102 BACTERIA 



be invisible. His ingenious experiments, reported in the 

 Transactions of the Royal Society of Edinburgh (vol. xxxv.), 

 have demonstrated that by supersaturation of air the invis- 

 ible dust particles may become visible. As is now well 

 known, Dr. Aitken has been able to prove that fogs, mists, 

 and the like do not occur in dust-free air, and are due to 

 condensation of moisture upon dust particles. But it should 

 be remembered that, though dust forms a vehicle for bac- 

 teria, dusty air is often comparatively free from bacteria. 

 Hence, after all, the necessary conditions for dissemination 

 of bacteria in air are two, namely, some degree of air-cur- 

 rent and dry surfaces. 



This latter condition is one of essential importance. Bac- 

 teria cannot leave a moist surface either under evaporation 

 or by means of air-currents. 1 Only when there is consider- 

 able molecular disturbance, such as splashing, can there 

 possibly be microbes transmitted to the surrounding air. 

 This fact, coupled with the influence of gravitation, is the 

 reason why sewer gas and all air contained within moist 

 perimeters is almost germ-free; whereas from dry surfaces 

 the least air-current is able to raise countless numbers of 

 organisms. Quite recently this principle has been admir- 

 ably illustrated in two series of investigations made upon 

 expired and inspired air. In a report to the Smithsonian 

 Institution of Washington (1895) upon the composition of 

 expired air, it is concluded that " in ordinary quiet respira- 

 tion no bacteria, epithelial scabs, or particles of dead tissue 

 are contained in the expired air. In the act of coughing or 

 sneezing such organisms or particles may probably be thrown 

 out." The interior of the cavity of the mouth and external 

 respiratory tract is a moist perimeter, from the walls of 

 which no organisms can rise except under molecular dis- 



1 Flttgge has lately attempted to demonstrate that an air current having a 

 velocity of four metres per second can remove bacteria from surfaces of liquids 

 by detaching drops of the liquid itself. 



