change of volume generally takes place-that is, the sum of the volumes of the reacting substances is almost equal to the sum of the resultant substances.

6. Hence it is impossible to judge the volume of the component substances from the volume of a compound, although it is possible to do so from the product of substitution. 7. The replacement of H, by sodium, Na, and by barium, Ba, as well as the replacement of SO, by Cl, scarcely changes the volume, but the volume increases with the replacement of Na by K, and decreases with the replacement of H by Li, Cu, and Mg.

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8. There is no foundation for comparing volumes in a solid and liquid state at the socalled corresponding temperatures-that is, at temperatures at which the vapour tension is equal in each case. The comparison of volumes at the ordinary temperature is sufficient for finding a regularity in the relations of volumes (this deduction was developed with particular detail by me in 1856).

9. Many (Perseau, Schröder, Löwig, Playfair and Joule, Baudrimont, Einhardt) have sought in vain for a multiple proportion in the specific volumes of solids and liquids.

10. The truth of the above is seen very clearly in comparing the volumes of polymeric substances. The volumes of their molecules are equal in a state of vapour, but are very different in a solid and liquid state, as is seen from the close resemblance of the specific gravities of polymeric substances. But as a rule the most complex polymerides are denser than the simpler.

11. We already know that the oxides of light metals have a less volume than the metals, whilst that of magnesium hydroxide is considerably greater, which is explained by the stability of the former and instability of the latter. In proof of this we may cite that the volume of barium (36) is greater than that of its stable hydroxide (sp. gr. 4'5,. vol. 30) as well as of those of the true alkalis. The volumes of the salts of magnesium and calcium are greater than the volume of the metal, with the single exception of the fluoride of calcium. With the heavy metals the volume of the compound is always greater than the volume of the metal, and moreover, for such compounds as silver iodide, AgI (d = 5·7), and mercuric iodide, HgI, (d=6°2, and the volumes of the compounds 41 and 73), the volume of the compound is greater than the sum of the volumes of the component elements. Thus the sum of the volumes Ag+1=36, and the volume of AgI=41. This stands out with particular clearness on comparing the volumes K + I = 71 with the volume of KI, which is equal to 54, because its density = 3:06.

12. In such kinds of combinations, between solids and liquids, as are solutions, alloys, isomorphous mixtures, and similar feeble chemical compounds, the sum of the reacting substances is always very nearly that of the resulting substance, but here the volume is either slightly larger or smaller than the original; speaking generally, the amount of contraction depends on the force of affinity acting between the combining substances.

CHAPTER XVI

ZINC, CADMIUM, AND MERCURY

THESE three metals give, like magnesium, oxides RO, which form feebly energetic bases, and like magnesium they are volatile. The volatility increases with the atomic weight. Magnesium can be distilled at a white heat, zinc at a temperature of about 930°, cadmium about 770°, and mercury about 360°. Their oxides, RO, are more easily reducible than magnesia, and mercuric oxide is the most easily reducible. The properties of their salts, RX, are very similar to the properties of MgX. Their solubility, power of forming double and basic salts, and many other qualities are in many respects identical with those of MgX. The greater or less ease with which they are oxidised, the instability of their compounds, the density of the metals and their compounds, their scarcity in nature, and many other properties, gradually change with the increase of atomic weight, as might be expected from the law of periodicity. Their principal characteristics, compared with magnesium, already find expression in the fact that zine, cadmium, and mercury are heavy metals.

Zine stands nearest to magnesium in atomic weight and in properties. Thus zinc sulphate, or white vitriol, easily crystallises with seven molecules of water, ZnSO4,7H2O. It is isomorphous with Epsom salts, and parts with difficulty with the last molecule of water; it forms double salts-for instance, ZnK2(SO4)2,6H,O, exactly as magnesium sulphate does.1 Zinc oxide, ZnO, is a white powder, almost in

1 Zinc sulphate is often obtained as a bye-product-for instance, in the action of galvanic batteries containing zinc and sulphuric acid. When the anhydrous salt is heated it forms zinc oxide, sulphurous anhydride, and oxygen. The solubility in 100 parts of water at 0=43, 20°=53, 40°=634, 60° = 74, 80° 844, 100=95 parts of anhydrous zine sulphate that is to say, it is nearly expressed by the formula 43 +0:52t.

An admixture of iron is often found in ordinary sulphate of zinc in the form of ferrous sulphate, FeSO4, isomorphous with the zinc sulphate. In order to separate it, chlorine is passed through the solution of the impure salt (the ferrous salt then passes into ferric), the solution is then boiled, and zinc oxide is afterwards added, which, after some time has elapsed, precipitates all the ferric oxide. Ferric oxide of the form RO3 is displaced by zinc oxide of the form RO.

