Cleaning the surface of the target produced little if any 

 difference in the magnitude of the effect ; neither was any 

 variation observed when a light target was backed by a heavy 

 metal plate. 



The mobility of the ions was roughly determined by a 

 modification of Langevin's method (J.J. Thomson, Conduction 

 of Electricity through Gases, 2nd Ed., p. 64). The distance of 

 electrode from target was 1*9 cm. and the P.D. 600 volts. 

 The curve showing the number of ions collected when the 

 electrometer circuit was closed at different intervals after 

 impact of the bullet is shown in fig. 3. This curve, for — ve 

 ions, shows three tolerably distinct kinks at A> B, C, due 

 to species of ions having mobilities 2xl0~ 4 cm. /sec. per 

 volt/cm., 4xl0" 4 cm. /sec. per volt/cm., and l*2xl0- 3 

 cm. /sec. per volt /cm. The coefficient of recombination was 

 found to be of the mean value 38 e. 



The dependence of the ionization produced on the velocity 

 of the bullet was investigated by allowing the bullet to 

 impinge after penetrating varying thicknesses of lead-foil, in 

 doing which its velocity was, of course, reduced. The velocity 

 was measured by the ballistic method. The following table 

 exhibits the results : — 



tocity. 



Ionization. 



V 2 



V 2 

 I 



V 



I 



126 



33 



16013 



485 



382 



106 



26 



11293 



434 



408 



96 



17 



9271 



545 



565 



69 



11 



4888 



444 



627 



The amount of ionization is seen to be, within the limits of 

 experimental error, proportional to the square of velocity of 

 impact, i.e., to the energy of impact. 



It was also sought to discover whether any radiations of 

 a penetrating character akin to /3 or y radiations from radio- 

 active substances were associated with the impact of the 

 bullet. To test for this effect the target was separated from 

 the ionization chamber by a cylinder made of metal near the 

 target and of wire-gauze covered with thin tissue-paper 

 farther away (see fig. 4) to prevent any ions produced at the 

 target from reaching the electrode. Under these conditions 

 th£ charge received by the electrometer was so nearly equal 

 to the natural leak that the existence of an ionizing radiation 

 is jmade very improbable. Thus, with paper removed from 

 wire-gauze the mean electrometer reading was 240, with 

 paper covering gauze the reading was only 1*8, the natural 

 leak being of the same order as this last. 



