42 BACTERIOPHAGES 



Elford's data by increasing his empirical factor to 0.83 (Lea, 

 1946). Nevertheless, Elford's data were for a long time the 

 best information available as to size and diversity of size of dif- 

 ferent phages. The relative sizes he determined were remark- 

 ably exact. This is shown in Table II, which lists Lea's correc- 

 tions to Elford's estimates. 



The method is still useful. Now that a number of phages of 

 known size are available, it should be possible to estimate the 

 comparative size of an unknown phage with considerable pre- 

 cision. The usefulness of the method is all the greater since it 

 does not require the phage to be purified or concentrated. For 

 some of the smaller phages, not yet identifiable in the electron 

 microscope, probably no better method is available. 



Ultrafiltration is also useful for concentrating and purifying 

 phages (Elford, 1938) and for isolating phage-specific materials 

 smaller in size than phage particles (Burnet, 1933b; DeMars, 

 1955). 



3. Analytical Centrifugation 



Centrifugal measurements of particle size of phages were also 

 attempted some years ago. Ingenious methods were developed 

 to follow the sedimentation of infective particles by plaque counts 

 under conditions in which no sedimentation boundary is es- 

 tablished. This work is reviewed by Elford (1938). The ap- 

 plication of the modern optical ultracentrifuge to the study of 

 phages is described by Hook, Beard, Taylor, Sharp, Beard 

 (1946), Putnam (1950), and Taylor, Epstein, and Lauffer 

 (1955). 



Sizes of some phage particles calculated from sedimentation 

 constants are given in Table II. These are computed without 

 correction for water content and departure from spherical 

 shape, which means that they are underestimates that have, in 

 fact, no exact meaning. From combined sedimentation and dif- 

 fusion measurements, the true particle weight can be calculated. 

 In this way Taylor, Epstein, and Lauffer (1955) found the dry 

 weight of a particle of T2 to be 3.3 X 10"^" grams. Putnam's 



