RECENT ADVANCES IN SCIENCE 23 



and, consequently, the value of the solvent pressure. As a 

 matter of fact, the free or unoccupied space in a liquid is about 

 one-fifth to one-tenth of the total volume occupied by the 

 liquid. 



It is thus seen that the osmotic pressure may be explained 

 as due to a difference between the value of the solvent pressure ir 

 in the pure solvent and the value of the solvent pressure w' in 

 the solution. This is the new concept of osmotic pressure, 

 especially for solutions which approximate to the ideal case. 

 In the case of non-ideal solutions the osmotic pressure is a 

 more complex phenomenon, but the underlying idea is the 

 same. It is not proposed to enter into the case of non-ideal 

 solutions in this place. One point only remains to be con- 

 sidered in connection with the ideal solution. 



It has been stated, that the osmotic pressure depends upon 

 the difference of the ir values. This must not be taken as 

 meaning that the osmotic pressure P is simply tt — tt'. The 

 actual relation between them is easily shown to be, in the case 

 of a dilute solution : 



P^RTtt-it' 



V tt' 



where V is the volume of one grammolecule of the solvent and 

 R is the gas constant. Tinker has shown further that this 

 expression leads at once to the van 't Hoff law for the osmotic 

 pressure. The new concept of osmotic pressure is therefore 

 quite in agreement with the striking experimental facts 

 demonstrated by van 't Hoff and is capable of accounting for 

 them in a satisfactory manner. We are therefore no longer 

 tied to the bombardment view which attributes the osmotic 

 effects to a simple gas pressure exerted by the solute. Several 

 other relationships, which cannot be discussed here, may like- 

 wise be deduced, with the help of the concept of solvent pressure. 

 The position is therefore a hopeful one, and it may not be 

 long before we possess a really comprehensive and satisfactory 

 theory of dilute solutions not simply from the thermodynamic 

 standpoint, but from the molecular standpoint as well. 



Addendum. — The problem of Osmotic Pressure was recently 

 the subject of a discussion held by the Faraday Society. The 

 reader is referred to the report of this meeting which will appear 

 in the next issue of the Transactions of the Faraday Society. 



