of oxygen to convert it into the acid, that is, to completely oxidize the original pound of solid carbon. By this combination, 10,100 units of heat are generated, as already stated, and since the carbon is now in the gaseous state, the whole of that quantity will be set free. Hence the temperature of the resulting 3²⁄₃ lb. of carbonic acid will be

32 + 4400 + 10,100 0·17 × 3·667 = 23,516°.

It will be seen from the foregoing considerations that if 1 lb. of pure carbon be burned in 2²⁄₃ lb. of pure oxygen, 3²⁄₃ lb. of carbonic acid is produced, and 14,500 units of heat are liberated; and further, that if the gas be confined within the space occupied by the carbon and the oxygen previously to their combination, the temperature of the product may reach 23,516° Fahr.

In the oxidation of hydrogen, one atom of oxygen combines with two atoms of the former substance; the resulting body is water. As the weight of hydrogen is to that of oxygen as 1 is to 16, 1 lb. of the former gas will require for its oxidation 8 lb. of the latter; and since the two substances enter into combination, the product, water, will weigh 1 + 8 = 9 lb. By this union, 62,032 units of heat are generated. Of this quantity, 8694 are absorbed in converting the water into steam, and 53,338 are set free. The specific heat of steam at constant volume being 0·37, the temperature of the product of combustion, estimated as before, will be

32 + 53,338 0·37 × 9 = 16,049°.

Hence it will be observed that if 1 lb. of hydrogen be burned in 8 lb. of oxygen, 9 lb. of steam will be produced, and 53,338 units of heat will be liberated; and further, that the temperature of the product may reach 16,049°.

Gases generated by an Explosion.

—It was shown in the preceding paragraph that in the combustion of carbon, one atom of oxygen may unite with one atom of carbon to form carbonic oxide, or two atoms of oxygen may unite with one atom of carbon to form carbonic acid. When the combination takes place according to the former proportions, the reaction is described as “imperfect combustion,” because the carbon is not fully oxidized; but when the combination is effected in the latter proportions, the combustion is said to be “perfect,” because no more oxygen can be taken up. The products of combustion are in both cases gaseous. Carbonic oxide, the product of imperfect combustion, is an extremely poisonous gas; it is this gas which is so noisome in close headings, and in all ill-ventilated places, after a blast has been fired. A cubic foot of carbonic oxide, the specific gravity of which is 0·975, weighs, at the mean atmospheric pressure, 0·075 lb., so that 1 lb. will occupy a space of 13·5 cubic feet. Thus 1 lb. of carbon imperfectly oxidized will give 2¹⁄₃ lb. of carbonic oxide, which, at the mean atmospheric pressure of 30 inches and the mean temperature of 62° Fahr., will occupy a space of 13·5 × 2¹⁄₃ = 31·5 cubic feet. The product of perfect combustion, carbonic acid, is a far less noxious gas than the oxide, and it is much more easily expelled from confined places, because water possesses the property of absorbing large quantities of it. In an ill-ventilated but wet heading, the gas from a blast is soon taken up. Carbonic acid is a comparatively heavy gas, its specific gravity relatively to that of common air being 1·524. Hence a cubic foot at the ordinary pressure and temperature will weigh 0·116 lb., and 1 lb. of the gas under the same conditions will occupy a space of 8·6 cubic feet. Thus if 1 lb. of carbon be completely oxidized, there will result 3²⁄₃ lb. of carbonic acid, which will fill a space of 8·6 × 3²⁄₃ = 31·5 cubic feet. It will be observed that, though an additional pound of oxygen has been taken up during this reaction, the product occupies the same volume as the oxide. In complete combustion, therefore, a contraction takes place.

In the oxidation of hydrogen, as already pointed out, one atom of oxygen combines with two atoms of the former substance to form water. In this case, the product is liquid. But the heat generated by the combustion converts the water into steam, so that we have to deal with this product also in the gaseous state, in all considerations relating to the effects of an explosion. A cubic foot of steam, at atmospheric pressure and a temperature of 212° Fahr., weighs 0·047 lb.; 1 lb. of steam under these conditions will, therefore, occupy a space of 21·14 cubic feet. Thus the combustion of 1 lb. of hydrogen will produce 9 lb. of steam, which, under the conditions mentioned, will fill a space of 21·14 × 9 = 190·26 cubic feet.

