Niobium and tantalum occur as acids in rare minerals, and are mainly extracted from tantalite and columbite, which are found in Bavaria, Finland, North America, and in the Urals. These minerals are composed of the ferrous salts of niobic and tantalic acids; they contain about 15 per cent. of ferrous oxide in isomorphous mixture with manganous oxide, in combination with various relative amounts of tantalic and niobic anhydrides. These minerals are first fused with a considerable amount of potassium bisulphate, and the fused mass is boiled in water, which dissolves the ferrous salt and leaves an insoluble residue of impure niobic and tantalic acids, &c. This raw product is then treated with ammonium sulphide, in order to extract the tin and tungsten, which pass into solution. The residue containing the acids (according to Marignac) is then treated with hydrofluoric acid, in which it entirely dissolves, and potassium fluoride is added to the resultant hot solution; on cooling, a sparingly-soluble double fluoride of potassium and tantalum separates out in fine crystals, while the much more soluble niobium salt remains in solution. The difference in the solubility of these double salts in water acidified with hydrofluoric acid (in pure water the solution becomes cloudy after a certain time) is so great that the tantalum compound requires 150 parts

the bivanadates by the action of a solution of acetic acid; hence they in this respect also resemble many chromates, which are also formed directly by the removal of an alkali by an acid, although it would be nitric and not acetic acid.

Vanadium was discovered at the beginning of this century by Del-Rio, and afterwards investigated by Sefström, but it was only in 1868 that Roscoe established the above formulæ of the vanadic compounds. The researches made by Roscoe were preceded by those of Marignac in 1865, on the compounds of niobium and tantalum, to which were also ascribed different formulæ from those now recognised. Tantalum was discovered simultaneously with vanadium by Hatchett and Ekeberg, and was afterwards studied by Rose, who in 1844 discovered niobium in it. Notwithstanding the numerous researches of Hermann (in Moscow), Kobell, Rose, and Marignac, still there is not yet any certainty as to the purity of, and the results obtained for, the compounds of these elements. They are difficult to separate from each other, and especially from the cerite metals and titanium, &c., which accompany them. Before the investigations of Rose the highest oxide of tantalum was supposed to belong to the type TaX-that is, its composition was taken as TaO3, and to the lower oxide was ascribed a formula TaO2. Rose gave the formula TaO, to the higher oxide, and discovered a new element called niobium in the substance previously supposed to be the lower oxide. He even admitted the existence of a third element occurring together with tantalum and niobium, which he named pelopium, but he afterwards found that pelopic acid was only another oxide of niobium, and he considered it probable that the higher oxide of this element is NbO2, and the lower Nb2O3. Hermann found that niobic acid which was considered pure contained a considerable quantity of tantalic acid, and besides this he admitted the existence of another special metallic acid, which he called ilmenic acid, after the locality (the Ilmen mountains of the Urals) of the mineral from which he obtained it. V. Kobell recognised still another acid, which he called dianic acid, and these different testimonies were only brought in agreement in the sixties by Marignac. He first of all indicated an accurate method for the separation of tantalic and niobic compounds, which are always obtained in admixture.

=

of water for its solution, and the niobium compound only requires 13 parts of water. The Greenland columbite (sp. gr. 5·36) only contains niobic acid, and that from Bodenmais (Bavaria, sp. gr. 6·06) almost equal quantities of tantalic and niobic acids. Having isolated tantalic and niobic salts, Marignac found that the relation between the potassium and fluorine in them is very variable-that is, that there exist various double salts of fluoride of potassium, and of the fluorides of the metals of this group, but that with an excess of hydrofluoric acid both the tantalum and niobium compounds contain seven atoms of fluorine to two of potassium, whence it must be concluded that the simplest formula for these double salts will be K2RF, RF,2KFthat is, that the type of the higher compounds of niobium and tantalum is RX5, and hence is similar to phosphoric acid. A chloride TaCl, may be obtained from pure tantalic acid by heating it with charcoal in a current of chlorine. This is a yellow crystalline substance, which melts at 211°, and boils at 241°; its vapour density with respect to hydrogen is 180, as would follow from the formula TaCl,. It is completely decomposed by water into tantalic and hydrochloric acids. Niobium pentachloride may be prepared in the same manner; it fuses at 194°, and boils at 240°. When treated with water this substance gives a solution containing niobic acid, which only separates out on boiling the solution. Delafontaine and Deville found its vapour density to be 9.3.51

