CHLORIDES OF SULPHUR.

197

description of the methods of preparing them would be out of place in an elementary manual.

245. Compounds of Sulphur and Chlorine.-Chlorine and sulphur combine with one another directly and readily, forming several different compounds, whose properties vary in accordance with the varying proportions of chlorine and sulphur which they respectively contain.

246. Dichloride of Sulphur (SCI) is the best-known of the compounds of chlorine and sulphur, and is often called simply chloride of sulphur.

It can be prepared by passing a current of dry chlorine through a flask or tubulated retort containing flowers of sulphur. The chlorine is rapidly absorbed by the sulphur, and care must be taken lest the mass become too hot. The reddish-yellow liquid, obtained as the result of the reaction, is a solution of sulphur in dichloride of sulphur; by distilling it the excess of sulphur can be separated.

Dichloride of sulphur is a yellowish-brown liquid of 1.68 specific gravity, and boiling, without decomposition, at 144°. It emits a peculiar odor, which has been likened to that of sea-plants; its vapor excites tears, and its taste is acid, acrid, and bitter. It fumes strongly in the air, being decomposed by the moisture of the air with evolution of chlorhydric acid. It is decomposed by water, but can be mixed with bisulphide of carbon and with benzine. It is remarkable as a powerful solvent of sulphur; 100 parts of dichloride of sulphur can take up about 70 parts of sulphur at the ordinary temperature; on slowly cooling the hot saturated solution, beautiful crystals of sulphur are deposited. Dichloride of sulphur is used in a process of vulcanizing caoutchouc, known as the cold process.

247. Chloride of Sulphur (SCI).-This compound is formed when sulphur is treated with an excess of dry chlorine, or when a current of chlorine is passed into dichloride of sulphur; the dichloride requires some 278 times its own bulk of chlorine gas, and absorbs it very slowly. Chloride of sulphur is a red liquid, of 1.625 specific gravity. It exhales suffocating and irritating fumes of chlorine and the dichloride, since it slowly decomposes when kept. The decomposition is particularly rapid in a strong light; and so much gas is evolved that a tightly-stoppered bottle

containing chloride of sulphur will explode, after a time, if it be placed in sunlight. On being heated, the liquid gives off so much chlorine at 50° that it seems to boil; but the temperature gradually rises to 64°, which appears to be the real boiling-point of the liquid. It is slowly decomposed by water.

The density of its vapor has been found to be 53; admitting that the gas is composed of one volume of sulphur vapor and two volumes of chlorine, condensed to two volumes of vapor, the calculated specific gravity of its vapor would be 51.5. In view of the instability of the compound, the experimental result is sufficiently near coincidence with the calculated number to make it certain that the composition of the gas is really as above stated.

248. The other compounds of sulphur with chlorine, and with chlorine and oxygen, need not here be discussed; and the same remark applies to the compounds formed by the union of sulphur with iodine, bromine, fluorine, and nitrogen. The compounds of sulphur with carbon, phosphorus, arsenic, and the metals will be treated of hereafter.

CHAPTER XIV.

SELENIUM AND TELLURIUM.

249. These elements are rare, and of little or no industrial importance; but to the chemist they are exceedingly interesting, on account of the close resemblance they bear to sulphur. The three elements, sulphur, selenium, and tellurium, constitute a group which is equally remarkable with that formed by chlorine, bromine, and iodine. (See § 152.)

250. Selenium, Se, is never found in any considerable quantity in any one place. Traces of it occur in many varieties of native sulphur, and in various metallic sulphides. It is now obtained chiefly from the sulphides of iron, copper, and zinc. These sulphides often contain minute traces of selenium, though the quantity is sometimes so small that it can hardly be detected

PROPERTIES OF SELENIUM.

199

by the ordinary methods of analysis. When these sulphides are burned for the purpose of manufacturing sulphuric acid, or in metallurgical operations, the selenium goes off with the sulphurous acid produced by the combustion, and is deposited either in the dust-flues of the furnaces or upon the floors of the leaden chambers at the sulphuric-acid works. In this way the selenium from hundreds of tons of the pyritous ores is collected and concentrated to a comparatively small bulk. The deposit taken from the leaden chambers of some sulphuric-acid works contains as much as from 2 to 10 per cent. of selenium. The methods of obtaining pure selenium from these deposits are founded upon the facts that by treatment with nitric acid or aqua regia the selenium can all be oxidized and converted into selenious acid (SeO), that selenious acid is soluble in water, and that when a solution of it is treated with sulphurous acid, the selenious acid is reduced and pure selenium deposited.

