84 J. van't Hoff on the Function of Osmotic Pressure 



P 



The approximately constant quotients ^ point conclusively 



to this proportionality between pressure and concentration. 



Comparison of Osmotic Pressures by Physiological Methods. 

 — Observations of de Vries (" Eine Methode zur Analyse der 

 Turgorkraft," Pringsheim's Jahrh. xiv.) show that equal 

 changes of concentration of solutions of sugar, and of potas- 

 sium sulphate and nitrate, exercise equal influence on the 

 osmotic pressure. This osmotic pressure was compared, by 

 physiological methods, with that of the contents of a plant- 

 cell ; the protoplasmal envelope contracts when it is immersed 

 in solutions possessing great attraction for water. By a 

 systematic comparison of the three bodies mentioned, using 

 the same cells, three isotonic liquids (i. e. liquids exhibiting 

 the same osmotic pressure) were obtained. Cells of a differ- 

 ent plant were then made use of, and so four isotonic series 

 were constructed which showed a similar proportion in their 

 concentrations ; this is exhibited in the following table, where 

 the concentrations are expressed in gram-molecules per 

 litre : — 



Series. KN0 3 . 



Ci 2 H 22 O u . 



K 2 S0 4 . 



KN0 3 =1. 



C 12 H 22 O u . 



K 2 S0 4 . 



I. . 0-12 



— 



0-09 



1 



— 



0-75 



II. . 0-13 



0*2 



0*1 



1 



1-54 



0-77 



III. . 0-195 



0-3 



0-15 



1 



1-54 



0-77 



IV. . 0-26 



0-4 



— 



1 



1-54 



— 



Theoretical Proof. — These observations render highly 

 probable the existence of proportionality between osmotic 

 pressure and concentration, and the theorem may be com- 

 pleted by a theoretical proof which is, indeed, almost self- 

 evident. Regarding osmotic pressure as due to a kinetic 

 cause (i. e. as produced by impacts of the dissolved molecules), 

 there must exist a proportionality between the number of im- 

 pacts in unit time and the number of molecules in unit volume. 

 The proof is therefore exactly the same as that for Boyle's 

 law. If, on the other hand, osmotic pressure be regarded as 

 the outcome of an attraction for water-molecules, its value is 

 evidently proportional to the number of attracting molecules 

 in unit volume, provided (and this is taken for granted in 

 sufficiently dilute solutions) the dissolved molecules exercise 

 no attraction on each other, and each one exerts its own 

 special attractive action, uninfluenced by its neighbours. 



III. Gay-Lussac's Law for Dilute Solutions. 



While the proportionality between concentration and os- 

 motic pressure is self-evident, so long as temperature remains 



