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TELLUROUS AND TELLURIC ACIDS.

vapour of tellurium is thus mechanically carried forward, and it is condensed in drops and flexible crystalline needles in the cooler parts of the apparatus. According to Mitscherlich, tellurium, when solidified after fusion, exhibits a rhombohedral cleavage, a circumstance which appears to indicate its isomorphism with arsenicum and antimony. Tellurium is a bad conductor of heat and of electricity. When heated strongly in the air it takes fire, burns with a blue flame edged with green, and emits a peculiar characteristic odour, whilst thick white fumes of tellurous anhydride are produced. Like sulphur and selenium, tellurium is soluble in cold concentrated sulphuric acid, to which it gives a fine purple-red colour; on dilution it is precipitated unchanged. Minute quantities of tellurium, when taken internally, impart a persistent and intolerable odour of garlic to the breath.

Tellurium forms two oxides, (FeO2; FeO3) which correspond in composition to sulphurous and sulphuric anhydrides.

(439) Tellurous Acid (H,Fe ̧=179).—Tellurium is readily dissolved by nitric acid of sp. gr. 125. If the solution be poured into water immediately, a white, bulky hydrate of tellurous acid subsides. It is slightly soluble in water, reddens litmus, and combines with the alkaline bases; these compounds are soluble. Tellurous acid has a bitter metallic taste: its anhydride may be obtained by gently heating the hydrate, or by boiling the nitric acid solution, when it is deposited in crystalline needles, which are very slightly soluble in water. The anhydride (Fe0,=161)

2

fuses easily, forming a transparent glass, which is yellow while hot, but becomes white and crystalline on cooling. Tellurous anhydride possesses considerable volatility: if fused with hydrate of potash, tellurite of potassium is formed. Tellurites may be formed of three classes-normal salts, M,Fe,; acid salts, MHTe, and hyperacid salts, or quadritellurites, MHTeOH,TeÐ ̧. The tellurites of the alkali-metals are soluble, those of the alkaline earths very sparingly so. This anhydride also, like many of the metallic anhydrides, combines with the stronger acids: the compounds which it thus furnishes have a metallic taste, and are said to act powerfully as emetics. Its salts with oxalic and tartaric acid are soluble. All the soluble salts in which tellurium acts as a base are decomposed if mixed with hydrochloric acid and heated with sulphurous acid reduced tellurium is precipitated under these circumstances. With sulphuretted hydrogen a black sulphide of tellurium is produced.

(440) Telluric Acid (H,Fe,=195) is obtained by gently heating tellurium or tellurous acid with nitre. A tellurate of potas

TELLURETTED HYDROGEN.

213

sium is formed, from which the acid is transferred to barium, and the barium is separated by sulphuric acid. It crystallizes in striated hexagonal prisms, which have a nauseous metallic taste; they exert but a feeble action on litmus. These crystals are composed of (H,Fe,,2 H,O). If heated nearly to redness they furnish telluric anhydride, and then assume an orange-yellow colour. This anhydride (Fe=177) is completely insoluble in water, and in nitric and hydrochloric acids, as well as in alkaline solutions. Telluric acid has but a feeble chemical attraction for bases, but, like selenic acid, it forms three classes of salts which may be represented by the general formulæ, M,Te,; MHTe,; and MHTEO H2FeO. Their solutions, when acidulated, yield a black precipitate with sulphuretted hydrogen. When telluric acid, or one of its salts, is heated to redness, oxygen is disengaged and the telluric is converted into tellurous anhydride, or the tellurate into a tellurite of the basyl.

Two chlorides, FeCl, and FeCl, have been obtained by the direct action of chlorine upon tellurium: both of them are volatile; the vapour of the bichloride is of a violet colour; they are decomposed by a large quantity of water.

(441) TELLURETTED HYDROGEN: H,Te=131; Sp. Gr. 4'489; Atomic Vol.; ; or HTe=655.-The most interesting compound of tellurium is that which it forms with hydrogen. It is a gaseous body analogous to sulphuretted hydrogen, and is possessed of feebly acid properties. It may be obtained by decomposing the alloy of tellurium with zinc or tin, by means of hydrochloric acid. The gas which escapes burns with a blue flame; it reddens litmus, and has an odour which cannot be distinguished from that of sulphuretted hydrogen: with water it forms a colourless solution, which becomes brown by exposure to the air, owing to the oxidation of the hydrogen and separation of tellurium. Telluretted hydrogen precipitates most of the metals from their solutions, in the form of tellurides which have a close analogy with the corresponding sulphides. The tellurides of the alkaline metals are soluble in water.

Tellurium, whether in the form of a soluble tellurite or in that of a tellurate, is thrown down from its solutions in the reduced form by zinc or iron; neutral solutions of the salts of both its acids are also reduced by ferrous sulphate and by stannous chloride; in these cases the tellurium falls in brown flocculi. The tellurates of the alkaline metals when heated to redness in a tube with charcoal are reduced to tellurides, which are soluble in water, and form a red liquid.

CHAPTER VIII.

§ I. PHOSPHORUS: P=31.*

Atomic Vol., or; Theoretical Sp. Gr. of Vapour, 4'284; Observed Sp. Gr. of Vapour, 4'50.

(442) Natural Relations of the Phosphorus Group.-Phosphorus is described here for the sake of convenience, and not because it exhibits any relation to the sulphur group; it has, however, a close connexion with arsenic and antimony, two bodies which will be described with the metals.

