18 Prof. R. Threlfall on the Electrical 



It may possibly be objected that some of the precautions 

 taken are flnnicking, and that (as in the manufacture of the 

 oxygen) an unnecessary variety of absorptive materials are 

 used. The reply I should make to such a criticism is that the 

 complete operation of any one reagent is always to some 

 small extent hypothetical, and I consider that by varying the 

 reagents one helps to get rid of the chance of the persistence 

 of impurities so small in amount as to be beyond the range of 

 ordinary analysis, i. e. beyond the point up to which we are 

 certain of the action of the reagents. 



Testing the Gas for Hydrocarbons and Hydrogen. 



In order to avoid the impurities introduced by the presence 

 of internal electrodes, a tube was prepared with external 

 electrodes. It was about 35 centim. long, of wmich length 

 20 centim. were made of capillary tube. In order to fill this 

 with the pure gas, the apparatus was exhausted three times, 

 as far as the chromous chloride U-tube, each exhaustion re- 

 quiring seven or eight hours' work with the Geissler pump, 

 after a good water-pump had got rid of the greater part of 

 the gas. 



The pressure of the remaining gas was imperceptible by 

 inspection of the pump-gauge (of which a description will be 

 given), but the pump continued on each occasion to show a 

 just appreciable bubble of gas when the mercury flowed into 

 the barometer-tube. The pumping was done at times separated 

 from each other by three or four days in order to give the 

 residual gas from the walls of the vessel time to come off ; of 

 course this gas w T as removed as it appeared. The pressure I 

 should judge was, on each occasion, about of the order of "01 

 millim. Several bottles of gas were also forced through the 

 apparatus at the ordinary pressure. 



Finally, on Feb. 19, 1891, the tube together with another 

 for experimental purposes were fused off — the former at about 

 "1 millim. pressure (the latter will be dealt with later). The 

 tube had previously been heated till the glass began to soften 

 and the walls to fall in, and the gas admitted had been for 

 several days in contact with the phosphorus pentoxide. The 

 only objection to the external electrode tube is that I have 

 hitherto failed to produce a line-discharge in it — thouo-h of 

 course Prof. J. J. Thomson's method (Phil. Mag. vol. xxxii. 

 1891) would give it in an endless tube without electrodes, 

 and if the method had been known at the time I should have 

 employed it. However, I got a bright bluish- white discharge 

 in the narrow tube (the larger ends being golden), and this 



