CRYSTALLOIDS. 29 



(since unlike electricities attract each other) are called kathions, 

 and the negatively charged ions that travel to the anode, anions. 

 Hydrogen and the metallic elements belong to the group of 

 kathions ; oxygen, the halogens and all acid groups, to the anions. 

 These facts may be more clearly understood from the following 

 equations : 



In water, or in a solution of a non-electrolyte, molecules of 

 H 2 or non-electrolyte exist thus: 



H 2 H 2 H 2 



H,0 H 2 H 2 



H 2 H 2 H 2 



In a solution of an electrolyte, the molecules split into ions 

 thus: 



Na* 01- Na+ Cl- Na + Cl- 



Na* Cl- Na + Cl- Na + Cl~ 



Na+ Cl- Na+ Cl- Na + Cl~ 



When an electric current passes through a solution of an 

 electrolyte, the ions arrange themselves thus : 



Kathode" Anode* 



Na + Na + Na + Cl- Cl- Cl~ 



.Na + Na + Na+ Cl~ Cl- Cl~ 



Na + Na + Na + Cl- Cl- Cl~ 



To return to osmotic pressure, the ions influence this as if they 

 were molecules, so that when we dissolve, say, sodium chloride 

 in water, the osmotic pressure is almost twice what it should be, 

 because every molecule has split into two ions. 



Osmotic Phenomena in Cells. Over and over again we shall 

 have to refer to these physico-chemical processes in explaining 

 physiological phenomena. For the present it may make matters 

 clearer if we consider how osmosis explains the behavior of cells 

 when suspended in different solutions. The cell wall acts as a 

 semipermeable membrane. Thus, if we examine red blood cor- 

 puscles suspended in different saline solutions under the micro- 

 scope, we shall observe that they shrink or crenate when the solu- 



