576

NITRATES OF MERCURY.

beautiful red precipitate which is formed, wash it carefully with water and dry it in the air. Place a portion of the dry red powder in a porcelain capsule; invert over the capsule a small glass funnel, and heat the capsule moderately upon a sand-bath; the iodide will melt, sublime, and finally be deposited upon the cold walls of the funnel in yellow crystals. On rubbing these crystals with a glass rod, their color will change again to red. Indeed the change of color often occurs of itself as the crystals cool, without friction. The composition of the iodide is neither changed by the sublimation nor by the friction; the change of color is due to a change of crystalline form-mercuric iodide being dimorphous, and exhibiting a red color in its octahedral form, and a yellow color when crystallized in rhombic prisms.

The change of coloration may be shown in another way, by dissolving some of the precipitated iodide in alcohol. The alcoholic solution is colorless and appears to contain the yellow modification of the iodide; on pouring it into water, iodide of mercury is precipitated as a yellow powder, which soon changes to red.

678. Sulphates of Mercury.-There is a sparingly soluble sulphate of dioxide of mercury (Hg,SO), a normal sulphate of the protoxide (HgSO), and a basic sulphate of the protoxide (of composition 3HgO,SO,). Normal mercuric sulphate may be prepared by dissolving metallic mercury in an excess of boiling concentrated sulphuric acid, and evaporating the solution to dryness. It is the material from which many other compounds of mercury are derived. It is decomposed by water; an insoluble trisulphate is thrown down, while but a small proportion of mercury remains dissolved in the dilute sulphuric acid which is formed.

There are at least

Both of the

679. Nitrates of Mercury are numerous. four nitrates of the dioxide, and as many of the protoxide namely the normal salts and three basic salts in either case. normal salts are soluble in water, and are commonly kept in the laboratory as examples of the mercury salts. The nitrate of the dioxide is prepared by digesting an excess of metallic mercury in cold moderately strong nitric acid. The solution should be kept in closed bottles containing a few globules of metallic mercury. The nitrate of the protoxide may be readily obtained by dissolving red oxide of mercury in an excess of nitric acid.

680. Amalgams.-Mercury unites with most of the other metals, forming alloys, many of which are pasty, or liquid when the proportion of mercury contained in them is large. These

alloys are commonly called amalgams, in contradistinction to the ordinary solid alloys of the other metals, in which mercury has no place. The liquid amalgams are true solutions of other metals, or of solid amalgams, in the fluid mercury. The so-called silvering of mirrors is an amalgam of tin.

Mercury may be detected in almost any soluble salt of the element by introducing into a solution of the salt a piece of clean copper.

Exp. 356.-Place a drop of a solution of either of the nitrates or chlorides of mercury upon a clean copper coin and rub the liquid over its surface. A white coating of metallic mercury will be deposited upon the metal.

681. Copper and mercury are classed together partly because of certain resemblances between the two metals, but also because neither of them falls naturally into either of the other groups of elements. They are alike in that they are not readily acted upon by air, excepting at high temperatures, that they do not decompose water at any temperature, and that they both form two salifiable oxides, and two chlorides of analogous composition. They are both acted upon in the same way by nitric and by sulphuric acids, the acid being reduced to a lower degree of oxidation, while the metal is dissolved, as has been seen in Exp. 96. As the formula of their compounds have doubtless already suggested, mercury and copper are univalent, like the alkali-metals, in the mercurous and cuprous compounds, but bivalent in the mercuric and cupric compounds.

CHAPTER XXXII.

TITANIUM-TIN.

TITANIUM.

682. This comparatively rare metal is found in several minerals, such as rutile and titaniferous iron, in the condition of titanic acid, TiO. None of its compounds are employed in the arts, and

the element itself is here mentioned mainly on account of the analogies which it bears to tin. Titanic acid is isomorphous with stannic acid (§ 685), and resembles it closely in its chemical deportment. Sesquioxide of titanium, Ti,O,, corresponds to sesquioxide of tin, Sn,O, (§ 685); and in the same way that the latter may be regarded as a stannate of tin, SnO,SnO,, the titanium compound may be considered a titanate of titanium, TiO, Tio,.

The bisulphide of titanium, and the bichloride, bibromide, and bifluoride, correspond in like manner to the tin compounds.

TIN.

683. Though by no means widely diffused in nature, and though ores of it occur in but few localities, tin is one of the metals which have longest been known to man. The fact admits, however, of ready explanation; for the specific gravity of the ores is high, and the metal is easily reduced from them by simple heating with charcoal. From the manner of the occurrence of many of these ores, in the beds of torrents, it is evident that their great weight would be likely to attract attention, and that their behavior towards fire would soon be noticed. The simplest possible metallurgical operation, and the one most likely to suggest itself to savage men, is the heating of a heavy stone in a wood fire.

