1879.] On Electrical Discharges through Rarefied Gases. 21 



situ on its liquefaction, or else was, as the result of the great floods 

 consequent upon the bursting of lake barriers, carried successively to 

 lower levels, leaving here and there banks of sand and gravel at 

 various heights on the hill sides. These destructive floods combined 

 with the incessant river inundations due to the same general thaw of 

 the great ice-sheet, carried down and spread out in the valleys and 

 plains the great beds of gravel and sand, which, with the modifications 

 since brought about by long continued fluviatile action, have given 

 rise to various forms of escars, terraces, and other less defined 

 accumulations of these detrital materials. 



May 8, 1879. 



t THE PRESIDENT (followed by Lord LINDSAY, Vice-President) 



in the Chair. 



The Presents received were laid on the table, and thanks ordered for 

 them. 



The following Papers were read : — 



I. " On the Sensitive State of Electrical Discharges through. 

 Rarefied Gases." By William Spottiswoode, P.R.S., and 

 S. Fletcher Moulton, late Fellow of Christ's College, 

 Cambridge. Received April 2, 1879. 



(Abstract.) 



It has frequently been remarked that the luminous column pro- 

 duced by electric discharges in vacuum tubes sometimes displays great 

 sensitiveness on the approach of the finger, or other conductor, to the 

 tube. This is notably the case when with an induction coil a very 

 rapid break is used, or when with any constant source of electricity 

 an air-spark is interposed in the circuit leading to the tube. The 

 striking character of the phenomena, and the opportunity which they 

 showed for affecting the discharge from the outside during its passage, 

 led the authors of this paper to consider that a special examination of 

 this sensitive state would be desirable. 



All the circumstances under which sensitiveness is produced appear 

 to agree in requiring, first, that there should be a rapid intermittence 

 in the current leading to the tube ; and secondly, that the individual 

 intermittent discharges should be small in quantity and extremely 

 brief, if not instantaneous, in duration. Both these requirements are 

 fulfilled by the methods used in the present investigation, viz., a 



