DISSEMINATION OF BACTERIA IN AIR. 127 



there is no doubt that disease germs can be disseminated by 

 means of the air. The possibility of this has been shown ex- 

 perimentally 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, and phthisis; 

 and here also probably whooping-cough, typhus fever, and measles 

 are to be added, though the morbific agents are unknown. In 

 the case of phthisis, the alighting 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 tuber- 

 culosis. Apart from direct infection from individuals, however, 

 pathogenic bacteria may be spread in some cases from the splash- 

 ing of infected water, as from a sewage outfall. This possibility 

 has to be recognised especially in . j cases of typhoid and 

 cholera. Besides infection through fluid particles, infection can 

 be caused in the air by dust coming, say, from infected skin or 

 clothes, etc. Fliigge, in dealing with this subject in an experi- 

 mental inquiry, distinguishes between large particles of dust 

 which require an air current moving at the rate of i cm. per 

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

 be kept in suspension by currents moving at from i to 4 mm. 

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

 they cannot be displaced by currents of air except when these 

 are moving at, at least, 5 m. 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 dis- 

 placed, as by the waving of an arm, breathing, etc. With regard 



