564 
Rev. P. J. Kirkby on the 
during a given discbarge are estimated to be 4 or 5 per cent, 
except for pressures below '8 mm., when tbe results are 
much more accurate. The pressures were all measured upon 
a McLeod gauge. 
Table I. is illustrated by the curve in fig. 2 3 in which all 
the points lie satisfactorily near the curve except two, the 
determinations of which were more uncertain than that of 
the others. 
Fie. 2. 
The curve is entirely different from corresponding curves 
of the other common gases. When pis greater than 1*5 mm. 
the values of Y are only between one-tenth and one-fifth of 
the electric force in a similar positive column in the similarly 
dense gases nitrogen or carbon monoxide. Again, at the 
pressure l - 75 mm., Y reaches a minimum with the curiously 
small value 4*5 volts per cm. 
The third peculiar and most interesting feature of the 
curve is the violent discontinuity which Y undergoes at the 
pressure *8 mm. That the discontinuity is real and not 
apparent is shown by the fact that Y then passes suddenly 
from about 11 to 20 without seeming to assume any inter- 
mediate value, and even more by the fact that a complete 
change seems to come over the discharge, which now settles 
down into the greatest steadiness. Thus the curve is broken 
and not merely very steep. The change occurs during a 
