450 BACTERIOLOGY. 



media cannot leave the moist surface. For the same rea- 

 son, the breath of a person is practically free from bac- 

 teria; or, at all events, it does not take up and carry out 

 the organisms that may be present in the air-passages. 

 The breath of a consumptive, therefore, is free from the 

 specific germ of this disease. It is only in case of violent 

 expulsion of air, as in a fit of coughing, that particles of 

 moisture with the contained organisms may be forcibly 

 ejected from the air passages. 



When air is passed over a moist surface, instead of 

 taking up bacteria, it is usually deprived of those that are 

 present. The suspended particles, whether mere grains of 

 dust or the much smaller bacterial cells, when once brought 

 into contact with a moist surface become fixed to that sur- 

 face. Hence it is, that the farther one goes away from the 

 shore the less numerous the air germs become. The air 

 at sea, at some distance from the land, may be said to be 

 practically free from organisms. 



The suspended particles in the air are made up, in the 

 first place, of the relatively heavy grains of dust. The 

 surface of these dust particles is covered with such organ- 

 isms as chanced to dry down. To a less extent the sun 

 motes can be considered as the carriers of bacteria. The 

 free organism, either single or, more often in short 

 threads or groups, constitutes the finest particle in the 

 air. 



The atmospheric germs, by no means, belong exclu- 

 sively to the group of bacteria. While this form is quite 

 common, yet the yeast is frequently met with and the spores 

 of moulds are very abundant. Inasmuch as these several 

 forms of living matter exist in the air as dried particles it 

 is evident that they do not multiply. On the contrary, 

 desiccation, when prolonged, tends to destroy many organ- 

 isms and this unfavorable action is still further accentuated 

 by the direct germicidal effect of sun-light. 



