Our saltpetre, which is commonly called nitrum, and sometimes, though more rarely, sal nitræ, is a neutral salt composed of a peculiar acid, named the acid of saltpetre or nitric acid, and that alkali called potash. The characters by which it is most readily distinguished from other salts are its cooling taste, its fusibility when exposed to a small degree of heat, and in particular its so-called deflagration; that is, the property it has when placed in the fire, or on an ignited body, or when melted in a crucible, with a combustible substance, of suddenly bursting into a very bright flame, by which it loses its acid, and nothing remains but a carbonate of potash. The principal use of it is in making gunpowder, and for the preparation of that acid known under the name of aquafortis[1191], which is employed in various ways.
Native saltpetre is so rare, that Cronstedt was not acquainted with it. At present, however, it is known to occur in the East Indies, in the lower part of Italy, also in Portugal[1192], Spain, America, and some other countries[1193]. But almost all the saltpetre obtained in Europe is produced partly by nature and partly by art. The putrefaction of organic bodies gives rise, under certain circumstances, to nitric acid, which in general combines with calcareous earth wherever it finds it, and forms the so-called earthy saltpetre. This is decomposed by potash, and the latter uniting with the acid forms common saltpetre. Sometimes also it is found that the nitric acid, instead of being united with calcareous earth, is combined with soda, which produces the so-called cubic saltpeter[1194]. Both these saline substances, but the earthy more frequently than the cubic, are often found on effloresced walls; and both are then comprehended under the common names of Mauersalz or Mauerbeschlag, sal murale.
This efflorescence on walls was observed, in all probability, at a very early period, especially as it is produced in many parts in great abundance, and as it makes itself perceptible by the decay of walls, which it seems to corrode. It is the plague or leprosy of houses mentioned in the Mosaic code of laws. As the ancients were so much inclined to expect medicinal virtue in all natural bodies, there is reason to think that they soon collected and made trial of this saline incrustation. That this indeed was actually the case, and that they gave the name of nitrum to this saline mass, may be proved from their writings. Their nitrum, however, must have been exceedingly various in its properties. For this incrustation is not always calcareous saltpetre; it is often soda[1195], mixed with more or less calcareous earth; and sometimes it consists of salts of sulphuric acid. In modern times, on closer examination, other salts of nitric acid have been found in the incrustation of walls, such as flaming saltpetre or nitrate of ammonia, bitter saltpetre or nitrate of magnesia; but of these no mention can be expected in the works of the ancients.
Substances so different ought not indeed to have been all named nitrum; but before natural history began to be formed into a regular system, mankind in general fell into an error directly contrary to that committed at present. Objects essentially different were comprehended under one name, if they any how corresponded with each other even in things accidental. Whereas at present every variety, however small, obtains a distinct appellation; because many wish to have the pleasure, if not of forming new species, at any rate of giving new names. The elephant and rhinoceros were formerly called oxen; the sable and ermine were named mice, and the ostrich was distinguished by the appellation of sparrow. In the like manner, calcareous saltpetre and alkali might be called nitrum. The ancients, however, gave to their nitrum some epithets, but they seem to have been used only to denote uncommon varieties.
Now, as the ancients were not acquainted with any accurate method of separating and distinguishing salts, it needs excite no wonder that they should ascribe to their nitrum properties which could not possibly be united in a salt, and much less exist in our saltpetre. But as they were neither acquainted with aquafortis nor manufactured gunpowder, and as no particular use of calcareous saltpetre was known, the nitrum most valuable to them must have been that which consisted chiefly of soda, and which consequently could be employed in washing, in painting, and in glass-making.
It is well known that in warm countries this alkali effloresces here and there from the earth, particularly in a dry soil, and even in such quantity as to be employed in commerce. Hence it may be readily comprehended why this effloresced salt, which is very often mixed with common salt, obtained the name of nitrum.
The important discovery, that a similar salt, having the like properties, and applicable to the same uses, named at present soda, may be obtained from the ashes of certain plants, was first made, in my opinion, by the ancient Egyptians or Arabians[1196]. This salt also, at least by the Greeks, was named nitrum, or considered as a species of it. By the incineration of the plants this salt was rendered slightly caustic; and it then became moist in the air, and deliquesced when not preserved in very close vessels. It was therefore like those salts which are obtained, in the same manner, from the ashes of all other plants; though the latter are essentially different from the former, and in the course of time obtained the peculiar appellation of potash or pearlash. One can hardly be surprised that the ancients were not able to distinguish soda from potash, especially as they were both obtained from vegetable ashes.
But were the ancients, under the ambiguous name of nitrum, acquainted with our saltpetre? There is certainly reason to think that it became known to them by lixiviating earths impregnated with salts. There are, as already said, not only in India but also in Africa, and particularly in Egypt, earths which, without the addition of ashes or potash, give real saltpetre, like that of the rubbish-hills on the road from new to old Cairo[1197], and like the earth in some parts of Spain. It is a knowledge only of this natural kind of saltpetre, which required no artificial composition, that can be allowed to the ancients, as it does not appear by their writings that they were sufficiently versed in chemistry to prepare the artificial kind used at present.
But even admitting that they had our saltpetre, where and by what means can we be convinced of it? Is it to be expected that any of the before-mentioned characters or properties of this salt should occur in their writings? They neither made aquafortis nor gunpowder; and they seem scarcely to have had any occasion or opportunity to discover its deflagration and the carbonization thereby effected, or, when observed, to examine and describe it. No other use of our saltpetre which could properly announce this phenomenon has yet been known. How then can it be ascertained that under the term nitrum they sometimes meant our saltpetre?
Those inclined to believe too little rather than too much, who cannot be satisfied with mere conjectures or probabilities, but always require full proof, will acknowledge with me, that the first certain accounts of our saltpetre cannot be expected much before the invention of aquafortis and gunpowder. It deserves also to be remarked, that the real saltpetre, as soon as it became known, was named also nitrum; but, by way of distinction, either sal nitrum, or sal nitri, or sal petræ. The first appellation, from which our ancestors made salniter, was occasioned by an unintelligible passage of Pliny, which I shall afterwards point out. The two other names signify, like sal tartari, sal succini, a salt which was not nitrum but obtained from nitrum. Sal-nitri, therefore, or salniter, was that salt which, according to the representation of the ancients, was separated by art from nitrum, yet was essentially different from the nitrum or soda commonly in use. Biringoccio says expressly, that the artificial nitrum, for the sake of distinction, was named, not nitrum, but sal nitrum.