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.

[Illustration:  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 passed over pressing rollers, and emerge snow white, ready to be dyed or to be used as white fabric.