4:2 . Introduction. 



the juices being forced from the fruit by the pressure 

 of the dissolved sugar as will be explained in the next 

 section. 



58. Osmotic Pressure. The power which causes the swell- 

 ing of the dried grains and fruits and that which causes 

 the shrinkage of the fresh fruit, in the cases cited in (57), 

 is known as osmotic pressure and is caused in fundamental- 

 ly the same manner as that of gaseous or steam pressure, 

 but in this case by molecules of substances in solution 

 moving in the same manner as the molecules of gas move 

 when developing gaseous pressure. 



59. Conditions Under Which Osmotic Pressure Becomes 

 Manifest. In order that the molecules of a dissolved sub- 

 stance may exert pressure analogous to gases they must 

 be dilute solutions, so that the individual molecules of 

 the kind manifesting the pressure are too far from one 

 another to be influenced by their individual attractions ; 

 besides this, in order that the pressure may become mani- 

 fest, the dissolving substance and the substance dissolved 

 must be separated by a membrane through which the mole- 

 cules of one of the fluids pass more readily than the other, 

 as represented in Fig. 6. 



FIG. 6. Illustrating the principle of osmotic pressure. 



