APPENDIX 201 



become dissociated into sodium ions, -which are charged positively with elec- 

 tricity, and chlorine ions, which are charged negatively with electricity. 

 Similarly a solution of hydrochloric acid in water contains free hydrogen 

 ions and free chlorine ions. Sulphviric acid is decomposed into hydrogen 

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

 ion may be a group of atoms, like SO,, 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, ion is equal to the 

 positive charge of two hydrogen ions. We can''thus speak of monovalent, 

 divalent, trivalent &c. ions. 



Ions positive h- 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,, &c. 



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

 Trivalent : — Al, Bi, Sb, Fe (in ferric salts), &c. 



An-ions. Monovalent : — CI, Br, I, OH, NO3, &c. 

 Divalent : — S, Se, SO,, kc. 



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 niunber 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. Sub- 

 stances 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. 



The conception of electrolytes which we owe to Arrhenius is extremely 

 important in \\evf of the question of osmotic pressure which we shall be con- 

 sidering immediately ; because the act of dissociation increases the number 

 of particles moving in the solution and so increases the osmotic pressure, for 

 injthis 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 Dr. Ringer showed that contractile tissues (heart, cilia, &c.) 

 continue to manifest then- activity in certain saline solutions. Indeed, as 

 HoweU puts it, the cause of such rhythmical action is the presence of these 

 inorganic substances in the blood or lymph which usually bathes them. In 

 the case of the heart, the sinus, or venous end of the heart is peculiarly 

 susceptible to the stimulus of the inorganic salts, and the rhjrthmical 

 peristaltic waves so started travel thence over the rest of the heart muscle. 



Loeband his fellow workers have confirmed these statements but interpret 

 them now as ionic action. Contractile tissues will not contract in pure 

 solutions of non-electrohi;es (like sugar, vurea, albumin). But different con- 

 tractile tissues differ in the nature of the ions which are most favourable 



