SODIUM 525 



acid), because they contain only one atom of hydrogen capable of 

 being replaced by the metal. Also, as will be shown in the subse- 

 quent chapters, sodium belongs to the univalent metals, exchangeable 

 for hydrogen atom for atom ; amongst metals sodium may, like 

 chlorine amongst the non-metals, serve as a representative of the 

 univalent properties. Most of the other elements which are not 

 capable of forming acid salts are bivalent, from whence it may 

 be understood that in a bibasic acid for instance, carbonic, H 2 CO 3 , 

 or sulphuric, H. 2 SO 4 the hydrogen may be exchanged, atom for atom, 

 for sodium, and yield an acid salt by means of the first substitution 

 and a normal salt by means of the second for instance, NaHSO 4 , 

 and Na. 2 SO 4 , whilst such bivalent metals as calcium or barium do 

 not form acid salts because one of their atoms at once takes the place 

 of both hydrogen atoms, forming, for instance, CaCO 3 and CaSO 4 . 

 It may be expected, from what has been mentioned above, that 

 bivalent metals easily form acid salts with acids containing more than 

 two atoms of hydrogen for instance, with tribasic acids, such as 

 phosphoric acid, H 3 PO 4 and actually such salts do exist ; but all 

 such relations are complicated by the fact that the character of the 

 base very often changes and becomes weakened with the increase of 

 valency and the change of atomic weight, and feebler bases (for 

 instance, silver oxide), although corresponding with univalent metals, 

 do not form acid salts, and the feeblest bases (for instance, CuO, PbO) 

 easily form basic salts, and, notwithstanding their valency, do not form 

 any acid salts which are in any degree stable for instance, which are 

 undecomposable by water. In addition, basic and acid salts ought to be 

 regarded rather as compounds similar to crystallo-hydrates, because 

 such acids as sulphuric form with sodium not only an acid and a 

 normal salt, as might be expected judging from the valency of 

 sodium, but in addition to these also salts containing a greater 

 quantity of acid. In sodium sesquicarbonate we saw an example of 

 such compounds. Taking all this into consideration, we ought to say 

 that the property of more or less easily forming acid salts more 

 readily accords with the energy of the base than with the valency, 

 and it is truest of all to state that the faculty of a base to form acid and 

 basic salts is its characteristic, just as the faculty of forming com- 

 pounds with hydrogen is a characteristic property of elements. In 

 this respect sodium hydroxide distinguishes itself by the facility with 

 which it forms acid salts and an absence of the faculty of forming 

 basic salts. This property is also shared with sodium by the bases 

 formed from the metals potassium and lithium. The metals lead and 

 copper do not form acid salts, but easily give basic salts. Barium,. 



