286 THE MICROSCOPICAL NEWS. 
constant watchfulness, and bestowing great care upon domestic 
filtration, it is probable that the process will not only entirely fail 
to purify the water, but will actually render it more impure than 
before. For the accumulation of putrescent organic matter upon 
and within the filtering material furnishes a favourable nest for the 
development of minute worms and other disgusting organisms, 
which not unfrequently pervade the filtered water; whilst the 
proportion of organic matter in the effluent water is often con- 
siderably greater than that present before filtration.” 
The carbon blocks in ordinary use should be frequently renewed, 
but as in many forms of filter this is inconvenient and expensive, 
we have pleasure in bringing Maignen’s “Filter Rapide” to the 
notice of our readers. This filter consists of a stoneware case and 
a filtering frame of the same material. The frame is covered with 
a special filtering cloth, and the filtering medium is automatically 
deposited on the cloth by being mixed with the first liquid put into 
the filter. It is this layer of filtering medium which produces 
perfect filtration. Various kinds of filtering medium, such as 
powdered charcoal, or magnesia, or paper pulp, may be used, and 
in some cases the sediment in the liquid to be filtered is sufficient 
to act as a filtering medium. 
These filters are very useful for trying experiments in the labor- 
atory. They are fitted with the canvas cloth, which is suitable for 
filtering most liquids, but woollen and asbestos cloths may also be 
had for strong acids or strong alkalies. 
This filtration of water will not remove the dissolved impurities, 
but all the animal and vegetable debris will be left behind, together 
with such living organisms as may exist. 
In the paper by Dr. Percy Frankland, already quoted, the 
following paragraphs appear to us to require modification, if not 
absolute contradiction ; this must be left till our next number, 
except the last sentence, which we think very generally states the 
case of nearly all water supplies in this country. Dr. Frankland 
writes :— 
“The doctrine known as the self-purification of river water is 
one of the most remarkable of the theories which have been started 
to soothe the conscience of the river-polluter on the one hand, and 
of the purveyor of polluted river-water on the other. 
“As its name implies, this doctrine alleges that noxious organic 
matters discharged into running water are rapidly destroyed in the 
course of a few miles’ flow. A doctrine more utterly dogmatic 
than this it is difficult to conceive, inasmuch as it not only does 
violence to all previous knowledge concerning the properties of 
organic substances in general, but is unsupported by any facts or 
observations. On the contrary, the late Rivers Pollution Com- 
missioners conclusively proved that water once polluted by sewage 
