CHAP. XL] ELECTRICITY AND MAGNETISM. 385 



hydrogen owing to secondary action of the metal upon 

 the water. From strong solutions of caustic potash and 

 soda Davy succeeded in obtaining metallic sodium and 

 potassium, which were before unknown. If electrodes of 

 mercury are employed, an amalgam of either of these 

 metals is readily obtained at the kathode. The so- 

 called <2;;z;/z<?;zzV;/z-amalgam is obtained by electrolysing a 

 warm, strong solution of salammoniac between mercury 

 electrodes. 



(d.) Substances can be arranged in a definite series 

 according to their electrolytic behaviour ; each substance 

 on the list behaving as a kathion (or being " electroposi- 

 tive ") when electrolysed from its compound with any 

 other on the list. In such a series the oxidisable metals, 

 potassium, sodium, zinc, etc., head the list ; after which 

 come the less oxidisable or " electronegative" metals ; then 

 carbon, boron, phosphorus, iodine, chlorine, sulphur, and 

 lastly oxygen. 



(e.) From a solution of mixed metallic salts the least 

 electropositive metal is deposited first, unless the current 

 be very strong. 



(f.) The liberated ions appear only at the elec- 

 trodes. 



(g.) For each electrolyte a minimum electromotive- 

 force is requisite, without which complete electrolysis 

 cannot be effected. (See Art. 413.) 



(h.) If the current be of less electromotive-force than 

 the requisite minimum, electrolysis may begin, and a, 

 feeble current flow at first, but no ions will be liberated, 

 the current being completely stopped as soon as the 

 opposing electromotive-force of polarisation has risen to 

 equality with that of the electrolysing current. 



(/.) There is no opposing electromotive-force of polar- 

 isation when electrolysis is effected from an anode of the 

 same metal that is being deposited at the kathode. The 

 feeblest cell will suffice to deposit copper from sulphate of 

 copper if the anode be a copper plate. 



2 C 



