General Science Part 19

212_a_. Source of Soda. An enormous quant.i.ty of sodium carbonate, or soda, as it is usually called, is needed in the manufacture of gla.s.s, soap, bleaching powders, and other commercial products. Formerly, the supply of soda was very limited because man was dependent upon natural deposits and upon ashes of sea plants for it. Common salt, sodium chloride, is abundant, and in 1775 a prize was offered to any one who would find a way to obtain soda from salt. As a result of this, soda was soon manufactured from common salt. In the most recent methods of manufacture, salt, water, ammonia, and carbon dioxide are made to react. Baking soda is formed from the reaction. The baking soda is then heated and decomposed into washing soda or the soda of commerce.

CHAPTER XXI

FERMENTATION

213. While baking powder is universally used for biscuits and cake, it is seldom, if ever, used for bread, because it does not furnish sufficient gas to lighten the tough heavy ma.s.s of bread dough. Then, too, most people prefer the taste of yeast-raised bread. There is a reason for this widespread preference, but to understand it, we must go somewhat far afield, and must study not only the bread of to-day, but the bread of antiquity, and the wines as well.

If grapes are crushed, they yield a liquid which tastes like the grapes; but if the liquid is allowed to stand in a warm place, it loses its original character, and begins to ferment, becoming, in the course of a few weeks, a strongly intoxicating drink. This is true not only of grape juice but also of the juice of all other sweet fruits; apple juice ferments to cider, currant juice to currant wine, etc.

This phenomenon of fermentation is known to practically all races of men, and there is scarcely a savage tribe without some kind of fermented drink; in the tropics the fermented juice of the palm tree serves for wine; in the desert regions, the fermented juice of the century plant; and in still other regions, the root of the ginger plant is pressed into service.

The fermentation which occurs in bread making is similar to that which is responsible for the transformation of plant juices into intoxicating drinks. The former process is not so old, however, since the use of alcoholic beverages dates back to the very dawn of history, and the authentic record of raised or leavened bread is but little more than 3000 years old.

214. The Bread of Antiquity. The original method of bread making and the method employed by savage tribes of to-day is to mix crushed grain and water until a paste is formed, and then to bake this over a camp fire. The result is a hard compact substance known as unleavened bread. A considerable improvement over this tasteless ma.s.s is self-raised bread. If dough is left standing in a warm place a number of hours, it swells up with gas and becomes porous, and when baked, is less compact and hard than the savage bread. Exposure to air and warmth brings about changes in dough as well as in fruit juices, and alters the character of the dough and the bread made from it. Bread made in this way would not seem palatable to civilized man of the present day, accustomed, as he is, to delicious bread made light and porous by yeast; but to the ancients, the least softening and lightening was welcome, and self-fermented bread, therefore, supplanted the original unleavened bread.

Soon it was discovered that a pinch of this fermented dough acted as a starter on a fresh batch of dough. Hence, a little of the fermented dough was carefully saved from a batch, and when the next bread was made, the fermented dough, or leaven, was worked into the fresh dough and served to raise the ma.s.s more quickly and effectively than mere exposure to air and warmth could do in the same length of time. This use of leaven for raising bread has been practiced for ages.

Grape juice mixed with millet ferments quickly and strongly, and the Romans learned to use this mixture for bread raising, kneading a very small amount of it through the dough.

215. The Cause of Fermentation. Although alcoholic fermentation, and the fermentation which goes on in raising dough, were known and utilized for many years, the cause of the phenomenon was a sealed book until the nineteenth century. About that time it was discovered, through the use of the microscope, that fermenting liquids contain an army of minute plant organisms which not only live there, but which actually grow and multiply within the liquid. For growth and multiplication, food is necessary, and this the tiny plants get in abundance from the fruit juices; they feed upon the sugary matter and as they feed, they ferment it, changing it into carbon dioxide and alcohol. The carbon dioxide, in the form of small bubbles, pa.s.ses off from the fermenting ma.s.s, while the alcohol remains in the liquid, giving the stimulating effect desired by imbibers of alcoholic drinks.

The unknown strange organisms were called yeast, and they were the starting point of the yeast cakes and yeast brews manufactured to-day on a large scale, not only for bread making but for the commercial production of beer, ale, porter, and other intoxicating drinks.

