jyo 
Nature's Scavengers. 
[April, 
above mentioned exercise a very beneficial action upon its 
condition, and if they do not succeed in bringing it to a state 
suitable for domestic use, they, in millions of cases, prevent 
it from tainting the air. But it is said, and not without a 
foundation of truth, that if the pollution becomes excessive 
Nature’s scavengers either beat an ignominious retreat or die 
at their posts. We must admit that there are in England, 
and probably in other populous countries, waters so contami- 
nated as to be habitable by nothing higher than Fungi and 
Infusoria. A certain French physicist has even proposed 
the presence of certain species of plants and animals, as 
furnishing a scale by which the sanitary condition of a river 
might be approximately estimated. It may therefore be in- 
teresting to give a case in point, which was observed and 
carefully studied by the present writer in the spring and 
summer of 1868. With a view of throwing light upon a 
scheme in contemplation for purifying the refuse waters of a 
large dye-works, we visited a branch of the Calder and 
Hebble Navigation, which extends from the bottom of the 
town of Halifax down to the village of Salter Hebble, about 
ij miles off, where it joins the main trunk of the Calder. 
The descent is so steep that the canal is merely a series of 
pools separated by locks, and at its origin in the town it is 
fed not with water, but with sewage pumped up from the 
shallow River Hebble, which receives the domestic and 
manufacturing refuse of the town.* Having inspected the 
basin, where the water was very foul and turbid, and emitted 
a disgusting odour, we walked along the towing-path, care- 
fully noting the phenomena presented. For the first two or 
three locks there was no material change ; no animal life 
could be detected in the water, and no plants were seen ex- 
cept sewage fungus, which had here and there attached 
itself to the stonework. At last, when about half the 
descent had been accomplished, water-weeds began to make 
their appearance — at first few, and by no means flourishing. 
They rapidly became more numerous, both in individuals 
and in species, and more luxuriant. The water exhibited a 
corresponding improvement in colour and odour, and when 
we had arrived at the last pool — above Salter Hebble — we 
recognised a variety of water-inseCts. The common whirli- 
gig beetle (Gyrinus natator) was spinning round amidst the 
duckweed; an Acilius and several Colymbetes plunged into 
deep water as we approached, and the water-scorpion was 
disporting himself on the surface. None of these inseCts 
515 We understand that these arrangements have been since altered. 
