APPENDIX 297 



direct sunlight. The most recent and conclusive experi- 

 ments of all, however, are those of Professor H. Marshall 

 AYard l with the very resistent and virulent spores of Bacillus 

 nutlimns, confirming and extending the results obtained by 

 some previous observers with the same bacterium. Having 

 found that repeated exposure to sunlight destroyed the 

 spores in a few c.cm. of Thames water containing a very 

 large number, while a few weeks of bright daylight greatly 

 lessened them, he proceeded to make a series of accurate 

 experiments as follows : Agar plates of anthrax were made 

 in Petri's capsules, using the virulent and resistent spores 

 obtained by keeping some c.cm. of distilled water, saturated 

 with material from an old agar culture, at 56 C. for 24 

 hours. To the bottom of each plate was attached a zinc 

 stencil having a letter cut out, the rest of the capsule was 

 covered with dull black paper, and the letter surface ex- 

 posed to direct or reflected sunlight for 2 to 6 hours, after 

 which the plate was kept in the incubator at 20 C. for 48 

 hours. The agar was then found to be grey and cloudy 

 from the development of a multitude of colonies, but the 

 space exposed to light remained quite clear, showing the 

 form of the letter, and the development of the colonies in 

 its neighbourhood were greatly retarded, owing to the action 

 of reflected light, when the letter was large. These results 

 were also obtained with other bacteria, as well as with the 

 spores of fungi; and similar, though less marked, effects 

 were produced by the electric arc light, so much so that 

 Marshall Ward thinks that it may yet prove to be an 

 efficient disinfecting agent. 



The action of coloured light When a plate culture of 

 anthrax spores is exposed to the solar spectrum the bacteri- 

 cidal action is found to be strongest at the blue-violet end 

 (Ward), and certain chromogenic bacteria have been 



1 Royal Soc. Proc., vol. Hi. No. 318, p. 393. 



