CLASSIFICATION OF THE METALS. 337 In treating of the groups of the non-metallic and electronegative elements, it has been remarked that the electronegative character of those belonging to the same group is most strongly marked in those which have the lowest combining number; chlorine, for example, being more active than bromine, and bromine than iodine. With the basylous or electropositive elements, the reverse generally holds good; the basic power of rubidium, for example, being greater than that of potassium, that of potassium greater than that of sodium, and that of sodium being superior to the basic power of lithium. I. The metals of the alkalies; these are five in number-viz., 1. Potassium 4. Cœsium 2. Sodium 3. Lithium These metals present a close analogy in properties; sodium, which is intermediate in properties between potassium and lithium, possesses a combining number which is the arithmetic mean of the two for 39+7=23. In like manner the atomic weight of rubidium (85.3) is intermediate between that of cœsium and potassium 133+3986. A similar remark is applicable to the intermediate member of some of the other groups. 2 2 The corresponding salts formed by the metals of the alkaline group are isomorphous only when they contain equal atomic proportions of water of crystallization. With these metals will be described the salts of ammonium: they are isomorphous with the salts of potassium, and indeed present the closest analogy with them. The metals of the alkalies are distinguished by the following characters:-They are monad or uniequivalent, and displace therefore I atom of hydrogen from the acids. They are soft, easily fusible, and volatile at high temperatures: they have an intense attraction for oxygen, and become tarnished immediately that they are exposed to the air: when thrown upon water they decompose it at all temperatures with rapid disengagement of hydrogen: they each form at least two oxides, but only one of these, that with the smallest proportion of oxygen, forms salts: the general formula of this basic oxide is M2O. These basic oxides combine with water with great avidity, yielding soluble hydrates of the general formula MHO; their aqueous solutions are powerfully caustic and alkaline. When they have once combined with water, the compound thus obtained cannot be rendered anhydrous by heat alone. In these metals the basylous quality, or their capacity for saturating the acids, is developed to the highest degree. The hydrated alkalies, 338 CLASSIFICATION OF THE METALS. : when exposed to the air, either in the solid form or in solution, absorb carbonic acid rapidly each alkali forms with this acid two salts, a normal carbonate and an acid carbonate commonly known as the bicarbonate, both of which are freely soluble in water. The metals of the alkalies combine with sulphur in several proportions; all of these compounds, also, are soluble. With chlorine they form but a single chloride; but their oxides have the power of combining with chlorine, and forming compounds possessed of bleaching properties. Lithium, from the sparing solubility of its carbonate, forms the connecting link between this group and the one which follows it. viz., II. The metals of the alkaline earths are three in number— 1. Barium 1 2. Strontium 3. Calcium. These metals are dyad, or biequivalent, 1 atom usually displacing 2 atoms of hydrogen from its combinations. They decompose water at all temperatures with great rapidity; with the exception of barium, they each form but one oxide, and this oxide combines with water with avidity, but the hydrate may be decomposed by ignition. The hydrated oxide is soluble to a certain extent in water, and is capable of forming salts by its reaction upon acids. They each furnish but one chloride, which assumes the form of N'Cl. The metals of this group are also powerfully basylous. They form several sulphides which are soluble in water; the protosulphides being less so than those which contain higher proportions of sulphur. With chlorine their oxides form bleaching compounds. Their carbonates are insoluble in pure water, but are soluble to a small extent in water charged with carbonic acid. They form insoluble phosphates and oxalates. The corresponding salts of these metals are in many cases isomorphous. III. Metals of the earths; ten in number-viz., The oxides of this class are insoluble in water; several of them are dissolved by solutions either of the caustic alkalies or of their carbonates. The phosphates of this group are insoluble in water. Aluminum and glucinum do not decompose water at ordinary temperatures unless the metals are in a very finely divided state; CLASSIFICATION OF THE METALS. 339 the other metals of this group are scarcely known in an isolated form. The basylous character of this group of metals is much less marked than that of the preceding ones. When salts of these metals in solution are mixed with sulphide of ammonium, precipitates consisting of hydrated oxides instead of sulphides are formed, whilst sulphuretted hydrogen escapes. Many of these metals are very rare, and their properties have been but imperfectly examined. Zirconium is closely allied to silicon. Aluminum, by the isomorphism of its oxide with sesquioxide of iron, the volatility of its chloride, its slight attraction for carbonic acid, and other peculiarities, connects this group with the group of iron metals. IV. Magnesian metals, three in number: 1. Magnesium | 2. Zinc 3. Cadmium. Magnesium is usually reckoned as one of the metals of the alkaline earths; but from its power of resisting oxidation at the ordinary temperature of the air, its volatility at high temperatures, the isomorphism of its salts with those of zinc, the sparing solubility of its oxide and its sulphide, the solubility of its sulphate, and several other particulars, it stands in closer relation to zinc and