(3) Potassium azide, KN₃, throws down thorium hydroxide from boiling neutral or faintly acid solutions. Ceric salts if present must be previously reduced; zirconium must be previously removed by oxalic acid. (4) Precipitation may be effected with potassium iodate in strong nitric acid solution. Here also ceric salts must be reduced before applying the test. Zirconium also gives the test; the precipitate must therefore be washed and warmed with oxalic acid, in which thorium iodate is insoluble, whilst zirconium iodate is soluble. The methods of estimating thorium are given in Chapter XVIII. PART III THE TECHNOLOGY OF THE ELEMENTS CHAPTER XVII THE INCANDESCENT MANTLE INDUSTRY--HISTORICAL AND GENERAL INTRODUCTION The group of elements which we are considering can be divided, from the point of view of technical application, into two classes. The first of these contains one element only, titanium, which in its technology, as in its chemistry, stands apart from the others; it will, accordingly, be treated in a separate chapter. The second class contains the yttrium and cerium metals, with zirconium and thorium; the technical importance of these elements is due chiefly to the use of their oxides in illumination, to a small extent in Nernst lamps, and to a much greater extent in the so-called Incandescent Lighting. The manufacture of incandescent mantles[487] is a large and ever-extending industry, intimately bound up with the older process of coal-distillation, with its innumerable ramifications; indeed, it may be said that but for the ingenious invention of Dr. Auer, illumination by means of coal-gas would to-day have been almost obsolete. The discovery which resulted in the production of the familiar incandescent mantle of the present day may be regarded as the culmination of a century’s effort to increase the value of coal-gas as an illuminating agent. In the present chapter it is proposed to outline the history of these endeavours, and to give a short general account of Auer’s work and its results. [487] The term ‘incandescent mantle’ is not, perhaps, scientifically very desirable. It is used here, not only on account of its general acceptance, but also because there seems to be no brief and convenient term which might be used in its stead.