266 Trans. Acad. Sci. of St. Louis. 



ter, as, for example, in testing the water from wells, or 

 estimating the efficiency of a filter; but when we are dealing 

 with the waters of a river system, problems of the most 

 complex character will often arise. 



It must be borne in mind that in an investigation along bac- 

 teriological lines, we are dealing with living matter. That 

 bacteria are present not only in water, but also in the air 

 and soil, and in fact they are [so -widely distributed that we 

 may consider them present wherever organic matter is found. 

 Certain bacteria are pathogenic or disease producing, but the 

 vast majority are harmless and take an active and most impor- 

 tant part in the life history and development of the universe. 



Without the activity of bacteria, plant life could not well 

 exist, for it has been found that the nitrogen so essential to 

 metabolism is in great part supplied through the agency of 

 certain bacteria which are present in decaying matter. Sew- 

 age contains countless numbers of bacteria, which through 

 their activity and the action of their enzymes, produce grad- 

 ual but definite changes in the organic matter. These 

 changes in the organic matter taking place by definite steps 

 are what is ordinarily called " decay " or putrefaction, and 

 when due to bacteria are designated by the term bacterio- 

 lysis in contradistinction to the term hydrolysis which is used 

 to designate similar changes when due to the action of 

 chemical agents. As a common illustration of such changes, 

 the waste product, urea, may be taken. By the action of 

 certain bacteria (M. ureae) a fermentative process takes 

 place in which urea in the presence of water is converted into 

 ammonium carbonate. The changes may be represented thus : 



NH 3 

 rr / + 2 H 2 = (NH.O2CO3. 



\NH 3 



In sewage and in soil, other bacteria being present, addi- 

 tional changes known as nitrification take place. In the 

 successive changes a process of oxidation occurs in which, in 

 the case of certain substances, ammonium compounds are 

 formed, and these in turn are further converted to nitrites 

 and to nitrates. In the form of nitrates plants are capable 



