V' = 161.82x6001/273 = 3557 litres

or at the temperature of explosion 1 gram-molecule of nitro-glycerine produces 3,557 litres of permanent gas.

[Footnote A: According to the law of Charles, the volume of any gas varies directly as its temperature on the absolute scale, provided the pressure remains constant. Knowing the temperature on the centigrade scale, the corresponding temperature on the absolute scale is obtained by adding 273 to the degrees centigrade.]

~Pressure or Crusher Gauge.~--There are many forms of this instrument. As long ago as 1792 Count Rumford used a pressure gauge. The so-called crusher gauge was, however, first used by Captain Sir Andrew n.o.ble in his researches on powder. Other forms are the Rodman[A] punch Uchatius Eprouvette, and the crusher gauge of the English Commission on Explosives.

They are all based either upon the size of an indent made upon a copper disc by a steel punch fitted to a piston, acted upon by the gases of the explosive, or upon the crushing or flattening of copper or lead cylinders.

[Footnote A: Invented by General Rodman, United States Engineers.]

[Ill.u.s.tration: FIG. 55.--PRESSURE GAUGE.]

Berthelot uses a cylinder of copper, as also did the English Commission, but in the simpler form of apparatus mostly used by manufacturers lead cylinders are used. This form of apparatus (Fig. 55) consists of a base of iron to which four uprights _a_ are fixed, set round the circ.u.mference of a 4-inch circle; the lead plug rests upon the steel base let into the solid iron block. A ring _c_ holds the uprights _d_ together at the top.

The piston _b_, which rests upon the lead plug, is a cylinder of tempered steel 4 inches in diameter and 5 inches in length; it is turned away at the sides to lighten it as much as possible. It should move freely between the uprights _d_. In the top of this cylinder is a cavity to hold the charge of explosive. The weight of this piston is 12-1/4 lbs. The shot _e_ is of tempered steel, and 4 inches in diameter and 10 inches in length, and weighs 34-1/2 lbs. It is bored through its axis to receive a capped fuse.

The instrument is used in the following manner:--A plug of lead 1 inch long and 1 inch in diameter, and of a cylindrical form, is placed upon the steel plate between the uprights _a_, the piston placed upon it, the carefully weighed explosive placed in the cavity, and the shot lowered gently upon the piston. A piece of fuse, with a detonator fixed at one end, is then pushed through the hole in the shot until it reaches the explosive contained in the cavity in the piston. The fuse is lighted. When the charge is exploded, the shot is thrown out, and the lead cylinder is more or less compressed. The lead plugs must be of a uniform density and h.o.m.ogeneous structure, and should be cut from lead rods that have been drawn, and not cast separately from small ma.s.ses of metal.

[Ill.u.s.tration: FIG. 56.--_b_, STEEL PUNCH; _c_, LEAD CYLINDER FOR USE WITH PRESSURE GAUGE.]

The strength of the explosive is proportional to the work performed in reducing the height of the lead (or copper) plug, and to get an expression for the work done it is necessary to find the number of foot-pounds (or kilogrammetres) required to produce the different amounts of compression.

This is done by submitting exactly similar cylinders of lead to a crushing under weights acting without initial velocity, and measuring the reduced heights of the cylinders; from these results a table is constructed establishing empirical relations between the reduced heights and the corresponding weights; the cylinders are measured both before and after insertion in the pressure gauge by means of an instrument known as the micrometer calipers (Fig. 57).[A]

[Footnote A: An instrument called a "Foot-pounds Machine" has been invented by Lieut. Quinan, U.S. Army. It consists of three boards, connected so as to form a slide 16 feet high, in which a weight (the shot of the pressure gauge) can fall freely. One of the boards is graduated into feet and half feet. The horizontal board at the bottom, upon which the others are nailed, rests upon a heavy post set deep in the ground, upon which is placed the piston of the gauge, which in this case serves as an anvil on which to place the lead cylinders. The shot is raised by means of a pulley, fixed at the top of the structure, to any desired height, and let go by releasing the clutch that holds it. The difference between the original length and the reduced length gives the compression caused by the blow of the shot in falling, and gives the value in foot-pounds required to produce the different amounts of compression. (Vide _Jour. U.S. Naval Inst._, 1892.)]

[Ill.u.s.tration: FIG. 57.--MICROMETER CALIPERS FOR MEASURING DIAMETER OF LEAD CYLINDERS.]

~The Use of Lead Cylinders.~--The method of using lead cylinders to test the strength of an explosive is a very simple affair, and is conducted as follows:--A solid cast lead cylinder, of any convenient size, is bored down the centre for some inches, generally until the bore-hole reaches to about the centre of the block. The volume of this hole is then accurately measured by pouring water into it from a graduated measure, and its capacity in cubic centimetres noted. The bore-hole is then emptied and dried, and a weighed quant.i.ty (say 10 grms.) of the explosive pressed well down to the bottom of the hole. A hole is then made in the explosive (if dynamite) with a piece of clean and rounded gla.s.s rod, large enough to take the detonator. A piece of fuse, fitted with a detonator, is then inserted into the explosive and lighted. After the explosion a large pear- shaped cavity will be found to have been formed, the volume of which is then measured in the same way as before.

