THE ENERGY OF MOLECULES IN SOLUTION 143 



number of molecules it contains in unit space, any method which, will 

 give us information 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 temperature to 100 C. On the other hand, Barger has 

 suggested an ingenious method in which the alteration of vapour- 

 tension is made the basis of a method for determining the osmotic 

 pressure of small quantities of fluids at ordinary temperatures. And 

 this method may find important applications in physiology. 



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 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 are equal. A series of trials is made with 

 different strengths of salt solution until this equality is established. In this 

 method only minimal quantities of material are required, and the determination 

 of the aqueous tension is made at ordinary temperatures. 



The method, however, which is of greatest value in physiology 

 is the measurement of the depression of freezing-point. The 

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

 thermometer reading to T ^o C. (Fig. 22). A solution freezes at 

 a lower temperature than pure water, and the depression of freezing- 

 point is proportional to the number of molecules present. Thus the 

 freezing-point of a 1 per cent, solution of NaCl is O61 C. The 

 depression of freezing-point is generally represented by the Greek 

 letter A. This method has the advantage that the fluids are in 

 most cases in no wise altered by the process of freezing, and it can 

 be applied to solutions containing coagulable proteins which would 

 be irretrievably altered by any considerable rise of temperature. 

 The depression of freezing-point can be converted directly into 

 osmotic pressure by multiplying the depression of freezing-point 

 observed by the factor 122*7. Thus a 1 per cent, solution of 

 sodium chloride with A = O61 will have an osmotic pressure of 

 0-61 x 122-7 = 74-847 metres of water. 



