lonization produced in Gases by Réntgen Rays, — 36d 
that the pressure in it might be varied while the other con- 
ditions remained constant. The bulb was exhausted until 
the pressure was just low enough for Rontgen rays to be 
produced which were strong enough to produce an appre- 
ciable effect in the leak apparatus. The air was then exhausted 
from the one cylinder until the ionization in it balanced that 
in the hydrogen in the other cylinder. The pressure in the 
bulb was then slightly lowered by means of the pump, and 
thereby harder rays were produced, and on doing this it was 
found that the balance was destroyed. The deflexion of the 
electrometer-needle was in the direction which indicated that 
the harder rays produced more ionization in hydrogen com- 
pared with air than did the softer rays. More air had to be 
let into the cylinder containing air in order to balance the 
ionization in the hydrogen. When the balance was again 
restored by thus letting in more air, the pressure in the bulb 
was again lowered, still further changing the type of rays 
given out. This again caused the balance to be destroyed 
and the deflexion of the electrometer-needle to be in the same 
direction as before. The balance was restored again as before 
and the same process repeated. The pressure in the bulb 
was continually lowered and observations taken at intervals 
in this manner, until the pressure was so low that the coil 
would no longer produce a steady discharge in the bulb. 
These experiments were repeated several times and the same 
result was always obtained, namely, that the harder the rays 
the greater was the ionization which they produced in 
hydrogen as compared with that which they produced in 
air. 
Hxperiments were made to see if there could be found any 
relation between the hardness of the rays and the ratio of the 
ionization produced by them in air and in hydrogen. To 
test this, the penetrability of the rays at the different pres- 
sures of the air in the bulb was tested by measuring the 
absorption of the rays by a known thickness of aluminium, 
It was, however, found to be very difficult to obtain definite 
results by this method, for the pressure in the bulb varied so 
rapidly, especially at the higher pressures, owing simply to 
the running cf the bulb, that when a balance had be2n 
obtained for the ionization produced by rays of a given 
penetrability, the rays changed before sufficient readings could 
be taken to measure this penetrability to any degree of 
accuracy. [t was necessary therefore, to get consistent 
results, that simultaneous measurements should be made 
by some means. The following method was therefore 
adopted :—In parallel with the Réntgen-ray bulb was placed 
