64 BACTEEIOLOGY. 



altitudes and that m mid-ocean ; the air exhaled from the lungs; 

 the deeper layers of the soil and the water coming from such 

 depths ; the internal fluids, tissues and secretions of healthy, nor- 

 mal animals and plants are practically the only places in nature 

 ivhere bacteria are not present. 



As pointed out on p. 58, the ordinary air always contains 

 bacteria which, as minute dry particles, are carried about by 

 the 'movements of the atmosphere. In a perfectly tight 

 room, free from currents, the suspended organisms will 

 readily settle to the floor, owing to the fact that they are 

 specifically heavier than air. A similar tendency exists in 

 the open air and as a result the lower layers of the atmos- 

 phere will contain more bacteria than the higher ones. 

 Moreover, the air is purified by the falling rain or snow, 

 which drag down to the earth the suspended solid particles. 

 It is evident, therefore, that the atmosphere on the tops of 

 high mountains will be practically free from bacteria. 



The air in mid-ocean may also be considered as free 

 from suspended particles, including bacteria. This is due to 

 the washing, as it were, of the winds or currents of air in 

 their passage over the water. The bacteria in the air on 

 coming into contact with a moist surface are held back. 

 This fact holds true not only in the case of the air which 

 passes over large surfaces of water but also in the case of 

 expired air. No matter how many hundreds or thousands 

 of bacteria may be present in the air which is drawn into 

 the lungs the expired air is always practically free from 

 organisms. The latter adhere to the moist, mucous mem- 

 brane of the mouth, nose, throat and bronchi, and conse- 

 quently, the expired air contains very few or no bacteria. 



It may be stated as a general and most important rule 

 that the bacteria present in liquids or on moist surfaces 

 cannot leave such places and enter the air. The most deadly 

 organism can be studied in the laboratory, as it grows on 

 moist culture media, without any danger in this respect. For 

 the same reason j the breath of a consumptive is free from 



