latter salt is a simple chloride of the metal sodium, and thus, from the formation of these salts being precisely analogous to that of sea-salt, the term haloid, or salt-like, has arisen.

The second class of salts, viz., oxysalts, are compounds of an oxyacid with a metallic oxide; the oxyacid being in by far the largest number of cases a compound of an elementary non-metallic substance with oxygen, while the base, or metallic oxide, is formed by the metal employed, which has taken the requisite oxygen either from the acid or from its combined water.

As examples of these oxyacids, it may be stated that sulphuric acid is a compound of sulphur and oxygen; nitric, of nitrogen and oxygen; oxalic, of carbon, also with oxygen, and so on: in all these cases bearing in mind that the acid exists as a hydrate, that is to say, combined with water.

If some iron be digested in sulphuric acid, it will be dissolved, and a salt formed; and, in effecting this, for each equivalent of iron dissolved, one equivalent of water is decomposed; its oxygen passes to the iron, and combines with it, its hydrogen escaping as gas. An equivalent of the acid at the same time takes the newlyformed oxide of iron, and one equivalent of sulphate of iron is the product; so that, it will be observed, the salt is, properly speaking, a sulphate of oxide of iron.

Again, when silver is dissolved in nitric acid, to form nitrate of silver, a similar oxidation of the metal is the first change; but, in the case of nitric acid, this is effected at the expense of a portion of the acid itself. Thus, for every 3 equivalents of silver dissolved, 1 equivalent of nitric acid is decomposed; it furnishes 3 out of its 5 equivalents of oxygen to the 3 equivalents of metal; these 3 equivalents of oxide of silver are then taken by 3 of undecomposed acid, and form the same number of nitrate of silver, while the remaining elements of the equivalent of decomposed acid, viz. NO, escaping, seizes 2 equivalents of oxygen from the air (on coming in

contact with it), and so produces the dense red fumes of nitrous acid observed in cases of solution of metals in nitric acid. These are illustrations of the formation of oxysalts.

The third class of salts, or sulphur salts, have been established by Berzelius; they are exactly similar to the oxysalts, if we imagine the oxygen in them removed, and replaced by sulphur. In fact, instead of an oxide and oxyacid, we have combining a sulphide, or sulphur base, with a sulphur acid, producing bodies having all the characters of salts, viz., crystalline form, and (in many cases) solubility in water, and so forth.

The oxysalt carbonate of potash (for example) would be represented by the symbol KO,CO,. Now, if the equivalent of oxygen be removed from the base, and be replaced by 1 of sulphur, and the 2 equivalents of oxygen in the acid also by 2 of sulphur, we get a compound which actually exists in a crystalline state, and affords a good illustration of a sulphur salt. These are far from being an important class.

The examples thus given of these three divisions of salts illustrate one theoretical view of their constitution, but at the same time divide them as seen, dependent upon their different modes of formation. But there is another theory, by which the two first and principal classes are brought into one category, called the binary theory of salts. This starts with the fact that all acids, when in a state capable of combining, so as to form salts, contain hydrogen, and consequently, in place of being regarded as an acid, plus water, may be viewed as an acid radicle, plus H. Under this aspect, then, sulphuric and nitric acids, in place of being symbolized as SOHO, and NO,HO, would be written SO„H, and NOH, respectively.

Thus it will be seen that the oxygen acids are brought into an analogous state to the hydroacids; and then, in the formation of a salt, the changes become the same in each case, viz., the simple removal of the hydrogen, and replacement of it, not by an oxide, but by the metal

itself. Thus, in the formation of chloride of zinc, the chlorine, as has been shown, unites directly with the zine, and hydrogen is evolved. So by this theory, when we form sulphate of iron, instead of FeO uniting with SO, Fe itself would simply remove H, and unite with SO..

These are, however, only theoretical views, which, while they serve to simplify our knowledge on these points, could be met by showing many inconsistencies in them. And Dr. Miller justly observes, 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."

