CHEMISTRY AND ITS DEVELOPMENT 217 



between the old atomic hypothesis and the facts of chemical 

 composition. 



After the relation between the known metals and their 

 oxides was established, Lavoisier himself, and others, be- 

 gan to suspect the true nature even of oxides whose metals 

 were not yet known in the free state, and attempts began 

 to be made to decompose these oxides so as to isolate their 

 metallic elements. About the beginning of the nineteenth 

 century, Sir Humphry Davy (1778-1829) undertook to 

 investigate the effect of the galvanic current on chemical 

 compounds. In 1807-08 he succeeded in decomposing 

 caustic potash and caustic soda, obtaining from them the 

 metals potassium and sodium. 



About the same time Seebeck similarly decomposed the 

 oxides of calcium, barium, strontium and magnesium, 

 obtaining these metals in the form of their amalgams i.e. 

 combinations with mercury. From these amalgams Davy 

 isolated the metals themselves and gave them their present 

 names. From the metals Davy turned his genius to the 

 non-metallic elements. Chlorine, known since 1774, re- 

 mained unrecognized as an element, and was generally con- 

 sidered as the oxide of hydrochloric acid. In 1811 Davy 

 clearly demonstrated its elementary nature ; and when, 

 soon afterwards, Courtois discovered iodine, Davy showed 

 that this substance, too, so similar to chlorine, must be con- 

 sidered as an element. Davy also was the first to demon- 

 strate clearly the elementary nature of nitrogen and even 

 of fluorine (from the similarity of hydrofluoric to hydro- 

 chloric acid, and of the fluorides to the chlorides), although 

 the latter element was not then known in the free state, 

 and remained unknown until 1887. The value of Davy's 

 contributions can be readily appreciated if we remember 

 that the substances he was dealing with are among the 

 commonest in the entire range of chemistry, and if we 

 imagine how much confusion would suddenly ensue in all 

 departments of the science if we were to forget their exist- 

 ence or their true nature. 



