10 ■ Prof. R. Threlfall on the Electrical 



plate whose edges were fused into the tube. The blue liquid 

 was forced through the plug by a pressure of carbonic acid 

 made from marble, which had been boiled for a good many 

 hours at reduced pressure in water before being placed in the 

 Kipp's apparatus. This carbonic acid always passed through 

 a wash-bottle of blue litmus purified in the way to be described 

 by a preliminary experiment. The filter acted so perfectly 

 that the liquid passing through it was brilliantly clear and 

 free from zinc- dust, which I greatly feared. In these preli- 

 minary operations I was practically forced to make use of some 

 rubber-stoppers, and this is the worst fault of the method . How- 

 ever, all the stoppers used were well coated with pure paraffin; 

 and though they might have given off some hydrocarbons, 

 still I was unable to detect any trace of them in the nitrogen 

 finally obtained. Some rubber-tubing was also employed to 

 make joints, but the same precautions were taken with it, and 

 in all cases the sulphur was well cleaned off. The filtered 

 liquid was finally got into a retort with a ground-on con- 

 denser, and was concentrated to one third of its original 

 volume in a brisk current of carbonic acid. 



Towards the end of the operation the stream of carbonic 

 acid was stopped, and a partial vacuum was created in the 

 retort and condenser by a water-pump, so that the final con- 

 centration took place under reduced pressure. These pre- 

 cautions were considered sufficient to remove the hydrogen 

 dissolved in the liquid as well as any traces of volatile sulphur 

 compounds resulting from supposed impurities in the potas- 

 sium bichromate. The resulting liquid was a clear blue 

 syrup, and was transfer! ed at once to the absorption-apparatus. 

 Before going into any details as to other precautions, I will 

 describe the arrangements adopted for the continuous pre- 

 paration of the nitrogen by this process. These arrange- 

 ments will be most easily understood by a reference to the 

 diagram (PI. I.) , though taps and clips are exaggerated for 

 clearness, g is a pipe projecting through the window of 

 the laboratory, and terminating close to the wall at a height 

 of about three metres from the ground. The University 

 paddock separates the laboratory by nearly a quarter of 

 a mile from the nearest road on this side, but the road on 

 the other side lies within 150 yards. The elevation of 

 the laboratory is 120 feet above sea-level. Consequently 

 the composition of the air is probably more nearly that of 

 typical " country air w than of town air. H is a U-tube 

 with paraffin (M.P. 51° C.) covered cork stoppers, and is 

 tightly packed with cotton-wool, from which the dust was 

 blown before the tube was mounted. F is a two-way tap, 



