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 substance[1202]. 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 strengthened[1203]. 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 salt[1204], 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 Theophrastus[1205] 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 Theophrastus[1206] that nitrum was said to be produced from oak-ashes; and Pliny[1207], 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 nitrum[1208]. 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 Rhine[1209]. Pliny says[1210] that oak timber had before been burnt for that purpose. In another place he mentions a similar process among the Gauls and the Germans[1211], as Tacitus does among the Hermanduri and the Catti[1212]. The former also states, on the authority of Theophrastus, that the Umbri burnt salt in the like manner[1213]. 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 Aristotle[1214].
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 Denkwerth[1215], 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 Brussels[1216] 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.
Boerhaave[1217], 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.