144 THE PHYSICS OF VIRUSES 



observed. The relative proportions of slow-inactivating and 

 fast-inactivating components are not, however, fixed, but as 

 the temperature is increased, the fast reacting component in- 

 creases in relative importance. Woese attributes this to an 

 equilibrium between two forms of virus, both of which can be- 

 come inactivated. He suggests that the equilibrium is condi- 

 tioned by the amount of bound water remaining in the virus. The 

 reaction statistics are given in the table. 



Thermal Inactivation of NDV Hemagglutination 



State of virus AH^ (calories/mole) AS^ (calories/mole/°C) 



Wet 1^25,000 320 



Dry (fast component) '23. 000 —4.8 



(slow component) '■22. 000 +0 2 



In addition to these thermal studies, Woese has observed the 

 effect of deuteron bombardment, in the dry state, on NDV 

 hemagglutination. The radius of the unit which is responsible 

 for this virus property lies between 40 A and .50 A and so has a 

 molecular weight of the order of 1>00,000. 



Nature of the Virus Surface 



By way of a conclusion to this chapter, a brief description of 

 the virus surface, as deduced from the work described, is now 

 given. The surface is covered with many similar groupings, each 

 carrying a charge. These grouj^ings are doubtless of the familiar 

 type of organic acid-base combination, and so acquire a charge 

 in a manner dependent on the hydrogen-ion concentration. The 

 number of such groupings is certainly large, of the order of 

 thousands, and when a similar group of the opposite charge is 

 presented near the virus, a very strong foice pulls the two 

 together. 



These groups are probably part of, or close to, enzymatic 

 elements, which can catalyse an irreversible reaction which may 

 either bind the virus on to a cell or, in the case of elution from 



