48 BACTERIOPHAGES 



able shrinkage on drying. Analytical centrifugation, supple- 

 mented by measurements of diffusion, hydration, and wet density, 

 also yields precise information. Weight measurements of puri- 

 fied preparations, related directly or indirectly to electron- 

 microscopic counts of particle number, together with independ- 

 ent estimates of chemical purity, are now feasible. 



All the available methods have been applied only to T2. For 

 this phage the particle "diameter" in m/x is 65 to 95 in electron 

 micrographs, 75 by sedimentation and diffusion, and 86 to 110 

 computed from the weights per infective particle, all referring to 

 dried particles. These estimates are consistent with an equiva- 

 lent spherical diameter of about 90 irifx and other characteristics 

 of the natural particles as determined by hydrodynamic methods. 



In addition, very simple methods, such as ultrafiltration or 

 measurements of sensitivity to X-rays or beta rays, suffice to 

 classify one phage in relation to others. These methods are 

 available in instances where the other methods fail. 



