CH. XXII. ] THEORY OF SOLUTIONS 321 



Osmotic Phenomena. 



The investigations of physical chemists during recent years have given us new 

 conceptions of the nature of solutions, and these have important bearings on the 

 explanation of osmotic phenomena, and so are interesting to the physiologist. 



Water is the fluid in which soluble materials are usually dissolved, and at 

 ordinary temperatures it is a fluid the molecules of which are in constant movement ; 

 the hotter the water the more active are the movements of its molecules ? until when 

 at last it is converted into steam, the molecular movements become much more 

 energetic. Perfectly pure water consists of molecules with the formula H 2 O, and 

 these molecules undergo practically* no dissociation into their constituent ions, and 

 it is for this reason that pure water is not a conductor of electricity. 



If a substance like sugar is dissolved in the water, the solution still remains 

 incapable of conducting an electrical current. The sugar molecules in solution are 

 still sugar molecules ; they do not undergo dissociation. 



But if a substance like salt is dissolved in the water, the solution is then capable 

 of conducting electrical currents, and the same is true for most acids, bases, and salts. 

 These substances do undergo dissociation, and the simpler materials into which 

 they are broken up in the water are called ions. Thus, if sodium chloride is dissolved 

 in water a certain number of its molecules become dissociated into sodium ions, 

 which are charged positively with electricity, and chlorine ions, which are charged 

 negatively with electricity. Similarly a solution of hydrochloric acid in water con- 

 tains free hydrogen ions and free chlorine ions. Sulphuric acid is decomposed into 

 hydrogen ions and ions of SO 4 . The term ion is thus not equivalent to atom, for 

 an ion may be a group of atoms, like SO 4 , in the example just given. 



Further, in the case of hydrochloric acid, the negative charge of the chlorine 

 ion is equal to the positive charge of the hydrogen ion ; but in the case of the 

 sulphuric acid, the negative charge of the SO 4 ion is equal to the positive charge of 

 two hydrogen ions. We can thus speak of monovalent, divalent, trivalent, etc., 

 ions. 



Ions positively charged are called kat-ions because they move towards the kathode 

 or negative pole ; those which are negatively charged are called an-ions because they 

 move towards the anode or positive pole. The following are some examples of each 

 class : 



Kat-ions. Monovalent : H, Na, K, NH 4 , etc. 



Divalent : Ca, Ba, Fe (in ferrous salts), etc. 



Trivalent : Al, Bi, Sb, Fe (in ferric salts), etc. 

 An-ions. Monovalent : Cl, Br, I, OH, NO 3 , etc. 



Divalent : S, Se, SO 4 , etc. 



Roughly speaking, the greater the dilution the more nearly complete is the 

 dissociation, and in a very dilute solution of such a substance as sodium chloride 

 we may consider that the number of ions is double the number of molecules of the 

 salt present. 



The ions liberated by the act of dissociation are, as we have seen, charged with 

 electricity, and when an electrical current is led into such a solution, it is conducted 

 through the solution by the movement of the ions. Substances which exhibit the 

 property of dissociation are known as electrolytes. 



The liquids of the body contain electrolytes in solution, and it is owing to this 

 fact that they are able to conduct electrical currents. 



This conception of electrolytes which we owe to Arrhenius is extremely impor- 

 tant in view of the question of osmotic pressure, because the act of dissociation 

 increases the number of particles moving in the solution, and so increases the 

 osmotic pressure, for in this relation an ion plays the same part as a molecule. 



Another physiological aspect of the subject is seen in a study of the actions of 

 mineral salts in solution on living organisms and parts of organisms. Many years 

 ago Ringer showed that contractile tissues (heart, cilia, etc.) continue to manifest 

 their activity in certain saline solutions. We have already seen (p. 256) that Howell 

 considers the cause of rhythmical action in the heart is the presence of these 

 inorganic substances in the blood or lymph which bathes it. 



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