Cadmium Vapour Arc Lamp. 355 



The vapour-pressure of the mercury being so much higher 

 than that of the cadmium, resulted in the electric energy 

 being almost entirely carried by the mercury, and the usual 

 brilliant mercury spectrum resulted. 



In view of the preceding facts, it is evident that a 

 serviceable brilliant cadmium-vapour lamp might be ob- 

 tained by alloying the cadmium with a suitable element of 

 lower vapour-pressure. Through the courtesy of Dr. W. F. 

 Hillebrand, a quantity of the little-known element, gallium, 

 was obtained. The material was in a very impure con- 

 dition, containing approximately 10 per cent, indium. The 

 freezing-point was below 22° C, at which temperature it was 

 a liquid with a viscosity less than that of mercury. A study 

 of the impure material was made by Dr. G. E. F. Lundell, 

 who succeeded in obtaining the gallium in a relatively pure 

 condition. 



Crude gallium was dissolved in aqua regia, treated with 

 sulphuric acid and famed to remove nitric acid. After 

 dilution, small amounts of lead sulphate were filtered off. 

 The solution was then diluted, treated with hydrogen 

 sulphide and filtered to remove the hydrogen sulphide group 

 of elements. The filtrate was boiled to expel hydrogen 

 sulphide and treated with ammonium hydroxide. The 

 precipitate was filtered oft', dissolved, and reprecipitated 

 three times to free it from zinc. The final separation from 

 indium was based on the solubility of gallium hydroxide in 

 a solution of sodium hydroxide and the insolubility of indium 

 hydroxide in that reagent. The sodium hydroxide separa- 

 tion was carried through three times. The deposition of 

 gallium was finally carried out by electrolysis of the alkaline 

 solution as recommended by Uhler and Browning*. 



The purified gallium had a freezing-point of approximately 

 30° 0. This surprising fact has since been verified by the 

 careful work of Richards f, who has definitely fixed this 

 temperature at 30°' 8 0. Regarding the boiling-point of 

 this element, but little is known. The few experiments 

 which have been made are in agreement that it is above 

 1500 degrees C. This property should make it an ideal sub- 

 stance for the purpose in hand, provided it would alloy with 

 cadmium. The first experiment demonstrated that it united 

 with the cadmium with the utmost ease. In fact, the 

 addition of a few drops to ten or fifteen cubic centimetres 

 of cadmium completely changed the texture of the latter, 



* Am. .lour. Sci. cxcii. ixlii. Fourth Series) p. 389 (1916). 

 t J Miir. Am. Oliein. !Soc. vol. xli. p. 181. 



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