4 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES 



part of their volume at C. for every degree C. that their temperature 

 is raised.* 



The pressure of a gas is measured by connecting a pressure gauge or 

 manometer with the vessel which contains the gas. Now, it is plain 

 that if the 22.4 liters, which is the volume occupied by a gram-molecular 

 quantity, were compressed so as to occupy a volume of 1 liter, its pressure 

 would be 22.4 times that of 1 atmosphere, or 22.4 x 760 mm. Hg the 

 temperature remaining constant. Under these conditions we must im- 

 agine that the molecules of gas are crowded together by the compression, 

 and if we further conceive of these molecules as being in constant mo- 

 tion, then we can understand why the pressure should increase just in 

 proportion as we confine the space in which they can move. 



One other property of gases must be borne in mind namely, their 

 tendency to diffuse from places where the pressure is high to places 

 where it is low until the pressure is the same throughout. 



OSMOTIC PRESSURE 



These fundamental facts regarding the behavior of gases suggested 

 to van't Hoff the hypothesis that molecules of dissolved substances must 

 behave in a similar manner to those of gases. To put this hypothesis to 

 the test, it is necessary that we have some method for measuring the 

 pressure of dissolved molecules. We can not, as in the case of a gas, 

 use an ordinary manometer, for this would measure only the pressure 

 of the solvent on the walls of its container and would tell us nothing of 

 the pressure of the dissolved molecules. We must use some filter or 

 membrane that will allow the molecules of the solvent but not those of 

 the dissolved substance to pass through it. It is evident that if such a 

 filter is placed, for example, between a solution of sugar in water and 

 water alone, the molecules of the latter will diffuse into the solution 

 until this has become so diluted that the pressure of the dissolved mol- 

 ecules is equal on both sides of the membrane. Such a membrane is 

 called semipermeable ; the diffusion of molecules through it is called 

 osmosis, and the pressure which is generated, the osmotic pressure. If 

 we prevent the water molecules from actually diffusing by opposing 

 a pressure which is equal to that with which they tend to diffuse through 

 the membrane, we can tell the magnitude of the osmotic pressure (Fig. 1). 



In applying these facts to test the hypothesis that molecules in solution 



*This implies that at -273 C. the gas would occupy no volume. Before this temperature is 

 reached, however, the liquefaction of the gas sets in. The temperature -273 C. is known as absolute 

 zero. An observed temperature phis 273 is called the absolute temperature. Another way of stat- 

 ing the above law is therefore that the volume is directly proportional to the absolute temperature. 

 At 273 C. the volume of a gas at C. would be doubled, or if expansion were prevented the 

 pressure would be doubled. 



