one of the great divisions of natural bodies, but which has never yet been accurately defined. The characteristic marks of salt have usually been reckoned its power of affecting the organs of taste, and being soluble in water. But this will not distinguish salt from quicklime, which also affects the sense of taste, and dissolves in water; yet quicklime has been universally reckoned an earth, and not a salt. The only distinguishing property of salts, therefore, is their crystallization in water; however, this does not belong to all salts; for the nitrous and marine acids, though allowed on all hands to be salts, are yet incapable of crystallization, at least by any method hitherto known. Several of the imperfect neutral salts also, such as combinations of the nitrous, muriatic, and vegetable acids, with some kinds of earths, crystallize with very great difficulty. However, by the addition of spirit of wine, or some other substances which absorb part of the water, keeping the liquor in a warm place, &c. all of them may be reduced to crystals of one kind or other. Salt, therefore, may be defined a substance affecting the organs of taste, soluble in water, and capable of crystallization, either by itself or in conjunction with some other body; and, universally, every salt capable of being reduced into a solid form, is also capable of crystallization per se. Thus the class of saline bodies will be sufficiently distinguished from all others: for quicklime, though soluble in water, cannot be crystallized without addition either of fixed air or some other acid; yet it is most commonly found in a solid state. The precious stones, bafaltes, &c. though supposed to be formed by crystallization, are nevertheless distinguished from salts by their infusibility and insolubility in water.
But acids and alkalis, and combinations of both, when in a concrete form, are salts, and of the purest sort. Hence we conclude, that the bodies, to which the name of salts more properly belongs, are the concretions of those substances; which are accordingly called acid salts, alkaline salts, and neutral salts. These last are combinations of acid and alkaline salts, in such proportion as to render the compounds neither four nor alkaline to the taste. This proportionate combination is called saturation: thus the common kitchen salt is a neutral salt, composed of marine acid and mineral alkali combined together to the point of saturation. The appellation of neutral salts is also extended to denote all those combinations of acids, and any other substance with which they can unite, so as to lose, wholly or in great measure, their acid properties.
But although this general definition of salts is commonly received, yet there are many writers, especially mineralogists, who confine the denomination of salts in the manner we first mentioned, viz. to those substances only which, besides the general properties of salts, have the power of crystallizing; that is, of arranging their particles so as to form regularly-shaped bodies, called crystals, when the water superfluous to their concrete existence has been evaporated.
The ancient chemists asserted that salt was one of the component principles of metals, and indeed of every thing else: a doctrine which was attempted to be revived by the late Dr Price of Guildford, who thought it probable that the basis of all imperfect metals is saline, because Mr Scheele had lately extracted a real acid from arsenic, which, by the addition of a proper quantity of phlogiston, becomes a semimetal. But here the argument will hold only with regard to the ferrometals, all of which are volatile in the fire, and therefore may possibly have a volatile basis, such as all acids are in some degree: but some of the imperfect metals, as tin and copper, may be reduced to a calx equally refractory with quicklime itself; and even zinc, though volatile in close vessels, is yet capable of being reduced to an exceedingly refractory calx called flowers of zinc; and it is to be observed, that the regulus of arsenic, even in its most perfect metallic form, cannot be calcined like other metals. The common opinion that metals have an earthy, rather than a saline basis, seems to be well founded.
The origin of salts is very much, or rather totally, unknown. Some eminent chemists, particularly Stahl, have supposed that the number of substances truly and essentially saline is very small; nay, that there is but one saline principle in nature. This principle they suppose to be the vitriolic acid, as being the most simple and indestructible of them all. Stahl delivers his opinion on this subject in the following words: "That he considers the vitriolic acid as the only substance essentially saline; as the only saline principle which, by uniting more or less intimately with other substances that are not saline, is capable of forming an innumerable multitude of other saline matters, which nature and art show us; and, secondly, that this saline principle is a secondary principle, composed only by the intimate union of two primary principles, water and earth.
In support of this theory Mr Macquer argues in the following manner: "Every true chemist will easily discover that this grand idea is capable of comprehending by its generality, and of connecting together, all the phenomena exhibited by saline substances. But we must at the same time acknowledge, that when we examine the proofs upon which it is founded, although it has a great appearance of truth by its consistency with the principles of chemistry, and with many phenomena, yet it is not supported by a sufficient number of facts and experiments to ascertain its truth. We might here examine what degree of probability ought to be granted to this theory of salts; but this could not be properly accomplished, without entering into long details, and penetrating into the depths of chemistry. We are therefore obliged to relate only what is most essential to be known concerning this grand hypothesis. We may perceive at once, that the former of those propositions, upon which is founded the theory which we mentioned, cannot be demonstrated, unless it be previously proved that every saline matter, excepting pure vitriolic acid, is nothing but this same acid differently modified, the primary properties of which are more or less altered or disguised by the union contracted with other substances. But we confess, that chemists are not capable of proving decisively this opinion; which, however, will appear very probable from the following reflections.
"First, Of all saline matters known, none is so strong, so unalterable, so eminently possessed of saline properties, as vitriolic acid."
The vitriolic acid, when combined with other substances, forms vitriolic salts, which vary both in specific names and properties, according to the various substances with which the acid is combined. Thus the vitriolic acid, combined with mineral alkali, forms the salt called Glauber's salt, or sal mirabile. When it is combined with calcareous earths, it forms vitriolic salts with bases of calcareous earth, which are commonly called selenites. When combined with argillaceous earths, it forms alum. When combined with metals, it forms vitriolic salts with metallic bases, to which the general name vitriols is given; and in commerce are commonly called copperas. The vitriols principally used are, 1. The martial vitriol; called also English vitriol, green vitriol, or green copperas, which is a combination of vitriolic acid with iron. 2. The vitriol of copper, called also blue vitriol, Cyprian vitriol, or blue copperas; which is a combination of vitriolic acid and copper. 3. The vitriol of zinc, called also white copperas, and Goslar vitriol, which is a combination of the same acid with a semimetal called zinc. It is a property peculiar to the vitriolic acid, that all the combinations of it, with those substances with which it can form neutral salts, are susceptible of crystallization.
"Secondly, Amongst the other saline substances, those which appear most active and most simple, as nitrous and marine acids, are at the same time those whose properties most resemble the properties of vitriolic acid."
The nitrous acid, combined with all the substances with which it can mix, forms saline substances, in general called nitrous salts; specifying each particular salt by the name of the substance united to the acid. Thus nitrous acid, with fixed vegetable alkali, forms a saline substance called nitre, or saltpetre. With mineral alkali, forms cubic or quadrangular nitre. When mixed with metallic substances, forms metallic nitrates, which are specified nitre of gold; nitre of silver, or lunar nitre, lunar crystals, and crystals of silver, nitrous crystals of mercury; nitre of copper, &c.
"Thirdly, We may give to vitriolic acid many of the characteristic properties of nitrous acid, by combining it in a certain manner with the inflammable principle, as we see in the volatile sulphureous acid; and even, according to an experiment of Mr Piech, related in a memoir concerning the origin of nitre, which gained the prize of the academy of Berlin, vitriolic acid, mixed with vegetable and animal matters susceptible of fermentation, is really transformed into a nitrous acid by the putrefaction of these matters. See Chemistry, no 720.
