Ordinary dynamite consists of a mixture of 75 per cent. of nitro-glycerine and 25 per cent. of kieselguhr. The guhr as imported (Messrs A. Haake & Co. are the chief importers) contains from 20 to 30 per cent. of water and organic matter. The water may be very easily estimated by drying a weighed quant.i.ty in a platinum crucible at 100 C. for some time and re-weighing, and the organic matter by igniting the residue strongly over a Bunsen burner. Before the guhr can be used for making dynamite it must be calcined, in order not only to get rid of moisture, but also the organic matter.

A good guhr should absorb four times its weight of nitro-glycerine, and should then form a comparatively dry mixture. It should be pale pink, red brown, or white. The pink is generally preferred, and it should be as free as possible from grit of all kinds, quartz particles, &c., and should have a smooth feeling when rubbed between the finger and thumb, and should show a large quant.i.ty of diatoms when viewed under the microscope. The following was the a.n.a.lysis of a dried sample of kieselguhr:--Silica, 94.30; magnesia, 2.10; oxide of iron and alumina, 1.3; organic matter, 0.40; moisture, 1.90 per cent.

The guhr is generally dried in a reverberatory m.u.f.fle furnace. It is spread out on the bottom to the thickness of 3 or 4 inches, and should every now and then be turned over and raked about with an iron rabble or hoe. The temperature should be sufficiently high to make the guhr red hot, or the organic matter will not be burnt off. The time occupied in calcining will depend of course upon the quality of the guhr being operated upon. Those containing a high percentage of water and organic matter will of course take longer than those that do not. A sample of the calcined guhr should not contain more than 0.5 per cent. of moisture and organic matter together.

After the guhr is dry it requires to be sifted and crushed. The crushing is done by pa.s.sing it between iron rollers fixed at the bottom of a cone or hopper, and revolving at a moderate speed. Beneath the rollers a fine sieve should be placed, through which the guhr must be made to pa.s.s.

The kieselguhr having been dried, crushed, and sifted, should be packed away in bags, and care should be taken that it does not again absorb moisture, as if it contains anything above about five-tenths per cent. of water it will cause the dynamite made with it to exude. The guhr thus prepared is taken up to the danger area, and mixed with nitro-glycerine.

The nitro-glycerine used should be quite free from water, and clear, and should have been standing for a day or two in the precipitating house. The guhr and nitro-glycerine are mixed in lead tanks (about 1-1/2 foot deep, and 2 to 3 feet long), in the proportions of 75 of the nitro-glycerine to 25 of the guhr, unless the guhr is found to be too absorbent, which will cause the dynamite to be too dry and to crumble. In this case a small quant.i.ty of barium sulphate, say about 1 per cent., should be added to the guhr. This will lessen its absorbing powers, or a highly absorptive sample of guhr may be mixed with one of less absorptive power, in the proportions found by experiment to be the best suited to make a fairly moist dynamite, but one that will not exude.

The mixing itself is generally performed in a separate house. In a series of lead-lined tanks the guhr is weighed, placed in a tank, and the nitro- glycerine poured on to it. The nitro-glycerine may be weighed out in indiarubber buckets. The whole is then mixed by hand, and well rubbed between the hands, and afterwards pa.s.sed through a sieve. At this stage the dynamite should be dry and powdery, and of a uniform colour.

It is now ready to be made up into cartridges, and should be taken over to the cartridge huts. These are small buildings surrounded with mounds, and contain a single cartridge machine. Each hut requires three girls--one to work the press, and two to wrap up the cartridges. The cartridge press consists of a short cylinder of the diameter of the cartridge that it is intended to make. Into this cylinder a piston, pointed with ivory or lignum vitae wood, works up and down from a spring worked by a lever. Round the upper edge of the cylinder is fastened a canvas bag, into which the powdery dynamite is placed by means of a wooden scoop, and the descending piston forces the dynamite down the cylinder and out of the open end, where the compressed dynamite can be broken off at convenient lengths. The whole machine should be made of gun-metal, and should be upright against the wall of the building. The two girls, who sit at tables placed on each side of the press, wrap the cartridges in parchment paper. From these huts the cartridges are collected by boys every ten minutes or a quarter of an hour, and taken to the packing room, where they are packed in 5-lb.

