i88i.]_ with reference to Polluted Waters. 143 
or in the dilute sewage of London and Paris. When once 
established, however, it is able to live on in relatively pure 
water, It need scarcely be said that this fungus, containing 
no chlorophyll, gives off no oxygen on exposure to light, and 
consequently contributes nothing to the purification of the 
water which it inhabits. It does, of course, withdraw a 
certain amount of organic matter from the water to form its 
own tissues, but this on its ultimate decomposition is re- 
stored to the stream. I am not aware that it is ever eaten 
by any animal — certainly not by fishes, inserts, crustaceans, 
or mollusca. Infusoria swarm among its fibres, but pro- 
bably as a place of shelter. Nor can I learn that it has 
ever been tried as an article of human diet. It might not 
be impertinent to express the wish that a certain gentleman 
who wrote to the papers proposing rats — foci of trichina ! — - 
as food for the destitute, would kindly make an experiment 
on sewage-fungus in corpore vilissimo. It is certainly nitro- 
genous, possibly nutritious and delicate, and just as possibly 
poisonous. I have frequently found this fungus growing to 
the bodies of dead fishes, floating in streams of doubtful 
purity, and I fear it is capable of attaching itself to these 
creatures whilst still living and bringing about their death. 
We have now to turn to the animal world and ask whether 
the presence or absence of fishes can be said, in general 
terms, to be solely and mainly dependent upon the greater 
or smaller proportion of free oxygen in the water ? That 
such oxygen is an essential of life for them must be known 
even to the most careless proprietor of an aquarium. We 
may therefore say that if oxygen is wanting fishes will be 
wanting also. But can we draw the converse inference, that 
if fish are wanting oxygen must be deficient ? By no means; 
there are various substances which occasionally find their 
way into rivers, and which prove very extensively destruc- 
tive to fish, but which are not likely to effedt any decrease in 
the proportion of dissolved oxygen. We may take an in- 
stance given on very good authority, and briefly noticed in 
the “ Journal of Science” for 1880, page 213. Dr. Auerbach, 
during an entire summer observed certain water-beetles— 
from his description probably the “ whirligig” ( Gyrinus 
natator) — living in tanks full of a saturated solution of 
sodium sulphate (Glauber’s salt). When alarmed these 
little beetles dived down and took refuge among the crystals 
that were forming, just as they would do among the water- 
plants in a pool. But a little of this liquid, thus harmless 
to inserts, happened to escape from the tanks by leakage 
and found its way along a ditch into a river at some distance, 
