SSX WILLIAM SIEMENS, F.R.S. 355 



that at all pressures the discharge in gases is not a current 

 in the ordinary acceptation of the term, but is of the nature of a 

 disruptive discharge. Even in an apparently perfectly steady dis- 

 charge in a vacuum tube, when the strata as seen in a rapidly 

 revolving mirror are immovable, he has shown that the discharge 

 is a pulsating one ; but, of course, the period must be of a very 

 high order. 



At the Royal Institution, on the occasion of his lecture, he pro- 

 duced, in a very large vacuum tube, an imitation of the Aurora 

 Boreal is ; and he has deduced from his experiments that the 

 greatest brilliancy of Aurora displays must be at an altitude of 

 from thirty-seven to thirty-eight miles a conclusion of the highest 

 interest, and in opposition to the extravagant estimate of 281 miles, 

 at which it had been previously put. 



The President of the Royal Society has made the phenomena of 

 electrical discharge his study for several years, and resorted in his 

 important experiments to a special source of electric power. In a 

 note addressed to me, Dr. Spottiswoode describes the nature of his 

 investigations much more clearly than I could venture to give 

 them. He says : " It had long been my opinion that the dis- 

 symmetry, shown in electrical discharges through rarefied gases, 

 must be an essential element of every disruptive discharge, and 

 that the phenomena of stratification might be regarded as magni- 

 fied images of features always present, but concealed under ordi- 

 nary circumstances. It was with a view to the study of this 

 question that the researches by Moulton and myself were under- 

 taken. The method chiefly used consisted in introducing into the 

 circuit intermittence of a particular kind, whereby one luminous 

 discharge was rendered sensitive to the approach of a conductor 

 outside the tube. The application of this method enabled us to 

 produce artificially a variety of phenomena, including that of 

 stratification. We were thus led to a series of conclusions 

 relating to the mechanism of the discharge, among which the 

 following may be mentioned : 



" 1. That a stria, with its attendant dark space, forms a physical 

 unit of a striated discharge ; that a striated column is an aggre- 

 gate of such units formed by means of a step-by-step process ; 

 and that the negative glow is merely a localised stria, modified by 

 local circumstances. 



A A 2 



