THE CIRCULATION OF NITROGEN IN NATURE. 71 
microbes shall be so developed as to deal with the sewage 
in a much more rapid and complete manner than is possible 
under natural conditions. It is not part of my scheme to 
pursue this subject further, but it is interesting to note that 
one of the latest developments is a return to the oldest form 
of sewage receptacle, the cesspit, in which a very remark- 
able amount of purification is effected by micro-organisms. 
I think perhaps too little attention has been paid by 
sanitary engineers to the total difference in conditions 
prevailing in the first and second stages of purification. In 
the first stage it is required to break down organic matter 
by means of micro-organisms which revel in it. The purifica- 
tion is carried to completion by the nitrifying organisms, 
which require a minimum of organic matter and an abund- 
ance of air. 
The second point I wish to draw attention to is the effect 
of this purified sewage upon health. This question is 
constantly brought before us in connection with drinking- 
water in this way : A water yields to chemical analysis no 
evidence of the presence of organic matter or even of 
ammonia, but contains nitrates and the mineral con- 
stituents of sewage ; it is evidently mixed with sewage 
that has undergone the changes of which I have been 
speaking during its passage through two or three yards of 
subsoil. Is it to be regarded as wholesome water? 
In the early days, when the dangerous properties of 
sewage-polluted water were attributed to some indefinite 
organic matter, it was argued that when nitrification was 
complete all dangerous matter must be burnt up, and the 
water devoid of its original injurious pollution. 
More cautious observers at that time, whilst admitting 
that a completely nitrified sewage might be innocent, 
pointed out that a slight change in meteorological conditions 
