Nitro-Explosives: A Practical Treatise Part 23

Judson powder, R.R.P. | 25 | Emmensite (No. 259) | 30 | Rack-a-rock | 32 | KClO_{3} 79 parts, | | C_{6}H_{5}(NO)_{2} 21 parts.

Bellite | 50 | Forcite No. 1 | 61 | Kieselguhr dynamite No. 1 | 64 | 75 per cent. nitro-gycerine.

Atlas powder No. 1 | 74 | __________________________|____________|_________________________

CHAPTER IX.

_DETERMINATION OF THE RELATIVE STRENGTH OF EXPLOSIVES._

Effectiveness of an Explosive--High and Low Explosives--Theoretical Efficiency--MM. Roux and Sarrau's Results--Abel and n.o.ble's--n.o.bel's Ballistic Test--The Mortar, Pressure, or Crusher Gauge--Lead Cylinders-- The Foot-Pounds Machine--n.o.ble's Pressure Gauge--Lieutenant Walke's Results--Calculation of Pressure Developed by Dynamite and Gun-Cotton-- Macnab's and Ristori's Results of Heat Developed by the Explosion of Various Explosives--Composition of some of the Explosives in Common Use for Blasting, &c.

~The Determination of the Relative Strength of Explosives.~--Explosives may be roughly divided into two divisions, viz., those which when exploded produce a shattering force, and those which produce a propulsive force.

Explosives of the first cla.s.s are generally known as the high explosives, and consist for the most part of nitro compounds, or mixtures of nitro compounds with other substances. Any explosive whose detonation is very rapid is a high explosive, but the term has chiefly been applied to the nitro-explosives.

The effectiveness of an explosive depends upon the volume and temperature of the gases formed, and upon the rapidity of the explosion. In the high explosives the chemical transformation is very rapid, hence they exert a crus.h.i.+ng of shattering effect. Gunpowder, on the other hand, is a low explosive, and produces a propelling or heaving effect.

The maximum work that an explosive is capable of producing is proportionate to the amount of heat disengaged during its chemical transformation. This may be expressed in kilogrammetres by the formula 425Q, where Q is the number of units of heat evolved. The theoretical efficiency of an explosive cannot, however, be expected in practice for many reasons.

In the case of blasting rock, for instance:[A]--1. Incomplete combustion of the explosive. 2. Compression and chemical changes induced in the surrounding material operated on. 3. Energy expended in the 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 proportion of useful work has been estimated to be from 14 to 33 per cent.

of the theoretical maximum potential.

[Footnote A: C.N. Hake, Government Inspector of Explosives, Victoria, _Jour. Soc. Chem. Ind._, 1889.]

For the purposes of comparison, manufacturers generally rely more upon the practical than the theoretical efficiency of an explosive. These, however, stand in the same relation to one another, as the following table of Messrs Roux and Sarrau will show:--

MECHANICAL EQUIVALENT OF EXPLOSIVES.

Theoretical Work Relative in Kilos. Value.

Blasting powder (62 per cent. KNO_{3}) 242,335 1.0 Dynamite (75 per cent. nitro-glycerine) 548,250 2.26 Blasting gelatine (92 per cent. nitro-glycerine) 766,813 3.16 Nitro-glycerine 794,563 3.28

Experiments made in lead cylinders give-- Dynamite 1.0 Blasting gelatine 1.4 Nitro-glycerine 1.4

Sir Frederick Abel and Captain W.H. n.o.ble, R.A., have shown that the maximum pressure exerted by gunpowder is equal to 486 foot-tons per lb. of powder, or that when 1 kilo, of the powder gases occupy the volume of 1 litre, the pressure is equal to 6,400 atmospheres; and Berthelot has calculated that every gramme of nitro-glycerine exploded gives 1,320 units of heat. MM. Roux and Sarrau, of the Depot Centrales des Poudres, Paris, by means of calorimetric determinations, have shown that the following units of heat are produced by the detonation of--

Nitro-glycerine 1,784 heat units.

Gun-cotton 1,123 "

Pota.s.sic picrate 840 "

which, multiplied by the mechanical equivalent per unit, gives--

Nitro-glycerine 778 metre tons per kilogramme.

Gun-cotton 489 " "

Picrate of potash 366 " "

~n.o.bel's Ballistic Test.~--Alfred n.o.bel was the first to make use of the mortar test to measure the (ballistic) power of explosives. The use of the mortar for measuring the relative power of explosives does not give very accurate results, but at the same time the information obtained is of considerable value from a practical point of view. The mortar consists of a solid cylinder of cast iron, one end of which has been bored to a depth of 9 inches, the diameter of the bore being 4 inches. At the bottom of the bore-hole is a steel disc 3 inches thick, in which another hole has been bored 3 inches by 2 inches. The mortar (Fig. 54) itself is fitted with trunnions, and firmly fixed in a very solid wooden carriage, which is securely bolted down to the ground. The shot used should weigh 28 lbs., and be turned accurately to fit the bore of the mortar. Down its centre is a hole through which the fuse is put.

