THE CELL 



49 



this narrow, hardly visible red line will broaden, and at 

 the end of this lecture will be several inches in breadth ; 

 while in several hours, or it may be days, the whole liquid 

 will be a uniform red colour. Apparently both liquids 

 interpenetrate each other. This depends on motion 

 — peculiar to their particles, and therefore invisible — 

 on their tendency to spread in space ; otherwise we 

 cannot explain how, in spite of the force of gravity, 

 the lighter particles sink to the bottom while the 

 heavier rise to the surface. 



Different substances are endowed in a different degree 

 with this property of diffusion — in 

 other words, particles of different 

 substances move with different 

 velocities. This is best demon- 

 strated with gases. This vessel 

 (fig. 17) made of very porous clay 



(a) is joined below to a glass tube 



(b) immersed at its lowest end in 

 water coloured red. Both vessel 

 and tube contain air. The object 

 of this apparatus is to demonstrate 

 the slightest change in the volume 

 of air, contained both in the vessel 

 and the tube. If by any chance 

 the volume increases, air will begin 

 to escape in bubbles through the 

 coloured liquid. On the other hand 

 if the volume of air in the apparatus 

 decreases, the coloured liquid will 

 rise in the tube. In the meantime 

 neither happens because the air in- 

 side the vessel is just like that 

 outside. But if we surround the 

 vessel with another kind of air, 

 with another gas, it is clear that a mutual interchange 

 of gases will take place through the porous wall, 



D 



Fig. 17. 



