DISCUSSION 175 



would be inclined to feel that differences of the order of 1 per cent may well be 

 present. 



Allen: 



I agree with Platzman that the concept of ionization in liquid water is a very- 

 vague one and that conclusions drawn on this basis are apt to be incorrect. The 

 effect of phase on the stopping power of water is hard to explain. One would 

 expect frona the theory that any differences would be more pronounced in ionic 

 soUds than in hquids. It is my impression that the stopping power of mica 

 from experiment agrees well with that predicted by the theory. I should like 

 to ask Platzman if there is any theoretical reason to feel that water is abnormal 

 from this point of view. 



Platzman : 



The problem is not one of invoking a reason, but of assessing its magnitude: 

 we know that the effects being discussed exist, but we do not possess unquestion- 

 able knowledge concerning their magnitudes. The case of mica cited by Allen 

 is important, for it is the only one in which the stopping power of an inorganic, 

 non-metallic solid has been accurately measured. We may regard it as throwing 

 additional doubt on the great abnormality in water. Yet even here there is room 

 for question: the cooperative effects, involving as they do only the valence 

 electrons, are progressively more suppressed as the mean atomic weight of the 

 atoms of the medium increases (it is much greater for mica than for water) ; and 

 the individual stopping powers of the atoms constituting mica, and the velocity 

 dependences of these stopping powers, are hardly known with sufficient cer- 

 tainty to permit a conclusive statement on the magnitude of the deviation from 

 the additivity rule. 



Eyeing : 



One would expect less straggling in liquids than in gases. 



Appleyard : 



I should hke to draw attention to some aspects of the "cooperative effect" in 

 water — if it exists. 



1. In the early work reported by Platzman, it was reported that other as- 

 sociated, polar liquids show similar, though smaller, effects. 



2. Water vapor appears to have the same stopping power as a stoichio- 

 metric mixture of hydrogen and oxygen. The Bragg law is therefore not called 

 into question here. 



3. Any such anomaly of the magnitude generally indicated by all the ex- 

 periments preceding those of de Carvalho would correspond to a decrease in the 

 parameter (the mean excitation energy /) characterizing the stopping power of 

 the outer electrons of the molecule by a factor of nearly 2, because the stopping 

 power is rather insensitive to /. The effect would be much smaller for faster 

 ionizing particles which lose more of their energy by head-on collisions whose 

 behavior is independent of the medium. In the absorption of ultraviolet light, 



