THE UNITED STATES SERVICE MAGAZINE. VOL. I.-MAY, 1864.-NO. V. ON SALTPETRE. BY JAMES с. воотн, (Melter and Refiner, U. S. Mint.) GUNPOWDER-whether used in war, the chase, or in blasting -is well known to be a mechanical mixture of three substances, saltpetre, sulphur, and charcoal; the two former being products of the mineral kingdom, and the latter being obtained from the vegetable kingdom. While charcoal can be obtained in unlimited quantity in every cultivated part of the globe, and sulphur is sufficiently abundant and widely distributed, either in its native state or in such combinations as readily yield it, the supply of saltpetre is comparatively restricted in quantity, and still more in locality. Hence its preparation by human ingenuity has been found necessary, and may at any time be demanded by the commercial blockade of a nation. Dismissing, therefore, the consideration of sulphur and charcoal, we may advantageously, perhaps, devote a few pages to saltpetre,-by inquiries into its nature, the extent of its natural production, the assistance which art has hitherto contributed to its generation, its extraction and purification, and, lastly, into the theory of its mode of formation. History.-Saltpetre appears to have been first mentioned under that name by Geber, in the eighth century, as sal perœ Entered according to Act of Congress, in the year 1864, by C. B. RICHARDSON, in the Clerk's 449 (rock-salt); but as its history is involved in that of gunpowder, the earliest notices of the last, or indeed of analogous, fire-compositions, embrace the history of saltpetre. While we have complacently trusted for ages to the current belief that gunpowder was invented by Berthold Schwartz, the German monk and alchemist, as far back as 1320, or by Roger Bacon, the great English friar, in 1270,-for the latter undoubtedly describes the fire-crackers of the boys, and mentions even the constituents of the "imitation of thunder,"-it was reserved for the deepsearching criticism of the nineteenth century (the most striking mental characteristic of the European mind in this century) to prove that it was known and used long before the thirteenth century, even by Europeans; that it is certain the Chinese were well acquainted with fire-works ("devouring fire," "earththunder") at the beginning of the Christian era, and which proved our pastime and annoyance on July 4, 1863; that the Greek fire was introduced from the East to the Byzantines in the seventh century; that the Crusaders were terrified at the fireworks of their Saracen enemies; that a treatise on pyrotechny has been discovered, written by Hassan Abramman, who speaks of Chinese fires and applies Chinese epithets to them; that Marcus Græcus, between the ninth and twelfth centuries, gives minute directions for making rockets in a work on pyrotechny, entitled "The Book of Fires, for Burning an Enemy by Sea or Land." It appears, therefore, that the Arabs, the great colporteurs of the arts from the indefinite East, conveyed gunpowder, the civilizing scourge and blessing, from China to our semi-civilized forefathers. Improvements in the thunder-mixtures of the Chinese progressed slowly in Europe during many centuries, until, by the westward march of empire across Asia, through Europe, and across America, they are again brought to compete with those of our earliest fire-work instructors, the Chinese; but these have remained so stereotyped for more than one thousand years, that their "devouring fire," unaided by foreigners, will not compare with the lightning and volcano effects of the cannon of their pupils. Chemical Characteristics.-Ordinary combustion is the union of the oxygen of the air with a combustible; and since the atmosphere contains four-fifths of nitrogen (a comparatively inert body in this respect), and only one-fifth oxygen, combustion in pure oxygen gas is intense, and usually brilliant. In like manner, when we employ a solid body, containing an enormous bulk of oxygen condensed in a small space, with a combustible, the intensity of combustion is enormously increased, often to explosive violence, even in the open air. Such a solid is saltpetre. A mixture of saltpetre and charcoal, ignited, deflagrates and burns vividly. A mixture of saltpetre and sulphur, with some pearlash, slowly heated, explodes with violence. Saltpetre (sal petræ, nitre, nitrate of potash, of potassa or potassium) is composed, according to the later views of the chemist, of potassium and nitric residue, with the formula KNO,, but according to the dualistic view, of nitric acid and potassa. We shall follow the former in the present essay, as more conformable to fact. It contains nearly one-half its weight of oxygen (over forty-seven per cent.), all of which is available for deflagrating the charcoal of gunpowder, because the sulphur seizes the potassium. But the conversion of carbon (charcoal) into carbonic acid gas, by the oxygen of the nitre, by no means represents all the force which the ignition of gunpowder exerts on adjacent bodies; for, in the new arrangement of the elements by the combustion, nitrogen, finding nothing to unite with, suddenly flies from the solid to the gaseous state, adding nearly fourteen per cent. (of the weight of nitre) to the gaseous products of combustion. Hence sixty-one per cent. of the nitre suddenly fly from their rigid crystal walls into a huge gaseous bulk, which is enormously expanded by the intense heat of combustion, simultaneous with its evolution. Substitutes for Saltpetre or Gunpowder.-Before wasting our time in discussing the manufacture of saltpetre,-the supply of which we have asserted to be quite restricted in locality, let us first consider whether other nitrates, or similar salts, might not be substituted for it, by fulfilling the requisite conditions of vividly supporting combustion and evolving a large amount of gas. Nitrate of sodium (cubic nitre, soda saltpetre) has the same formula as saltpetre, Na NO,, except that the cheaper element sodium takes the place of potassium, fulfils the conditions. laid down, and even yields a larger volume of gas. Moreover, it is found much nearer home, on the plain of Atacama, on the border between Peru and Chili; while almost all our saltpetre is derived from the valley of the Ganges. It is also found in large quantity on Atacama, and is now the chief source of the nitric acid (aquafortis) of the world. Why, then, can we not use it in place of saltpetre? If gunpowder becomes moist, its value is impaired or destroyed. Now, even