WATER, AND ITS USES^ , , 101 



a blotter or air into water, the phenomenon is called absorption. If 

 the result of the diffusion or absorption is a homogeneous liquid, the 

 phenomenon is preferably called solution. Freedom of the molecules 

 to intermingle is common in all these phenomena. 



In science a formal statement of a general truth of wide application 

 is known as a law. To the extent to which a law is applicable its state- 

 ments must always be in accord with the facts of any particular case. 



An hypothesis is a supposition advanced to give a clearer under- 

 standing of any phenomenon, or any group of closely related phe- 

 nomena. It is always subject to change as additional information con- 

 cerning these phenomena is acquired, but it is always based upon what 

 is known to be true. 



Where the diffusion of like states of matter takes place through a 

 porous partition, as when the liquids within and without the cells in 

 plants and in animals intermingle, the phenomenon is known as os- 

 mosis. The less dense a gas is, or the less dense a liquid is, the more 

 rapidly it gets through. By reason of an unequal rate of diffusion 

 through porous partitions there is developed what is called an osmotic 

 pressure of varying value. 



Some substances such as white of egg, glue, and cooked starch, 

 osmose very slowly if at all even as thin liquids. They are known as 

 colloids. 



Exercises 



1. Give several illustrations of the phenomena (a) of diffusion; (b) of absorp- 

 tion. In which of these cases, if any, is there also a solution? Why so? 



2. What length of time can man go without water and (a) not suffer; (6) not 

 die? With abundance of water all about them why do men die of thirst 

 at sea? 



3. Why is the "lime" in waters used as drink and in cooking likely to be 

 harmful to the body? Name some of the grains which as foodstuffs 

 are chief sources of supply of bone material. 



4. What is meant by the phenomenon of osmosis ? How is it to be accounted 

 for? Explain osmotic pressure. 



5. That osmosis may go on freely throughout the body what must be true 



of the condition of the tissues? What is meant by (a) the lymph; (6) 

 the lymphatic (lim-fat'-ik) system of the body? 



6. Since the starchy foods from plants, and albuminous foods like white 

 of egg, do not when in solution osmose readily, what change is necessary 

 during digestion besides just dissolving them? 



