WEIGHT-NORMAL SYSTEM FOR SOLUTIONS. 109 



points, but not at higher temperatures. In other words, they can be calcu- 

 lated correctly up to the temperature above which the previously constant 

 ratios of osmotic to gas pressure were found to begin to decline in value, 

 and no further. So far as known at the present time, all the osmotic 

 pressures of cane-sugar solutions at and above the temperatures at which 

 the various ratios of osmotic to gas pressure become unity, can be correctly 

 calculated from a normal molecular depression of the freezing-points, i. e., 

 from a supposed molecular depression of about 1.85°. The tempera- 

 ture area within which the depressions of the freezing-points can be 

 employed for the determination of osmotic pressure is much smaller 

 in the case of glucose than in that of cane sugar. The boiling-point 

 method of determining osmotic pressure is also hampered by restric- 

 tions. It is limited in its applicability to a wholly unknown tem- 

 perature area — "unknown" because no one can predict at what lower 

 temperature hydration, or some equivalent phenomenon, will manifest 

 itself. If the freezing and boiling points of a solution were both nor- 

 mal, it would probably be practicable to calculate from either of them 

 the osmotic pressure at any intermediate temperature. But if one of 

 them is abnormal — and one or the other is usually abnormal in the 

 case of aqueous solutions — -this can not be done. 



But the vapor-tension method of determining osmotic pressure — 

 unlike the freezing and boiling point methods — is not of restricted appli- 

 cability. It can be employed at all temperatures between the freezing 

 and boiling points of solutions. All that has been said in praise of the 

 comprehensive character of the vapor-tension method is true enough — 

 theoretically; but if anyone who is enthusiastic for it will once attempt 

 to apply it, he will soon discover for himself some of the reasons why it 

 has not come into general use for the measurement of osmotic pressure. 

 The most needed agent for the investigation of solutions at the present 

 time is a really practicable precision-method for the measurement of 

 vapor pressure at all temperatures. 



In the foregoing pages, the author has frequently spoken of "hydra- 

 tion" as something which may account for apparently abnormal con- 

 duct on the part of solutions. He has employed this explanation of 

 excessive-constant and excessive-declining ratios of osmotic to gas pressure 

 because it is the simplest and most statisfactory one at hand, and not 

 because he is fully convinced of its entire correctness. 



