146 TransactiouH of tlie Boijal Society of South Africa. 
Tube free of colour. 
Temperature 14-3° C. 
Discharge passed through Vapour umder Pressure of 2-6 mm. 
Intervals of Time. 
1 min. 
3 
6 
9 
13 
18 
21 
40 
Pressure as per Gauge. 
0-037 mm. 
0-175 
0-234 
0-289 
0-325 
0-339 
0-338 
0-339 
Discharge stopped. 
Tube free of colour. 
It appears reasonable to suppose that this coloration was the result of 
chemical action of cathode rays on the glass. According to J. J. Thomson 
(" Conduction of Electricity through Gases," 2nd edition, p. 623) such 
coloration rapidly disappears on mere exposure to water vapour. In the 
present instance, however, the passage of a discharge through the water 
vapour was found to be essential for the vanishing of the coloration ; e.g., 
the pressure of gas in the apparatus being initially 0-000 mm., vapour 
under its saturation pressure (12 mm.) was admitted, and allowed to stand 
for twenty hours. At the end of this time there was no apparent diminu- 
tion in the coloration, while the McLeod gauge indicated a pressure of gas 
of 0-001 mm., showing that no appreciable dissociation or reduction of the 
vapour had taken place. On passing the discharge, however, a consider- 
able amount of the coloration had already disappeared at the end of two 
minutes and was fast continuing to do so. 
On passing the discharge through vapour under pressures greater than 
about 2-5 mm., the pressure corresponding to unstability of the colora- 
tion, the gauge readings indicated increases in pressure to permanent 
maximum values. (Curves 10, 11, 12, 13, 14.) These maximum 
values did not become continually greater as the pressure of vapour 
admitted became greater. They differed little from one another except 
when the pressure of vapour admitted approached saturation pressure 
(11 to 12 mm.). 
