PRODUCED IN GASES BY RONTGEN RAYS. 
217 
It is seen that in this case the values of the velocity change less rapidly as the 
values of T become large ; but for the smaller values of T the change is more rapid 
than it was when the smaller inner cylinder was used. The points on the curves are 
not advantageously distributed, and so do not allow of a very accurate projection of 
the lines to T = 0 ; but from those drawn it is seen that the results are but slightly 
smaller than those obtained with the smaller inner cylinder. This is considered a 
good agreement even if it is left out of account that an additive correction is still to 
be made. 
An alteration which was tried to test the effect of surface ionization was a change 
in the material of the inner surface of the outer cylinder. The aluminium part DTP 
(fig. 2) of the outer cylinder was coated on its inner surface with a layer of tin-foil. 
The rays in penetrating the cylinder now had a tin instead of an aluminium surface 
in contact with the air. J. Perrin has shown (see § 4 (10) ) that what he calls the 
coefficient of the increased ionization at a metal surface is for tin in air '6 as against 
•0 for aluminium in air. The effect varies with the state of the surface. In these 
experiments the aluminium surface used was an ordinary unpolished surface, while 
the tin surface used was that of bright tin-foil. It was thought that if an increase 
of the ionization near the metal surface has any marked effect upon the value of the 
velocity obtained, the difference should be observed by this new arrangement. 
The results obtained are given in Table VIII., dry air being used as before. The 
smaller inner cylinder having a diameter of 1 centim. was used. 
Table VIII.—Dry Air. Summary for Tin Surface. 
Reference 
X. 
U. 
A. 
T. 
Tempera- 
Gas 
Ionic velocity. 
number. 
ture. 
pressure. 
Negative. 
Positive. 
1 
5-22 
8-56 
- 6-06 
•6 
°C. 
13-9 
77-3 
1-42 
2 
5-22 
8-61 
+ 7-82 
•6 
13-9 
77-3 
— 
1-105 
3 
5-22 
18-2 
+ 15-2 
•29 
14-4 
77-4 
— 
1-20 
4 
5-22 
18-2 
-10-8 
•29 
14-4 
77-4 
1-68 
— 
5 
2'62 
17-3 
- 19-6 
•15 
1 
77-2 
1-76 
— 
6 
2-62 
18-5 
_ 21-2 
•14 
16-5 
77-2 
1-73 
— 
7 
2-62 
18-5 
+ 29-4 
•14 
16-5 
77-2 
1-25 
1 
The points are plotted as squares on the curves in fig. 8, which represent corre¬ 
sponding values when the aluminium surface was used. It is seen that the points 
for the negative ions agree very well with the curve. The points for the positive 
ions are 2 to 3 per cent, below the values for the aluminium surface. Taking both 
results into consideration it was concluded, if the addition of a tin surface changed 
the values of the velocities by but such a small amount, that originally when the 
VOL, CXCY.- A. 2 F 
