THE ETI1ER AND PONDERABLE MATTER 



the size of molecules. One of these is based upon the 

 phenomena of contact electricity ; another upon the 

 wave-theory of light; and another upon capillary at- 

 traction, as shown in the tense film of a soap-bubble ! 

 No one of these methods gives results more definite than 

 that due to the kinetic theory of gases, just outlined ; 

 but the important thing is that the results obtained by 

 these different methods (all of them due to Lord Kelvin) 

 agree with one another in fixing the dimensions of the 

 molecule at somewhere about the limits already men- 

 tioned. We may feel very sure indeed, therefore, that 

 the ultimate particles of matter are not the unextended, 

 formless points which Boscovich and his followers of the 

 last century thought them. 



IV 



Whatever the exact form of the molecule, its outline is 

 subject to incessant variation ; for nothing in molecular 

 science is regarded as more firmly established than that 

 the molecule, under all ordinary circumstances, is in a 

 state of intense but variable vibration. The entire en- 

 ergy of a molecule of gas, for example, is not measured 

 by its momentum, but by this plus its energy of vibra- 

 tion and rotation, due to the collisions already referred 

 to. Clausius has even estimated the relative importance 

 of these two quantities, showing that the translational 

 motion of a molecule of gas accounts for only three- 

 fifths of its kinetic energy. The total energy of the 

 molecule (which we call " heat ") includes also another 

 factor, namely, potential energy, or energy of position, 

 due to the work that has been done on expanding, in 

 overcoming external pressure, and internal attraction 



245 



