MATTER 



13 



two phases, in other words, tend to change places, one being at one time 

 dispersed in the other, at another time the other in the one. When the 

 colloid droplets are scattered in the watery dispersion medium, it is a 

 thin jelly; but when they swell, press upon each other, and the dispersion 

 medium is restricted to the crevices between them, the whole becomes 

 thick and tends to set or become firm (Fig. 2). The ability to change 

 from one state to another and back again, over and over, causes a colloidal 

 emulsion +o be called revemihle. A gelatin suspension in water forms 

 such an emulsion. 



A B C D 



Fig. 2. — Diagrams to illustrate the change of a colloidal emulsion from sol to gel. In 

 A the droplets of the disperse phase (not stippled) are shown scattered through the dis- 

 persion medium (stippled) and the emulsion is a sol; in B the droplets are shown taking up 

 liquid and swelling; in C this is continued until they press upon one another; in D the drop- 

 lets are so crowded as to become continuous and to have become in fact the dispersion 

 medium, while that which was the dispersion medium is now in droplets and has become 

 the disperse phase. The emulsion has become a gel. 



20. Reactions. — Whenever two substances are brought together and 

 a change occurs which involves a recombination of the atoms in a manner 

 different from that which previously existed, this change is termed a 

 reaction. Reactions vary in speed and in the results attendant upon 

 them with the character of the substances reacting. Some substances 

 have the power to cause a reaction without themselves entering into it 

 or being affected by it. Such substances are known as catalyzers, 

 catalytic agents, ferments, or enzymes; the effect is called catalysis, ferment 

 action, ox fermentation. A small amount of a digestive ferment is capable, 

 if given time, of causing the digestion of any amount of the substance on 

 which it acts and would itself be found undiminished in quantity and 

 unchanged in character at the end of that time. Each ferment acts on a 

 particular substance or on similar substances and is most active at a 

 certain temperature or in a medium of a certain degree of acidity or 

 alkalinity. 