Usually in an explosion a large quantity of nitrogen gas is liberated. This gas, which is not in itself noxious, has a specific gravity of 0·971, so that practically a cubic foot will weigh 0·075 lb., and 1 lb. will occupy a space of 13·5 cubic feet, which are the weight and the volume of carbonic oxide. Other gases are often formed as products of combustion; but the foregoing are the chief, viewed as the results of an explosion, since upon these the force developed almost wholly depends.

Force developed by an Explosion.

—A consideration of the facts enunciated in the foregoing paragraphs will show to what the tremendous energy developed by an explosion is due. It was pointed out that the combustion of 1 lb. of carbon gives rise to 31·5 cubic feet of gas. If this volume of gas be compressed within the space of 1 cubic foot it will obviously have a tension of 31·5 atmospheres; that is, it will exert upon the walls of the containing vessel a pressure of 472 lb. to the square inch. If the same volume be compressed into a space one-eighth of a cubic foot in extent, say a vessel of cubical form and 6 inches side, the tension will be 31·5 × 8 = 252 atmospheres, and the pressure 472 × 8 = 3776 lb. to the square inch. Assuming now the oxygen to exist in the solid state, and the two bodies carbon and oxygen to occupy together a space of one-eighth of a cubic foot, the combustion of the carbon will develop upon the walls of an unyielding containing vessel of that capacity a pressure of 252 atmospheres. Also the combustion of 1 lb. of hydrogen gives rise, as already remarked, to 190·26 cubic feet of steam; and if combustion take place under similar conditions with respect to space, the pressure exerted upon the containing vessel will be 22,830 lb., or nearly 10·5 tons, to the square inch, the tension being 190·26 × 8 = 1522 atmospheres.

The force thus developed is due wholly to the volume of the gas generated, and by no means represents the total amount developed by the explosion. The volume of the gases evolved by an explosion is estimated for a temperature of 62°; but it was shown in a former paragraph that the temperature of the products of combustion at the moment of their generation is far above this. Now it is a well-known law of thermo-dynamics that, the volume remaining the same, the pressure of a gas will vary directly as the temperature; that is, when the temperature is doubled, the pressure is also doubled. By temperature is understood the number of degrees measured by Fahrenheit’s scale on a perfect gas thermometer, from a zero 461°·2 below the zero of Fahrenheit’s scale, that is, 493°·2 below the freezing point of water. Thus the temperature of 62° for which the volume has been estimated is equal to 461·2 + 62 = 523°·2 absolute.

It was shown that the temperature of the product of combustion when carbon is burned to carbonic oxide is 9718° Fahr., which is equivalent to 10179°·2 absolute. Hence it will be observed that the temperature has been increased 10179°·2 523°·2 = 19·45 times. According to the law above enunciated, therefore, the pressure will be increased in a like ratio, that is, it will be, for the volume and the space already given, 3776 × 19·45 = 73,443 lb. = 32·8 tons to the square inch.

When carbon is burned to carbonic acid, the temperature of the product was shown to be 23,516° Fahr., which is equivalent to 23977·2 absolute. In this case, it will be observed that the temperature has been increased 23977·2 523·2 = 45·83 times. Hence the resulting pressure will be 3776 × 45·83 = 173,154 lb. = 77·3 tons to the square inch. It will be seen from these pressures that when combustion is complete, the force developed is 2·36 greater than when combustion is incomplete; and also that the increase of force is due to the larger quantity of heat liberated, since the volume of the gases is the same in both cases. If we suppose the carbon burned to carbonic oxide in the presence of a sufficient quantity of oxygen to make carbonic acid, we shall have 31·5 cubic feet of the oxide + 15·7 cubic feet of free oxygen, or a total volume of 42·7 cubic feet of gases. If this volume be compressed within the space of one-eighth of a cubic foot, it will have a tension of 42·7 × 8 = 341·6 atmospheres, and will exert upon the walls of the containing vessel a pressure of 5124 lb. to the square inch. The temperature of the gases will be 32 + 4400 0·190 × 3·667 = 6347° Fahr. = 6808°·2 absolute, the mean specific heat of the gases being 0·190; whence it will be seen that the temperature has been increased 6808°·2 523·2 = 13·01 times. According to the law of thermo-dynamics, therefore, the pressure under the foregoing conditions will be 5124 × 13·01 = 66,663 lb. = 29·8 tons to square inch. So that, under the conditions assumed in this case, the pressures developed by incomplete and by complete combustion are as 29·8 to 77·3, or as 1 to 2·59.

Similarly, when hydrogen is burned to water, the temperature of the product will be, as shown in a former paragraph, 16,049 Fahr. = 16510·2 absolute; and the pressure will be 22,830 × 16510·2 523·2 = 720,286 lb. = 321·1 tons to the square inch.