"Fourthly, The marine acid, although its principles are less known than those of the nitrous acid, may be approximated to the character of vitriolic and nitrous acids by certain methods. This acid, after it has been treated with tin and other metallic matters, is capable of forming either with spirit of wine, as vitriolic acid does, which it cannot do in its natural state; and when iron is dissolved in it, it seems to be approximated to the nature of nitrous acid. Reciprocally, the approximation of vitriolic acid to the character of marine acid seems not impossible. Having once distilled very pure vitriolic acid upon a considerable quantity of white arsenic, I was struck with a strong smell like that of marine acid, which was not either that of arsenic or of vitriolic acid; for this has no smell when it is pure."
The marine acid, combined with various matters, forms marine salts, or simply salts, specified by the names of their particular bases. The sea-salt, or kitchen salt, and sal gem, are combinations of marine acid and mineral alkali. When this acid is combined with volatile alkali, it forms sal ammoniac (a.) With metals it forms metallic salts, called salt of gold, salt of copper, &c., according to the various metals combined with the acid. The salt of silver is also called luna cornea; the salt of lead is often called plumbum cornueum; and the salts of antimony, and of arsenic, are known by the names of butter of antimony, and butter of arsenic.
"Fifthly, Oily vegetable acids become so much stronger, and more similar to vitriolic acid, as they are more perfectly deprived of their oily principle, by combining them with alkalis, earths, or metals; and afterwards by separating them from these substances by distillation, and especially by frequently repeating these operations. They might perhaps be reduced to a pure vitriolic acid, by continuing sufficiently this method: and reciprocally, vitriolic and nitrous acids, weakened by water, and treated with much oily matters, or still better with spirit of wine, acquire the characters of vegetable acids. We may see a remarkable instance of this in Mr Pott's dissertation De acido nitri vino. [The most remarkable experiment in which is related under the article Chemistry, no 781.]
"Sixthly, The properties of fixed alkalis seem to be very different from those of acids in general, and consequently of vitriolic acid. Yet if we consider that a large quantity of earth enters their composition; that much of it may be separated by repeated solutions and calcinations; and also, that by depriving these saline substances of their earthy principles, they become less fixed, more deliquescent, and, in a word, more similar to vitriolic acid in this respect;—we shall not think it improbable, that fixed alkalis owe their saline properties to a saline principle, of the nature of vitriolic acid, but much disguised by the quantity of earth, and probably of inflammable principle, to which it is united in these combinations. The properties of volatile alkalis, and the transformation of fixed alkalis, or of its materials, into volatile alkali in putrefaction, and in several distillations, seem to show sufficiently that they are matters essentially saline, as fixed alkalis are, and that their volatility which distinguishes them proceeds from their containing a less quantity of earth, but more attenuated, and a portion of very subtle and volatile oil, which enters their composition. [For some other particulars relating to the transmutation of salts, see Chemistry, no 784.]
"Besides these principal facts, there are many others, too numerous to be even slightly mentioned here; they may be found scattered in the works of chemists, particularly of Stahl. But persons who would collect and compare all the experiments relating to this subject,
(a) Ammoniacal salts is also a general name given to all neutral salts composed of an acid saturated with a volatile alkali. subject, ought to know, that many of them are not sufficiently ascertained; and that perhaps a greater number of them have not been sufficiently prosecuted, and are, properly speaking, only begun. We must even acknowledge, that many of those experiments which we have mentioned have not been sufficiently prosecuted.
"The second fundamental proposition of the theory of salts, namely, 'That the vitriolic acid is compounded of only the aqueous and earthy principles,' is, like the first, supported by many facts which give it a degree of probability, but which do not amount to a complete demonstration. This proposition may be supported by the following considerations.
"First, Experience constantly shows, that the properties of compound bodies are always the result of those of the component parts of these bodies, or rather they are the properties of these component bodies modified by one another.
"Thus, if a body be composed of two principles, one of which is fixed and the other volatile, it will have a less degree of fixity than the former, and a less volatility than the latter. If it be composed of two principles, one of which is specifically heavier than the other, its specific gravity will be greater than that of one of them, and less than that of the other. The same observation is applicable to all the other essential properties, excepting those which destroy each other; as, for instance, the tendency to combination, or the dissolving power; for these latter properties are weakened to much more in the compounds as their principles are more strongly united, and in more just proportion.
"We observe, nevertheless, that the properties of compound bodies are not always exactly intermediate betwixt the properties of the component bodies; for, to produce this mean, the quantities of each of the component parts must be equal, which is the case in few or no compounds.
"Besides, some particular circumstances in the manner in which the principles unite with one another, contribute more or less to alter the result of the combined properties: for instance, experience shows, that when several bodies, particularly metals, are united together, the specific gravities of which are well known, the alloy formed by such union has not the precise specific gravity which ought to result from the proportion of the allayed substances; but that in some alloys it is greater and in others less. But we are certain, on the other side, that these differences are too inconsiderable to prevent our distinguishing the properties of the principles in the compounds which they form, especially when they have very different properties.
"These things being premised, when we examine well the properties of vitriolic acid, we shall easily find that they partake of the properties of the aqueous and of the earthy principles.
"First, When this acid is as pure as we can have it, it is like the purest water and the purest vitrifiable earths, free from colour or smell, and perfectly transparent.
"Secondly, Although we cannot deprive the vitriolic acid of all the water superabundant to its saline essence, and therefore its precise specific gravity has not been determined, we know that when it is well concentrated, it is more than twice as heavy as pure water, and much less heavy than any earthy substance.
"Thirdly, This acid is much less fixed than any pure earth, since, however well it may be concentrated, it may always be entirely distilled; for which purpose a much stronger degree of heat is requisite than for the distillation of pure water.
"Fourthly, We do not know the degree of solidity of vitriolic acid, or the adhesion of aggregation, which its integrant parts have one to another, because for this purpose the vitriolic acid ought to be deprived of all superabundant water; but if we judge of it by the solid consistence of this acid when highly concentrated, as we see from the vitriolic acid called glacial, the integrant parts of this acid seem susceptible of a much stronger adhesion than those of pure water; but much less than those of earth, as we see from the intance of hard stones.
"Fifthly, The union which this acid contracts with water and with earths, shows that these substances enter into its composition; for we know, that in general compounds are disposed to unite superabundantly with the principles which compose them. All these properties of vitriolic acid, which so sensibly partake, and much more than any other acid, of the properties of earth and of water, are sufficient to induce us to believe that it is composed of these two principles; but it has one very eminent property, which is common with it to neither water nor pure earth, which is, its violent and corrosive taste. This property is sufficient to raise doubts, if we could not explain it from principles, which seem certain and general, relating to the combination of bodies.
