SIZE AND MORPHOLOGY OF BACTERIOPHAGES 43 



(1950) estimate for T6 is very similar. This corresponds to a 

 particle diameter of 75 mju, which should be understood as the 

 diameter of a phage particle dried and compressed to a sphere 

 of density 1.5. These hydrodynamic measurements of the 

 particle weight of T2 and T6 are the only satisfactory ones of 

 their kind for any phage. 



Taylor, Epstein, and Lauffer (1955) also measured the wet 

 density of T2 by sedimentation in a sucrose gradient. The 

 density found, 1.27, calls for a water content of 37.5 per cent, a 

 natural particle weight of 5.3 X 10"^*^ grams, and a spherical 

 diameter of 92 m^u. If anything this water content is probably 

 underestimated, because sucrose may extract water from phage 

 (Anderson, Rappaport, and Muscatine, 1953). 



Phages T2 and T6 exist in two reversibly interconvertible 

 forms. One, sedimenting and diffusing the more rapidly, pre- 

 dominates at pH 5, the other at pH 7. According to Taylor, 

 Epstein, and Lauffer (1955), these probably differ only in shape. 

 It is possible that these difTerences reflect different states of the 

 tail fibrils described earlier. 



4. Diffusion Constants 



Measurements of diffusion coefficients are needed for the 

 proper interpretation of sedimentation constants, and also lead 

 directly to an equivalent particle diameter computed on the as- 

 sumption of spherical shape (Putnam, 1950). Early applica- 

 tions of diffusion measurements by the porous disk method to 

 phages (Northrop, 1938; Kalmanson and Bronfenbrenner, 

 1939) yielded erroneous results and improbable theories. 

 What was measured was not diffusion but flow of solvent 

 through the disk (Hershey, Kimura, and Bronfenbrenner, 1 947) . 

 Poison (1948) measured reasonable diffusion constants of phages 

 T3 and T4 but Poison and Shcpard (1949) studying the same two 

 phages again decided that T4 must be self-motile. In all these 

 attempts, efforts were made to measure diffusion of infective 

 particles in dilute solutions. None of them proved very suc- 

 cessful. 



