Читать книгу Nitro-Explosives: A Practical Treatise - P. Gerald Sanford - Страница 22

[Footnote A: See also Berthelot, Comptes Rendus, 1900, 131[12], 519- 521.]

Every one employed in the buildings should wear list or sewn leather shoes, which of course must be worn in the buildings only. The various houses should be connected by paths laid with cinders, or boarded with planks, and any loose sand about the site of the works should be covered over with turf or cinders, to prevent its blowing about and getting into the buildings. It is also of importance that stand pipes should be placed about the works with a good pressure of water, the necessary hose being kept in certain known places where they can be at once got at in the case of fire, such as the danger area laboratory, the foreman's office, &c. It is also desirable that the above precautions against fire should be tested once a week. With regard to the heating of the various buildings in the winter, steam pipes only should be used, and should be brought from a boiler-house outside the danger area, and should be covered with kieselguhr or fossil meal and tarred canvas. These pipes may be supported upon poles. A stove of some kind should be placed in the corner of each building, but it must be entirely covered in with woodwork, and as small a length of steam pipes should be within the building as possible.

In the case of a factory where nitro-glycerine and dynamite are manufactured, it is necessary that the work-people should wear different clothes upon the danger area than usual, as they are apt to become impregnated with nitro-glycerine, and thus not very desirable or safe to wear outside the works. It is also necessary that these clothes should not contain any pockets, as this lessens the chance of matches or steel implements being taken upon the danger area. Changing houses, one for the men, and another for the girls, should also be provided. The tools used upon the danger area should, whenever the building is in use, or contains explosives, be made of phosphor bronze or brass, and brass nails or wooden pegs should be used in the construction of all the buildings.

[Illustration: FIG. 2.—MELSENS SYSTEM OF LIGHTNING CONDUCTORS.]

~Lightning Conductors.~—The Explosive Substances Act, 38 Vict. ch. 17, clause 10, says, "Every factory magazine and expense magazine in a factory, and every danger building in a magazine, shall have attached thereto a sufficient lightning conductor, unless by reason of the construction by excavation or the position of such magazine or building, or otherwise, the Secretary of State considers a conductor unnecessary, and every danger building in a factory shall, if so required by the Secretary of State, have attached thereto a sufficient lightning conductor."

The exact form of lightning conductor most suitable for explosive works and buildings has not yet been definitely settled. Lightning-rod engineers favour what is known as the Melsens system, due to Professor Melsens, of Brussels, and Professor Zenger, of Prague, but first suggested by the late Professor Clerk-Maxwell. In a paper read before the British Association, Clerk-Maxwell proposed to protect powder-magazines from the effects of lightning by completely surrounding or encasing them with sheet metal, or a cage of metallic conductors. There were, however, several objections to his system as he left it.

Professor Melsens[A] has, while using the idea, made several important alterations. He has multiplied the terminals, the conductors, and the earth-connections. His terminals are very numerous, and assume the form of an aigrette or brush with five or seven points, the central point being a little higher than the rest, which form with it an angle of 45°. He employs for the most part galvanised-iron wire. He places all metallic bodies, if they are of any considerable size, in communication with the conducting system in such a manner as to form closed metallic circuits. His system is illustrated in Fig. 2, taken from Arms and Explosives.

[Footnote A: Belgian Academy of Science.]

This system is a near approximation to J.C. Maxwell's cage. The system was really designed for the protection of powder-magazines or store buildings placed in very exposed situations. Zenger's system is identical with that of Melsens, and has been extensively tried by the Austrian military authorities, and Colonel Hess has reported upon the absolute safety of the system.

[Illustration: Fig. 3.—FRENCH SYSTEM OF LIGHTNING CONDUCTORS.]

The French system of protecting powder-magazines is shown in Fig. 3, where there are no brush terminals or aigrettes. The French military authorities also protect magazines by erecting two or more lightning-rods on poles of sufficient height placed close to, but not touching, the walls of the magazine. These conductors are joined below the foundations and earthed as usual.

In the instructions issued by the Government, it is stated that the lightning-rods placed upon powder-mills should be of such a height, and so situated, that no danger is incurred in igniting the powder-dust in the air by the lightning discharge at the pointed rod. In such a case a fork or aigrette of five or more points should invariably be used in place of a single point.

[Illustration: FIG. 4_a_.—GOVERNMENT SYSTEM OF LIGHTNING CONDUCTORS FOR

LARGE BUILDINGS.]

[Illustration: FIG. 4_b_.—GOVERNMENT SYSTEM OF LIGHTNING CONDUCTORS FOR

SMALL BUILDINGS.]

In Fig. 4 (a and b) is shown the Government method for protecting buildings in which explosives are made or stored. Multiple points or aigrettes would be better. Lord Kelvin and Professor Melsens favour points, and it is generally admitted that lightning does not strike buildings at a single point, but rather in a sheet; hence, in such cases, or in the event of the globular form being assumed by the lightning, the aigrette will constitute a much more effective protection than a single point. As to the spacing of conductors, they may, even on the most important buildings, be spaced at intervals of 50 feet. There will then be no point on the building more than 25 feet from the conductor. This "25-feet rule" can be adhered to with advantage in all overground buildings for explosives.

