142 On Living Organisms [March, 
— they are able to live in running water where the amount 
of pollution is exceedingly small — much smaller than the 
proportion specified as to be tolerated in the “recommenda- 
tions” of the Rivers’ Pollution Commission. If it be asked 
how this is ascertained, I reply that I know a small stream 
which down to a certain point is perfectly free from sewage- 
fungus, as I have satisfied myself by frequent and careful 
inspection. At that point it receives a stream of sewage, 
about one-sixth of its own volume, and purified to such an 
extent that one of the highest authorities on water-analysis, 
after repeatedly examining samples taken at haphazard, has 
pronounced it to fall well within the limits of the recom- 
mendations just referred to. Yet the stream, after receiving 
this infinitesimal proportion of fecal matter displays here 
and there a tuft of sewage-fungus, along with a most lux- 
uriant growth of green water-plants. It must further be 
noticed that on pursuing the stream for a few hundred yards 
the fungus disappears, its pabulum having possibly been 
destroyed by the oxygen evolved by the higher vegetation. 
But whilst water may thus be too pure to supply the trace 
of nitrogenous nourishment required by sewage-fungus, it 
may be found, if not too impure, at least not to possess the 
right kind of impurity. I have made frequent and minute 
examinations of the sewage of Leeds, and of the results of 
the various processes which have been adopted for its purifi- 
cation, but I have never seen a trace of sewage-fungus either 
brought down the sewers from the town, or floating in the 
tanks, or fixed in the cut-flow channel, or where the latter 
opens into the river Aire. This absence was equally distinct 
whatever was the nature and the success of the process 
adopted. 
Again, the London sewage at the southern outfall has 
never, in as far as I have had the opportunity of observing, 
contained a particle of sewage-fungus. 
The same may be said of the sewage of Paris, which I 
have had prolonged opportunities of examining at Genne- 
villiers. On the other hand, a ditch at Wimbledon which 
received the sewage of the district, some of it treated and 
some in its original state, contained in the autumn of 1875 
some beautifully characteristic specimens. At Aylesbury, 
after heavy rain, it has been swept down from the sewers of 
the town in great quantities, and at Hertford it has also 
been observed in great perfection. 
Hence we may probably conclude that it grows by prefer- 
ence in the rich sewage of small residential towns rather 
than in the waste waters of the great manufacturing centres, 
