CHAPTER VI 
PHYSICAL AND CHEMICAL (PROPER TIES 
OF. VIRUSES 
In the previous chapter we examined briefly some of the methods of 
purifying and isolating viruses, and in this chapter we shall consider 
some of the chemical and physical properties of such purified viruses. 
Tobacco Mosaic Virus 
The virus of tobacco mosaic is one of the most infectious and stable 
viruses known, and has been found to be still infectious after being 
kept in extracted sap for a period of thirty-four years. It occurs in 
high concentration in the tobacco plant and about 2 g per litre of sap 
can be obtained; additional virus is liberated by incubating the leaf 
residues with snail-gut enzymes (Bawden and Pirie, 1946). The 
amount of virus, however, which occurs in plants affected with some 
mutant strains of tobacco mosaic virus is considerably less than that 
referred to above. 
If a purified solution of tobacco mosaic virus of about 1-2 per cent is 
left standing, it separates into two layers of which the lower is the 
more concentrated. Under polarized light, the lower layer is spon- 
taneously birefringent whilst the upper layer only becomes so when 
shaken. This separation into layers occurs when the concentration is | 
too great to allow free movement of the particles which arrange 
themselves into small boat-shaped drops called “microtactoids.” 
These are of slightly higher density and so fall to the bottom. Bernal 
and Fankuchen (1937) found this bottom layer to be a jelly with all 
the rods the same distance apart, and Bernal considers each rod of 
the tobacco mosaic virus to be itself a small crystal. 
If the bottom layer is allowed to dry gradually in a thin cell, the 
distance between the rods gets less and eventually the rods dry, 
forming a dry gel. The centres of the particles are separated by 15-2 my, 
but on drying completely the distance is reduced to 15 mu and this 
is taken as the diameter of the rod. 
On treatment with dilute alkalis, the tobacco mosaic virus particles 
tend to split down into smaller and smaller particles, and sub-units 
tend to form with some regularity (Schramm, 1947), but these small 
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