THE PHYSICAL PROPERTIES OF INFECTIVE PARTICLES 271 



be interpreted in terms of the well-established theory for moving 

 boundaries. 



Electrophoresis is likely to prove of greatest value to virologists in studies 

 of the homogeneity of a given preparation. Different tests can be applied. 

 First of all, a single boundary should be observed over as wide a range of 

 pH and ionic strengths as can be employed. Such tests are necessarily limited 

 to conditions under which the virus is stable. The observation of a single 

 boundary at one pH is not a sufficient basis for conclusions about homo- 

 geneity. Different materials may have identical mobilities under one set of 

 conditions, and the differences become manifested only when the pH or 

 ionic strength is altered. Secondly, the rate of spreading of the boundary is 

 analyzed quantitatively. In effect such homogeneity tests in electrophoresis 

 involve measurement of an apparent diffusion coefficient. If this is indepen- 

 dent of time, homogeneity is indicated. As in sedimentation, there are sharpen- 

 ing effects which make such tests illusory. One simple test is the so-called 

 reversible boundary-sharpening test. After the boundary has migrated a 

 considerable distance, the polarity of the current is reversed. Any sharpening 

 of the boundary upon its return to its original position is conclusive evidence 

 of inhomogeneity. If, instead, the boundary continues to broaden after the 

 current is reversed, the spreading is attributable to diffusion. Most delicate 

 of the various tests is the analysis of boundary spreading during electro- 

 phoresis at the average isoelectric point of the material. If all of the mole- 

 cules are identical the spreading should be governed solely by diffusion. Any 

 heterogeneity, however, leads to a marked increase in the rate of spreading 

 of the boundary. Experiments performed at the isoelectric point obviate 

 many of the anomalies resulting from the movement of boundaries. 



B. Optical Methods 



1. Light Scattering 



Light scattering is one of the phenomena that may be observed v>'hen 

 particles, such as viruses, interact with radiation. Without exception, when 

 radiation is incident upon a particle an event generally describable as scatter- 

 ing occurs in which some of the radiant energy is diverted from its incident 

 linear path to follow paths that have the particle itself as the source of the 

 scattered energy. Four kinds of physical methods involvmg scattering of 

 radiation have been demonstrated to be useful in the study of viruses. Two 

 of these, light scattering (see Stacey, 1956; Edsall, 1953) and low-angle 

 X-ray scattering, are essentially similar in principle and in method, differing 

 only in the wavelengths of the radiation used. In both, the observed radia- 

 tion originates as wavelets that are scattered from nearly independent par- 

 ticles in a suspension with only moderate coherence among the wavelets. A 

 VOL. I — 19 



