Gases by penetrating Réntgen and Radium Rays. 618 
Gas Relative Previous | Present 
Density. values. | values. 
a oe | 07 ae | 4g 
ee eee 10nd 0 
2S 2 See 12 eae ea a 
Oe 2-2 8 2-3 
Chileroform. ...¢.:.....3-. Se 43 32 4:6 
Carbon Tetrachloride ...... 53 45 | 4:9 
Methyl Lodide ............... 50 72 | 13°5 
* Strutt ‘11; J. J. Thomson °33; Rutherford °5; Perrin ‘026. 
It will be seen that hard rays show a closer agree- 
ment than soft rays with the density law which holds 
for the y rays of radium. It is probable that a still harder 
bulb would further reduce the value obtained for methyl 
iodide. By interposing an additional lead screen it was 
shown that the value obtained depended more on the hardness 
of the bulb than on the amount of lead penetrated. This 
has been proved more directly by McClung (‘ Nature, 17th 
March, 1904), who has obtained lower ionization values, except 
in the case of hydrogen, by increasing the hardness of the 
bulb employed. 
The curves for pressure and current are shown in fig. 3, 
for air, H,S, and H. Their curvature indicates a con- 
Siieeable Eriousk of secondary radiation; this will be 
considered later. The portion of the curve between 18 and 
50 mms. was found to be a straight line (fig. 4), and the 
readings for the gases used were taken between these limits. 
The value for hydrogen relative to air seemed to be constant 
for all pressures. The curve for H,S intersected the curve 
for air. The scale of the diagram is too small to show this, 
but the values obtained were : 
Pressure in mms. Value relative to air. 
760 2, 
218 26 
63 eO5 
a0 "90 
. y Rays of Radium. 
Observations were made in a manner similar to those for 
Rontgen rays. The current-pressure curve between 30 and 
