358 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 

 Gay-Lussac'sZ<TZ/0/' Volumes may be summed up as follows : 



" In every chemical change in which gases are concerned, 

 the volumes absorbed and liberated are in simple ratios to one 

 another." 



This law is not exact, as the following examples show : 



Hydrogen : oxygen = 2*00268 : i 



Hydrogen : hydrogen chloride = 1*00790 : 2 

 Hydrogen : nitrogen = 3*00172 : i 



It would probably be true at zero pressure, the deviations 

 shown above being due to the lesser compressibility of hydro- 

 gen as compared with the other gases. 



C. AVOGADRO'S HYPOTHESIS. 



Dalton, about 1802, whilst trying to form a mental picture of 

 the atmosphere, concluded that the particles of nitrogen, oxygen, 

 water- vapour, etc., were " all of the same size," in the sense that 

 each occupied an equal space in the gas. Later he was obliged 

 to reject this view, because nitric oxide occupied as much space 

 as the nitrogen and oxygen of which it was composed ; the con- 

 traction, which must accompany the production of one " com- 

 pound atom " from two or more " simple atoms " of equal volume, 

 did not in fact occur. 



Avogadro, in i8ir, got over this difficulty by suggesting that 

 the molecules of a simple elementary gas might contain two or 

 more atoms. Thus, if the decomposition of nitric oxide were 

 represented by the equation 



2NO -> N 2 + O 2 



2 mols. i inol. i mol. 



it would produce no change in the number of molecules and no 

 alteration in the volume of the gas. 



The possibility of halving the molecules of oxygen and 

 nitrogen is confirmed by the production of 2 volumes of steam 

 from i volume of oxygen and of I volume of nitrogen from 2 

 volumes of ammonia. 



The halving of the hydrogen molecule was proved by the 

 presence of i^ volumes of hydrogen in i volume of ammonia, 