251. In its properties and in its chemical behavior, selenium resembles sulphur in many respects, while in others it is like tellurium. Like sulphur and oxygen, it occurs in distinct allotropic modifications (§§ 162, 196). The precipitate obtained by mixing solutions of sulphurous and selenious acids is of a deep red color, almost like that of cinnabar. But, after having been fused and suddenly cooled, selenium appears as a brilliant black mass, amorphous, like glass, and of 4-3 specific gravity. When fused selenium has been slowly cooled, it appears as a dark-grey, very brittle, crumbling mass, of crystalline or granular structure and a metallic lustre like that of lead; the specific gravity of this variety is 4.81. The amorphous or vitreous modification of selenium does not conduct electricity; but the granular or crystalline variety conducts it, and the more readily in proportion as it is hotter. The specific heat of selenium, at the ordinary temperature, is 0.0746, being the same for both the vitreous modification and that with metallic lustre. The vitreous variety is soluble in bisulphide of carbon, but the granular variety is insoluble in that liquid.

Selenium melts readily upon being heated, and the liquid thus

obtained boils at about 700°, being converted into a dark yellow vapor, the specific gravity of which has been found to be 82.3. The atomic weight of selenium has been determined to be 79.5. This discrepancy between the vapor-density and the atomic weight is to be ascribed to the imperfection of the experimental determinations. Of itself, selenium has neither taste nor odor. When heated in the flame of a lamp, it burns with a beautiful blue flame and exhales a peculiarly offensive odor, like that of putrid horseradish, selenious acid, SeO,, being the chief product of the reaction.

Selenium combines with most of the elements, usually in the same way as sulphur, though not always, since it is a weaker chemical agent than sulphur; its compounds are as a rule somewhat less stable than the corresponding sulphur compounds. With oxygen it forms selenious acid, SeO,, and selenic acid, SeO,,-analogous to sulphurous and sulphuric acids respectively. Besides these, there is a lower oxide, SeO (?); it is a colorless gas, having the strong and disagreeable odor like horseradish before mentioned.

252. Both selenious and selenic acids form numerous salts, which closely resemble the corresponding sulphites and sulphates in composition and in many of their properties. Normal seleniate of potassium, for example, K,SeO,, cannot be distinguished, by its external appearance, from sulphate of potassium, K,SO,,-the crystalline form of the two bodies, as well as their texture, color, and lustre, being identical. If solutions of these two salts be mixed, neither of the salts can subsequently be crystallized out by itself when the solution is evaporated; the crystals obtained will be composed of sulphate of potassium and seleniate of potassium mixed in the most varied proportions. Bodies which are thus capable of crystallizing together in all proportions, without alteration of the crystalline form, are said to be isomorphous (like-formed). The formula of the two isomorphous salts, just mentioned, differ only in this-that the one contains the atom Se, where the other contains the atom S. It is therefore possible to replace 32 parts by weight of sulphur by 79.5 parts of selenium, or 79.5 of selenium by 32 of sulphur, without changing the crystalline form of the salts; it follows that 32 parts by

ATOMIC VOLUME-TELLURIUM.

32

79.5

=

201

weight of solid sulphur, and 79-5 parts of solid selenium, occupy the same space. That this is actually the case may be shown by comparing the quotients obtained by dividing the atomic weights of the two elements by their specific gravities; these quotients will be found to be equal, or as nearly equal as the limits of error of the physical determinations involved will permit. The specific gravity of prismatic sulphur is 1.91, or, in other words, one cubic centimetre of solid sulphur weighs 1.91 gramme; the specific gravity of crystalline selenium is 4.81, or one cubic centimetre of selenium weighs 4.81 grammes; 32 grammes of sulphur will, therefore, occupy 16.75 cubic centimetres; 79-5 grammes of selenium will occupy 481 = 16.53 cubic centimetres. What is true of grammes, is true of any parts by weight, and ultimately of the atoms. This quotient, obtained by dividing the atomic weight of an element by its specific gravity, is called the atomic volume of the element; it must be borne in mind that the standard of specific gravity for liquids and solids is water, for gases hydrogen, and, therefore, that the atomic volume of a solid or liquid must not be directly compared with that of a gas. Two elements whose atomic volume is the same can be exchanged in their compounds without alteration of crystalline form, precisely as a brick or stone taken out of a wall can be replaced by another of the same size and shape without changing the form of the wall.

253. With chlorine, selenium forms two compounds, SeCl and SeCl, the first of which is analogous to dichloride of sulphur. With hydrogen it forms a compound, H,Se, called selenhydric acid, or seleniuretted hydrogen, which is perfectly analogous to sulphuretted hydrogen, HS, but possesses a still more disagreeable odor. In its action upon solutions of the metallic salts, upon metals and metallic oxides, selenhydric acid behaves like sulphydric acid, a selenide of the metal being always formed.

254. Tellurium (Te) occurs in nature even more rarely than selenium. Sometimes it is found in the free state, but more generally in combination with the heavy metals, such as gold, silver, lead, copper, and bismuth. It is one of the few elements with regard to which chemists have, at times, been in doubt whether