I have already told you that acids are characterised by what we
term an acid reaction upon certain vegetable and artificial colours,
whilst bases or basic substances in solution, especially alkalis,
restore those colours, or turn them to quite another shade; the acids do
the one thing, and the alkalis and soluble bases do the opposite. The
strongest and most soluble bases are the alkalis--soda, potash, and
ammonia. You all know, probably, that a drop of vitriol allowed to fall
on a black felt hat will stain that hat red if the hat has been dyed
with logwood black; and if you want to restore the black, you can do
this by touching the stain with a drop of strong ammonia. But the use
of a black felt hat as a means of detecting acidity or alkalinity would
not commend itself to an economic mind, and we find a very excellent
reagent for the purpose in extract of litmus or litmus tincture, as well
as in blotting paper stained therewith. The litmus is turned bright red
by acids and blue by alkalis. If the acid is exactly neutralised by,
that is combined with, the alkaline base to form fully neutralised
salts, the litmus paper takes a purple tint.
Pages:
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120