106 BACTERIA 



human tissues. Two cultivations of lowly virulent bacilli 

 were therefore grown by Mr. Shattock in flasks upon a 

 favourable medium over which was drawn sewer air. This 

 was continued for two weeks or five weeks respectively. Yet 

 no increased virulence was secured. Such experiments re- 

 quire ample confirmation, but even from this it will be seen 

 that sewer air does not necessarily have a favouring influence 

 upon the virulence of the bacilli of diphtheria. 



It should be noted that the bacilli of diphtheria are cap- 

 able of lengthened survival outside the body, and are readily 

 disseminated by very feeble air-currents. The condition 

 necessary for their existence outside the body for any period 

 above two or three days is moisture. Dried diphtheria 

 bacilli soon lose their vitality. It is probably owing to this 

 fact that the disease is not as commonly conveyed by air as, 

 for example, tubercle. 1 



The influence of gravity upon bacteria in the air may be 

 observed in various ways, in addition to its action within a 

 limited area like a sewer or a room. Miquel found in some 

 investigations in Paris that, whereas on the Rue de Rivoli 

 750 germs were present in a cubic metre, yet at the summit 

 of the Pantheon only 28 were found in the same quantity 

 of air. At the tops of mountains air is germ-free, and bac- 

 teria increase in proportion to descent. As Tyndall has 

 pointed out, even ultra-microscopic cells obey the law of 

 gravitation. This is equally true in the limited areas of a 

 laboratory or warehouse and in the open air. 



The conditions which affect the number of bacteria in the 

 air are various. After a fall of rain or snow they are very 

 markedly diminished ; during a dry wind they are increased. 

 In open fields, free from habitations, they are fewer, as 

 would be expected, than in the vicinity of manufactories, 

 houses, or towns. A dry, sandy soil or a dry surface of 

 any kind will obviously favour the presence of organisms in 



1 Annali d" 1 Igiene Sperimetitale, vol. v. (1895), fasc. 4. 



