148 DISEASES OF GLASSHOUSE PLANTS 
Properties of the Virus 
The infectious virus has been studied critically by 
Allard (2) and others, who have made important deduc- 
tions. It was found that the virus of tobacco mosaic 
retained its infectivity after passing through Berkefeld, 
Chamberland, and Kitasato filters, but was non-infec- 
tious after filtration through filters with fine pores, such 
as the Livingstone atmometer cup or a layer of powdered 
talc seven-eighths of an inch in thickness. Thus 91 per 
cent of the plants inoculated became infected after the 
virus had been passed through a Chamberland filter, 
63 per cent when a Berkefeld filter was used, and 40 per 
cent with the Kitasato filter. The cucumber mosaic 
virus has been found to pass a Berkefeld filter but not a 
Chamberland. The virus may be destroyed rapidly by 
treatment with 80 per cent alcohol or 4 per cent formalde- 
hyde. The virus is destroyed in thirty-one days when 
treated with formaldehyde of a strength greater than 1 in — 
800. Carbolic acid and mercuric chloride have little 
effect on the infectivity of the virus. When heated to 
boiling point the infective principle is quickly destroyed, 
but lower temperatures are only doubtfully effective. 
The virus may be diluted to 1 in 1,000 without losing its 
infectivity, and it is claimed that infection has been 
obtained with dilutions as low as 1 in 10,000. ‘The juice 
has been kept for fifteen months and has retained its 
infectivity, although in an advanced state of putrefaction. 
Experiments with the virus of cucumber mosaic have 
shown it to be less resistant to killing agents than is that 
from the tobacco. Heating to a temperature of 70° C. and 
treatment with 0-5 per cent solution of copper sulphate, 
formaldehyde, or phenol has been found to destroy its in- 
fectivity, as has a 0-5 per cent solution of mercuric chloride. 
Transmission of the Disease 
The researches of previous workers have shown that 
once the infectious principle has been introduced into a 