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soluble in water, like magnesia, from which, however, it is distinguished by its solubility in solutions of sodium and potassium hydroxides.3 Zinc chloride is decomposed by water, combines with ammonium chloride, potassium chloride, &c., just like magnesium chloride. Altogether the resemblance between zinc and magnesium compounds-particularly when

? Zinc oxide is obtained both by the combustion and oxidation of zinc, and also by the ignition of some of its salts-for instance, those of carbonic and nitric acids; it is likewise precipitated by alkalis from a solution of ZnX, in the form of a gelatinous hydroxide. The oxide produced by roasting zinc blende (by burning in the air when the sulphur is converted into sulphurous anhydride) contains various impurities. For purification, the oxide is mixed with water, and the sulphurous anhydride formed by roasting the blende is passed through it. Zine bisulphite, ZnSO3,H,SO,, then passes into solution. If a solution of this salt be evaporated, and the residue ignited, zinc oxide, free from many of its impurities, will remain.

Zine oxide is a light white powder, used as a paint instead of white lead; the basic salt, corresponding with magnesia alba, is used for the same purpose (Vol. I. p. 592).

3 For the solution of one part of the oxide 55400 parts of water are required. Nevertheless, even in such a weak solution, zinc oxide (hydroxide, ZnH2O2) changes the colour of red litmus paper. Zinc oxide is obtained in the wet way by adding an alkali hydroxide to a solution of a zinc salt-for instance: ZnSO4+2HKO = K2SO,+ZnHO. The gelatinous precipitate of zinc hydroxide is soluble in an excess of alkali, which clearly distinguishes it from magnesia. This solubility of zinc hydroxide in alkalis is due to the power of zinc oxide to form a compound, although an unstable one, with alkalis-that is to say, points to the fact that zinc oxide already partly belongs to the intermediate oxides. The oxides of the metals above-mentioned do not show this property. The property which metallic zinc itself has of dissolving in caustic alkali with the disengagement of hydrogen (the solution is facilitated by contact with platinum or iron) depends on the formation of such a compound of the oxides of zinc and the alkali metals. The solution of zinc hydrox ide, ZnH2O, in potash (in a strong solution), proceeds when these hydrates are taken in proportion to ZnH2O2+KHO. If such a solution be evaporated to dryness, water extracts only caustic potash from the fused residue. When a solution of zine hydroxide in strong alkali is mixed with a large mass of water, nearly all the oxide of zinc is precipitated; and, therefore, in weak solutions, a large quantity of the alkali is required to effect solution, which points to the decomposition of the zinc-alkali compounds by water. If strong alcohol be added to a solution of zinc oxide in sodium hydroxide, the crystallohydrate, 2Zn(OH)(ONa),7H2O, separates.

• Zinc chloride, ZnCl2, is generally used in the arts in the form of a solution obtained by dissolving zinc in hydrochloric acid. This solution is used for soldering metals. The reason why it is thus employed is understood from its properties. When evaporated the combination with the water of crystallisation first separates, which, however, on being further heated, loses all traces of water, and forms an oily mass of anhydrous salt which solidifies on cooling. This substance melts at 250°, and commences to volatilise at about 400°. The soldering of metals-that is, the introduction of an easily-fusible metal between two contiguous metallic objects-is hindered by any film of oxide upon them; and, as heated metals easily oxidise, they are therefore difficult to solder. Zinc chloride is used to prevent the oxidation. It fuses on being heated, and, covering the metal with an oily coating, prevents contact with the air; but even if any oxide has formed, the free hydrochloric acid generally existing in the zine chloride solution dissolves the oxide formed, and in this way the metallic surface of the metals to be soldered is preserved fit for the adhesion of the liquid solder, which, on cooling, binds the objects together. Much zinc chloride is used also for steeping wood (telegraph-posts and sleepers on railways) in order to preserve them from decaying quickly; this preservative action is in all probability mainly due to the poisonous character of zinc salts (corrosive sublimate is still

both are in solution-is so great that it exceeds the resemblance which exists between magnesium and calcium compounds.

Zinc, like many heavy metals, is often found in nature in combination with sulphur, forming the so-called zinc blende, ZnS. It sometimes exists in large masses, often crystallised in cubes; it is frequently translucent, and has a metallic lustre, although this is not so clearly developed as in many other metallic sulphides with which we shall here

more poisonous, and a still better agent to preserve wood from decay), because decay is brought about by the action of lower organisms.

