Oxygen may unite in a small propor tion with some colouring substances, with out weakening their colour, or changing it to another : thus indigo, which becomes green by uniting with an alkali, with lime, or a metallic oxide, resumes its colour, and quits those substances, when it (recovers a small portion of the oxy gen which it had lost. The liquor of the whelk, employed to dye purple, is naturally yellowish ; but when exposed to the air, and more especially in the sunshine, it quickly passes through vari ous shades, and at length assumes that colour so precious in the eyes of the an cients.
It may be considered as a general fact, that colours become brighter by their union with a small portion of oxygen ; for this reason it is found necessary to air stuffs, when they come out of the bath, and sometimes even to take them out of it from time to time, expressly for this purpose ; but in some cases the quantity of oxygen, which, thus becoming fixed, contributes to the brightness of the co lour, is very inconsiderable, and its dete rioration soon commences.
The action of the air affects not only the colouring matter and the stuffs, but also metallic oxide, when they are employed as intermedes, because the oxides are deprived at first by the colouring matter of part of their oxygen, and absorb it afterwards from the air. Those oxides, then, whose colour varies in proportion to their quantity of oxygen, cause changes of colour in the stuff in this manner.
Thus the blue given to wool, by suL phate of copper and logwood, soon changes into a green by the action of the air; because the copper, which is blue, when combined with a small portion of oxygen, becomes green, by its union with a larger quantity.
Colouring matter, in a state of combi nation with most substances, is less liable to be changed by the air than when un combined ; hut there are some excep tions; for alkalies produce a contrary ef fect : they darken the colours to which they are added, and are found by experi ment to promote the absorption of air, and in proportion as this takes place, the colours in general become more and more brown. This is consonant to the effect they produce on other substances, such as sulphur, for they favour the absorp tion of air, because they have a strong at traction for the substance which is the result of that absorption.
Of the differences between Wool, Silk, Cot ton, and Linen, and the operations by .which they are prepared for dyeing.
Wool and silk are animal substances, cotton and linen are vegetable. produc tions. Animal substances have a greater disposition to combine with other sub: stances than those of vegetable origin ; hence they are more liable to be destroy ed by different agents, and are more dis posed to combine with colouring parti cles. l3erthollet accounts for these pro perties by their principles being more disposed to assume a gaseous form, and having less cohesive fOrce among them selves. Thus the pure or caustic alkalies destroy animal substances, because they combine with them, and thereby lose their causticity. For this cause animal substances cannot bear legs, and alkalies should be used with great caution in the processes for dyeing them ; whereas no dangeris to be apprehended from the use of alkalies with vegetable sub stances. Nitric and sulphuric acids have also considerable action on animal sub stances.
Silk appears to bear some resemblance to vegetable substances, by being less disposed to combine with colouring par ticles, and by resisting the action of alka lies and acids more powerfully ; but though the action of these substances on silk is weaker than upon wool, they should still be employed with great caution, because the brightness of co smoothness of its surface, which should therefore be preserved unimpaired.
Cotton withstands the action of acids better than flax or hemp, and is dif ficultly destroyed even by the nitric acid.
Of Wool.
Wool is naturally covered with a kind of grease called suint, which preserves it from moths, so that it is not scoured until it is about to be dyed or spun. In order to scour wool, it is put for about a quar ter of an hour into a kettle containing a sufficient quantity of water, mixed with a fourth part of putrid urine, heated to such a degree as the hand can just bear, and it is stirred from time to time with sticks; it is then taken out and put to drain. It is next carried in a large basket to a stream of running water, where it is mov ed about until the grease is entirely sepa rated, and no longer renders the water turbid ; it is then taken out, and again left to drain. It sometimes loses in this operation more than a fifth of its weight.