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EXPERIMENT STATION BULLETINS 223 



This subject presents treatment under three distinct heads, although a 

 few media of distribution may scarcely be classified, and will accordingly 

 be considered separately. These three heads are: 



1. Air. 



2. Water. 



3. Soil. 



Air. The air can hardly be regarded as a natural home for bacteria, 



notwithstanding the fact that many are found there. Most of 

 the species found in the atmosphere are non-infectious; very few path- 

 ogenic bacteria have had their presence demonstrated by direct examina- 

 tion. Organic matter and moisture are the measure of bacterial life in the 

 air about us. The dust which rises from the streets, laden with pulverized 

 sputa of people with infectious diseases, and filled with the otfal of horses, 

 disseminates through the air a multitude of bacteria. The material con- 

 veyed directly to the air from the lungs of the inhabitants and the 

 sweepings from houses and shops provide a fertile soil for bacteria. The 

 ware-room where rags are handled and skins are carted about, numbers its 

 victims every year, the one dying of cholera, the other of anthrax. The 

 city has the largest number of bacteria, mid-ocean the smallest, Where 

 life is so active and every foot covered by habitation, it is a natural result 

 that the air of the city should furnish its equally dense proportion of life. 

 Mid-ocean on the contrary has little opportunity of feeding micro-organisms 

 although moisture may be in abundance. The air is consequently practi- 

 cally free from bacteria. The extremes have been drawn. The country 

 with its scattered dwellings, the plain with its single house, the mountain 

 with its bare rock, — all of which appear mid-way between the city and 

 mid- ocean, — are not able to muster a regiment of bacteria to antagonize man. 

 There are the winds and currents of the air to carry these fine particles of 

 life over miles of country. Cinders from a volcano have been estimated 

 to pass through hundreds of miles of air before they fell, and cinders are 

 hardly comparable with bacteria. It is impossible to state the influence of 

 currents of air upon bacteria, yet it must be great. Influenced greatly as 

 they may be by currents of air and wind, bacteria always gravitate towards 



the ground and will eventually reach the soil. 

 Water. Water may contain very many or very few bacteria; it rests 



wholly upon the kind and supply of food. Elvers flowing down 

 the sides of mountains covered with dense wildernesses, lakes whose banks 

 are free from inhabitants, and seas which measure thousands of miles in 

 extent can boast of a scarcity of bacteria. There is but little decaying 

 matter to be transported by these waters, and little opportunity for gather- 

 ing sewage of habitable districts to supply the little nourishment required 

 for the growth of bacteria. From the other side, view the conditions ex- 

 isting along streams in our thickly settled states where town after town 

 pours its sewage into them and where there are only two or three miles 

 intervening between the towns for purification; also regard the borders of 

 our lakes, where large cities have their intake pipe for their water supply 

 only a few yards away from the outlet of the sewerage system. The water 

 about the shores of lakes is usually very quiet and there is little chance 

 for bacterial life to vanish. Here is the best food imaginable for growing 

 bacteria. The waters of such places are pregnant witti these forms, some 

 of which are detrimental and some hM,rmless. Bacteria are associated with 

 habitation; they are essential for the reduction of the organic wastes given 



