XV THE MOLECULAR THEORY 353 



Avogadro and Cannizzaro as to the number of atoms 

 present in the molecules of various gases and vapours have 

 been confirmed in a very striking way by independent 

 physical methods. It is well known that the heat required 

 to raise the temperature of a gas is greater when the gas is 

 allowed to expand than when its volume is kept constant ; 

 the ratio y of the two heat-capacities can be calculated to be 

 5/3=1-67 for an ideal or "perfect gas." Experiment has 

 shown that this value is rarely attained in the case of 

 elements, and never in the case of compounds, and further 

 that the value of y diminishes as the number of atoms in 

 the molecule increases. Typical values are as follows : 



Substance. Ratio 7. 



Hg, He 1-67 



H 2 , N 2 , Oo, CO, HC1, HBr, HI 1-41 V 



C1 2 , Br 2 , I.,, C1I I -29 to I -32 / 



C0 2 , N 2 0,N0 2 i'3i 



It will be seen from the table that the value 1*4 is 

 characteristic for the lighter diatomic gases and the figure 

 i '3 for the heavier diatomic and the lighter triatomic gases, 

 whether elementary or compound. This observation supplies 

 a remarkable proof of the correctness of Avogadro's 

 fundamental proposition, that the molecules of the 

 elements nitrogen and oxygen are of the same complexity as 

 those of the compound nitric oxide, that hydrogen and 

 chlorine are as complex as hydrogen chloride, and so 

 forth. 



Special interest attaches to the case of mercury, which falls 

 into a special class, giving a value of y which is much higher 

 than the figure i '4 characteristic of the diatomic gases. In 

 the case of so heavy a molecule this can only mean that the 

 number of atoms is less than two ; the molecule must 



1 The value of 7 seems to depend on the moment of inertia of the 

 molecules : the hydride HI has probably a smaller moment of inertia 

 than C1 2 , although its weight is greater. 



A A 



