190 PRINCIPLES OF CHEMISTRY 



compounds with hydrogen, in which the hydrogen may be replaced by 

 a metal forming substances which, in their reactions and external 

 characters, resemble the salts formed from oxides. Table salt, NaCl, 

 is an example of this. It may be obtained by the replacement of hydro- 

 gen in hydrochloric acid, HC1, by the metal sodium, just as sulphate 

 of sodium, Na a SO.,, may be obtained by the replacement of hydrogen 

 in sulphuric acid, H. 2 SO 4 , by sodium. The exterior appearance of the 

 resulting products, their neutral reaction, and even their saline taste, 

 show their mutual resemblance ; as the acid reaction, the property of 

 saturating bases, the capacity of exchanging their hydrogen for some 

 metal, and the acid taste, show the common properties belonging to 

 hydrochloric and sulphuric acids. 



To the fundamental properties of salts yet another must be added 

 namely, that they are more or less decomposed by the action of a galvanic 

 current. The results of this decomposition are very different, accord- 

 ing to whether the salt be taken in a fused or dissolved state. But 



O 



the decomposition may be so represented, that the metal appears at the 

 electro-negative pole (like hydrogen in the decomposition of water, or 

 its mixture with sulphuric acid), and the remaining parts of the salt 

 appear at the electro- positive pole (where the oxygen of water appears). 

 If, for instance, an electric current acts on an aqueous solution of sodium 

 sulphate, then the sodium appears at the negative pole, and oxygen 

 and the anhydride of sulphuric acid at the positive pole. But in the 

 solution itself the result is different, for sodium, as we know, decom- 

 poses water with evolution of hydrogen, forming caustic soda ; conse- 

 quently hydrogen will be evolved, and caustic soda appear at the 

 negative pole : while at the positive pole the sulphuric anhydride 

 immediately combines with water and forms sulphuric acid, and there- 

 fore oxygen will be evolved and sulphuric acid formed round this 

 pole. 55 In other cases, when the metal separated is not able to decom- 

 pose water, it will be deposited in a free state. Thus, for example, in 

 the decomposition of copper sulphate, copper separates out at the 

 cathode, and oxygen and sulphuric acid appear at the anode, and 

 if a copper plate be attached to the positive pole, then the oxygen 

 evolved will oxidise the copper, and the oxide of copper will dissolve in 

 the sulphuric acid which is formed around this pole ; hence the copper 

 will be dissolved at the positive, and deposited at the negative, pole 



55 This kind of decomposition maybe easily observed by pouring a solution of sodium 

 sulphate in a U-shaped tube and inserting electrodes in both branches- If the solution 

 lae coloured with an infusion of litmus, it will easily be seen that it turns blue round the 

 electro-negative pole, owing to the formation of sodium hydroxide, and red at the 

 electro-positive pole, from the formation of sulphuric acid. 



