288 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



These two actions were regarded by Daniel! in 1840 as due to 

 a decomposition of the salt into its two radicals, e.g. 



Na 2 SO 4 -> 2Na + SO 4 ; 



the radicals then acted on the water, so that caustic soda and 

 hydrogen were produced together at the negatively-charged 

 electrode (Faraday's kathode] where the current left the liquid, 

 whilst sulphuric acid and oxygen were produced together at the 

 positively charged electrode (Faraday's anode] where the 

 current entered the liquid, thus : 



Kathode: 2Na + 2H 2 O -> 2NaOH + H 2 

 Anode : SO 4 + H 2 6 -> H 2 SO 4 + |O 2 



Davy, in 1807, after having failed to separate a metal from 

 aqueous caustic potash, passed an electric current across a piece 

 of solid potash, slightly moistened on the surface. The potash 

 was melted by the heat of the current andgave a light inflam- 

 mable metal, which Davy called potassium. A similar metal, 

 sodium, was separated from caustic soda. These metals were 

 intensely active ; they combined directly with chlorine to form 

 salt-like substances, and had so great an attraction for oxygen 

 that they were able to decompose water at ordinary temperatures, 

 liberating hydrogen and reproducing the caustic alkali ; they 

 were used constantly during the succeeding years as the most 

 powerful agents for extracting oxygen from substances in which 

 its presence was suspected. 



Thus boron, the characteristic non-metal of borax (sodium 

 borate) and of ^nV<2W(prepared by Homberg in 1702 and com- 

 monly known as " sedative salt "), was isolated in 1808, by Davy, 

 and by Gay-Lussac and Thenard, by heating boric anhydride 

 with potassium. Silicon, which exists as an oxide (silica or silicic 

 anhydride} in quartz and sand, and in the mineral silicates, was 

 prepared by Gay-Lussac and Thenard in 1809 by the action of 

 potassium on the fluoride. Aluminium, the characteristic metal 

 of alumina, of alum, and of clay, was isolated by Wohler in 1827 

 by the action of potassium on the chloride. 



Davy, in 1808, separated a new series of metals from the 

 alkaline earths magnesia, lime, strontia and baryta. The 

 earths were mixed with oxide of mercury, covered with naphtha 

 and submitted to the action of the electric current. Amalgams 

 were obtained, from which the mercury was distilled off, more or 



