ILLUSTRATIONS OF THE DYNAMICAL THEORY OF GASES. 405 



copper of equal thickness. It would be almost impossible to establish the value 

 of the conductivity of a gas by direct experiment, as the heat radiated from the 

 sides of the vessel would be far greater than the heat conducted through the 

 air, even if currents could be entirely prevented*. 



PART III. 



ON THE COLLISION OF PERFECTLY ELASTIC BODIES OF ANY FORM. 



When two perfectly smooth spheres strike each other, the force which acts 

 between them always passes through their centres of gravity ; and therefore their 

 motions of rotation, if they have any, are not affected by the collision, and 

 do not enter into our calculations. But, when the bodies are not spherical, 

 the force of compact will not, in general, be in the line joining their centres 

 of gravity ; and therefore the force of impact will depend both on the motion 

 of the centres and the motions of rotation before impact, and it will affect 

 both these motions after impact. 



In this way the velocities of the centres and the velocities of rotation 

 will act and react on each other, so that finally there will be some relation 

 established between them ; and since the rotations of the particles about their 

 three axes are quantities related to each other in the same way as the three 

 velocities of their centres, the reasoning of Prop. IV. will apply to rotation as 

 well as velocity, and both will be distributed according to the law 



dN A7 1 --, 



-j- =N 7=6 '. 

 ax av /7r 



* [Clausius, in the memoir cited in the last foot-note, lias pointed out two oversights in this 

 calculation. In the first place the numbers have not been properly reduced to English measure, 

 and have still to be multiplied by '4356, the ratio of the English pound to the kilogramme. The 

 numbers have, further, been calculated with one hour as the unit of time, whereas Maxwell has 

 used them as if a second had been the unit. Taking account of these circumstances and using his 

 own expression for the conduction which differs from (59) only in having ^ in place of J.on the 

 right-hand side, Clausius finds that the resistance of a stratum of air to the conduction of heat is 

 1400 times greater than that of a stratum of lead of the same thickness, or about 7000 times greater 

 than that of copper. Ed.] 



