1900.] on the Effect of Physical Agents on Bacterial Life. 451 



in the air. We used Dr. Aitken's Dust-counter, which not only 

 renders the dust particles visible, but gives a means of counting 

 them in a sample of air. In an open suburb of London we found 

 20,000 dust particles in 1 cubic centimetre of air ; in a yard in the 

 centre of London about 500,000. The dust contamination we found 

 to be about 900 per cent, greater in the centre of London than in a 

 quiet suburb. In the open air of London there was on an average 

 just one organism to every 38,300,000 dust particles present in the 

 air, and in the air of a room, amongst 184,000,000 dust particles, only 

 one organism could be detected. 



These figures illustrate forcibly the poverty of the air in micro- 

 organisms even when very dusty, and likewise the enormous dilution 

 they undergo in the atmosphere. Their continued existence is 

 rendered difficult through the influence of desiccation and sunlight. 

 Desiccation is one of nature's favourite methods for getting rid of 

 bacteria. Moisture is necessary for their development and their vital 

 processes, and constitutes about 80 per cent, of their cell-substance. 

 When moisture is withdrawn most bacterial cells, unless they pro- 

 duce resistant forms of the nature of spores, quickly succumb. The 

 organism of cholera air-dried in a thin film dies in three hours. 

 The organisms ,of diphtheria, typhoid fever and tuberculosis show 

 more resistance, but die in a few weeks or months. 



Dust containing tubercle bacilli may be carried about by air 

 currents, and the bacilli in this way transferred from an affected to a 

 healthy individual. It may, however, be said that drying attenuates 

 and kills most of these forms of life in a comparatively short time. 

 The spoxes of certain bacteria may, on the other hand, live for many 

 years in a dried condition, e.g. the spores of anthrax bacilli which 

 are so infective for cattle and also for man (wool-sorters' disease). 

 Fortunately few pathogenic bacteria possess spores, and, therefore, 

 drying by checking and destroying their life is a physical agent that 

 plays an important role in the elimination of infectious diseases. 

 This process is aided by the marked bactericidal action of sunlight. 

 Sunlight, which has a remarkable fostering influence on higher plant 

 life, does not exercise the same influence on the bacteria. With few 

 exceptions we must grow them in the dark in order to obtain succes- 

 ful cultures ; and a sure way of losing our cultures is to leave them 

 exposed to the light of day. Direct sunlight is the most deadly 

 agent, and kills a large number of organisms in the short space of 

 one to two hours ; direct sunlight proves fatal to the typhoid bacillus 

 in half an hour to two hours, to the diphtheria bacillus in half an 

 hour to one hour, and to the tubercle bacillus in a few minutes to 

 several hours. Even anthrax spores are killed by direct light in 

 three and a half hours. Diffuse light is also injurious, though its 

 action is slower. By exposing pigment-producing bacteria to sun- 

 light colourless varieties can be obtained, and virulent bacteria so 

 weakened that they will no longer produce infection. The germicidal 

 action of the sun's rays is most marked at the blue end of the spec- 



