Page:EB1922 - Volume 31.djvu/73

Rh

Thus we have in explosives a store of molecular energy in a condition of unstable equilibrium, requiring some form of external energy to release it. This may consist of ignition, friction, percus- sion or the action of a detonator, which imparts a violent shock to the explosive and at the same time emits a flash of flame. - Mercury Fulminate. For stable high explosives a detonator is almost invariably used. Indeed, it is almost impossible to cause an insensitive explosive such as amatol to detonate without such an initial impulse. The discovery of fulminate of mercury by Howard about the year 1800 had a far-reaching effect on military and industrial explosives. This sensitive chemical compound is readily caused to detonate by heat, friction, or percussion.

It is consequently manufactured only under the greatest pre- cautions in small quantities at a time. It is made by first dissolving mercury in nitric acid, and then pouring the solution into alcohol. A vigorous reaction takes place, and after a time the mercury fulminate separates out. It has then to be washed and finally dried very carefully at a low temperature.

Mercury fulminate revolutionized the methods of bringing about explosion, being first used in percussion caps for igniting gunpowder, and thereby displacing the cumbersome and uncertain method of flint and steel. At a considerably later date its value as a detonator or igniting agent for more stable high explosives became recognized, for which purpose it is now mainly used. When required simply for ignitory purposes a mixture with potassium chlorate, which causes a larger and hotter flame, is generally employed.

In order to appreciate the function of the detonator it is necessary to consider that in an explosive substance each molecule in its decomposition gives out a surplus of energy, and so provides the initial impulse required to decompose the neighbouring molecules. When, however, a high explosive such as T N T is merely ignited, the decomposition propagates itself slowly at first, and may cease altogether owing to external cooling; in any case, the velocity of decomposition increases but gradually, and it is only after a con- siderable quantity has decomposed that detonation ensues. As much as five tons of T N T have been known to burn off without explosive violence, though this is by no means always the case.

The particular value of fulminate of mercury as a detonating agent is due to the fact that the explosion wave is in the first place very easily initiated in it by heat or friction, and in the second place is accelerated to its maximum almost instantaneously, so that com- plete detonation of the bulk immediately ensues, and the detonation is similarly imparted to any high explosive, with which the fulminate is in contact. Owing to the sensitiveness of the fulminate, not more than about 10 grains of the detonating substance is em- ployed in artillery shell. In order to communicate the detonation to the stable high explosive in a shell, it is usual to " step up " the detonation wave. Thus the fulminate detonates a core of an ex- plosive of intermediate sensitiveness such as tetryl (tri-nitro-phenyl- methyl nitro-amine), and this detonates the main high explosive. Similarly, when it is desired to detonate a slab of wet guncotton, it is necessary to insert a " primer " of dry guncotton between the de- tonator and the wet guncotton.

Detonators of standard sizes are made for commercial blasting purposes; thus the size known as No. 8, containing 30-9 grains of fulminate, is in common use for blasting, and was used during the war in Mills grenades and trench-mortar bombs, where the shock of discharge is very much less than in a gun.

Another compound which has come into use to a considerable extent as a detonating substance is lead azide (PbNe). This is an example of an explosive containing no oxygen or combustible matter its explosion is due to a simple disruption of the molecule into lead and nitrogen.

Properties of High Explosives. The investigation of the be- haviour of explosives on detonation is attended by considerable difficulties. Some account of recent methods is given by Sir R. Robertson in the Journal of the Chemical Society, 1921, vol. cxix, p. I, from which the appended data are taken.

Important advances have been made in methods of measure- ment of the time-pressure curve of high explosives. The explosive is detonated at one end of a suspended steel bar and causes a wave

of compression to travel along the bar. This is reflected at the far end as a wave of tension which causes a disc lightly attached to be projected into a ballistic pendulum, whereby the momentum de- veloped overa very small time interval, usually about five millionths of a second, is obtained.

Explosives in the Future. It is natural to inquire what are likely to be the future developments of explosives. If the history of the application of explosives be broadly reviewed, it is some- what striking that the materials used for explosive purposes in the World War of 1914-8 were practically all chemical compounds which have been known for at least 50 years. Indeed, the history of the last century has been much more concerned with dis- coveries relating to the methods of application of explosives than the discovery of new explosive compounds. The popular im- agination readily accepts stories of new explosives of fabulous violence, but experience shows that it is not in such directions that research has met with its greatest successes. Until about the middle of last century gunpowder held the field, although gun- cotton, nitro-glycerine, picric acid,mercury fulminate, ammonium nitrate, and the chlorates and perchlorates were all known com- pounds. Only one of these namely, mercury fulminate was used at all, and this only in its capacity as a simple igniter. The successive steps which led to the utilization of one after another of the modern explosives were first directed towards the nitric esters nitro-glycerine and nitro-cellulose. Nitro-glycerine was brought into a form in which it could be practically used, by absorbing it into the pores of kieselguhr and later by incorpo- rating it into gelatinized explosives, thus giving rise to extremely powerful combinations. In the utilization of nitro-cellulose, the initial problem was to bring it into a sufficiently stable condition to render it safe against spontaneous explosion. The discovery of the conversion of nitro-cellulose to a gelatinous condition by treatment with solvents led to valuable blasting explosives such as gelatine dynamite, and, more important still, formed the basis of the modern smokeless propellants.

The method of initiating the detonation of high explosives by mercury fulminate dates from 1867, and opened the way to the ultimate utilization of very insensitive explosives for blasting and military purposes. The importance of this discovery will be realized when it is considered that it rendered possible the use of a wide range of ammonium nitrate and other mixtures for indus- trial purposes, and the use of T N T and amatol for military purposes in the World War. Many more steps in the investigation of detonation were, however, necessary before the mechanism of gun-shells was so perfected as to give efficient detonation com- bined with perfect safety; and although the use of aromatic nitro compounds, as represented by picric acid, for shell purposes was introduced about 1886, it is only in the present century that the methods of detonation have been so perfected as to render these high explosives an outstanding factor in warfare.

The number of new explosives which have been patented is enormous, but these consist almost entirely of different mixtures of known ingredients, nor is it likely that any spectacular dis- covery will be made in the nature of a new compound of unprece- dented power. In the first place, granted that the oxygen is correctly balanced against the carbon and hydrogen, the chemical energy can only be increased by lowering the heat of formation. This might be done to some extent, and compounds of somewhat

Behaviour of Explosives on Detonation.

Explosive.

CO r*

11

&""

38. SJ

Heat evolved (cal. pergrm.)

Stability test c.c. per hr. per kilo, after 40 hrs. in a vacuum.

Relative insensitive- ness to impact (Picric Acid = 100).

Velocity of Detonation.

Density.

Metres per sec.

80 C. 120 C. 140 C.

Tri-nitro-toluene Tri-nitro-benzene Tri-nitro-phenol (Picric Acid) Tetryl. Tetra-nitro-aniline Nitro-cellulose (13% I. N) .... Nitro-glycerine Mercurv Fulminate

728 820 744 794

875 713

924 940 914 1,090

982 1,478

9 o 0-6 8 18 5,000 3,660 2-5

H5 107

IOO

70

86 23 13

10

1-57

1-63 1-63

1-2

loose

6,950

7,250 7,520

7,3oo 3,000