To indigo carmine solution add sodium bisulphite solution sufficient to discharge the blue color. To one portion of the product add dilute sulphuric acid, to a second ammonium hydroxide.

Sulphurous acid, H2SO3, is known only in solution. Its anhydride is the gas sulphur dioxide, SO2. Being dibasic, sulphurous acid forms acid salts, e.g. sodium bisulphite, NaHSO3, as well as the normal salts, such as sodium sulphite, Na2SO3. The anhydride, the acid, and the acid sodium salt are all used as bleaching agents in the textile industries. The gaseous anhydride, sulphur dioxide, finds the widest The usual method of bleaching wool and silk is to burn sulphur in iron or brick pans in a chamber in which the goods are suspended. The process is known technically as "stoving." As a little sulphuric acid is formed in the burning of sulphur, thorough washing should follow stoving.

use.

For bleaching on a small scale sodium bisulphite is convenient. The goods may either be steeped for some hours in a fairly strong (about 15 per cent) solution of the bisulphite, then passed through very dilute hydrochloric acid, and finally washed with water; or a weak solution of the bisulphite may be acidified with hydrochloric acid and the goods soaked several hours in the mixture, which, of course, contains free sulphurous acid.

Wool bleached with sulphurous acid or bisulphite is readily affected by alkalies, the natural yellow color returning on washing with soap or soda.

A more permanent bleach is obtained by the use of hydrogen peroxide. Black or brown wools and hair cannot be bleached white, but assume a golden color when treated with peroxide.

Blueing

In addition to bleaching, i.e. chemical alteration of the coloring matters, there is another device used, both industrially and in the household, to give yellowish goods a pure

white appearance. Blue and yellow, being complementary colors, neutralize each other in their optical effect. Yellowish goods can therefore be made white by treatment with a suitable quantity of blue coloring matter. The materials used for this purpose are:

1. Ultramarine, a complex compound of the elements sodium, aluminium, silicon, sulphur, and oxygen. Originally found as a rather rare mineral, lapis lazuli, ultramarine is now made synthetically from sodium sulphate, clay, and sulphur by heating with a carbonaceous reducing agent, such as charcoal or tar. Ultramarine is insoluble in water, but in a finely divided condition remains suspended long enough to be evenly distributed over the goods. It is not affected by air, light, or alkalies, but is decomposed by dilute acids.

2. Indigo, a dye formerly, and to some extent still, obtained by the fermentation of the juices of certain species of plants known as Indigofera, but now manufactured in a purer form from coal-tar products. Indigo is insoluble in water, but can be suspended in it in the same way as ultramarine. By treatment with fuming sulphuric acid it can be converted into indigo carmine (sulphindigotic acid), a soluble product retaining the blue color. It is not decolorized by acids, soap, or soda, and is fast to light. Suspended indigo can be decolorized by treatment with sodium bisulphite and zinc dust, indigo carmine by sodium bisulphite alone.

3. Soluble Coal-tar Products. Blues of this class can often be recognized by making one portion of the solution acid and another alkaline and comparing colors. The two will usually differ in shade. The best coal-tar blues are not actual dyes, which would permanently color the textile, but rather substances which will readily wash out, e.g. the alkali blues and indigo carmine.

All the above are good stable blues. The soluble blues

including indigo carmine - have the advantage over the insoluble that they give a more even coloring to the fabric. Some of them, however, are treated with oxalic acid in the laundry to set them in the goods, and the acid, drying in the fabric, corrodes the textile fibers.

A cheap but very objectionable laundry blue is Prussian, or Berlin, blue. This is a compound of iron, carbon, and nitrogen, the chemical name for which is ferric ferrocyanide. It is decomposed by alkalies with production of ferric hydroxide, Fe(OH)3. Goods treated with it are apt to show rust stains, particularly if they contain any soap or soda when blued.

Experiment 165.

Materials:

Ultramarine.

Prussian blue.

Indigo.

Soluble blues.

Soap solution.

Place a minute quantity of each blue in a separate test tube. Fill with water and shake until the blue is evenly distributed through the water.

Test portions of each liquid with the following solutions:

(a) Dilute hydrochloric acid. Note the odor from the ultramarine. Moisten a piece of filter paper with lead acetate solution and hold at the mouth of the test tube.

(b) Sodium hydroxide. Compare the colors with those of the acidified solutions from (a).

(c) Sodium carbonate.

(d) Soap, boiling.

Allow the main portions to stand for a few days and note which form sediments.

CHAPTER XLIII

DYEING

DYEING consists in attaching a colored substance to the fibers of the textile in such a manner that it is not readily removed by rubbing or washing. Whether dyeing involves a chemical union between the fiber and the coloring matter is a disputed question. There are some facts which appear to indicate that such combinations of fiber and dye do occur in some instances. The animal fibers (and leather), being protein and having, therefore, basic and acid radicles, will combine directly with certain dyes which are acid and basic, whereas the same dyes will not become attached to the vegetable fibers. On the other hand, the product of the deposition of the dye in the fiber does not appear to have properties distinct from those of the dye and of the fiber, which ought to be the case if actual combination has occurred; nor does the combination seem to occur in the definite proportions in which substances react in chemical processes, but this may be because the enormous size of the protein molecules makes possible an almost unlimited number of compounds. The fact that cotton fibers from which the lumen is absent refuse to take dyes furnishes another argument in favor of a physical theory of dyeing.

The protein fibers are so different in character from the cellulose fibers, and the diversity of chemical nature among dyestuffs is so great, that it is not likely that all cases of dyeing can be explained in the same way. Physical phenomena may play the more important part in some instances, chemical phenomena in others.

Many dyes that will not attach themselves directly to a

given kind of fiber will dye the fiber after it has been first. treated with a substance called a mordant (from the Latin, mordeo, I bite). The mordant is used to attach to the fiber a compound capable of combining with the dye to form an insoluble product. The insoluble product is known as a lake.

A dye that will dye fibers without the intervention of a mordant is called a substantive dye.

A dye that will only attach itself to the fiber through the intervention of a mordant is called an adjective or mordant dye.

Practically all the modern dyes are organic compounds. A buff color of iron oxide is sometimes produced by treating the fabric first with an iron solution (such as ferrous sulphate), then with an alkali (e.g. sodium hydroxide or sodium carbonate), and then exposing it to the air to oxidize the ferrous hydroxide, Fe(OH)2, formed at first, to ferric hydroxide, Fe(OH)3. "Khaki" is obtained by adding a chromium salt (chrome alum) to the iron solution and treating in this way, thus producing a mixture of ferric and chromic oxides. Manganese brown, composed of an oxide (or hydroxide) of manganese, can be similarly produced from solutions of manganese salts. Chrome yellow is produced as a precipitate in the fibers when the goods are treated first with lead acetate and then with potassium dichromate. These inorganic dyes are sometimes called "mineral pigments."

Of the organic dyes a few are still obtained from plant or animal sources. Logwood, a dye extracted from the heart wood of a large South and Central American tree, the Haematoxylon campechianum (literally, blood-red wood of Campeachy), is used extensively in the silk industry and in calico printing for the production of a full black. Quercitrin bark and Persian berry are still used in dyeing yellow, although better effects are, as a rule, obtained with synthetic dyes.

Natural indigo, obtained by exposing to the air the juice