sistence of the oil or fat. The compounds containing a large proportion of stearin and palmitin, like tallow, form hard soaps, while those in which olein predominates, as the soft fats and oils, produce soap of softer consistence. The glycerin which is retained in soft soap also adds to its fluidity. Soap has a powerful affinity for water, and may retain from 50 to 60 per cent. of it and still continue solid; hence dealers frequently keep it in damp places where it will absorb moisture. It is soluble in fresh water, but, with the exception of cocoa-soap, is insoluble in salt-water.

338. Mode in which Soap acts in Cleansing.-As water, having no affinity for oily substances, will not dissolve them, of course it cannot alone remove them from surfaces to which they may adhere. The oily matters which are constantly exuding from the glands of the skin, uniting with the outer dust, form a film over the body. The alkali of the soap acts upon the oil during ablution, partially saponifies it, and renders the unctuous compound freely miscible with water, so as to be easily removed. The cuticle or outer layer of the skin is chiefly composed of albumen, which is soluble in the alkalies. The alkali of the soap, therefore, dissolves off a portion of the cuticle with the dirt; every washing with soap thus removing the old face of the scarf-skin and leaving a new one in its place. The action of soap in cleansing textile fabrics is of a similar nature. Alkalies not only act upon greasy matter, but, as is well known, dissolve all organic substances. In the case of soap, however, the solvent power of the alkali is in part neutralized, thus preserving both the texture and color of the fabric exposed to its action. The oily nature of the soap also increases the pliancy of the articles washed.

§ 3. Lithium, Rubidium, Casium.

339. History.-These three rare elements are found associated with potassium and sodium, to which they are closely allied in all their chemical relations. Lithium is a

brilliant, silver-white metal, softer than lead, remarkably light, having a specific gravity of 0.578. We have already referred to its distribution (201).

Rubidium, also a soft, white metal, was discovered by means of the spectroscope. Its spectrum contains two dark-red lines; hence its name, from the Latin rubidus, meaning dark-red. Rubidium has been found in the ashes of many plants, in mineral water, in tobacco-leaves, in coffee, tea, cocoa, and crude tartar.

Casium was discovered at the same time with rubidium. The name comes from cæsius, sky-blue, and has reference to two blue lines in the spectrum.

CHAPTER XIV.

SILVER.-GOLD.-BORON.

§ 1. Silver and its Compounds.

SILVER.-Symbol, Ag. (Argentum). Atomic Weight, 108; Quantivalence, I. and III.; Specific Gravity, 10.5.

Silver is obtained from its some of which the ore is

340. Metallic Silver (Molecule) (Ag-Ag).-This wellknown metal is frequently found native in fibrous crystalline masses. It is also found in combination with chlorine, sulphur, arsenic, or antimony. The principal silver-mines are those of Mexico and Peru. ores by various processes, in roasted with common salt, by which argentic chloride is formed; then, together with water, iron-scraps, and mercury, put into casks which revolve on their axes. The iron removes the chlorine, and the mercury amalgamates with the silver, from which it is afterward freed by distillation (III.).

From plumbic sulphide, which is by far the most important source of silver, it is obtained by first smelting for lead, and then volatilizing the lead by cupellation, a pe

culiar process, conducted in a furnace, the shallow, basinshaped bottom of which is covered with a thick layer of bone, ashes, marl, or some other porous, infusible material. When the lead, alloyed with a small quantity of silver, is melted on this hearth, in a current of air, most of the lead oxidizes; the oxide or litharge melts, and, being absorbed by the cupel, leaves the silver pure.

341. Properties and Uses.-Silver is the whitest of the metals, with a bright, metallic lustre. It is very malleable, ductile, and tenacious. It may be extended into leaves not exceeding 1000 of an inch, or 0% of a millimetre, in thickness; and one grain, or of a gramme, may be drawn out into 400 feet, or 122 metres of wire. Silver does not oxidize in the air at any temperature, but absorbs oxygen when melted, holding it mechanically, and giving it off on solidifying. It is a good conductor of heat and electricity, and its polished surface is one of the best reflectors of light. Silver is chiefly consumed in the manufacture of alloys used for coinage and silver-plate. Being too soft, pure silver is never employed for these purposes.

