372 S. GARD AND O. MAALOE 



sharply. Pollard's interpretation of this temperature effect is that: "... 

 transfer of energy through the virus to a sensitive part, or damage to a 

 larger part of the virus, is facilitated when irradiating at high temperatures." 

 As mentioned earlier in this section, close-range indirect effects may also be 

 considered responsible. Pollard and co-workers have presented a number of 

 other determinations of target volumes (Pollard, 1953, 1954); these will be 

 discussed in Section II B, 2, b, together with attempts to estimate size and 

 shape of the targets. 



Epstein and Englander (1954) compared X-ray inactivation of the tem- 

 perate phage A and of a virulent mutant thereof, and concluded that the loss 

 of ability to lysogenize associated with the mutation is accompanied by a 

 small, but significant diminution of the target volume. As a supplement to 

 the data previously compiled by Epstein (1953) they point out that the 

 phages A and T2 have the same DNA/target-volume ratio. 



To conclude this section, we shall mention briefly that X- and y-rays are 

 being used in attempts to sterilize virus-containing material. General aspects 

 of "electronic food sterilization" have been discussed by Nickerson et al. 

 (1953). A series of ammal viruses and phages have been subjected to the high 

 intensity electron beam from a 3Mv "Capacitron"^ it was found that the dose 

 required to sterilize was inversely related to the size of the virus (Huber, 

 1952). Jordan and Kempe (1956) have used gamma rays from a cobalt-60 

 source to sterihze poliovirus-containing material (in these experiments, 

 "sterilization" represented a reduction to roughly 10"^ of the original as 

 surviving particles). The mam points of this investigation are that crude 

 material requires more irradiation than semi-purified preparations, and that 

 no antigenic change could be demonstrated after applying three times the 

 "sterilizing dose." 



6. a-Rays and Other Heavy-Particle Rays. The potential usefulness of these 

 densely ionizing radiations in virus research was discussed by Jordan (1940); 

 WoUman et al. (1940) showed that, according to theory, the Do (in Roentgens) 

 for a-rays was several times greater than for X-rays. Radon in solution was 

 used to obtaui uniform effect. 



Lea and co-workers (1940-1944) proceeded to develop a practical procedure 

 for drying viruses in thin films in order to permit the use of poorly penetrating 

 radiations. In his classic monograph. Lea (1947) strongly emphasizes that the 

 target theory requires the fulfillment of three different criteria: two of these 

 have been discussed, i.e., exponential inactivation and dose-rate independ- 

 ence. The third demands that the efficiency per ionization (the ionic yield) 

 decreases with increasing density of ionization along the track of the particle. 

 If the average number of primary ionizations produced in the target by an 



^ Electronized Chemicals Corp., Brooklyn, N.Y. 



