148 Transactions of the. lloyal Society of South Africa. 
The discbarge was passed through vapour under various pressures, 
ranging to saturation pressure, for certain temperatures, the method of 
procedure being as follows : x\fter exhausting the apparatus included 
between the traps and J to 0*000 mm., the requisite amount of vapour 
was introduced, and trap closed to separate the discharge tube, McLeod 
gauge, &c., from any moisture that might rise to the surface of the mercury 
at b during the course of the experiment. At the end of each experiment, 
immediately after the discharge had been stopped, the water vapour was 
absorbed, and the apparatus again exhausted to prevent any appreciable 
occlusion of gas in the walls of the tube, which would be driven out by the 
discharge in the next experiment, and so cause error. In order to 
maintain the temperature constant the days were chosen cooler by two or 
three degrees than the required temperature, and the temperature kept up 
by regulating a number of glow-lamps. In this way it was found possible 
to prevent the temperature from varying by more than 0*1° C. 
Following is a series of experiments conducted at the temperature 
19*8° C. The series is typical of others : — 
(1) At 1.0 P.M. Discharge passed theough II2O under 0-6 mm. 
Pressure. 
Times. 
Pressures. 
Temperature. 
Remarks. 
1.1 p.m. 
1.3 
1.5 
I.IO 
1.14 
1.36 
0-050 mm. 
0-060 
0-055 
0-047 
0-041 
0-036 
19-80° C. 
19-80 
19-80 
19-80 
19-80 
19-80 
Faint green fluorescence. 
Appearance of coloration. 
Signs of more pronounced 
fluorescence. 
(2) At 3.19 P.M. Discharge passed through H2O under 2-1 mm. 
Pressure. 
Times. 
Pressures. 
Temperature. 
Ptemarks. 
3.21 p.m. 
3.26 
3.29 
3.32 
3.40 
3.52 
I 4.2 
4.11 
0-095 mm. 
0-196 
0-217 
0-220 
0-219 
0-217 
0 213 
0-208 
19-80° C. 
19-85 
19-85 
19 85 
19-85 
19-80 
19-80 
19-80 
Very narrow striations 
with faint fluorescence. 
Striations less distinct. 
Coloration still present. 
