OSMOTIC PRESSURE 155 



with the osmotic pressures obtained by the direct method. Several improvements were 

 introduced in order to increase its accuracy. 



When great sensibility is not required, Barger's method (1904) will be found 

 very useful and easily carried out. Suppose that, in Fig. 49 (upper figure), we 

 have a means of observing the change* in volume of the two solutions, and that 

 we take as one of them a solution whose osmotic pressure is known, say cane- 

 sugar, and that we change its concentration until no change occurs, on standing, 

 in the volume of either of the solutions. Then the vapour pressure of the 

 unknown solution is equal to that of the known sugar solution. Barger introduces 

 alternate drops of the two solutions into a capillary tube, and observes the change 

 in length of the various drops by measurement under a microscope. 



It is clear that much time is saved by knowing beforehand the approximate osmotic 

 pressure of the solution to be measured. In an application of this method to solutions of 

 Congo-red (1911, ii. p. 233), I found no difficulty in distinguishing between concentrations 

 of 0-020 and 0'023 molar. 



The boiling point of a solution also depends on its osmotic pressure, and this 

 method is frequently in use by chemists, but is rarely applicable to physiological 

 problems on account of changes in solutes produced by the high temperature 

 required. 



On the other hand, the method of freezing point determinations is of great 

 value, although not so sensitive as direct measurements. A decimolar solution in 

 water lowers the freezing point by only 0'184, so that a very sensitive thermo- 

 meter must be used. In fact, 0- 001, a quantity difficult to measure with accuracy, 

 corresponds to an osmotic pressure of 0-012 atmosphere, or about 9'1 mm. 

 of mercury, a pressure easy of measurement, especially with a manometer 

 containing a liquid of low density. 



Solutions which have the same osmotic pressure have the same freezing point ; 

 for the freezing point is that temperature at which the solid solvent (ice) and the 

 solution are capable of existing together, so that they must have the same vapour 

 pressure, otherwise isothermal distillation would occur. Solutions have a lower 

 vapour pressure than the pure solvent, hence the ice with which they are in 

 equilibrium at their freezing points must have a lower vapour pressure than pure 

 ice in equilibrium with water, in other words, it must be at a lower temperature. 



It is scarcely necessary to remind the reader that ice has an appreciable vapour pressure, 

 which decreases as the temperature falls, theoretically as far as absolute zero, at which 

 temperature water vapour, like all gases, ceases to exist as such. This fact enables desiccation 

 of tissues to be carried out below their freezing points, as in the method of Altmann 

 (page 17 above). 



In connection with the measurement of the freezing points of solutions there 

 are two important laws to be kept in mind. The law of Blagden (1788) states 

 that the lowering of the freezing point is proportional to the concentration of the 

 solution, and that of Raoult (1883) states that equi molecular quantities of various 

 substances in the same solvent lower its freezing point by the same amount. 



For further theoretical treatment see Nernst's book (1911, p. 146), and for practical 

 details of the methods used, see Findlay's monograph (1906, pp. 110-123), Nernst's book 

 (1911, pp. 259-263), and the monographs of Raoult (1900-1901). Guye and Bogdan (1903) have 

 modified the ordinary Beckmann apparatus in such a way as to make it available for smaller 

 volumes of solutions, 1'5 c.c. instead of 10-20 c.c. This renders the apparatus of more use 

 in physiological work, where it is not always possible to obtain sufficient liquid for the 

 usual form of apparatus. A further modification, by which even less solution is required, is 

 described by Burian and Drucker (1910). It appears, nevertheless, to give accurate results. 



The value in degrees by which the freezing point of a solution is lower than 

 that of water is denoted by the sign A- 



There is still another method of measurement of osmotic pressure which has 

 been used for physiological liquids, viz., that of the effect of dissolved substances 

 on the critical solution temperature. Many liquids are able to dissolve each other 

 to a limited extent, as, for example, phenol and water. Above a certain tempera- 

 ture these two liquids are miscible in all proportions, but, as the temperature 

 falls, phenol separates out as a distinct phase in an opalescence to begin with. 

 This temperature is altered by dissolved substances and in proportion to their 



