Colours have been divided into substantive and adjective. Substantive colours are such as do not require the intervention of any other substance to fix them
permanently, their attraction for the cloth being sufficient. Adjective colours require the intervention of some substance which has an attraction both for
the colouring matter and stuff to be dyed. This intervening substance is called a mordant. It consists of a metallic salt, with which the substance to be
dyed is first impregnated, and afterwards passed through s solution of the colouring matter.
The mordants most commonly employed are sulphate of iron, muriate of tin, sulphate of aluminc and acetate of iron.
545. Different mordants are used for different colours, as well as for different kinds of cloih.
lllus. Blaik is produced by astringents and salts of iron. The stuff is first soaked in a strong solution of galls, and afterwards passed through a solution
of sulphate of iron in an infusion of logwood.
2. Reds are chiefly produced from madder, and the colouring matter is fixed by a mordant of muriate of tin, &c.
546. Fixed Oils. Vegetable oils are of two kinds, fixed and volatile.
547' Fixed oil is found in the seeds of plants only, and is almost entirely confined to those which have two colytedons, as linseed, almonds, walnuts,
olives, &c.
548. These oils are obtained by pressure. They are viscid, nearly insipid, and generally congeal at a higher temperature than that required to' freeze water.
Obs. The fixed oils, with a few exceptions, undergo little other change by exposure to air, than that of becoming somewhat more viscid, aDd acquiring'a
degree of rancidity. This change is owing to absorption of oxygen, for rancid oils redden vegetable blues, and therefore contain a quantity of free acid.
Some few of the fixed oils, on exposure to air, become covered with a thin pellicle, occasioned partly by the evaporation of their volatile particles and
partly by the absorption of oxygen. These are called drying oils, and are used in painting. Of these oils, the most common is linseed oil, rape oil, and hemp
seed oil.
Thealkplies combine with the fixed oils, and form soap.
The fixed oils are composed of carbon, hydrogen and oxygen.
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