fraction of this exposure, perhaps 5000 r or something of the sort. 



The point is that because there is only one place where the catalase is, 

 namely, on the slide, the time for diffusion is important, and if the time for dif- 

 fusion is longer than the time for recombina,tion, then the radical is not effec- 

 tive. By calculating for plane surfaces one would come out with a theoretical 

 figure. 



I don't really want to spend time on this part since I want to get to the 

 details of direct action studies. What we have done has been to pick up where 

 Lea, Smith, Holmes and Markham (7) left off. They investigated the effect of 

 X-rays on dry myosin and dry ribonuclease and measured what is called their 

 inactivation volume. This is the volume within which one ionization, randomly 

 distributed, will cause inactivation or loss of function. It is to be thought of as 

 a parameter and it is only by chance or, by what I hope to bring out, by some 

 process that we would like to be able to describe, that this parameter agrees in 

 any way with anything known about the molecule at all. 



Lea and his associates found that in both of these molecules something 

 like the molecular volume was involved in the figure for the inactivation volume, 

 and that if, in particular, one allowed for the way in which ionization comes in 

 clusters, the calculated molecular weights based on this method of inactivation 

 agreed tolerably with the figures that were accepted at the time. Lea did not 

 follow this up in the years before he died, and when we began our work on ir- 

 radiation of viruses, it was suggested by Dr. Forro that we should study the ef- 

 fect of radiation on enzymes as well. 



The experiments are threefold in character and quite elaborate. First 

 of all, whatever you study has to be brought into a condition whereby it can be 

 dried and handled stably. For most enzymes and antigens, this is easy. In fact, 

 most of these things seem to have a higher stability in the dry than in the wet 

 state. For example, catalase can be heated to lOO^C when dry but is quite tem- 

 peramental when wet. These are then irradiated with fast deutrons, slow deu- 

 trons, alpha particles, fast electrons of over 500, 000 volts, and also with elec- 

 trons of limited penetration, of energies below 4000 volts. 



We have brought every piece of physical equipment that we could to bear 

 on this major type of study. That is one advantage of being a physicist of some 

 reputation. You can get hold of apparatus that otherwise is a little hard to get 

 your hands on. We have not hesitated to go right after it and we have studied 

 what is a surprisingly large array of things. 



We have found, first of all, that you can consider a molecule as having 

 an inactivation volume and an inactivation cross-section, depending upon whether 

 you deal with ionization that is random in volume or with ionization that is con- 

 fined to dense swaths and so can be considered as a sort of linear problem. The 

 two usually go together, although not exactly. It is not a perfect fit unless you 

 start to introduce other factors. 



But the first thing to say is that in no case when we calculate the molec- 

 ular weight do we come out with something wildly wrong. As a matter of fact, 

 we have had some remarkable successes. For instance, we insisted that the 

 molecular weight of urease would be somewhere in the neighborhood of 100,000 

 and we held to that in the face of opinion that it was 480,000. Well, later deter- 

 minations are giving 100,000. We hit some things rather accurately. For pep- 

 sin we find a figure of 39, 000 as against the accepted value of 36, 000. For oth- 

 ers we haven't done so well. We come out with a figure of 31,000 for trypsin, 



