23 



watery solution (called A), on the other hand, is frequently made 

 use of (E., p. 172), since direct measurements of osmotic pressure 

 are difficult. The vapour pressure is also used for the same 

 purpose (P., p. 1 54). The fact that the vapour pressure of a solution 

 is lower than that of the solvent can be foreseen from consideration 

 of the energetics of the process. Imagine two vessels in an 

 enclosed space, one containing water, the other a sugar solution. 

 The pressure must be lower over the latter in order that the 

 osmotic energy of the whole system may be lowered by distillation 

 of water to dilute the solution. The reason why the vapour 

 pressure is less is of the same nature as that discussed in the 

 preceding pages. Air may be regarded as a semi-permeable 

 membrane to a non-volatile solute, since it is permeable to water 

 vapour, not to the solute, which has no vapour. A greater part of 

 the surface of the water is occupied by molecules escaping to the 

 air than in the case of the solution, where a part of it is occupied 

 by the molecules of the solute. 



We have spent much time on the question of osmotic pressure, 

 because it is a difficult one ; but clear ideas upon it are of great 

 importance. 



Electrolytic Dissociation 



Pursuing our investigations on the osmotic pressure of the red 

 blood corpuscles, we shall find that we are led to another very 

 important characteristic of certain substances in solution in water. 

 When isotonic solutions of various materials were tested on plant 

 cells and blood corpuscles, it was found that some of them, although 

 of equal osmotic pressure, were lower in molecular concentration 

 than sugar. Thus, sodium chloride (E., p. i/o), if taken in 0.3 molar 

 strength, was too strong and caused the cells to shrink. The 

 correct value was found to be 0.9 per cent., or o. 1 54 molar ; that is, 

 a little more than half its expected value. In other words, these 

 particular substances behaved as if they were split up, or " dis- 

 sociated," into a larger number of smaller molecules, each of them 

 acting as a separate molecule. 



But what can these smaller parts be? They cannot be ordinary 

 sodium and chlorine, because free sodium immediately reacts 

 violently with water, forming caustic soda, and if there were free 

 chlorine in a solution of sodium chloride, it would easily be detected. 

 On further examination, it was noticed that all of these anomalous 

 substances were such as had been found to conduct electricity when 

 in solution ; they were salts, acids, or bases. Those that behaved 

 normally were organic compounds and non-conductors. 



When an electrical current is passed through a solution of 



