§1] PHYSICAL ACTION OF SOLUTIONS 71 



brane permits the free passage of water, but not of the dissolved 

 substance, or rather, of the dissolved substance but slowly. 

 Under these conditions, the water flows more rapidly towards 

 the solution containing the greater number of molecules (per cc). 

 The theory of this movement is that upon the side containing 

 the greater number of molecules of salt fewer water molecules 

 will in a given time strike the membrane than upon the other 

 side ; and since the number passing through is proportional 

 to the number striking, relatively fewer molecules of water 

 will consequently pass out, and so there will be a resultant flow 

 of water to that side ; and if the mass of water is confined, it 

 will exert great pressure. 



This phenomenon of osmosis plays an important part in 

 organic life. Thus, under certain conditions, cells take in the 

 surrounding water, so that their walls are put under tension 

 (turgescence). The tension thus gained may be considerable, 

 amounting to 6 or 7 atmospheres. Under other conditions the 

 cells give up their water to the surrounding medium, thus 

 losing their turgescence. This occurs when they are put into 

 certain solutions of KNOg or NaCl. The relation between the 

 density of the internal and external fluids thus determines the 

 internal pressure experienced by the cell. 



A quantitative method of determining this pressure in the 

 presence of various solutions has been employed by Pfefpee 

 ('77). Solutions of different di-y salts in different proportions, 

 enveloped by a semi-permeable membrane, were placed in pure 

 water, and the pressure upon a column of mercury determined. 

 It was found, for example, that with a 1% solution of cane 

 sugar a pressure of 47.1 cm. of mercury* was produced ; with 

 a 1% solution of Kg^^i' ^ pressure of 193 cm. of Hg. He 

 concluded, as a result of his various experiments, (1) that the 

 pressure is proportional to the concentration of the solution, 

 and (2) that as the temperature rises the pressure increases. 



De Veies ('84) made a noteworthy advance, using plant 

 cells as objects of experimentation and subjecting them to 

 various solutions of substances freed of water. He determined 

 the degree of concentration which a solution of KCl must have 



* The pressure of 76 cm. of mercury equals that of 1 atmosphere. 



