oXVdKN AND ITS SALINK ( '( >.M l',I NATI< >NS 191 



that is, a transfer of copper from the positive to the negative pole 

 ensues. The galvanoplastic art (electrotyping) is based on this 

 principle/" 1 Therefore the most radical and general properties of salts 

 (including also such salts as table salt, which contains no oxygen) may 

 be expressed by representing the salt as composed of a metal M and a 

 haloid X that is, by expressing the salt by MX. In common table 

 salt the metal is sodium, and the haloid an elementary body, chlorine. 

 In sodium sulphate, Na 2 SO 4 , sodium is again the metal, but the 

 complex group, S0 4 , is the haloid. In sulphate of copper, CuSO 4 , the 

 metal is copper, and the haloid the same as in the preceding salt. 

 Such a representation of salts expresses with great simplicity the 

 capacity of every salt to enter into saline double decompositions with 

 ntlicr salts ; consisting in the mutual replacement of the metals in the 

 salts. This exchange of their metals forms the fundamental property 

 of salts. If there be two salts with different metals and haloids, and 

 they be in solution or fusion, or any other manner, brought into con- 

 tact, then the metals of these salts will always partially or wholly 

 exchange places. If we designate one salt by MX, and the other by 

 NY, then we either partially or wholly obtain from them new salts, 

 MY and NX. Thus we saw in the Introduction, that on mixing 

 solutions of table salt, NaCl, and silver nitrate, AgNO 3 , a white 

 insoluble precipitate of silver chloride, AgCl, is formed, and a new salt, 

 sodium nitrate, NaNO 3 , is obtained in solution. If the metals of salts 

 exchange places in reactions of double decomposition, it is clear that 

 metals themselves, taken in a separate state, are able to act on salts, as 

 zinc evolves hydrogen from acids, and as iron separates copper from 

 copper sulphate. When, to what extent, and which metals displace each 

 other, and how the metals are distributed between the haloids, all this we 

 will discuss later on, guided by those reflections and deductions which 

 Berthollet introduced into the science at the beginning of this cen- 

 tury. 



According to the above observations, an acid is nothing more than 

 a salt of hydrogen. Water itself may be looked on as a salt in which 



56 In other cases the decomposition of salts by the electric current may be accom- 

 panied by much more complex results. Thus, when the metal of the salt is capable of a 

 higher degree of oxidation, such a higher oxide may be formed at the positive pole by 

 the oxygen which is evolved there. This takes place, for instance, in the decomposition 

 of salts of silver and manganese by the galvanic current, peroxides of these metals being 

 formed. If the metal separated at the negative pole acts on a salt occurring in the 

 solution, then it may do so at this pole, and in this manner the phenomena of the action 

 of a current on a salt are in many cases rendered remarkably complicated. But all the 

 phenomena as yet known may be expressed by the above law that the current decom- 

 poses salts into metals, which appear at the negative pole, and into the remaining com- 

 ponent parts, which appear at the positive pole. 



