VIRUS GENETICS, MULTIPLICATION, AND PHYSICS 207 



An approximate value for the force, F, between two such 

 plate double-layers is 



F = i>4>tth-T 



nPri 



+ 1 



«— i«' 



This force depends on the concentration of ions and also on j). 

 The greater the value of j), the more rapidly does the force fall 

 off with distance. 



This type of force is remarkably interesting from a biological 

 viewpoint, and it is undoubtedly one of the factors in the 

 organization of large molecules into the pattern of a virus or a 

 chromosome. A living cell which is metabolizing is the seat of 

 rai)id energy turnover, of rai)id molecular change, and of 

 synthesis. It is surrounded by a membrane capable of selective 

 passage of ions. As a result, the ionic strength of the inside of a 

 cell is continually changing as the develoj)ment of the cell 

 proceeds. The expression for the force between two large mole- 

 cules (which, for the present, we can think of as represented by 

 planes) is dependent critically on the ionic concentration and 

 also on p, which contains both the concentration and the 

 valence. Thus the degree of repulsion between two large mole- 

 cules is under control by the condition of the cell. We shall see 

 shortly that attractive forces of a different character exist, and 

 the balance between these will determine whether two large 

 molecules remain near each other or will be repelled apart. We 

 have seen that Luria's mutation studies indicate that virus 

 multiplication consists of an exponential growth involving some- 

 thing like a growth, division, and separation. The separation is 

 almost certainly constrained to take place because of the 

 repulsive force due to ionic atmospheres. 



Evidence for this repulsion as a factor in virus action was ob- 

 tained by Bernal and Fankuchen (1941) in their X-ray studies 

 of tobacco mosaic virus. 



The preparations of this virus showed strong birefringence, 

 and the orientation causing this persisted even on drying. In 

 order to study the process, they constructed a small-angle X-ray 

 diffraction camera, using slits in place of circular openings in 



