The time for planting cotton naturally varies in different climates. In the United States, in February, the fields are cleared of the last year's stalks, and the ground is plowed and prepared for rows about four feet apart. The seeds are planted between March 1 and June 10, although rarely as late as June, and the plant comes up in about fifteen days. Thirty-five to forty-five days later the first buds appear, and after sixty-five days more the crop is ready to pick. Thus, four months is about the shortest duration of time between the planting and the ripening of the seeds.
Cultivation of the crop is required when the plants are young, and as considerable loss is suffered from various pests and diseases, a great deal of attention has been given in the last few years to methods of fighting these destroyers. The cotton worm and other insects, fungi, and diseases cause much loss every year, but the cotton-boll weevil is probably the most serious pest.
The importance of the cotton crop to this country has been very great, and its importance to our Southern states can hardly be esti-mated. Cotton is the largest textile industry, and as a world industry ranks very high. The importance to other countries of the cotton crop of the United States is shown by the fact that during our Civil War it was only by the greatest effort that a terrible famine was prevented among the cotton-mill hands in parts of England. At the present time, England and other European countries are encouraging in every possible way cotton production in their own colonies which lie within the cotton belt, in order that they may not be so dependent on the crop of the United States.
We have said that the physical structure of a fiber determines whether or not it may be used in textile manufacture. The peculiar structure of cotton which makes it suitable for spinning is the twist which the fiber takes on ripening. Cotton fibers are short in comparison with others used for manufacture, being three-fourths of an inch to two inches in length, the greater number varying from one to one and one-half inches. Were it not for the characteristic spiral twist which helps hold the fibers together, it would be extremely difficult to spin. Under the microscope the cotton fiber appears like a twisted ribbon, with the edges thicker than the central part. Cross sections show an irregular form, somewhat oval, with thick walls and an opening in the center. Different fibers vary as to the number of twists. There are sometimes present unripe fibers, showing no internal structure and no spiral twist, while different varieties of ripe fibers also vary in amount of twist. The size of the cotton fiber also varies greatly. Sea Island is the finest fiber, while the coarsest cottons sometimes reach a diameter six or seven times greater. In tensile strength cotton stands between silk and wool, and in elasticity it is below either, although more elastic than linen.
The most common color of the cotton fiber is a creamy white, but it is sometimes golden or quite yellow, as Nankin cotton. The luster of the fibers varies in different kinds of cotton, and is most marked in the long, silky Sea Island.
The hygroscopic quality of a fiber is its ability to absorb water. Cloth may absorb moisture in two ways. It may merely take the moisture up in the spaces between the threads of the cloth, giving it a damp feeling, or the fiber may absorb the moisture into itself. It is this second quality which we call the hygroscopic quality. Ordinarily, cotton holds from five to eight per cent of this hygroscopic moisture, although it may, in a moist atmosphere, hold much more. The spinning of cotton is greatly affected by the amount of moisture present, therefore artificial humidity is usually maintained in the spinning room. Cotton which has been freed from the vegetable wax adhering to it is much more hygroscopic, and is known as absorbent cotton. Cotton withstands heat better than animal fibers. At 250° C. it begins to turn brown. It burns freely in the air, with a luminous flame.
Fig. 18. Cotton Fibers.
Certain chemical actions distinguish vegetable from animal fibers. Cotton is largely cellulose. The surface of the fiber is protected by a layer of vegetable wax and oil, and there is a small amount of coloring matter. The natural impurities are very small, being from four to five per cent, and consisting of small particles of seed, leaves, and dust. In its chemical behavior the reactions of cotton are those of cellulose. Mineral acids destroy the fiber; very concentrated sulphuric acid dissolves it, while weaker acids disintegrate it, so that the fiber may be reduced to dust when dried. Dilute acids do not affect the cotton while wet. Concentrated caustic alkalies bring about a change in cellulose known as mercerization. This will be referred to again. Dilute solutions of alkalies have no injurious effect on cotton unless exposed to the air, when they weaken it.
These reactions are important in the bleaching and dyeing of cotton as well as in some household processes. A stain may be removed from cotton cloth with a dilute acid and the cloth be unharmed. If, however, that acid is left on the cloth until it dries thoroughly and becomes more concentrated, it may destroy the fiber. If a hot iron is put on the cloth before the acid is thoroughly removed, a hole will doubtless be the result. Cleaning powders containing free alkali must be thoroughly removed or they weaken the fiber. Cotton does not take dyestuffs readily. In most cases an assistant is required.