1894.] Electric Strength of Mixtures of Nitrogen, fyc. 263 



The a priori theory of the constant temperature of the crater is so 

 attractive, that the author is inclined to attribute this phenomenon, 

 not to any actual change of the luminosity of the crater, or to any 

 wandering of the luminous area, as is seen with a long, unsteady arc, 

 but to the refraction of the light by heated vapour. All experiments, 

 such as enclosing the arc in a small chamber of transparent mica, or 

 the use of magnets, or an air blast, have failed to produce any 

 effect. A distortion of the image of the crater while the patch re- 

 volves, has been looked for, but nothing distinguishable from changes 

 of luminosity has been seen. 



An unexpected difficulty is thus introduced in the use of the arc as 

 a standard of light, and one which may interfere with its use under 

 some circumstances as a steady and continuous source of light. The 

 author is further examining this phenomenon, with the view of 

 ascertaining its nature, and of finding practical conditions under 

 which it is absent or negligible. 



VIII. " The Electric Strength of Mixtures of Nitrogen and 

 Hydrogen." By Miss P. G. FAWCETT. Communicated by 

 Professor J. J. THOMSON, F.R.S. Received June 21, 1894. 



The experiments described in this paper were undertaken at 

 Professor Thomson's suggestion, and have beeu carried out with the 

 advantage of his advice and help. 



The immediate object of the experiments was to determine the 

 electromotive force required to produce a spark between two flat 

 parallel metal plates in a mixture of hydrogen and nitrogen in differ- 

 ent proportions and at different pressures. 



The hydrogen used was obtained by electrolysis of water, as it was 

 found that that obtained in the ordinary way from zinc and hydro- 

 chloric acid was liable to contain impurities which seriously affected 

 its electric strength. 



The two gases were collected over water in a graduated cylindrical 

 gas-holder, and were allowed to stand for some hours to give them 

 time to mix before being put into the apparatus. The mixture was 

 passed through sulphuric acid, and also through cotton wool to 

 remove dust. 



The electromotive force was supplied by a battery of storage cells, 

 each of about 2 volts, and was measured simply by counting the 

 number of cells. The strength of the cells was measured by a 

 quadrant electrometer. 



At very low pressures it was found that, unless special precautions 

 were taken to prevent the discharge passing anywhere except between 