cardboard boxes, which are then further packed in deal boxes lined with indiarubber, and fastened down air tight. The wooden lids are then nailed down with bra.s.s or zinc nails, and a label pasted on the outside giving the weight and description of the contents. The boxes should then be removed to the magazines. It is well to take a certain number of cartridges from the packing house at different times during the day, say three or four samples, and to test them by the heat test. A sample cut from a cartridge, about 1 inch long, should be placed under a gla.s.s shade, together with water (a large desiccator, in fact), and left for some days.

A good dynamite should not, under these conditions, show any signs of exudation, even after weeks.[A]

[Footnote A: For a.n.a.lysis of dynamite, see chapter on "a.n.a.lysis," and author's article in _Chem. News_, 23rd September 1892.]

~Properties of Kieselguhr Dynamite.~--One cubic foot of dynamite weighs 76 lbs. 4 oz. The specific gravity of 75 per cent. dynamite is, however, 1.50. It is a red or grey colour, and rather greasy to the touch. It is much less sensitive to shock than nitro-glycerine, but explodes occasionally with the shock of a rifle bullet, or when struck. The addition of a few per cent. of camphor will considerably diminish its explosive qualities to such an extent that it can be made non-explosive except to a very strong fulminate detonator. The direct contact of water disintegrates dynamite, separating the nitro-glycerine, hence great caution is necessary in using it in wet places. It freezes at about 40 Fahr. (4 C.), and remains frozen at temperatures considerably exceeding that point. When frozen, it is comparatively useless as an explosive agent, and must be thawed with care. This is best done by placing the cartridges in a warming pan, which consists of a tin can, with double sides and bottom, into which hot water (130 Fahr.) can be poured. The dynamite will require to be left in for some considerable time before it becomes soft. On no account must it be placed on a hot stove or near a fire, as many serious accidents have occurred in this way.

Frozen dynamite is a hard ma.s.s, with altered properties, and requires 1.5 grm. of fulminate instead of 0.5 grm. to explode it. Thawing may also cause exudation of the nitro-glycerine, which is much more sensitive to shock, and if accidentally struck with an iron tool, may explode. It is a dangerous thing to cut a frozen cartridge with a knife. Ramming is even more dangerous; in fact it is not only dangerous, but wasteful, to use dynamite when in a frozen state.

Dynamite explodes at a temperature of 360 Fahr., and is very sensitive to friction when hot. In hot countries it should never be exposed to the rays of the sun. It should, however, not be kept in a damp or moist place, as this is liable to cause exudation. Sunlight, if direct, can cause a slow decomposition, as with all nitro and nitric compounds. Electric sparks ignite, without exploding it, at least when operating in the open air.

Dynamite, when made with neutral nitro-glycerine, appears to keep indefinitely. Sodium or calcium carbonate to the extent of 1 per cent. is often added to dynamite to ensure its being neutral. If it has commenced to undergo change, however, it rapidly becomes acid, and sometimes explodes spontaneously, especially if contained in resisting envelopes.

Nevertheless, neutral and well-made dynamite has been kept for years in a magazine without loss of its explosive force. If water is brought into contact with it, the nitro-glycerine is gradually displaced from the silica (guhr). This action tends to render all wet dynamite dangerous.

