388 RADIATION BIOLOGY 



what conHictinf^ results. Cohlentz and Fulton (1924) concluded that the 

 law did not apply, since they observed that a reduction in intensity to one- 

 liftieth required an increase in exposure of seventy-five- to eightyfold to 

 obtain the same killing elYect. These same workers, however, observed 

 no (lirtcrcncc in inactivation when the same total energy was given in one 

 contiinu)Us dose or in as many as sixteen intermittent doses with varying 

 int(M-vals between. (Jates (19291)) studied the elTect on an approximately 

 fourfold variation in intensity and observed small differences in survival 

 at the two intensities. The difference in survival at the two intensities 

 diminished as the survival ratio approached zero. Lea and Haines (1940) 

 observed no intensity effect on the inactivation of B. mesentericus spores 

 and A', coll when the intensity was varied a hundredfold. Roller (1939), 

 in his studies of th(> lethal effects on air-borne bacteria, observed reci- 

 procity when the intensity of 2537 A ultraviolet radiation was varied 

 about fifteen hundredfold. The most extreme variations in intensity 

 were those employed by Rentschler ct al. (1941 ) w^ho gave the same dose of 

 2537 A ultraviolet in periods of time varjdng from a few microseconds to 

 several minutes. No effect in intensity was observed as long as the total 

 length of exposure w^as small relative to the generation time of the bac- 

 teria. Therefore it appears that the reciprocity law holds approximately 

 for the inactivation of bacteria by ultraviolet radiation if all other factors 

 are held constant. 



For very low intensities given over long periods of time, the deviations 

 from reciprocity are greater. Since the energy emitted in the various 

 lines of the mercury-arc spectrum varies widely, it is difficult to avoid 

 differences in intensity of the various wave lengths in action-spectrum 

 studies. However, it would appear that the variation in intensity could 

 have only a very minor influence on the results obtained at the different 

 wave lengths. 



RELATION OF TEMPERATURE 



The early somewhat conflicting results of studies concerned with the 

 relation of temperature to the bactericidal efficiency of ultraviolet have 

 been discussed by Duggar (193()). Bayne-Jones and Van der Lingen 

 (1923) observed temperature coefficients {Q^n) of 1.06 and 1.04 for the 

 temperature ranges 2°-12°C and 30°-40°C, respectively. Gates (1929b) 

 similarly observed a small temperature coefficient of about 1.1. Exposure 

 at 5° and 37°C had no effect in the studies of Rentschler et al. (1941). 

 Heinmets and Taylor (1951) have studied the effect of temperatures as 

 low as — 50°C. No pronounced influence of temperature is apparent 

 until -35°C when survi\al begins to dei^rease rapidly for a given dose. 

 The low temperature coefficients ol)served agree well with those expected 

 on the hypothesis that the bactericidal effects of ultraviolet result from a 

 primary, simple photochemical reaction. 



