140 Transactions of the Boyal Society of South Africa. 
(see Fig. 2). The object of this was to allow of the introduction of definite 
small quantities of vapour into the apparatus. Vapour under various 
pressures ranging to saturation pressure was introduced into the 
exhausted apparatus and absorbed, in order to test for the presence of 
any unabsorbable gas carried over with the vapour. The results were 
satisfactory. After the first two trials the pressures indicated by the 
gauge after the vapour had been absorbed varied from O'OOl mm, 
downwards. This method of admitting vapour was adopted. 
Experimental Arrcmgements. — -Fig. 2 shows the complete apparatus as 
finally used. The trap T served to separate the pump and P2O5 bulb 
from the rest of the apparatus, so that the X-ray tube and the McLeod 
gauge might be connected without the inclusion of any P2O5 bulb. It 
also served as a manometer for measuring the pressure of vapour intro- 
duced into the X-ray tube. The capillary C was added as a means of 
introducing air or other gases quite independently of the introduction 
of water vapour. The slope of the tube tt was for the prevention of its 
obstruction by mercury carried over from the trap T3 when this was 
opened for the introduction of the vapour ; for at this stage the trap 
separated two regions of the apparatus, in one of which was a pressure 
of 0-000 mm., and in the other of which was saturation pressure of 
water vapour. 
The X-ray tubes first used were of the ordinary type, with aluminium 
cathode and platinum anode and anticathode combined. An ordinary 
10-inch spark induction coil, with platinum contact-breaker, was used for 
the discharge, worked by accumulators giving 8 volts. The spark-length 
(between points) in air was about 2-5 cm. 
Experiments were first made with air. The air was admitted through 
the capillary, and the variation in quality of the X-rays tested by observing 
the sharpness and intensity of a shadow cast on a barium platino-cyanide 
screen. These experiments yielded no satisfactory results, the behaviour 
of the various tubes being very inconsistent. A characteristic incon- 
sistency was in the reduction of pressure produced by the discharge. This 
phenomenon was present in most of the tubes, but it was very much more 
marked in some cases than in others. In one instance the discharge dis- 
posed of the air as fast as it was introduced through the capillary, so that, 
with the capillary broken, the tube was giving good rays for several minutes. 
Blackening of the tubes through the disintegration of the electrodes was 
rapid and considerable, more especially when the pressure of air in the 
tube was too high for the passage of the discharge to be attended by any 
fluorescence of the glass. Attempts were made to diminish this effect by 
varying the form of X-ray tube. The form of tube shown in Fig. 3 is that 
which proved to be the most successful. The anode was made in 
the form of a long spiral of aluminium, with the object of exposing 
