THE PHYSICAL PROPERTIES OF INFECTIVE PARTICLES 229 



among the viscosities of various fluids, the effect on viscosity caused by the 

 addition of small amounts of macromoleculcs is well understood both quali- 

 tatively and quantitatively. For virologists, viscometry can be particularly 

 rewarding. In conjunction with other measurements it provides information 

 about the size, shape, and hydration of the virus particles or other materials 

 being investigated. Under special circumstances it is useful in the detection 

 of impurities whose presence can be revealed by other methods only with 

 great difficulty. Measurements can be made simply and rapidly; viscometry, 

 as a result, finds wide application in the study of the kinetics of synthesis 

 or degradation of macromoleculcs like viruses or nucleic acids. From these 

 kinetic data inferences can be drawn about the structure of the macro- 

 molecules. Unlike most other techniques now widely applied in the study of 

 macromolecules, viscometry can be practised with inexpensive apparatus 

 most of which is still designed and constructed in the investigator's own 

 laboratory. For certain types of materials, of which deoxyribonucleic acid 

 is the paramount example, such apparatus is inadequate and recourse is 

 made to elaborate, intricate instruments just becoming available commer- 

 cially. 



Viscometry as applied to solutions of macromolecules involves a com- 

 parison of the solution of interest with the pure solvent. From such measure- 

 ments, the increment in viscosity is determined and this is related to 

 properties of the solute. Rarely, if ever, is the absolute viscosity of the solu- 

 tion of interest. All theories for the viscosity of solutions originate with 

 the definition of viscosity as a measure of the amount of energy required 

 to maintain a certain rate of flow of the Hquid. More commonly, viscosity is 

 considered a measure of the resistance to flow. In pure liquids this internal 

 resistance or friction is a function of the attractive and repulsive forces 

 among the molecules of the liquid; these cannot be specified with sufficient 

 exactness to furnish a reliable theory for the viscosity of pure liquids. Such 

 considerations do not enter treatments of the viscosity of solutions, however, 

 as is shown by the following brief discussion. 



6. Viscosity of Solutions. When macromolecules are added to a solvent, 

 the flow patterns normally present during the movement of the liquid are 

 disturbed. Instead of the adjacent layers of liquid gliding over one another 

 at slightly different rates, as in so-called laminar flow, the layers are forced 

 to move aromid the particles which may be considered as obstructions. 

 Owing to the differences in velocities among neighbouring stream lines, the 

 particles are caused to rotate. Particles which are not rigid may be deformed. 

 Thus, in maintaining the flow of a solution there is a dissipation of energy 

 over and beyond that required for the production of flow of a pure liquid; 

 this additional energy is reflected experimentally as an enhanced viscosity 

 of the solution relative to the solvent. This problem was first investigated 



