FIG. 148.

emitting white vapors of a garlic odor and shines in the dark, whence its name, phosphorus, bearer of light. It must be handled with caution, as the burns it produces are deep and difficult to heal. It is insoluble in water; partially scluble in ether, bu dissolves readily in carbonic disulphide and various oils. It melts at 44° C., forming a viscid, oily liquid. In warm weather phosphorus is flexible, and may be bent without breaking, but near the freezing-point of water it becomes brittle, exhibiting a cystalline fracture. On ac

count of its inflammability it is kept under water. It crystallizes from its solution, in forms of the monometric system (Fig. 148). Solutions of phosphorus, as well as the solid itself, are luminous in the dark. Phosphorus is a violent poison. The chief use of this substance is in the manufacture of friction-matches; and vast quantities are consumed in this way among all civilized nations.

369. Red Phosphorus.-This substance may be obtained by exposing ordinary phosphorus to sunlight, or heating it to near its boiling-point in an atmosphere free from oxygen. As thus prepared, red phosphorus, also known as amorphous phosphorus, is a red powder, of about 2.18 specific gravity, much less fusible than ordinary phosphorus, but reverting into the latter at about 260° C. It exhales no vapor or odor; oxidizes but very slowly in the air, does not change oxygen into ozone, is chemically indifferent, may be handled with impunity, or carried exposed in the pocket, and is not poisonous. Phosphorus forms three compounds with hydrogen, but only one is of importance to the ordinary student.

370. Hydric Phosphide (H,P) (Phosphuretted Hydrogen). This is a colorless gas, with a very offensive odor, like rotten fish. It is found in Nature, being produced in small quantities by the decay of animal matter, and ap

FIG. 149.

pears to be the cause of the will o' the wisp. It may be prepared by heating small fragments of phosphorus with a strong solution of caustic potash in a retort. The end of the retort-tube dips beneath water, and, as the gas passes out in bubbles, it rises to the surface and takes

fire

spontaneously.

If some pieces of cal

Wreaths of Flame.

cic phosphide are thrown into a glass of water, the same thing takes place. Double decomposition with the water produces hydric phosphide, which ignites at the surface and forms beautiful wreaths of vapor (Fig. 149). Pure hydric phosphide (H,P) is, however, not spontaneously inflammable, this property being due, in this case, to the admixture of a minute quantity of a liquid compound (HP). It is poisonous. In many cases the combining weights and the unit-volume weights are identical. But the atomic weight of phosphorus is 31; while the specific gravity of its vapor has been found to be 62.1. The volumetric composition of hydric phosphide will be readily understood by reference to the accompanying diagram :

371. Phosphoric Pentoxide (P,O,).-When phosphorus

FIG. 150.

Combustion of Phosphorus.

is burned in dry oxygen (Fig. 150), the dense, white vapors which are formed condense upon the glass in snow-like flakes. This is phosphoric pentoxide. It has a powerful attraction for moisture, absorbing it from the air, or, if brought into contact with water, seizing it with such violence as to emit a hissing

sound. By the union of phosphoric pentoxide with water, there are formed three distinct acids: meta-phosphoric acid (HPO,), pyro-phosphoric acid (HP,O,), and orthophosphoric acid (H,PO).

§ 3. Arsenic and its Compounds.

ARSENIC.-Symbol, As. Atomic Weight, 75; Quantivalence, III. and V. Molecular Weight, 300; Molecular Volume, 2; Specific Gravity, 5.79.

372. Arsenic. This element is found native, but most of the arsenic of commerce is obtained by the decomposition of the arsenides. The compounds are heated in retorts of earthen-ware; the arsenic sublimes and collects in iron tubes and earthen receivers.

Arsenic is a brittle metal, of a steel-gray, or nearly white color. The coarse, gray powder, sold under the name of "fly-poison," or "cobalt," is an impure arsenic. When arsenic is heated in a close vessel to 356° F., it volatilizes without fusion, giving off a dense, colorless vapor, having the peculiar odor of garlic, and corresponding to the formula As,. If heated in the open air it takes fire, burning with a blue flame, with formation of arsenic trioxide. It is highly poisonous.

373. Hydric Arsenide (H,As.) (Arseniuretted Hy

FIG. 151.

drogen). This gas may be formed by decomposing an alloy of arsenic and iron with dilute sulphuric acid, or by introducing a solution of arsenic into a flask in which hydrogen is being evolved. It burns with a bluish-white flame, is highly poisonous, and of a disgusting odor. The formation of this gas is used for the detection of arsenic by Marsh's test. Fig. 151 shows the form of an apparatus which answers, in a rough way, very well for this purpose. Bits of zinc and a little water are placed in the vessel, which is provided with a cork through which a tube is inserted. Sulphuric acid is now poured in through the funnel-tube, and the evolution of hydrogen commences. After the air has been completely expelled from the flask, the gas may be lighted at the jet. If the solution containing arsenic be now poured in through the funnel-tube, the color of the flame immediately changes, and a cold, white surface, held so as to cut the flame in half, is stained with a black or brown spot by the deposition of metallic arsenic. Antimony produces a similar effect, but a solution of calcic or sodic hypochlorite dissolves the arsenical stain, leaving that made by antimony unchanged. This is a very delicate test, but great care should be taken that the sulphuric acid and zinc do not contain any previous traces of arsenic. It is estimated that of a grain of arsenious oxide in one hundred grain measures of the solution may be detected by this test.

Marsh's Test.

374. Arsenic Trioxide (As,O,) (Arsenious Acid).This compound occurs native, but is also prepared on a large scale by roasting certain ferric arsenides, and other arsenical ores. Thus obtained, it constitutes the wellknown white arsenic or ratsbane of commerce, a white

solid body, capable of existing in three isomeric forms. It is soluble in about ten parts of hot water, the solution having a slightly sweetish taste, and acid reaction. It also dissolves readily in hot hydric chloride, and in solutions of the alkaline arsenites. It is used in dyeing and calico-printing, in glass-making, and for the preparation of arsenical soap, which is employed for preserving stuffed animals. Though a violent, corrosive poison, it is used in medicine; its most effectual antidotes are the moist hydrated ferric oxide and caustic magnesia. Ortho-arsenic acid (H,ASO,) is formed by oxidizing arsenious oxide by means of nitric acid. It has strongly acid properties, decomposing the carbonates with effervescence. Arsenic disulphide (As,S,) has been known since very remote times, and is found native as realgar, a mineral crystallizing in translucent, oblique, rhombic prisms, of beautiful orange-yellow, or ruby-red, color. It is produced artificially, is used as a pigment, and in the preparation of the pyrotechnical mixture known as "Bengal whitelight." Arsenic trisulphide (As,S,), familiarly known as "orpiment," is found native, but is also prepared artificially. It is a bright-yellow substance, and is used in dyeing to reduce indigo, and also in the preparation of "rusma," a paste employed to remove the hair from skins.

§ 4. Antimony and Bismuth.

ANTIMONY.-Symbol, Sb. (Stibium). Atomic Weight, 122; Quantivalence, III. and V.; Molecular Weight, 488 (?); Molecular Volume, 2; Spe. cific Gravity, 6.78.

375. Antimony is found native, though most of the antimony of commerce is obtained from the trisulphide (Sb,S,). It exists in two modifications; ordinarily it is a brittle, brilliant, bluish-white metal, crystallizing in rhombohedrons. The other form is obtained by electrolysis, and