'54 



PRINCIPLES OF GENERAL PHYSIOLOGY 







osmotic pressure as that of the inner solution, so that no movement of the meniscus takes place. 

 The concentration of the sugar solution can then be ascertained by an appropriate method, say 

 by specific gravity or rotatory power, and its osmotic pressure is obtained from the measure- 

 ments of Morse and others. The method is only applicable when the membrane is not easily 

 permeable to the solute whose osmotic pressure is to be determined, and it must obviously 

 mil le acted on chemically by solvent or solute in contact with it. According to Walden 

 (1892, p. 708) such membranes are permeable to nearly all inorganic salts. The substances 

 tested by Fouard were lactose, glucose, mannite, asparagine, and quinine tartrate. Apparently 

 the tannin-gelatine membrane was impermeable to these, but it is the most permeable of all the 

 precipitation membranes tested by Walden (see page 113 above) ; the least permeable was that 

 of copper ferrocyanide. 



Vapour Pressure. That a solution of any substance must have a higher 

 vapour pressure than that of the pure solvent can readily be seen by the following 



consideration due to Arrhenius 

 (1901, p. 33). Suppose two vessels, 

 W and S (Fig. 49), situated in a 

 closed space filled with air. W con- 

 tains a dilute solution of a non- 

 volatile solute in water, and S a 

 stronger solution of the same solute. 

 Water will pass from W to S, since 

 the air may be regarded as a semi- 

 permeable membrane, permeable to 

 water as vapour, impermeable to the 

 non-volatile solute. The pressure of 

 water vapour over W must, therefore, 

 be greater than over S, otherwise it 

 would not pass from the one place to 

 the other. Further, suppose that W 

 and S, instead of being in separate 

 vessels, are in one vessel but separ- 

 ated by a membrane, permeable to 

 the solvent, impermeable to the 

 solute. The water, as we know, 

 passes to the stronger solution until 

 the osmotic pressure of the two is 

 the same. Now, if the pressure of 

 water vapour were greater over S 

 than over W, water would continu- 

 ally distil over to W nd pass through 

 the membrane to S, equilibrium 

 would never be attained, and we 

 should have a "perpetually auto- 

 matic cyclic process, i.e., a perpetuum 

 mobile, which would perform work 

 at the expense of the heat of the 

 , environment, which is contrary to 



the second law of thermodynamics" (Nernst, 1911, p. 132). 



The method of calculating the exact quantitative relation between vapour pressure and 

 osmotic pressure is beyond the scope of this book, and may be found in that of Nernst 

 (1911, pp. 132-137). 



In practice, various methods of determining the vapour pressure of a solution are adopted. 

 It may be measured directly by introduction of the solution into a Torricellian vacuum and 

 measuring the fall of the mercury column, or by a differential method, determining the 

 difference of pressure over the solvent and the solution. An apparatus for use in physiological 

 work is described by Friedenthal (1903). The method has the disadvantage that the solutions 

 are in mcuo, so that dissolved gases must be removed previously ; but, on the other hand, it 

 can be used at the temperature of the organism from which the solutions were obtained, 

 an advantage over the freezing point method. Another method is -that suggested 

 by Ostwald and investigated by James Walker (1888). This depends on the fact 

 that, when an indifferent gas is bubbled through a solution, the amount of the solvent removed 

 by the gas is proportional to the vapour pressure of the solution. This method was employed 

 by Berkeley and Hartley (1906, 2) to compare the vapour pressures of cane-sugar solutions 



B 



FIG. 49. DIAGRAMS TO ILLUSTRATE THE RELATION 

 OF VAPOUR PRESSURE TO OSMOTIC PRESSURE. 



W, Water. 



S, A solution in water. 



In A, the liquids are separated by air. In B, there is also 

 a semi-permeable membrane, with which they are both 

 in contact. 



(After Arrhenius.) 



