PETRI'S SAND-FILTER METHOD 129 



consist of felted masses of threads, from which are thrust into the 

 air special filaments, and in connection with these the spores are 

 formed. By currents of air these latter can easily be detached, 

 and may float about in a free condition. With the bacteria, on 

 the other hand, the case is different. Usually these are growing 

 together in little masses on organic materials, or in fluids, and 

 it is very much by the detachment of minute particles of the 

 substratum that the organisms become free. The entrance of 

 bacteria into the air, therefore, is associated with conditions 

 which favour the presence of dust, minute droplets of fluid, etc. 

 The presence of dust in particular would specially favour a large 

 number of bacteria being observed, and this is the case with the 

 air in many industrial conditions, where the bacteria, though 

 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, 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 

 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, 

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