It will be observed, from a consideration of the foregoing facts, that a very large proportion of the force developed by an explosion is due to the heat liberated by the chemical reactions which take place. And hence it will plainly appear that, in the practical application of explosive agents to rock blasting, care should be taken to avoid a loss of the heat upon which the effects of the explosion manifestly so largely depend.

Section II.—Nature of Explosive Agents.

Mechanical Mixtures.

—In the preceding section, it was shown that an explosion is simply the rapid oxidation of carbon and hydrogen. To form an explosive agent, the problem is, how to bring together in a convenient form the combustible, carbon or hydrogen, and the oxygen required to oxidize it. Carbon may be obtained pure, or nearly pure, in the solid form. As wood charcoal, for example, that substance may be readily procured in any needful abundance; but pure oxygen does not exist in that state, and it is hardly necessary to point out that only the solid form is available in the composition of an explosive agent. In nature, however, oxygen exists in the solid state in very great abundance in combination with other substances. Silica, for example, which is the chief rock constituent, is a compound of silicon and oxygen, and the common ores of iron are made up chiefly of that metal and oxygen. The elementary constituents of cellulose, or wood fibre, are carbon, hydrogen, and oxygen; and the body known as saltpetre, or nitrate of potash, is compounded of potassium, nitrogen, and oxygen. But though oxygen is thus found in combination with many different substances, it has not the same affinity for all. When it is combined with a substance for which its affinity is strong, as in the silica and the iron oxide, it cannot be separated from that substance without difficulty; but if the affinity be weak, dissociation may be more easily effected. The former combination is said to be “stable,” and the latter is, in contradistinction, described as “unstable.” It will be evident on reflection that only those compounds in which the oxygen exists in unstable combination can be made use of as a constituent part of an explosive agent, since it is necessary that, when required, the oxygen shall be readily given up. Moreover, it will also appear that when one of these unstable oxygen compounds and carbon are brought together the mixture will constitute an explosive agent, since the oxygen which is liberated by the dissociation of the unstable compound will be taken up by the carbon for which it has a stronger affinity. Saltpetre is one of those compounds, and a mixture of this body with charcoal constitutes gunpowder. The means employed to dissociate the elements of saltpetre is heat. It is obvious that other compounds of oxygen might be substituted for the saltpetre, but this body being easily procurable is always employed. The chlorate of potash, for example, is less stable than the nitrate, and therefore an explosive mixture containing the former substance will be more violent than another containing the latter. For the violence of an explosion is in a great measure determined by the readiness with which the oxygen is given up to the combustible. But the chlorate is much more costly than the nitrate. As, however, the force developed is greater, the extra cost would perhaps be compensated by the increased effect of the explosion. But the instability of the chlorate is such that friction or a moderately light blow will produce explosion in a mixture containing that substance, a circumstance that renders it unfit to be the oxidizer in an explosive agent in common use. The nitrate is therefore preferred on the ground of safety. Saltpetre, or nitrate of potash, consists, as already pointed out, of the metal potassium in combination with the substances nitrogen and oxygen. Of these, the last only is directly concerned in the explosion; but the two former, and especially the nitrogen, act indirectly to intensify its effects in a manner that will be explained hereafter.

The chemical formula for nitrate of potash is KNO₃, which signifies that three atoms of oxygen exist in this body in combination with one atom of nitrogen and one atom of kalium or potassium. As the atomic weights of these substances are 16, 14, and 39 respectively, the weight of the molecule is 101, that is, in 101 lb. of nitrate of potash there are 39 lb. of potassium, 14 lb. of nitrogen, and (16 × 3) = 48 lb. of oxygen. Hence the proportion of oxygen in nitrate of potash is by weight 47·5 per cent. It will be seen from this proportion that to obtain 1 lb. of oxygen, 2·1 lb. of the nitrate must be decomposed.

The carbon of gunpowder is obtained from wood charcoal, the light woods, such as alder, being preferred for that purpose. The composition of the charcoal varies somewhat according to the degree to which the burning has been carried, the effect of the burning being to drive out the hydrogen and the oxygen. But, generally, the composition of gunpowder charcoal is about 80 per cent. carbon, 3·25 per cent. hydrogen, 15 per cent. oxygen, and 1·75 per cent. ash. Knowing the composition of the charcoal, it is easy to calculate the proportion of saltpetre required in the explosive mixture.