5

51 If niobic acid be mixed with a small quantity of charcoal and ignited in a stream of chlorine, then a difficultly-fusible and difficultly-volatile oxychloride, NbOC15, separates. The vapour density of this compound with respect to air is 75, and this vapour density gives a perfect certainty to the accuracy of the formulae given by Marignac, and shows the quantitative analogy between the compounds of niobium and tantalum, and those of phosphorus and arsenic, and consequently also of vanadium. In their qualitative relations (as is seen from the correspondence of the atomic weights also), the compounds of tantalum and niobium exhibit a great analogy with the compounds of molybdenum and tungsten. Thus zinc, when acting on acid solutions of tantalic and niobic compounds, gives a blue coloration, exactly as it does with those of tungsten and molybdenum (titanium also). These acids form the same large number of salts as those of tungsten and molybdenum. The anhydrides of the acids are also insoluble in water, but as colloids are sometimes held in solution, just like those of titanic and molybdic acids. Furthermore, niobium is in every respect the nearest analogue of molybdenum, and tantalum of tungsten. Niobium is obtained by reducing the double fluoride of niobium and sodium, with sodium. It is difficult to obtain in a pure state. It is a metal on which hydrochloric acid acts with some energy, as also does hydrofluoric acid mixed with nitric acid, and also a boiling solution of caustic potash. Tantalum, which is obtained in exactly the same way, is a much heavier metal. It is infusible, and is only acted on by a mixture of hydrofluoric and nitric acids. Rose in 1868 showed that in the reduction of the double fluoride, NbF,2KF, by sodium, a greyish powder is obtained after treating with water. The specific gravity of this powder is 68, and he considers it to be niobium hydride, NbH. Neither did he obtain metallic niobium when he reduced with magnesium and aluminium, but an alloy AlNb, having a sp. gr. of 4'5.

Niobium, as far as is known, unites in three proportions with oxygen. NbO, which is

formed when NbOF3,2KF is reduced by sodium; NbO2, which is formed by igniting niobic acid in a stream of hydrogen, and niobic anhydride, Nb2O5, a white infusible substance, which is insoluble in acids, and has a specific gravity of 45. Tantalic anhydride closely resembles niobic anhydride, and has a specific gravity of 72. The tantalates and niobates present the type of ortho-salts-for example, Na2HNьО4,6H2O, and also of pyro-salts, such as K ̧HNb2O7,6H2O, and of meta-salts-for example, KNьO5,2H2O. And, besides these, they give salts of a more complex type, containing a larger amount of the elements of the anhydride; thus, for instance, when niobic anhydride is fused with caustic potash it forms a salt which is soluble in water, and crystallises in monoclinic prisms, having the composition KgNbO19,16H2O. There is a perfectly similar isomorphous salt of tantalic acid. Tantalite is a salt of the type of metatantalic acid, Fe(TaO3)2. The composition of Yttrotantalite appears to correspond with orthotantalic acid.

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CHAPTER XX

SULPHUR, SELENIUM, AND TELLURIUM

3

THE acid character of the higher oxides RO, of the elements of the VI. group is still more clearly defined than that of the higher oxides of the preceding groups, whilst feeble basic properties only appear in the oxides R3 of the elements of the even series, and then only for those elements having a high atomic weight-that is, under those two conditions in which, as a rule, the basic characters increase. Even the lower types RO, and R2O3, &c., formed by the elements of the VI. group, are acid anhydrides in the uneven series, and only those of the elements of the even series have the properties of peroxides or even of bases.

