PHYSICAL AND CHEMICAL PROPERTIES OF VIRUSES — 59 
these substances, at concentrations below 4 M, inactivate the viruses 
in neutral solution. The two exceptions mentioned above form 
reversible, fibrous precipitates. 
Loss of Infectivity without Alteration of Serological and 
Physical Properties 
Certain treatments of plant viruses induce loss of infectivity without 
alteration of serological and physical properties. These are irradiation 
with ultra-violet or X-rays and treatment with formaldehyde, nitrous 
acid, or hydrogen peroxide. 
Tobacco mosaic virus and potato virus X, when inactivated by these 
methods, form liquid crystals, and show anisotropy of flow in a 
manner similar to normal infective preparations. In the same way 
tomato bushy stunt virus, similarly inactivated, will form the charac- 
teristic dodecahedra when precipitated with ammonium sulphate. 
Ross and Stanley (1938) claim that partial reactivation of formalized 
tobacco mosaic virus is possible. The virus, after being inactivated, 
could have a certain proportion of activity restored by dialysis at pH 3. 
For example, three preparations which had had their activity reduced 
to IO per cent, I per cent, and o-I per cent were found after re-acti- 
vation to possess approximately 20 per cent, Io per cent, and 1 per 
cent of their original activity. Ross and Stanley consider that two 
simultaneous reactions occur, one reversible and the other irreversible. 
The fact that the addition of formaldehyde results in a simultaneous 
decrease in activity of amino groups and of reducing groups whilst 
with the removal of the formaldehyde the number of these groups 
is increased together with an increase of virus activity, suggests that 
certain of these groups play at least a partial role in the structure 
necessary for virus activity. 
Inhibition of Infectivity 
Certain substances have the power of inhibiting the activity of some 
viruses, especially plant viruses. Johnson (1938) showed that certain 
micro-organisms produced a substance which inhibits the activity of 
tobacco mosaic virus. Smith (1939) and Black (1939) demonstrated 
that there existed an inhibitor in the tissues of insects which prevented 
the infection of plants by tobacco mosaic and other viruses. This 
explained the reason why it was never possible to infect a plant by 
inoculating it with the juices of virus-carrying insects. Black further 
showed that inhibitor and virus could be separated and infectivity 
