For example, if we
place on a paper filter some bone black, and filter through it some
indigo solution, after first warming the latter with some more of the
bone black, the liquid comes through clear, all the indigo being
absorbed in some peculiar way, difficult to explain, by the bone black,
and remaining on the filter. This power of charcoal also extends to
gases, and to certain noxious dissolved organic impurities, but it is
never safe to rely too much on such filters, since the charcoal can at
length become charged with impurities, and gradually cease to act. These
filters need cleaning and renewing from time to time.
LECTURE V
ACIDS AND ALKALIS
_Properties of Acids and Alkalis._--The name acids is given to a class
of substances, mostly soluble in water, having an acid or sour taste,
and capable of turning blue litmus solution red. All acids contain one
or more atoms of hydrogen capable of being replaced by metals, and when
such hydrogen atoms are completely replaced by metals, there result
so-called neutral or normal salts, that is, neutral substances having no
action on litmus solution.
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