MODE OF ACTION OF ULTRA-VIOLET LIGHT 105 



Ultra-violet light generated by a Cooper Hewitt Mercury vapour lamp has 

 been used for the sterilization of drinking water. Foulds (1911) found that it 

 was quite easy to ensure a 99 per cent, reduction in the bacterial count, including 

 all Bad. coli, by this method. For further information, see Thresh and Beale 

 (1910). 



Mode of Action of Ultra-violet Light. — Ultra-violet light is peculiar in that its 

 penetrating power is very low. Even a thin layer of glass, such as a cover slip, 

 is able to filter off a large proportion of the rays. The same power is possessed 

 by proteins. It follows therefore that its action must be chiefly on the surface 

 of the body which it irradiates. Wesbrook (1896), by an ingenious series of experi- 

 ments, showed that when tetanus or cholera cultures were irradiated by the sun 

 in the presence of air, a consumption of oxygen took place. D'Arcy and Hardy 

 (1894) came to the conclusion that under the influence of ultra-violet light, some 

 oxidizing substance is produced — possibly ozone — which is responsible for its 

 bactericidal effect. This is very doubtful. There seems to be no question that 

 the destructive action of ultra-violet light is manifest in the absence of atmo- 

 spheric oxygen (Thiele and Wolf 1906, 1907, Blum 1932, Buchholz and v. Jeney 

 1935), so that ozone can hardly be responsible. Nor does it seem likely that 

 hydrogen peroxide is generated in sufficient quantities to prove lethal, since 

 Ehrismann (1930) found that, to produce a destruction of bacteria similar 

 to that caused by ultra-violet light, a concentration of 30 per cent. HgOj was 

 required. 



There is evidence that the ultra-violet rays act by inducing some change in 

 the protein molecule. 



Thus they are destructive, not only of organized cells, but of cellular products, such as 

 tetanus toxin (Kitasato 1891, Fermi and Pernossi 1894, Wesbrook 1896), and serum 

 complement (Sellards 1918, Brooks 1920). Dreyer and Hanssen (1907), working with solu- 

 tions of various albumins and globulins, exposed in very thin layers, found that the rays 

 caused a true coagulation of the proteins, which were no longer soluble in weak acids or 

 alkalies. Corroboration of this view is found in the experiments of Tchahotine (1921). 

 Working with the eggs of sea urchins, which are rich in lecithin, he found that if they 

 were stained with neutral red and then irradiated with ultra-violet light, the colour changed 

 after a time to yellow, indicating the presence of alkali within the cell. Further work 

 seemed to show that the action of the rays was on the superficial membrane of the cell, 

 which was rendered more permeable to the OH -ions of the medium ; these, on penetrating 

 the cell, were responsible for the change of the neutral red to yellow. In support of this, 

 it was found that if the eggs were irradiated in a neutral solution, no change in the coloiu" 

 of the dye took place. From these and further experiments he comes to the conclusion 

 that the rays act primarily on the superficial layer of the cell, coagulating its colloids, and 

 rendering it more permeable to the ions in the surrounding medium. Ehrismann (1930) 

 exposed saline suspensions of various bacteria to ultra-violet irradiation for 6 hours, and 

 observed that a decrease in the opacity occurred, accompanied by a fall in the total count. 

 No change in pH, however, was noticed. Agencies such as increased acidity, higher 

 temperatures, formol, and mercuric chloride, that tended to hasten the coagulation of 

 protein, partly or completely inhibited the clearing effect of the rays. Analysis showed 

 that the total nitrogen in the suspending fluid was increased, though the amino-nitrogen 

 figure remained unaltered. 



The conclusion appears to be that the rays produce a colloidal change in the 

 protoplasm leading to the solution of certain of its constituents. Whether true 

 autolysis occurs in addition is still doubtful. 



