of Zinc, Cadmium, and Mercury. 35 



mercury, and were used in order to investigate the method 

 employing a number of holes. Cadmium alloys readily 

 with platinum, and so these tubes could not be used with that 

 metal. 



Tubes with iron plate. — The thin steel plates in which holes 

 are drilled are mounted in iron collars ground flat to a 

 glass flange inside the tube. If necessary, the iron collar 

 is cemented with magnesia and silicate paste to the glass 

 flange. These tubes do not give such accurate results unless 

 the collar is well-cemented to the glass flange. 



Fused quartz with ground cap. — This is the form of the 

 apparatus which is of most general application. Messrs. 

 Johnson and Matthey succeeded in boring accurately cir- 

 cular apertures in very thin silica for me. Apertures of 

 different sizes and number can then be mounted on silica 

 caps which are ground to fit the silica tube. With such 

 tubes, it will now be possible to determine the vapour 

 pressures of many other metals, such as lead, tin, &c. 



In Knudsen's experiments, the mercury was condensed on 

 the sides of the tube by the cooling agent, and afterwards 

 shaken down into a small calibrated tube. In the following 

 experiments carried out on the vapour pressure of mercury 

 for the purpose of standardizing the glass tubes, the mercury 

 pellet was always weighed directly. 



In the case of the experiments on cadmium a different 

 method of deposition of the metal was used. The end of tbe 

 tube possessed a ground joint, through which passed a glass 

 tube and on to this was sealed the end of a thin test-tube. 

 The inside of the tube was cooled by a stream of water. 

 The cadmium deposited on tbe outside of this thin cooled 

 tube. In order to make sure of the u resistance^ of the 

 tube under such circumstances, the tubes were standardized 

 with mercury, allowing the vapour to condense on the same 

 tubes cooled with liquid air. 



The chief object of such a method was to obtain the 

 deposit so that it could be easily weighed. The end of the 

 glass tube is cut off after the experiment and weighed ; 

 the deposit is then dissolved off and the tube weighed 

 again. 



There are some objections to this method, and it has been 

 found better, on the whole, to allow the vapour to condense 

 on the walls of the tube, keeping them cool outside. This 

 can only be done when the metal deposits in such away that 

 it flakes off easily without loss ; the tube has to be detached 

 from the apparatus at the end of the experiment, the deposit 

 being then scraped off and collected. Both methods have 



D2 



