With regard to the composition of mantles, numerous proposals have been made. It is stated that thoria with 0·25 per cent. of uranic oxide, UO₃, gives a light almost equal to that of the Auer mantle. Zirconia with 0·40 per cent. of vanadium, in the form of the pentoxide, is said to give a splendid white light; the vanadium oxide slowly volatilises, but addition of an equivalent proportion of silica is said to prevent this. Langhans claims to have obtained a product equal in light-giving power to the Auer mantle, by using as impregnating fluid a solution of colloidal silica, obtained by the addition of nitric acid to a solution of sodium silicate, to which suitable quantities of rare earth nitrates are added. Bodies obtained by the use of very similar solutions give skeletons which are coming into extended employment for gas radiators. The ‘Sunlight’ mantles use a mixture of thoria (50 per cent.), alumina (40 per cent.), and chromium sesquioxide (10 per cent.). A direction of development in which some success has been attained is the introduction of self-lighting devices. The catalytic action of finely divided metals has been proposed in innumerable patents,[521] but these devices are unreliable, and it seems doubtful if chemical methods will ever be successfully applied to the problem. For the lighting of streets, shops, etc., the ‘by-pass’ system is employed; a tiny jet of gas burns continuously from a pin-hole nozzle, which is momentarily increased, when the main supply is turned on, to such an extent that the gas issuing from the burner is ignited.[522] [521] _Vide_, _e.g._ _D. R. P._ 158974 and 253550; _F._ 417934. [522] For automatic regulators for self-lighting, _vide_ _J. Gasbel_. 1910, ~53~, 490. An account of the innumerable forms of lamps and burners which have been introduced in the last twenty years would fill several volumes. The theoretical grounds on which improvements in this direction are based are outlined in an able article by Dr. H. Bunte, a recognised authority on incandescent lighting, which appeared recently;[523] for an account of some of the lamps which have been successfully applied, the reader is referred to a recent French publication.[524] [523] _J. Gasbel._ 1911, ~54~, 469; _vide_ also Pickering, _J. Gaslighting_, 1911, ~113~, 156. [524] _L’Éclairage à l’incandescence par le gaz_, Lévy, Part I. Ch. III. CHAPTER XX ARTIFICIAL SILK--ITS PRODUCTION AND USE IN THE MANTLE INDUSTRY