812 EXPLOSIVES . . . (14). This expression for the work done is of considerable practical value in the case of gunpowder, or any explosive which can be used as a propelling agent with heavy guns. Knowing the length and diameter of the bore, we can calculate the total maximum work due to a given weight of charge in expanding to that volume. This maximum is of course not attained in practice, and it is there fore necessary to multiply it by a ratio, or factur, dependent on the nature of the gun and projectile, the powder used, mode of ignition, ifcc. However, by making use of the results of actual experiment, this "factor of effect," or percentage of work realized, can be determined with much accuracy. Its value is greatest for very large guns, being 93 per cent, for the 38-ton gun, and becoming as low as about 50 per cent, in the case of the little 7-pr. mountain gun of 150 Ib. weight; the difference is chiefly due to the loss of heat by communication to the metal of the gun. (See Noble and Abel on "Fired Gunpowder.") Initial We can approximate to the "muzzle velocity," or the velocity, velocity at which the projectile leaves the bore, by substituting the value of W, found for the particular gun, in the ordinary wV 2 equation of work, W = , where V is the velocity, and w the weight of the projectile ; we thence obtain v w If W, the maximum work due to the expansion of the gaseous products in the volume of the bore, be multiplied by the factor of effect /, for the nature of gun and powder used, the result will be very nearly the mean observed velocity. Grapln- The mathematical expression for the work done by an sentation ex plsive substance in expanding from V to v, or / pdv, wor evidently denotes the area of a plane curve ; the work may therefore be graphically represented by the area enclosed by a curve, having for its ordinates the pressures in foot- tons, or atmospheres, and for abscissae the corresponding volumes or spaces occupied by the gases. Total If, in equation (14), we take the limits between V and theoretic infinity, we arrive at a very simple expression for the total work - theoretic work due to the indefinite expansion of a given weight of any explosive substance, v becoming indefinitely great compared to the original volume V, and we have W-J1I, (15), which may be called the " potential energy " of the ex plosive, being the product of the total quantity of heat generated by the explosion and the mechanical equivalent of heat. This .conclusion, within the assumptions made, is in strict accordance with the principle of the mutual convertibility of energy and heat. The following table shows the potential energy, in foot-tons, calculated from the heat of combustion for each explosive, determined by Roux and Sarrau, in the experi ments already referred to ; that for gunpowder is the mean given by five kinds. Potential Energy Explosive Substance. per rb. Foot-tons. Gunpowder 480 Gun-cotton 716 Nitro-glycerin 1139 Picrate of potash 536 Picrate of potash and saltpetre 615 Picrate and chlorate of potash 781 Chloride of nitrogen 216 The above figures naturally direct our attention to the small amount of work stored up in even the most violent explosive substance, compared with the potential energy of 1 Ib. of coal, which is about 4980 foot-tons. Noble and Abel point out that this great difference is not alone due to the fact that the coal draws its oxygen from the air, but also because the explosive has to expend a considerable amount of work in converting its condensed magazine of oxygen into gas, before it can combine with the carbon ; further, with reference to the economic value of the work done, that the oxygen used by the coal costs nothing, whereas much expense is incurred in condensing the oxygen into the explosive substance. The practical value of any explosive must depend greatly Practica upon the object to be attained. It is essential to distinguish value of between explosive force and effect; the more sudden the ex> action the more local will be the effect produced, and hence p the very violent explosive substances are useless as propel ling agents for heavy guns or small arms, since they would destroy the weapon before overcoming the inertia of the projectile. It is true that gun-cotton, prepared in various forms, and mixed with other substances to moderate its action, as well as a similar compound made from saw-dust, an inferior form of cellulose, are sometimes used with small arms ; but, in addition to a want of uniformity in action, the strain caused by such substances would be far too great in the large charges needed for heavy guns. Again, there are cases, even in mining or blasting operations, for instance, when it is desired to displace large masses of earth or soft rock, in which a comparatively slow explosive, such as gunpowder, would give better results than gun-cotton or dynamite. However, speaking generally, gunpowder in some one of its forms is far the most valuable as a pro pelling agent, while, for destructive purposes, the last- named substances are much more effective, especially when detonated. Laic. In 1860 an Act was passed " to amend the law concerning Law re- tlie making, keeping, and carriage of gunpowder and compositions of lating to an explosive nature, and concerning the manufacture and use of fire- explo- works " (23 and 24 Viet. c. 139), whereby previous Acts on the same sives. subject were repealed, and minute and stringent regulations intro duced. Gunpowder may only be manufactured in mills, lawfully used at the commencement of the Act, or duly licensed as in this Act provided ; other explosive compositions require a licence, and the precautionary rules as to quantity, distance from dwelling houses, &c., are set forth in minute detail. No person may sell fireworks without a licence, or to persons apparently under 16 years of age ; and throwing fireworks on the streets was made punishable by a penalty not exceeding 5. Other regulations deal with car riage by land and sea, search-warrants, inspections of mills, &c. Amending Acts were passed in 1861 and 1862. In 1875 was passed the " Explosives Act " (38 Viet. c. 17), which repeals the former Acts, and deals with the whole subject in a more comprehensive manner. " Explosives" are thus defined: (1) Gun powder, nitro-glycerin, dynamite, gun-cotton, blasting powders, fulminate of mercury or of other metals, coloured fires, and every other substance, whether similar to those above mentioned or not, used or manufactured with a view to produce a practical effect by explosion or a pyrotechnic effect, and including (2) fog-signals, fireworks, fuzes, rockets, percussion caps, detonators, cartridges, ammunition of all descriptions, and every adaptation or preparation of an explosive as above defined. Part i. deals with gunpowder; part ii. with nitre-glycerin and other explo sives ; part iii. with inspection, accidents, search, &c. ; part iv. with various supplementary provisions. In addition to the licence required for manufacturing gunpowder, it is provided that gun powder shall not be kept in any place except (1) a licensed factory, (2) a licensed magazine or store, or (3) premises registered for keeping gunpowder. Private persons may keep gunpowder for their own use to the amount of thirty pounds. Rules for the proper keeping of gunpowder on such registered premises are prescribed. The Act contains 122 sections, and applies to Scotland and Ireland as well as England. It was based on the report of a Committee of the House of Commons. Public opinion had been greatly excited on the sub ject by the terrible explosion on the Regent s Canal in 1874. Petroleum is governed by the Petroleum Act 1871 an annual Act, which has been included every year in the Expiring Acts Con tinuance Bill. In 1877 the "Fisheries (Dynamite) Act " was passed, whereby any person who uses dynamite or other explosive substance to catch or destroy fish in a public fishery, shall be liable, on summary con viction, to a fine not exceeding 20, or imprisonment for a term cf
not more than two months.