"We observe, then, concerning the property now in question, that is, of taste in general, that it can only be considered as an irritation made upon the organs of taste by rapid bodies; and if we reflect attentively upon it, we shall be convinced, that no substance that is not impressed by some impulse can irritate or agitate our sensible organs, but by a peculiar force of its integrant parts, or by their tendency to combination; that is, by their dissolving power. According to this notion, the taste of bodies, or the impression made upon our sensible organs by their tendency to combination, or by their dissolving power, are the same property; and we see accordingly, that every solvent has a taste, which is so much more strong as its dissolving power is greater; that those whose taste is so violent that it amounts to acrimony, corrosion, and causticity, when applied to any other of the sensible parts of our body besides the organs of taste, excite in them itching and pain.
"This being premised, the question is, How earths, in which we perceive no taste nor dissolving power, and water, which has but a very weak dissolving power, and little or no taste, should form by their combination a substance, such as the vitriolic acid is, powerfully corrosive and solvent?
"To conceive this, let us consider, first, that every part of matter has a power by which it combines, or tends to combine, with other parts of matter. Secondly, that this force, the effects of which are perceptible, in chemical operations, only among the very small molecules, or the integrant and constituent parts of bodies, seems proportional to the density or specific gravity of these parts. Thirdly, that this same force is limited in every integrant molecule of matter: that if we consider this force as not satisfied, and consequently as a simple tendency to combination, it is the greatest possible in an integrant molecule of matter perfectly inflated, or attached to nothing; and is the smallest possible, or none, when it is satisfied by its intimate combination with other parts capable of exhausting all its action; its tendency being then changed into adhesion.
"Hence we may infer, that the integrant parts of the earthy principle have essentially, and like all the other parts of matter, a force of tendency to union, or of cohesion in union, according to their condition; that as this earthy principle has a much more considerable density or specific gravity than all other simple bodies that we know, we may probably presume that its primary integrant molecules have a more considerable force of tendency to union, in the same proportion, than the integrant parts of other principles; that consequently when they cohere together, and form an aggregate, their aggregation must also be stronger and firmer than that of any other body. Accordingly we see, that the purest earthy substances, whose parts are united and form masses, such as, for instance, the stones called vitrifiable, are the hardest bodies in nature. We are not less certain, that as the tendency of the parts of matter to unite is so much less evident as it is more exhausted and satisfied in the aggregation, the parts of the earthy principle being capable of exhausting mutually all their tendency to union, we may thence infer, that every sensible mass of pure earthy matter must appear deprived of any dissolving power; of taste; in a word, of tendency to union from the firmness of its aggregation. But we may also infer, that when these primary integrant parts of the earthy principle are not united together in aggregation, then, resuming all the activity and tendency to union which are essential to them, they must be the strongest and most powerful of all solvents.
"There being premised, if we suppose again, with Stahl and the best chemists, that, in the combination of the saline principle or of vitriolic acid, the parts of the earthy principle are united, not with each other, as in the earthly aggregation, but with the primary parts of the aqueous principle, each to each, we may then easily conceive, that the primary integrant parts of the water, having essentially much less tendency to combination than those of earth, the tendency of these latter to union will not be exhausted, but satisfied only partly, by their combination with the former; and that consequently a compound must result, the integrant parts of which will have a strong dissolving power, as vitriolic acid is.
"We may see from hence how much mistaken chemists are, who, considering earth only in its aggregation, or rather not attending to this state, and not distinguishing it from that state in which the parts of this same earth are so separated from each other by the interposition of another body, that they cannot touch or cohere together, have considered the earthy principle as a substance without force or action, and have very improperly called that a passive principle, which of all others is the strongest, most active, and most powerful.
"However this general theory of salts may conform with the most important phenomena of chemistry, we must acknowledge, that it can only be proposed as a systematical opinion, till it be evidently demonstrated by the decisive means employed in chemical demonstrations, namely, by decomposition and recomposition: thus, if we could reduce vitriolic acid to earth and water, and make that acid by combining together these two principles, this theory would cease to be a system, and would become a demonstrated truth. But we must confess, that this theory is less supported by experiment than by argument, from the many difficulties that are inevitable in such inquiries. For on one side, we know that the simpler bodies are, the more difficult is their decomposition; and on the other side, the stronger the aggregation is, the greater is the difficulty of making it enter into a new combination. Thus, as vitriolic acid is very simple, since it is a compound of the first order, it ought strongly to resist decomposition; and as the aggregation of pure earth is the firmest that we know, it cannot easily be made to enter as a principle into a new combination with water to form a saline matter. The following are the principal experiments which have been made relative to the subject.
"First, We seem to be certain, from many proofs, that all saline substances, comprehending those that contain vitriolic acid, as vitriolated tartar, Glauber's salt, and other vitriolated salts which are sufficiently fixed to support a perfect drying, or rather calcination, being alternately dissolved, dried, and calcined a number of times, are more and more diminished in quantity, and that earth and water are separated from them each operation. But alkaline salts appear to be still more susceptible than any other saline matter of this kind of decomposition.
"Secondly, When nitre is burnt in close vessels, so that we may retain not only all that remains fixed after this burning, but also what exhales in vapours, as in the experiment of the clystus of nitre, we have a proof which seems decisive, that the mineral acid of this salt, which is not very far from the simplicity of vitriolic acid, is totally decomposed and reduced into earth and water. For if we examine the fixed residuum in the retort, we find that it is only the alkali that was contained in the nitre, charged with a superabundant earth, which is separable from it by solution and filtration. And if the liquor in the receiver, formed by the vapours condensed there, be examined, which ought to be nitrous acid; if this acid had not been destroyed, we find, that, so far from being acid, it is only pure water, sometimes even charged with a little fixed alkali, which had been raised by the force of the detonation. Thus nitrous acid is made to disappear in this experiment, and in its place we find only earth and water.
"Thirdly, The phenomena of limestone, which by calcination and extinction in water acquires saline properties that it had not before its attenuation by fire and its combination with water; and also the experiment of Beecher, who asserts, that if a vitrifiable stone be alternately made red-hot, and extinguished in water a number of times, it may be so attenuated that it shall be like a saline gelatinous matter; these, I say, show that saline matters are actually formed by the intimate combination of the very attenuated parts of earth with those of water. We find in the writings of Beecher and Stahl, and particularly in the Specimen Beecherranum of the latter author, many other observations and experiments tending to prove the same proposition; but we must confess, that none of the experiments we have mentioned, excepting that of the decomposition of of nitrous acid by burning, are absolutely decisive; principally because they have not been sufficiently repeated or prosecuted, nor carefully enough examined in all their circumstances."
