8. Iron, and some earthy substances.
1. The matter which prevents the moistening of cotton wool may be separated by means of alcohol, which, when evaporated, leaves thin yellowish scales, soluble in alkalies, in acids, and even in a large quantity of boiling water. For a long time the bleaching process commenced with the removal of this resinous stuff, by passing the cloth or the yarn through an alkaline ley. This was called scouring; it is now nearly laid aside.
2. The colouring matter of cotton seems to be superficial, and to have no influence on the strength of the fibres; for the yarn is found to be as strong after it has been stripped by caustic soda of its resinous and colouring matters, as it was before. The colouring matter is slightly soluble in water, and perfectly in alkaline leys. When gray calico is boiled in lime water, it comes out with a tint darker than it had before; whence it might be supposed that the colouring matter was not dissolved out, even in part. This, however, is not the case; for if we filter the liquor, and neutralise it with an acid, we shall perceive light flocks, formed of the resinous substance, united with the colouring matter. The dark colour of the cloth is to be ascribed solely to the property which lime possesses of browning certain vegetable colours. This action is here exercised upon the remaining colour of the cloth.
It may be laid down as a principle, that the colouring matter is not directly soluble by the alkalies; but that it becomes so only after having been for some time exposed to the joint action of air and light, or after having been in contact with chlorine. What change does it thereby experience, which gives it this solubility? Experiments made upon pieces of cloth placed in humid oxygen, in dry oxygen, in moist chlorine, and in dry chlorine, tend to show that hydrogen is abstracted by the atmosphere; for in these experiments proofs of dis-hydrogenation appeared, and of the production of carbonic acid. In all cases of bleaching by chlorine, this principle combines immediately with the hydrogen of the colouring matter, and forms muriatic acid, while the carbon is eliminated.
Undoubtedly water has an influence upon this phenomenon, since the bleaching process is quicker with the humid chlorine than with the dry; but this liquid seems to act here only mechanically, in condensing the particles of the gas into a solution. We should also take into account the great affinity of muriatic acid for water.
3. The weaver’s dressing is composed of farinaceous matters, which are usually allowed to sour before they are employed. It may contain glue, starch, gluten; which last is very soluble in lime-water.
4. When the dressing gets dry, the hand-weaver occasionally renders his warp-threads more pliant by rubbing some cheap kind of grease upon them. Hence it happens, that the cloth which has not been completely freed from this fatty matter will not readily imbibe water in the different bleaching operations; and hence, in the subsequent dyeing or dunging, these greasy spots, under peculiar circumstances, somewhat like lithographic stones, strongly attract the aluminous and iron mordants, as well as the dye stuffs, and occasion stains which it is almost impossible to discharge. The acids act differently upon the fatty matters, and thence remarkable anomalies in bleaching take place. When oil is treated with the acetic or muriatic acid, or with aqueous chlorine, it evolves no gas, as it does with the sulphuric and nitric acids, but it combines with these substances so as to form a compound which cannot be dissolved by a strong boiling ley of caustic soda. Carbonic acid acts in the same way with oil. On the other hand, when the oils and fats are sufficiently exposed to the air, they seize a portion of its oxygen, and become thereby capable of saponification, that is, very soluble in the alkalies.
5. When the hand-weaver’s grease continues in contact for a night with the copper dents of his reed, a kind of cupreous soap is formed, which is sometimes very difficult to remove from the web. Lime-water does not dissolve it; but dilute sulphuric acid carries off the metallic oxide, and liberates the margaric acid, in a state ready to be acted on by alkalis.
6. When cloth is boiled with milk of lime, the grease which is uncombined unites with that alkaline earth; and forms a calcareous soap, pretty soluble in a great excess of lime-water, and still more so in caustic soda. But all fats and oils, as well as the soaps of copper and lime, cease to be soluble in alkaline leys, when they have remained a considerable time upon the goods, and have been in contact with acetic, carbonic, muriatic acids, or chlorine. These results have been verified by experiment.
7. Cotton goods are sometimes much soiled, from being sewed or tamboured with dirty hands; but they may be easily cleansed from this filth by hot water.