12 Prof. R. Threlfall on the Electrical 



mercury ; and X is a little phosphorus-pentoxide tube re- 

 quired to protect the tap, which is lubricated with phosphorus 

 pentoxide, and which of course requires to be kept at the 

 proper degree of concentration. Y is a sulphur tube, and at 

 Z are silver and copper tubes. Beyond Z is seen a tube and 

 gauge of the type used in experimenting on the nitrogen at 

 low temperatures. 2 is a small phosphoric-acid guard-tube, 

 M* is a sulphur tube, and 12 and <£ are silver and copper 

 tubes separated from each other by long tubes of capillary 

 bore. This system divides at b, so that it may be put in 

 communication with a water-pump via d, a, and e, or a 

 Geissler pump through /, where there is another tap not 

 shown. The taps at F, W, cl, and / are fine samples of the 

 diagonal taps made by the successors of Geissler, of Bonn ; 

 they have all been reground with " optical " precision, and 

 are all lubricated (except Fj w T ith phosphorus pentoxide and 

 protected with mercury. The weak point in the system is at 

 the stoppers of L and K, which had to be lubricated with 

 tallow. In order to satisfy myself of the nature of the 

 tallow I began by purifying it with animal charcoal, and 

 finally by keeping it melted for eight hours in a vacuum. 

 After this the smell was almost gone and the tallow was 

 applied to the stoppers mentioned. The tap F was lubricated 

 with vaseline and beeswax also long melted in vacuo. I 

 feared that I should introduce hydrocarbons at these points, 

 but the convenience of using the tallow caused me to try it, 

 and undertake an examination of the gas in a vacuum-tube 

 in order to find whether any hydrocarbons were present or 

 not. The result was that no trace of the " three " bands 

 could be found, so that the enormous complications which 

 would have been required in the other event were happily 

 unnecessary. The stoppers of the large ammonia vessels 

 were of paraffin-coated rubber. It will be noticed that all 

 the points where tallow or rubber were used are to the right 

 of the main purifying system, which includes strong sul- 

 phuric acid, and the grinding of all the stoppers was so ac- 

 curate that an almost inappreciable film of tallow was all that 

 was necessary. It remains to describe the precautions taken 

 with the various reagents, beginning on the right. 



Ammonia.- — The examination of the ammonia sold as " pure 

 liquid ammonia"" which I made in connexion with the am- 

 monium bichromate experiments showed that nothing was to 

 be gained for the purpose of the main experiment by attempt- 

 ing any further purification. 



Copper. — At different times sheet gauze, wire, and turn- 

 ings were employed. The purity of the copper does not 