On this theory it is obvious to remark, that our author has omitted to mention the most active part of the composition of salts, namely elementary fire. Of this both acids and alkalis undoubtedly contain a great quantity in a very active state, as is evident from their performing the effects of fire when applied to certain substances; nay, from their actually bursting into flame when mixed with some kinds of oils. For an explanation of the reason of which, see Heat, and the various detached articles relative to that subject. Whatever doubts we may have of the power of mere water combined with mere earth to affect the organs of taste, we can have none that the element of fire is capable of so doing; and from the very tasting of these substances, we may be assured, that whatever gives that peculiar sensation to the tongue which we call acid or alkaline, gives also the other properties of the salt, whatever they may be. In alkalis, no doubt the greatest part of the composition is earth; but from what has been said on Quicklime, it appears, that mere earth, by the artificial action of fire alone, acquires all the properties of salt, that of crystallizing per se excepted: it seems probable therefore, that, in the more perfect operations of nature, the same materials are used; only the proportions are such, that the substance is more soluble, and its causticity greater, than even quicklime itself. With regard to acids, the earthy parts seem to be fewer; and in all probability the most considerable ingredient in their composition is water; but in what manner this element is united to that of fire so as to produce the peculiar phenomena of acids, cannot be explained.
The acid of tartar (the purest part of which, or that saline substance which first crystallizes by evaporation in the vessels in which it is purified, is called cream of tartar), and also all other concrete vegetable acids analogous to it, when mixed with various other substances, form compounds, generally called tartarous salts, or soluble tartaric, because they are dissolved by water more easily than the acid of tartar itself. Acetous salts, that is, all salts containing the acid of vinegar, are also combined with various bases, and form saline substances of different names; the principal of which are, the acetous salt of copper, called crystals of Venus, or verdigris, by the chemists, and distilled or crystallized verdigris in commerce; the acetous salt of lead, commonly called salt or sugar of lead; and the acetous mercurial salts. Sugar is an essential vegetable salt, of a pleasant sweet taste, containing a vegetable acid combined with earth and oil.
Potash is a fixed vegetable alkali, extracted from the ashes of wood. Concrete volatile alkalis are generally called volatile salts; although this name is sometimes also given to the volatile salt of amber, which is not an alkaline but an acid salt. Borax is a neutral saline matter, whose origin, whether animal or vegetable, is as yet unknown, its components being not sufficiently examined. It is soluble in water, and very nearly as crystallizable as alum. When borax is exposed to the fire, it first bubbles and foams very much, but afterwards it melts into a clear glass. When acids are combined with the alkaline part of borax, a substance of a singular na-
ture is separated from it, commonly called sedative salt. Although this substance acts as an acid in borax, by saturating its alkali, yet it has no acid taste, nor does it turn the tincture of heliotrope to a red, as other acids do. It is the property of borax to facilitate considerably the fusion of metals, of earths, and other minerals. Some species of stones and earths cannot be vitrified at all, except they are mixed with borax. For this property borax is commonly used as a flux (that is, a substance which facilitates the fusion of other bodies) in various manufactories; but especially in soldering metals, and in assaying ores. Phosphoric salts are combinations of alkaline, earthy, and metallic substances with the acid obtained from the phosphorus of urine. Besides the above-mentioned salts, there are several others to be met with in the writings of the chemical and medical authors; but, as they are of little consequence, we shall omit any account of them.
Some new neutral salts have been formed by the dephlogisticated marine, or, according to the new theory, the oxygenated muriatic acid.—This was first taken notice of by M. Berthollet, and the discovery is thus illustrated by Dr Dollfus, in Crell's Annals for the year 1788, vol. i. p. 319.
"In the month of November 1786 (says he), whilst I was preparing to translate Higgins's experiments respecting the acetous acid, I found the following amongst the numerous observations which that work contains, p. 180. 'The acid elastic fluid which issues, when two pounds of manganese are mixed and distilled with two or three of ordinary spirit of sea-salt, may all, except a small portion of phlogistic air, be condensed in a solution of fixed vegetable alkali; and the solution thus impregnated yields a considerable quantity of nitre, which crystallizes in the ordinary form, and detonates on red-hot coals. The solution at the same time yields regenerated sea-salt.' The part of this proposition which relates to the form of the crystals and to their detonation is sufficiently plain; but that I might have a still more complete conviction on the subject, I repeated the experiment upon a small scale.
"For this purpose I put into a vial an ounce of pulverized oxyd (calx) of manganese with an ounce and a half of muriatic acid, and by means of a bent tube I directed the vapour into another vial, which contained a solution of vegetable alkali. I then distilled by the gentle heat of a small lamp. From the vial containing the alkali went a second tube, for the purpose of carrying off the air which I hoped to obtain by this process.
"As soon as the oxygenated muriatic acid appeared, some air escaped through the tube, which showed all the properties of common atmospheric air; and as soon as all the air which the vials contained previous to the distillation had been expelled, no more such air appeared. The vapours of the oxygenated muriatic acid were absorbed by the solution of vegetable alkali, without the extrication of the smallest portion of carbonic acid (fixed air) from the alkali. As fast as the alkali, which adhered to the sides of the glass, absorbed the acid vapour, prismatic crystals appeared; and many more, which I obtained a few hours afterwards, were formed in the liquor. Although these crystals detonated in the fire, they had a taste very different from that of nitre. It was extremely pungent, and was rendered still more more intolerable by the suffocating odour of the nitro- muriatic acid (aqua regia). In order to complete the crystallization, I evaporated in the same vial the remain- ing liquor. As soon as the vapour appeared, a quanti- ty of carbonic acid was disengaged, and afterwards some atmospheric air. The salt which I obtained by crystallization after the evaporation was a true muriat of potash, which did not detonate in the fire. Prob- ably Mr Higgins performed the operation in the way I have described; but he was too hasty in concluding this salt to be nitre merely because it detonated. I gave an account of this experiment to Mr Kirwan at the time, and soon after communicated it to Professor Gadolin, who offered to assist me in repeating the expe- riment.
"We agreed to employ crystallized carbonat of soda (mild mineral alkali); and the following was the result of our experiment. We dissolved some of this carbo- nat in a large quantity of water, and we employed two or three hours a day, for several successive days, in in- troducing into the solution as much oxygenated muri- atic gas as was sufficient entirely to saturate it; we then poured the saline liquor into a glass basin, and left it covered over to evaporate spontaneously. After some time a number of prismatic crystals were formed, which detonated in the fire like nitre. They occasioned a brown precipitate from a solution of iron in sulphuric or vitriolic acid; and mixed with sal ammoniac, they gave out a strong ammoniacal odour, accompanied with some effervescence, which was to be attributed to the extrication of fixed air during the mixture. The re- maining part of the liquor evaporated again, produced fresh crystals, which, though they certainly had a faint smell of oxygenated muriatic acid, in reality consisted partly of muriat of soda (common salt), and partly of uncombined soda; for they did not detonate, and they precipitated iron of a light green colour. The liquor which appeared above these crystals, however, had not yet entirely lost the smell of the oxygenated muriatic acid. Since this, M. Gadolin has made the following experiment, which he communicated to me. He put two drams of magnesia, saturated with carbonic acid, into an ounce and a half of water, into which he intro- duced during several hours a quantity of oxygenated muriatic gas. The water evidently acquired the odour of the oxygenated muriatic acid. He filtered the liquor, and washed and dried that part of the magnesia which had not been dissolved, and which weighed one dram 4-5ths, so that the water was found to have dissolved 1-5th of a dram. As soon as the liquor began to boil, a strong effervescence was occasioned, some oxygenated muriatic gas was disengaged, and a small quantity of carbonat of magnesia was precipitated. When the liq- uor had become cool, it was filtered, that it might be separated from the precipitated powder. It had still the same odour; and on being again heated, an efferves- cence similar to the first took place, and a fresh quantity of carbonat of magnesia was separated. This phenome- non appeared every time M. Gadolin boiled the liquor after its cooling, till at last he had evaporated it to dry- ness, when there still remained a small quantity of mag- nesia. Hence M. Gadolin concludes, that water, oxy- genated muriatic acid, and carbonat of magnesia, form a combination which heat does not decompose till the vapour of the water carries off the oxygenated muriatic acid, at which time the carbonat of magnesia is precipi- tated. In consequence of what we have now related, we ought to reckon, in addition to the two salts disco- vered by M. Berthollet, another salt, to which, accord- ing to the new French nomenclature, might be given the name murias oxygenatus magnesia liquidus, because we cannot obtain it in a concrete form. The oxygen- ated muriatic acid appears to enter into a very differ- ent, or at least into a much more intimate, combi- nation with the metals; a subject which greatly merits the attention of the chemist.
