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 quantity 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 constituent 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 passes 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