The grains, rye, corn, rice, wheat, from which meal is made, contain only a small quant.i.ty of sugar, but, on the other hand, they contain a large quant.i.ty of starch which is easily convertible into sugar. Upon this the tiny yeast plants in the dough feed, and, as in the case of the wines, ferment the sugar, producing carbon dioxide and alcohol.

The dough is thick and sticky and the gas bubbles expand it into a spongy ma.s.s. The tiny yeast plants multiply and continue to make alcohol and gas, and in consequence, the dough becomes lighter and lighter. When it has risen sufficiently, it is kneaded and placed in an oven; the heat of the oven soon kills the yeast plants and drives the alcohol out of the bread; at the same time it expands the imprisoned gas bubbles and causes them to lighten and swell the bread still more. Meanwhile, the dough has become stiff enough to support itself. The result of the fermentation is a light, spongy loaf.

216. Where does Yeast come From? The microscopic plants which we call yeast are widely distributed in the air, and float around there until chance brings them in contact with a substance favorable to their growth, such as fruit juices and moist warm batter. Under the favorable conditions of abundant moisture, heat, and food, they grow and multiply rapidly, and cause the phenomenon of fermentation. Wild yeast settles on the skin of grapes and apples, but since it does not have access to the fruit juices within, it remains inactive very much as a seed does before it is planted. But when the fruit is crushed, the yeast plants get into the juice, and feeding on it, grow and multiply. The stray yeast plants which get into the sirup are relatively few, and hence fermentation is slow; it requires several weeks for currant wine to ferment, and several months for the juice of grapes to be converted into wine.

Stray yeast finds a favorable soil for growth in the warmth and moisture of a batter; but although the number of these stray plants is very large, it is insufficient to cause rapid fermentation, and if we depended upon wild yeast for bread raising, the result would not be to our liking.

When our remote ancestors saved a pinch of dough as leaven for the next baking, they were actually cultivating yeast, although they did not know it. The reserved portion served as a favorable breeding place to the yeast plants within it; they grew and reproduced amazingly, and became so numerous, that the small ma.s.s of old dough in which they were gathered served to leaven the entire batch at the next baking.

As soon as man learned that yeast plants caused fermentation in liquors and bread, he realized that it would be to his advantage to cultivate yeast and to add it to bread and to plant juices rather than to depend upon accidental and slow fermentation from wild yeast.

Shortly after the discovery of yeast in the nineteenth century, man commenced his attempt to cultivate the tiny organisms. Their microscopic size added greatly to his trouble, and it was only after years of careful and tedious investigation that he was able to perfect the commercial yeast cakes and yeast brews universally used by bakers and brewers. The well-known compressed yeast cake is simply a ma.s.s of live and vigorous yeast plants, embedded in a soft, soggy material, and ready to grow and multiply as soon as they are placed under proper conditions of heat, moisture, and food. Seeds which remain on our shelves do not germinate, but those which are planted in the soil do; so it is with the yeast plants. While in the cake they are as lifeless as the seed; when placed in dough, or fruit juice, or grain water, they grow and multiply and cause fermentation.

CHAPTER XXII

BLEACHING

217. The beauty and the commercial value of uncolored fabrics depend upon the purity and perfection of their whiteness; a man's white collar and a woman's white waist must be pure white, without the slightest tinge of color. But all natural fabrics, whether they come from plants, like cotton and linen, or from animals, like wool and silk, contain more or less coloring matter, which impairs the whiteness. This coloring not only detracts from the appearance of fabrics which are to be worn uncolored, but it seriously interferes with the action of dyes, and at times plays the dyer strange tricks.

Natural fibers, moreover, are difficult to spin and weave unless some softening material such as wax or resin is rubbed lightly over them.

The matter added to facilitate spinning and weaving generally detracts from the appearance of the uncolored fabric, and also interferes with successful dyeing. Thus it is easy to see that the natural coloring matter and the added foreign matter must be entirely removed from fabrics destined for commercial use. Exceptions to this general fact are sometimes made, because unbleached material is cheaper and more durable than the bleached product, and for some purposes is entirely satisfactory; unbleached cheesecloth and sheeting are frequently purchased in place of the more expensive bleached material. Formerly, the only bleaching agent known was the sun's rays, and linen and cotton were put out to sun for a week; that is, the unbleached fabrics were spread on the gra.s.s and exposed to the bleaching action of sun and dew.

[Ill.u.s.tration: FIG. 158.--Preparing chlorine from hydrochloric acid and manganese dioxide.]

