THE KINETIC THEORY OF MATTER. 133 



the force is doubled, or TJ is a constant. Anything which hurries the 

 rearrangement will lessen the viscosity. For instance, we have every 

 reason to suppose that rise of temperature increases the energy of the 

 molecules, and therefore enables them to get free from each other more 

 frequently. This agrees with our knowledge that the viscosity of a 

 liquid decreases as the temperature rises. 



Kinetic Theory Of Gases. In applying the kinetic theory to 

 gases we can go into much greater detail. The molecules in a gas are 

 much farther apart on the average than in solids or liquids. A 

 cubic centimetre of water at 100 C. forms about 1600 cubic centi- 

 metres of steam at 100 C. and 760 mm. pressure. Hence, the molecules 

 of the steam are about \/1600, say 12 times as far apart as those of the 

 water. And this increased distance results in an almost complete 

 absence of cohesion, as is shown by the experiments of Gay-Lussac and 

 Joule (p. 119), in which a gas is allowed to expand without doing 

 external work. The change of temperature is only infinitesimal, showing 

 that practically no work is done against the mutual actions between the 

 molecules. The extreme rapidity of gaseous diffusion shows that the 

 molecules are in very rapid motion. But as there are an enormous 

 number of molecules, even in a very small space, and as they are not 

 mere points, but have some volume of their own, they must continually 

 be colliding with each other, and under the term "collision" we must 

 include every case in which two molecules come sufficiently within each 

 other's sphere of action to influence each other's motion. We need not 

 necessarily suppose that in a collision the force between two molecules is 

 repulsive. We may illustrate this by the motion of a comet which comes 

 into our system from outer space. Drawn by the sun's attraction it 

 rushes inwards, travels round the sun, and then rushes away to outer 

 space again. From our present point of view this is a " collision," and 

 the collision of molecules may perhaps be similar. The collisions in gas at 

 ordinary pressure cannot, however, occupy a very appreciable fraction of 

 the time of travelling about, for this would be inconsistent with the 

 absence of cohesion. When, in Joule's experiment, there are 22 times 

 as many molecules in a given space as in ordinary air, there are many 

 more collisions in a given time for each molecule ; in fact, the number 

 for a given gas per second is nearly proportional to its density. If these 

 collisions occupied an appreciable fraction of the time in ordinary air 

 they would, therefore, occupy a still larger fraction in the com- 

 pressed air. At any one instant, then, an appreciable fraction of 

 the total number of molecules would be in collision, and hence work 

 would have to be done to separate them, that is, to lessen the number 

 within each other's sphere of action. In other words, there would be 

 cohesion. 



We shall, in the first place therefore, regard the collisions as instan- 

 taneously altering the velocities and directions of motion of the colliding 

 molecules. We shall not attempt to inquire what goes on in a collision, 

 and in speaking of the velocities of the molecules we shall regard only 

 the velocities when free from each other's actions. 



When a collision occurs, the velocities and directions of motion of 

 the two colliding bodies are changed ; but we shall suppose that the 

 total enei'gy of the motion of translation remains the same after the 



