84 SCIENCE PRIMERS, [§ V. 



If you put peas into a bladder and tie the neck, the 

 peas will not get out until the bladder is untied or 

 torn. But if you were* to put a solution of sugar or 

 of salt into the bladder, and place the bladder with 

 its neck tied ever so tightly in a basin of pure water, 

 you would find that very soon the water in the basin 

 would begin to taste of sugar or salt — and that 

 without your being able to discover any hole, however 

 small, in the bladder. By putting various substances 

 in the bladder, you will find that soHd particles 

 and things which will not dissolve in water keep 

 inside the bladder, whereas sugar and salt, and many 

 other things which dissolve in water, will make their 

 way through the bladder into the water outside, 

 and will keep on passing until the water in the basin 

 is as strong of sugar or salt as the water in the 

 bladder. This property which membranes such as 

 a bladder have of letting certain substances pass 

 through them is called osmosis. You will at 

 once see how important a part it plays in your 

 own body. It is by osmosis chiefly that the raw 

 nourishing material in the blood gets into the little 

 islets of flesh lying, as we have seen, in the meshwork 

 of the capillaries. It is by osmosis chiefly that the 

 worn-out stuff from the same islets gets back into the 

 blood. It is by osmosis chiefly that food gets out of 

 the stomach into the blood. It is by osmosis chiefly 

 that the waste, worn-out matters are drained away 

 from the blood, and so cast out of the body altogether. 

 By osmosis the blood nourishes and purifies the flesh. 

 By osmosis the blood is itself nourished and kept 

 pure. 



There are two chief things by which the blood, and 