The probability of this proposition is strengthened by the theory of M. Berthollet; according to which the mercury in corrosive muriat of mercury (corrosive sub- limate) is combined with the oxygenated muriatic acid, so as not to be separated from it without great difficulty.
Common Salt, or Sea-Salt, the name of that salt ex- tracted from the waters of the ocean, which is used in great quantities for preserving provisions, &c.
It is a perfect neutral salt, composed of marine or muriatic acid, saturated with mineral alkali. It has a saline but agreeable flavour. It requires about four times its weight of cold water to be dissolved, and nearly the same quantity of boiling water, according to Macquer. But according to Kirwan, it only requires 2½ its weight of water to be dissolved in the tempera- ture of sixty degrees of Fahrenheit. This salt always contains some part formed with a calcareous base; and, in order to have it pure, it must be dissolved in distilled water; then a solution of mineral alkali is to be poured in it until no white precipitation appears; then by filtrating and evaporating the solution, a pure common salt is produced. Its figure is perfectly cubic, and those hollow pyramids, or tremies as the French call them, as well as the parallelopipeds formed sometimes in its crystallization, consist all of a quantity of small cubes disposed in those forms. Its decrepitation on the fire, which has been reckoned by some as a characteristic of this salt, although the vitriolated tartar, nitrous lead, and other salts, have the same property, is owing chiefly to the water, and perhaps also to the air of its crystal- lisation.
Its specific gravity is 2.120 according to Kirwan. The acid of tartar precipitates nothing from it. One hundred parts of common salt contain thirty-three of real acid, fifty of mineral alkali, and seventeen of water. It is commonly found in salt water and salt springs, in the proportion of even thirty-five per cent. It is found also in coals, and in beds of gypsum. This salt is un- alterable by fire, though it fuses, and becomes more opaque; nevertheless a violent fire, with the free access of air, causes it to evaporate in white flowers, which stick to the neighbouring bodies. It is only decom- posed, as Macquer affirms, by the vitriolic and nitrous acid; and also by the boracic or sedative salt. But although nitre is decomposed very easily by arsenic, this neutral marine salt is nowise decomposed by the same. According to Mongez, the fixed vegetable alkali, when caustic, decomposes also this marine salt. It pre- serves from corruption almost all sorts of animal food much better for use than any other salt, as it preserves them without destroying their taste and qualities; but when applied in too small a quantity, it then forwards their corruption.
Of this most useful commodity there are ample stores on land as well as in the ocean. There are few countries which which do not afford vast quantities of rock or fossil salt. Mines (A) of it have long been discovered and wrought in England, Spain, Italy, Germany, Hungary, Poland, and other countries of Europe. In several parts of the world, there are huge mountains which wholly consist of fossil salt. Of this kind are two mountains in Russia, near Astracan; several in the kingdoms of Tunis and Algiers, in Africa; and several also in Asia; and the whole island of Ormus in the Persian gulf almost entirely consists of fossil salt. The new world is likewise stored with treasures of this useful mineral, as well as with all other kinds of subterranean productions. Moreover, the sea affords such vast plenty of common salt, that all mankind might thence be supplied with quantities sufficient for their occasions. There are also innumerable springs, ponds, lakes, and rivers, impregnated with common salt, from which the inhabitants of many countries are plentifully supplied therewith. In some countries which are remote from the sea, and have little commerce, and which are not blest with mines of salt or salt-waters, the necessities of the inhabitants have forced them to invent a method of extracting their common salt from the ashes of vegetables.
The muriatic salt of vegetables was described by Dr Grew under the title of lixiviated marine salt. Leeuwenhoek obtained cubical crystals of this salt from a lixivium of soda or kelp, and also from a solution of the lixivial salt of cardus benedictus; of which he hath given figures in a letter to the Royal Society, published in No. 173. of their Transactions. Dr Dagner, in AG. Acad. N. C. vol. v. obf. 150. takes notice of great quantities of it which he found mixed in potatoes. And the ingenious Dr Fothergill extracted plenty of it from the ashes of fern: See Medical Essays, vol. v. article 13.
The muriatic salt which the excellent Mr Boyle extracted from sandiver, and supposed to be produced from the materials used in making glass, was doubtless separated from the kelp made use of in that process. Kunckel also informs us, that he took an alkaline salt; and after calcining it with a moderate fire, dissolved it in pure water, and placing the solution in a cool cellar, obtained from it many crystals of a neutral salt. He supposes, that the alkaline salt was by the process converted into this neutral salt. But it is more reasonable to believe, that the alkaline salt which he applied was not pure, but mixed with the muriatic salt of vegetables, which by this process was only separated from it.
It is doubtless chiefly this muriatic salt which, in some of the inland parts of Asia, they extract from the ashes of duck-weed and of Adam's fig-tree, and use for their common salt.
That they are able in those countries to make common salt to profit from vegetables, ought not to be wondered at; since in Delhi and Agra, capitals of India, salt is so scarce as usually to be sold for half-a-crown a pound. We may therefore give some credit to Marco Polo, when he informs us, that in the inner parts of the same quarter of the world, in the province of Caindu, lying west of Tebeth, the natives used salt instead of money, it being first made up in cakes, and sealed with the stamp of their prince; and that they made great profit of this money by exchanging it with the neighbouring nations for gold and musk. We are also told by Ludolfus, in his Historia Ethiopicata, that in the country of the Abyssines there are mountains of salt, which when dug out is soft, but soon grows hard; and that this salt serves them instead of money to buy all things. The same is confirmed by Ramusio.
Mr Boyle discovered common salt in human blood and urine. "I have observed it (says Mr Brownrigg), not only in human urine, but also in that of dogs, horses, and black cattle. It may easily be discovered in these, and many other liquids impregnated with it, by certain very regular and beautiful starry figures which appear in their surfaces after congelation. These figures I first observed in the great frost in the year 1729. The dung of such animals as feed upon grass or grain, doth also contain plenty of common salt."