The results thus obtained are only relative, but are of considerable value for comparing dynamites among themselves (or gun-cottons). Experiments in lead cylinders gave the relative values for nitro-glycerine 1.4, blasting gelatine 1.4, and dynamite 1.0. (Fig. 58 shows sections of lead cylinders before and after use.)

[Ill.u.s.tration: FIG. 58.--LEAD CYLINDERS BEFORE AND AFTER USE.]

Standard regulations for the preparation of lead cylinders may be found in the _Chem. Zeit._, 1903, 27 [74], 898. They were drawn up by the Fifth International Congress of App. Chem., Berlin. The cylinder of lead should be 200 mm. in height and 200 mm. in diameter. In its axis is a bore-hole, 125 mm. deep and 25 mm. in diameter. The lead used must be pure and soft, and the cylinder used in a series of tests must be cast from the same melt. The temperature of the cylinders should be 15 to 20 throughout.

Ten grms. of explosive should be used and wrapped in tin-foil. A detonator with a charge of 2 grms., to be fired electrically, is placed in the midst of the explosive. The cartridge is placed in the bore-hole, and gently pressed against the bottom, the firing wires being kept in central position. The bore-hole is then filled with dry quartz sand, which must pa.s.s through a sieve of 144 meshes to the sq. cm., the wires being .35 mm.

diameter. The sand is filled in evenly, any excess being levelled off. The charge thus prepared is then fired electrically. The lead cylinder is then inverted, and any residues removed with a brush. The number of c.c. of water required to fill the cavity, in excess of the original volume of the bore-hole, is a measure of the strength of the explosive. The results are only comparable if made with the same cla.s.s of explosive. A result is to be the mean of at least three experiments. The accuracy of the method depends on (_a_) the uniform temperature of the lead cylinder (15 to 20 C. 7); (_b_) on the uniformity of the quartz sand; (_c_) on the uniformity of the measurements.

[Ill.u.s.tration: FIG. 59.--n.o.bLE'S PRESSURE GAUGE.]

~n.o.ble's Pressure Gauge.~--The original explosive vessels used by Captain Sir A. n.o.ble in his first experiments were practically exactly similar to those that he now employs, which consists of a steel barrel A (Fig. 59), open at both ends, which are closed by carefully fitted screw plugs, furnished with steel gas checks to prevent any escape past the screw. The action of the gas checks is exactly the same as the leathers used in hydraulic presses. The pressure of the gas acting on both sides of the annular s.p.a.ce presses these sides firmly against the cylinder and against the plug, and so effectually prevents any escape. In the firing plug F is a conical hole closed by a cone fitting with great exactness, which, when the vessel is prepared for firing, is covered with fine tissue paper to act as an insulator. The two firing wires GG, one in the insulated cone, the other in the firing plug, are connected by a very fine platinum wire pa.s.sing through a gla.s.s tube filled with meal powder. The wire becomes red-hot when connection is made with a Leclanche battery, and the charge which has previously been inserted into the vessel is fired. The crusher plug is fitted with a crusher gauge H for determining the pressure of the gases at the moment of explosion, and in addition there is frequently a second crusher gauge apparatus screwed into the cylinder. When it is desired to allow the gases to escape for examination, the screw J is slightly withdrawn. The gases then pa.s.s into the pa.s.sage I, and can be led to suitable apparatus in which their volume can be measured, or in which they can be sealed for subsequent chemical a.n.a.lysis.

The greatest care must be exercised in carrying out experiments with this apparatus; it is particularly necessary to be sure that all the joints are perfectly tight before exploding the charge. Should this not be the case, the gases upon their generation will cut their way out, or completely blow out the part improperly secured, in either case destroying the apparatus.

The effect produced upon the apparatus when the gas has escaped by cutting a pa.s.sage for itself is very curious. The surface of the metal where the escape occurred presents the appearance of having been washed away in a state of fusion by the rush of the highly heated products.

~The Pressure Gauge.~--The pressure is found by the use of a little instrument known as the pressure gauge which consists of a small chamber formed of steel, inside of which is a copper cylinder, and the entrance being closed by a screw gland, in which a piston, having a definite sectional area, works. There is a gas check E (Fig. 60) placed in the gland, and over the piston, which prevents the admission of gas to the chamber. When it is desired to find the pressure in the chamber of a gun, one or more of these crushers are made up with or inserted at the extreme rear end of the cartridge, in order to avoid their being blown out of the gun when fired. This, however, often takes place, in which case the gauges are usually found a few yards in front of the muzzle. The copper cylinders which register the pressure are made 0.5 inch long from specially selected copper, the diameters being regulated to give a sectional area of either 1/12 or 1/24 square inch.

[Ill.u.s.tration: FIG. 60.--CRUSHER GAUGE. _E_, GAS CHECK.]

