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 berepresented thus: 
i + 2 H20 = (NH4)sCOs. 
\NHs 
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 
