MOLECTTLES. 



If we wish to form a mental representation of what is going on among 

 the molecules in calm air, we cannot do better than observe a swarm of bees, 

 when every individual bee is flying furiously, first in one direction and then 

 in another, while the swarm, as a whole, either remains at rest, or sails slowly 

 through the air. 



In certain seasons, swarms of bees are apt to fly off to a great distance, 

 and the owners, in order to identify their property when they find them on 

 other people's ground, sometimes throw handfulls of flour at the swarm. Now 

 let us suppose that the flour thrown at the flying swarm has whitened those 

 bees only which happened to be in the lower half of the swarm, leaving those 

 in the upper half free from flour. 



If the bees still go on flying hither and thither in an irregular manner, 

 the floury bees will be found in continually increasing proportions in the upper 

 part of the swarm, till they have become equally diffused through every part 

 of it. But the reason of this diffusion is not because the bees were marked 

 with flour, but because they are flying about. The only effect of the marking 

 is to enable us to identify certain bees. 



We have no means of marking a select number of molecules of air, so 

 as to trace them after they have become diffused among others, but we may 

 communicate to them some property by which we may obtain evidence of their 

 diffusion. 



For instance, if a horizontal stratum of air is moving horizontally, molecules 

 diffusing out of this stratum into those above and below will carry their 

 horizontal motion with them, and so tend to communicate motion to the 

 neighbouring strata, while molecules diffusing out of the neighbouring strata 

 into the moving one will tend to bring it to rest. The action between the 

 strata is somewhat like that of two rough surfaces, one of which slides over 

 the other, rubbing on it. Friction is the name given to this action between 

 solid bodies ; hi the case of fluids it is called internal friction, or viscosity. 



It is, in fact, only another kind of diffusion a lateral diffusion of momentum, 

 and its amount can be calculated from data derived from observations of the 

 first kind of diffusion, that of matter. The comparative values of the viscosity 

 of different gases were determined by Graham in his researches on the tran- 

 spiration of gases through long narrow tubes, and their absolute values have 

 been deduced from experiments on the oscillation of discs by Oscar Meyer and 

 myself. 



