DISTRIBUTION OF BACTERIA BY AIR 151 



numerous, may be quite innocuous. Great numbers of bacteria 

 thus may not indicate any condition likely to injure health, and 

 this may be true also even when the bacteria come from the 

 crowding together of a number of healthy human beings. On 

 the other hand, there is no doubt that disease germs can be 

 disseminated by means of the air. The possibility of this 

 has been shown experimentally by infecting the mouth with the 

 b. prodigiosus, which is easily recognised by its brilliantly 

 coloured colonies, and then studying its subsequent distribution. 

 Most important here is the infection of the air from sick persons. 

 The actions of coughing, sneezing, speaking, and even of deep 

 breathing, distribute, often to a considerable distance, minute 

 droplets of secretions from the mouth, throat, and nose, and these 

 may float in the air for a considerable time. Even five hours 

 after an atmosphere has been thus infected evidence may be 

 found of bacteria still floating free. Before this time, however, 

 most of the bacteria have settled upon various objects, where 

 they rapidly dry, and are no longer displaceable by ordinary air 

 currents. The diseases of known etiology where infection can 

 thus take place are diphtheria, influenza, pneumonia, plague of 

 the pneumonic type, and phthisis. In the case of phthisis, the 

 deposition of tubercle bacilli has been demonstrated on cover- 

 glasses held before the mouths of patients while talking, and 

 animals made to breathe directly in front of the mouths of 

 such patients have become infected with tuberculosis. Apart 

 from direct infection from individuals, however, pathogenic 

 bacteria may be spread in some cases from the splashing of 

 infected water, as from a sewage outfall. This possibility has 

 to be recognised especially in the cases of typhoid and cholera. 

 Besides infection through fluid particles, infection can be caused 

 in the air by dust coming from infected skin or clothes, 

 etc. Fliigge, in dealing with this subject in an experimental 

 inquiry, distinguishes between large particles of dust which 

 require an air current moving at the rate of 1 centimetre per 

 second to keep them suspended, and the finer dust which can be 

 kept in suspension by currents moving at from 1 to 4 milli- 

 metres per second. In the former case, when once the particles 

 settle they cannot be displaced by currents of air except when 

 these are moving at, at least, 5 metres per second, but the 

 brushing, shaking, or beating of objects may, of course, distribute 

 them. In the case of the finer dust the particles will remain for 

 long suspended, and when they have settled can be more easily 

 displaced, as by the waving of an arm, breathing, etc. With re- 

 gard to infection by dust, a most important factor, however, is 



