Osmotic Phenomena 29 



each of the different solutes dissolved separately to form 

 a volume of solution equal to the original volume. As in 

 gases, these latter pressures are termed partial pressures. 

 This principle may be formulated again as follows : The 

 total osmotic pressure of a dilute solution of mixed solutes is 

 the sum of the partial osmotic pressures of the component 

 solutes. As was seen in the last chapter, however, for more 

 concentrated solutions this principle does not seem to hold. 

 But more work needs to be done here before we may be 

 positive. 



The principles of Boyle and Gray-Lussac would hold per- 

 fectly true only for an ideal gas, i. e., one without any fric- 

 tion between its particles. Such a gas does not exist, although 

 hydrogen approaches this condition very nearly. These 

 principles, however, hold very nearly true for all ordinary 

 gases as long as they are not nearing the point of condensa- 

 tion into a liquid. But, as has been stated, in the vicinity 

 of this critical point, whether it be approached because of 

 increase in pressure or fall in temperatnre, they do not hold 

 true. In the study of osmotic pressure it is found that a 

 similar breaking down of the same principles occurs when 

 the solute becomes too concentrated. At high concentration 

 the principles of gas pressure no more apply to osmotic pres- 

 sure than they do to gas pressure itself. 



Just what is the action of the membrane in osmotic phe- 

 nomena is not known. In many respects it acts like a sieve 

 or filter, to prevent the passage of large particles, but allow 

 smaller ones to go through unhindered. In some cases, 

 however, the chemical nature of the membrane seems to 

 come into play ; it seems to react chemically with the solute 

 particles, taking them up on one side and giving them off 

 on the other. But for a discussion of the principles and 

 general phenomena of osmotic pressure, a knowledge of the 

 exact method by which the membrane acts seems not to be 