Naturalists, observing the great variety of forms under which this salt appears, have thought fit to rank the several kinds of it under certain general classes; distinguishing it, most usually, into rock or fossil salt, sea-salt, and brine or fountain salt. To which classes, others might be added, of those muriatic salts which are found in vegetable and animal substances. These several kinds of common salt often differ from each other in their outward form and appearance, or in such accidental properties as they derive from the heterogeneous substances with which they are mixed. But when perfectly pure, they have all the same qualities; so that chemists, by the closest inquiries, have not been able to discover any essential difference between them; for which reason we shall distinguish common salt after a different manner, into the three following kinds, viz., into rock or native salt, bay salt, and white salt.
By rock salt, or native salt, is understood all salt dug out of the earth, which hath not undergone any artificial preparation. Under the title of bay salt may be ranked all kinds of common salt extracted from the water wherein it is dissolved, by means of the sun's heat, and the operation of the air; whether the water from which it is extracted be sea-water, or natural brine drawn from wells and springs, or salt water stagnating in ponds and lakes. Under the title of white salt, or boiled salt, may be included all kinds of common salt extracted by evaporation from the water wherein it is dissolved; whether this water be sea water, or the salt water of wells, fountains, lakes, or rivers; or water of any sort impregnated with rock-salt, or other kinds of common salt.
The first of these kinds of salt is in several countries found so pure, that it serves for most domestic uses, without any previous preparation (triturate excepted); for of all natural salts rock-salt is the most abundantly furnished by nature in various parts of the world, being found in large masses, occupying great tracts of land. It is generally formed in strata under the surface of the earth.
(A) Amongst the salt mines of chief note are those of Northwich in Cheshire, Altemonte in Calabria, Hall in Tyrol, Cardona in Catalonia; also those stupendous mines at Wilczka of Poland, and Soowar in Upper Hungary; of which see accounts in Phil. Trans. No 61. and 413. earth, as in Hungary, Moscovy, Siberia, Poland, Calabria, Egypt, Ethiopia, and the East Indies. "In England (says Magellan), the salt mines at Northwich are in a high ground, and contain it in layers or strata of various colours, of which the yellow and brown are the most plentiful, as I have observed on the spot, which I visited in June 1782, in company with my worthy and learned friend Mr Volta, professor of Natural Philosophy in the University of Pavia, and well known by his great abilities, and many discoveries in that branch of knowledge. The mine into which we descended was excavated in the form of a vast dome or vault underground, supported by various columns of the salt, that were purposely left to support the incumbent weight. And the workmen having lighted a number of candles all round its circumference, it furnished us with the most agreeable and surprising sight, whilst we were descending in the large tub, which serves to bring up the lumps that are broken from the mine, &c. See the description of the famous salt-mines of Wilieczka in Poland, by Mr Bernard, in the Journal de Physique, vol. 16, for 1780, pag. 459, in which the miraculous tales concerning those subterranean habitations, villages, and towns, are reduced to their proper magnitude and estimate." But the English fossil salt is unfit for the uses of the kitchen, until by solution and coction it is freed from several impurities, and reduced into white salt. The British white salt also is not so proper as several kinds of bay salt for curing fish and such flesh-meats as are intended for sea provisions, or for exportation into hot countries. So that for these purposes we are obliged, either wholly or in part, to use bay salt, which we purchase in France, Spain, and other foreign countries.
However, it does not appear that there is any other thing requisite in the formation of bay salt than to evaporate the sea-water with an exceedingly gentle heat; and it is even very probable, that our common sea-salt by a second solution and crystallization might attain the requisite degree of purity. Without entering into any particular detail of the processes used for the preparation of bay-salt in different parts of the world, we shall content ourselves with giving a brief account of the best methods of preparing common salt.
At some convenient place near the sea-shore is erected the saltern. This is a long, low building, consisting of two parts; one of which is called the fore-house, and the other the pan-house, or boiling-house. The fore-house serves to receive the fuel, and cover the workmen; and in the boiling-house are placed the furnace, and pan in which the salt is made. Sometimes they have two pans, one at each end of the saltern; and the part appropriated for the fuel and workmen is in the middle.
The furnace opens into the fore-house by two mouths, beneath each of which is a mouth to the ash-pits. To the mouths of the furnace doors are fitted; and over them a wall is carried up to the roof, which divides the fore-house from the boiling-house, and prevents the dust of the coal and the ashes and smoke of the furnace from falling into the salt pan. The fore-house communicates with the boiling-house by a door, placed in the wall which divides them.
The body of the furnace consists of two chambers, divided from each other by a brick partition called the mid-feather; which from a broad base terminates in a narrow edge nigh the top of the furnace; and by means of short pillars of cast iron erected upon it, supports the bottom of the salt pan; it also fills up a considerable part of the furnace, which otherwise would be too large, and would consume more coals than, by the help of this contrivance, are required. To each chamber of the furnace is fitted a grate, through which the ashes fall into the ash-pits. The grates are made of long bars of iron, supported underneath by strong cross bars of the same metal. They are not continued to the farthest part of the furnace, it being unnecessary to throw in the fuel so far; for the flame is driven from the fire on the grate to the farthest part of the furnace; and from thence passes together with the smoke, through two flues into the chimney; and thus the bottom of the salt pan is everywhere equally heated.
The salt pans are made of an oblong form, flat at the bottom, with the sides erected at right angles; the length of some of these pans is 15 feet, in breadth 12 feet, and the depth 16 inches; but at different works they are of different dimensions. They are commonly made of plates of iron, joined together with nails, and the joints are filled with a strong cement. Within the pan five or six strong beams of iron are fixed to its opposite sides, at equal distances, parallel to each other and to the bottom of the pan, from which they are distant about eight inches. From these beams hang down strong iron hooks, which are linked to other hooks or clamps of iron firmly nailed to the bottom of the pan; and thus the bottom of the pan is supported, and prevented from bending down or changing its figure. The plates most commonly used are of malleable iron, about four feet and a half long, a foot broad, and the third of an inch in thickness. The Scots prefer smaller plates, 14 or 15 inches square. Several make the sides of the pan, where they are not exposed to the fire, of lead; those parts, when made of iron, being found to consume fast in rust from the steam of the pan. Some have used plates of cast iron, five or six feet square, and an inch in thickness; but they are very subject to break when unequally heated, and shaken (as they frequently are) by the violent boiling of the liquor. The cement most commonly used to fill the joints is plaster made of lime.
The pan, thus formed, is placed over the furnace, being supported at the four corners by brickwork; but along the middle, and at the sides and ends, by round pillars of cast iron called taplins, which are placed at three feet distance from each other, being about eight inches high, and at the top, where smallest, four inches in diameter. By means of these pillars the heat of the fire penetrates equally to all parts of the bottom of the pan, its four corners only excepted. Care is also taken to prevent the smoke of the furnace from passing into the boiling-house, by bricks and strong cement, which are closely applied to every side of the salt pan. In some places, as at Blyth in Northumberland, besides the common salt pans here described, they have a preparing-pan placed between two salt pans, in the middle part of the building, which in other works is the fore-house. The sea-water being received into this preparing-pan, is there heated and in part evaporated by the flame and heat conveyed under it through flues from the two furnaces of the salt pans. And the hot water, as occasion requires, is conveyed through troughs... from the preparing-pan into the salt pans. Various other contrivances have been invented to lessen the expense of fuel, and several patents have been obtained for that purpose; but the salt-boilers have found their old methods the most convenient.