The principal ore of tin is the binoxide, called tin-stone. In order to extract the metal from it, the tin-stone is mixed with powdered coal and heated upon the hearth of a reverberatory furnace in a reducing flame. The reduced metal melts readily, and is then run out of the furnace into iron moulds. Tin is a lustrous white metal, soft, malleable and ductile, though not very tenacious. Its ductility varies greatly with the temperature; at 100° the metal may be drawn into thin wire, but at 200° it is very brittle. When a bar of tin is bent, it emits a peculiar crackling sound, and if the bending be repeated, the metal becomes decidedly warm. These phenomena appear to depend on the disturbance of interlaced crystals contained in the bar, and upon the friction of these crystals one against the other. Tin always exhibits a great tendency to assume crystalline form in passing from the liquid to the solid condition. Upon this peculiarity is founded a method of ornamenting tinned iron.

Exp. 357.-Heat a piece of common tinned iron over the gas-lamp until the tin has melted, thrust the plate into cold water in order that the tin may harden quickly, then remove the smooth surface of the metal by rubbing it first with a bit of paper moistened with dilute aqua regia, and then with paper wet with soda-lye. By this treatment there will soon be laid bare a new surface covered with beautiful crystalline figures, like frost upon a window-pane. The plate should then be washed thoroughly with water, dried quickly, and covered with some transparent varnish.

The same crystalline structure can be brought out, though less conspicuously, by removing the outside polished surface of almost any piece of tin plate by means of warm dilute aqua regia, without first heating the plate as in this experiment.

Tin does not tarnish in the air at ordinary temperatures, no matter whether the air be moist or dry; but when strongly heated it oxidizes rapidly, and even burns with a brilliant white light. The specific gravity of tin is about 73; its atomic weight is 118. It melts at about 230°-at a lower temperature than any of the other common metals. At very high temperatures it is slightly volatile.

On account of its brilliant lustre, and its power of resisting atmospheric action, tin is largely employed for coating other metals, copper, for example, as in ordinary pins, cooking-vessels, and bath-tubs and iron, as in common sheet-tin, of which the so-called tin ware is manufactured.

Exp. 358. Thoroughly clean the surface of a copper coin, or of a small piece of sheet-copper, by means of dilute sulphuric acid; place the copper over the gas-lamp, and melt upon it a bit of tin as large as a pea. Rub the melted tin over the copper with a rag. It will not adhere to the copper; for although the latter was once carefully cleaned, it afterwards became coated with oxide of copper in such manner that the tin could not come in contact with the metal.

Repeat the experiment as before; but when the tin has melted, strew over the copper some finely powdered chloride of ammonium. On now rubbing the tin against the copper, the two metals will adhere firmly. The chlorine of the chloride of ammonium unites with the copper of the oxide of copper to form fusible, volatile chloride of copper, while ammonia and water are set free, as may be perceived by the odor. The excess of tin should be wiped off with a rag, so that a smooth surface may be left upon the coin,

When in contact with dilute acids, or with alkalies, tin slowly absorbs oxygen from the air and goes into solution. Of the strong acids, nitric acid acts upon it violently, with formation of insoluble hydrated binoxide of tin; a certain amount of water is decomposed as well as the nitric acid, in this reaction, and some nitrate of ammonium is formed; the ammonium comes from the union of the nascent hydrogen and nitrogen of the deoxidized water and nitric acid (see § 92). Hot concentrated chlorhydric acid gradually dissolves tin, and gives off hydrogen. Boiling concentrated sulphuric acid converts it into sulphate of tin, with evolution of sulphurous acid; but dilute sulphuric acid has no action upon it out of contact with the air. When heated with concentrated soda or potash lye, tin slowly dissolves, with formation of soluble stannate of sodium or of potassium, and evolution of hydrogen.

There are two prominent oxides of tin, a protoxide and a bioxide, besides intermediate oxides compounded of these two. The binoxide occurs, moreover, in combination, in different isomeric modifications.

684. Protoxide of Tin (SnO) may be obtained as a black powder by heating its hydrate in an atmosphere of carbonic acid or other inert gas. The hydrated protoxide is prepared by adding the solution of an alkaline carbonate to a solution of tin in chlorhydric acid; the hydrate is thrown down as an insoluble precipitate while carbonic acid escapes :—

=

2

SnCl, + Na,CO, + HO SnH2O2 + 2NaCl + CO,. The hydrate rapidly absorbs oxygen from the air when moist, but is tolerably permanent when dry. The anhydrous oxide undergoes no change in air at the ordinary temperature; when touched with a glowing coal, it takes fire and burns vividly, being converted into the binoxide. The hydrate also burns in the same way when lighted. It is remarkable that the anhydrous oxide is more readily soluble in acids than the hydrate; but in alkaline lyes only the hydrate is soluble. Most of the salts of protoxide of tin greedily absorb oxygen from the air and from many oxygenated substances. They are much employed as reducing agents.

Exp. 359.-To 5 or 6 c. c. of a solution of corrosive sublimate add a few drops of protochloride of tin, and heat the mixture; a gray