104 BACTERIA 



body. Dental caries will occur to everyone's mind as a dis- 

 ease possibly due to bacteria. As a matter of fact, probably 

 acids (due to acid secretion and acid fermentation) and 

 micro-organisms are two of the chief causes of decay of 

 teeth. Defects in the enamel, inherent or due to injury, 

 retention of debris on and around the teeth, and certain 

 pathological conditions of the secretion of the mouth are 

 predisposing causes, which afford a suitable nidus for putre- 

 factive bacteria. The large quantity of bacteria which a 

 decayed tooth contains is easily demonstrated. 



From the two series of experiments which we have now 

 considered we may gather the following facts : 



(a) That air may contain great numbers of bacteria which 

 may be readily inspired. 



(b) That in health those inspired do not pass beyond the 

 moist surface of the nasal and buccal cavities. 



(c) That here there are various influences of a bactericidal 

 nature at work in defence of the individual. 



(d) That expired air contains, as a rule, no bacteria what- 

 ever. 



The practical application of these things is a simple one. 

 To keep air free from bacteria, the surroundings must be 

 moist. Strong acids and disinfectants are not required. 

 Moisture alone will be effectual. Two or three examples at 

 once occur to the mind. 



Anthrax spores are conveyed from time to time from 

 dried infected hides and skins to the hands or bodies of 

 workers in warehouses in Bradford and other places. If the 

 surroundings were moist, and the hides moist, anthrax 

 spores and all other bacteria would not remain free in the 

 air. 



The bacilli or spores of tubercle present in sputum in great 

 abundance cannot, by any chance whatever, infect the air 

 until, and unless, the sputum dries. So long as the ex- 

 pectorated matter remains on the pavement or handkerchief 



