104 DISINFECTION 



violet and the extreme ultra-violet. No other portion of the spectrum had any 

 effect whatever. Their work has been confirmed by Browning and Russ (1917), 

 who used a quartz spectrometer illuminated by an arc of pure tungsten, which is 

 very rich in ultra-violet rays. The spectrum was thrown across a gelatin or agar 

 plate seeded with staphylococci, the plate incubated after irradiation, and then 

 used as an ordinary photographic negative for producing positive contact prints. 

 An exposure of 6 minutes was sufficient to destroy the organisms in that region 

 illuminated by rays of 2380 to 2940 Angstrom units. (An Angstrom unit is equal 

 to 1 X 10"' mm., i.e. 1 /10,000,000th mm. One m^ is equal to 1 X 10"^ mm., i.e. 

 l/l,000,000th mm., or l/l,000th f.i, or 10 Angstrom units.) There was a sharp 

 line of demarcation at 2960 A.U. ; rays of a longer wave length than this were 

 almost devoid of germicidal action. It was found that the rays between 2960 

 and 2100 A.U. were highly germicidal, but that the most active were those 

 between 2800 and 2540 A.U. As the limit of retinal sensibility is reached when 

 the wave length falls to 3970 A.U., it follows that the most actively germicidal 

 portion of the spectrum is in that part which is invisible to the human eye 

 (Fig. 20). More recent work by Gates (1930), Ehrismann and Noethling (1932), 

 Buchholz and v. Jeney (1935), Prudhomme (1937), and Rouyer and Servigue (1938), 

 has shown that two of the most active wave lengths are 2650 and 2530 A.U. 

 Different organisms appear to vary in their susceptibihty to different wave lengths. 

 Ultra-violet light is lethal, not only to bacteria, but to other unicellular organisms 

 such as amoebae (Barr 1923), and to the tissue cells of animals and plants. It is, 

 in fact, a protoplasmic poison. Though the short rays are undoubtedly the most 

 active, the other rays of sunlight are not completely devoid of germicidal power ; 

 they require, however, very much longer to produce their lethal efiect (Thiele and 

 Wolf 1907). Laroquette (1918) found that blue was more active than yellow, and 

 yellow than red ; green light was the poorest of all. The time necessary for destruc- 

 tion of micro-organisms by ultra-violet light depends on the intensity of the light, 

 the distance of the source of illumination, and the nature of the medium in which 

 the organisms are exposed. In general, the Bunsen-Roscoe law holds true ; that 

 is, within given limits the product of the intensity of irradiation and the length 

 of exposure is constant. The temperature of the organisms at the time of exposure, 

 provided that it is within the normal limits of viability, seems to have no effect 

 on the action of ultra-violet light (Rentschler et al. 1941). 



The germicidal effect of sunlight under natural conditions varies greatly. Its 

 action is complex, due partly to the actinic and partly to the calorific rays, which 

 act by dehydration. Apart from its action on the organisms themselves, it has 

 an action on the medium in which they are growing. In southern lands, the 

 combined effect of the ultra-violet rays and the heat rays render sunlight highly 

 efficient as a germicidal agent ; thus Semple and Greig (see Hewlett 1909) in India 

 found that Salm. typhi exposed to the sun on pieces of white drill cloth were killed 

 in 2 hours ; controls kept in the dark were still alive after 6 days. In the civihzed 

 smoke-covered towns of the north, the ultra-violet rays are often very weak, being 

 largely filtered off by the impurities in the atmosphere, thus depriving sunlight of 

 most of its activity. That light under these conditions is not entirely without 

 effect, however, has been shown by Garrod (1944), w^ho found that in hospital wards 

 hsemolytic streptococci could be readily demonstrated in dust from the darker 

 portions of the ward, but not from dust on window sills and other parts exposed 

 to diffuse daylight. 



