36 



INTRODUCTION TO CYTOLOGY 



emulsion of oil in an aqueous medium (vinegar) with egg protein as an 

 emulsifier. Milk also is stabilized largely by proteins. When water- 

 soluble sodium oleate is formed as the emulsifier in an olive oil-water 

 mixture by adding NaCl, the oil disperses in the water; this occurs even if 

 the water constitutes only 1 per cent of the whole, the water forming a 

 series of thin films separating the large oil droplets. When oil-soluble 

 calcium oleate is formed by including CaCU, the water disperses in the 

 oil. By employing both sodium and calcium salts, varying their ratio, 

 the important change known as phase reversal may be produced at will 

 in either direction (Fig. 17). At a certain critical ratio there is a very 

 delicate balance between the two conditions, and it is a biologically 

 significant fact that the ratio at which colloidal proteins and lipides form 



A 





Fig. 17. — Diagram illustrating phase reversal in a colloidal emulsion. A, aqueous 

 phase. B, oil or other non-aqueous phase. C, interfacial film formed by emulsifying 

 agent. {After Clowes, 1916.) 



such balanced emulsions is about the same as that in which the two classes 

 of salts occur in both blood and sea water. 



In many colloidal systems the finely divided substance is in the form 

 of filaments or thin films rather than globules or small granules. Further- 

 more, these may extend throughout the mass as a network or platework, 

 neither phase being wholly discontinuous. The structure of such an 

 "interlacing system" is illustrated on a large scale by bread or a rubber 

 bath sponge. What is essential to all types of systems is the enormous 

 extent of the interface, and to obtain this condition it is therefore only 

 necessary to make at least one dimension of the dispersed matter small. 

 It can readily be calculated that a cube of matter 1 centimeter in each 

 dimension and exposing 6 square centimeters of surface would expose 60 

 square centimeters if cut into 1-millimeter cubes, 6 square meters if cut 

 into 1-/X cubes, and 6,000 square meters if cut into l-m^u cubes. 



The Colloidal Nature of Protoplasm. — The work of the past few years 

 has made it abundantly clear that protoplasm is a colloidal system. This 

 is manifest in its general physical properties. Its viscosity, surface- 

 tension phenomena, power of adsorption, and high electrical resistance 



