144 MOLECULAR MOTION AND ITS ENERGY 60 



/ as from the table in 55 we see is really the case for some 

 gases. 



For gases whose molecules contain three or more atoms the 

 number of degrees of freedom q is to be taken still greater, 

 since the structure of such a molecule cannot be imagined 

 to be that of a figure of revolution. The position of such a 

 figure as a triangle or tetrahedron is not completely deter- 

 mined when only the position of its centroid and the 

 direction of one axis are given ; there are thus more 

 elements required for its determination. If, therefore, we 

 assume q to be greater than 5 or 6, the ratio of C to c 

 becomes smaller, and with increasing q approaches more 

 and. more the limit 1, as observation also teaches. 

 y This theory therefore gives a satisfactory account of all 

 the principal circumstances. In spite of this we cannot 

 think that by it the question has been exhaustively treated. 

 For there is a weighty objection to this theory, which H._T 1 

 Eddy l has pointed out. Since the bonds of the atoms by 

 which they are bound together in the molecule allow of 

 neither perfect freedom nor perfect fastness, it does not seem 

 admissible simply to count the kinds of movability ; the 

 7^ degrees of freedom cannot be introduced as all of equal 

 value, but must be brought into the calculation differently 

 weighted. An atom in a molecule has not the same degree 

 of freedom of its motions as the centroid of the molecule, 

 and a limited freedom must not be counted as a perfectly 

 unlimited freedom. 



As we shall easily see, this objection amounts to the 

 /same thing as the opinion already mentioned in 56, that 

 the energy of the limited motion of an atom cannot be equal 

 to the energy of a molecule, but must be smaller an 

 opinion which in the first edition of this book was shown to 

 be in accordance with experiment. 



On these grounds, at the Aberdeen meeting of the British 

 Association in 1885, a great number of prominent investi- 

 gators denied, or at least threw doubt upon, the validity of 



1 Sclent. Proc. Ohio Mech. Inst. 1883, p. 42 ; Journ. Franklin Inst. [3] 

 Ixxxv. pp. 339, 409 ; Ohio Mech. Inst. 1883, p. 82. 



