RARIFIED OA8BS 



U> the g* to slide must be to diminish the action of all tangential stresses 

 oa the surface, without affecting the normal stresses, and in course of time 

 to wt up current* MMpi* wr the surfaces of solid bodies, thus completely 

 <WtrorinjT the simplicity of oar first solution of the problem. 



14. Wbja ternal forces, such as gravity, act on the gas, and when the 

 thermal phenomena produce differences of density in different parts of the vessel, 

 flp the well-known convection currents are set up. These also interfere with 

 the simplicity of the problem and introduce very complicated effects. All that 

 we know is that the rarer the gas and the smaller the vessel the less is 

 the effect of the convection currents, so that in Mr Crookes' experiments 

 they play a very small part 



We now proceed to the calculations : 



(1) Encounter between Two Molecules. 



The motion of the two molecules after an encounter depends on their 

 motion before the encounter, and is capable of being determined by purely 

 dynamical methods. If the encounter of the molecules does not cause rotation 



>r vibration in the individual molecules, then the kinetic energy of the centres 

 of "H"P of the two molecules must be the same after the encounter as it was 

 before. 



This will be true on the average, even if the molecules are complex 

 systems capable of rotation and internal vibration, provided the temperature is 

 constant. If, however, the temperature is rising, the internal energy of the 

 molecules is, on the whole, increasing, and therefore the energy of translation 



it" their centres of mass must be, on an average, diminishing at every encounter. 

 The reverse will be the case if the temperature is falling. 



But however important this consideration may be in the theory of specific 

 heat and that of the conduction of heat, it has only a secondary bearing on 

 the question of the stresses in the medium ; and as it would introduce great 

 complexity and much guesswork into our calculations, I shall suppose that the 

 jzas here considered is one the molecules of which do not take up any sensible 

 amount of energy in the form of internal motion. Kundt and Warburg* 

 have shewn that this is the case with mercury gas. 



* Pogg. Ann., clvii. 1876, p. 353. 