218. An Artificial Bleaching Agent. While the sun's rays are effective as a bleaching agent, the process is slow; moreover, it would be impossible to expose to the sun's rays the vast quant.i.ty of fabrics used in the civilized world of to-day, and the huge and numerous bolts of material which daily come from our looms and factories must therefore be whitened by artificial means. The substance almost universally used as a rapid artificial bleaching agent is chlorine, best known to us as a const.i.tuent of common salt.

Chlorine is never free in nature, but is found in combination with other substances, as, for example, in combination with sodium in salt, or with hydrogen in hydrochloric acid.

The best laboratory method of securing free chlorine is to heat in a water bath a mixture of hydrochloric acid and manganese dioxide, a compound containing one part of manganese and two parts of oxygen. The heat causes the manganese dioxide to give up its oxygen, which immediately combines with the hydrogen of the hydrochloric acid and forms water. The manganese itself combines with part of the chlorine originally in the acid, but not with all. There is thus some free chlorine left over from the acid, and this pa.s.ses off as a gas and can be collected, as in Figure 158. Free chlorine is heavier than air, and hence when it leaves the exit tube it settles at the bottom of the jar, displacing the air, and finally filling the bottle.

Chlorine is a very active substance and combines readily with most substances, but especially with hydrogen; if chlorine comes in contact with steam, it abstracts the hydrogen and unites with it to form hydrochloric acid, but it leaves the oxygen free and uncombined. This tendency of chlorine to combine with hydrogen makes it valuable as a bleaching agent. In order to test the efficiency of chlorine as a bleaching agent, drop a wet piece of colored gingham or calico into the bottle of chlorine, and notice the rapid disappearance of color from the sample. If unbleached muslin is used, the moist strip loses its natural yellowish hue and becomes a clear, pure white. The explanation of the bleaching power of chlorine is that the chlorine combines with the hydrogen of the water and sets oxygen free; the uncombined free oxygen oxidizes the coloring matter in the cloth and destroys it.

Chlorine has no effect on dry material, as may be seen if we put dry gingham into the jar; in this case there is no water to furnish hydrogen for combination with the chlorine, and no oxygen to be set free.

219. Bleaching Powder. Chlorine gas has a very injurious effect on the human body, and hence cannot be used directly as a bleaching agent. It attacks the mucous membrane of the nose and lungs, and produces the effect of a severe cold or catarrh, and when inhaled, causes death. But certain compounds of chlorine are harmless, and can be used instead of chlorine for destroying either natural or artificial dyes. One of these compounds, namely, chloride of lime, is the almost universal bleaching agent of commerce. It comes in the form of powder, which can be dissolved in water to form the bleaching solution in which the colored fabrics are immersed. But fabrics immersed in a bleaching powder solution do not lose their color as would naturally be expected. The reason for this is that the chlorine gas is not free to do its work, but is restricted by its combination with the other substances. By experiment it has been found that the addition to the bleaching solution of an acid, such as vinegar or lemon juice or sulphuric acid, causes the liberation of the chlorine.

The chlorine thus set free reacts with the water and liberates oxygen; this in turn destroys the coloring matter in the fibers, and transforms the material into a bleached product.

The acid used to liberate the chlorine from the bleaching powder, and the chlorine also, rot materials with which they remain in contact for any length of time. For this reason, fabrics should be removed from the bleaching solution as soon as possible, and should then be rinsed in some solution, such as ammonia, which is capable of neutralizing the harmful substances; finally the fabric should be thoroughly rinsed in water in order that all foreign matter may be removed. The reason home bleaching is so seldom satisfactory is that most amateurs fail to realize the necessity of immediate neutralization and rinsing, and allow the fabric to remain too long in the bleaching solution, and allow it to dry with traces of the bleaching substances present in the fibers. Material treated in this way is thoroughly bleached, but is at the same time rotten and worthless. Chloride of lime is frequently used in laundry work; the clothes are whiter than when cleaned with soap and simple washing powders, but they soon wear out unless the precaution has been taken to add an "antichlor" or neutralizer to the bleaching solution.

220. Commercial Bleaching. In commercial bleaching the material to be bleached is first moistened with a very weak solution of sulphuric acid or hydrochloric acid, and is then immersed in the bleaching powder solution. As the moist material is drawn through the bleaching solution, the acid on the fabric acts upon the solution and releases chlorine. The chlorine liberates oxygen from the water. The oxygen in turn attacks the coloring matter and destroys it.

