533 
*^ 5 *] Self-Purification of Waters and Soils. 
V. ON THE SELF-PURIFICATION OF WATERS 
AND SOILS. 
E J\ EMICH has carried out a series of experiments on 
• the behaviour of water when allowed to stand 
. exposed to the air, and also when shaken up with 
air. He has further experimented with sterilised water, and 
on the behaviour of waters submitted to the adtion of ozone 
and hydrogen peroxide. The self-purification of water — in 
other words, the destruction of its organic and inorganic 
impurities — may be the consequence either of a purely 
chemical process (oxidation) or of a biological process. It 
appears that on exposure to, or agitation with, air the self-puri- 
fication of water takes place only if it has not been previously 
sterilised by boiling, and protected afterwards against the 
entrance of germs. But if a sterilised water has been sub- 
sequently exposed to the air, or mixed with ordinary water, 
it undergoes the same changes as waters which have not 
been sterilised ; its oxidisable power and its proportion of 
ammonia decrease, whilst nitrous or nitric acid is formed. 
If, therefore, the development of organisms in the water is 
rendered impossible, self-purification is also impossible, the 
latter process being dependent on the former. 
Dired oxidation by atmospheric oxygen certainly does not 
take place. Ozone and hydrogen peroxide may possibly co- 
operate in the natural purifying process, but they play in 
any case a subordinate part. 
In connection with the conclusions reached in the fore- 
going paragraph, the disappearance of the free dissolved 
oxygen in rivers, where impurities are introduced, must not 
be forgotten. This disappearance is an analytical faCt, and 
if the missing oxygen has not served direCtly for the oxida- 
tion of the impurities, it must have been transferred to them 
by the aCtion of the microbia. 
The kind of living beings which effeCI the purification of 
waters, converting more complicated compounds into simpler 
ones, — in other words, de-organising or mineralising them, 
— will differ greatly according to circumstances. Such a 
change of species has actually been observed in one and the 
same water-couse in the different stages of its pollution. 
It must be remarked that, as far back as 1869, M. Muller 
instituted experiments on the self-purification of contami- 
nated waters, and pronounced it a vital process. He was 
