72 



THE VERTEBRATE ANIMAL: METABOLISM 



talloid solutions, such as those of salt and sugar, which readily- 

 diffuse through parchment-Uke membranes, and yield crystals or 

 definitely formed masses when evaporated. Colloidal systems 

 are a subdivision of what are known as " disperse systems," 

 examples of which are famihar to all. Fog is a disperse system in 

 which water in small droplets is dispersed in the air. In this 

 instance the air is known as the " dispersion medium " while the 

 water droplets constitute the " dispersed phase." Foam is a 

 disperse system in which air bubbles are the dispersed phase and 

 water the dispersion medium; smoke, a disperse system in which 

 the disperse phase is made up of soot particles and the dispersion 

 medium is air. A, muddy stream, in which the water is a disper- 

 sion medium for solid particles like soil, is a suspension type of 

 disperse system. 



When both disperse phase and dispersion medium are liquid, we 

 have an emulsion, as, for example, when oil and water are shaken 



together so that fine droplets 

 |.vi-,^-j : :;-s;:^.;--.g;"g^-^ of oil are dispersed in the 



water. Such an emulsion 

 may be temporary, but by 

 appropriate methods small 

 amounts of oil can be held 

 in permanent emulsion in 

 water. Another emulsion 

 commonly made is mayon- 

 naise salad dressing, in 

 which oil is dispersed in egg 

 yolk. A familiar natural 

 emulsion is milk. In this 

 instance, cream, in the form 

 of very fine fat droplets con- 

 stituting the dispersed phase, 

 is held in a dispersion 

 medium formed by the solution of the other constituents in 

 water. When milk is churned, what is known as " reversal of the 

 phases " occurs, and in butter we find droplets of whey making 

 the dispersed phase in a dispersion medium of butter-fat (Fig. 42). 

 The " setting " of gelatine may also be interpreted as a reversal 

 of phase in an emulsion. 



These examples indicate the conception of disperse systems. 



Fig. 42. — Diagrams to illustrate re- 

 lation of dispersed phase to dispersion 

 medium in a disperse system. 



In the diagram on the left the dispersed 

 phase is shown in black and the dispersion 

 medium in white; on the right, the dispersed 

 phase is white and the dispersion medium is 

 black. If in both diagrams black represents 

 the fatty substances of milk and white the 

 solution of the other constituents in water, 

 the diagrams illustrate the "reversal of 

 phases" that occurs when milk is churned to 

 form butter. 



