The Paris Seivage Irrigation at Gennevilliers. 
115 
In France, all hygienic chemists now employ no other method 
for testing the purity of water, than by ascertaining the quantity 
of oxygen it contains in solution, through a most simple and 
ready process invented by M. Gerardin, one of the hygienic 
inspectors of Paris. 
It is a remarkable fact that sewage, or even ordinary stagnant 
water, after having undergone rapid decomposition when in a 
state of repose, recovers a condition of comparative purity, after 
a while, under the action of air and light. Sewage-water when 
enclosed in a glass jar will soon decompose, and emit bad 
smells, but after some time it will gradually get clear, owing to 
the precipitation of the solid matter which it held in sus- 
pension. One of the first tokens of this restoration of purity is 
the appearance of vegetable life in the shape of algae, such as 
Oscillaria viridis and Palmella. After a while, the noxious 
smell will disappear ; and limpidity and purity are restored. 
M. Gerardin, on being invited by the Sewer Commissioners to 
make microscopic and chemical analyses of the Seine water 
fouled by the influx of the sewage, was led to abandon the long 
and tedious processes of the laboratory, and try if it were not 
possible to test the alteration of water by means of some che- 
mical reagent, such as permanganate of potash : a salt the use 
of which had often been recommended to that effect. 
It was in 1856 that M. Monnier first proposed the use of per- 
manganate of potash to test the presence of organic matter in 
water, relying on the property of that salt to get discoloured by 
substances having a great affinity for oxygen. 
In practising with this reagent, M. Gerardin met with two 
difficulties, which led him to give it up : — First, the solution 
of that salt is of a light pink colour, which it was impossible 
to distinguish in the turgid samples of water that were to be 
analysed. Secondly, permanganate of potash indicates the oxi- 
dising capability of organic matter, rather than the degree of 
its decomposition and its influence on the condition of water. 
However, the numerous experiments made with that reagent, 
led M. Gerardin to this important conclusion : that the discolo- 
ration of the solution of permanganate of potash was owing to its 
oxidising the organic matter in the water. That matter, whether 
in solution or suspension, has therefore more or less affinity for 
oxygen, and if so, it must easily absorb the oxygen dissolved in 
water. When water contains its normal quantity of dissolved 
oxygen, it may be considered pure, and capable of sustaining 
the life of fishes and plants. When the quantity of dissolved 
oxygen diminishes, fishes whose respiration is active can no 
\ longer live in such water, whilst those of a lower order, whose 
respiration is less active, can still exist. This is the reason why 
