48 Introduction. 



inotic pressure will tend to force this substance into the 

 plant tissues and the plant is helpless to prevent this en- 

 trance. 



63. Dissociation of Salts in Solution. There is a large 

 class of substances which, when they go into solution in 

 water, increase its electrical conductivity. It is also true 

 that the osmotic pressure which they may develop is 

 greater than can be explained on the basis of the number 

 of molecules which were contained in the salt before its 

 solution occurred. To account for both the greater electric 

 conductivity and the higher osmotic pressure in such cases 

 it has been assumed that, at the time of solution, more 

 or less of the molecules dissolved separate into two groups, 

 each of which may take part in developing osmotic press- 

 ure, making it greater than it could otherwise be. 



When a very dilute solution of potassium nitrate, for ex- 

 ample, is made, it is supposed that the molecules are 

 broken into two groups, each of which may absorb heat 

 energy and so strike a greater number of blows per unit 

 of time against the confining membrane, and in this way 

 produce a higher pressure. The two ions, as they are 

 called, act like two hammers and each is able to absorb 

 and deliver more energy when moving separately than 

 when combined as a single but heavier hammer. 



