PHYSICAL AND CHEMICAL PROPERTIES OF VIRUSES — 53 
1938). Although it now seems to have disappeared completely from 
the tomato crops of this country, the virus is to be found in labora- 
tories in many parts of the world where, because of its scientific 
interest, it has been intensively studied. ) 
Bawden and Pirie purified this virus in the first instance by heating 
the sap extracted from young infected tomato plants to 60°C, spinning 
off and discarding the precipitate and adding ammonium sulphate. 
However, since heating greatly reduces the infectivity of the virus 
without apparently causing any other change in its nature, the 
method of purification was revised and the heating omitted. 
Stanley (19402) used Datura plants as a source of virus in preference to 
tomatoes. 
Purified solutions of tomato bushy stunt virus are opalescent and 
show no flow birefringence, such as is characteristic of tobacco mosaic 
virus, since the particles are spherical. The virus is soluble in water 
and dilute salt solutions over the whole pH range in which it is stable 
and it does not, therefore, precipitate at its isoelectric point which is 
pH 4:1. On the addition, at room temperature, of saturated ammo- 
nium sulphate to the opalescent virus solution, an amorphous deposit 
separates out. This is better carried out at o°C than at room temperature 
since the virus is more soluble in the cold. 
The crystals produced by this method are rhombic dodecahedra, 
but Cohen (1942) has shown that by the use of heparin and other 
substances in the crystallization process the virus forms prisms which 
appear to be isotropic. 
The elementary analysis of purified preparations made by ammonium 
sulphate precipitation has given the following values: carbon 48-5 
percent, lyydrogen “7:7 per cent, nitrogen, I6'I per. cent, 
sulphur 0-6 per cent, phosphorus 1-5 per cent. Nothing has yet 
been published on the amino-acid composition of the protein part of 
the virus. 
Nucleic acid of the ribose type has been isolated from a preparation 
of bushy stunt virus purified by centrifugation and the carbohydrate 
and phosphorus are largely present in this form. There appears to 
be about 17 per cent of nucleic acid. 
No enzymes are known which will digest the bushy stunt virus 
and it can therefore be incubated with trypsin to assist the purification 
processes. 
The infectivity of the purified tomato bushy stunt virus is 10~§ g 
per millilitre. 
