LITMIR.BIZ

Скачать книгу

this earth we shall have occasion to say more, when we come to treat of metallic substances: but in the mean time it must be owned, that the truth of this opinion is so far from being proved by a sufficient number of experiments, that the very existence of such a mercurial earth is not yet well established; and therefore, that we may not exceed the bounds of our knowledge, we shall content ourselves with delivering here the properties which characterize the Acid in question, and by which it is distinguished from the two others considered above.

      When it is combined with absorbent earths, such as lime and chalk, it forms a neutral salt that does not crystallize, and, when dried, attracts the moisture of the air. If the absorbent earth be not fully saturated with the Marine Acid, the salt thereby formed has the properties of a fixed Alkali: and this is what made us say, when we were on the subject of those salts, that they might be imitated by combining an earth with an Acid. The Marine Acid, like the rest, hath not so great an affinity with earths as with fixed Alkalis.

      When it is combined with the latter, it forms a neutral salt which shoots into cubical crystals. This salt is inclined to grow moist in the air, and is consequently one of those which water dissolves in equal quantities, at least as to sense, whether it be boiling hot or quite cold.

      The affinity of this Acid with Alkalis and absorbent Earths is not so great as that of the vitriolic and nitrous Acids with the same substances: whence it follows, that, when combined therewith, it may be separated from them by either of those Acids.

      The Acid of Sea-salt, thus disengaged from the substance which served it for a basis, is called Spirit of Salt. When it contains but little phlegm it is of a lemon colour, and continually emits many white, very dense, and very elastic vapours; on which account it is named the Smoaking Spirit of Salt. Its smell is not disagreeable, nor much unlike that of saffron; but extremely quick and suffocating when it smokes.

      The Acid of Sea-salt, like the other two, seems to have a greater affinity with the Phlogiston, than with fixed Alkalis. We are led to this opinion by a very curious operation, which gives ground to think that Sea-salt may be decomposed by the proper application of a substance containing the Phlogiston.

      From the Marine Acid combined with a Phlogiston results a kind of Sulphur, differing from the common sort in many respects; but particularly in this property, that it takes fire of itself upon being exposed to the open air. This combination is called English Phosphorus, Phosphorus of Urine, because it is generally prepared from urine; or, only Phosphorus.

      This combination of the Marine Acid with a Phlogiston is not easily effected; because it requires a difficult operation in appropriated vessels. For these reasons it does not always succeed; and Phosphorus is so scarce and dear, that hitherto Chymists have not been able to make on it the experiments necessary to discover all its properties. If Phosphorus be suffered to burn away in the air, a small quantity of an acid liquor may be obtained from it, which seems to be spirit of salt, but either altered, or combined with some adventitious matter; for it has several properties that are not to be found in the pure Marine Acid; such as, leaving a fixed fusible substance behind it when exposed to a strong fire, and being easily combined with the Phlogiston so as to reproduce a Phosphorus.

      Phosphorus resembles sulphur in several of its properties: it is soluble in oils; it melts with a gentle heat; it is very combustible; it burns without producing soot; and its flame is vivid and bluish.

      From what has been said of the union of the Acid of Sea-salt with a fixed Alkali, and of the neutral salt resulting therefrom, it may be concluded that this neutral salt is no other than the common kitchen-salt. But it must be observed, that the fixed Alkali, which is the natural basis of the common salt obtained from sea-water, is of a sort somewhat differing from fixed Alkalis in general, and hath certain properties peculiar to itself. For,

      1. The basis of Sea-salt differs from other fixed Alkalis in this, that it crystallizes like a neutral salt.

      

      2. It does not grow moist in the air; on the contrary, when exposed to the air, it loses part of the water that united with it in crystallization, by which means its crystals lose their transparency, become, as it were, mealy, and fall into a fine flour.

      3. When combined with the vitriolic Acid to the point of saturation, it forms a neutral salt differing from vitriolated tartar, first, in the figure of its crystals, which are oblong six-sided solids; secondly, in its quantity of water, which in crystallization unites therewith in a much greater proportion than with vitriolated tartar; whence it follows, that this salt dissolves in water more readily than vitriolated tartar; thirdly, in that it flows with a very moderate degree of heat, whereas vitriolated tartar requires a very fierce one.

      If the Acid of Sea-salt be separated from its basis by means of the vitriolic Acid, it is easy to see that, when the operation is finished, the salt we have been speaking of must be the result. A famous Chymist, named Glauber, was the first who extracted the Spirit of Salt in this manner, examined the neutral salt resulting from his process, and, finding it to have some singular properties, called it his Sal mirabile, or wonderful Salt: on this account it is still called Glauber's Sal mirabile, or plainly Glauber's Salt.

      4. When the basis of Sea-salt is combined with the nitrous Acid to the point of saturation, there results a neutral salt, or a sort of nitre, differing from the common nitre, first, in that it attracts the moisture of the air pretty strongly; and this makes it difficult to crystallize; secondly, in the figure of its crystals, which are parallelopipeds; and this has procured it the name of Quadrangular Nitre.

      Common salt, or the neutral salt formed by combining the Marine Acid with this particular sort of fixed Alkali, has a taste well known to every body. The figure of its crystals is exactly cubical. It grows moist in the air, and, when exposed to the fire, it bursts, before it melts, into many little fragments, with a crackling noise; which is called the Decrepitation of Sea-salt.

      That neutral salt mentioned above, which is formed by combining the Marine Acid with a common fixed Alkali, and called Sal febrifugum Sylvii, hath also this property.

      India furnishes us with a saline substance, known by the name of Borax, which flows very easily, and then takes the form of glass. It is of great use in facilitating the fusion of metallic substances. It possesses some of the properties of fixed Alkalis, which has induced certain Chymists to represent it, through mistake, as a pure fixed Alkali.

      By mixing borax with the vitriolic Acid, Mr. Homberg obtained from it a salt, which sublimes in a certain degree of heat, whenever such a mixture is made. This salt has very singular properties; but its nature is not yet thoroughly understood. It dissolves in water with great difficulty; it is not volatile, though it rises by sublimation from the borax. According to Mr. Rouelle's observation, it rises then only by means of the water which carries it up: for, when once made, it abides the fiercest fire, flows and vitrifies just as borax does: provided care be taken to free it previously from moisture by drying it properly. Mr. Homberg called it Sedative Salt, on account of its medical effects. The sedative salt hath the appearance, and some of the properties, of a neutral salt; for it shoots into crystals, and does not change the colour of violets; but it acts the part of an Acid with regard to Alkalis, uniting with them to the point of saturation, and thereby forming a true neutral salt. It also acts, like the Acid of vitriol on all neutral salts; that is, it discharges the Acid of such as have not the vitriolic Acid in their composition.

      Since Mr. Homberg's time it hath been discovered, that a sedative salt may be made either with the nitrous or with the marine Acid; and that sublimation is not necessary to extract it from the borax, but that it may be obtained by crystallization only. For this latter discovery we are indebted to Mr. Geoffry, as we are to Mr. Lemery for the former.

      Since that time M. Baron d'Henouville, an able Chymist, hath shewn that a sedative salt may be obtained by the means of vegetable Acids; and hath lately demonstrated, in some excellent papers published in the collection of Memoirs written by the correspondents of the Academy of Sciences, that the sedative salt exists actually and perfectly in the borax, and that it is not produced by mixing Acids with that saline substance, as it seems all the Chymists before

Скачать книгу