THE ENERGY OF MOLECULES IN SOLUTION 



127 



INDIRECT METHODS OF MEASURING OSMOTIC PRESSURE. Equi- 

 molecular solutions have the same osmotic pressures. Since the osmotic 

 pressure of a solution is therefore directly dependent on the number of 

 molecules it contains in unit space, any method which will give us informa- 

 tion as to the number of molecules present will also enable us to determine 

 the osmotic pressure. Other properties of solutions which, like the osmotic 

 pressure, are functions of the number of molecules 

 present, are vapour-tension, boiling-point, freezing- 

 point. The presence of a substance in solution in 

 water diminishes its vapour-tension at any given 

 temperature, raises its boiling-point, and depresses 

 its freezing-point, and the extent of the deviation 

 from distilled water is proportional to the number 

 of dissolved molecules present. The determination 

 of the rise of boiling-point, though much employed 

 by chemists, is of very little value in physiology, 

 owing to the fact that nearly all the fluids of the body 

 are seriously modified in character by a rise of tem- 

 perature to 100 C. On the other hand, Barger has 

 suggested an ingenious method in which the altera- 

 tion of vapour-tension is made the basis of a method 

 for determining the osmotic pressure of small quan- 

 ities of fluids at ordinary temperatures. And this 

 method may find important applications in phy- 

 siology. 



BARGER'S METHOD. Drops of the fluid, the vapour- 

 tension of which it is desired to ascertain, are drawn up 

 into a tube (1-5 mm. in diameter), so as to alternate with 

 small drops of cane sugar solution of known content (Fig. 21). 

 Water in a state of [vapour will pass from the solution of c 

 which the vapour-tension is the higher. By observing the 

 edge of a drop under a magnification of 65 diams., it can 

 be easily seen whether it has grown or diminished in size. 

 If the edge of the drop remains stationary, it shows that the 

 vapour- tension and the osmotic pressure of the two fluids 

 equal. A series of trials- is made with different strengths 

 )f salt solution until this equality is established. In 

 lis method only minimal quantities of material are re- 

 quired, and the determination of the aqueous tension is 

 le at ordinary temperatures. 



The method however, which is of greatest value 



physiology, is the measurement of the depression of freezing-point. 



e depression of freezing-point can be converted directly into osmotic 

 >ressure by multiplying the depression of freezing-point observed by the 

 ictor 122 '7. Thus a 1 per cent, solution of sodium chloride with A = 0'61 

 ill have an osmotic pressure of 0'61 X 1227 = 74'847 metres of water. 



The determination is carried out in a Beckmann's apparatus with a thermometer 

 ?ading to n \ - C. (Fig. 22). A solution freezes at a lower temperature than pure water, 



FIG. 



22. Beckmann's 

 apparatus for determi- 

 nation of freezing-point. 



