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POLYBASIC ACIDS-DOUBLE SALTS.

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normal tartrate of potassium (KH), a salt which no longer affects the colour either of litmus or of turmeric paper, is produced. An equivalent quantity of carbonate of potassium may be substituted for caustic potash with equal effect, as it will be decomposed, and the carbonic anhydride will be expelled with effervescence. Carbonate of sodium may be substituted for the carbonate of potassium, but in this case a different salt, known as Rochelle salt, the tartrate of sodium and potassium (KNa¤ ̧H ̧Ð ̧ 4 H2O) will be formed by the following reaction; NaH¤¤ ̧+ KHЄH2O+H2+€0,+KNa€,HO.

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Acid salts, however, though generally formed, like cream of tartar, from a dibasic acid which has reacted with I atom only of a powerful base, the place of the second atom of metal being supplied by an atom of hydrogen, are not always so produced: an acid sulphate of potassium, which is anhydrous, may be obtained (K,SOSO); and the acid chromate or bichromate of potash always occurs in the anhydrous form. Such salts have been distinguished from ordinary acid salts by the term anhy. dro-salts, or salts containing an anhydride.

(555) Double Salts.-The foregoing description of the polybasic acids has presented us with certain cases in which double salts are formed. There are several varieties of double salts. The most common are those which are produced by the union of two dissimilar metals with the same acid radicle. These varieties, however, are confined within certain limits. It is not possible to form double salts ad libitum, by bringing 2 equivalents of any acid in contact with 1 equivalent each of any two bases. Chemists assume that when two different metallic monads, such as sodium and potassium, combine with the same acid radicle in the proportion of 1 atom of each metal to form a double salt (like Rochelle salt), the acid in question is dibasic. The larger number of double salts which have been produced are thus formed by the combination of different metals with polybasic acid radicles. The socalled bicarbonates, binoxalates, and many other similar compounds, prove, on examination to be, as we have already shown, true double salts of this class, analogous to normal or neutral salts in composition. Some other considerations relating to the basicity of acids and to the polybasic acids will be more conveniently deferred until the nature of the organic acids has been discussed.

The formation of another remarkable series of double salts, particularly investigated by Graham, appears to be directly connected with the mode in which water attaches itself to certain salts,

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In most cases the water of crystallization may be expelled from a salt by exposing it to a temperature not exceeding 212°. This, however, does not always happen: sometimes all the water of crystallization may thus be expelled with the exception of a single atom, which requires a much higher heat for its expulsion, although in these cases it does not appear to act in any degree as a base. Under these circumstances it was found that this last atom of water might readily be displaced by adding to the salt an equivalent of certain anhydrous salts. An excellent illustration of such a method of the formation of double salts is afforded in the case of a certain class of the sulphates. All the sulphates of metals isomorphous with magnesium are capable of forming double salts of this nature with some anhydrous sulphate not isomorphous with this class-such, for instance, as the sulphate of potassium.

When sulphate of magnesium (MgSO, H2O. 6 H2O) is heated to 212°, six out of the seven atoms of water are expelled, but the seventh atom is retained until the temperature is raised considerably. If, however, sulphate of magnesium and sulphate of potassium be separately dissolved in water in equivalent proportions, mixed while hot, and allowed to crystallize, a new double salt (MgSOK2SO. 6 H2O) is deposited, having the same crystalline form as sulphate of magnesium, but it contains only 6 atoms of water of crystallization. The seventh atom has been displaced by the sulphate of potassium, and this portion of water has hence been termed by Graham, constitutional, or saline water.

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There is another well-known variety of double salts, in which it is not necessary that the component salts should be formed from oxides of the same, class, or even contain a similar number of equivalents of the radicle of the acid. In this way sulphates of sesquioxides often unite with the sulphates of protoxides to form well-characterized double salts: a striking example of this kind is afforded in the important tribe of alums. Common alum consists of a combination of sulphate of potassium with sulphate of aluminum and water of crystallization (KAl 2 SO4, 12 H2O) : numerous other salts, having the same crystalline form, and of similar composition, might be mentioned.

There are a few instances of two different acid radicles being united to one basyl, but they are neither sufficiently numerous nor important to merit lengthened notice, and are more frequently met with among natural than artificial combinations.

