Positive Column in Oxygen. 561 
less than 5 mm. between 4'5 and 20 volts per cm. in these 
experiments. Now the cathode fall is about 400 volts, so 
that if a small change of a few per cent, in the field near 
the cathode occurs between two observations, it can easily 
introduce an error of 1 cm. in D. Moreover, it is necessary 
to keep the current accurately the same, since a very small 
error in reading it corresponds to a much larger one in D. 
The electrical arrangement is represented diagrammatically 
in fig. 1. ABPQ represents the discharge-tube, supported 
in such a position that its base dipped under the mercury 
filling a large jar J. The side tube T connected the dis- 
charge-tube through a drying-tube to a mercury pump ; and 
to an electrolysis apparatus where oxygen was generated 
and dried. It may be stated here that the oxygen was pre- 
pared by the electrolysis of pure barium hydrate, which 
insures its purity*. 
Fiar. 1. 
T=< 
The figure represents the tube partly filled by the mercury 
— the shaded part — when the oxygen was reduced to a low 
pressure. A is the fixed cathode connected to a platinum 
* H. B. Baker, Trans. Chem. Soc. 1902 ; vol. lxxxi. 
