124 THE PHYSICS OF VIRUSES 



solution of sheep red cells. This is quite a sensitive, but laborious, 

 method. 



The recent discovery that long polypeptides form a not highly 

 specific and reversible combination with viruses has added a 

 secondary aspect to serological work. The polypeptides are 

 under better control as regards size and length than are anti- 

 bodies and so can be used in different classes of experiment. 



Hemagglutination. Certain animal viruses agglutinate red 

 cells in a manner which is relatively simple to study. Hemag- 

 glutination may be followed by elution, wherein the virus releases 

 itself from the red cells. Both properties are surface properties 

 and both can be studied. 



Adsorption 



Although by far the greatest degree of study has gone into 

 virus serology, it is simplest to consider the nature of adsorption 

 because this forms the best introduction to the study of virus 

 surfaces. 



Bacterial viruses lend themselves very simply to adsorption 

 studies. The virus and bacteria are mixed so that a known 

 amount of virus is initially present and there is an excess of 

 bacteria. After a certain time, which is usually varied, the 

 mixture is spun in an ordinary laboratory centrifuge and the 

 supernatant examined for virus. Any change in the amount of 

 virus is ascribed to adsorption on the bacteria. The results of 

 this type of experiment show that the rate of adsorption is 

 proportional to the concentration of bacteria, B, and to the 

 concentration of unattached virus, U, so that 



dU 

 dt 



hUB 



which, on integration, gives the familiar relation 



Iny^ = k2Bt (5.2) 



Measurements of U/Uo, the percent virus left unattached as a 

 function of time, thus enable Jc2 to be measured if B is known. 



