A History of Inventions, Discoveries, and Origins, Volume II (of 2)

SALTPETRE. GUNPOWDER. AQUAFORTIS

In examining the question, whether Theophrastus, Pliny, and in general the ancient Greeks and Romans, were acquainted with our saltpetre, or at what period it became known, I shall perhaps meet with as little success as those who have preceded me in the same research1190. I shall therefore be satisfied if competent judges allow that I have contributed anything new that can tend to illustrate the subject.

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 aquafortis1191, 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 Portugal1192, Spain, America, and some other countries1193. 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 saltpeter1194. 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 soda1195, 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 Arabians1196. 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 Cairo1197, 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.

The name nitrum is of great antiquity, and seems to have been conveyed from Egypt and Palestine to Greece, and thence to Italy and every part of Europe. For it is evidently the neter mentioned by the prophet Jeremiah, chap. ii. ver. 22; and which occurs also in the Proverbs of Solomon, chap. xxv. ver. 20. But whether the name nitrum, as Jerome says1198, be derived from the Egyptian province Nitria, whence it was exported in great abundance, or the name of the province was derived from nitrum, is a question of little importance in regard to this research. Nitron is mentioned by Herodotus, where he describes the Egyptian method of embalming dead bodies1199; by some of the Greeks the word was written and pronounced litron. In the same manner people say nympha and lympha. In order to avoid confusion, I shall here call the nitrum of the ancients nitrum, and the nitrum of the mineralogists saltpetre.

In the course of time men became acquainted with the purer, more useful, and cheaper mineral alkali which was furnished, under the name of soda, by the Moors and inhabitants of the southern countries, who had learned the method of preparing it. The vegetable alkali also was always more and more manufactured in woody districts, as an article in great request, and sold under the name of potash, cineres clavellati. All knowledge of the impure alkali from the incrustation of walls was then lost; and as there was no further need of guarding against confusion, it was not longer thought worth while to name saltpetre sal nitri: it was called nitrum; and the oldest signification of this word being forgotten, it was admitted without further examination, that the nitrum of the ancients was nothing else than our saltpetre.

In the sixteenth century some learned Europeans, while travelling through the East, heard the name natrum given to the mineral alkali which was then exported as an article of commerce, and introduced in their works this transformation of the ancient word nitrum. This appellation was employed by the systematic mineralogists, who, giving themselves little trouble about the original meaning of words, and taking care only to avoid confusion, called the mineral alkali also natrum, and applied the name of nitrum to saltpetre. As far as I know at present, it was first stated by Peter Bellon and Prosper Alpinus1200, that the mineral alkali was in the East called natrum. The former returned in 1549, and the latter was still in Cairo in 1580.

This word was adopted in mineralogy by Linnæus, in the year 1736, as the name of a species, in which he comprehended for the first time the alkaline incrustation found on walls. In this he is followed by Wallerius, who includes also the mineral alkali from the East. Afterwards the word natrum was employed in the same sense by all mineralogists.

It deserves here to be remarked, that Boyle had even examined and determined the difference between the fixed and volatile alkalies; but that mineralogists and chemists, till the latest periods, believed that all fixed alkali arose, or at least was obtained, by the incineration of plants. The difference between the mineral and vegetable alkalies was first defined, in a proper manner, by the exertion of the German chemists Pott, Model and Marggraf1201; especially after the last had proved, in the year 1758, that the basis of common salt was not, as had before been generally believed, an alkaline earth, but a fixed alkali, to which, because it was in many of its properties different from the fixed vegetable alkali, he gave the name of fixed mineral alkali. Soon after this substance was discovered in mineral springs; and Model and others have shown that it is not essentially different from that which in the East is called natrum.

It is singular, and yet may be accounted for, that since that time many have spoken of the nitrum and natrum of the ancients, though they are only different pronunciations of the same word; and natrum is never found in the works of the Greeks or the Romans, and not even in writings of the middle ages.

But if the greater part of what I have here said should be considered only as conjecture, it must nevertheless be acknowledged that it is deduced from the nature of the thing; and when impartially compared with what we read in the ancients, the latter I hope will be better understood than it hitherto has been; the impropriety of many readings will become apparent, and the truth of this conjecture be admitted.