342. Argentic Chloride (AgCl.) (Chloride of Silver) is occasionally found native in mines, and is called horn silver, from its tough, horny texture. It may be prepared artificially by adding a solution of common salt to a solution of argentic nitrate, and appcars as a white powder which darkens in color on exposure to light. Argentic bromide (AgBr) and argentic iodide (AgI) resemble the previous compound, and are found in a similar way.

343. Argentic Monoxide (Ag,O) (Silver Oxide).—This substance is best prepared by mixing concentrated hot solutions of argentic nitrate and potassic hydrate. It is a black or bluish-black, heavy powder, slightly soluble in water. It is also formed when silver is heated in the air, but at a red heat, or even lower temperatures, it is completely decomposed, with formation of metallic silver and oxygen gas.

344. Argentic Nitrate (AgNO,) (Lunar Caustic).— This interesting salt may be obtained by dissolving metallic silver in nitric acid; colorless, anhydrous crystals being formed, which are readily soluble in an equal weight of cold water. These crystals, when melted and cast into small sticks, form the lunar caustic of surgery. Argentic nitrate solution is decomposed by organic matter, with separation of black, finely-divided metallic silver, the reac tion being most rapid when taking place in the light. Advantage is taken of this property in making indelible ink and hair-dye. A solution of potassic cyanide removes the stain thus produced.

§ 2. Gold.

Symbol, Au. (Aurum). Atomic Weight, 196.6; Quantivalence, I. and III.; Specific Gravity, 19.34.

345. Gold (molecular symbol of metal probably [Au Au]). -This is one of the most widely diffused of the metals, and generally occurs in minute grains, though sometimes in masses weighing many pounds. In 1851 a lump weighing 106 pounds was found in Australia, embedded in a matrix

FIG. 140.

of quartz. It sometimes occurs in crystalline form, as shown in Fig. 140. As found in Nature, gold is rarely pure, but generally mixed with a variable quantity of silver. Gold is separated from all the constituents of its cres except silver, by amalgamation with mercury. It is obtained from silver by boiling the alloy in nitric acid, which dissolves out the silver, leaving the gold pure. In this operation, in order to prevent the silver from being mechanically protected from the action of the acid, it is necessary that there should be three times as much silver

Crystal of Gold.

as gold. As the gold constitutes only one quarter of the mass, the process is known as quartation.

346. Properties.-Gold is the only metal of a yellow color, it has a brilliant lustre, and high specific gravity. It is the most malleable of the metals, and to its ductility there is scarcely a limit; when pure it is nearly as soft as lead. It fuses at about 1200 C., and does not oxidize in the air at any temperature. Gold is not dissolved by any single acid, but is acted upon by chlorine, or any solution which liberates the gas. Its usual solvent is a mixture of four parts of hydric chloride and one of hydric nitrate, called, on account of its power to dissolve gold, aqua regia. The compounds of gold have little chemical interest to the ordinary student. AuCl, is somewhat used in the laboratory, and a solution of auric cyanide in potassic cyanide is used in electro-gilding.

§ 3. Boron and its Compounds.

BORON. -Symbol, B. Atomic Weight, II.; Quantivalence, III.; Molecular Weight, 22 (?); Molecular Volume, 2; Specific Gravity, 2.68.

347. Boron. This substance is not found native, but may be prepared by decomposing fused boric oxide (B,O,) with sodium or aluminium. It exists in several modifications, being either brown, amorphous, and slightly soluble in water, or crystalline, and entirely insoluble. One of the two crystalline modifications known is exceedingly hard and brilliant, resembling the diamond in this respect.

348. Boric Acid, HBO,.-This acid is found as a natural constituent of hot mineral springs, the principal locality being the "lagoors" of the volcanic district of Tuscany, where the acid issues from the earth with jets of steam, and is collected by throwing the jets into water. The acid is afterward separated from the water by evaporation in leaden pans so arranged that they are heated by the vapors as they escape from the earth. It is