338 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



have ever made, gave 1*97 hydrogen to i oxygen " (A.C.R. 

 IV. 27). 



C. AVOGADRO'S HYPOTHESIS 



Avogadro (1811) revives Dalton's discarded hypo- 

 thesis. The suggestion as to the spacing of gaseous atoms, 

 which Dalton had considered and rejected in 1808, was 

 revived three years later, in 1811, by Amadeo Avogadro, 

 (1776-1856) Professor of Physics at Turin, as the most 

 obvious, perhaps even the only possible, method of 

 explaining Gay-Lussac's Law of Volumes. AVOGADRO'S 

 HYPOTHESIS, as it is now generally called, states that : 



" When their temperatures and pressures are equal, equal 

 volumes of different gases contain equal numbers of 

 molecules" 



Later researches have proved the substantial truth, as 

 well as the usefulness of this hypothesis. But it is now 

 recognised that (like Gay-Lussac's Law of Volumes) it is 

 strictly accurate only when applied to gases under very low 

 pressures ; if applied to gases at atmospheric pressures, a 

 correction must be made for their unequal compressi- 

 bilities. 



Avogadro postulates complex molecules in gaseous 

 elements. The merit of Avogadro's paper on the " Relative 

 Masses of Elementary Molecules," consists in the fact that 

 he was able to find a remedy for the defects which 

 had proved fatal to the hypothesis as discussed and 

 condemned by Dalton. Avogadro suggested that all 

 difficulties would disappear if the molecules of an elementary 

 gas consisted of groups of atoms, similar to those which 

 formed the molecules of a compound gas. 



If the molecules of an element were indivisible they 

 could never give rise to more than an equal number of 



