44 BACTERIOPHAGES 



Diffusion of phage T6 across an optically visible boundary was 

 carefully studied by Goldwasser and Putnam (1951). They 

 found that the particles diffused like spheres 94 mfx in diameter, 

 in excellent agreement with physical predictions. Similar re- 

 sults were obtained by Taylor, Epstein, and Lauffer (1955) for 

 T2. 



5. Electrophoresis 



The mobility of particles in an electric field yields information 

 about the sign and density of charges on their surfaces, but does 

 not permit any estimate of size. Only phage T6 has been stud- 

 ied in this way (Putnam, 1950). The particles were negatively 

 charged down to pH 5.2, below which measurements were not 

 attempted. Only one rather diffuse boundary was observed for 

 the best preparations. Since reversal of the electric field did not 

 sharpen the boundary, no evidence for electrophoretic in- 

 homogeneity was obtained. The relatively slow mobility sug- 

 gested a protein, rather than a nucleic acid, surface. 



6. Ionizing Radiations 



Certain radiations such as electrons, protons, deuterons, 

 neutrons, alpha particles, gamma rays, and X-rays are called 

 ionizing radiations because in traversing matter they produce ion 

 pairs by ejecting electrons from atoms. This ionization is 

 often followed by chemical change, which if it occurs in an es- 

 sential molecule can result in death of cells and viruses. All 

 viruses and many single-celled organisms are inactivated by 

 ionizing radiations as an exponential function of the dose of 

 radiation. This has been interpreted as indicating that a 

 single ionization is sufficient to inactivate (Lea, 1 946) . One may 

 readily determine the dose of radiation which will produce an 

 average of one lethal event per organism; that is the 37 per cent 

 survival dose. Since the density and something about the 

 spatial distribution of ionizations produced by a given dosage 



