THE RARE METALS 271 



The percentage of tungsten in tool steel varies with the manu- 

 facturers. Some use from 1$ to 3$ per cent.; others from 16 to 

 20 per cent, of tungsten. According to C. A. Edwards, the hard- 

 est steel recorded contained 19.37 per cent, tungsten. Tungsten 

 is added to steel in the form of an alloy of tungsten and iron 

 carrying from 40 to 82 per cent, of the former metal. Alloys 

 of tungsten with copper and aluminum are well known, and of 

 considerable technical value. A small quantity of tungsten 

 added to aluminum greatly improves its resistance to erosion, 

 and increases its tensile strength. Tungsten is used in the 

 manufacture of crucibles for electric furnaces. Powdered tung- 

 sten is mixed with carbonaceous matter in the form of a paste, 

 pressed into the desired shape, and sintered. Tungsten is used 

 as a filament in incandescent electric lamps. The extreme 

 whiteness of the light renders it far superior to that of the carbon 

 incandescent lamp, which it is rapidly supplanting. It is far 

 more efficient than the tantalum lamp. The drawback is the 

 brittleness of the filament, and much material is lost in shipment. 

 The advantages are its whiter-light, its longer life, and its use in 

 either alternating or direct currents. Metallic tungsten has been 

 used in arc-lamp electrodes. Tungsten is used in rendering 

 curtains, draperies and papers fire-proof. It is used as a mordant 

 in dyeing, also in weighting delicate fabrics. As sodium tung- 

 state has approximately the same ratio of expansion for moderate 

 temperatures as platinum, it is used for sealing platinum appa- 

 ratus for making water determinations in rock analysis. Tung- 

 sten is used as a pigment in the manufacture of glass, also of gold 

 and violet bronze powders. Calcium tungstate is used as a 

 screen to make X-rays visible. 



Economics. The production of tungsten is so closely related 

 to that of pig iron, from which tungsten steel is manufactured, 

 that the output for 1908 was far below that of 1907. The value 

 of its production is as follows: 1905, $268,676; 1906, $348,867; 

 1907, $890,048; 1908, $229,995; 1909, $559,900; 1910, $844,526, 

 and 1911, $407,985; 1912, $492,000. 



Titanium 



Properties. Titanium, symbol Ti, is a rare metal, extremely 

 difficult to isolate in a pure state, owing to the fact that it unites 

 directly with nitrogen, forming a nitride. Its melting point is 