Phosphorus, arsenic, and antimony afford good instances of the terequivalent or triad group of elements, as in most cases when in combination they represent 3 atoms of hydrogen, though sometimes they are quinquequivalent, or represent 5 atoms. These three elements are indeed related much in the same way as sulphur, selenium, and tellurium; each of them unites with hydrogen, and forms a gaseous compound in which 6 volumes of hydrogen combine with I volume of the vapour of the other element-the compound which is formed occupying the space of 4 volumes-these gaseous compounds exhibiting a tendency to alkalinity. Each of these elements unites with oxygen in the proportion of 2 atoms with 3, and 2 with 5 atoms of oxygen, forming compounds in which the acid character is less and less marked as the atomic weight of the combustible element increases. The isomorphous relations of arsenious anhydride and oxide of antimony have long been known, and the corresponding tribasic phosphates and arseniates offer some of the most striking exemplifications of isomorphism. Chlorine unites with the members of this group in the proportion of 3 atoms to 1 atom of phosphorus, arsenic, or antimony. Bismuth is also related to this group by the composition and character of its oxides and chloride; although no bismuthated hydrogen is. at present known. Nitrogen, as already pointed out, is connected with the phosphorus group by its combination with hydrogen (H,N), and by its formation of anhydrides with 3 and with 5 atoms of oxygen. An interesting isomorphous relation exists between the members of the sulphur and those of the phosphorus group; sulphur being isomorphous with arsenic, as is shown in

*The vapour volume of phosphorus appears to be tetratomic; for if the molecule of free phosphorus be taken as P, it will furnish two volumes of vapour. Arsenic resembles phosphorus in this respect.

PHOSPHORUS-SOURCES OF.

215

the correspondence in form between crystals of iron pyrites (FeS), and those of mispickel (FeSAs). (See note, Part I. p. 131.)

The following table exhibits some of the corresponding compounds of the 5 triads just mentioned :

:

A gradation of properties is observed in these elements,and particularly in the three intermediate ones: phosphorus is the least dense, the most fusible and volatile; next follows arsenic, and then antimony, in the order of their atomic weights. The acid properties of the oxidized compounds are most marked in nitrogen, then in phosphorus; they are weaker in arsenic, still weaker in antimony, and are scarcely apparent in bismuth. The compounds with hydrogen follow the same order: ammonia is a powerful base and requires a high temperature for its decomposition, phosphuretted hydrogen a very feeble base; in arseniuretted hydrogen the basic character is not perceived, although manifest in some of its derivatives, and the same thing is true of antimony; each of the three hydrides last mentioned being in succession more easily decomposed by simple exposure to heat, whilst the attraction of bismuth for hydrogen is so feeble that its hydride is unknown.

(443) Phosphorus was discovered by Brandt in 1669. It is never met with in nature in the uncombined state, but it occurs in small proportion as phosphate of calcium, as a constituent of the primitive and volcanic rocks, by the gradual decay of which it passes into the soil: from the soil it is extracted by plants, which accumulate it, particularly in their seeds, in quantity sufficient for the support of the various tribes of animals which they supply with food. In the animal system it is collected in large amount, and when combined with oxygen and calcium, as phosphate of

216

PHOSPHORUS-EXTRACTION.

calcium, it forms the principal earthy constituent of the bones of the vertebrata. Phosphorus also appears to be essential to the exercise of the higher functions of the animal, since it exists as a never-failing ingredient in the substance of which the brain and nerves are composed. It is likewise contained in albumen and in fibrin in small proportions, and is present in the form of phosphates of the metals of the alkalies and of the earths in the urine and solid excrements of animals.

Extraction. Phosphorus was originally extracted from the salts contained in urine, but it is now obtained almost exclusively from the bones of animals. In order to prepare it, bones were formerly always burned to whiteness by calcining them in an open fire for some hours, then reduced to powder; but now the gelatin of the bones is first economized by heating them, under pressure, with water; or the bones are distilled in closed vessels, the ammonia and volatile products are collected, whilst the bone black is employed in sugar-refining, and after it has become useless for this purpose, it is burned in the open fire. Three parts of bone-ash obtained by any of these methods are mixed with 2 of concentrated sulphuric acid, and 18 or 20 parts of water. The mixture is allowed to stand for two or three days, after which it is placed upon a strong linen filter, and the acid liquid is separated from the sulphate of calcium by pressure; the residue is further washed with water, and the washings are added to the filtered solution. In this process the sulphuric acid is added in such quantity as partially to decompose the phosphate of calcium; twothirds of the calcium are removed by it in the insoluble form, as sulphate of calcium, the remaining third being left as an acid salt, in combination with the whole of the phosphoric acid, with which it forms a compound readily soluble in water, frequently described as superphosphate of lime (H ̧¤a 2 PO). The reaction may be thus expressed in symbols :

This acid solution is evaporated to the consistence of a syrup, then mixed with one-fourth of its weight of charcoal, and heated to incipient redness in an iron pot, stirring constantly. The mass, when dry, is transferred to an earthen retort (a, fig. 310), which is covered externally with a thin paste, consisting of a mixture of equal parts of borax and fire-clay, with a view of rendering the retort less porous. It is then exposed to a heat which is slowly raised to a full red. Phosphorus gradually rises in vapour, and is