200 



Lord Rayleigh. 



[Jan. 16, 



convey the electrodes : one is the siphon for the supply of alkali, 

 while the fourth and fifth are for the withdrawal and introduction of 

 the gas, the former being bent up internally, so as to allow almost 

 the whole of the gaseous contents to be removed. The fifth tube, by 

 which the gas is returned, communicates with the fall-tube of the 

 Topler pump, provision being made for the overflow of mercury. In 

 this way the gas, after weighing, could be returned to the working 

 vessel at the same time that the globe was exhausted. It would be 

 tedious to describe in detail the minor arrangements. Advantage 

 was frequently taken of the fact that oxygen could always be added 

 w r ith impunity, its presence in the working vessel being a necessity 

 in any case. 



When the nitrogen had been so far removed that it was thought 

 desirable to execute a weighing, the gas on its way to the globe had 

 to be freed from oxygen and moisture. The purifying tubes contained 

 copper and copper oxide maintained at a red heat, caustic soda, and 

 phosphoric anhydride. In all other respects the arrangements were 

 as described in the memoir on the densities of the principal gases,* 

 the weighing globe being filled at 0°, and at the pressure of the 

 manometer gauge. 



The process of purification with the means at my command proved 

 to be extremely slow. The gas contained more nitrogen than had 

 been expected, and the contraction went on from day to day until I 

 almost despaired of reaching a conclusion. But at last the visible 

 contraction ceased, and soon afterwards the yellow line of nitrogen 

 disappeared from the spectrum of the jar discharge. f After a little 

 more sparking, a satisfactory weighing was obtained on May 22, 

 1895 ; but, in attempting to repeat, a breakage occurred, by which a 

 litre of air entered, and the whole process of purification had to be 

 re-commenced. The object in view was to effect, if possible, a series 

 of weighings with intermediate sparkings, so as to obtain evidence 

 that the purification had really reached a limit. The second attempt 

 was scarcely more successful, another accident occurring when two 

 weighings only had been completed. Ultimately a series of four 

 weighings were successfully executed, from which a satisfactory con- 

 clusion can be arrived at. 



* < Koy. Soc. Proc.,' vol. 53, p. 134, 1893. 



+ Jan. 29. — When the argon is nearly pure, the arc discharge (no jar connected) 

 assumes a peculiar purplish colour, quite distinct from the greenish hue apparent 

 while the oxidation of nitrogen is in progress and from the sky blue observed when 

 the residue consists mainly of oxygen. 



