When the dissociated vapor cools, the parted gases recombine to form solid chloride of ammonium.

It is obvious that the phenomena of dissociation interfere fatally with one of the common methods of arriving at the weight and structure of the molecule of a volatile compound; the indirect method of getting at the volumetric composition of a substance from its ponderal composition and the specific gravity of its vapor becomes impracticable whenever the vapor of the compound under examination is liable to dissociation, inasmuch as experiment cannot determine beyond a doubt the real vapor-density of such a body.

301. Bromides of Phosphorus.-When a piece of phosphorus is dropped into bromine, the two elements combine with explosive violence, the burning phosphorus being thrown about in a highly dangerous manner. There are two bromides of phosphorus, PBr, and PBr,, corresponding to the two chlorides. The terbromide is liquid at ordinary temperatures and the quinquibromide solid.

302. Iodides of Phosphorus.-Iodine and phosphorus unite directly, when brought in contact with one another, and so much heat is developed by their union, that a portion of the phosphorus will take fire if the mixture be in contact with the air. There are two iodides of phosphorus, both of them solid at the ordinary temperature; their composition is respectively PI, and PI,. It will be noticed that, while the teriodide corresponds to the terchloride and terbromide, the other compound is a biniodide, of which there is known neither a bromine nor a chlorine analogue. The fact is interesting as illustrating the general truth that, when in any group or family of elements we compare the behavior of its several members, analogy ceases to be a sure guide, in proportion as the individuals compared are more widely separated in the natural series. Chlorine and bromine stand next to one another in the family or series of elements to which

SULPHIDES OF PHOSPHORUS.

241

they belong; and as we have just seen, their behavior, as regards phosphorus, is well-nigh identical; but iodine, one step further removed from chlorine than bromine is, enters into new combinations not altogether conformable to those of chlorine.

303. Sulphides of Phosphorus.-There is a definite sulphide of phosphorus corresponding to each of the oxides, and in addition to these there are two other compounds, which may be represented by the formulæ PS, and P,S,. Sulphur and phosphorus may also be melted together in any proportion. The sulphides of phosphorus are exceedingly inflammable, taking fire even more readily than phosphorus itself, and they are all more readily fusible than either of the two elements of which they are composed. They may be prepared by heating sulphur under water in contact with melted phosphorus. The union of the two elements is attended with development of much heat, and sometimes with dangerous explosions. It is well, therefore, to operate only upon small quantities, and to add the sulphur gradually to the phosphorus.

CHAPTER XVII.

ARSENIC.

304. Compounds of arsenic have been known from very early times. The element is sometimes found native, but much more frequently associated with other metals and with oxygen and sulphur. The metals in connexion with which it most commonly occurs are iron, cobalt, nickel, and copper. Ferruginous ores and deposits, in particular, are rarely free from traces of arsenic. In small quantity, arsenic is very widely distributed.

The greater part of the arsenic of commerce is prepared from a native arsenide and sulphide of iron (arsenical pyrites) corresponding to the formula FeAsS, and from the arsenides of nickel and cobalt. Metallic arsenic is obtained directly from the mineral of the formula FeAsS by heating it in earthen tubes laid

R

242

PROPERTIES OF ARSENIC.

horizontally in a long furnace; a tube, made by rolling up a piece of thin sheet iron, is inserted in the mouth of each earthen retort, and an earthen receiver is luted on to this iron tube. The arsenic condenses principally in the iron tube, in the form of a compact, whitish, crystalline mass, which is detached, when cold, by unrolling the sheet iron. The metal is also indirectly obtained by reducing the arsenious acid (As,O,) which results from roasting (heating in a current of air) arsenides, like those of cobalt and nickel; this oxide is heated with charcoal in earthen crucibles covered with conical iron caps, or inverted crucibles, into which the reduced metal sublimes. The metal obtained by the second process is gray and pulverulent, instead of whitish and coherent.

FeAsS FeS + As ;

As20, +3C2As + 3CO.

3

305. Arsenic is a brittle solid, of a steel-gray color and a metallic lustre. Its specific gravity has been variously given at from 5.62 to 5.96. Like the metals, it is a good conductor of electricity. It crystallizes in acute rhombohedrons, and in octahedrons also, thus taking on forms of both the monometric and hexagonal systems, as do phosphorus, the preceding member, and antimony, the succeeding member of this family. At a dull red heat it volatilizes without previous fusion; the vapor is colorless, and possesses a characteristic odor resembling that of garlic. The specific gravity of this vapor is 150, while the atomic weight of the element is 75; arsenic, therefore, resembles phosphorus, and differs from all the other elements heretofore studied, in that its atomic weight is not identical with its unit-volume weight; two combining proportions by weight of arsenic occupy the same volume as one combining proportion of hydrogen; its symbol, As, represents its atomic weight, but only half the weight of the unit-volume of its vapor. At the ordinary temperature the compact metal does not tarnish by exposure to dry air, but a moistened powder of arsenic is slowly converted by the air into a mixture of arsenious acid and metallic arsenic. At a red heat the metal burns with a whitish flame, producing a white smoke of arsenious acid. When thrown, in fine powder, into chlorine gas, it takes fire spontaneously and is converted into

ARSENIC AND HYDROGEN.

