The Effect of the Electric Discharge on Water Va])our. 145 
maximum in this case was greater while the decrease in the pressure 
was less. (Curve 6.) 
As the pressure of gas produced approached its maximum value the 
coloration on the tube began to disappear ; at the end of the experiment, 
however, the greater portion was still present. 
The discharge was passed through vapour under 2*6 mm pressure, the 
temperature being 14*5° C. The pressure again immediately began to 
increase, while the coloration, from the instant of starting the discharge, 
disappeared rapidly. When the pressure as indicated by the gauge had 
reached the value 0*308 mm., the last traces of colour had vanished. 
After this point the pressure did not vary much. (Curve 7.) Three and 
a half hours afterwards (the discharge having passed continuously) the 
pressure as indicated by the gauge was 0*307 mm., while the tube had 
remained perfectly free of coloration. In this case, then, we have again a 
rise in pressure to a maximum value ; but the maximum is permanent ; 
there is no subsequent falling off of the pressure. 
The tube being now free of coloration, the above experiments were 
repeated : 
The discharge was passed through vapour under 0*9 mm. pressure, 
(temperature = 14° C). The pressure increased to a maximum value 
of 0*07 mm., after which it did not appreciably diminish. Until the 
maximum had been reached, the tube remained perfectly free of colora- 
tion. After this faint traces of returning coloration began to appear. 
(Curve 8.) 
The discharge w^as passed through the same quantity of vapour. The 
coloration became more apparent, and the pressure increased to a higher 
maximum than before, after which it diminished for a short time. 
(Curve 9.) 
The discharge was passed through vapour under a pressure of 2*7 mm. 
As in the previous parallel case, the coloration immediately began to 
vanish, while the pressure as indicated by the gauge rose to a maximum 
and permanent value of 0*34 mm., when the last traces of coloration 
disappeared. 
The coloration, then, occurred if the pressure under which vapour was 
admitted was less than about 2*5 mm. If the coloration was present and 
the discharge was passed through vapour under a pressure just greater 
than 2*5 mm., the coloration disappeared, while the pressure of gas in the 
apparatus increased to a permanent maximum value. Further, as the 
following results show, if the discharge is passed through the same 
quantity of vapour, while the tube was free of colour, the pressure, as 
before, increased to a permanent maximum value, while there was at no 
time any coloration. 
