THE PHYSICAL PROPERTIES OF INFECTIVE PARTICLES 225 



discussed here. Other methods, although interesting and promising, will be 

 omitted, or discussed only briefly, because of the relative rarity of their 

 application. 



When virus particles are harvested from infected tissues and purified it is 

 found that they are composed mainly of a nucleoprotein and that their size 

 range is between about 200 and 3000A. The particles associated with a given 

 disease are frequently found to be so uniform in size and shape that they can 

 be conveniently termed macromolecules, but the spatial arrangements of 

 their protein and nucleic acid portions are mostly miknown. Although we 

 may hope eventually to discover something about the ^structure of viruses 

 at the level of their amino acid and nucleotide configurations by applying 

 the methods of organic chemistry, it remains true that most of the physical 

 examinations of viruses have dealt with their properties as colloidal par- 

 ticles, either in the intact or disintegrated form. In this domain of size interest 

 centers primarily in three physical parameters: shape, size, and mass. How- 

 ever, as will be discussed later, when shape and size are referred to there is 

 no universally accepted concept as to what is meant by the words. They are 

 always circumscribed in their meaning by consideration of the ways in which 

 they are measured. Thus, the X-ray crystallographer determines the size of 

 a virus as it exists in a unit cell of a crystal; the hydrodynamicist determines 

 the effective size of the particle as it plows its way through a solution; the 

 electron microscopist finds its shape and size, when dry, by direct photo- 

 graphy. If virus particles were hard objects, like marbles, these distinctions 

 among methods would be irrelevant, but a virus particle may be more like a 

 sponge with water bound both internally and externally. It has amphoteric 

 properties which may confer upon it different effective diameters, depending 

 upon concentration and the ionic environment. We should not be surprised 

 if we find that the same virus jiarticles apparently have different shapes and 

 sizes in the hands of different experimenters, aU of whom have performed 

 their work correctly. In addition to these inherent discrepancies there are 

 the uncertainties arising from the difficulties of producing truly mono- 

 disperse suspensions of viruses. The presence of impurities, of dimerization, 

 disintegration, and aggregation provides ample opportunity for apparent 

 disagreements among the values obtained for shape and size. There is no 

 a priori reason to believe that a virus suspension can be prepared in a way 

 such that it is composed only of physically identical particles. Although, for 

 example, bushy stunt virus is known to have a molecular weight of 9 X 10'', 

 to contain 16 % by weight of ribonucleic acid, and 8.1 % of the amino acid, 

 threonine, all we know is that these values are averaged over billions of 

 particles. Individual particles may differ from the average figures by several 

 per cent, but we would be unaware of this fact. 



Another parameter of viruses that interests physical chemists is their 