cadmium. Magnesium, zinc, and cadmium are dyads; they all burn with flame when heated in air to a sufficiently high temperature, and each of these metals forms but a single oxide, chloride, and sulphide, as well as but a single class of salts with the radicles of the acids. V. Metals more or less analogous to iron; six in number. They are all dyads or biequivalent metals. These metals, when heated to dull redness, decompose the vapour of water if it be transmitted over them, and become converted into oxides, whilst hydrogen escapes: they are also soluble with effervescence and ́ evolution of hydrogen in diluted sulphuric or in hydrochloric acid. The protoxides of these metals are powerful bases: they have the general formula N". They yield hydrates, usually of the form N",H,, and lose their water readily when heated. These protoxides, with the exception of that of uranium, are dissolved more or less freely by ammonia, especially if chloride of ammonium be present in the solution. Each of the metals of this group forms a sesquioxide, which, excepting in the case of those 340 CLASSIFICATION OF THE METALS. 2 3 of cobalt and nickel, reacts with acids and forms corresponding salts they also form an oxide of the form NO,N,, corresponding with the magnetic oxide of iron (Fe,Fe¿Ð ̧). Several of the metals of this group-viz., iron, chromium, and manganese— form teroxides or even higher oxides, which are very soluble in water, and furnish powerful metallic acids. Hydrated sulphides of these metals are produced by the addition of a solution of sulphide of potassium or of ammonium to a solution of their salts; the precipitate so occasioned is insoluble in excess of the alkaline sulphide. The chromic salts, however, are precipitated as hydrated oxide of chromium, not as sulphide. Sulphuretted hydrogen gas, when transmitted through the solutions of these metals acidulated with sulphuric acid, occasions no precipitate of sulphide, excepting in the case of the salts of cadmium. Corresponding salts of the protoxides of this group are isomorphous and the salts formed by the sesquioxides with the same acid are likewise isomorphous with each other. Chromium and manganese also exhibit an isomorphous relation to the sulphur group, inasmuch as the corresponding sulphates, chromates, and manganates have the same crystalline form. In the case of manganese a singular connexion with the halogens is exhibited in the isomorphism of the permanganates with the corresponding perchlorates and periodates. VI. Metals which yield powerful acids when their higher oxides are combined with water; of these there are ten, as follow :-viz., The first four of these metals are biequivalent or dyads under certain circumstances, but tetrad or quadrequivalent in other more usual cases; the next three are sometimes tetrads, but often hexads, or equivalent to six atoms of hydrogen; whilst the last three, though usually triads, are occasionally, as in pentachloride of antimony, pentad or quinquequivalent. A close parallelism in properties exists between tin and titanium, corresponding compounds such as tinstone (Sn¤,) and rutile (Fi,) being isomorphous ; they each yield a liquid volatile perchloride, and in this particular, as well as in their powerful attraction for fluorine, they exhibit considerable analogy with silicon: columbium and tantalum also are similarly related to each other, CLASSIFICATION OF THE METALS. 341 and to silicon; they both furnish an anhydride with 2 atoms of oxygen, form a volatile tetrachloride, and yield definite compounds with fluorine. Molybdenum, vanadium, and tungsten, have likewise certain analogies with each other, but they are less strongly marked; and the properties of vanadium are not truly intermediate between those of molybdenum and tungsten. Protoxide of tin is a powerful base, but basic qualities are nearly wanting in the oxides of the other metals in this group. The metals included in this class decompose water when its vapour is driven over them at a red heat (with the exception of arsenic, which is more allied in character to phosphorus than to the metals), but they do not evolve hydrogen when treated with diluted sulphuric acid, owing to their want of basylous power. Many of them, owing to this tendency to form acids, decompose water with evolution of hydrogen in the presence of a powerful base, such as potash. The metallic acids formed by these metals are, with the exception of arsenic acid, nearly insoluble in water. The persulphides of this group of metals are soluble in the sulphides of the alkaline metals, and in many cases form crystallizable compounds with them. VII. The next group contains but three metals:—viz., 2. Lead 3. Thallium. 1. Copper They are not related to each other by any strong chemical resemblances; copper and lead exert no decomposing action upon water, even at a full red heat; all form powerfully basic oxides. Copper and lead exhibit a considerable tendency to the formation of subsalts ; they are not dissolved by either diluted sulphuric or hydrochloric acid; they are precipitated from acid solutions by sulphuretted hydrogen, and their sulphides do not combine with the sulphides of the alkaline metals. In the case of thallium the precipitation is incomplete; copper forms salts which are isomorphous with those derived from the protoxides of the metals in the iron group, and in the compounds which it forms with carbonic acid displays a close correspondence with magnesium and zinc, as well as with cobalt and nickel; and lead in some of its compounds is isomorphous with those of the alkaline earths, but in chemical properties it is more allied to mercury and silver. Thallium is uniequivalent, copper is so occasionally, as in the subchloride, but in the majority of cases it, like lead, is biequivalent. VIII. The last group consists of the noble metals, of which there are nine-viz., |