CHEMICAL COMPOSITION 93 



densities per cm. divided by phage concentrations in visible 

 particles per ml., plaque-forming particles per ml., and mg. of 

 phosphorus per ml., insofar as the data given permit. In many 

 instances values are given for the best of numerous preparations 

 that vary considerably. These variations depend in part on 

 variable content of noninfective phage particles in the prepara- 

 tions but, as the data of Luria, Williams, and Backus (1951) 

 show, depend also on other impurities that contribute to the 

 absorbancy or light scattering by the preparation. 



TABLE XI 



Extinction Coefficients of Phage Preparations 



° Herriott and Barlow (1952). 

 ^ Luria, Williams, and Backus (1951). 



' These figures are about 20 per cent low relative to the others owing to cor- 

 rection for scattering. 



^ Hershey, Dixon, and Chase (1953). 



* Hershey, Kamen, Kennedy, and Gest (1951). 



•^Putnam, Miller, Palm, and Evans (1952). 



" Cohen and Arbogast (1950b). 



" Smith and Wyatt (1951). 



Measurements of extinction provide a rapid and convenient 

 method of assay of phage during purification, and the extinction 

 per plaque-forming particle of the final product is an excellent 



