H Prof. Chattoek, Miss Walker, and Mr. Dixon on the 



at its other end by a micrometer-screw S with a large 

 divided head. P is the discharging point and R the ring, 

 both of them supported by ^lass-sheathed wires and capable 

 of motion in a vertical direction. 



The apparatus having been completely filled with tur- 

 pentine, mercury was introduced at F until it stood at AA, 

 a layer of water separating it from the turpentine in each 

 limb ; the cross wire of the microscope M was then focussed 

 on the mercury surface in the right-hand limb. Change of 

 level due to discharge was measured by turning S until the 

 mercury surface again appeared on the cross wire, after 

 which it was of course a simple matter to calculate the 

 pressure of the discharge. 



The chief difficulty met with was the tendency for ions to 

 pass from the ring to the point, and so reduce, and in some 

 cases even reverse, the pressure. Brass rings were very 

 troublesome in this respect, no doubt owing to chemical 

 action of the turpentine on the brass ; for which reason 

 platinum was adopted. Dust particles in the liquid also 

 reduced the pressures, presumably by playing the part of 

 secondary discharge-points ; and the pressures were found to 

 rise as the purity of the turpentine was increased. 



In Curves I. are plotted the pressures in arbitrary units as 

 ordinates, observed in the purest turpentine we were able to 

 obtain, between a platinum point and ring ; tbe distances 

 between these, in centimetres, being the abscissse. The 

 current was about 0*2 microampere, and each point is the 

 mean of 24 observations. 



The full curve is for positive discharge, the dotted for 

 negative. Both curves show the existence of " back-dis- 

 charge" from the ring, this being most marked for the 

 negative. It will be seen that at a distance of 2 centimetres 

 the curves have become practically straight, so that velocities 

 should be calculated for distances above this value (see 

 previous paper). Unfortunately, the effect of conduction 

 along the sides of the tube then makes itself felt, the 

 pressures at 2*4 centimetres falling below their proper values, 

 and the curves at still greater distances tending downwards. 

 It is thus unsafe to go beyond 2 centimetres, and we have 

 therefore calculated V from tangents drawn to the curves at 

 this point. The results are thus possibly rather too high, and 

 must be regarded as approximate only in the absence of 

 experiments with a wider tube, which there is no immediate 

 prospect of our being able to make. The following are the 

 values obtained : — 



c. V-f. V-. V-/V4- ; 



0-2 <00013 <O0015 1-15 



