_Filtration._--We hear a great deal about filtration or filters as
universal means of purifying water. Filtration, we must remember, will,
as a rule, only remove solid or suspended impurities in water. For
example, if we take some ivory black or bone black, and mix it with
water and afterwards filter the black liquid through blotting-paper, the
bone black remains on the paper, and clear, pure water comes through.
Filtering is effective here. If we take some indigo solution, however,
and pour it on to the filter, the liquid runs through as blue as it was
when poured upon the filter. Filtering is ineffective here, and is so
generally with liquids containing matters dissolved in them. But I said
"generally," and so the question is suggested--Will filtration of any
kind remove matters in solution? This question I will, in conclusion,
try to answer. Bone charcoal, or bone black, has a wonderful attraction
for many organic matters such as colours, dyes, and coloured impurities
like those in peat water, raw sugar solutions, etc.
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