204 



THE PHYSICS OF VIRUSES 



which approach closely to the molecule. However, these, in turn, 

 attract more positive charges behind them, and so an atmosphere 

 results in which, on the whole, there is more negative charge near 

 the molecule, but in a rather diffuse way, with positive charge 

 mixed in with it. This is called a double-layer, which is an ap- 

 proximately correct description. The forces due to double-layers 

 have been treated in colloid theory, notably by Verwey and 



® 



Fig. 8.6. The ionic atmosphere around a bacterium, drawn schematically to 

 show the character of a double-layer. 



Overbeek (1948). Two cases have been worked out in some de- 

 tail — the cases of flat sheets and of spheres. A skeleton account of 

 the theory is given here, less for the reason of deducing the re- 

 sults quoted than because it enables an understanding of the 

 reason for the forces to be gained. A very clear general descrip- 

 tion of ionic processes and of the Debye-Hiickel theory is given 

 by Gurney (1946) which can serve as a background for the 

 following material. 



The double-layer is due to a fixed set of charges on the virus 

 surface which influences a charge distribution in the solution con- 



