388 Observations on Polluted Waters. [July, 
speed with which the impurities in a water are destroyed is 
greater where it is spread out in shallow layers to the aClion 
of air and light, and where its transparency is not interfered 
with by suspended matter ; that the self-purification of 
sewage is of no practical value, since during the greater 
part of the process it is exceedingly offensive, and since to 
let it stand in shallow settling tanks would be ruinously 
costly. 
The speed of purification would doubtless be increased no 
little if it were in motion instead of at rest. Further, not 
even the most polluted river contains the same proportion of 
impurities as the undiluted and untreated sewage used in 
some of the above-mentioned experiments. It will likewise 
be admitted that in a river, well stocked with water-plants, 
the oxidation of the impurities will be more rapid than 
where such vegetation has first to be developed. On the 
other hand, there are in practice circumstances met with 
which must interfere with the combustion of the organic 
matter held in solution. In the sewage of manufacturing 
towns and mining districts there may be poisons which pre- 
vent vegetation of any kind, chlorophyllaceous or otherwise. 
.The great opacity of certain sewages will also retard purifi- 
cation by excluding the light. 
Bearing all the above faCts in mind, we shall find that the 
conflict between the authorities above mentioned is probably 
more apparent than real. There are many rivers in the manu- 
facturing districts of England polluted to such a pitch that 
vegetation in them is impossible, and the great self-purifying 
agency is thus suspended. I can very well imagine that an 
analysis of the water of the Irwell, at its junction with the 
Mersey, will show the same impurities, and substantially in 
the same proportion, as are found in the river where it leaves 
Manchester. I should expeCt that the waters of the Aire or 
of the Calder will, in like manner, show no improvement in 
10 or 15 miles from their most polluted point. I should ex- 
peCt this to be the case because the flora and the fauna of 
the water remain the same. But with the Vesle, at Rheims, 
the case is plainly different. We read of green aquatic 
vegetation reappearing below the town, and becoming more 
vigorous within the space of a few miles. Hence the means 
for the purification of the water being present, we need feel 
no surprise that the effeCl is produced. If Prof. Frankland 
had made his observations on the Vesle he would have come 
to the same conclusions as his French confrere ; and if the 
latter had been sent to study the Mersey, the Irwell, or the 
Aire, he would in turn have given it as his opinion that 