pure nitrate of sodium is hygroscopic; i.e. it attracts moisture from the air when the latter is not very dry. Hence, a gunpowder made with it, instead of its potassium congener, would be likewise hygroscopic, and become moistened in storage under ordinary circumstances. Nitrate of sodium, therefore, must be rejected as a constituent of gunpowder, unless some means, now unknown, be devised to prevent its absorbing water. In like manner, the nitrates of calcium and magnesium cannot be employed, because of their deliquescent character, although they are found in nature, and might be made economically. By the more modern theory, 0=16; by the older, 0=8. Most other metallic nitrates are deliquescent, and therefore useless for our purpose. Nitrate of lead, perhaps, might be used; but it would be more expensive than saltpetre, would only yield twenty-nine per cent. oxygen, and, altogether, thirty-eight per cent. gas, instead of forty-seven and sixty-one per cent. of these respectively yielded by saltpetre; and, worse than all, while common gunpowder has to expend some of its force in projecting thirty-nine per cent. of its saltpetre as potassium, a nitrate-of-lead gunpowder would have to waste some of its power in projecting sixty-two per cent. of its nitric salt as lead; i.e. would have to project a sixty-two-pound shot before acting on the iron ball or shell. We know of no other nitrate available; but there is a closely analogous salt (chlorate of potassium) that might be employed. Its formula, K CI O,, shows that it contains a large amount of oxygen, which we know it readily gives up, and which renders it a powerful supporter of combustion. It is also a very dry salt. It cannot, however, be substituted for saltpetre; for it is more costly, evolves less gaseous matter, and, above all, yields up its oxygen so easily that a gunpowder made with it would be more dangerous to friend than foe. The first experiment made by that great chemist, Berthollet, to make a gunpowder with it at Essonne, in France, resulted in a fearful explosion, with loss of life, and has not been repeated. This leads us to consider whether other explosive bodies may not be employed instead of gunpowder, such as the fulminates, chloride of nitrogen, &c. In the first place, their preparation is attended with imminent danger, and their storage and handling would not be less dangerous-we do not exaggerate in saying a thousandfold more so-than gunpowder. But, in the second place, a more important reason is, that their explosion is almost instantaneous, while that of gunpowder is progressive. A heavy door may be closed by the finger, if time be given to overcome the vis inertia; but a sudden blow with the fist would probably hurt the hand rather than close the door. A ball rammed down upon powder in the chamber of a gun is clearly movable if time be allowed to overcome its vis inertia ; but for the first moment of time it is as fixed and immovable as the solid walls of the gun. If the powder were instantaneously explosive, its whole gaseous power would, in a moment of time and in all directions alike, act against sides and ball, and would burst the gun as easily as move the ball. By the progressive combustion of gunpowder, the first small portions of gas generated begin to move the ball; while the others, added in succession, continually increase the velocity of projection to the end of the tube or bore. We may, therefore, dismiss all violent explosives from further consideration. The only question remaining is, whether a substitute for gun powder may not be provided, of similar progressive explosiveness, and in other respects of equal or greater value in its cost, the volume of gas evolved, the security of storage, &c. Two substances now offer themselves,-a powder prepared from ferro-prussiate and chlorate of potash and sugar, and gun-cotton. The best composition and proportions for the former have not been sufficiently elaborated; but, as far as experiment has gone, it is too liable to explosion from accident. Moreover, the dried prussiate has over fifty-seven per cent. solid matter to waste some of the force of the powder in being ejected, or, if not ejected, it would foul a gun seriously. Gun-cotton seems likely to prove a formidable rival to gunpowder in the hands of the Austrians; but time is yet needed to ascertain whether in every respect it be equal or superior to it. Until thorough, varied, and long-continued experiment shall have found a perfectly reliable substitute, the experience of centuries in the manufacture of saltpetre-gunpowder is not to be thrown away. The discussion leads to the conclusion, that, according to our present experience, nothing can be substituted for gunpowder or for the saltpetre which it contains. We may not, therefore, waste our time unprofitably in considering the manufacture or extraction of saltpetre more minutely. Moreover, even if its possible rival, gun-cotton, should supersede it, a study of the preparation of saltpetre is just as important, because the question of the supply of nitric acid is the leading question in nitre and gun-cotton. Natural Supply of Saltpetre.-Any nitrate may be easily converted into nitrate of potassium by wood-ashes; and, as the other metals allied to potassium are abundantly distributed, and generally all mixed together, we must look for natural alkaline nitrates associated together, chiefly those of potassium, sodium, calcium, and magnesium. We find nitrates almost everywhere on the globe, in the soil, in rain, river, and spring waters, more abundant in boggy soils in temperate regions, and still more so in partly wet and even sandy soils in warmer climates. The more recent improvements in detecting the presence of a nitrate have revealed this unlooked-for wide distribution; and yet, in spite of this diffusion, there are but few localities on the globe where it has been profitably extracted. We prove its presence almost everywhere, but in such minute proportion to the soil or water containing it, that there is no ground to hope that we can obtain it everywhere. Again, while we thus find nitric acid in some nitrate everywhere, in like manner do we find potassium claiming an equally wide diffusion, but only abundant in some feldspar or greensand localities, rather less in the kaolin derivatives of feldspar, and still less in clays and other rock masses. In proof of its universal distribution, we need only cite land-plants, whose ashes yield potash, and constitute the supply of potassium for all |