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the hydriodic, and the hydrobromic, into the composition of which hydrogen enters; yet these bodies were found in other respects to correspond perfectly with the above definition, and to possess all the characters of powerful acids. To meet this objection, the theory was modified, and the acids were divided into two great classes, the first of which comprised the oxyacids, such as the sulphuric, nitric, and others of analogous composition, in which it was supposed that the acid properties depended on the presence of oxygen; the second class was formed by the hydracids, such as the hydrochloric and hydriodic acids, in which hydrogen was an essential component. It was noticed, that when bodies belonging to either of these classes combine with metallic compounds, and form neutral combinations, the acids do not unite directly with the metals without evolution of gas; with their oxides, on the contrary, combination appears to take place directly : diluted sulphuric acid, for example, has no action upon metallic copper, but it quickly dissolves its oxide, forming the blue solution of sulphate of copper. On applying heat so as to render the salt anhydrous, it was found that the salts of the oxyacids (of which sulphate of potash, KO,SO,, may be taken as the type, adopting for the present the equivalents O 8, S 16, and the old notation) might be represented under the form MO,SO,, which supposes the union of 1 equivalent of the anhydride with 1 equivalent of a metallic oxide; while a hydracid (such for instance as hydrochloric acid) if made to act upon a base such as soda, yields a body like common salt (NaCl), which when dry contains neither hydrochloric acid nor soda, the radicle of the acid being left in combination with the metal itself: NaO,HO + HCl yielding NaCl + 2 HO. Thus, in the case of the salts of the hydracids, it will be observed that the oxygen of the oxide is precisely sufficient to convert the hydrogen of the acid into water: this union, indeed, actually takes place, and the water so formed is expelled on the application of heat. When, therefore, a hydracid acts upon a base, a true double decomposition occurs.

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In consequence of this supposed difference in constitution, it was proposed to subdivide salts into two classes,-the first, like nitrate of potash, KO,NO,, being formed by the union of an oxide, such as potash, with an oxyacid, or anhydride as we now term it, such as the nitric; these were termed oxysalts: the other class being produced by the combination of a metal with the characteristic element in a hydrogen acid. The salts of the second class, being composed upon the same plan or type as sea-salt, were termed haloid salts (from aλs, sea-salt). This distinction is still

368

BINARY HYPOTHESIS OF SALTS.

recognized by many chemical writers. The supposition that a salt consists of an anhydride united to a base still in many cases affords the simplest explanation of many chemical decompositions.

(549) Binary Hypothesis of Salts.-The foregoing observations seem to prove that there is a marked difference between the composition of the oxyacid and the hydracid series of salts. The separation of salts into two classes, one consisting of the salts of the oxyacids, and the other of those of the hydracids, is not, however, indispensable. A hypothesis was advanced by Davy and by Dulong, which reduces all salts to the hydracid type. Upon this view-frequently termed the binary theory of salts-all the hydrated acids are regarded as salts containing hydrogen in the place of a metal, so that hydrogen acts the part of a feeble basyl towards a group of elements, or a single element, which forms the radicle of the salt. It has already been shown that those of the oxyacids which can be obtained as anhydrides, such for example as the sulphuric, the nitric, the phosphoric, the carbonic, and the boracic, do not as anhydrides possess the properties generally admitted to constitute the true acid character. Sulphuric anhydride, for instance, does not redden dry litmus; it may be moulded in the fingers without injury; but when once it has passed into the hydrated form, which it speedily does by absorbing moisture from the air, it corrodes all organized substances with great activity. Carbonic anhydride is also without action on litmus. When such compounds have entered into combination with water they may be represented as hydracids, by a slight modification of the ordinary formula; e.g., nitric acid (HO,NO,) may be expressed as (H,NO), corresponding with hydrochloric acid (H,Cl): each atom of these bodies, when heated in contact with a base or a metallic oxide, gives off 1 atom of water, in a manner precisely analogous to the hydracids already examined. One equivalent of oil of vitriol treated with 1 equivalent of oxide of lead would thus produce an equivalent of sulphate of lead and an equivalent of water; H,SO+PbO becoming Pb,SO+HO.

Many chemists indeed now regard the compounds which were previously considered as hydrated acids as salts composed of a compound radicle (consisting of the anhydride + an equivalent of oxygen) united with an equivalent of hydrogen. The other salts of the acid would be formed from these hydrogen compounds by the displacement of the hydrogen by an equivalent amount of each of the different metals which enter into the composition of the various salts, and which are indicated by their respective

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names. In accordance with this view, we have already given a simple explanation of the liberation of hydrogen when diluted sulphuric acid is acted upon by zinc; the zinc merely entering into combination with the radicle of the acid, and displacing the hydrogen; so that (resuming the notation which regards →=16 and water H2O) H2SO+Zn become ZnSO4 + H2; and the reaction is, upon this view, analogous to that of the same metal upon hydrochloric acid; 2 HCl + Zn=ZnCl2 + H2.

4

A comparison of a few of the so-called hydracids with some of the hydrated oxyacids will show the similarity between them; whilst the corresponding anhydrides will be at once seen to belong to an entirely distinct group of compounds :

A few reactions between certain bases on the one hand with some of the hydracids, and on the other with certain hydrated oxyacids, will enable us to complete the parallel :

In each case the salt is formed by the substitution of an equivalent amount of metal for hydrogen, whilst a corresponding quantity of water is liberated and occupies the place of the metallic oxide originally employed.

