THE THYSICAL PROPERTIES OF INFECTIVE PARTICLES 227 



reduced. Our criteria of purity are much more stringent tlian heretofore. 

 The time is ripe, it seems, for a reexamination of many viruses previously 

 studied. Therefore, the major part of this chapter is devoted to the funda- 

 mentals of methods themselves rather than to a compilation and evaluation 

 of the large amount of available data on different viruses. 



II. Physical Methods 

 A. Hydrodynamic and Thermodynamic Methods 



1. Introdiiction 



Often in the study of viruses, and particularly with the larger animal 

 viruses, cursory examination of electron micrographs or ultracentrifuge 

 patterns reveals evidence of polydispersity with respect to the size of the 

 virus particles. Such variations in particle size often are not evident from 

 measurements by certain techniques like viscometry or light scattering, and 

 it is necessary, therefore, to consider average properties such as an average 

 diameter or an average molecular weight. (We shall temporarily ignore the 

 semantic paradox implied by referring to the molecular weight of a poly- 

 disperse material.) Polydispersity may arise from the presence in a pre- 

 paration of a discrete number of monodisperse fractions, or it may result 

 from a collection of particles ranging in size in a continuous manner. For the 

 former, methods are available whereby each component can be considered 

 separately and its size determined. In the latter situation, however, physical 

 methods give some kind of average molecular weight, which average we now 

 consider. 



When a solution is polydisperse, it is convenient to think of it as consisting 

 of a number (from one to infinity) of monodisperse fractions. If we designate 

 any such fraction as the i^^ component, the yiumher average molecular weight 

 may then be defined as: 



J.nMi 



where n, represents the number (or number fraction) of molecules of the i^^ 

 type and M^ is the molecular weight of that component. The symbol, Z, is a 

 summation sign; i.e. J^nMi = >ii^i + nj^l^ + W3M 3 + . . . Since n,!/.- is 

 the weight (or weight fraction) c^, of the i*^ component, the number average 

 molecular weight can be expressed in the alternative from 



In the determination of a colligative property of a solution, such as the 

 osmotic pressure, every molecule contributes equally, regardless of its weight, 

 and hence the relevant molecular weight is M^. 



