PROPERTIES OF MATTER 47 



sions. These coarse dispersions are suspensions ol one li(]uicl in an- 

 other and may be indefinitely stable, or they may be stabilized by emul- 

 sifying agents. The latter are materials adsorbed at the interface; by 

 their presence they prevent cohesion of the dispersed droplets. 



Common emidsions include milk, in which liquid fat is dispersed in 

 an aqueous phase; biuter, in which aqueous phase is dispersed in fat; 

 and the microscopically visible oil droplets seen in blood after butter 

 has been eaten, as well as ointments, salad dressing, and cosmetic 

 creams. 



Populations and Ecology 



Little is known about the special physical and chemical properties of 

 coarse dispersions, although many branches of engineering are largely 

 concerned with them. The well-developed technicpies for studying 

 ordinary solutions apply to some extent to colloidal systems but almost 

 not at all to coarse dispersions. Hence few natural laws are known 

 describing the behavior of these systems, and theoretical development 

 has hardly begun. 



Yet in one special area a start has been made in work on coarse dis- 

 persions. A group of individuals of a given species is called a popula- 

 tion, or if this group exerts a degree of conscious self-control it may 

 be called a society. In the latter case the social sciences study the 

 communal behavior of the group, the effect of the society on the in- 

 dividual and the converse, the result of environmental or psychologi- 

 cal stresses, economics, and so forth. The more general problems of 

 populations consider some of these same aspects. In one respect the 

 general problems of popidations are perhaps a little simpler than 

 those of societies, since populations may show little conscious group 

 self-control. Individuals may function consciously on occasion but 

 with little real regard for the entire group. Thus chance conditions 

 produce more nearly reproducible results. Population studies might 

 involve human beings, rats, protozoa, or almost any other form of 

 life, including even non-motile types like plants. 



One special phase of this field concerns the interactions of pojiida- 

 tions. When the environments of two populations ovei laj), one group 

 affects the other directly or indirectly. The complete history of a 

 group of plants is directly affected by the presence of grazing animals. 

 Even those not eaten may be tram[)Ied. One or more species able to 

 survive repeated damage will be automatically favored, filling in 

 vacancies left by species imable to withstand the stress. The reverse is 

 also true, for the nature of the plants will determine the presence or 



