PHYSICAL AGENTS AND BACTERIAL LIFE. 241 



to harbor bacteria, are more important points than air disinfection, 

 and this fact has been recognized in modern surgery. In an investi- 

 gation, in conjunction with Mr. Lunt, an estimation was arrived at 

 of the ratio existing between the number of dust particles and bacteria 

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

 the air 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 centimeter of air; in a yard in the 

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

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

 (iniet 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 favorite methods for getting rid of bac- 

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 When moisture is withdrawn most bacterial cells, unless they pro- 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

 rents, 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 spores 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 success- 

 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 



VOL. LVIII— 16 



