SODIUM 583 



H 2 C0 3 ,' or sulphuric, H 2 S0 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, 

 NaHS0 4 , and Na 2 S0 4 , whilst such bivalent metals as calcium and 

 barium do not form acid salts because one of their atoms at once 

 takes the place of both hydrogen atoms, forming, <for example, CaC0 3 

 and CaSCV 8 "* 



We have seen the transformation of common salt into sodium 

 sulphate, of this latter into sodium carbonate, and of sodium carbonate 

 into caustic soda. Lavoisier still regarded sodium hydroxide as an 

 element, because he was unacquainted with its decomposition with the 

 formation of metallic sodium, which separates the hydrogen from water, 

 reforming caustic soda. 



The preparation of metallic sodium was one of the greatest dis- 

 coveries in chemistry, not only because through it the conception of 

 elements became broader and more correct, but especially because in 

 sodium, chemical properties were observed which were but feebly shown 

 in the other metals more familiarly known. This discovery was made 

 in 1807 .by the English chemist Davy by means of the galvanic 

 current. By connecting with the positive pole (of copper or carbon) 

 a piece of caustic soda (moistened in order to obtain electrical con- 

 ductivity), and boring a hole in it filled with mercury connected with 

 the negative pole of a strong Volta's pile, Davy observed that on passing 

 the current a peculiar metal dissolved in the mercury, less volatile 

 than mercury, and capable of decomposing water, again forming 

 caustic soda. In this way (by analysis and synthesis) Davy demon- 

 strated the compound nature of alkalis. On being decomposed by the 

 galvanic current, caustic soda disengages hydrogen and sodium at the 



53 bi n might be expected, from what has been mentioned above, that bivalent metals 

 would easily form acid salts with acids containing more than two atoms of hydrogen for 

 instance, with tribasic acids, such as phosphoric acid, HjPC^ 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 ; the feebler bases (like silver oxide), although corresponding with 

 univalent metals, do not form acid salts, while the feeblest bases "(CuO, PbO, &c.) easily 

 form basic salts, and notwithstanding their valency do not form acid salts which are in 

 any degree stable that is, which are undecomposable by water. 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 ex- 

 pected from the valency of sodium, but also salts containing a greater quantity of acid. 

 In sodium sesquicarbonate we saw an example of such, compounds. Taking all this into 

 consideration, we must say that the property of more or less easily forming acid salts 

 depends more upon the energy of the base than upon its valency, and the best statement 

 is that the capacity of a base for forming acid and basic salts is characteristic, just 

 as the faculty of forming* compounds with hydrogen, is characteristic of elements. 



