nitrate, 117 parts concentrated sulphuric acid, and 30 parts of water, are placed in a glass retort which is supplied with a receiver B, kept cool by cold water flowing over it from the tube i, by means of a netting (Fig. 145). With the application of heat, the nitrate is decomposed, and the acid distils over into the receiver. The change is thus shown:

NaNO, + H2SO, = NaHSO,+HNO,.

4

That is, one molecule of sodic nitrate and one of sulphuric acid furnish one molecule of nitric acid and one of hydrosodic sulphate.

360. Properties.-Nitric acid is a colorless, mobile liquid, of 1.52 specific gravity, fuming in contact with the air, and possessed of an intensely sour taste and peculiar sweetish-nauseous pungent smell. It becomes solid at very low temperatures, and boils at 86° C., with partial decomposition. It is highly corrosive, and stains of a yellow color the skin, nails, and many other animal substances; it is therefore used to produce yellow patterns upon woolen fabrics. It is also employed for etching on copper, for assaying or testing metals, and, by dyers and calicoprinters, as a solvent for tin. In consequence of its large proportion of oxygen, it corrodes the metals with great energy, and hence is the most powerful of oxidizing agents. It ignites powdered charcoal and oil of turpentine, and oxidizes phosphorus so rapidly as to produce an explosion.

361. A mixture of chlorohydric acid with nitric acid constitutes the aqua regia, or royal water, of the alchemists, so named from the power it possesses of dissolving gold, the "king of metals," a property due to the presence of chlorine, which, at the moment of its formation, attacks metals with great energy. The proportions for the mixtures are four measures of hydric chloride to one of nitric acid.

362. Ammonic Chloride (NH,Cl) (Sal-ammoniac).-This substance is found native in many volcanic regions, in the vicinity of burning coal-mines, and in guano-deposits.

FIG. 146.

Formation of Ammonic Chloride.

When hydric chloride and ammonia are brought together, they form dense white clouds of sal-ammoniac, as may be seen in Fig. 146. The reaction is expressed thus:

When a solution of ammonic hydrate is neutralized by hydric chloride, crystals of ammonic chloride are produced, which have a sharp taste, and dissolve in thrice their weight of cold water. Sal-ammoniac is chiefly obtained by neutralizing the ammoniacal liquor of the gas-works by hydric chloride. On evaporating the resulting solution, the salt appears in the form of the tough, fibrous crystals of commerce. It is volatilized by heat. Mixed with lime, which decomposes it and expels the ammonia, it is used in smelling-bottles.

363. Ammonic Hydrate (NH ̧O) (Aqua Ammonia).—This compound is prepared by passing ammonia gas (NH,) into water, which absorbs it rapidly to the extent of 700 times its own volume. The gas is evolved by gently heat

FIG. 147.

ing a mixture of slacked lime and sal-ammoniac, and passing it through a series of bottles. In making solutions of the absorbable gases several difficulties have to be guarded against. The action in the evolution-flask is liable to various interruptions, while the water present in the apparatus rapidly absorbs the gas. This creates a partial vacuum, and the consequence is, that the water in the jar flows back into the flask, thus putting an end to the process; also, if the gas is generated fast

Woulfe's Bottles.

er than it is absorbed, there arises the danger of an explosion, unless there is a free outlet to the apparatus. These dangers are obviated by the arrangement known as Woulfe's bottles (Fig. 147).

The flask in which the gas is generated is provided with a safety tube which serves both as a means of introducing a liquid and as a protection against the above-mentioned accidents. When the liquid is poured in, a portion of it is retained in the bend of the tube, acting there as a valve to prevent the access of air to the flask. Each bottle has an upright tube in the middle neck which acts as a safetytube, allowing the air in case of a vacuum to pass in, or the liquid in flow out, if the pressure of the gas becomes too great. The other tubes serve to connect the bottles

with the flask and with each other.

364. Properties.-Ammonic hydrate is a colorless, transparent liquid, of 0.85 specific gravity, with the pungent odor of ammonia, and a sharp, burning taste. As usually met with, it is rarely a definite compound, being liable to contain either more water or gas than the above formula implies. A saturated solution freezes between -38° and

-41° C., forming shining, flexible, needle-shaped crystals. It boils at 130° F., but is, at the same time, decomposed with evolution of ammonia. The graphic formula of ammonic hydrate is (H) N-O-H. The single atom of hydrogen which is linked to the one oxygen-atom is replaceable by negative radicles; the resulting compounds are termed ammoniacal salts. These salts are remarkable as being isomorphous with certain potassic compounds, and when the formulas of any two of these isomorphous salts are compared, it is found that the atomic group HN exactly corresponds in function to the radical potassium. Thus in comparing the formulas of the two well-known isomorphous salts

Potash-alumn K
Ammonia-alum HN

AIS,O. + 12H,O, and
AIS,O,+ 12H2O,

we readily observe this analogy. On account of this remarkable fact, it has been assumed that the atomic group, HN, or its double molecule (H,N), must be very similar in character to potassium, and possessed of metallic properties. This radical has been named ammonium.

.

365. Ammonic Nitrate, (H,N) NO,, is formed in small quantities during thunder-storms, and is sometimes contained in rain-water. Ammonic sulphate, (H,N), SO,, is a valuable fertilizer. Several ammonic carbonates are known, and the commercial ammonic carbonate, when purified, constitutes the volatile salts, or smelling-salts, of the shops.

§ 2. Phosphorus and its Compounds.

PHOSPHORUS.-Symbol, P. Atomic Weight, 31; Molecular Weight, 124; Molecular Volume, 2; Quantivalence, I., III., V.; Specific Gravity,

1.82.

366. Distribution.-Phosphorus is found in Nature chiefly in combination with calcium. It is a never-failing constituent of the plants used by man and the domestic animals. It is an equally important ingredient of animal

skeletons, which owe their strength to calcic and magnesic phosphates, while it also exists in other combinations in the blood, flesh, milk, and other tissues, and secretions of animals. Phosphorus exists in several allotropic states. Ordinarily, it is a white solid, with a faint-yellow tinge, and almost transparent; when exposed to light under water, it gradually becomes white, opaque, and scaly. Exposed to direct sunlight under water, phosphorus becomes covered with a red coating, and the same modification is formed when ordinary phosphorus is heated to a temperature below 250° C., in a gas which has no action upon it. None of the modifications are found native.

387. Ordinary Phosphorus (P).-This interesting body was discovered in 1669 by Brandt, who obtained it by distilling the residue of evaporated urine with charcoal. Most of the phosphorus of commerce is obtained by the decomposition of the bones of animals, which consist largely of calcic phosphate (Ca,P,O,). The bones are first burned, and, the organic matter being consumed, they are reduced to powder and soaked in concentrated sulphuric acid. This decomposes the phosphate, with formation of insoluble calcic sulphate, and soluble acid calcic phosphate (CaH ̧P ̧0 ̧). The solution of this compound, after being separated from the sulphate, evaporated to sirupy consistence, mixed with charcoal, and heated in an iron pot, is distilled at a brightred heat. The carbon unites with the oxygen, liberating the phosphorus, which rises in vapor, and is condensed in water in the shape of yellow drops. These are melted under water and forced into tubes, thus forming the ordinary stick-phosphorus.

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368. Properties.-Phosphorus is a soft, colorless, halftransparent, waxy solid, so extremely inflammable that it takes fire in the open air by the heat of the slightest friction, and burns with great violence, emitting a brilliant flame, and dense, white fumes of phosphoric pentoxide. If quietly exposed to the air, it undergoes slow oxidation,