XIBIZING SUBSTANCES 225 



(OC1)~. In other words, a solution of bleaching powder in water 

 acts like mixed solutions of calcium chloride and calcium hypo- 

 chlorite. Only the calcium hypochlorite is concerned in the 

 bleaching process. When the solution is exposed to the air, it 

 gradually absorbs carbon dioxide (see p. 336). This dissolves in 

 the solution to form carbonic acid H^COa, which by double decom- 

 position liberates hypochlorous acid : 



2CaCl(QCl) + H 2 CO 3 <=* CaCl 2 + CaCO 3 + 2HOC1. 



If now materials which it is desired to bleach by oxidation 

 are introduced into the solution, the HOC1 is used up, stopping 

 the backward reaction and carrying the decomposition finally 

 to completion. 



Bleaching. Cotton and linen, in their original states, are not 

 pure white. Bleaching is therefore an extensive and most impor- 

 tant industry. The yarn or cloth must first be freed from cotton- 

 wax and tannin, since the former would hinder the action of the 

 bleaching agent, and both would also make the subsequent dyeing 

 uneven. The material is therefore first boiled with dilute caustic 

 soda-solution, and washed with water. The goods are then first 

 " chemicked " in cold bleaching powder solution; next " soured " 

 by immersion in very dilute sulphuric or hydrochloric acid; and 

 finally washed with extreme thoroughness. 



The final washing, to remove all traces of chlorine and bleach- 

 ing powder, is absolutely necessary. If not removed, the hypo- 

 chlorous acid acts gradually upon the cotton or linen, and " rots " 

 it. Bleaching agents, when used in the household, carelessly, are 

 liable to cause extensive damage from this cause. A dilute solu- 

 tion of sodium thiosulphate (photographers' " hypo ") is often 

 used, as " antichlor," to interact with and remove the last traces 

 of chlorine. 



Cotton and linen (C 6 HioO 5 ) tf are rather indifferent chemical 

 substances (p, 398), and stand brief contact with dilute chlorine- 



