THE RATIO OF THE SPECIFIC HEATS OF GASES. 281 



Theory of Gases, second edition, p. 49)and others have pointed 

 out. It is generally assumed that the perfect elasticity of 

 the molecules is a consequence of their not actually hitting 

 each other in an encounter, but circling round each other 

 under the influence of a mutual attraction or repulsion, 

 like a comet round the sun. If the least distance between 

 two molecules during an encounter is great compared with 

 their size, the resultant force will pass approximately 

 through the centre of mass of each, and very little 

 energy of rotation will be communicated to either by the 

 encounter, so that any appreciable change in the rotational 

 energy may require a much longer time than the very 

 small fraction of a second afforded by the sound waves used 

 in Kundt's method. 



The important question, however, is whether a poly- 

 atomic gas can have /3 equal to zero. It is difficult to 

 believe that this could be the case, when of the forty certainly 

 polyatomic gases whose y's are known not one has a value 

 of /3 anything approaching zero. With the exception of 

 mercury, /3 is least for the diatomic gases, but even here it 

 is "67, and for the. gases of higher atomicity it reaches 3 or 

 4. The greater the complexity of the molecule the greater 

 is the internal energy, and for no capacity for internal 

 energy we seem to be driven to assume the simplest 

 possible molecule — a single atom. 



After all, in our present state of ignorance regarding 

 the status of the atom in the molecule, the argument is 

 little more than an argument from analogy. Such an 

 argument is the stronger, the greater the number of points 

 of agreement between the two objects or phenomena com- 

 pared, and the fewer the points of disagreement. The 

 question at issue seems to be just the sort of case where the 

 argument may break down, for argon differs in such a 

 remarkable way from all other known substances that it 

 would be unsafe to deny the possibility of further eccen- 

 tricities in the dynamics of its molecule. Consider the 

 diatomic gases in the table. Six of them have y near 1*41, 

 and hence /3 near '67 — four have y near 1*3, and hence /3 

 near 1*2. If such a large range as this is possible with 



