The Germicidal Action of Ultra-Violet Radiation. 35 



transmitted light. From tMs negative a transparency was made, and this 

 was employed for the print. 



Photographs taken with long exposures always show the presence of 

 scattered radiation, and it is interesting to observe that the organisms appear 

 to be stimulated in their growth in the region just between two portions 

 which have been exposed to the full radiation. The increase of bacterial 

 growth was confirmed microscopically, but it is not necessarily due to the 

 scattered radiation. This stimulation is only seen satisfactorily in the original. 



The effect of prolonging the exposure from 6 to 24 minutes is to increase 

 somewhat the range of lethal action, but only to a comparatively slight 

 extent. A more prolonged exposure brings out prominently the region of 

 the spectrum at which germicidal action practically stops. In order to 

 increase the amount of radiation reaching the bacterial film, the slit was 

 widened ; this had the effect of causing the lines of the spectrum to overlap 

 sufficiently to form a practically continuous band of radiation. 



The procedure followed was otherwise identical with that which has just 

 been described ; the bacterial film was first exposed to the whole range of 

 spectral lines, then incubated, to see what effect had been produced upon the 

 organisms by the various constituents of the beam. 



The central strip of fig. 2 shows the result of a prolonged exposure 

 (about 3J hours) of the organisms to the tungsten radiation. It will be 

 seen that germicidal action, indicated by the black region, occurs throughout 

 a region of wave-lengths extending from about 2960 to 2150 A.U. The 

 most striking feature is the sharp line of demarcation occurring in the 

 region of the former wave-length. Ether vibrations of wave-length 

 2960 A.U. have a marked germicidal action ; an increase of 1 or 2 per 

 per cent, in this wave-length is, practically speaking, sufficient to bring one 

 to a region of the spectrum devoid of such germicidal action. 



A control experiment was made to decide whether the radiation had any 

 effect upon the agar. A portion of it was irradiated, and then inoculated 

 with a suspension of an agar culture of Etxpliylococcus pyogenes aureus, which 

 had been washed three times by centrifuging with sterile 0'8 per cent, 

 sodium chloride solution. IsTo difference was detected in the density of 

 growth of the organisms over the irradiated and the non-irradiated portions 

 of the agar. 



The existence of this sharp line of demarcation strongly suggested that 

 selective absorption was playing some part. This was put to the test by 

 examining the absorption spectrum of a suspension of the organisms which 

 were used in the experiment just described. A small quartz vessel was 

 filled with the bacterial emulsion and placed in front of the slit of the 



