of Gases and Molecular Force. 509 



The object of the present paper is to show that the whole 

 of the discrepancy between theory and experiment disappears 

 if in the theory account is taken of molecular force. Accord- 

 ing to the usual presentation of the kinetic theory, the 

 molecules are supposed to be spheres colliding with coefficient 

 of restitution unity, molecular force is neglected because at 

 the average distance apart of the molecules in a gas it is very 

 small. Sow molecular attraction has been proved to exist, 

 and, though negligible at the average distance apart of mole- 

 cules in a gas, it is not negligible when two molecules are 

 passing quite close to one another, it can cause two molecules 

 to collide which in its absence might have passed one another 

 without collision ; and the lower the velocities of the mole- 

 cules, the more effective does molecular force become in 

 bringing about collisions which would be avoided in its 

 absence : thus molecular force cannot be neglected in inves- 

 tigating the relation between viscosity and molecular velocity 

 or temperature. 



Molecular force alone without collisions will not cany us 

 far in the explanation of viscosity of gases as known to us in 

 nature, because in all experiments, on the viscosity of gases 

 there is a solid body which either communicates to the gas 

 motion parallel to its surface or destroys such motion, so that 

 the molecules of the gas must collide with the molecules of 

 the solid; for if the molecules of gas and solid act on one 

 another only as centres of force, then each molecule of gas 

 when it comes out of the range of the molecular force of the 

 solid must have the same kinetic energy as when it went in, 

 so that without collision between molecules of gas and solid 

 there can be no communication of motion to the gas. If, 

 then, molecules of gas and solid collide, molecules of gas 

 must collide amongst themselves. 



Of course, if this difficulty about communicating motion to 

 a number of centres of force is ignored, then, as Maxwell does, 

 we can proceed to trace viscosity in the gas as due to the fact 

 that when two centres pass close to one another they deflect 

 each other's path through an angle depending on the rela- 

 tive velocity and nearness of approach: thus the centres which 

 leave the surface of a moving solid with their thermal velo- 

 cities of agitation compounded with the velocity v of the solid, 

 have the resultant velocities deflected in so haphazard a 

 manner that at a certain distance from the solid they are 

 uniformly distributed in all directions, and thus the energy 

 of the velocity v is converted into heat, and there is viscous 

 action between the successive layers of gas. And this holds 

 whether the force be attractive or repulsive ; hence we see 