Salts combine with each other, and produce a class called double salts. In these, two distinct bases are united with one acid. It may be first in the way of the combination of two neutral salts of the same acid, as, for instance, sulphate of copper with sulphate of potassa, where a perfectly definite crystalline salt may be obtained by dissolving and mixing together equivalent proportions of the two component salts.

Again, the sulphates of copper and of iron may be so united; and although the crystals of the first contain 7 equivalents of water, while those of the latter contain 5, so complete will be the union that the new salt will agree in this respect with the one containing the larger amount of water.

The haloid salts will combine in like manner, and so afford some very important double salts. An example may be given in the double chloride of platinum and potassium, which consists of PtCl2+ KCl.

Double salts may be formed also by union of those of bases of different degrees of oxidation. Thus the alums are all compounds of sulphates of protoxides with sulphates of sesquioxides. For instance, iron alum is KO,SO,+Fe,,,3SO,; and chrome alum is the same, substituting persulphate of chromium for the iron salt

of the former. Lastly, ordinary alum is a sulphate of potassa with sesquisulphate of alumina. Thus the formula is KO,SO,+A1,0,3SO,; and in all these examples there are found 24 equivalents of water.

When a metallic salt has been formed by the solution of a metal, the simple evaporation, so as to drive off a portion of water, causes it to assume the solid state in certain regular mathematical forms, called crystals; and these forms are always the same (with certain modifications) in the same salt. Thus sulphate of iron always crystallizes in oblique rhombic prisms; sulphate of copper in rhombohedral forms; nitrate of silver in four or six sided tables; chloride of sodium in cubes; chloride of barium in flat, four-sided crystals, bevelled at their edges; all, it will be perceived, distinct, and, in the same salt, constant forms.

In cases of artificial crystallization, the more slow the process, the finer and more definite will be the crystalline forms. Hence, by exposing a strong solu tion to the air, so as to allow the water to evaporate spontaneously, we fulfil the conditions to perfection; and we therefore find in nature crystalline forms the most perfect where the deposit of solid matter has been so exceedingly slow that it is effected in the most regular manner: thus many of our ordinary ores contain the most beautiful crystalline portions, some being entirely crystalline; and even the metals themselves are frequently found native in perfect crystals.

It has been stated that the same salts crystallize uniformly in the same definite forms; but this law 'admits of exceptions, the reason of which may be here explained.

If we take an ordinary crystal, and with a knife attempt to split it, it will be found that this can be effected only in certain directions, and in such a clean facet will be obtained; but in all others (if we succeed at all, and do not actually crush it) we get only an irregular, broken surface. Now the planes in which the operation can be effected are called planes of cleavage;

and as all planes have one or more imaginary axes, around which their particles are supposed to have been built up, we can cleave a crystal around these in the same relative directions, until we have altered its mathematical form altogether, and obtained an equally regular one, which would hence be called the secondary one of the crystal.

Suppose, for example, a cube be taken, and, starting from a central point upon its upper face, the four solid angles be cleft off successively, the cleft surface being formed in each case from the point just mentioned,

Fig. 5.

down to the centre of each edge; then, if we turn the crystal upside down, and repeat the operation, starting from the same point of the opposite face, a regular octohedron would result. Again, by similar means, but by removing the twelve edges of the perfect cube, instead of the angles, we should obtain the dodecahedron; or, lastly, from the same primary form, a tetrahedron may be obtained, by cleaving off alternate angles only.

Now it will be readily perceived that in nature's laboratory a slight disturbing force may frequently come into play during the aggregation of particles going to form a crystal, and may hence interfere with the completion of the perfect primary form. Thus a great variety of secondary ones will arise, for in the progress of building up a crystal, which is, of course, the reverse action to the kind of dissection above described, we may see how growth may accidentally be stopped in some directions; indeed, it very commonly is so; and hence we seldom get either primary or secondary forms quite perfect. But the disturbing force being some internal one, acting upon the ultimate particles, has influence alike upon all parts around each axis, except where hindered by external interference, as where a crystal rests upon the vessel in