13 



The explanation of this fact may possibly be found in 

 the existence of an electrical double layer on the surface of 

 target or bullet, from which the ions are set free in propor- 

 tion to the energy lost by the bullet on striking the surface. 

 Thus, the number of ions formed will increase with the 

 amount of kinetic energy destro} T ed up to the point at which 

 the "double layer'" in region of impact is entirely disin- 

 tegrated. If this explanation hold, however, it is obvious 

 that a critical velocity should also exist above which no fur- 

 ther variation of ionization with velocity should occur. No 

 evidence of such a limit was obtained with the speeds 

 employed. 



In the present state of ignorance with regard to the 

 causes and mechanism of ionization in general, no complete 

 explanation can be offered of the above results. The cause 

 is no doubt to be sought in the destruction of kinetic energy 

 at the moment of impact, and the transformation of this 

 energy into energy of intra-molecular vibration. It may 

 plausibly be assumed that the internal energy of a certain 

 number of molecules would be increased to the limit at which 

 instability and consequent ionization result. 



It is more difficult, perhaps, to offer even a general ex- 

 planation of the inequality in number of the positive and 

 negative ions which occurs when one or both of the bodies is 

 an insulator. Such an inequality might naturally be con- 

 nected with the existence of an electric field, due to the 

 electrification of the colliding bodies : but the fact that the 

 total charge in the air when glass beads fall upon a target 

 of any material, whether this latter be electro-positive or 

 electro-negative to glass, is invariably negative, renders this 

 mode of explanation improbable. The same inequality has. 

 of course, been observed in the Lenard effect, and the same 

 explanation will probably cover both cases. 



It may be suggested, in conclusion, that this effect plays 

 a part in certain other phenomena, e.g., in the luminescence 

 of meteoric swarms, such as Saturn's rings, and possibly in 

 tribo-luminescence. An attempt was made to reproduce the 

 well-known luminescence of a vacuum tube containing mer- 

 cury by substituting glass beads. This was unsuccessful. 



Nevertheless, the apparent ease with which bodies 

 electrify by friction in high vacua may be associated with 

 the absence of such ionization as we have investigated above. 