243

chloride of arsenic (AsCl1). Bromine, iodine, and sulphur also combine readily with arsenic, when aided by a gentle heat. Nitric acid and aqua regia convert the metal into arsenic acid (As,O,); chlorhydric acid has little action upon it. Dilute sulphuric acid has no action upon the metal; but the concentrated acid has the same effect upon arsenic as upon phosphorus (§ 276); arsenious acid is formed and sulphurous acid escapes :—

3H,SO,+2As As,0, + 3SO, + 3H ̧0.

=

2

Some fatty oils dissolve arsenic to a slight extent, as they do phosphorus. Metallic arsenic unites by fusion with most metals, forming alloys which the arsenic tends to make hard or brittle. In the manufacture of shot a little arsenic is added to the lead to facilitate the formation of regular globules.

306. Arsenic and Hydrogen.-Arsenic forms two combinations with hydrogen; one of these is an unstable, brown solid of uncertain composition; the other is a well-known gas whose constitution is represented by the formula AsH,, and which is therefore analogous in composition to ammonia (NH ̧) and phosphuretted hydrogen (PH). The solid hydride is so obscure a substance that nothing need here be said of it, except that it is supposed to contain two atoms of hydrogen and one of arsenic (AsH2?).

307. Arseniuretted Hydrogen.-This very dangerous gas may be prepared in an impure state by decomposing, with sulphuric acid diluted with three parts of water, an arsenide of zinc obtained by fusing together equal weights of powdered arsenic and granulated zinc :

3H SO, + Zn,As=3ZnHSO,+ AsH ̧.

4

As it is not possible to prepare the precise alloy Zn, As, the arseniuretted hydrogen thus obtained is always mixed with hydrogen. The arsenide of sodium can be decomposed by water, with evolution of arseniuretted hydrogen :

3H,0 + Na ̧As = 3NaHO + AsH ̧.

The same remark, however, applies to this reaction as to the preceding one; the product is contaminated with an indeterminate quantity of free hydrogen. Arseniuretted hydrogen seems also to be formed whenever the oxides of arsenic, or compounds of

244

ARSENIURETIED HYDROGEN.

these oxides, are brought in contact with nascent hydrogen. A mixture of arseniuretted hydrogen and hydrogen may be readily obtained by acting upon zinc by dilute chlorhydric or sulphuric acid in which arsenious acid has been dissolved.

308. Arseniuretted hydrogen is a colorless gas, having a fetid odor; even when very much diluted with air, it is intensely poisonous, and fatal results have repeatedly followed its accidental inhalation. In experimenting with this deadly gas, the greatest care is required not to inhale the least portion of it. it has been condensed at -40° to a transparent liquid, but it has never been solidified. The gas is soluble in water at the ordinary temperature only to the extent of one-fifth of its volume, and neither the gas nor its aqueous solution has any action upon blue or red litmus-paper. In spite, therefore, of its strong resemblance to ammonia in composition, some of its physical properties are strikingly unlike those of that very soluble and intensely alkaline gas. Arseniuretted hydrogen burns in the air with a whitish flame, forming water and a white smoke of arsenious acid; but if a cold body, like a piece of porcelain, for example, be introduced into a jet of the burning gas, the hydrogen alone will burn, and the arsenic will be deposited in the metallic state upon the porcelain surface, forming a lustrous black spot. This effect is precisely similar to the deposition of soot on a cold body held in the flame of a candle. It is also decomposed when caused to pass through tubes heated to dull redness, metallic arsenic being deposited as a brown or blackish mirror, while hydrogen gas escapes. This decomposition is a good illustration of the dissociation of gases (§ 300). Chlorine in excess reacts violently upon it, forming terchloride of arsenic (AsCl,) and chlorhydric acid :—

When, however, chlorine acts on an excess of arseniuretted hydrogen, there are formed chlorhydric acid and metallic arsenic ; flame accompanies this reaction. The reactions of bromine and iodine are similar to, but less violent than, those of chlorine. We recall, in this connexion, the decomposition of ammonia by chlorine, with formation of chlorhydric acid and liberation of