Stuffs chemically considered being composed of three or four elements,
already in a state of reciprocal saturation, have but a feeble
attraction for simple substances. We know in fact, that the latter
combine only with each other, or with binary compounds, and that in the
greater number of cases where they exert an action upon more complete
compounds, it is by disturbing the arrangement of their elements, and
not by a resulting affinity with the whole together.
3, 4. Although stuffs may in a general point of view be considered as
neutral in relation to colouring reagents, yet experience shows that
they are more disposed to combine with acid than with alkaline
compounds; and that consequently their nature seems to be more alkaline
than acid. By steeping dry wool or other stuff in a clean state in an
alkaline or acid solution of known strength, and by testing the liquor
after the stuff is taken out, we shall ascertain whether there be any
real affinity between them, by the solution being rendered more dilute
in consequence of the abstraction of alkaline or acid particles from it.
Wool and silk thus immersed, abstract a portion of both sulphuric and
muriatic acids; but cotton and flax imbibe the water, with the rejection
of a portion of the acid. The acid may be again taken from the stuffs by
washing them with a sufficient quantity of water.
5. The affinity between saline bodies and stuffs may be ascertained in
the same way as that of acids, by plunging the dry stuffs into solutions
of the salts, and determining the density of the solution before the
immersion, and after withdrawing the stuffs. Wool abstracts alum from
its solution, but it gives it all out again to boiling water. The
sulphates of protoxide of iron, of copper and zinc resemble alum in this
respect. When silk is steeped for some time in solution of protosulphate
of iron, it abstracts the oxide, gets thereby dyed, and leaves the
solution acidulous. Wool put in contact with cream of tartar decomposes
a portion of it; it absorbs the acid into its pores, and leaves a
neutral salt in the liquor. The study of the action of salts upon stuffs
is at the present day the foundation of the theory of dyeing; and some
of them are employed immediately as dye-drugs.