Binary compounds are such as consist of single atoms of two elements only; chloride of sodium (NaCl), therefore, is a binary compound; and if all salts be assimilated to this type, it is assumed that the grouping of their molecules resembles that which occurs in this binary compound. It has indeed been supposed that all salts consist of two portions: one comprising the distinctive constituents of the acid, and consisting either of a non-metallic elementary substance (chlorine, Cl, for example), or else an equivalent compound body (such as sulphion, SO), which is termed the radicle of the salt; the other is either a metal (sodium, Na, for instance), or else a compound like ammonium (H ̧N), equivalent to a metal, termed the basyl of the salt. Attention has already

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OBJECTIONS TO THE BINARY HYPOTHESIS.

been directed to the bearing of the electrolysis of saline compounds (286), upon this theory of their constitution.

(550) Objections to the Binary Hypothesis.-Notwithstanding the ingenuity of the foregoing hypothesis, and the advantages which it offers in the explanation of certain modes of decomposition, it is open to many serious objections; and indeed it cannot be regarded as a correct representation of the composition of a salt under all circumstances. In fact, none of the compound radicles, SONO„„, have been obtained in an isolated form, nor is it probable that they ever will be.

It also appears to be highly improbable that a body of such powerful chemical attractions as potash should, in carbonate of potassium for example, part with its oxygen to a substance which, like carbonic anhydride, exhibits no tendency to further oxidation, so that K,,, should become K„¤Ð ̧.

2

The conclusion which is most probable is this: viz., that a salt, when once formed, must be regarded as a whole; it can no longer be looked upon as consisting of two distinct parts, but as a new substance, maintained in its existing condition by the mutual actions of all the elements which compose it. These different elements are not all united with each other in every direction with an equal amount of force. As in a crystal there are certain directions in which the mass admits of cleavage with greater facility than in others, and as two or three different directions of cleavage may be found in the same crystal by varying the direction in which the force is applied, so in the same salt there are directions in which it yields to the application of chemical force more readily than in others; and according as that chemical force is applied in one way or in another, the compound splits up into simpler substances, the nature of which will vary according to the mode which has been selected for effecting its decomposition.

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3

For example, if the solution of a powerful acid, such as nitric acid, be poured upon carbonate of potassium, carbonic anhydride is liberated abundantly, and nitrate of potassium is produced; K ̧¤Ð ̧+2 HNÐ ̧=2 KNO ̧+€0,+H2; but if another portion of the same carbonate of potassium be mixed with charcoal, and heated in an iron retort to whiteness, metallic potassium and carbonic oxide are the results; K ̧€0,+2¤=K2+3 €0. Again, -3 if a solution of carbonate of potassium be subjected to electrolysis by the aid of the voltaic battery, the salt splits up into potassium (which is immediately oxidized by the water in the midst of which it is liberated), and into €, which is as instantly resolved into oxygen gas and carbonic anhydride; 2 K,Є0, becoming

SULPHO-SALTS-VARIETIES OF SALTS.

2

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2 K2+2 €0, and 2 K2+4 H2 giving 4 KHO+2 H2, whilst 2ЄO becomes 2 €0, +2. The probability therefore is, that neither the old nor the new view is absolutely correct, but that each may in turn well represent the salt when subjected to the influence of particular circumstances. It may therefore readily be conceded that the binary theory may in certain cases elucidate the decompositions observed, notwithstanding the difficulties which prevent its adoption as a correct representation of the molecular arrangement of saline compounds in general when in a quiescent state. On the other hand, it may often be convenient to represent certain salts as compounds of the anhydride and the base : carbonate of calcium, for example, may sometimes be written Єa,¤¤, although the empirical formula Єa¤¤, may generally be preferred. So, again, in the case of the sulphates. Sulphate of magnesium may, for instance, be often written MgO,SO,, although we may generally adopt the form MgSO, which involves no theory of its constitution.

(551) Sulpho-Salts.-The preceding remarks have been made with almost exclusive reference to those salts into the composition of which oxygen enters. There is, however, a numerous series of compounds parallel to these oxy-compounds, but in which sulphur enters into combination with the metal; and for each atom of oxygen in the series of the oxy-salts an equivalent of sulphur is substituted in the corresponding compound in the sulphur series. Generally speaking, the sulphur-salts are of subordinate importance to the oxy-compounds; many of them are decomposed by admixture with water, and they have been the subject of much less study and research than the oxy-salts. Many chemists regard these compounds as salts in which the electropositive sulphides, such as the protosulphides of potassium, &c., act the part of bases; and the electronegative sulphides, such as the higher sulphides of arsenic and antimony, act as acids. No doubt their molecular constitution is analogous to that of the oxy-salts.

(552) Varieties of Salts.—Salts are usually spoken of as neutral, acid, or basic; but though these terms are in general use, there is some ambiguity in the manner in which they are applied.

(553) Neutral, or Normal Salts.-The idea of neutrality implies that the peculiar characters of the acid and of the alkali have each disappeared as a result of chemical combination, and one of the usual means by which this neutralization in properties is judged of, consists in observing the effect which is produced upon certain vegetable colours when mixed with a solution of the salt. The blue colour of litmus, for example, is changed to red by