1881.] with reference to Polluted Waters. 141 
But we must now turn to the growth which is considered 
most characteristic of polluted waters. Everyone talks of 
“sewage-fungus,” and yet there are redoubted “sanitary 
reformers” and patentees of processes for the purification of 
sewage who have never seen this dreaded plant, and who 
suppose that it must have something of the appearance of 
a mushroom. This is a complete error; in form and colour 
it is not unlike a bundle of tow, with the fibres running 
parallel and ending in loose tufts. Suppose such a bundle 
fixed at one end to a stick or a stone or to the earth at some 
little depth under water, and swaying to and fro in the 
current, and you have a very fair resemblance of sewage- 
fungus. The chief difference is that this unholy and un- 
lovely plant has a greater specific gravity than hemp, and 
tends to sink rather than rise if not kept in a horizontal 
position by the stream. The colour, too, is modified by the 
particles of dirt which get entangled in the fibres. What 
concerns us are neither the structure, nor the affinities, nor 
the chemical composition of this fungus, but the conditions 
under which it exists and its value as a sign of water-pollu- 
tion. It is, in the first place, as far as I have been able to 
see and to learn, peculiar to running water. I have never 
seen it at the sides of any pond or reservoir whether of pure 
or of polluted water, nor at the bottom of such pools when 
emptied. If planted in an aquarium for experimental pur- 
poses it hangs straight down from the stick or root to which 
it has been found attached, and whether kept in the light or 
the dark it shows no disposition to flourish. Moving water 
is, therefore, essential to its growth. If preserved in the 
dark, however, it undergoes no change for months, and 
seems unaffected by the most powerful chemical agents ex- 
cept in enormous proportions. Chromic acid, for instance, 
is not reduced by it even on prolonged contact. If, however, 
sewage-fungus is placed in still water and exposed to strong 
light, green confervse fix themselves upon it, overspread it, 
and seem gradually to effeCt its destruction. I have observed 
similar cases in shallow trenches in which partially purified 
sewage was flowing, but where water is still strongly charged 
with animal matter the fugus appears to hold its own, 
especially if the depth and turbidity of the stream interfere 
with the free aCtion of light. It need scarcely be said that 
sewage-fungus is never found in pure mountain streams, or 
even in the brooks and ditches of rural districts which re- 
ceive the drainage of cultivated lands, except they are con- 
nected with some sewer. But if portions of the plant are 
swept down out of a sewer — e.g., by a violent storm of rain 