It has been observed that a dynamite made with wood sawdust can be moistened and then dried without marked alteration, and from 15 to 20 per cent. of water may be added to cellulose dynamite without depriving it of the power of exploding by strong detonator (this is similar to wet gun-cotton). It is, however, rendered much less sensitive to shock. With regard to the power of No. 1 dynamite, experiments made in lead cylinders give the relative value of No. 1 dynamite, 1.0; blasting gelatine, 1.4; and nitro-glycerine, 1.4. The heat liberated by the sudden explosion of dynamite is the same as its heat of combustion,[A] and proportionate to the weight of nitro-glycerine contained in the mixture. The gases formed are carbonic acid, water, nitrogen, and oxygen.

[Footnote A: Berthelot, "Explosives and their Power."]

The "explosive wave" (of Berthelot) for dynamite is about 5,000 metres per second. At this rate the explosion of a cartridge a foot long would only occupy 1/24000 part of a second, while a ton of dynamite cartridges about 7/8 diameter, laid end to end, and measuring one mile in length, would be exploded in one-quarter of a second by detonating a cartridge at either end.[A] Mr C. Napier Hake, F.I.C., the Inspector of Explosives for the Victorian Government, in his paper, "Notes on Explosives," says: "The theoretical efficiency of an explosive cannot in practice be realised in useful work for several reasons, as for instance in blasting rock--

"1. Incomplete combustion.

"2. Compression and chemical changes induced in surrounding material.

"3. Energy expended in cracking and heating of the material which is not displaced.

"4. The escape of gas through the blast-hole and the fissures caused by the explosion.

"The useful work consists partly in displacing the shattered ma.s.ses. The proportion of useful work obtainable has been variously estimated at from 14 to 33 per cent. of the theoretical maximum potential."

[Footnote A: C.N. Hake, "Notes on Explosives," _Jour. Soc. Chem. Ind._, 1889.]

Among the various forms of dynamite that are manufactured is carbo- dynamite, the invention of Messrs Walter F. Reid and W.D. Borland. The base is nitro-glycerine, and the absorbent is carbon in the form of burnt cork. It is as cheap as ordinary dynamite, and has greater explosive force, seeing that 90 per cent. of the mixture is pure nitro-glycerine, and the absorbent itself is highly combustible. It is also claimed that if this dynamite becomes wet, no exudation takes place.

Atlas powder is a dynamite, chiefly manufactured in America at the Repanno Chemical Works, Philadelphia. It is a composition of nitro-glycerine, wood-pulp, nitrate of soda, and carbonate of magnesia. This was the explosive used in the outrages committed in London, by the so-called "dynamiters." Different varieties contain from 20 to 75 per cent. of nitro-glycerine.

The Rhenish dynamite, considerably used in the mines of Cornwall, is composed of 70 parts of a solution of 2 to 3 per cent. of naphthalene in nitro-glycerine, 3 parts of chalk, 7 parts of sulphate of barium, and 20 of kieselguhr.

Kieselguhr dynamites are being largely given up in favour of gelatine explosives. The late Colonel Cundill, in his "Dictionary of Explosives,"

gives a list of about 125 kinds of dynamites. Many of these, however, are not manufactured. Among the best known after the ordinary No. 1 dynamite are forcite, ammonia dynamite, litho-fracteur, rendock, Atlas powder, giant powder, and the various explosive gelatines. They all contain nitro- glycerine, mixed with a variety of other substances, such as absorbent earths, wood-pulp, nitro-cotton, carbon in some form or other, nitro- benzol, paraffin, sulphur, nitrates, or chlorates, &c. &c.

~Blasting Gelatine and Gelatine Dynamite.~--The gelatine explosives chiefly in use are known under the names of blasting gelatine, gelatine dynamite, and gelignite. They all consist of the variety of nitro- cellulose known as collodion-cotton, i.e., a mixture of the penta- and tetra-nitrates dissolved in nitro-glycerine, and made up with various proportions of wood-pulp, and some nitrate, or other material of a similar nature. As the gun-cotton contains too little oxygen for complete combustion, and the nitro-glycerine an excess, a mixture of the two substances is very beneficial.

