1886.] Passage of an Electric Discharge through Nitrogen. 331 



Fig. 1. 



then covered with mercury, which prevents any leakage between the 

 electrode and the tube, which we found frequently happened if this 

 precaution was not taken. The tubes were cleaned before being used 

 by filling them (1) with aqua regia, which was boiled in the tube, (2) 

 with caustic potash, (3) distilled water, (4) very pure alcohol. After 

 this they were carefully heated and dried. This was the treatment 

 adopted for the greater part of the tubes, some of them, however, 

 were treated with boiling sulphuric acid in addition. 



The sulphuric acid in the gauge was boiled with sulphate of 

 ammonia before being used ; when the sulphate was first added to the 

 acid, the acid became dark, but it was boiled for about half a day 

 until it was quite colourless, and its volume about one-fourth of its 

 original value. 



The levels of the liquid in the legs of the gauge were read by a 

 cathetometer ; when the liquid was sulphuric acid the readings could 

 be made accurately enough by placing a sheet of white paper behind 

 the gauge and illuminating it by a gas flame. "When, however, the 

 liquid in the gauge was mercury a different course was adopted. In 

 the first place, the gauge-tube had to be much larger to prevent 

 mistakes arising from the sticking of the mercury to the sides of the 

 tube. The gauge used for mercury was of the shape shown in fig. 2 ; 

 the diameter of the tube where the free surface of the mercury came 

 into contact with it was about 2 cm. A different method of reading 

 the levels of the mercury in the legs of the gauge had also to be 

 adopted, because it was found that when the gas flame was in front 



