44 



PRINCIPLES OF ANIMAL BIOLOGY 



into contact with one another may diffuse to an appreciable extent 

 provided the surfaces of the soHds have been long enough in contact. 

 The mixtures resulting from the diffusion of solids into liquids or solids, 

 or of liquids into liquids, are known as solutions. When a solid is dis- 

 solved in a liquid, the liquid is called the solvent, and the solid the 

 solute. 



Osmosis. — As was said in the discussion of diffusion, miscible liquids 

 diffuse into each other so that a homogeneous mixture is finally formed. 



If a semi-permeable membrane, by which is 

 meant one that is permeable to solvents but 

 more or less impermeable to dissolved substan- 

 ces, be interposed between water and a sub- 

 stance dissolved in water, it may be noted that 

 the water passes through the membrane while 

 the passage of the substance in solution occurs 

 less readily, or not at all.^ Animal membranes, 

 such as the tissue of the urinary bladder, often 

 do not even approximate the characteristics of 

 an ideal semi-permeable membrane, yet they 

 usually resist the passage of certain substances 

 through them. If a piece of bladder is stretched 

 over the open end of a funnel containing sugar 

 solution, and the funnel is inverted in a dish 

 of pure water (Fig. 23) the water will pass 

 through the membrane causing the solution to 

 rise in the tube, and some of the sugar may pass 

 through to the water of the dish. The rise of 

 solution in the tube is a measure of a pressure 

 F ' 23 — D- r m of existing in the sugar solution. This pressure in 

 apparatus used to illustrate the solution is known as osmotic pressure, and is 

 osmosis. T, inverted thistle ^^^ ^^ ^^^ presence of molecules of sugar dis- 



tube covered with animal ^ " 



membrane and containing a pcrscd through the Water. 



solution of sugar in water; Qsmotic pressure is in fundamental respects 

 V, vessel of water. ^ ^ 



not unlike gas pressure. As stated above, when 



a gas is placed in a closed chamber the molecules of the gas quickly 



disperse to fill the whole chamber. When dispersion is checked by 



the walls of the container, pressure is exerted upon those walls and 



throughout the gas. The nature of the mutual repulsion of the 



molecules of a gas need not be discussed, but the resulting expansion 



of the gas, and its pressure against any object preventing expansion 



^ Ideally a semi-permeable membrane does not permit the passage of dissolved sub- 

 stances at all. Since, however, an ideal semi-permeable membrane probal^ly does not 

 exist, the term is loosely applied to membranes which greatly retard the passage of 

 substances in solution and thus approximate the requirements of the definition. 



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