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 mass 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 grass and exposed to the bleaching action of sun and dew.

[Illustration:  FIG. 158.—­Preparing chlorine from hydrochloric acid and manganese dioxide.]