PHYSICAL AND CHEMICAL PROPERTIES OF VIRUSES 61 
The X-ray diffraction method is only applicable to viruses obtain- 
able in a crystalline or semi-crystalline form. In a tomato bushy stunt 
virus crystal, or a tobacco mosaic virus gel, the virus particles are 
arranged in a regular array or pattern. This array serves as a dif- 
fraction grating to a beam of X-rays, and measurement of the angles 
at which the X-rays of known wave-length are diffracted enables 
the plan of the pattern and the repeat interval to be established. 
Thus, in the case of wet crystals of bushy stunt virus, it has been 
found that the pattern is a body-centred 
cubic lattice of edge 394 A. In other 
words, if one imagines the crystal to be 
built up out of cubes of this size, then 
at the centre of each cube and at every 
corner there is either a virus particle, 
or an identical group of virus particles. 
There will thus be two particles, or 
groups, per cube of edge 39°4 mu, and 
knowing the density of the crystal, the 
molecular weight of the particle, or group, can be calculated 
(see Fig. 1). Calculated by this method the particle size of the 
tomato bushy stunt virus is 34. mu when wet. When the crystal is 
dried it shrinks and the size of the unit cell also decreases to 31°6 my, 
the dry virus particle being about 27-6 my in diameter. 
In the method of determining size by sedimentation and diffusion, 
the experiments consist in determining Sy) and Dyo, the sedimentation 
and diffusion constants respectively. The sedimentation constant is 
determined by measuring the rate at which particles sediment through 
water in a gravitational field, but as this rate would be very slow in 
the earth’s gravitational field, it is necessary to put the solution into 
an ultracentrifuge, the fields used being 2000-18,000 x g. The 
diffusion constant is a measure of the rate of movement of the virus 
particles through water, impelled by Brownian movement. For a 
spherical, non-hydrated virus, the partial specific volume of which 
is known, measurement of either Sj) or Dgy alone gives the size of 
the virus. With both together, however, a much better estimate of 
the size is obtained. 
The method of determining the size of a virus by filtration through 
collodion membranes of graded porosity depends upon (a) deter- 
mining the pore size of each membrane used in terms of a conventional 
measure known as the “average pore diameter” (A.P.D.), (b) relating 

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