The following is the method of making an experiment:--A piece of hard wood is turned in the lathe to exactly fit the hole in the steel disc at the bottom of the bore. This wooden cylinder itself contains a small cavity into which the explosive is put. Ten grms. is a very convenient quant.i.ty.

Before placing in the mortar, a hole may be made in the explosive by means of a piece of gla.s.s rod of such a size that the detonator to be used will just fit into it. After placing the wooden cylinder containing the explosive in the cavity at the bottom of the bore, the shot, slightly oiled, is allowed to fall gently down on to it. A piece of fuse about a foot long, and fitted with a detonator, is now pushed through the hole in the centre of the shot until the detonator is embedded in the explosive.

The fuse is now lighted, and the distance to which the shot is thrown is carefully measured. The range should be marked out with pegs into yards and fractions of yards, especially at the end opposite to the mortar. The mortar should be inclined at an angle of 45. In experimenting with this apparatus, the force and direction of the wind will be found to have considerable influence.

[Ill.u.s.tration: FIG. 54.--MORTAR FOR MEASURING THE BALLISTIC POWER OF EXPLOSIVES. _A_, Shot; _B_, Steel Disc; _C_, Section of Mortar (Cast Iron); _D_, Wooden Plug holding Explosive (_E_); _F_, Fuse.]

Mr T. Johnson made some ballistic tests. He used a steel mortar and a shot weighing 29 Ibs., and he adopted the plan of measuring the distance to which a given charge, 5 grms., would throw the shot. He obtained the following results:--

Range in Feet.

Blasting gelatine (90 per cent. nitro-glycerine and nitro-cellulose) 392 Ammonite (60 per cent. Am(NO_{3}) and 10 per cent. nitro-naphthalene) 310 Gelignite (60 per cent. nitro-gelatine and gun-cotton) 306 Roburite (AmNO_{3} and chloro-nitro-benzol) 294 No. 1 dynamite (75 per cent. nitro-gelatine) 264 Stonite (68 per cent. nitro-gelatine and 32 per cent. wood-meal) 253 Gun-cotton 234 Tonite (gun-cotton and nitrates) 223 Carbonite (25 per cent. nitro-gelatine, 40 per cent. wood-meal, and 30 per cent. nitrates) 198 Securite (KNO_{3} and nitro-benzol) 183 Gunpowder 143

~Calculation of the Volume of Gas Evolved in an Explosive Reaction.~--The volume of gas evolved in an explosive reaction may be calculated, but only when they are simple and stable products, such calculations being made at 0 and 760 mm. Let it be required, for example, to determine the volume of gas evolved by 1 gram-molecule of nitro-glycerine. The explosive reaction of nitro-glycerine may be represented by the equation.

C_{3}H_{5}O_{3}(NO_{2})_{3} = 3CO_{2} + 2-1/2H_{2}O + 1-1/2N_{2} + 1/4O_{2} By weight 227 = 132 + 45 + 42 + 8 By volume 2 = 3 + 2-1/2 + 1-1/2 + 1/4

The weights of the several products of the above reactions are calculated by multiplying their specific gravities by the weight of 1 litre of hydrogen at 0 C. and 760 mm. (0.0896 grm). Thus,

One litre of CO_{2} = 22 x .0896 = 1.9712 grm.

" H_{2}O = 9 x " = 0.8064 "

" N_{2} = 14 x " = 1.2544 "

" O_{2} = 16 x " = 1.4336 "

The volume of permanent gases at 0 and 760 mm. is constant, and a.s.suming the gramme as the unit of ma.s.s, is found to be 22.32 litres. Thus:--

Volume of 44 of CO_{2}, at 0 and 760 mm. = 44/1.9712 = 22.32 litres.

18 " H_{2}O " " = 18/0.8044 = 22.32 "

28 " N_{2} " " = 28/1.2544 = 22.32 "

32 " O_{2} " " = 32/1.4366 = 22.32 "

Therefore

132 grms. of CO_{2} at 0 C and 760 mm. = 22.32 x 3 = 66.96 litres.

45 " H_{2}O " " = 22.32 x 2-1/2 = 55.80 "

42 " N_{2} " " = 22.32 x 1-1/2 = 33.48 "

8 " O_{2} " " = 22.32 x 1/4 = 5.58 "

____________

161.82 "

Therefore 1 gram-molecule or 227 grms. of nitro-glycerine when exploded, produces 161.82 litres of gas at 0 C and 760 mm.

To determine the volume of gas at the temperature of explosion, we simply apply the law of Charles.[A] Thus--

V : V' :: T : T' or V' = VT'/T

in which V represents the original volume.

V' " new volume.

T " original temperature on the absolute scale.

T' " new temperature of the same scale In the present case T' = 6001.

Therefore subst.i.tuting, we have