36 
POPULAR SCIENCE REVIEW. 
most essential, which, not unadvisedly we think, we have re- 
frained from noticing till now. This quality is muddiness, 
opalescence, or any want of perfect clearness. Of course the 
matters suspended may be harmless, but even so their presence 
still remains offensive to the eye. Usually, however, such waters 
contain organic matter, and in most cases the solid particles them- 
selves which give opalescence to the water are of this kind 
and still in possession of vitality, either as fragments of living 
organisms, or as perfect, organised beings themselves. Such 
living particles are too probably full of power for mischief to 
make their ingestion in any case safe. They may be capable of 
development into human parasites ; or they may give rise, it is 
believed, to specific diseases (such as cholera, supposing them to 
be derived from the intestines of cholera patients through the 
admission of sewage into rivers used, like the Thames, to supply 
water for domestic use) ; or, during the putrefaction they have to 
undergo after their vitality has passed away, they may very likely 
give rise to fever and to functional disorders of the nervous and 
alimentary systems. 
Having thus entered at some length into what water is and 
what water is not to be regarded as a pure drinking-water, we 
shall now take under consideration the nature and effects of the 
different forms of domestic filters provided for domestic use. 
The proprietors and patentees of the various kinds of filters have, 
with the exception of Mr. Lipscombe, of Regent Street, sent in, 
on request, samples of the filters they offer to the public, in order 
that they may be reported upon. 
The primary and essential effect of a filter should be to rid 
the water, passed through it of all suspended matters. But 
in modern filters another effect is endeavoured to be produced, 
in order to meet the requirement that water containing objec- 
tionable matters in solution should be freed from them. 
Many persons, when they see a water quite clear, seem to 
imagine that it must be in a good state for drinking. They 
should remember, however, that substances which entirely dissolve 
in water do not diminish its clearness. Hence a clear bright 
water may, despite its brightness, be charged with a poison or 
substances more or less injurious to health, such as soluble 
animal matters. 
One of the most perfect filters is formed of carefully prepared 
unsized paper ; but this is evidently unfitted for general use, 
and is only employed in chemical operations. 
Beds of material in coarse powder, pervious to water, and 
unacted upon by it, such as silicious sand, form the means 
usually resorted to. In other cases porous plates and blocks 
are employed, such as we have presently to notice. 
By forming the bed of sufficient thickness, and using 
