34 



Drs. C. H. Browning and S. Russ. 



of Marshall Ward,* Wesbrook,t and D'Arcy and Hardy,j conferred upon such 

 investigations an exactness which had been previously entirely lacking, both 

 as regards knov^ledge of the most effective germicidal portion of the visible 

 rays, and also with regard to the essential chemical processes associated with 

 such action. 



The present investigation deals with one particular region of ultra-violet, 

 radiation, which we find has much greater germicidal action than any part, 

 of the visible spectrum, and an attempt is made to correlate such germicidal 

 action with the physical phenomenon of selective absorption. 



The quartz spectrometer (Design C), made by Messrs. Hilger. is so 

 constructed that photographs may be obtained ranging from wave-lengths 

 7000-2100 A.U. ;§ this region of the spectrum is spread over a length of 

 about 19 cm. (it varies slightly with each instrument). 



The radiation emitted by an arc of pure tungsten, when focussed on the 

 slit of the instrument and received on the photographic plate, is seen to 

 consist of a series of bright lines very closely packed together extending as 

 far as 2100 l.U. Thus it is a very suitable source of intense ultra-violet 

 radiation over a wide range. 



In order to determine what part of this radiation exerts germicidal action,, 

 the following procedure was employed : A glass plate, similar to the photo- 

 graphic plate used in this instrument, was coated with a thin layer of 

 nutrient agar ; then over this surface a thin layer of a living bacterial 

 emulsion {Staphylococcus pyogenes mireus) was painted. The plate thus 

 inoculated was then placed in the carrier of the instrument, and the shutter- 

 opened to allow the radiation from the arc to fall upon a narrow strip of' 

 the bacterial film. After an appropriate exposure the plate was removed 

 from the spectrometer, and then incubated at 37° C. for 48 hours in order 

 to see, by the resulting growth, what effect had been produced upon the 

 organisms by the various constituents of the beam. 



Fig. 1 shows the germicidal effect of the rays upon Staphylococcus pyogenes 

 aureus for three different times of exposure of the organisms, viz., 6, 12, and 

 24 minutes. The radiation utilised, as shown by its action on a photographic 

 plate, extended right across the bacterial film ; the germicidal action is, 

 however, restricted to a region which ranges from a wave-length of 2940 to 

 about 2380 A.U. The black lines indicate the bactericidal action. The 

 illustration was obtained by photographing the bacterial film by means of 



* Marshall Ward, 'Phil. Trans.,' B, 1894, p. 961. 



t Wesbrook, ' Jouru. Path, and Bact.,' vol. 3, p. 70 (1896). 



X D'Arcy and Hardy, ' .Journ. Physiology,' vol. 17, No. 5, p. 390 (1894). 



O O 



§ A.U. signifies Angstrom units, the standard by which wave-lengths are measured.. 