2

any

Sulphur is the representative of the VI. group both owing to the fact that the acid properties of the group are sharply defined in it, and also because it is more widely distributed in nature than of the other elements belonging to this group. As an element of the uneven series of the VI. group, sulphur gives HS, sulphuretted hydrogen, SO3, sulphuric anhydride, and SO2, sulphurous anhydride. And in all of them we find acid properties-SO, and SO, are anhydrides of acids, and H2S is an acid, although a feeble one. As an element sulphur has all the properties of a true non-metal; it has not a metallic lustre, does not conduct electricity, is a bad conductor of heat, is transparent, and combines directly with metals-in fact, all the properties of the non-metals, like oxygen and chlorine. Furthermore, sulphur exhibits a great qualitative and quantitative resemblance to oxygen, especially from the fact that it, like oxygen, combines with two atoms of hydrogen, and forms compounds like oxides with metals and non-meta's. In this sense and aspect sulphur is bivalent, if the halogens are univalent. The chemical character of sulphur is expressed by the fact

1 The character of sulphur is very clearly defined in the organo-metallic compounds. Without lingering over this vast subject, which belongs to the province of organic chemistry, I think it will be enough for our purpose to compare the physical properties of the ethyl compounds of mercury, zinc, sulphur and oxygen. They have the common

VOL. II.

that it forms a very slightly stable and feebly energetic acid with hydrogen, which, on the one hand, resembles water in its atomic composition, and, on the other hand, the halogen acids in its property of forming salts. The salts corresponding with this acid are the sulphides, just as the oxides correspond to water or the chlorides to hydrochloric acid. However, as we shall afterwards see more fully, the sulphides are more analogous to the former than to the latter. But although combining with metals, like oxygen, sulphur also forms chemically stable compounds with oxygen, and this fact impresses a peculiar character on all the relations of this element.2

Sulphur belongs to the number of those elements which are very widely distributed in nature, and occurs both free and combined in various forms. The atmosphere, however, is almost entirely free from compounds of sulphur, although a certain amount of them should be present, if only from the fact that sulphurous anhydride is emitted from the earth in volcanic eruptions. Sea and river water generally contain more or less sulphur in the form of sulphates. The beds of gypsum, sodium sulphate, magnesium sulphate, and the like are formations of undoubtedly aqueous origin. The sulphates contained in the soil are the source of the sulphur found in plants, and are indispensable to their growth. Among vegetable substances, the proteïds always contain from one to two per cent. of sulphur. From plants these albuminous substances, together with their sulphur, pass into the animal organism, and therefore the decomposition of animal matter is accompanied by the odour of sulphuretted hydrogen, as the product into which the sulphur passes in the decomposition of the albuminous substances. Thus a rotten egg emits sulphuretted hydrogen. Sulphur occurs largely in nature, as the various insoluble sulphides of the metals. Iron, copper, zinc, lead, antimony, arsenic, &c., occur in nature combined with sulphur. These sulphides frequently have a metallic lustre, and in the majority of cases cccur crystallised, and also very often several sulphides occur combined or mixed together in these crystalline compounds. If they are yellow and have a metallic lustre they are called pyrites. Such are, for example, composition (C2H5),R, where R = Hg, Zn, S, or O. They are all volatile: mercury ethyl, Hg(C2H5)2, boils at 159°, its sp. gr. is 2:444, molecular volume 106; zinc ethyl boils at 118°, sp. gr. 1882, volume 101; ethyl sulphide, S(C2H5)2, boils at 90°, sp. gr. 0·825, volume 107; common ether, or ethyl oxide, O(C2H3)2, boils at 35°, sp. gr. 0-736, volume 101, in addition to which diethyl itself, (C2H5)=C4H10, boils about 0, sp. gr. about 0.62, volume about 94. Thus the substitution of Hg, S, and O scarcely changes the volume, notwithstanding the difference of the weights; the physical influence, if one may so express oneself, of these elements, which are so very different in their atomic weights, is almost alike.

*Therefore in former times sulphur was known as the amphid element.