The specific gravity of solutions (see my Treatise,' p. 50) containing p per cent. of zinc chloride, ZnCl,:

The last line shows the change of specific gravity for 1° in ten-thousandth parts for temperatures near 15. More accurate determinations of Cheltzoff, personally communicated by him, led him to conclude that solutions of zinc chloride follow the same laws as the solutions of sulphuric acid, which will be treated in Chapter XX.: (1) from H2O to ZnCl,,120H ̧O 8=S1+92·85p+0·1748p2; (2) from thence to ZnCl2,40H ̧O 8=S1, +93′96p -00126p2; (3) from thence to ZnCl2,25HO 8=114815+96:45 (p-1589) +0:4567 (p-15-89); (4) from thence to ZnCl2,10H2O s=122121+10482 (p=23·21)+0·7992 (p-23-21); (5) from thence to p=65 p.c. s=146063+14096 (p-43′05) +1·4905 (p-4305), where s is the specific gravity of the solution at 15, containing p p.c. of ZnCl, by weight, if the water at 4° =10000, and where S,,=9991 6 (specific gravity of water at 15°). The compound of zinc chloride with hydrochloric acid has been mentioned in Vol. I. page 450.

Zinc chloride has a great affinity for water; it is not only soluble in it, but in alcohol, and on being dissolved in water becomes considerably heated, like magnesium and calcium chlorides. Zine chloride is capable of taking up water, not only in a free state, but also in chemical combination with many substances. Thus, for instance, it is used in organic researches, for removing the elements of water from many of the organic compounds.

When mixed with zinc oxide it forms with remarkable ease a very hard mass of zinc oxychloride, which is applied in the arts; for instance, in painting, to resist the action of water, or for cementing such objects as are destined to remain in water. Zinc oxychloride, ZnCl2,3ZnO,2H ̧O( = Zn ̧OCl2,2ZnH2O) is also formed from a solution of zinc chloride by the action of a small quantity of ammonia on it after heating the precipitate obtained with the liquid for a considerable time; the admixture of ammonium salts with a mixture of a strong solution of zinc chloride with its oxide makes a similar mass, which does not solidify so rapidly, and is therefore more useful for similar purposes. Moisture and cold do not change the hardened mass of oxychloride, and it also resists the action of many acids, and a heat of 3002, which makes it a useful cement for many purposes. A solution of magnesium chloride with magnesium oxide forms a similar oxychloride. The mass solidifies best when there is an even quantity by weight of zinc in the chloride and oxide, and, therefore, when it has the composition ZnOCI. In preparing such a cement, naturally zinc oxide alone may be taken, and the requisite quantity of hydrochloric acid added to it.

5 This mineral has been given the name of 'mock-ore,' on account of its having the appearance (considerable density, 4'06, &c.) of ordinary metallic ores; it deceived the first miners, because it did not, like other ores, give metal when simply roasted in air and fused with charcoal. The white zinc oxide, formed by burning the vapours of zinc, was also called 'nihil album,' or 'white nothing,' on account of its lightness.

after become acquainted. The ores of zinc also comprise the carbonate, calamine, and silicate, siliceous calamine.

Metallic zinc (spelter) is most frequently obtained from the ores containing the carbonate"-that is, from calamine, which is sometimes found in thick veins for instance, in Poland, Galicia, in some places on the banks of the Rhine, and in considerable masses in Belgium and England. In Russia beds of zinc ore are met with in Poland and the Caucasus; but the output is small. In Sweden, as early as the fifteenth century, the Belgian calamine was worked up into an alloy of zinc and copper (brass), and Paracelsus produced zinc from calamine; but the technical production of the metal itself, long ago practised in China, only commenced in Europe in 1807 in Belgium, when the Abbé Donnet discovered that zinc was volatile.

From that time the production increased until it is now about 140 million kilograms in Germany alone.

The reduction of metallic zinc from its ores is based on the fact that zinc oxide7 is easily reduced by charcoal at a red heat: ZnO + C = Zn+CO. The zinc thus obtained is in a finely-divided state and impure, being mixed with other metals reduced with it. The refining depends on the zinc being converted into vapour at a white heat, from which it easily passes into a liquid or solid state. The distillation is carried on in earthenware retorts, filled with a mixture of the divided ore and charcoal. The vapours of zinc and gases formed during the reaction escape by means of a pipe leading downwards, and are led to a space where the vapours are

* Let us mention here that by the word ore is meant a hard, heavy substance dug out of the earth, which is used in metallurgical works for obtaining the usual heavy metals long known and used. The natural compounds of sodium, or magnesium, are not called ores, because magnesium and sodium have not been long ago obtained in works. The heavy metals, those which are easily reduced and do not easily oxidise, are exclusively those which are directly applied and obtained in works. Ores either contain the metals themselves (for instance, ores of silver or bismuth), and the metals are then in a native state, or else their sulphur compounds (blende, mock-ore, pyrites-as, for instance, galena, PbS; zinc blende, ZnS; copper pyrites, CuFeS) or oxides (as the ores of iron), or salts (calamine, for instance). Zinc is incomparably rarer than magnesium, and is only well known because it is transformed from its ores into a metal which finds direct use in many branches of industry.

7 Ores, when extracted from the earth by the miners, are often enriched by sorting, washing, and other mechanical operations. The sulphurous ores (and likewise others) are then generally roasted. Roasting an ore means heating it to a red heat in air. The