CHEMISTRY AND PHYSICS. BT 
that they need to do their work, notwithstanding that the food is plenty. If the 
change has been carried to the nitrate stage, the alge cannot appropriate its 
results in the absence of light and oxygen. Sometimes there may occur an ap- 
parent retrograde movement. If, in the case of a water rich in nitrates, there be 
an addition of organic matter capable of decomposition, together with the organ- 
isms capable of doing the work, they may take the little oxygen that they need 
from the matter already oxydized, so that an analysis would show a less quan- 
tity of nitrates and an increase in the amounts of nitrites or ammonia. 
A right understanding of the changes through which the nitrogen. goes is 
essential to a correct interpretation of any water analysis. A strong tendency to 
stability in the form of the nitrogen is a favorable indication. A state of change 
is a condition of danger. Large amounts of ammonia or nitrites indicate an in- 
complete purification, and also a recent contamination. High nitrates indicate 
the oxidation of the organic matter, but also that there was a previous con- 
tamination at some time. If large numbers of bacteria are found, this points to 
an abundant food supply and to an active state of change. 
The chief difficulty in drawing conclusions from the presence of nitrogen lies 
in its instability and the rapidity with which the changes occur. There is a sub- 
stance, however, chlorine, that preserves its character through all the changes 
under which water is modified. Almost all waters have some amount of this 
substance present, and this is fairly constant for any given locality. When this 
normal amount is determined by examinations of waters known to be free from 
pollution, the finding of any excess over this normal in any water may be fairly 
assumed as indicating organic pollution; and inasmuch as chlorine always forms 
one of the constituents of sewage, such pollution can be traced to the discharge 
of a town’s sewage into a stream, or the leakage of the contents of a cesspool 
into a private well. 
The interpretation of a biological analysis of a water is a much more difficult 
matter than that of a chemist’s investigation. The science of bacteriology is SO 
new that we do not as yet know just what to look for on the one hand, or, on the 
other, just how to doit. But our knowledge is being added to faster than one 
can easily keep pace with it. So far, the only biological analysis that has come 
into general use, outside of laboratories devoted chiefly to some special scientific 
investigation, is a quantitative one, being simply an estimation of the number of 
the bacteria in the water, made from a count of those present in a small volume 
of it. As has already been said, if large numbers are found, there is indicated a 
strong activity and rapid change. But what we want to know is much more 
than this: just what species are at work, what are their life-histories, how they 
act and react on each other, which ones are detrimental to us and which ones 
are our helpers, and how we can put them to work. 
We have learned how to isolate and study the habits of many of these mi- 
crobes, but not all of them. We know, in a general way, how our friends, the 
saprophytic bacteria, or the scavengers that accomplish the work of putrefaction, 
do their work. The nitrifying organisms, also our friends, we are fairly well ac- 
quainted with. By a combination of chemical and biological methods, the expert 
can often trace a sewage contamination for long distances. Indeed, a microbe 
or microbes that surely indicate such source of impurity can be found long after 
the chemist fails to find any trace of such contamination, because of the degree 
of its dilution in the water. Many of the germs of water-borne diseases, such as 
typhoid, cholera, and malaria, are known, and have been studied. We also know, 
in a general way, that the conditions which are favorable to the growth and de- 
velopment of the minute organisms which are our friends are not favorable to 
