12 



III. Summary and Discussion of the Experimental 

 Results. 

 The results of the experiments described above sufficiently 

 establish — 



(1) That ionization is a general consequence of the col- 



lision of solid bodies in air. 

 The Lenard effect is thus produced both by solid and 

 liquid bodies. 



(2) That for bodies of a given kind the amount of 



ionization is proportional to the energy of col- 

 lision. 



(3) That the amount of ionization depends in general on 



the nature of the colliding bodies, or at least on 



the nature of their surfaces. 

 The constancy of the effect when a leaden bullet strikes 

 a metal target is perhaps explicable on the assumption that 

 the ions are formed mainly, if not entirely, at the surface 

 of the lead, invariably the softer metal. 



(4) That the number of positive ions produced when two 



metallic bodies collide is equal to the number of 

 negative, but when one of the bodies is an insu- 

 lator the numbers are unequal. 



(5) That the ions formed are of very small mobility, com- 



parable with that of the large ions in the air, 

 those produced in the slow oxidation of phos- 

 phorus, the splashing of liquids, etc. 



(6) That the ions are produced mainly, if not entirely, 



at those portions of the surface of the bodies where 

 contact takes place in collision. 

 The evidence for this last statement is to be found in the 

 additive law for targets of thickness giving the maximum 

 effect, when placed in series. Thus, if an ionization x be pro- 

 duced by a target of thickness a, and an ionization y for a 

 target of thickness b, the bullet having already traversed a 

 target of thickness a outside the chamber, then the total 

 ionization due to both targets placed within the chamber is 

 found to be x and y, whereas the effect due to a single target 

 of thickness a and b is simply x. The "fatigue" effect ex- 

 hibited when lead shot falls upon a metal target also finds 

 ready explanation on the assumption that the ionization is a 

 surface effect. 



(7) When a leaden bullet perforates a metal target the 

 ionization increases with thickness of target up to 

 a certain maximum, after which increasing the 

 target, even to the point at which the bullet no 

 longer pierces it, produces no further effect. 



