20 ELEMENTS OF BIOLOGY 



of one or more substances finely divided into particles greater than 

 one molecule and suspended in another substance. Under this defi- 

 nition a cloud is a colloid, consisting of water finely divided and 

 suspended in air. The nature of the suspending medium may often 

 be the chief factor in determining much of the physical nature of a 

 colloidal system. Two common substances illustrate this clearly. 

 Cream is essentially a suspension of finely divided droplets of oil 

 suspended in water; butter is an oily mass containing finely divided 

 water droplets (Fig. 2). It is a familiar fact that cream is highly 

 fluid, while butter much more closely resembles a solid. The dit- 

 ference in physical properties between cream and butter is obvi- 

 ously due to a simple diflFerence in the 

 physical arrangement of the two sub- 

 stances, oil and water. Where water is the 

 suspending medium the fluidity of the 

 system resembles that of water. Where 



CREAM BUTTER ^-j "^ ^^ suspcudiug medium the char- 



FiG. 2. — Diagram illus- . . 111 c \ \ 



trating the difference in dis- acteristics resemble tnose OI the Oil. 



tribution of water and oil g^^. ^^^^^ and butter are systems 



in cream and butter. ^ 



in which essentially only two substances 

 are concerned; that is to say, the system is diphasic. Protoplasm 

 is much more complicated. Suppose a system consisting of a 

 liquid finely divided into masses greater than one molecule and 

 suspended in another liquid in which it does not dissolve. Fur- 

 ther, suppose to this is added small quantities of salts, a trace 

 of acid, a trace of alkali, and some fats. Further, suppose that a 

 variety of colloidal particles and large molecules of various sorts 

 are added, some to the suspending medium and some to the finely 

 divided substances. The result is a complicated, polyphasic, colloidal 

 system. Protoplasm is regarded as a polyphasic system of still 

 greater complexity. The chief suspending medium, or continuous 

 phase, or external phase, as it is called, is water. The suspended 

 substances, or internal phase, or discontinuous phase, are chiefly 



