196 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
of a gas through which the primary rays pass. But, whether these 
high-speed secondary rays are scattered primary rays, or are true sec- 
ondary rays, they must in their turn produce electrons of slow speed 
in the gas through which they pass; and so, directly or indirectly, by 
primary or secondary or tertiary or rays still more transformed, 
eventually the great majority of the electrons set free in the ioniza- 
tion chamber of ordinary experiment are of the slow-speed type. 
In the case of the a rays there is abundant evidence that their im- 
pact on, or emergence from, solid surfaces causes the ejection of slow- 
speed electrons. (J. J. Thomson, Cambridge Phil. Soc. Trans., Feb- 
ruary, 1905; Rutherford, “ Nature,” March 2, 1905; Logeman, Proc. 
Roy. Soc., September, 1906.) Now, it is generally characteristic of 
all these electric radiations that they are concerned with the in- 
dividual atoms and molecules, and that they do not recognize any 
difference between the atom in the solid and the atom in the gaseous 
condition. Consequently, there is every reason to suppose that the 
heavy ionization caused by an a particle in traversing a gas consists 
in the production of the same slow-speed electrons as are set free 
from a solid, and indeed no trace of faster-moving electrons has ever 
been found. The slow-speed electrons originated by a rays have been 
called § rays, and the term may be applied to all such slow-speed elec- 
trons as we are now considering. 
Again, it has been shown by Fuchtbauer (Phys. Zeit., November 1, 
1906) that 8 rays are emitted from a metal surface struck by canal rays; 
and here also there is every reason to suppose that gas molecules 
struck by such rays emit the same 8 particles. The same author has 
shown by a direct comparison that the velocity of these particles is 
the same as that of the 6 rays displaced ‘by cathode rays, i. e., about 
3.3108 em./sec., or the velocity due to about 20 volts, a velocity only 
slightly larger than that found by Lenard. 
As regards B and y rays, it is true that is has not been definitely 
proved that most of the ionization which they cause is of the 8 type. 
But this may be inferred from well-known experiments, such as 
those of Durack (Phil. Mag., May, 1903), or McClelland (Trans. 
Roy. Dub. Soc., February, 1906). When a pencil of £ radiation is 
allowed to cross an ionization chamber normally, and fall upon the 
opposite wall, it gives rise to a secondary ionization, less in quantity, 
but not much less in speed than the primary. A tertiary radiation 
is caused by the secondary rays if they impinge on the walls of the 
chamber, and there will doubtless be still further derivations. But 
it appears that the quantity of the derived radiations dies away 
much more quickly than the speed. Thus the chamber is crossed 
and recrossed (a few times) by electrons of high speed, able to tra- 
verse an average path of about 100 em. in air at atmospheric pres- 
