144 
On Living Organisms . 
[March, 
where it proved fatal to a multitude of fish. It can certainly 
not be contended that a small quantity of such a solution, 
added to the water of a river, would either seize hold of and 
expel the free oxygen ; yet we see that it turned the scale 
between life and death. 
Cream of lime thrown into a stream will, as is known, 
seize upon carbonic acid, leaving oxygen unaffected. Yet 
this addition is well known as fatal to fish. 
The presence of fish is certainly no proof of the absence of 
foecal matter. At Kingston, just where the sewage of the 
town entered the Thames in 1875 — it may possibly since 
have been diverted — I have seen fish darting about in 
numbers, and I learn that the sewer-mouth was a favourite 
spot for anglers. 
Among amphibious animals frogs are found in pure waters, 
or in those but slightty polluted, and the same may be said 
of newts. If either excrementitious or manufacturing refuse 
is introduced in appreciable quantities they withdraw. 
The presence of aquatic inseCts is not a character from 
which any definite conclusion can be drawn. When manu- 
facturing refuse has been largely introduced they are gene- 
rally absent. Thus, I have never seen either larvae or adult 
inseCts in the Bridgewater Canal or the Irwell below Man- 
chester, in the notorious Sankey Brook, in the Ayre below 
Leeds, the Kelvin Water, and similarly polluted streams. 
But in rivers largely fouled with putrescent vegetable refuse 
or foecal matter they abound. On the other hand, in the 
very purest water they are necessarily absent as finding 
there no food. It would be interesting to find what is the 
minimum of impurity at which the larvae of gnats and blood- 
worms (Cheironomus plumosus) are able to exist, and what is, 
if any, their maximum limit. 
They are frequently found in water-butts and cisterns 
which have no other source of contamination but the organic 
matter suspended in the air. Still I should suggest that no 
water in which these larvae are present should ever be used 
for domestic purposes, since their excretions, as far as I have 
been able to observe, set up in organic matter decomposi- 
tion of an offensive and probably dangerous type. The 
character of their juices may possibly explain the irritating, 
and in some cases even dangerous, effects of the bites of 
gnats, mosquitoes, and pollution-fed Diptera in general. 
Water-beetles, such as Acilius sulcatus , Colymbetes sp., &c., 
and also certain Hemiptera, may be found in water which, 
from the absence of known sources of pollution and from 
chemical and microscopical examination, may be pronounced 