Were I here to relate everything that we read of nitrum, in order to compare it with nature and to examine it thoroughly, I should be obliged to extend this article to a greater length than might be agreeable to the reader. I shall therefore give only the principal proofs of my assertion, premising, that doubts which might be excited by single passages not here mentioned, will, on a closer comparison, vanish without my assistance. But I maintain that those who wish to explain the old names of natural objects must relate everything said of them, and not that alone which is favourable to their opinion, and which may be often contradicted by what was purposely or accidentally concealed. The first part of such an examination is always a careful collection from the writings of the ancients of all the predicates of the natural object, the systematic name of which one is endeavouring to prove.

There is reason however to conjecture, that the ancients, in the history of their impure nitre, the manner of obtaining which the Romans at least had no opportunity themselves of seeing, for Pliny says expressly that it was not procured in Italy, fell into many errors and mistakes, which at present cannot all be explained.

Hence it happened that the ancients did not understand the art of purifying the salt which they obtained from minerals; and therefore they were obliged to use it in the same impure state in which they found it. On this account they considered each natural mixture as a peculiar kind; gave to the greater part of them, or those most useful, particular names; and of these recommended for different purposes those which, according to their purity or mixture, or according to other circumstances, were the most convenient. It is not probable that all these varieties could be again found out or defined; and it seems to be of little importance, when it is known that the names denote nothing more than the varieties of a mineral.

In this examination it is to be regretted that the book of Theophrastus, in which he expressly treated of nitrum, has not been preserved. But it may be believed, even without the testimony of Pliny, that he was one of the most accurate and acute naturalists among the ancients, and that he gave the best account of this substance1202. It must, however, be admitted that Pliny thoroughly understood this author, and gave a correct extract from him, and that the transcriber fell into no mistake.

That the nitrum of the ancients was an alkali more or less impure, but not saltpetre, has been long admitted by those who had the least knowledge of mineralogy, as well as by the most sagacious physicians. The grounds for this opinion, as far as I have yet learned, are as follows: more indeed might be found, but these are sufficient to afford a complete proof. Galen, a cautious writer, says that nitrum was in general burnt, by which means its effects were strengthened1203. Had it been saltpetre, it is impossible that the ancients should not in burning it have observed its deflagration, and this property is too remarkable not to have been mentioned. But nothing is to be found that can with any probability be supposed to allude to it.

But should it be admitted without any grounds that it was not an alkali but saltpetre which they burnt, it must certainly have been carbonized; for a burning body may easily have fallen into the crucible, and in general nitrum seems to have been burnt in an open fire, like our lime, because Pliny, speaking of the Egyptian, considers the contrary as somewhat uncommon. Physicians then, at any rate, must have observed, that a body very different both in its appearance and effects was produced from saltpetre by burning, but which could not be used for any other purpose than that salt. Of this however we do not find the least intimation.

But nitrum was undoubtedly soda, and on that account when burnt must have become more caustic as well as stronger in most of its effects, and in this respect similar to potash, since [owing to impurities] it in the same manner became moist and deliquesced in the air. What Pliny relates of the Egyptian nitrum becomes then intelligible. The latter, he says, was transported in pitched vessels, because it would otherwise have deliquesced; and he afterwards adds, that it was burnt before it was sent off. Had he known that the latter was the cause and the former the effect, he would have mentioned the latter first; but his whole extract, in regard to nitre, is written in general without order. The vessels, no doubt, were of clay; but whether he means in what he adds that they were not burnt but only baked in the sun, or that before they were filled they were completely dried in the sun, has been determined by no commentator. To me the latter is the more probable. Pliny also mentions another circumstance in regard to the burning of the Egyptian nitrum; namely, that it must be done in a close vessel, otherwise it would decrepitate or fly off. This is perfectly intelligible, when it is considered that it contained a great deal of common salt, which alone possessed the property of decrepitating; and it is well known in mineralogy that native soda, and even that which in modern times has been introduced into our collections from Tripoli, and of which I have in my possession a specimen, contains common salt1204, and often in cubic crystals. Pliny had just reason to add, that nitrum otherwise does not properly decrepitate. The ancients were well acquainted with the resemblance of their nitrum to lime, and especially of that which was burnt. On this account, because the Egyptian was exported after it had been burnt, it could easily be mixed with quicklime, or, as Pliny says, be adulterated. But the proof which he gives he does not seem to have thoroughly understood. The Egyptian must at all times have been caustic (pungens) even without lime; but that which was mixed with lime could not so speedily or completely dissolve on the tongue as that which was pure, and left behind it more earth. What he says of a test by the smell, I cannot understand in any other manner than that burnt lime, when moistened with water, diffused that disagreeable vapour observed in apartments the walls of which have been newly plastered; though when the quantity is small this is hardly perceptible.

