CHAPTER VI 

 OSMOTIC PRESSURE 



THE fact that the metal palladium allows hydrogen to pass freely through it, 

 while refusing such passage to nitrogen, enabled Ramsay (1894, p. 206) to make 

 an interesting experiment. A vessel of palladium was filled with nitrogen and 

 connected to a mercury manometer. It was then immersed in an atmosphere 

 of hydrogen and the mercury was seen to rise steadily in the manometer. Why 

 did this happen? 



The reason is that hydrogen passes through the walls of the vessel until its 

 concentration o? pressure becomes equal within and without ; but, as the nitrogen 

 cannot escape to give room for the hydrogen which enters, the amount of gas 

 inside the closed vessel must increase and the total pressure rise. 



The fact can also be shown by the use of a membrane of water, or, rather, a parchment- 

 paper membrane soaked in water. Such a membrane is freely permeable to carbon dioxide, 

 because the gas is soluble in water, but almost impermeable to oxygen and nitrogen. If, 

 therefore, we take a bell-shaped vessel, and tie over the large end a wet parchment-paper 

 membrane, connect the interior to a manometer, and then immerse the vessel in carbon 

 dioxide, the pressure will rise rapidly inside for similar reasons as in the case of hydrogen and 

 palladium. 



Now we know, by what is usually known as Dalton's Law, that, in a mixture of 

 gases at a certain pressure, this pressure is divided between the different gases in 

 proportion to their relative volumes, or, in other words, the total pressure of a mix- 

 ture of gases is equal to the sum of the pressures which each alone would exercise 

 if it alone filled the vessel. Suppose that we have a mixture of nitrogen and 

 carbon dioxide consisting of one-fifth nitrogen and four-fifths carbon dioxide at 

 atmospheric pressure. The partial pressure of the nitrogen is one-fifth of 760 mm., 

 that is, 152 mm. of mercury; this is also called its "tension." If such a mixture 

 is put in a vessel as described above and pure carbon dioxide placed on the outer 

 side of the membrane, the pressure will rise by carbon dioxide passing in until 

 its tension is equal on both sides. But, since gases are compressible, the relative 

 volume of the nitrogen will have been diminished by the process, so that it is 

 better for the sake of description to imagine that, before immersion in the carbon 

 dioxide atmosphere, we have raised the internal pressure by forcing in more of 

 the gaseous mixture until the manometer reads 152 mm., that is, until the 

 pressure is increased by the tension of the nitrogen while that of the carbon 

 dioxide is that of the atmosphere. By this means we avoid the further inflow of 

 carbon dioxide, and we find that the gauge remains stationary at 152 mm. of 

 mercury, if the barometer stands at 760 mm. We have thus a measurement of 

 the tension of nitrogen in the mixture. Of course, the pressure will not remain 

 indefinitely at this point, since nitrogen is not absolutely insoluble in water, and 

 it will therefore pass very slowly through the membrane, until the composition of 

 the mixture is the same on both sides and no pressure will be shown on the 

 manometer. 



Let us now take an analogous experiment with a liquid system. We have seen 

 in the preceding chapter that a membrane of copper ferrocyanide is freely 

 permeable to water, while refusing passage to cane-sugar in solution in water. 

 Pfeffer (1877) made a number of experiments in which the membrane was 

 supported in the pores of a clay cell in order that it might be able to withstand 

 the pressures developed. He found that these pressures, spoken of in the case of 



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