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SCIENCE PROGRESS 



length. It is well to remember that the picture we have in 

 mind at this stage is that of a molecule as a small sphere, which 

 can collide with and rebound from other spheres. That is 

 to say, we are thinking of a molecule as possessing a definite 

 though extremely minute surface. In the next section we 

 shall see that this idea is too crude, for, when we come to think 

 of it, a surface is, after all, a geometrical and not a physical 

 concept. 



The next magnitude with which we have to deal is that 

 known as the mean free path of a molecule in the gaseous 

 state, that is, the mean or average distance which a molecule 

 traverses between two consecutive collisions with other mole- 

 cules. In the following table are given the values of the mean 

 free paths of a few gases at normal temperature and pressure 

 together with the values of the collision frequency, that is, 

 the number of collisions which on the average any single mole- 

 cule experiences in one second. This latter quantity is ex- 

 tremely large, one hydrogen molecule, for example, colliding 

 with its neighbours some nine thousand million times per 

 second. 



It will be observed that under moderate pressures the 

 mean free path of a molecule is very much greater than the 

 actual diameter of a molecule, in fact, several hundred times 

 as great. Under these conditions, therefore, the molecule is 

 very small compared with the mean free space allotted to it, 

 so that, in a preliminary treatment of the gaseous state, it 

 is quite justifiable to neglect the actual volume of the mole- 

 cules themselves compared with the total volume occupied by 

 the gas. With a simplification such as this it is an easy 

 matter to arrive at the well-known gas law, PV = RT, where 

 P is the pressure exerted by the gas, V its total volume, T 

 the absolute temperature, and R a constant having the same 

 numerical value for one grammolecular weight of any gas. 

 If, however, the pressure be greatly increased, say by com- 

 pressing the gas, the mean free path will diminish, and as the 



