PHYSICAL AND CHEMICAL PROPERTIES OF VIRUSES — 57 
type. If this is so, the virus is different from all other animal viruses 
thus far examined. 
Inactivation of Viruses 
There are various methods of inactivating viruses and they include 
the permanent loss of infectivity due to denaturation of the protein; 
loss of infectivity without alteration of the serological and physical 
properties of the virus; inhibition of infectivity. 
Denaturation 
This is brought about by heating and ageing, by chemicals, parti- 
cularly oxidizing agents, protein precipitants or reagents causing 
wide changes in pH. 
With plant viruses the conventional test consists of subjecting the 
virus in extracted sap to a range of temperatures for ten minutes; the 
temperature at which the virus loses infectivity is known as the 
“thermal inactivation point.” However, as Bawden has pointed out, 
this term does not mean very much unless the conditions under which 
the heating is carried out are precisely defined, the pH of the virus 
solution having a marked effect on the thermal inactivation point. 
The “loss” of infectivity is also rather vague since different results 
will be obtained for different dilutions of a virus. 
With some plant viruses there is general agreement among workers 
as to the temperature at which infectivity is lost, but with others there 
may be wide discrepancies. This can be explained by the relation 
between loss of infectivity and denaturation which differs in the various 
viruses. For example, in potato virus X, loss of infectivity is correlated 
with denaturation whereas with tomato bushy stunt virus the loss of 
infectivity is not necessarily closely associated with denaturation. 
In consequence, in addition to the temperature, other factors such as 
concentration of inoculum and susceptibility of test plants must be 
taken into account (Bawden and Pirie, 1943). 
Similarly, loss of infectivity in potato virus X is correlated with a 
fall in serological titre but with bushy stunt virus there is no reduction 
in serological titre unless the virus is heated above 80°C. 
The same differences are found between the behaviour of viruses 
when inactivated by ageing in vitro. Thus, after a few wecks standing 
at room temperature, purified preparations of potato virus X are non- 
infective, fail to react with antiserum or to show anisotropy of flow. 
On the other hand, sap, from plants infected with tomato bushy 
