radicle, to an acid (hydric salt). Water combines directly with many substances, especially those which are crystallizable from aqueous solutions. Thus held in combination, it is termed water of crystallization. From this, it appears that the radicle oxygen contained in water is capable of performing a tetradic linking function, which view may be graphically expressed by assigning to it the formula H-O-H, instead of H-O-H.

IV

401. Hydric Dioxide (H,O,) is a transparent, colorless, syrupy liquid, of 1.452 specific gravity, which does not solidify at -30° C., and may be evaporated at low temperatures in a vacuum. It has an astringent taste, a decided odor, and possesses active bleaching properties. It is a very unstable compound, decomposing slowly at 15° C., while higher temperatures, or the contact of various substances, causes it to separate into water and oxygen with explosive violence. It may be regarded as free hydroxyl (HO), composed of two atoms of a compound monad radical.

§ 2. The Atmosphere.

402. Its Composition. It was not until the year 1774 that Lavoisier pointed out the true composition of the atmosphere. Up to this time it was spoken of as one of the four elements; but the careful observations of Priestley and Scheele, and their discovery of oxygen gas, prepared the way for a knowledge of its exact composition. It is now regarded as a mixture of several gases, nitrogen and oxygen constituting its bulk-the one incapable of sup porting combustion or respiration, and the other essential to life.

That the air is made up of these gases may be ascertained both by analysis and synthesis. That it is a mixture and not a chemical compound is made manifest by the facts that its components are not united in the ratio of their atomic weights, and that each gas dissolves in water, independently of the other; but the analyses of air collected from different parts of the earth, and at different heights, show a remarkable uniformity in its composition. In addition to the oxygen and nitrogen present in the atmosphere, there is always a small proportion of aqueous vopor, carbonic dioxide, and ammonia.

403. The proportion of watery vapor in the atmosphere varies with the temperature. It usually ranges from the

to the of the bulk of the air. By passing known quantities of air through carefully-weighed tubes of potassic hydrate, the carbonic dioxide is absorbed, and its proportion determined. It varies from 3 to 6 parts in 10,000 of air, and averages about one volume in 2,500. The quantity is variable within the limits above stated. It increases as we rise from the earth, and is less after a rain, which washes it down from the air; it increases during the night, and diminishes after sunrise, is less over large bodies of water than over large tracts of land, and is more abundant in the air of towns than in that of the country.

404. The Carbonic Acid which is poured into the atmosphere in prodigious quantities and from innumerable sources, is as necessary to the vegetable world, as is oxygen to the animal world. It is absorbed by the leaves, and minute as is its proportion, if it were withdrawn, the vegetable world would quickly perish. Liebig has shown that the air contains minute traces of ammonia, which are washed down, and may be detected in rain-water. Traces of nitric acid have also been frequently detected. This substance is thought to be formed by electricity, every flash of lighting which darts across the sky combining &

portion of the oxygen and nitrogen along the line of its course, and forming this acid. The saline particles of the ocean-waves, as they are dashed into foam and spray, are carried by the winds far inland. All these substances are brought down by the rains, and help to quicken the growth of vegetation.

405. Resulting Properties.-Each of the constituents of the air is essential to the present order of things. Oxygen is preeminently its active element. To duly restrain this activity, the oxygen is diluted and weakened by four times its bulk of the negative element, nitrogen. Their properties are thus perfectly adjusted to the requirements of the living world. Were the atmosphere wholly composed of nitrogen, life could never have been possible; were it to consist wholly of oxygen, other conditions remaining as they are, the world would run through its career with fearful rapidity; combustion once excited, would proceed with ungovernable violence; animals would live with hundred-fold intensity, and perish in a few hours.

406. The Atmosphere and the Living World. The rclations of the atmosphere to living beings, the stability of its composition, and the wonderful forces that are displayed within it, are full of surpassing interest. The vegetable world is derived from the air; it consists of condensed gases that have been reduced from the atmosphere to the solid form by solar agency. On the other hand, animals, which derive all the material of their structure from plants, destroy these substances while living, by respiration, and when dead, by putrefaction, thus returning them again in the gascous form to the air whence they came.

CHAPTER XVII.

THE SULPHUR GROUP.-SULPHUR, SELENIUM, TELLURIUM.

§ 1. Sulphur and its Compounds.

VI.;

SULPHUR.-Symbol, S. Atomic Weight, 32; Quantivalence, II, IV., Molecular Weight, 64; Molecular Volume, 2; Specific Gravity, 2.05. 407. Modifications of Sulphur.-Sulphur, like oxygen, is capable of existing in several different modifications. At ordinary temperatures it is solid, or nearly so. At 115° C., it melts to a pale-yellow liquid. As the temperature rises this liquid becomes viscid, until, between 200° and 250° C., it is too thick to flow. At a still higher temperature it again becomes fluid, and finally boils at 440° C. The density of the vapor then diminishes gradually, until, at 1000° C., a point is reached where it is 32 times as great as that of hydrogen at the same temperature. Sulphur in all its forms is insoluble in water and alcohol, a poor conductor of heat, and a non-conductor of electricity. When heated in the air to 260° C., it takes fire, burning with a pale-blue flame. The vapor cf sulphur supports combustion, many metals taking fire in it, and burning actively. When combined with metals or positive radicals, sulphur is a dyad, but in other combinations it may be either a tetrad or hexad.

408. Ordinary Modification (Sa)-In this form, sulphur is one of the oldest known substances, being mentioned in the Bible, and in the writings of the ancients. It exists abundantly in Nature; is found in various volcanic regions, as in the island of Sicily, where it is mincd in immense quantities for the market. It is deposited by many springs and small lakes, being produced by the decomposition of hydric sulphide. The sulphur of commerce is prepared from the impure native material, by subjecting it to a rough distillation in earthen retorts which separates it from min

eral impurities. It is also obtained from a native ferric sulphide. This is generally done by piling the ferric sulphide with wood, in large heaps in the open air, and setting these on fire. A portion of the ferric sulphide burns, and, through the heat attending its combustion, the remainder is also decomposed with the liberation of sulphur, which volatalizes and collects in the fluid state, in basin-shaped cavities on the surface of the heap. In commerce, sulphur exists in forms due to the different modes of its preparation: first, as roll-sulphur or brimstone, obtained by running melted sulphur into moulds; second, as flour of sulphur, a pale yellow gritty powder, obtained by sublimation; and, third, as milk of sulphur, produced by the decomposition of solutions of certain sodic and potassic sulphides with acids. 409. Properties.--In its ordinary modification sulphur

FIG. 161

Sulphur-Crystals.

is a brittle, yellow solid, crystallizing in transparent right rhombic octohedra, or allied forms (Fig. 161). It is soluble in carbonic disulphide, and the crystals may be obtained from this solution by evaporation.

410. Oblique Prismatic Sulphur (S). This modification may be obtained by melting ordinary sulphur in a crucible, allowing it to cool until a crust is formed, then breaking the crust and pouring out the still fluid portion. The walls of the crucible will then be found lined with a mass of transparent yellowish-brown, needle-shaped crystals (Fig. 162). They are oblique rhombic prisms, or modifications of these, have a specific gravity of 1.98, and in the course of a few days pass spontaneously into the ordinary octahedral modification. They are readily soluble in carbonic disulphide.

411. Plastic Sulphur (Sy). This variety is produced by heating melted sul

FIG. 162.

Crystals by Fusion.