86 THE MANURE HEAP AND SEWAGE. 
dant at the end than at the beginning of the treatment. But for 
reasons as yet little understood, the reverse is the case. The 
number of bacteria in the treated sewage appears to be always less 
than in the raw sewage. The amount of reduction in bacteria is by 
no means constant. Sometimes it is comparatively small. In a 
series of tests upon the sewage of London, treated in this way, a 
reduction of only about 32 per cent, was found (7,000,000 to 5,000,- 
ooo). In other cases the reduction is greater, and sometimes there 
is found a number as high as 9,000,000 per c.c. in the raw sewage, 
and only from 5,000 to 10,000 in the treated product. Something 
evidently is at work destroying the bacteria, but its efficiency varies 
widely in different instances. 
Whether these methods of treating sewage destroy its dangerous 
nature as well as its offensiveness is not easy to answer. The 
danger in sewage comes primarily from the disease bacteria it may 
contain, foremost among which is the typhoid bacillus. The 
bacterial treatment greatly reduces the number of bacteria, but does 
not by any means eliminate them. Does it eliminate the disease 
germs? So far as evidence goes to-day it seems that the typhoid 
bacillus is eliminated by the treatment, and the effluent from such 
beds fails to show typhoid bacteria, even when they have been pur- 
posely put in the sewage. Bacteriologists who have had confidence 
in the efficacy of the purification have not hesitated to drink 
freely of the water from such a sewage filter bed. It is certain, 
therefore, that the treatment greatly improves the healthfulness 
of the sewage. But that it removes all danger from it cannot be 
positively stated. 
Such a disposal of sewage means, of course, a complete loss of the 
nitrogenous material, for no method is adopted for utilizing the 
wasted nitrates. But this fact is no longer regarded so seriously as 
it was a few years ago. We have learned that there are efficient 
forces in nature for bringing back from the atmosphere the nitrogen 
dissipated from the soil, and it is a matter of less significance to throw 
away the sewage nitrogen than it appeared to be when the only 
known source of nitrogen was supposed to be the fixed nitrogen of 
the soil. Since the soil can readily replace its lost nitrogen through 
