142 MOLECULAR MOTION AND ITS ENERGY 60 



three coordinates of its centroid and by two angles, which 

 determine the direction of the joining line of the two atoms. 

 The formula therefore becomes 



I 



c 5(1 + h)' 



and gives the value 



which has been found for atmospheric air and other 

 diatomic gases if for these gases also may be put 



We should arrive at this same result also if we considered 

 v the molecules to be rigid bodies, shaped like any figures of 

 revolution we like ; for their place and position in space are 

 also determined by five coordinates. 



But against this Pirogoff l has raised the justifiable 

 objection that a figure of revolution can rotate not only 

 about its axis of symmetry, but also about another axis at 

 right angles to the first as well. A figure of revolution and 

 also a diatomic molecule would thus have more than five 

 kinds of free movability. 



In spite of this objection, we may suppose that inside a 



diatomic molecule no shifting of the atoms towards each 



other occurs when the heating is kept within the limits 



p> within which the observations have been made. For this 



r speaks the fact that for diatomic gases at least for atmo- 



spheric air and carbon monoxide the value of the ratio 



varies with the temperature only very inconsiderably. 



If, therefore, the molecules of diatomic gases behave as 

 solid bodies, in which the parts suffer no relative motion, 

 only such motions of the atoms as consist in a common 

 rotation of all the atoms about their centroid can occur in 

 addition to the translatory motion of the molecule as a 

 whole. In monatomic molecules, as we have seen, such 

 rotations do not also occur ; in their case there is nothing 

 but the rectilinear translatory motion of the centroid. 



1 Fortschr. d. Physik. 1886, 2. Abth. p. 247. 



