272 Common Science 



Diffusion in liquids. Diffusion takes place in liquids, 

 as you know. For when you put sugar in coffee or tea 

 and do not stir it, although the upper part of the tea or 

 coffee is not sweetened, the part nearer the sugar is 

 very sweet. If you should let the coffee or tea, with 

 the sugar in the bottom, stand for a few months, it 

 would get sweet all through. Diffusion is slower in 

 liquids than in gases, because the molecules are so very 

 much closer together. 



Osmosis. One of the most striking and important 

 facts about diffusion is that it can take place right 

 through a membrane. Try this experiment : 



Experiment 86. With a rubber band fasten a piece of 

 parchment paper, made into a little bag, to the end of a 

 piece of glass tubing about 10 inches long. Or make a small 

 hole in one end of a raw egg and empty the shell ; then, to 

 get the hard part off the shell, soak it overnight in strong 

 vinegar or hydrochloric acid diluted about i to 4. This 

 will leave a membranous bag that can be used in place of 

 the parchment bag. Fill a tumbler half full of water colored 

 with red ink, and add enough cornstarch to make the water 

 milky. Pour into the tube enough of a strong sugar solution 

 to fill the membranous bag at its base and to rise half an 

 inch in the tube. Put the membranous bag down into the 

 pink, milky water, supporting the tube by passing it through 

 a square cardboard and clamping it with a spring clothespin 

 as shown in Figure 151. Every few minutes look to see what 

 is happening. Does any of the red ink pass through the 

 membrane? Does any of the cornstarch pass through? 



This is an example of diffusion through a membrane. 

 The process is called osmosis, and the pressure that 

 forces the liquid up the tube is called osmotic pressure. 