Blasting gelatine consists of collodion-cotton and nitro-glycerine without any other substance, and was patented by Mr Alfred n.o.bel in 1875. It is a clear, semi-transparent, jelly-like substance, of a specific gravity of 1.5 to 1.55, slightly elastic, resembling indiarubber, and generally consists of 92 per cent. to 93 per cent. of nitro-glycerine, and 7 to 8 per cent. of nitro-cotton. The cotton from which it is made should be of good quality. The following is the a.n.a.lysis of a sample of nitro-cellulose which made very good gelatine:-

Soluble cotton 99.118 per cent.

Gun-cotton 0.642 "

Non-nitrated cotton 0.240 "

Nitrogen 11.64 "

Total ash 0.25 "

The soluble cotton, which is a mixture of the tetra- and penta-nitrates, is soluble in ether-alcohol, and also in nitro-glycerine, and many other solvents, whereas the hexa-nitrate (gun-cotton), C_{12}H_{14}O_{4}(ONO_{2})_{6}, is not soluble in the above liquids, although it is soluble in acetone or acetic ether. It is very essential, therefore, that the nitro-cotton used in the manufacture of the gelatine explosives should be as free as possible from gun-cotton, otherwise little lumps of undissolved nitro-cotton will be left in the finished gelatine.

The non-nitrated or unconverted cotton should also be very low, in fact considerably under 1/2 per cent.

The nitro-cotton and the nitro-glycerine used should always be tested before use by the heat test, because if they do not separately stand this test, it cannot be expected that the gelatine made from them will do so.

It often occurs, however, that although both the ingredients stand this test separately before being mixed, that after the process of manufacture one or other or both fail to do so.

The nitro-cotton most suitable for gelatine making is that which has been finely pulped. If it is not already fine enough, it must be pa.s.sed through a fine bra.s.s wire sieve. It will be found that it requires to be rubbed through by hand, and will not go through at all if in the least degree damp. It is better, therefore, to dry it first. The percentage of nitrogen in the nitrated cotton should be over 11 per cent. It should be as free as possible from sand or grit, and should give but little ash upon ignition, not more than 0.25 per cent. The cotton, which is generally packed wet in zinc-lined wooden boxes, will require to be dried, as it is very essential indeed that none of the materials used in the manufacture of gelatine should contain more than the slightest trace of water. If they do, the gelatine subsequently made from them will most certainly exude, and become dangerous and comparatively valueless. It will also be much more difficult to make the nitro-cotton dissolve in the nitro-glycerine if either contains water.

In order to find out how long any sample of cotton requires to be dried, a sample should be taken from the centre of several boxes, well mixed, and about 1,000 grms. spread out on a paper tray, weighed, and the whole then placed in the water oven at 100 C., and dried for an hour or so, and again weighed, and the percentage of moisture calculated from the loss in weight. This will be a guide to the time that the cotton will probably require to be in the drying house. Samples generally contain from 20 to 30 per cent. of water. After drying for a period of forty-eight hours, a sample should be again dried in the oven at 100 C., and the moisture determined, and so on at intervals until the bulk of the cotton is found to be dry, i.e., to contain from 0.25 to 0.5 per cent. of moisture. It is then ready to be sifted. During the process of removing to the sifting house and the sifting itself, the cotton should be exposed to the air as little as possible, as dry nitro-cotton absorbs as much as 2 per cent. of moisture from the air at ordinary temperatures and average dryness.

The drying house usually consists of a wooden building, the inside of which is fitted with shelves, or rather framework to contain drawers, made of wood, with bra.s.s or copper wire netting bottoms. A current of hot air is made to pa.s.s through the shelves and over the surface of the cotton, which is spread out upon them to the depth of about 2 inches. This current of air can be obtained in any way that may be found convenient, such as by means of a fan or Root's blower, the air being pa.s.sed over hot bricks, or hot-water pipes before entering the building. The cotton should also be occasionally turned over by hand in order that a fresh surface may be continually exposed to the action of the hot air. The building itself may be heated by means of hot-water pipes, but on no account should any of the pipes be exposed. They should all be most carefully covered over with wood-work, because when the dry nitro-cotton is moved, as in turning it over, very fine particles get into the air, and gradually settling on the pipes, window ledges, &c., may become very hot, when the slightest friction might cause explosion. It is on this account that this house should be very carefully swept out every day. It is also very desirable that the floor of this house should be covered with oilcloth or linoleum, as being soft, it lessens the friction.

