Positive Column in Oxygen, 565 
discharge, if the pressure of discontinuity is approached very 
gradually. I have twice noticed immediately after the sudden 
rise in Y the appearance of peculiar striee so close together 
that there were 10 within 1*8 cm. That was with the zinc 
electrodes. It is very seldom indeed that they appear with 
oxygen. 
To investigate the discontinuity in Y more thoroughly, I 
connected to the apparatus a large glass tube which could 
be filled partly or wholly with mercury like the barrel of a 
mercury pump. With this arrangement the pressure could 
be varied, and. without introducing new oxv£en, could be 
made to pass backwards and forwards across the point or 
discontinuity. It was then observed that Y passed from its 
low value to its high and back again from high to low, 
keeping pace with the pressure. This is illustrated by the 
following simultaneous values of p and Y, which represent a 
series of experiments upon the same mass of oxygen : — 
•95 mm., 10; -84,11-4; -765,19-2; -802,20-0; 
SS, 11-6 ; -81, 20. 
It is evident, therefore, that the discontinuity is not brought 
about because the gas has been vitiated by the discharge. It 
must be due to some inherent instability in the oxygen when 
the pressure is slightly greater than '8 mm. 
But when a quantity of oxygen had been subjected to a 
long series of discharges, the pressure can be raised con- 
siderably beyond '8 mm. where the discontinuity first 
occurred. Thus on one occasion, after the pressure was 
reduced below the discontinuity point, it was gradually 
raised from "79 mm. to 1'07 without Y's abruptly changing, 
and Y reached the abnormally high value 23. The same 
phenomenon was observed with the silver electrodes. 
The electric discharge therefore tends to prevent the 
oxvgen from recovering the condition roughly defined by 
p = '$, Y=ll. It is natural to attribute this tendency to 
ozone ; especially since the fall of pressure during each of 
these two series of observations showed the conversion of a 
considerable amount — probably more than 15 per cent. — of 
oxvgen into ozone. 
We may therefore conclude that the presence of ozone 
checks the repassage of the oxygen through the discontinuity. 
When the pressure falls below *8, the slope of the curve 
p-Y is very steep : but there is no discontinuity whatever 
here. On the contrary, Y is nowhere determined so easilv 
as when p lies between *5 and *8 mm. The discharges for 
this range of pressures are very steady : the readings of D 
