CHAPTER VII. 



INVESTIGATION OF AIR, SOIL, AND WATER. 



1. Air. 



The purpose of bacteriological investigations of the air is to as- 

 certain the number and kind of micro-organisms inhabiting the 

 strata of air surrounding us. 



The entire method of culture on solid media now in use de- 

 pends, as will be remembered, upon the fact that there are devel- 

 oped on the surface of slices of boiled and exposed potatoes a series 

 of divers bacterial colonies owing their origin to germs dropped 

 from the air. The soiling of plate cultures established the fact 

 that the air contains vast quantities of micro-organisms. 



The quaint ideas of the past in regard to the enormous distribu- 

 tion of bacteria in the atmosphere were disproved by a more accu- 

 rate examination. It is well known that on admitting a ray of the 

 sun into a dark space, the illumined strata of air teem with small 

 particles of organic or inorganic origin, the so-called "sun-dust." 

 Every one of these particles was supposed to be either a germ or a 

 bearer of one; an obviously erroneous view, as will be seen by a 

 simple consideration, even without direct proofs to the contrary. 



It is not easy for bacteria to rise into the air. It has been 

 seen, during the discussion of the causes of tuberculosis and chol- 

 era, that a voluntary rising of the bacteria is absolutely impos- 

 sible and that they cannot be torn from a medium on which they 

 have once become firmly rooted, even by the strongest draught 

 of air. The medium on which the micro-organisms are found and 

 have been thriving, must dry up complete^ and crumble into fine 

 powder or dust before they become a plaything for air-currents to 

 scatter broadcast. But as the majoritj^ of them cannot endure 

 such a drying without harm, it will be obvious that the quantity of 

 micro-organisms, proved to exist in the air by bacteriological in- 

 vestigation, does not by any means come up to the former exag- 

 gerated opinions. 



It is a matter of course that these conditions cannot be ascer- 

 tained by the microscope alone. The method of cultivation has, 

 therefore, been resorted to and applied in various forms. 



