PRODUCED IN GASES BY RONTGEN RAYS. 
'Z07 
the gasometer scale the volume emitted was obtained, and with the aid of the 
pressure readings which were taken the average velocity of the gas stream in the 
apparatus could be calculated. 
An interval of about three minutes was allowed as a rest for the tube, as this made 
it much more constant over a large number of readings. In the mean time, if 
necessary, gas was forced back from the gas bag into the gasometer. Guided by the 
previous electrometer deflection the potential of the outer cylinder was now changed, 
and the whole process repeated. In this way a number of readings were taken, such 
that the electrometer deflections ranged from some value down to near zero. These 
were taken in such an order that at first, say, a descending series of readings was 
obtained, and then immediately afterwards an ascending series. In this manner it is 
possible to detect any uniform changes which are taking place in the intensity of the 
rays, for in that case the two series of points would lie on curves of different 
inclinations. 
It was seen in § 5 that the time of passage of the ions from one cylinder to 
the other could be varied by changing the velocity of the gas stream, and also by 
changing the distance X. Both of these were employed in practice, and it was found 
that the values of the velocity obtained diminished as the time increased; but they 
were practically the same for two different values of X if the velocity of the gas 
stream was changed in the same ratio, i.e., if the time of passage of the ions was 
the same. 
J. S. Townsend* has recently observed that the rate of diffusion of the ions 
depends upon the moisture in the gas. In these experiments the gases were used 
both dry and saturated with aqueous vapour, and it was found that the velocity was 
different in the two cases. 
For saturating a gas with aqueous vapour it was forced, in passing between the 
gasometer and the apparatus, to bubble through a water bottle and then to pass 
through a long horizontal tube half filled with water. After the gas had been passed 
several-times back and forth between the gasometer and the gas bag, and before any 
readings were taken, the water bottle was cut out so as to avoid any unsteadiness in 
the pressure due to the bubbling. 
For drying a gas the above arrangement was replaced by one in which the gas 
had to pass through a long, horizontal glass tube, partly filled with concentrated 
sulphuric acid, and then through a large volume of calcium chloride. In order to 
allow a sufficiently rapid stream with the small pressures used the calcium chloride 
was placed in a large, wide bottle, the gas entering above and leaving by a protected 
funnel-shaped tube near the bottom. It thus had to traverse a considerable length of 
calcium chloride, and on account of the large area of the bottle the velocity through 
it was small. 
* J, S, Townsend, ‘Phil. Trans.,’ A, vol. 193, 