If I understand Theophrastus1205 properly, he seems to say, that if nitrum be burnt as soon as it is dug up, it communicates heat to water in the same manner as lime. It may here be seen how great a resemblance the ancients found between their nitrum, alkaline earth and lime.

The similarity of wood-ashes to the nitrum of the ancients, which they acknowledged, proves also that it was in reality an alkaline salt. We are told by Theophrastus1206 that nitrum was said to be produced from oak-ashes; and Pliny1207, who borrowed from this writer, remarks that it was certain the ashes of that wood were nitrous. He ascribes also to burnt wine-lees the nature and properties of nitrum1208. Nay he considers as a kind of nitrum those saline ashes which, in many countries destitute of salt, were used for seasoning food, and which were prepared by pouring sea-water or salt brine over burning piles of wood, gradually and in small quantities, so that the fire was not extinguished, by which means the water evaporated, leaving the salt behind, but mixed indeed with charcoal, ashes, earth, and alkaline salts; consequently it must have been moist, or at any rate nauseous, if not refined by a new solution. This method of preparing or boiling salt, which perhaps is the oldest, has been mentioned by various writers; but many of them, through ignorance or neglect, have not told us that sea-water or brine was employed, as they speak in such a manner as if any kind and even fresh water had been used for that purpose.

Varro relates that he saw this process employed on the Rhine1209. Pliny says1210 that oak timber had before been burnt for that purpose. In another place he mentions a similar process among the Gauls and the Germans1211, as Tacitus does among the Hermanduri and the Catti1212. The former also states, on the authority of Theophrastus, that the Umbri burnt salt in the like manner1213. It is however certain that Pliny and other ancient writers often quote from Theophrastus what, at present, is not to be found in the works of that naturalist, but in those of his preceptor Aristotle1214.

Pliny adds, that this paltry method of obtaining salt had been long given up; and this indeed was the natural consequence of increased civilization. It is however certain that it was long continued in many countries, and in some still exists.

About two centuries ago the inhabitants of the province of Zeeland, descendants perhaps of the Catti, used no other salt than what they obtained in the like manner, from mud thrown up by the sea, which they burned and moistened with sea-water, as we are told by Lemnius, who was himself a native of that country. Boxhorn says, in his annotations on the above-quoted passage of Tacitus, that he saw a painting at Zirkzee, in which the whole process was represented. It is probable that salt was boiled exactly in the same manner as at some of the Sleswic islands, described by Denkwerth1215, from whose account it is seen that the glebæ marinæ, of which Lemnius speaks, consisted of mud mixed with roots growing in them; and that the salt when afterwards refined was called there Frisic, in all probability because the inhabitants had learned to make it from their ancestors the Frieslanders. I remember somewhere to have read that salt was made for a long time in this manner by the so-called Wurst-Frieslanders, in the country of Wurst, belonging to the duchy of Bremen. The inhabitants also of the Austrian part of Moldavia, or Buccowina as it is called, still use a salt, which they do not boil, but burn with their superfluous wood, in the like manner from the brine of a saline spring. A member of the former Academy of Brussels1216 took the trouble to examine the process as described by the ancients, and obtained, as might certainly have been expected, a highly alkaline kind of common salt, similar to that which Pliny, not without reason, considered as a sort of nitrum, because undoubtedly it may oftener have been an alkaline carbonate than common salt.

Boerhaave1217, in quoting the passages of the ancients, did not reflect that, during the incineration of the wood, salt water was poured over it. He considered the whole process as a burning of potash, and thought that the salt obtained was fit for use only because it was made according to the manner of Tachenius. That indeed gives a carbonated salt, which is almost saponaceous, and so mixed with various parts of the burnt plants that it is much milder, consequently fitter for use than common soda or pearlash can be; but that salt was not so much of the Tachenian kind as a species of common salt superabundant in alkali.

If the nitrum was carbonated alkali, there is reason to suppose that the ancients must have occasionally mentioned in their writings that it effervesced with acids. With the mineral acids indeed they were not acquainted; but they had vinegar, and that nitrum produced with this an effervescence had been known in the oldest times. A very clear allusion to this circumstance is found in the book of Proverbs, chap. xxv. ver. 20; where Luther however translates the word by chalk. Jerome, whose explanation I have already quoted, was in some degree acquainted with this phænomenon; and therefore to him the comparison of Solomon was intelligible1218. But at present I can produce no proofs from Greek writers; though they might have occurred during the use of nitrum in medicine, in consequence of which it was often put into vinegar.