PHYSICO-CHEMICAL INTRODUCTION 



29 



of the blood are present, as in whipped blood, the conductivity is 

 reduced to about half. 



We shall see how various ions are supposed to play important parts 

 in the body functions. For example, the excised muscles remain contrac- 

 tile and the heart beats when bathed with a solution containing a certain 

 concentration of sodium, potassium, and calcium ions. The hydrogen 

 (H) and the hydroxyl (OH) ions .also are important. These when com- 

 bined yield a molecule of water. The free H ion in aqueous solution 

 possesses the property of endowing a substance with acidity e.g., 



HC1 /H and Cl); the OH ion, on the other hand, gives alkalinity 



e.g., caustic potash (K and OH). Various reactions will only take 



place when a free H ion is present for example, the splitting 



of cane-sugar into dextrose and levulose and it 



is found that the rate of this change depends 



upon the concentration of the H ions. So, too, 



it is suggested that the free H ion in the blood 



plays a part in exciting the respiratory centre 



and determining inspiration. The immunizing 



properties of the blood are closely connected 



with the concentration of H ions. 



Crystalloids and Colloids. Thus far attention 

 has been paid only to such characteristics as the 

 osmotic activity and the conduction of the electric 

 current by various bodies. Another distinction 

 between substances may now be pointed out 

 that is, the readiness with which they crystallize 

 from water; and those which crystallize readily 

 e.g., sodium chloride, sugar also diffuse readily 



through animal membranes, and are known as 



ii -j mu I.- L L 11- -J.U j-.cc The haemoglobin m the 



crystalloids. Those which crystallize with diffi- parchment tube does 



10. 17. To SHOW 

 DIALYSIS OF A CRYS- 

 TALLOID, BUT NOT 

 OF A COLLOID. 



not diffuse out, the 

 chloride does. 



culty, or not at all, are characterized by low 



diffusive power or absolute inability to pass 



through animal or vegetable membranes. Such 



bodies are termed colloids, from the gummy nature of many 



bodies belonging to the group e.g., gums, starches, etc. 



This difference may be demonstrated by placing a mixture of a 

 colloid and crystalloid in a tube of parchment, and placing the tube 

 in distilled water e.g., a solution containing the red pigment of blood 

 (haemoglobin) and sodium chloride. The haemoglobin does not pass 

 through the membrane, and the water outside remains uncoloured. 

 But a test for chloride shows the presence of this in the water after 

 a short time (Fig. 17). 



Some crystalloids are electrolytes and ionize; others are non- 

 electrolytes and do not ionize. All, however, form true solutions in 

 water. In contradistinction to the last property of these bodies, we 

 have a group of substances which are quite insoluble in water when 

 in bulk, but which, if finely divided by mechanical means, can be 



