GUNPOWDER 329 siura, and 840 Ibs. of chloride of sodium, would be refined by simple recrystallization, some- what as follows : Half the mass would be dis- solved in 1,200 Ibs. of water, and heated to 100 C., while the remainder was added. This would dissolve also all the chloride of potas- sium, but leave a large part (516 Ibs.) of the chloride of sodium. From the solution the or- ganic impurities are removed by adding a little glue, boiling, and skimming, after which the temperature is allowed to fall to 18 0., at which point about 4,452 Ibs. of saltpetre will be precipitated, 6 Ibs. of chloride of sodium, and no chloride of potassium. A further re- crystallization will reduce still more the amount of impurity. This is the outline of the process adopted. It is modified by operations intend- ed to hinder the formation of large crystals of saltpetre, which always contain enclosed por- tions of the mother liquor. The final product should never contain more than T $ of 1 per cent, of chlorides. In fine sporting powders the proportion of chlorides is usually not more than yfoi or sometimes T -oVo? of 1 P er cent - After a thorough drying by heat, the saltpetre is ground to the finest possible state, either with millstones or in mills specially construct- ed. Charcoal is the principal form of carbon capable of giving an explosive gunpowder. Graphite or anthracite, when substituted, yields a compound which burns with some vigor, but not explosively. The causes of this peculiarity are not known with .certainty. The wood selected for the charcoal is usually the young shoots of alder and willow. The bark being removed, they are placed -in iron retorts and subjected to distillation, expelling the greater part, sometimes nearly the whole, of the hydrocarbons. Three grades of char- coal are recognized: red charcoal, containing about 8 or 9 per cent, of hydrocarbons, and having a strong red tinge; brown charcoal, containing 4 to 5 per cent, of hydrocarbons; and black charcoal, containing about 1 to 2 per cent. The red charcoal makes the strong- est powder, and is generally used for the sport- ing variety ; the black, and sometimes the brown, is preferred for heavy ordnance. The sulphur selected is refined brimstone ; flowers of sulphur is objectionable because it contains sulphuric acid. The mixture is commenced by pulverizing the charcoal and sulphur to- gether. They are rolled in barrels with small iron balls for about six hours, and are ultimate- ly reduced to extreme minuteness. The salt- petre is then added, and another rolling with zinc or copper balls is given. The mixture is then carried to the mill, where it is moistened with water, and placed in a large circular pan or trough, in which iron wheels, weighing sev- eral tons and having broad treads, are rolled by machinery, triturating and kneading the powder into the most intimate mechanical union. The milling is the dangerous part of the process, and a year seldom passes at a pow- der manufactory without' one or more explo- der sions at the wheel mills, though the precautions are such that these accidents are seldom dis- astrous. The powder withdrawn from the wheel trough is very lumpy and irregular ; it is therefore reduced by a breaker to a fine meal, in which condition it is transferred to a very powerful press. The meal is stacked into the form of a cheese, 2 or 2 ft. square and 3 or 4 ft. high, consisting of layers of powder 1 to 2 in. thick, separated by plates of copper or vulcanite. The press reduces the bulk of the powder nearly one half, and delivers it in sheets five eighths of an inch to an inch thick, which, in hardness, lustre, and fracture, re- semble thick slates. The degree of pressure to be given is one of the most important consid- erations in the process of manufacture ; since the ultimate density of the powder is deter- mined by it, and this in turn determines the rate of combustion. By merely varying the degree of pressure, the powder may be made either violently and destructively explosive, or mild and easy in its action. The press cakes are broken into grains bypassing them through corning rolls, and the different sizes of grains are separated by passing the whole over a suc- cession of sieves, with varying mesh. The grains thus obtained are sharply angular, and require rounding and smoothing to prevent their thin edges from being ground into dust by the wear of transportation and handling. While still moist, the powder (now granulated) is put into rolling-barrels, slowly revolved from 6 to 24 hours, and then withdrawn smooth, lustrous, and free from angularity. Sometimes the glazing is heightened by the addition of a minute quantity of graphite. A single table- spoonful of this substance will impart its pecu- liar lustre to half a ton of fine powder. Its presence has no appreciable effect upon the ac- tion of the powder, nor upon its preservation. The last operation, drying, takes place in a room heated by steam to 130 or 140 F. Much progress has been made in the past 20 years in diminishing the destructive effects of gunpowder upon large cannon. A brief con- sideration of the relations between the pressure of a body of gas and the volume it occupies will readily indicate that the elastic force of gases generated by the combustion of gunpow- der in a gun must be many times greater at the breech than at the muzzle ; that is, greater in the earlier than in the latter stages of discharge. It is the maximum pressure which involves danger to the gun; and the principle upon which this progress has been based consists in keeping the maximum pressure well within the limit of safety, and increasing as far as pos- sible all pressures below that limit. To ac- complish this, advantage has been taken of the following properties : The combustion of a car- tridge is not instantaneous, but progressive. It must first be ignited throughout, by flame from the vent, which diffuses itself through the interstices between grains, enveloping progres- sively the whole. Secondly, each grain must
