THE IONIC THEORY 163 



water to dilute this salt in the blood plasma, and thus a 

 condition of hydraemia, i.e. excess of water in the blood, is 

 brought about. This stimulates the kidneys and there is 

 accelerated excretion of dilute urine. Such salts are there- 

 fore used as saline diuretics. 



Other salts taken by the mouth in concentrated solutions 

 do not cause diuresis for the reason that little of the salt is 

 absorbed into the blood. In other words, the intestinal wall 

 has a vital selective power and whilst it allows the passage of 

 many salts (e.g. the saline diuretics) it resists the passage of 

 others. These latter remain in the intestine, the wall of 

 which acts as a semi-permeable membrane, water is drawn 

 out from the blood and in turn from the tissues, and so the 

 intestinal contents become very liquid. These salts, there- 

 fore, are used as saline purgatives. 



THE IONIC THEORY. In the succeeding pages frequent 

 references will be found to the action of ions, and the 

 actions of many drugs are only explainable by an under- 

 standing of the theory of ionisation. 



When a salt, such as sodium chloride, is in watery solution 

 of a known strength the osmotic pressure of that solution 

 can be theoretically calculated from its molecular concentra- 

 tion. It is found experimentally, however, that the osmotic 

 pressure is higher than this theoretical result, and from 

 analogies between the behaviour of gaseous compounds and 

 solutions of salts it is believed that the increase of pressure is 

 due to dissociation of the salt, into chlorine and sodium ions. 

 The whole of the molecules of the salt are not dissociated, 

 but a large number are, and thus the solution contains 

 NaCl molecules and Cl and Na ions, and since for the pur- 

 poses of osmosis these ions are just as effective as molecules, 

 the osmotic pressure of the solution rises. Acids, bases, and 

 salts all undergo dissociation in solution to a greater or less 

 extent, and depending on the degree of dissociation the 

 osmotic pressure of the solution is higher than if no ionisation 

 occurred. The ions are not present in the solution just in 

 the form of sodium and chlorine, for they have none of the 

 specific characters of these elements. They are believed to 

 be held bound by being charged with electricity, the metallic 

 ion having a positive or + charge, the acid ion a negative or 



