526 PEINCIPLES OF CHEMISTRY 



calcium, and silver form acid and basic salts with difficulty, although 

 they easily form normal ones. 



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 

 elimination of oxygen and its formation by means of metallic sodium, 

 which separates the hydrogen from water, forming caustic soda. The 

 preparation of metallic sodium was one of the greatest discoveries in 

 chemistry, not only because through it the conception of elements became 

 broader and more regular, 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 con- 

 necting with the positive pole (of copper or charcoal) a piece of moist 

 (in order to obtain electrical conductivity) caustic soda, and boring a hole 

 in it filled with mercury connected with the negative pole of a strong 

 Yolta'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 demonstrated the complexity of alkalis, 

 which up to that time had been regarded as undecomposable substances. 

 On being decomposed by the galvanic current, caustic soda disengages 

 (hydrogen and sodium at the negative pole and oxygen at the positive 

 pole. Davy showed that the metal formed volatilises at a red heat, 

 which is the most important physical property for extracting sodium, 

 'because all further methods are founded on the volatility of sodium. 

 Besides this Davy observed that sodium easily oxidises, that its 

 ^vapours take fire in air, and the latter circumstance was for a long time 

 .an obstacle to the easy preparation of this metal. The properties of 

 sodium were later on more thoroughly investigated by Gay-Lussac and 

 Thenard, who furnished easier means of obtaining sodium, and observed 

 'that metallic iron at a high temperature was capable of reducing sodium 

 from caustic soda. 36 Brunner latterly discovered that not only iron, 



36 Deville supposes that such a decomposition of sodium hydroxide by metallic iron 

 depends solely on the dissociation of the alkali at a white heat into sodium, hydrogen, 

 and oxygen. Here the part played by the iron is only that it retains the oxygen formed, 

 otherwise the decomposed elements would again reunite upon cooling, as in other cases 

 of dissociation. If it be supposed that the temperature at the commencement of the dis- 

 MK-iiition of the iron oxides is higher than that of sodium oxide, then the decomposition 

 may be explained by the hypothesis of Deville. Deville demonstrates his views by the 

 : following experiment : An iron bottle, filled with iron shavings, was heated in such a 

 ;way that the upper part became red hot, the lower part remaining cooler; sodium 



