The attraction of alumen for animal substances may be shewn by direct ex periment ; for if a solution of alum is mix ed with a solution of glue, on adding an alkali, the glue is precipitated in combi nation with the alumen.
The attraction ofalumen for most co louring substances may also be proved by experiment. If a solution of a co louring substance be mixed with a solu tion of alum, and an alkali be added, which decomposes the alum, the colour ing matter will be precipitated combined with the alum, and the liquor will re main clear. The matter. precipitated is called a lake. In this experiment too much alkali must not be added, because, alkalies are capable of dissolving most lakes.
No direct experiment has yet shewn that alumen attracts any vegetable sub stances, except colouring matters : its at traction to them seems much weaker than that which it has for animal substances; hence the acetite of alumen is a better mordant than alum for linen and cotton, as has been observed ; and upon this de pend the different means employed to increase the fixidity of the colouring par ticles in dyeing these substances.
Lime is the only earth, besides alum, which is employed in dyeing: the affinity of lime for cloth is sufficiently strong ; it is, hOwever, found to answer the purpose of a mordant less perfectly than alumen, on account of the colour, which is not so good. It is employed either in the state of lime-water, or of that of sulphate of lime dissolved in water.
Metallic oxides have so great an at traction for many colouring substances, that they quit the acids in which they were dissolved, and arc precipitated in combination with them they have also the property of uniting with animal sub stances; it is therefore natural, that they should serve as a bond of union between the colouring particles and animal sub stances ; but, besides the attraction of the oxides for colouring substances, and for animal matter, their solutions in acids possess qualities which render them more or less fit to act as mordants : thus, those oxides which easily part with their acids, such as that of tin, are capable of com bining with animal substances, without the aid of colouring particles : it is sitffi, cient to impregnate wool or silk with a solution of tin, although they be after. wards carefully washed, which is not the case with other metallic solutions.
Some metallic substances afford in combination only a white and colourless basis ; and some, by the admixture of their own colour, modify that which is proper to the colouring particles ; but in many metallic oxides, the colour varies according to the proportion of the oxy gen they contain, and the quantity of this is easily liable to change. Upon these circumstances their properties in dyeing principally depend.
The attraction of metallic oxides for substances of vegetable origin is much weaker than for animal substances, and we are even ignorant whether they are capable of contracting a real union with them or not: metallic solutions are there fore ill-fitted to serve as mordants for co. lours in linen or cotton, except iron, the oxide of which unites firmly with vege table substances, as is shewn by irort moulds, which are owing to a real com bination of this oxide. When the co. louring particles have precipitated a me tallic oxide from its menstruum, the su pernatent liquor contains the disengaged acid, which is commonly capable of dis solving a portion of the compound of colouring substance and oxide, so that the liquor remains coloured : but some times the whole'of the colouring matter is precipitated, when the proportions have been accurately adjusted : this pre cipitation is facilitated, and rendered more complete, by the presence of the stuff, which assists, by the tendency it has to unite with the compound of oxide and colouring matter.
Uncombined metallic oxides have also a very evident action on many colouring substances when boiled with them, and modify their colour ; the oxide of tin, in particular, increases the brightness and fixidity of several.
The compounds of oxides and colour mg substances may be compared to many other chemical compounds which are in soluble, when the principles of which they are formed are properly proportion ed, but which are capable of being su per-saturated by an excess of one of the principles, and thence of becoming solu ble. Thus, a metallic oxide, united with a colouring substance in excess, will pro duce a liquor, the colour of whioh will be modified by the oxide ; whereas, when the colouring matter is not in ex cess, the compound will be insoluble, or nearly so : these effects are very evident in the combination of iron with the as tringent principle.