230 A MANUAL OF PHYSIOLOGY 



gas is in a closed vessel, the molecules will be constantly striking its 

 sides and rebounding from them. If a very small opening is made 

 in the vessel, some molecules will occasionally hit on the opening 

 and escape altogether. If the opening is made larger, or the experi- 

 ment continued for a longer time with the small opening, all the 

 molecules will in course of time have passed out of the vessel into 

 the air, while molecules of the oxygen, nitrogen, and argon of the air 

 will have passed in. In a gas, then, not enclosed by impenetrable 

 boundaries, there is no restriction on the path which a molecule may 

 take, no tendency for it to keep within any limits. 



When two chemically indifferent gases are placed in contact with 

 each other, diffusion will go on till they are uniformly mixed. The 

 diffusion of gases may be illustrated thus . Suppose we have a, 

 perfectly level and in every way uniform field divided into two 

 equal parts by a visible but intangible line, the well-known whitewash 

 line, for instance. On one side of the line place 500 blind men in 

 green, and on the other 500 blind men in red. At a given signal let 

 them begin to move about in the field. Some of the men in green 

 will pass over the line to the ' red ' side ; some of the men in red 

 will wander to the ' green ' side. Some of the men may pass over^ 

 the line and again come back to the side they started from. But, 

 upon the whole, after a given interval has elapsed, as many green 

 coats will be seen on the red side as red coats on the green. And if 

 the interval is long enough there will be at length about 250 men in 

 red and 250 in green on each side of the boundary-line. When this 

 state of equilibrium has once been reached, it will henceforth be 

 maintained, for, upon the whole, as many red uniforms will pass 

 across the line in one direction, as will recross it in the other. 



In a liquid it is very different ; the molecule has no free path. In 

 the depth of the liquid no molecule ever gets out of the reach of 

 other molecules, although after an encounter there is no tendency to 

 return on the old path rather than to choose any other; so that any 

 molecule may wander through the whole liquid. Although the 

 average velocity of the molecules .is much less in the liquid state 

 than it would be for the same substance in the state of gas or vapour 

 (gas in presence of its liquid), some of them may have velocities 

 much above the average. If any of these happen to be moving near 

 the surface and towards it, they may overcome the attraction of the 

 neighbouring molecules and escape as vapour, But if in their 

 further wanderings they strike the liquid again, they may again 

 become bound down as liquid molecules. And so a constant inter- 

 change may take place between a liquid and its vapour, or between 

 a liquid and any other gas, until the state of equilibrium is reached, 

 in which on the average as many molecules leave the liquid to 

 become vapour as are restored by the vapour to the liquid, or as 

 many molecules of the dissolved gas escape from solution as enter 

 into it. 



For the sake of a simple illustration, let us take the case of a 

 shallow vessel of water originally gas-free, standing exposed to the 

 air. It will be found after a time that the water contains the atmo- 



