[553] Chem. Zeitg. 1911, 35, 1037.
[554] D. R. P. 212842, August, 1909; vide also Kreidl and Heller, E. 17862, 1909, and D. R. P. 228203, 1910.
Artificial silk itself is of a colloidal nature, the solidification of the filaments of cellulose during the manufacture being rather in the nature of a coagulation than of a precipitation; it is to this fact that the fibres owe their solid, rod-like structure, and it is probably this circumstance also which determines the very intimate absorption of the hydroxides or peroxides produced by fixing. It is well known that colloidal substances under some circumstances possess the property of clinging tenaciously to foreign bodies, exhibiting the so-called phenomenon of adsorption. The strength and elasticity of the oxide skeleton, obtained when the fixed and dried fabric is subjected to the operation of burning off, are presumably to be referred to such a relation between the cellulose of the fibres and the insoluble thorium and cerium compounds, precipitated by one of the methods of fixing described.
The Final Stages.
—The treatment of the fabric after impregnation and fixing differs only slightly from that used for the impregnated ramie and cotton products. The dried strips are cut into suitable lengths, and the head is drawn together with asbestos and threaded across. No tulle or gauze is required, the end being simply turned down before threading. After the ordinary strengthening process for the head (vide [p. 296]) the process of manufacture was, until recently, finished, the goods being sent out in the unburnt condition, on account of the difficulties of collodinisation. These have now been overcome, so that the mantles are burnt off and collodinised as usual. Burning off and shaping are now frequently effected in one operation by machine; the nature of the methods by which the fibre is made produces a uniform fabric, and if the earlier processes are carefully carried out, a uniform product is obtained, which is therefore suitable for machine treatment.
CHAPTER XXI
OTHER TECHNOLOGICAL USES OF THE CERIUM AND YTTRIUM ELEMENTS, ZIRCONIUM AND THORIUM
The technical uses of the members of this group of the elements we are considering, apart from the employment in the manufacture of incandescent mantles, are at present very restricted. Innumerable proposals for the employment of the compounds of cerium and the allied metals, which are obtained in such large quantities as by-products in the thorium industry, have been put forward, but the actual extent to which they are utilised is so small that only an insignificant fraction of the available quantities is annually required. In the metallic form, a limited application is found for various alloys, e.g. the so-called pyrophoric alloys, misch metal, and the magnesium and aluminium alloys. Various compounds of the elements, as well as some alloys, have been suggested for use in arc-lamp electrodes, and the use of the metals themselves, as well as of various salts, for the manufacture of flashlight powders, is protected by several patents. Investigations have been made to determine the value of the oxides and sulphates as catalysts in the contact process for the manufacture of sulphuric acid, and one patent states that the yield obtained is equal to that given by platinised asbestos. Cerium salts have been proposed for tanning, and in the preparation of enamels; cerium sodium sulphate is used in the catalytic oxidation of aniline to aniline black. The oxalate has a very slight use in medicine. The oxidising power of ceric salts is of some use in photography; ceric sulphate in acid solution is also said to be an efficient oxidising agent for aromatic hydrocarbons. On account of the deep colour of the higher oxide of praseodymium, didymium salts find a limited application for marking textiles.
Compounds of the yttrium group have at present no technical importance. They were formerly used to some extent for the manufacture of filaments for Nernst lamps, but with the introduction of metal filament lamps in electric lighting, the demand for Nernst lamps and consequently for the yttria oxides, has to a very great extent died away.
Zirconium and its compounds, on the other hand, promise to become of some technical importance. The metal received considerable attention in the earlier stages of experimental work on metallic filaments for electric lighting, but it has been shown that its melting-point is not sufficiently high to allow of extended use in this direction. The carbide has been proposed for the same purpose, but is even less suitable; this compound, however, on account of its great hardness, is likely to find employment as an abrasive, and in glass-cutting. The oxide, which occurs in nature in an impure form as the mineral [Baddeleyite] (q.v.), is employed in the manufacture of ‘Siloxide’ glass and of enamels, as a pigment and polishing agent, and in various forms of lamps, e.g. the Nernst and Bleriot lamps, the Drummond light, etc. Far more important, however, is its use for fire-resistant crucibles, furnace linings and supports, etc., for which its refractory nature renders it particularly suitable. On account of its high specific gravity and non-poisonous character, it has been proposed for use in the Röntgen ray examination of the human body. Quite recently, metallic zirconium has been employed in metallurgy; addition of small quantities, in the form of suitable alloys, is said to secure sound castings, with increased strength and resistance to acids.