1895.] Gondactivities of Badly Conducting Substances. 267 



with air at different pressures with the effect produced by bulbs 

 of the same size filled with solutions of sulphuric acid of different 

 strengths ; the conductivity of the gas was estimated by finding 

 which of the solutions produced the same effect on the luminosity 

 of the discharge of the bulb in B as the bulb containing the gas. 

 The bulb was connected to the air-pump by a flexible glass spiral 

 which allowed it to be lifted out of, or inserted, in the coil without 

 disturbing the connection with the pump. When the air in A 

 was at atmospheric pressure it produced no effect on the bulb 

 in B, indeed no effect was observed until the pressure in the bulb 

 in A was reduced to that due to 3 or 4 millimetres of mercury ; 

 at this pressure, though no discharge of any kind was visible in the 

 bulb, the pressure of the bulb produced a distinct effect upon the 

 luminosity of the bulb in B, and this effect rapidly increased 

 until at a pressure of between 1 and 2 millimetres it attained 

 its maximum value. During these stages there was no discharge 

 visible in the bulb in the coil A; on further diminishing the 

 pressure the effect on the bulb in B diminished until the pressure 

 got so low that the ring discharged began to appear in the bulb. 

 When this stage was reached a diminution in the pressure increased 

 the effect on the bulb in B ; the ring discharge in A got brighter 

 and brighter, and the conductivity seemed to go on increasing 

 as the pressure was reduced. It was only after very protracted 

 pumping that a stage was reached when a further diminution in 

 pressure produced a diminution in the conductivity. 



Thus if we represent the effect of the bulb in A upon the 

 luminosity of the one in B by the ordinate of a curve of which 

 the abscissa represents the rarefaction, we get a curve similar 

 to that shown in Fig. 3. The existence of the first maximum 



is very remarkable especially as it is not accompanied by any 

 luminosity of the gas. I have come to the conclusion that 

 although at this pressure the electromotive force is not sufficient 

 to send a ring discharge through the gas, yet there is a dark 

 discharge from the inside of the glass in the neighbourhood^ of 

 the coil of wire, due to the high potential which this coil attains 

 at some phases of the discharges of the jars. That there is an 



