378 E. A. Partridge — Atomic Weight of Cadmium. 



stated that zinc and cadmium can be distilled in vacuo, leaving 

 the impurities as a residue. This method has been employed 

 by Morse and Burton* for the purification of zinc. One of the 

 means employed by Stas in his classical investigation upon 

 atomic weights was the purification of silver by distillation. 

 "Whenever available, distillation ranks first among the processes 

 for purification at the disposal of the chemist. This was 

 affected in the case of cadmium in tube retorts of glass, about 

 30 cm in length and 20-25 mm in diameter. 



The tubes were closed at one end and drawn down at a point 

 about ll cm from the closed end, making a neck about 12 mm in 

 diameter. 100 grams of cadmium having been introduced, 

 the open end was drawn down to a size suitable for a connec- 

 tion with the mercury pump. The portion of the tube be- 

 tween the neck and the end connected with the pump serves 

 to condense and retain the distilled metal. 



After having been supported on a combustion furnace, the 

 retort was exhausted as completely as a rapidly acting three- 

 fall mercury pump would effect in one hour. The gauge, then 

 standing only a fraction of a mm. lower than the barometer. As- 

 the pump was self-acting this degree of exhaustion was readily 

 maintained during the entire operation. Heat was then slowly 

 applied and so managed that the greater part of the metal con- 

 densed and ran down the sides of the retort, only the more 

 volatile portion passing beyond the neck of the tube. A frac- 

 tional distillation was thus effected, cadmium alone passing 

 over. In an hour 60 grams of cadmium had collected in 

 the receiver, the distillation was then stopped and when entirely 

 cold the metal was removed by cutting open the tube. The 

 greater part of it collected as a bar at the bottom of the 

 receiver, while the sides and top were lined with crystals, 

 many of which were quite perfect. The residue in the retort 

 was covered with a brownish black scum, which was tested 

 spectroscopically and found to consist mainly of lead, iron and 

 thallium, with a little copper. The lines of zinc were once 

 very faintly seen. 



The cadmium thus purified was distilled a second time in the 

 same manner. The residue from this distillation, which was 

 pushed no further than the first, remained perfectly bright to 

 the end and when tested with the spectroscope did not reveal 

 a trace of foreign metals. All the cadmium used in this inves- 

 tigation was thus purified by double distillation. 



One of the methods used for the determination of the atomic 



weight of cadmium is that of Lenssen which, although good, 



was merely touched upon by him. His result was based upon 



only three experiments, using very small quantities of material. 



*Amer. Chem. Jour., x, 311. 



