5S8 The Positive Column in Oxygen. 
The observation of the discontinuity in Y at the pressure 
"8 mm. immediately recalled the discontinuity oi pv observed 
by C. Bohr* in oxygen at the pressure "7 mm., the history 
of which is very interesting. Bohr's observations were con- 
firmed by Baly and Bamsay f ; but in 1901 Lord Rayleigh i 
was unable to detect any signs o£ such an effect with his 
extremely accurate manometer, and, at a loss to account for 
the difference between Bohr's experience and his own, could 
" only suppose that it must be connected somehow with the 
quality of the gas complicated perhaps by interaction with 
the glass or the mercury/'' 
Now, according to Bohr, Boyle's law must be replaced, in 
the case of oxygen, for temperatures between 11 and 14, by 
(p + *109)r = k when p is greater than *7 mm., 
and by 
(p + '07)v = h when p is less than *7 mm. 
All the pressures given above were measured on a McLeod 
gauge : and the temperature of the laboratory ranged from 10° 
to 15°. Hence, if Bohr's results apply to them, they must be 
diminished by *109 when p is greater than *7 mm., and by *07 
when p is less than '7 mm. Moreover, the pressure *7 would 
be given by the McLeod gauge either as '809 or "77. Thus 
the pressure of Bohr's discontinuity agrees in the closest way 
with the pressure at which Y is discontinuous. Nevertheless, 
in view of Lord Rayleigh's conclusions, I have not a entured 
to make any corrections to the estimated values of my 
pressures. 
The observed discontinuity in Y points to a molecularly 
unstable condition reached by oxygen at the pressure in 
question, such that under the discharge the gas — or possibly 
only that part of it which is the seat of the discharge — 
passes from one to the other of two states of molecular equi- 
librium. In this connexion the suggestion of Sutherland § 
in explanation of Bohr's and Sir W. Crookes's anomalies is 
interesting, that oxygen tends spontaneously to pass into 
ozone at about "7 mm. pressure. This view receives some 
support from the conclusion above that the presence of ozone 
checks the return of the oxygen to the state defined by smaller 
values of Y. I have twice observed at the point of Y's dis- 
continuity curious falls of pressure which would be well 
explained by Bohr's results, and cannot but conclude that his 
* YVied. Ann. vol. xxvii. pp. 459-479 (1886). 
t Phil. Mag. vol. xxxviii. p. 301 (1894). 
% Phil. Trans. 1901, p. :205. 
§ Phil. Mag. vol. xliii. 1897, p. 201. 
