MOLECULES. 



Another way of tracing the diffusion of molecules through calm air is to 

 heat the upper stratum of the air in a vessel, and to observe the rate at 

 which this heat is communicated to the lower strata. This, in fact, is a third 

 kind of diffusion that of energy, and the rate at which it must take place 

 was calculated from data derived from experiments on< viscosity before any direct 

 experiments on the conduction of heat had been made. Professor Stefan, of 

 Vienna, has recently, by a very delicate method, succeeded in determining the 

 conductivity of air, and he finds it, as he tells us, in striking agreement with 

 the value predicted by the theory. 



All these three kinds of diffusion the diffusion of matter, of momentum, 

 and of energy are carried on by the motion of the molecules. The greater 

 the velocity of the molecules and the further they travel before their paths are 

 altered by collision with other molecules, the more rapid will be the diffusion. 

 Now we know already the velocity of the molecules, and therefore, by experiments 

 on diffusion, we can determine how far, on an average, a molecule travels without 

 striking another. Professor Clausius, of Bonn, who first gave us precise ideas 

 about the motion of agitation of molecules, calls this distance the mean path 

 of a molecule. I have calculated, from Professor Loschmidt's diffusion experi- 

 ments, the mean path of the molecules of four well-known gases. The average 

 distance travelled by a molecule between one collision and another is given in 

 the table. It is a very small distance, quite imperceptible to us even with 

 our best microscopes. Roughly speaking, it is about the tenth part of the 

 length of a wave of light, which you know is a very small quantity. Of 

 course the time spent on so short a path by such swift molecules must be 

 very small. I have calculated the number of collisions which each must undergo 

 in a second. They are given in the table and are reckoned by thousands of mil- 

 lions. No wonder that the travelling power of the swiftest molecule is but small, 

 when its course is completely changed thousands of millions of times in a second. 



The three kinds of diffusion also take place in liquids, but the relation 

 between the rates at which they take place is not so simple as in the case 

 of gases. The dynamical theory of liquids is not so well understood as that 

 of gases, but the principal difference between a gas and a liquid seems to be 

 that in a gas each molecule spends the greater part of its time in describing 

 its free path, and is for a very small portion of its time engaged in encounters 

 with other molecules, whereas, in a liquid, the molecule has hardly any free 

 path, and is always in a state of close encounter with other molecules. 



VOL. II. 47 



