The Germicidal Action of Ultra- Violet Radiation. 37 



the experiments in which the meningococcus was compared, " trypagar " was 

 employed as the medium.) After suitable exposures the agar plates were 

 incubated, and the range of wave-lengths over which a lethal action was 

 subsequently observed was measured for the different organisms. The 

 meningococcus was found to be susceptible over a slightly greater range than 

 the staphylococcus ; exposure of these two organisms to the same radiation 

 for the same time (20 minutes) resulted in a lethal action upon the meningo- 

 coccus over a range 2960-2240, the range for staphylococcus being 2960- 

 2320 A.U. 



In the case of the acid-fast bacillus, the result was reversed, the staphylo- 

 coccus being slightly more susceptible than the acid-fast bacillus. 



The Connection between Germicidal Action and Selective Absolution. 



The photograph which serves to illustrate the selective absorption of the 

 bacterial emulsion was obtained with an exposure of two minutes. A single 

 observation is obviously not sufficient to prove that the region beyond a 

 wave-length of 2960 ATI. is selectively absorbed. Therefore a series of 

 photographs was taken in which the exposure ranged from five seconds to 

 five minutes, and the fact that practically identical records resulted, except 

 in the respective densities of the spectral lines transmitted through the 

 bacterial emulsion, may be taken as evidence that wave-lengths shorter than 

 2960 A.U. are actually absorbed in a selective manner. By increasing the 

 exposures to as much as 20 minutes, only a few additional lines made their 

 appearance beyond the line of demarcation in question. 



It seems therefore that the conclusion may justifiably be drawn that ultra- 

 violet radiation between wave-lengths 2960 and 2100 A.U. is germicidal to 

 bacteria, and that rays over this range of wave-length are also particularly 

 absorbed by the substances of which such bacteria are composed. We have 

 found experimentally that such substances as human serum and egg albumen 

 also have a well marked absorption band for wave-lengths ranging from 

 3180 to 2100 A.U. (and also possibly beyond, but this we have not investi- 

 gated). If further we recall the fact that human skin in a layer as thin as 

 1/10 mm. is practically opaque to radiation over a very similar range of 

 wave-lengths, then we may look upon this region as one for which proto- 

 plasm has a particular power of absorption. The high degree of correlation 

 between the germicidal action of a particular portion of the ultra-violet 

 radiation with an enhanced degree of absorption of such radiation by the 

 organisms, which appears to have been clearly established, does not, of course, 

 explain the bactericidal effect, but the enquiry now takes on a physico- 

 chemical aspect as well as a purely biological one. Attention may be called 