Instances are common in which two different haloid salts unite with each other; compounds of this description are most usual between the chlorides, iodides, and bromides of the less oxi

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dizable metals with those of the metals contained in the alkalies and earths the double chloride of platinum and potassium (2 KCl,PtCl) and the double iodide of mercury and potassium (2 KI,HgI) are good instances of such compounds. Bonsdorff proposed to consider these compounds in the light of salts in which the chloride and iodide of the electronegative metal (platinum, gold, &c.) acted the part of an acid towards the electropositive chloride (chloride of potassium, sodium, &c.); but this view is not tenable. Such salts are never resolved by electric action into their constituent chlorides, and the acid reaction of the higher chlorides is not neutralized or modified by combination with the chlorides of the alkaline metals; in fact, the constituent chlorides themselves are salts.

Many double salts may be formed by fusion with each other, though they cannot be procured by the usual method of crystallization from a solution containing equivalent quantities of the two salts. Chloride of sodium, for example, may be melted with an equivalent amount of chloride of calcium, of strontium, or of barium; and in each case a compound salt is obtained, which has a much lower fusing-point than either of its component chlorides in a separate form; but the double salt is decomposed when it is dissolved in water.

(556) Subsalts.-A very different series of saline compounds still remains for consideration, and in these the proportion of base predominates over that of the acid; they are usually designated basic salts, or subsalts. The theory of the formation of these compounds is very imperfect. In many cases subsalts may be compared to salts which contain water of crystallization, the atoms of base in excess being assumed to be attached to the normal salt in a manner analogous to that by which the water of crystallization is retained in ordinary instances.

The tendency to the formation of subsalts is limited to certain acids and bases. It is indeed one of the peculiarities of the monad bases, such as the alkalies, and the oxides of silver and thallium, that they do not form basic salts; whilst the dyads, such as the oxides of copper, lead, mercury, and zinc, have a strong tendency to do so, while the oxides of the triads, when basic, such as oxide of antimony and bismuth, have a still greater propensity to form basic salts: no general rule can be laid down for the acids, but among the common acids, those which most frequently form basic salts are the sulphuric, nitric, carbonic, and acetic acids.

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Among basic sulphates, for example, are the following:

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Just as we have polybasic acids, so are there polyacid basyls— basyls, that is, which require more than one atom of a monobasic acid for their saturation. This is the case, for example, with the metals of the alkaline earths; but as yet the class of double salts, which they no doubt compose, has been scarcely examined.

(557) Oxychlorides, &c.—A class of compounds which resemble the subsalts more than any others, is presented to us in the bodies termed oxychlorides, oxyiodides, and oxycyanides. In these compounds, one atom of the chloride, of the iodide, or of the cyanide of a metal is united with one or more atoms of the oxide of the same metal. Turner's yellow, which is an oxychloride of lead (PbCl 7 PbO), is a well-known commercial article belonging to this class. Such combinations usually occur between oxides and chlorides or iodides of metals the pure chlorides or iodides o which never form any but anhydrous crystals.

Some salts enter into combination with other bodies, and form compounds which are in many respects anomalous; such for instance are the compounds of ammonia with many dry salts: 2 atoms of chloride of silver will in this manner absorb 3 atoms of ammonia. Many of the salts of copper exhibit a similar power (622).

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The metals of this class are soft, easily fusible, and volatile at high temperatures: they furnish several oxides, of which only one is basic. This oxide is caustic, and extremely deliquescent: the hydrate cannot be decomposed by ignition; it absorbs carbonic acid with avidity. The carbonates are soluble, so are the sulphides and hydrosulphates of the sulphides (see p. 337).

§ I. POTASSIUM: K'=39'1. Sp. Gr. 0·865; Fusing-pt. 144°5.

Native Compounds which contain Potassium.

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(558) Symbols of Mixtures of Isomorphous Compounds.—The formulæ employed above for felspar and mica require explanation, as the principle of notation adopted in these cases will be applied hereafter to the formulæ of a large number of minerals.*

It often happens that isomorphous bases displace each other in the same mineral without altering its form or mineralogical characters, or even without altering its general chemical formula. Mica, for example, may be regarded as a compound of 1 atom of a silicate of a protoxide of a metal with 3 atoms of a different silicate of a sesquioxide of a different metal. Let M stand for

The formula of the silicates are so complex, and the true function of silica in combination is at present so ill defined, that I have throughout formulated silica as present in combination with the bases, in accordance with the older view, retaining, however, the new atomic weights.

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