478 



PRINCIPLES OF CHEMISTRY 



may be prepared in the laboratory by passing a current of chlorine 

 through a cold mixture of water and lime (milk of lime). The mixture 

 must be kept cold, as otherwise 3Ca(C10) 2 passes into 2CaCl 2 

 + Ca(C10 3 ) 2 . In the manufacture of bleaching powder in large 

 quantities at chemical works, the purest possible slaked lime is taken 

 and laid in a thin layer in large flat chambers, M (whose walls are 

 made of Yorkshire flags or tarred wood, on which chlorine has no 

 action), and into which chlorine gas is introduced by lead tubes. The 

 distribution of the plant is shown in the annexed drawing (fig. 67). 



FIG. 67. Apparatus for the manufacture of bleaching powder (on a small scale) by the action of 

 chlorine, which is generated in the vessels C, on lime, which is charged into M. 



The products of the metalepsis of alkaline hydrates, NaCIO and 

 Ca(C10) 2 , which are present in solutions of ' Javelle salt ' and bleaching 



chloride is formed by the action of water on bleaching powder, and this is proved to be 

 the case by the fact that small quantities of water extract a considerable amount of 

 calcium chloride from bleaching powder. If a large quantity of water act on bleaching 

 powder an excess of calcium hydroxide remains, a portion of which is not subjected to 

 change. The action of the water may be expressed by the following formulae : From the 

 dry mass Ca 3 (HO) 6 Cl 4 there is formed lime, Ca(HO) 2 , calcium chloride, CaCl 2 , and a 

 saline substance, Ca(ClO) 2 . CasH^OeCl^CaHaOa + CaCLjOa + CaCls + aHaO. The re- 

 sulting substances are not equally soluble ; water first extracts the calcium chloride, 

 which is the most soluble, then the compound Ca(C10) 2 and ultimately calcium hydroxide 

 is left. A mixture of calcium chloride and hypochlorite passes into solution. On evapo- 

 ration there remains Ca 2 O 2 Cl 4 3H 2 O. The dry bleaching powder does not absorb more 

 chlorine, but the solution is able to absorb it in considerable quantity. If the liquid be 

 boiled, a considerable amount of chlorine monoxide is evolved. After this calcium 

 chloride! alone remains in solution, and the decomposition may be expressed as follows : 

 CaCl 2 + CaCl 2 O 2 + 2Cl 3 = 2CaCl 2 + 2Cl 2 O. Chlorine monoxide may be prepared in this 

 manner. 



It is sometimes said that bleaching powder contains a substance, Ca(OH) 2 Cl 2 , that is 

 calcium peroxide, CaO 2 , in which one atom of oxygen is replaced by (OH) 2 , and {he other 

 by C1. 2 ; but, judging from what has been said above, this can only be the case in the dry 

 state, and not in solutions. 



On being kept for some time, bleaching powder sometimes decomposes, with the 

 evolution of oxygen (because CaCl 3 O 3 =CaCl 2 + 2 , see p. 16); the" same takes place 

 when it is heated. 



