56 THE VEGETATIVE FUNCTIONS OF PLANTS 
sweet, showing that some of the dissolved sugar had 
passed, by Us own motion, up through the mass of the 
water to the top. This simple experiment illustrates diffu- 
sion of liquids. The dissolved sugar behaves in a manner 
quite similar to that of the gaseous "odor" of the musk. 
63. Osmosis. If, now, the denser sugar solution in the 
bottom of the tumbler were separated from the less 
dense water above by a porous membrane, such as a piece 
of bladder or parchment, the diffusion would take place 
through the porous membrane, and the water above would 
soon become sweet, as in the previous case. In other 
words, it is possible to have diffusion through a membrane. 
Diffusion through a membrane is osmosis. 
The conditions realized in the experiment described 
above are a denser liquid (in the bottom of the tumbler), 
separated from a less dense liquid (at the top of the 
tumbler) by a porous membrane. Moreover, not only 
would the sugar solution pass up through the membrane, 
but the water above would pass in the opposite direction, 
and more rapidly than the sugar solution. This would 
continue until the solution was of the same density 
(equal amounts of sugar in equal amounts of water) 
on both sides of the membrane. Thus the action of 
osmosis may be stated as follows : 
When two fluids (liquids or gases) of different densities 
are separated by a porous membrane, diffusion through the 
membrane will take place until equilibrium results. The 
diffusion will be more' rapid from the less dense to the more 
dense fluid. 
Or, again, osmosis may be denned as the interchange oj 
two fluids of different densities when separated by a porous 
membrane. 