Underground magazines should, whenever possible, also be protected, because, although less exposed than overground buildings, they frequently contain explosives packed in metal cases, and hence would present a line of smaller electrical resistance than the surrounding earth would offer to the lightning. The conductor should be arranged on the same system as for overground buildings, but be applied to the surface of the ground over the magazines.

In all situations where several conductors are joined in one system, the vertical conductors should be connected both at the top and near the ground line. The angles and the prominent portions of a building being the most liable to be struck, the conductors should be carried over and along these projections, and therefore along the ridges of the roof. The conductors should be connected to any outside metal on the roofs and walls, and specially to the foot of rain-water pipes.

All the lightning conductors should be periodically tested, to see that they are in working condition, at least every three months, according to Mr. Richard Anderson. The object of the test is to determine the resistance of the earth-connection, and to localise any defective joints or parts in the conductors. The best system of testing the conductors is to balance the resistance of each of the earths against the remainder of the system, from which the state of the earths may be inferred with sufficient accuracy for all practical purposes.

Captain Bucknill, R.E., has designed an instrument to test resistance which is based on the Post Office pattern resistance coil, and is capable of testing to approximate accuracy up to 200 ohms, and to measure roughly up to 2,000 ohms. Mr. R. Anderson's apparatus is also very handy, consisting of a case containing three Leclanché cells, and a galvanometer with a "tangent" scale and certain standard resistances. Some useful articles on the protection of buildings from lightning will be found in Arms and Explosives, July, August, and September 1892, and by Mr. Anderson, Brit. Assoc., 1878–80.

~Nitro-Glycerine.~—One of the most powerful of modern explosive agents is nitro-glycerine. It is the explosive contained in dynamite, and forms the greater part of the various forms of blasting gelatines, such as gelatine dynamite and gelignite, both of which substances consist of a mixture of gun-cotton dissolved in nitro-glycerine, with the addition of varying proportions of wood-pulp and saltpetre, the latter substances acting as absorbing materials for the viscid gelatine. Nitro-glycerine is also largely used in the manufacture of smokeless powders, such as cordite, ballistite, and several others.

Nitro-glycerol, or glycerol tri-nitrate, was discovered by Sobrero in the year 1847. In a letter written to M. Pelouse, he says, "when glycerol is poured into a mixture of sulphuric acid of a specific gravity of 1.84, and of nitric acid of a gravity of 1.5, which has been cooled by a freezing mixture, that an oily liquid is formed." This liquid is nitro-glycerol, or nitro-glycerine, which for some years found no important use in the arts, until the year 1863, when Alfred Nobel first started a factory in Stockholm for its manufacture upon a large scale; but on account of some serious accidents taking place, its use did not become general.

It was not until Nobel conceived the idea (in 1866) of absorbing the liquid in some absorbent earth, and thus forming the material that is now known as dynamite, that the use of nitro-glycerine as an explosive became general.

Among those who improved the manufacture of nitro-glycerine was Mowbray, who, by using pure glycerine and nitric acid free from nitrous acid, made very great advances in the manufacture. Mowbray was probably the first to use compressed air for the purpose of keeping the liquids well agitated during the process of nitration, which he conducted in earthenware pots, each containing a charge of 17 lbs. of the mixed acids and 2 lbs. of glycerol.

A few years later (1872), MM. Boutnny and Faucher, of Vonges,[A] proposed to prepare nitro-glycerine by mixing the sulphuric acid with the glycerine, thus forming a sulpho-glyceric acid, which was afterwards mixed with a mixture of nitric and sulphuric acids. They claimed for this method of procedure that the final temperature is much lower. The two mixtures are mixed in the proportions—Glycerine, 100; nitric acid, 280; and sulphuric acid, 600. They state that the rise of temperature upon mixing is limited from 10° to 15° C.; but this method requires a period of twenty-four hours to complete the nitration, which, considering the danger of keeping the nitro-glycerine in contact with the mixed acids for so long, probably more than compensates for the somewhat doubtful advantage of being able to perform the nitration at such a low temperature. The Boutnny process was in operation for some time at Pembrey Burrows in Wales, but after a serious explosion the process was abandoned.

[Footnote A: Comptes Rendus, 75; and Desortiaux, "Traité sur la Poudre," 684–686.]

Nitro-glycerine is now generally made by adding the glycerine to a mixture of sulphuric and nitric acids. The sulphuric acid, however, takes no part in the reaction, but is absolutely necessary to combine with the water that is formed by the decomposition, and thus to keep up the strength of the nitric acid, otherwise lower nitrates of glycerine would be formed that are soluble in water, and which would be lost in the subsequent process of washing to which the nitro-compound is subjected, in order to remove the excess of acids, the retention of which in the nitro-glycerol is very dangerous. Nitro-glycerol, which was formerly considered to be a nitro-substitution compound of glycerol, was thought to be formed thus—