Hollow copper cylinders are manufactured with reduced sectional areas for measuring very small pressures. It has been found that these copper cylinders are compressed to definite lengths for certain pressures with remarkable uniformity. Thus a copper cylinder having a sectional area of 1/12 square inch, and originally 1/2 inch long, is crushed to a length of 0.42 inch by a pressure of 10 tons per square inch. By subsequently applying a pressure of 12 tons per square inch the cylinder is reduced to a length of 0.393 inch. Before using the cylinders, whether for experimenting with closed vessels or with guns, it is advisable to first crush them by a pressure a little under that expected in the experiment.

Captain Sir A. n.o.ble used in his experiments a modification of Rodman's gauge. (Ordnance Dept., U.S.A., 1861.)

~By Calculation.~--To calculate the pressure developed by the explosion of dynamite in a bore-hole 3 centimetres in diameter, charged with 1 kilogramme of 75 per cent. dynamite, Messrs Vieille and Sarrau employ the following formula:--

P = V_{o}(1 + Q/273._c_)/(V - _v_).

Where V_{o} = the volume (reduced to 0 and 760 mm.) of the gases produced by a unit of weight of the explosive; Q the number of calories disengaged by a unit of weight of the explosive; _c_ equals the specific heat at constant volume of the gases; V the volume in cubic centimetres of a unit of weight of the explosive; _v_ the volume occupied by the inert materials of the explosive. The volume of gas produced by the explosion of 1 kilogramme of nitro-glycerine (at 0 and 760 mm.) is 467 litres.

V_{o} will therefore equal 0.75 x 467 = 350.25.

The specific heat _c_ is, according to Sarrau, .220 (_c_); and according to Bunsen, 1 kilogramme of dynamite No. 1 disengages 1,290 (Q) calories.

The density of dynamite is equal to 1.5, therefore

V = 1/1.5 = .666.

If we take the volume of the kieselguhr as .1, we find from above formula that

P = 350(1 + 1290/(273 x .222))/(.600 - .1) = 13,900 atmospheres,

which is equal to 14,317 kilogrammes per square centimetre. The pressure developed by 1 kilogramme of pure nitro-glycerine equals 18,533 atmospheres, equals 19,151 kilogrammes. Applying this formula to gun- cotton, and taking after Berthelot, Q = 1075, and after Vieille and Sarrau, V_{o} = 671 litres, and _c_ as .2314, and the density of the nitro-cellulose as 1.5, we have (V = O)

P = 671(1 + 1075/(273 x .2314))/.666 = 18,135 atmospheres.

To convert this into pressure of kilogrammes per square centimetre, it is necessary to multiply it by the weight of a column of mercury 0.760 m.

high, and 1 square centimetre in section, which is equal to increasing it by 1/30. It thus becomes

P^{k} = (1 + 1/30).

P^{k} = 18,135 x 1.033 = 18,733 kilogrammes.

The following tables, taken from Messrs William Macnab's and E. Ristori's paper (_Proc. Roy. Soc._, 56, 8-19), "Researches on Modern Explosives,"

are very interesting. They record the results of a large number of experiments made to determine the amount of heat evolved, and the quant.i.ty and composition of the gases produced when certain explosives and various smokeless powders were fired in a closed vessel from which the air had been previously exhausted. The explosions were carried out in a "calorimetric bomb" of Berthelot's pattern.[A]

[Footnote A: For description of "bomb," see "Explosives and their Power,"

Berthelot, trans. by Hake and Macnab, p. 150. (Murray.)]

Table Showing Quant.i.ty of Heat and Volume and a.n.a.lysis of Gas Developed per Gramme with Different Sporting and Military Smokeless Powders Now In Use

______________________________________________________________________ | | | | | Name of Explosive. | Calories | Permanent | Aqueous | Total Volume | | per grm. | Gases. | Vapour. | of Gas at 0 | | | | | and 760 mm. | ______________________|__________|___________|_________|______________| | | cc/grm | cc/grm | cc/grm | E.C. powder, English | 800 | 420 | 154 | 574 | S.S. powder | 799 | 584 | 150 | 734 | Troisdorf, German | 943 | 700 | 195 | 895 | Rifleite, English | 864 | 766 | 159 | 925 | B.N., French | 833 | 738 | 168 | 906 | Cordite, English | 1253 | 647 | 235 | 882 | Ballist.i.te, German | 1291 | 591 | 231 | 822 | Ballist.i.te, Italian | 1317 | 58l | 245 | 826 | and Spanish | | | | | ______________________|__________|___________|_________|______________|

The figures in column headed "Co-efficient of Potential Energy" serve as a measure of comparison of the power of the explosives, and are the products of the number of calories by the volume of gas, the last three figures being suppressed in order to simplify the results.

The amounts of water found were calculated for comparison as volumes of H_{2}O gas at 0 and 760 mm.

E.C. powder consists princ.i.p.ally of nitro-cellulose mixed with barium nitrate and a small proportion of camphor.

S.S. of nitro-lignine mixed with barium nitrate and nitro-benzene.

Troisdorf powder is gelatinised nitro-cellulose; rifleite gelatinised nitro-cellulose and nitro-benzene.