~Application to Arc-lamp Electrodes.~--During the last fifteen years, innumerable efforts have been made to adapt titanium and its compounds to the manufacture of arc-lamp electrodes, or pencils.[631] The spark-spectrum of titanium is very rich in lines, and in respect of light efficiency, the element is very suitable for the purpose; the experimental difficulties, however, have been very great, and though electrodes containing titanium compounds have been on the market for some years, the problem cannot be said to have been satisfactorily solved. The best pencils contain titanium carbide, but successful attempts have been made to use the oxide. As early as 1904, Weedon[632] proposed an electrode prepared by heating 7 parts (1 mol.) of the dioxide with 1 part of carbon to 1500°-2000°C.; the ‘sub-oxide’ produced was powdered, worked up into a paste with a suitable binding material, and forced through a nozzle. The sticks so obtained, after drying and baking in the usual manner, were said to give satisfactory results, but consumption is very rapid, and troublesome deposits of the dioxide are formed at the end of the electrode. The dioxide, which alone is a very bad conductor, enters directly into the composition of the so-called ‘magnetite’ pencils, which are best made[633] by fusing together magnetite, rutile, and chromite, in suitable proportions, with a little potassium fluoride, powdering the brittle mass, and using this to form a paste from which the pencils may be obtained as usual. These electrodes are said to give a very efficient and fairly steady arc. They have the disadvantage that tiny glowing particles are thrown off, which soon render the globes opaque; the addition of sulphur[634] to the powder during manufacture is said greatly to diminish this inconvenience. Pencils made in a similar manner from powdered ferro-titanium[635] do not appear to have come into use. [631] _Vide_, _e.g._ Ladoff, _J. Ind. Eng. Chem._ 1909, ~1~, 711. [632] _E._ 26921, 1904. [633] _E._ 2027, 1909. [634] _E._ 18965, 1909. [635] _U. S. P._ 840634, January, 1907.