List shoes should always be worn in this building, and a thermometer hung up somewhere about the centre of the house, and one should also be kept in one of the trays to give the temperature of the cotton, especially the bottom of the trays. The one nearest to the hot air inlet should be selected. If the temperature of the house is kept at about 40 C. it will be quite high enough. The building must of course be properly ventilated, and it will be found very useful to have the walls made double, and the intervening s.p.a.ce filled with cinders, and the roof covered with felt, as this helps to prevent the loss of heat through radiation, and to preserve a uniform temperature, which is very desirable.

The dry cotton thus obtained, if not already fine enough, should be sifted through a bra.s.s sieve, and packed away ready for use in zinc air-tight cases, or in indiarubber bags. The various gelatine compounds, gelignite, gelatine dynamite, and blasting gelatine, are manufactured in exactly the same way. The forms known as gelatine dynamite differ from blasting gelatine in containing certain proportions of wood-pulp and pota.s.sium nitrate, &c. The following are a.n.a.lyses of some typical samples of the three compounds:--

Gelatine Blasting Gelignite. Dynamite. Gelatine.

Nitro-glycerine 60.514 71.128 92.94 per cent.

Nitro-cellulose 4.888 7.632 7.06 "

Wood-pulp 7.178 4.259 ... "

Pota.s.sium nitrate 27.420 16.720 ... "

Water ... 0.261 ... "

The gelignite and gelatine dynamites consist, therefore, of blasting gelatine, thickened up with a mixture of absorbing materials. Although the blasting gelatine is weight for weight more powerful, it is more difficult to make than either of the other two compounds, it being somewhat difficult to make it stand the exudation and melting tests. The higher percentage of nitro-cotton, too, makes it expensive.

When the dry nitro-cotton, which has been carefully weighed out in the proportions necessary either for blasting gelatine or any of the other gelatine explosives, is brought to the gelatine making house, it is placed in a lead-lined trough, and the necessary quant.i.ty of pure dry nitro- glycerine poured upon it. The whole is then well stirred up, and kept at a temperature of from 40 to 45 C. It should not be allowed to go much above 40 C.; but higher temperatures may be used if the nitro-cotton is very obstinate,[A] and will not dissolve. Great caution must, however, be observed in this case. The mixture should be constantly worked about by the workman with a wooden paddle for at least half an hour. At a temperature of 40 to 45 the nitro-glycerine acts upon the nitro-cotton and forms a jelly. Without heat the gelatinisation is very imperfect indeed, and at temperatures under 40 C. takes place very slowly.

[Footnote A: Generally due to the nitro-cotton being damp.]

[Ill.u.s.tration: FIG. 30.--WERNER, PFLEIDERER, & PERKINS' MIXING MACHINE.]

The limit of temperature is 50 C. or thereabouts. Beyond this the jelly should never be allowed to go, and to 50 only under exceptional circ.u.mstances.

The tank in which the jelly is made is double-lined, in order to allow of the pa.s.sage of hot water between its inner and outer linings. A series of such tanks are generally built in a wooden framework, and the double linings are made to communicate, so that the hot water can flow from one to the other consecutively. The temperature of the water should be about 60 C. if it is intended to gelatinise at 45 C., and about 80 if at 50 C.; but this point must, of course, be found by experiment for the particular plant used. An arrangement should be made to enable the workman to at once cut off the supply of hot water and pa.s.s cold water through the tanks in case the explosive becomes too hot.