2 4 RESPIRA TION 



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 interchange may take place between a liquid and its 

 vapour, or between a liquid and any other gas, until the state of equi- 

 librium 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 atmospheric gases in 

 certain proportions in round numbers, about T foj of its volume of 

 oxygen and G of its volume of nitrogen (measured at 760 mm. mercury 

 and o C.). 



Now, let a similar vessel of gas-free water be placed in a large airtight 

 box filled with air at atmospheric pressure, and let the oxygen be all 

 absorbed before the water is exposed to the atmosphere of the box. 

 The latter now consists practically only of the nitrogen of the air, and 

 its pressure will be only about four-fifths that of the external atmo- 

 sphere. Nevertheless, the quantity of nitrogen absorbed by the water 

 will be exactly the same as was absorbed from the air. If the box 

 was completely exhausted, and then a quantity of oxygen, equal to that 

 in it at first, introduced before the water was exposed to it, the pressure 

 would be found to be only about one-fifth that of the external atmo- 

 sphere; but the quantity of oxygen taken up by the water would be 

 exactly equal to that taken up in the first experiment. 



