The name or word "mordant" indicates the empiricism, or our old friend
"the rule of thumb," of the age in which it was first created and used.
It serves as a landmark of that age, which, by the way, needed
landmarks, for it was an age of something between scientific twilight
and absolute darkness. _Morder_ in French, derived from the Latin
_mordere_, means "to bite," and formerly the users of mordants in dyeing
and printing believed their action to be merely a mechanical action,
that is, that they exerted a biting or corroding influence, serving to
open the pores of the fabrics, and thus to give more ready ingress to
the colour or dye.

Most mordants are salts, or bodies resembling salts, and hence we must
commence our study of mordants by a consideration of the nature of
salts. 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. Coloured reagents such as