342 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



regarded the molecule of water as " composed of a half- 

 molecule of oxygen with one molecule, or, what is the 

 same thing, two half-molecules of hydrogen" (A.C.R. IV. 

 footnote, p. 32). 



Nitric Oxide and Ammonia In the same way Avogadro 

 was able to show that since : 



1 volume of oxygen and i volume of nitrogen are 

 present in 2 volumes of nitrous gas 



the molecule of nitrous gas is " composed of a half- 

 molecule of oxygen and a half-molecule of nitrogen " 

 (A.C.R. IV. 36). So too in the case of ammonia, 

 Berthollet's observation that : 



2 volumes of ammonia are decomposed by sparking into 

 i volume of nitrogen and 3 volumes of hydrogen 



led Avogadro to conclude that the molecule of ammonia 

 is composed of one half-molecule of nitrogen combined with 

 three half-molecules of hydrogen. 



Muriatic Acid. Another case quoted by Avogadro is 

 that of gaseous muriatic acid. 



" It follows from the experiments both of Gay-Lussac and 

 Thenard, and of Davy, that muriatic acid gas is formed by 

 the combination of equal volumes of [chlorine] and 

 hydrogen, and that its volume is equal to their sum. This 

 means, according to our hypothesis, that muriatic acid is 

 formed of these two substances united molecule to molecule, 

 with halving of the molecule, of which we have had so 

 many examples." (A.C.R. IV. 44.) 



Here since : 



i volume of hydrogen and i volume of chlorine give 2 

 volumes of hydrogen chloride, 



the molecule of hydrogen chloride is composed of one 

 half-molecule of hydrogen and one half-molecule of chlorine. 