Thus far we have considered gunpowder as composed of charcoal and saltpetre only. But in this compound, combustion proceeds too slowly to give explosive effects. Were the chlorate of potash used instead of the nitrate, the binary compound would be sufficient. The slowness of combustion in the nitrate mixture is due to the comparatively stable character of that body. To accelerate the breaking up of the nitrate, a quantity of sulphur is mixed up with it in the compound. This substance possesses the property of burning at a low temperature. The proportion of sulphur added varies from 10 per cent. in powder used in fire-arms, to 20 per cent. in that employed for blasting purposes. The larger the proportion of sulphur, the more rapid, within certain limits, is the combustion. Thus ordinary gunpowder is a ternary compound, consisting of charcoal, saltpetre, and sulphur.

As the composition of charcoal varies, it is not practicable to determine with rigorous accuracy the proportion of saltpetre required in every case; a mean value is therefore assumed, the proportions adopted being about—

With these proportions, the carbon should be burned to carbonic acid, and the sulphur should be all taken up by the potassium. Powder of this composition is used for fire-arms. For blasting purposes, as before remarked, the proportion of sulphur is increased at the expense of the saltpetre, in order to quicken combustion and to lessen the cost, to 20 per cent. as a maximum. With such proportions, some of the carbon is burned to carbonic oxide only, and some of the sulphur goes to form sulphurous acid, gases that are particularly noisome to the miner.

It is essential to the regular burning of the mixture that the ingredients be finely pulverized and intimately mixed. The manufacture of gunpowder consists of operations for bringing about these results. The several substances are broken up by mechanical means, and reduced to an impalpable powder. These are then mixed in a revolving drum, and afterwards kneaded into a paste by the addition of a small quantity of water. This paste is subjected to pressure, dried, broken up, and granulated; thus, the mixing being effected by mechanical means, the compound is called a mechanical mixture. It will be observed that in a mechanical mixture the several ingredients are merely in contact, and are not chemically united. They may therefore be separated if need be, or the proportions may be altered in any degree. Mechanical mixtures, provided the bodies in contact have no chemical action one upon another, are stable, that is, they are not liable, being made up of simple bodies, to decompose spontaneously.

Chemical Compounds.

—In a mechanical mixture, as we have seen, the elements which are to react one upon another are brought together in separate bodies. In gunpowder, for example, the carbon is contained in the charcoal, and the oxygen in the saltpetre. But in a chemical compound, these elements are brought together in the same body. In a mechanical mixture, we may put what proportion of oxygen we please. But elements combine chemically only in certain definite proportions, so that in the chemical compound we can introduce only a certain definite proportion of oxygen. The oxygen in saltpetre is in chemical combination with the potassium and the nitrogen, and, as we have already seen, these three substances hold certain definite proportions one to another. That is, to every atom of potassium, there are one atom of nitrogen and three atoms of oxygen. Or, which amounts to the same thing, in 1 lb. of saltpetre, there are 0·386 lb. of potassium, 0·139 lb. of nitrogen, and 0·475 lb. of oxygen. Moreover, these elements occupy definite relative positions in the molecule of saltpetre. But in the mechanical mixture, the molecules of which it is made up have no definite relative positions. Even if the three substances—charcoal, saltpetre, and sulphur—of which gunpowder is composed, could be so finely divided as to be reduced to their constituent molecules, the relative position of these would be determined by the mixing, and it would be impossible so to distribute them that each should find itself in immediate proximity to those with which it was to combine. But so far are we from being able to divide substances into their constituent molecules, that when we have reduced them to an impalpable powder, each particle of that powder contains a large number of molecules. Thus, in a mechanical mixture, we have groups of molecules of one substance mingled irregularly with groups of molecules of another substance, so that the atoms which are to combine are not in close proximity one to another, but, on the contrary, are, many of them, separated by wide intervals. In the chemical compound, however, the atoms are regularly distributed throughout the whole mass of the substance, and are, relatively to one another, in the most favourable position for combining. Viewed from this point, the chemical compound may be regarded as a perfect mixture, the mechanical mixture being a very imperfect one. This difference has an important influence on the effect of an explosion. All the atoms in a chemical compound enter at once into their proper combinations, and these combinations take place in an inconceivably short space of time, while, in a mechanical mixture, the combinations are less direct, and are much less rapidly effected. This is the reason why the former is more violent in its action than the latter. The one is crushing and shattering in its effects, the other rending and projecting. The compound gives a sudden blow; the mixture applies a gradually increasing pressure. It is this sudden action of the compound that allows it to be used effectively without tamping. The air, which rests upon the charge, and which offers an enormous resistance to motion at such inconceivably high velocities, serves as a sufficient tamping.