374 S. GAED AND O. MAAL0E 



energy absorption by P atoms was not significantly more effective in inacti- 

 vating pliage Tl than was absorption by any other atom. 



3. Nonionizing Radiation 



a. Far UV Light {Wavelength below 3000 A). It has been mentioned that, in 

 contrast to X-rays, UV inactivates by direct effects only. This was demon- 

 strated by McKinley et al. (1926), who showed that phage, herjDes, and 

 Levaditi viruses, as well as bacteria, could be killed by direct irradiation but 

 not by being introduced into preirradiated medium, Inactivation by UV is 

 characteristically exponential. This was observed by Fisher and McKinley 

 (1927) and Baker and Nanavutty (1929) for phages, and has later been con- 

 firmed for a variety of viruses by Bawden and Kleczkowski (1953) for small 

 plant viruses, and by Fogh (1955), Dulbecco and Vogt (1955), and Stanley 

 et al. (1956) for poUoviruses. At low survival values the rate of inactivation 

 is sometimes observed to decrease; this may be due to the presence in the 

 virus population of a few relatively resistant particles. 



The Do dose (in ergs/mm.^) for inactivation by UV is usually found to be 

 independent of dose rate. A departure from this rule was observed by Eckart 

 (1954), who found that the D^ dose for inactivation of phage Tl by mono- 

 chromatic UV increased slightly with increasing dose rate (i.e., low dose 

 rates were most efficient). This effect has not been observed by others and 

 its nature is obscure. Another abnormality connected with dose rate was 

 noticed by Latarjet and Morenne (1951): in experiments with phage T2 

 irradiated with UV of very low intensity, a deviation from exponential in- 

 activation was observed which suggested a "3-hit" process. It is difficidt to 

 say what this means; published inactivation curves for T2 irradiated at 

 higher mtensities do not deviate nearly as much from exponentiality. 



The rate of inactivation by UV is usually a stable and characteristic 

 property of a virus, with the possible exception of host-ceU-induced modifica- 

 tions. Salk et al. (1940) claimed that influenza virus harvested from the lungs 

 of infected mice was more susceptible to UV than virus from tissue cultures, 

 and the sensitivity of certain actinophages was found to depend on the 

 strain on which the phage was propagated (Welsch and Minon, 1955b). 



All speculations about the mechanism of UV inactivation are based on 

 analysis of the relative efficiency of different wavelengths. The D^ dose de- 

 termined by irradiation with monochromatic UV is a convenient expression 

 of the efficiency at the wavelength (A) chosen. A plot of l/Dj, against A gives 

 a curve somewhat resembling an absorption spectrum; i.e., the curve has 

 maxima corresponding to the most effective A values and vice versa. "Action 

 spectra" of this type were determined for staphylococci and phages active on 

 this organism, as well as for vaccinia virus (Rivers and Gates, 1928; Sturm 

 et al., 1932; Gates, 1930, 1934). Close parallelism could be demonstrated 