Between the sides of the pan and walls of the boiling-house, there runs a walk five or six feet broad, where the workmen stand when they draw the salt, or have any other business in the boiling-house. The same walk is continued at the end of the pan, next to the chimney; but the pan is placed close to the wall at the end adjoining to the fore-house.
The roof of the boiling-house is covered with boards fastened on with nails of wood, iron nails quickly mouldering into rust. In the roof are several openings, to convey off the watery vapours; and on each side of it a window or two, which the workmen open when they look into the pan whilst it is boiling.
Not far distant from the saltern, on the sea-shore, between full sea and low-water marks, they also make a little pond in the rocks, or with stones on the sand, which they call their sump. From this pond they lay a pipe, through which, when the tide is in, the sea-water runs into a well adjoining to the saltern; and from this well they pump it into troughs, by which it is conveyed into their ship or cistern, where it is stored up until they have occasion to use it.
The cistern is built close to the saltern, and may be placed most conveniently between the two boiling-houses, on the back-side of the fore-house; it is made either of wood, or brick and clay; it sometimes wants a cover, but ought to be covered with a shed, that the salt-water contained therein may not be weakened by rains, nor mixed with soil and other impurities. It should be placed so high, that the water may conveniently run out of it, through a trough, into the salt pans.
Besides the buildings already mentioned, several others are required; as store-houses for the salt, cisterns for the bittern, an office for his majesty's salt officers, and a dwelling-house for the salt-boilers.
All things being thus prepared, and the sea-water having stood in the cistern till the mud and sand are settled to the bottom, it is drawn off into the salt-pan. And at the four corners of the salt-pan, where the flame does not touch its bottom, are placed four small lead pans called scratch pans, which, for a salt-pan of the size above-mentioned, are usually about a foot and an half long, a foot broad, and three inches deep; and have a bow or circular handle of iron, by which they may be drawn out with a hook, when the liquor in the pan is boiling.
The salt-pan being filled with sea-water, a strong fire of pit coal is lighted in the furnace; and then, for a pan which contains about 400 gallons, the salt-boiler takes the whites of three eggs, and incorporates them well with two or three gallons of sea water, which he pours into the salt-pan while the water contained therein is only lukewarm; and immediately stirs it about with a rake, that the whites of eggs may everywhere be equally mixed with the salt water.
Instead of whites of eggs, at many salterns, as at most of those near Newcastle, they use blood from the butchers, either of sheep or black cattle, to clarify the sea-water: And at many of the Scots salterns they do not give themselves the trouble of clarifying it.
As the water grows hot, the whites of eggs separate from it a black frothy scum, which rises to the surface of the water, and covers it all over. As soon as the pan begins to boil, this scum is all risen, and it is then time to skim it off.
The most convenient instruments for this purpose are skimmers of thin ash boards, six or eight inches broad, and so long that they may reach above half way over the salt-pan. These skimmers have handles fitted to them; and the salt-boiler and his assistant, each holding one of them on the opposite sides of the pan, apply them so to each other that they overlap in the middle, and beginning at one end of the pan, carry them gently forward together, along the surface of the boiling liquor, to the other end; and thus, without breaking the scum, collect it all to one end of the pan, from whence they easily take it out.
After the water is skimmed, it appears perfectly clear and transparent; and they continue boiling it briskly, till so much of the treth or aqueous part is evaporated, that what remains in the pan is a strong brine almost fully saturated with salt, so that small saline crystals begin to form on its surface; which operation, in a pan filled 15 inches deep with water, is usually performed in five hours.
The pan is then filled up a second time with clear sea-water drawn from the cistern; and about the time when it is half filled, the scratch-pans are taken out, and being emptied of the scum found in them, are again placed in the corners of the salt-pan. The scum taken out of these pans is a fine white calcareous earth found in the form of powder, which separates from the sea-water during its coction, before the salt begins to form into grains. This subtle powder is violently agitated by the boiling liquor, until it is driven to the corners of the pan, where the motion of the liquor being more gentle, it subsides into the scratch pans placed there to receive it, and in them it remains undisturbed, and thus the greatest part of it is separated from the brine.
After the pan hath again been filled up with sea-water, three whites of eggs are mixed with the liquor, by which it is clarified a second time, in the manner before described; and it is afterwards boiled down to a strong brine as at first; which second boiling may take up about four hours.
The pan is then filled up a third time with clear sea-water; and after that, a fourth time; the liquor being each time clarified and boiled down to a strong brine, as before related; and the scratch-pans being taken out and emptied every time that the pan is filled up.
Then, at the fourth boiling, as soon as the crystals begin to form on the surface of the brine, they slacken the fire, and only suffer the brine to simmer, or boil very gently. In this heat they constantly endeavour to keep it all the time that the salt corns or granulates, which may be nine or ten hours. The salt is said to granulate, when its minute crystals cohere together into little masses or grains, which sink down in the brine and lie at the bottom of the salt pan.
When most of the liquor is evaporated, and the salt thus lies in the pan almost dry on its surface, it is then time to draw it out. This part of the process is performed by raking the salt to one side of the pan into a long heap, where it drains a while from the brine, and is then filled out into barrows or other proper vessels, and carried into the store-house, and delivered into the custody of his majesty's officers. And in this manner the whole process is performed in 24 hours; the salt being usually drawn every morning.
In the store-house the salt is put hot into drabs, which are partitions like stalls for horses, lined on three sides and at the bottom with boards, and having a sliding-board on the fore-side to put in or draw out as occasion requires. The bottoms are made shelving, being highest at the back-side, and gradually inclining forwards; by which means the saline liquor, which remains mixed with the salt, easily drains from it; and the salt, in three or four days, becomes sufficiently dry; and is then taken out of the drabs, and laid up in large heaps, where it is ready for sale.
The saline liquor which drains from the salt is not a pure brine of common salt, but hath a sharp and bitter taste, and is therefore called bitter; this liquor, at some works, they save for particular uses, at others throw away. A considerable quantity of this bitter is left at the bottom of the pan after the process is finished; which, as it contains much salt, they suffer to remain in the pan, when it is filled up with sea-water. But at each process this liquor becomes more sharp and bitter, and also increases in quantity; so that, after the third or fourth process is finished, they are obliged to take it out of the pan; otherwise it mixes in such quantities with the salt, as to give it a bitter taste, and disposes it to grow soft and run in the open air, and renders it unfit for domestic uses.
After each process there also adheres to the bottom and sides of the pan a white stony crust, of the same calcareous substance with that before collected from the boiling liquor. This the operators call stone-feratch, distinguishing the other found in the lead-pans by the name of powdery-feratch. Once in eight or ten days they separate the stone-feratch from their pans with iron picks, and in several places find it a quarter of an inch in thickness. If this stony crust is suffered to adhere to the pan much longer, it grows so thick that the pan is burnt by the fire, and quickly wears away.
