86 THE PHYSICS OF VIRUSES 



phage. It is possible that it corresponds to a coiled-up sensitive 

 region of nucleoprotein. 



These illustrations have been chosen for a purpose. They 

 show that the broad assum])tion that the whole virus is sensitive 

 to one average primary ionization cannot be maintained and so 

 that no simple treatment in those terms can hold. They show 

 that certain types of radiation study seem capable of yielding 

 results which can be interpreted now — notably, high specific 

 ionization bombardment, which yields a cross section which is 

 an approximation to the area of the dry, unhydrated virus par- 

 ticle. They show the urgent need for a better understanding of 

 the biological action of primary ionization. This somewhat 

 gloomy note is in reality very misleading. Radiation, in all 

 prol)ability, does destroy the local region where it arrives. 

 Whether this fact will interfere with the j^roperty of the virus 

 being studied depends on the way the virus uses its various 

 parts. The fact that apparently all the parts of the virus are not 

 in all cases absolutely necessary for multiplication is, in the end, 

 going to make it possible, by radiation techniques, to find out 

 something about both morphology and function. We hope to 

 show, later in the chapter, how this can be approached. 



The Action of X-Rays 



Because the interest in this book is in viruses rather than in 

 radiation action we have treated one important class of experi- 

 ment at some length. In so doing we have ignored some of the 

 more usual techniques of ionizing radiation, and have not even 

 mentioned the most used type of radiation — X-rays — at all. 

 In order to consider a further range of important work, some 

 more description of the action of X-rays is necessary. We can 

 begin by a brief statement of the utility of X-rays and how they 

 are related to the first part of the chapter. 



X-rays are high-energy photons and so are to be classed with 

 gamma radiation. They act by reason of their electromagnetic 

 field and do so in two major ways, by photoelectric absorption 

 and by Compton recoil. In the former, the photon dies and the 

 whole energy is transmitted to one atom as excitation energy. 



