THE RADIOMETER. g 



creased velocity, and it is this more rapid motion which alone consti- 

 tutes the higher temperature. 



Consider, next, what must be the effect on the surface. A moment's 

 reflection will show that the normal pressure exerted by the molecular 

 storm, always raging in the atmosphere, is due not only to the impact 

 of the molecules, but also to the reaction caused by their rebound. 

 When the molecules rebound they are, as it were, driven away from the 

 surface in virtue of the inherent elasticity both of the surface and of the 

 molecules. Now, what takes place when one mass of matter is driven 

 away from another when a cannon-ball is driven out of a gun, for 

 example? Why, the gun kicks! And so every surface from which 

 molecules rebound must kick; and, if the velocity is not changed by the 

 collision, one-half of the pressure caused by the molecular bombardment 

 is due to the recoil. From a heated surface, as we have said, the mole- 

 cules rebound with an increased velocity, and hence the recoil must be 

 proportionally increased, determining a greater pressure against the 

 surface. 



According to this theory, then, we should expect that the air would 

 press unequally against surfaces at different temperatures, and that, 

 other things being equal, the pressure exerted would be greater the 

 higher the temperature of the surface. Such a result, of course, is 

 wholly contrary to common experience, which tells us that a uniform 

 mass of air presses equally in all directions and against all surfaces of 

 the same area, whatever may be their condition. It would seem, then, ' 

 at first sight, as if we had here met with a conspicuous case in which 

 our theory fails. But further study will convince us that the result is 

 just what we should expect in a dense atmosphere like that in which we 

 dwell ; and, in order that this may become evident, let me next call 

 your attention to another class of molecular magnitudes. 



It must seem strange indeed that we should be able to measure 

 molecular velocities, but the next point I have to bring to your notice 

 is stranger yet, for we are confident that we have been able to deter- 

 mine with approximate accuracy for each kind of gas-molecule the 

 average number of times one of these little bodies runs against its 

 neighbors in a second, assuming, of course, that the conditions of the 

 gas are given. Knowing, now, the molecular velocity and the number 

 of collisions a second, we can readily calculate the mean path of the 

 molecule that is, the average distance it moves, under the same con- 

 ditions, between two successive collisions. Of course, for any one 

 molecule, this path must be constantly varying; since, while at one 

 time the molecule may find a clear coast and make a long run, the very 

 next time it may hardly start before its course is arrested. Still, 

 taking a mass of gas under constant conditions, the doctrine of aver- 

 ages shows that the mean path must have a definite value, and an illus- 

 tration will give an idea of the manner in which we have been able to 

 estimate it. 



