Wells £ Foote — One Hundred Years of Chemistry. 273 



A Lack of Confidence in Avogadro's Principle. — One 

 reason why chemists were so slow in arriving at the 

 correct atomic weights and formulas was a partial loss 

 of confidence in Avogadro's principle. About 1826 the 

 young French chemist Dumas devised an excellent 

 method for the determination of vapor densities at high 

 temperatures, and his results and those of others showed 

 some discrepancies in the expected densities. For 

 example, the vapor density of sulphur was found to be 

 about three times too great, that of phosphorus twice too 

 great, that of mercury vapor and that of ammonium 

 chloride only about half large enough to correspond to 

 the values expected from analogy and other considera- 

 tions. Thus, one volume of oxygen with two volumes of 

 hydrogen make two volumes of steam, but only one-third 

 of a volume of sulphur vapor was found to unite with 

 two volumes of hydrogen to make two volumes of hydro- 

 gen sulphide. Berzelius saw clearly that the results 

 pointed to the existence of such molecules as S 6 , P 4 , and 

 Hg 1? but it was not generally realized in those days that 

 Avogadro 's rule is fundamentally reliable, and Berzelius 

 himself appears to have lost confidence in it on account 

 of these complications, for he did not apply Avogadro's 

 principle to decisions about atomic weights except in the 

 cases of substances gaseous at ordinary temperatures. 



Electro-chemical Theories. — The observation was 

 made by Nicholson and Carlisle in 1800 that water 

 was decomposed into its constituent gases by the 

 electric current. Then in 1803 Berzelius and Hisinger 

 found that salts were decomposed into their bases and 

 acids by the same agency, and in 1807 Davy isolated 

 potassium, sodium, and other metals afterwards, by a 

 similar decomposition. Since those early times a vast 

 amount of attention has been paid to the relation of 

 electricity to chemical changes, a relation that is evi- 

 dently of great importance from the fact that while 

 electric currents decompose chemical compounds, these 

 currents, on the other hand, are produced by chemical 

 reactions. 



Berzelius was particularly prominent in this direc- 

 tion, and in 1819 he published an elaborate electro-chem- 

 ical theory. He believed that atoms were electrically 

 polarized, and that this was the cause of their combina- 

 tion with one another. He extended this idea to groups 



