400 Prof. Thomson, On some experiments on the [May 10, 



the molecule takes to have its velocity reversed, the more likely 

 is it to be decomposed by the electric field. Now this time will 

 depend on the average time between two collisions and the in- 

 tensity of the force between the molecules : thus, for example, 

 if the molecules acted on each other like two perfectly elastic 

 spheres, that is, if when they collided they interchanged the 

 velocities parallel to the line joining their centres, the mean time 

 taken to reverse the motion of a molecule would be a little less 

 than twice the mean time between two collisions, for the motion 

 of the molecule will be reversed when it meets a molecule moving 

 in the opposite direction, and it is rather more likely to meet one 

 moving in the opposite direction than one moving in the same 

 direction as itself. Again, it will take longer to reverse the direc- 

 tion of motion when the changes brought about by a collision are 

 small than when they are large ; so that taking the ordinary view 

 of a collision as due to forces between the molecules, the greater 

 the forces between the molecules the smaller the time taken to 

 reverse the direction of motion. Other things however being the 

 same, it is evident that this time will vary inversely as the 

 number of collisions per unit time. The particles will be moving 

 very much longer in one direction when the gas is rare than when 

 it is dense ; so that the rarer the gas the more easily will the 

 molecules be dissociated : this explains why a rare gas is electrically 

 weaker than a dense one. There must however be a certain 

 density, such that if the rarefaction be carried beyond it the 

 electric strength of the gas will increase as the rarefaction in- 

 creases; for, according to our view, if there were no molecules 

 there would be nothing to discharge the energy : so that when the 

 density is zero the gas has infinite electric strength. This point 

 will depend, among other things, upon the energy required to 

 dissociate a molecule of the gas into atoms, other things being the 

 same ; the greater this energy the lower the critical pressure. It 

 will also depend upon the time during which the molecules are 

 moving in the same direction ; if we were to shorten this time 

 without altering the density of the gas or the energy required to 

 dissociate a molecule we should raise the critical pressure, for we 

 should hamper the efforts of the rarefaction to diminish the 

 electric strength, while we should not affect the causes which tend 

 to increase the strength. De la Hue and Miiller, Proceedings of the 

 Royal Society, 1882, have found that the size of the vessel con- 

 taining the gas has an effect upon the critical pressure. This is 

 just what we should expect if the above view were true, for the 

 size and shape of the tube will, when the pressure gets so low 

 that the distance traversed by the molecule while it is moving 

 continuously in one direction is comparable with the dimensions 

 of the tube, affect the time during which the molecules are 



