ELECTRIC RADIATIONS—BRAGG. 199 
appear to produce cathode rays of any speed from the air molecules 
which they traverse, or from the molecules of any gas consisting of 
atoms of small weight, and since they produce much ionization in 
some way or other, we may conclude fairly that they produce slow- 
speed ions themselves. Thus, whether they act directly or indirectly 
through cathode rays, the result is the same. The principal effect 
appears to be due rather to secondary than primary. As Sagnac re- 
marks (Ann. Chim. Phys., X XIII, p. 196), “ The transformation of 
X rays, by increasing the activity at any point, permits the detection 
there of very penetrating X rays, which would otherwise have passed 
unperceived.” 
In the case of the y rays, such evidence as we have is also in favor 
of the existence of slow-speed ions, as the result of their action. It 
is known that 6 rays of high speed originate where they strike the 
molecules of a solid body (Eve, Phil. Mag., December, 1904) ; such 
an action may, therefore, be expected in the case of gas molecules also. 
It is possible, however, that there may be a differential effect in re- 
spect to heavy and lght atoms, as in the case of the X rays. The 8 
rays will produce 8 rays in their turn; and if, as is probably the case, 
the y rays are themselves able to ionize, the product will consist of § 
rays, a conclusion which may be safely adopted from the analogies of 
the cathode rays on the one hand and the X rays and ultra-violet 
hight on the other. As in the case of the hard X rays, the existence 
of y rays is often made clear by the secondary effects which they 
produce, as has been shown by Becquerel. 
To sum up what has been said, the ionization which we measure in 
the ionization chamber is almost wholly due to the emission of slow- 
speed electrons from the atoms of the gas contained in the chamber 
or of the chamber walls; and this is true for all forms of radiation. 
Moreover, there is some evidence to show that the speed of the 8 
rays is almost independent of the cause and manner of their produc- 
tion. As has already been said, Fuchtbauer found the velocity of 
the 8 rays, caused by canal rays, to be about 3 310%, and the same 
in the case of cathode rays. Logeman found the velocity of the 8 
rays, emitted from a plate struck by a rays, to be such that they were 
deflected by a weak magnetic field. Ewers found (Phys. Zeit., 
March, 1906) the § rays of polonium to possess a speed of 3.25108. 
With these may be compared Lenard’s estimate, viz, 10°, of the speed 
with which the ions leave a plate struck by ultra-violet light. It 
seems probable that we have here a critical speed for the electron. 
Below this it is not able to leave the parent atom. If its velocity 
exceeds the critical amount it possesses powers of penetration and of 
causing ionization, the extent of these powers depending on the 
excess, 
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