314 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914. 
The range or distance traversed in a gas at ordinary pressures is a 
few centimeters. The following table, compiled by Geiger, gives the 
range in air at the temperature of 15° C.: 
Centimeters. Centimeters. 
Sl Oiramanmn al ese ee ae a ee te DOO) INO TEL UTTIA ONO eee hse een el aed 4.30 
(Want? cee eee se eee ar 290m Mh emana tio nase ass eee era 5. 00 
Jioniume eso et A SOON Riana ACES e.g eee 5. 70 
1 ESCVS TE V0 Oh ses Cama eS ge Rr eS 3:,80))| Ronin C202 Sec. ae se meee eee 4. 80 
areMamatlOMs nace eo ceaaae act Sate AAG PHORM Co: a. ee eee a oe 
adiqimavare nse sesccainse cece aaa A Owl vAdiOaGoMIUIN === 455 42 9a 4. 60 
UAT OR ope oer ee see 62947) PACE UTNE Ose ys eae se ee ee 4. 40 
IRAGAUINGH S22) Watieee ener Seu ae Sal PACE Ml ana tone eee eee eee 5. 70 
HI OTN ess Sere see ee ede ORS Pia Alt AGENT. S25. oi is ea a eres 6. 50 
RaGgioniorminimsssseee serene eee oe SE 87 beAc emmy Cee oak) 2 eves eee peer eee 5. 40 
It will be seen that the ray of greatest range is that proceeding from 
thorium C,, which reaches a distance of 8.6 centimeters. In: the 
uranium family the fastest ray is that of radium C. It attains 6.94 
centimeters. There is thus an appreciable dif- 
- ference between the ultimate distances traversed 
by the most energetic rays of the two families. 
The shortest ranges are those of uranium 1 and 2. 
The ionization effected by these rays is by no 
means uniform along the path of the ray. By 
examining the conductivity of the gas at dif- 
ferent points along the path of the ray the ioniza- 
tion at these points may be determined. At 
ply a Ge @ the limits of the range the ionization ceases. 
0. 2 In this manner the range is, in fact, determined. 
The dotted curve (fig. 1) depicts the recent in- 
vestigation of the ionization effected by a sheaf of parallel rays of 
radium C in air, as determined by Geiger. The range is laid out - 
horizontally in centimeters. The numbers of ions are laid out verti- 
cally. The remarkable nature of the results will be at once apparent. 
We should have expected that the ray at the beginning of its path, 
when its velocity and kinetic energy were greatest, would have 
been more effective than toward the end of its range when its energy 
had almost run out. But the curve shows that it is just the other 
way. The lagging ray, about to resign its ionizing properties, 
becomes a much more efficient ionizer than it was at first. The maxi- 
mum efficiency is, however, in the case of a bundle of parallel 
rays, not quite at the end of the range, but about half a centimeter 
from it. The increase to the maximum is rapid, the fall from the 
maximum to nothing is much more rapid. 
It can be shown that the ionization effected anywhere along the 
path of the ray is inversely proportional to the velocity of the ray at 
that point. But this evidently does not apply to the last 5 or 10 
lonisation 
