116 The Paris Sewage Irrigation at Gennevilliers. 
eels, for instance, will live in water so foul as to kill other fish, 
and leeches will thrive where even eels cannot exist. Among 
fresh-water shells, Unio pictorum will die before Cyclas cornea 
or BytJiinia impura. Pliysa fontinalis and Valvata piscinalis 
require aerated water, whilst Planorbis corneus thrives in impure 
water. 
The same rule obtains with vegetable life. Those alga; pos- 
sessing a superior organisation, that is, being provided with 
chlorophyl, their cells being ramified and well articulated, are only 
found in well-aerated water ; they thrive only in rapid streams, 
close to waterfalls and other running waters, whose surface is 
constantly renewed by a quick motion. Unicellular algae, on 
the contrary, are found in stagnant water, which is partly 
deprived of its oxygen through the presence of organic matter 
in a state of decomposition. 
From those considerations, M. Gerardin sought first to ascer- 
tain whether there existed any dissolved oxygen in waters 
which are notoriously foul, such, for instance, as those which 
issue from factories and sewers ; and the result of his analyses 
was that no oxygen was to be found in them. Thus the conclu- 
sion which that eminent chemist came to is, that the salubrious 
or noxious quality of water is in direct ratio with the quantity 
of dissolved oxygen it contains. 
A comparison between the state of the Seine water above, 
at, and below the influx of the sewage, will best illustrate 
M. Gerardin's theory. 
Above Paris, the average amount of dissolved oxygen comes 
to 9 cubic centimetres per quart ; through Paris, the average is 
7 ; below Paris, and up to Asnieres Bridge, it descends to 5 '34. 
At St. Denis, after the whole of the sewage has been disgorged 
into the river, there is not a trace of oxygen ; but as the stream 
goes down, and the sewage gets rid of its organic matter by pre- 
cipitation and fermentation, oxygen reappears by degrees^ vi/. : — • 
At Epinay, 1-05 ; at Bezons, 1-54 ; at Marly, 1-91 ; Maisons- 
Lafitte, 3-79 ; Poissy, 6-12 ; Mantes, 8-17 ; Rouen, 10-42. 
The quantity of nitrogen follows the same progression, but in 
an inverse ratio. So in those places where most oxygen is 
found, there is the least nitrogen, and vice versa. 
It would be highly interesting, no doubt, to develop here the 
practical purposes to which M. Gerardin has applied his sewage- 
water theory, especially as regards the refuse liquids from 
chemical factories, distilleries, fecula mills, «Scc. ; but it would 
extend this paper to an unreasonable length. Suffice it to say, 
that M. Gerardin has come to the conclusion that water that does 
not contain at least 8 cubic centimetres of dissolved oxygen 
per quart, is not fit for domestic uses. 
