From the mechanical and physical side the recent developments have been very marked. The introduction of the ‘inverted’ lamp was a tremendous step forward, and paved the way to the second great improvement, the use of ‘high-pressure’ gas, with which such successful results are being obtained. The form of lamp now coming into use for street lighting gives 1500 candle-power per mantle, and usually carries three mantles; each lamp thus develops 4500 candle-power. The purely mechanical devices which are now used to secure ‘automatic’ lighting are rapidly bringing this form of lamp into favour for street illumination. A full account of these developments would be entirely beyond the scope of the present work. In the following chapters, therefore, no complete treatment of the incandescent lighting industry can be given; but whilst the chemical aspect is treated at some length, many points of more purely technical character, which are connected with this, have also been included. CHAPTER XVIII THE CHEMICAL TREATMENT OF MONAZITE It has been stated in the previous chapter that the first Auer mantles were made of mixtures of various rare earth oxides, the mixture of thoria with 1 per cent. of ceria being first employed in October, 1891. The impetus given to the mantle industry by the success of the new mixture caused an immediate demand for thoria, which was at that time extracted from thorite (see p. 43). A ‘thorite-fever’ broke out along the coasts of Scandinavia, and the price of orangite rose to 600 marks per kilogram (about £13 10_s._ per pound avoirdupois), sinking again shortly to 80 marks[497] (about £1 16_s._ per lb). The discovery of the monazite sands of the Carolinas and Brazil, which at the present rate of consumption may be considered to be, for all practical purposes, inexhaustible, placed the industry on a firm basis, and the pure monazite, extracted from these deposits by the methods outlined in Chapter VII, is now almost the sole source of the thorium nitrate of commerce. Small quantities are obtained from thorianite, the separation of the pure material presenting, in this case, very little difficulty by reason of the solubility of the mineral in acids and the very high percentage of thoria. [497] _Vide_ Böhm, ‘Die Thorium Industrie,’ _Chem. Ind._ 1906, ~29~, 450 and 488.