126 Messrs. J. E. Barnard and H. de E. Morgan. [June 15, 



" Upon the Bactericidal Action of some Ultra-violet Radiations as 

 Produced by the Continuous-Current Arc." By J. E. 

 BARNARD and H. de R. MORGAN. Communicated by Sir 

 HENRY EOSCOE, F.R.S. Received June 15, Read June 18, 

 1903. 



(From the Jenner Institute of Preventive Medicine.) 



The experiments here described have been carried out with the 

 object of determining the effect on the vitality of bacteria, as the 

 result of exposure to the arc spectra of carbon and of various metals. 



The organisms experimented with have been the Bacillus coli 

 communis, B. prodigiosus, B. subtilis, Micrococcus tetragenus, Staphylococws 

 anreus and Bacillus tubercMlosix. 



The spectroscope was provided with quartz lenses and prism, and a 

 subsidiary quartz lens of 18-inch focus was used to project the image 

 of the arc on to the slit. The instrument was adjusted as for photo- 

 graphy, except that the slit was somewhat more open than is generally 

 the case for that purpose. The method of experimenting was to project 

 the spectrum on to an agar film, contained in an ordinary Petri dish, 

 over the surface of which had been spread with a sterilised brush an 

 active culture of one of the organisms mentioned. The plates were 

 exposed to the light directly after inoculation and were then incubated 

 for 24 hours at 37 C. The part of the plate on which no portion of 

 the spectrum was allowed to fall grew normally and served as u 

 control. 



It was found that the bactericidal action was entirely confined to 

 the ultra-violet region. With the spectrum projected as described, the 

 results were as follows : 



Length 

 Current. Width of slit. of exposure. 



11 amperes. 0'5 mm. 15 mins. Effect alight, but bands 



discernible. 



0-5 20 Effect well marked. 



0-5 45 Effect very marked, so 



that the bands almost 

 merged and formed a 

 sterile area. 



With exposures as long as two hours no effect whatever was obtained 

 in any other portion of the spectrum, even when the slit of the 

 spectroscope was opened to an extent which in photographing would 

 have been regarded as inadmissible. The active radiations lie in that 

 portion of the spectrum between wave-length 3287 and 2265. The 

 wave-lengths were determined by reference to the arc spectrum of 



