292 THE REALITIES OF MODERN SCIENCE 



ergs, and hence R/2J is 0.99 calorie. For conven- 

 ience * in discussion we shall use the approximate 

 value of 1 calorie. We then expect the specific heat, 

 C v , to be (3+n)E/2J or (3+n) calories, where n is 

 the number of degrees of freedom which the molecule 

 has in excess of the three of translation. For a mona- 

 tomic molecule, therefore, we put n = 3 and expect 

 C v to be (3+n) or 6 calories. 



Specific heat, we notice, is merely the name for a 

 rate, that of change of the total molecular energy per 

 degree of temperature. We expect this rate for a 

 monatomic molecule to be 6 calories, independent 

 of the temperature. Now, what are the facts? In 

 the first place, we find that specific heats are not in 

 general independent of the temperature, as we shall 

 see in more detail later. In the second place, we find 

 that the specific heat of monatomic gases is practically 

 3 instead of 6 calories. For argon it has been measured 

 over a very wide range of temperature and found to be 

 constant and of value 2.98 cal. Apparently all the 

 energy required to raise a mole of argon 1 C. is that 

 needed to increase its kinetic energy of translation. 

 According, however, to the simple mechanical con- 

 siderations of the last few pages any impacts, except 

 head-on, must cause some rotation of the molecules 

 unless the latter happen to be smooth spheres. Of 

 course, if the argon atom was practically a geometrical 

 point, that is, infinitely small, it could not rotate. But 



1 This convenience may not be credited to the calorists. In this 

 connection it is interesting to note that before Joule's classical ex- 

 periments on the "mechanical equivalent of heat," J. R. Mayer had 

 reasoned from the relation, C P C V = external work, and had ob- 

 tained a value of the calorie in mechanical units. 