In M. de Pagés's Travels round the World, we find the following important fact. "I had been anxious (says that author) to ascertain by comparison, whether sea-water contains salt in greater quantity under the torrid than under the other zones; and my experiments on this subject served to show, contrary to what I expected, that sea-water is impregnated with salt in less quantity within than without the tropics." These experiments were made on a hundred pounds of sea-water, taken at the depth of ten fathoms, and weighed in water-scales. M. de Pagés has given a table of these experiments, from which it appears that 100 lb. of sea-water in 46° 12' S. lat. gave 4½ lb. of salt, and in 1° 6' only 3½ lb.; and that in 74° N. lat. it gave 4½ lb. and in 4° 22' only 3½ lb., these being the highest and lowest latitudes in which the experiments were made, and also the greatest and least quantities of salt.
Duty on Salt, is a distinct branch of his majesty's extraordinary revenue, and consists in an excise of 3s. 4d. per buttel imposed upon all salt, by several statutes of King William and other subsequent reigns. This is not generally called an excise, because under the management of different commissioners: but the commissioners of the salt-duties have, by statute 1 Ann. c. 21. the same powers, and must observe the same regulations, as those of other excises. This tax had usually been only temporary: but by statute 26 Geo. II. c. 3. was made perpetual.
Triple Salts, a kind of salts formed by the union of three ingredients; the common neutrals being composed only of two. They are but lately discovered; and it is chiefly to the industry of Mr Bergman that we owe the knowledge we have of them. Sometimes we meet even with salts of four ingredients; in which case we call the resulting compounds quadruple salts. The most remarkable of these complicated substances are the following:
1. Aphronitrum, or mineral alkali, combined with a small quantity of calcareous earth. The three ingredients here are fixed air, pure alkali, and calcareous earth. "This salt (says Cronstedt) is so strongly united with the calcareous earth, that the latter enters with it into the very crystals of the salt; though, by repeated solutions, the earth is by degrees separated from it, and falls to the bottom after every solution." Cartheuer affirms, that, on throwing into its solution in water a fixed mineral alkali, the calcareous earth was precipitated; and on the contrary, by adding oil of vitriol, nitrous acid was expelled, and a Glauber's salt produced; "from which (says M. Magellan) it is evident that the aphronitrum is a triple salt arising from the combination of the nitrous acid with calcareous earth and mineral fixed alkali." Wallerius mentions three species of this salt; viz. one which contains only a mixture of calcareous earth with fixed mineral alkali. This, he says, is the aphronitrum of the ancients; but he thinks that it ought to be rather called apbronatrum, as they bestowed the name of natron upon the mineral alkali. The second species is that described by Cronstedt under the title of calcareous nitre. The third is that described by Hoffman under the title of aphronitrum jarense, into whose composition the vitriolic acid enters. It is a kind of Glauber's salt, and is frequently confounded with it.
The aphronitrum of Cronstedt is described by him as appearing on old walls and below vaults, or in places where it cannot be washed away by the rain. When it contains any considerable quantity of calcareous earth, it floats into rhomboidal crystals, a figure frequently affected by the calcareous earth when it floats into crystals; but when the aphronitrum is purer, it forms prismatic crystals. From these circumstances, M. Magellan thinks, that the aphronitrum is not only a triple but a multiple salt; as these pieces of old mortar, covered with this white frost, on ancient walls, are the very same from which the saltpetre-makers extract the mother water of nitre; after mixing with it the vegetable ashes to furnish the alkali.
2. Common salt with magnesia, or mineral alkali, contaminated by muriatic magnesia. This is a compound of common salt with magnesia, and is very deliquescent, owing to the compound of magnesia and spi- Salt; for neither mineral alkali nor pure sea salt are at all deliquescent in the air.
3. Vitriolated magnesia with vitriol of iron, or Epsom salt contaminated with copperas. This, according to M. Monet, is found in some mineral waters.
4. Native alum contaminated with copperas. This is sometimes found in the aluminous schistus, and effloresces in a feathery form, and is perhaps the plumose alum of the ancients.
5. Native alum contaminated with sulphur. Dr Withering informs us, that this salt is met with about Wednesbury and Bilston, two places in Staffordshire, where the coal-pits are on fire. It sublimes to the surface, whence it may be collected in considerable quantity during dry or frosty weather. Our author, however, does not certainly affirm that this is a true chemical union, but the parts, he says, cannot be distinguished by the eye. It is kept in a deliquescent state by an access of vitriolic acid.
6. Native alum contaminated by vitriolated cobalt. This is found in some of the mines of Herregund and Idria, where it shoots into long and slender filaments. M. Magellan supposes that this may be the trichites of the Greeks. On dissolving it in water, the presence of the vitriolic acid is discovered by adding a solution of terra ponderosa in muriatic acid; the phlogisticated alkali throws down a precipitate of cobalt, which forms a blue glass with cobalt or microcosmic salt.
7. Vitriol of copper with iron, the vitriolum ferreo-cupreum cyanicum of Linnaeus. It is also called Vitriol of Hungary, because found in plenty in that country. Its colour is that of blue mixed with green; but sometimes the one shade prevails, and sometimes the other.
8. Vitriol of copper, iron, and zinc, is prepared in Sweden from the water pumped out of the copper mines at Dalame. The copper does not precipitate from a solution of this salt by rubbing it on iron, as is the case with the common blue vitriol. Large crystals of this salt are often found in the water, the copper mines from whence it is prepared.
9. Vitriol of copper and zinc. This is a quadruple salt, styled by Linnaeus Vitriolum ferreo-zincum cupreum cyanicum. Its colour is blue inclining to green; and it does not precipitate the copper by rubbing it on iron, as the common blue vitriol does. It is called the blue vitriol of Goflar. Mongez makes a separate article of a compound salt mentioned by Wallerius, consisting also of a vitriolated copper with zinc, but whose crystals are of a fine red colour, found lately in the mines of Falun in Sweden. He adds, that the pale-blue colour of the former salt shows the predominance of the copper, by which it is necessarily distinguished from the latter, where the vitriol is over-saturated. M. Magellan, however, is of opinion, that the red colour is owing to a proper quantity of iron in a dephlogisticated state, which has been overlooked in that compound. To this kind also Wallerius refers the yellowish vitriol found in Hungary.
10. Vitriol of iron and zinc; the green vitriol from Goflar in the Hartz; the vitreolum zineo-ferreum viride of Linnaeus. It is of a pale-green colour.
Salt-Mines. See Salt.
Rock-Salt. See Salt.
Salt-Water, or Sea-water (Distillation of). See Sea-Water.
Neutral Salts. See Chemistry, no 172, 1182, and 1331.
Salt-Springs. Of these there are great numbers in different parts of the world, which undoubtedly have their origin from some of the large collections of fossil salt mentioned under the article Common Salt. See that article, and likewise Spring.