THE CELL 



51 



the coloured liquid in the beaker. I lift up the bell now ; 

 the conditions are altogether reversed ; hydrogen is 

 now inside the vessel, air outside it ; hydrogen moves 

 towards the outside, air passes in ; but particles of 

 hydrogen move more quickly than those of air, so the 

 volume inside the apparatus decreases, and you notice 

 the red liquid rising quickly in the glass tube (b) . 



Therefore gases, even more than liquids, are capable 

 of diffusion, i.e. are capable of permeating all spaces as 

 yet unoccupied by them. The hydrogen rushed into 



Fig. 18. 



the vessel only because the latter contained no hydrogen, 

 and later rushed out of the vessel only because none 

 was present in the air of this hall. Likewise all gaseous 

 matter and also matter dissolved in liquids tends to 

 occupy the whole space accessible to it, and to spread 

 uniformly through it. 



Now let us see in what relation the phenomena of the 

 diffusion of gases and liquids stand to our question 

 concerning the nutrition of the cell. Here is an appar- 

 atus reminding us pretty closely of a cell (fig. 18). It 

 is a thin bladder moistened with water and transparent 

 as glass, made out of a substance like cellulose or 

 rather actually made of cellulose itself, only a little 



