of Zinc, Cadmium, and Mercury* 37 



by means of a relay connected to a mercury thermometer 

 with a large bulb, and stirred by an asbestos fan. All six 

 sides of the uralite box were wound with spirals of resistance- 

 wire. The box was carefully insulated by means of slag 

 wool packed round between it and a large wooden box 

 outside. A box of this description could be kept auto- 

 matically at a constant temperature throughout within 

 0°"5 0. ; but when the tubes are inserted, the leakage of 

 heat up the tubes caused the temperature to fluctuate. In 

 order to overcome this difficulty, the tubes themselves were 

 wound outside closely with resistance- wire, for three or four 

 centimetres, between the position of the hole and the point 

 of deposition. This circuit was operated by the same relay 

 and the heating so regulated by a resistance, that it just 

 counteracted the leakage up the tube. 



The temperature itself was measured by mercury thermo- 

 meters, standardized and corrected for stem exposed. No 

 doubt further refinement would be obtained by the use of 

 resistance-thermometers, but this complication has been left 

 for subsequent work. 



Another important point is the maintenance of a good 

 vacuum. In the experiments with mercury, Knudsen 

 recommended washing the tubes out with hydrogen, so as to 

 be sure that the residual gas did not interfere with the 

 accuracy of the results, by colliding with the molecules of 

 vapour in their passage through the aperture. In the pre- 

 liminary experiments on mercury made in Professor Nernst's 

 laboratory (Berlin), the precaution was taken to boil the 

 mercury in a separate tube. After sealing the latter and 

 the main tube from the mercury pump, the mercury was 

 then tipped into the main tube and the side tube was sealed off. 



In the case of the experiments with such metals as cadmium 

 or zinc, or when metals such as iron are employed for the 

 materials in which the hole is bored, it is necessary to keep 

 a " good " vacuum throughout the experiment. This was 

 accomplished by means of a charcoal tube kept in liquid air, 

 The gas which is evolved is chiefly carbon oxides and water 

 vapour ; the trace of hydrogen has not been found to be 

 sufficient to cause detriment to the experiments. 



Besides this, a Rose oil-pump was found very convenient 

 for quickly obtaining a high vacuum : in some of the shorter 

 experiments, it was found best to fill the tube with nitrogen, 

 insert it in the baths, and when the temperature was constant, 

 to quickly pump out the nitrogen ; the time of starting in 

 this way could be reduced to one mintute. 



Weighings were made on a balance sensitive to 0*02 mg. 



