()XV(iKN AND ITS SALINE COMBINATIONS 101 



is often commenced by the preparation of oxygen by this method, and 

 of hydrogen by the aid of zinc and sulphuric acid, all the more as 

 thi'M- -uses enable many interesting and striking experiments to be 

 made. 15 



A solution of bleaching powder, which contains calcium hypo- 

 chlorite, CaCl 2 O 2 , evolves oxygen when gently heated with the ad- 

 dition of a small quantity of certain oxides for instance, cobalt 

 oxide, which in this case acts by contact (see Introduction). Of 

 itself, a solution of bleaching powder does not evolve oxygen when 

 heated, but it oxidises the cobalt oxide to a higher degree of oxidation ; 

 this higher oxide of cobalt in contact with the bleaching powder, decom- 

 poses into oxygen and lower oxidation products, and the resultant lower 

 oxide of cobalt with bleaching powder again gives the higher oxide, 

 which again gives up its oxygen, and so on. 16 The calcium hypo- 

 chlorite is here decomposed according to the equation CaCl 2 O 2 = 

 CaCl 2 + O 2 . In this manner a small quantity of cobalt oxide 17 is 

 sufficient for the decomposition of an indefinitely large quantity 

 of bleaching powder. 



with powdered manganese peroxide proceeds at so low a temperature (the salt does not 

 melt) that it may be effected in an ordinary glass flask. As the reaction is exothermal, the 

 decomposition of potassium chlorate with the formation of oxygen may probably be 

 accomplished, under certain conditions (for example under contact action), at very low 

 temperatures. Substances mixed with the potassium chlorate probably act partially in 

 this manner. 



15 Many other salts evolve oxygen by heat, like potassium chlorate, but they only 

 part with it either at a very strong heat (for instance, common nitre) or else are un- 

 suited for use on account of their cost (for instance, potassium manganate), or evolve 

 impure oxygen at a high temperature (for instance, zinc sulphate at a red heat gives 

 a mixture of sulphurous anhydride and oxygen), and are not therefore used in prac- 

 tice. 



1(1 Such is, at present, the only possible method of explaining the phenomenon 

 of contact action. In many cases, as here, it is supported by observations based on facts. 

 Thus, for instance, it is known, as regards oxygen, that often two substances rich in 

 oxygen retain it so long as they are separate, but directly they come into contact 

 free oxygen is evolved from both of them. Thus, an aqueous solution of hydrogen 

 peroxide (containing twice as much oxygen as water) acts in this manner on silver oxide 

 (containing silver and oxygen). This reaction takes place at the ordinary temperature, 

 and the oxygen is evolved from both compounds. To this class of phenomena may be 

 also referred the fact that a mixture of barium peroxide and potassium manganate with 

 water and sulphuric acid evolves oxygen at the ordinary temperature. It would seem 

 that the essence of phenomena of this kind is entirely and purely a property of 

 contact ; the distribution of the atoms is changed by contact, and if the equilibrium be 

 unstable it is destroyed. This is especially clear for substances which change exother- 

 inally that is, for those reactions which are accompanied by an evolution of heat. The 

 decomposition CaCLO.^ = CaCL> + O. 2 belongs to this class (like the decomposition of 

 potassium chlorate). 



17 Generally a solution of bleaching powder is alkaline (contains free lime), and, there- 

 VOL. I. M 