[Ill.u.s.tration: FIG. 159.--The material to be bleached is drawn through an acid _a_, then through a bleaching solution _b_, and finally through a neutralizing solution _c_.]

The bleached material is then immersed in a neutralizing bath and is finally rinsed thoroughly in water. Strips of cotton or linen many miles long are drawn by machinery into and out of the various solutions (Fig. 159), are then pa.s.sed over pressing rollers, and emerge snow white, ready to be dyed or to be used as white fabric.

221. Wool and Silk Bleaching. Animal fibers like silk, wool, and feathers, and some vegetable fibers like straw, cannot be bleached by means of chlorine, because it attacks not only the coloring matter but the fiber itself, and leaves it shrunken and inferior. Cotton and linen fibers, apart from the small amount of coloring matter present in them, contain nothing but carbon, oxygen, and hydrogen, while animal fibers contain in addition to these elements some compounds of nitrogen. The presence of these nitrogen compounds influences the action of the chlorine and produces unsatisfactory results. For animal fibers it is therefore necessary to discard chlorine as a bleaching agent, and to subst.i.tute a substance which will have a less disastrous action upon the fibers. Such a substance is to be had in sulphurous acid. When sulphur burns, as in a match, it gives off disagreeable fumes, and if these are made to bubble into a vessel containing water, they dissolve and form with the water a substance known as sulphurous acid. That this solution has bleaching properties is shown by the fact that a colored cloth dipped into it loses its color, and unbleached fabrics immersed in it are whitened. The harmless nature of sulphurous acid makes it very desirable as a bleaching agent, especially in the home.

Silk, lace, and wool when bleached with chlorine become hard and brittle, but when whitened with sulphurous acid, they retain their natural characteristics.

This mild form of a bleaching substance has been put to uses which are now prohibited by the pure food laws. In some canneries common corn is whitened with sulphurous acid, and is then sold under false representations. Cherries are sometimes bleached and then colored with the bright shades which under natural conditions indicate freshness.

Bleaching with chlorine is permanent, the dyestuff being destroyed by the chlorine; but bleaching with sulphurous acid is temporary, because the milder bleach does not actually destroy the dyestuff, but merely modifies it, and in time the natural yellow color of straw, cotton, and linen reappears. The yellowing of straw hats during the summer is familiar to everyone; the straw is merely resuming its natural color which had been modified by the sulphurous acid solution applied to the straw when woven.

222. Why the Color Returns. Some of the compounds formed by the sulphurous acid bleaching process are gradually decomposed by sunlight, and in consequence the original color is in time partially restored. The portion of a hat protected by the band retains its fresh appearance because the light has not had access to it. Silks and other fine fabrics bleached in this way fade with age, and a.s.sume an unnatural color. One reason for this is that the dye used to color the fabric requires a clear white background, and loses its characteristic hues when its foundation is yellow instead of white. Then, too, dyestuffs are themselves more or less affected by light, and fade slowly under a strong illumination.

Materials which are not exposed directly to an intense and prolonged illumination retain their whiteness for a long time, and hence dress materials and hats which have been bleached with sulphurous acid should be protected from the sun's glare when not in use.

223. The Removal of Stains. Bleaching powder is very useful in the removal of stains from white fabrics. Ink spots rubbed with lemon juice and dipped in bleaching solution fade away and leave on the cloth no trace of discoloration. Sometimes these stains can be removed by soaking in milk, and where this is possible, it is the better method.

Bleaching solution, however, while valuable in the removal of some stains, is unable to remove paint stains, because paints owe their color to mineral matter, and on this chlorine is powerless to act.

Paint stains are best removed by the application of gasoline followed by soap and water.

CHAPTER XXIII

DYEING

224. Dyes. One of the most important and lucrative industrial processes of the world to-day is that of staining and dyeing. Whether we consider the innumerable shades of leather used in shoes and harnesses and upholstery; the mult.i.tude of colors in the paper which covers our walls and reflects light ranging from the somber to the gay, and from the delicate to the gorgeous; the artificial scenery which adorns the stage and by its imitation of trees and flowers and sky translates us to the Forest of Arden; or whether we consider the uncounted varieties of color in dress materials, in carpets, and in hangings, we are dealing with substances which owe their beauty to dyes and dyestuffs.