34 



INTRODUCTION TO CYTOLOGY 



Each of the physically homogeneous constituents, or phases, may be 

 chemically complex; an aqueous phase, for example, may contain salts 

 and other compounds in solution. The different chemical substances, 

 including the solvent, which make up a phase, are called components. 

 Furthermore, more than two phases may be present; there may be 

 "polyphase systems"; but it will be sufficient for our purpose to deal only 

 with those having two phases. 



Colloidal systems differ widely in general properties according to the 

 liquid, solid, or gaseous nature of the phases. The following list of 

 familiar substances will help to give a picture of colloidal structure, 

 although some of them, because of the large size of their particles, are 

 only large-scale models of such structure. 



Liquid in liquid: mayonnaise 

 Solid in liquid: muddy water 

 Gas in liquid: foam 

 Liquid in solid: pearl 



Solid in solid: true ruby glass 

 Gas in solid: bread 

 Liquid in gas: fog 

 Solid in gas: smoke 



It is with the liquid colloidal systems that biology is chiefly concerned. 

 Typically these are uncrystallizable, considerably more viscous than 

 water, readily coagulable, only slightly or not at all osmotic, and poor as 

 electrical conductors. In all of these features they differ markedly 

 from true molecular solutions. It was formerly customary to classify 

 them as "suspensoids," in which the suspended matter is solid, and 

 "emulsoids," in which it is supposedly liquid. More adequate is the 

 classification into lyophobic and lyophilic systems. A lijophohic system 

 is one in which neither phase will dissolve in the other, whereas in a 

 lyophilic system the disperse phase and the dispersions medium are 

 more or less soluble one in the other (Martin Fischer). Gelatin in water 

 is a lyophilic system; gelatin in alcohol is a lyophobic system. It is 

 uncertain to what extent the dispersions medium actually dissolves in the 

 micelles in the solvation of a lyophilic colloid (as when gelatin swells in 

 water, a case of hydration) or only becomes closely "bound" in layers 

 about them (Gortner, 1929, p. 212.) 



Colloidal systems are referred to as sols if they flow readily and as 

 gels if they do not. Gels include jellies (true gels) formed by lyophilic 

 colloids, and coagula formed by precipitation in lyophobic ones. A sol 

 may be made to become a gel {gelation) or a gel to become a sol {peptiza- 

 tion) under certain conditions. In many cases such alterations are 

 reversible. What structural change occurs during gelation is not well 

 known, but there is evidence which indicates that an agglomeration or a 

 partial coalescence of the suspended particles occurs, with the resulting 

 formation of a reticulum or a spongework of threads. The consistency 

 of a colloidal system obviously may vary according to the relative volumes 

 of its phases, the closeness with which the dispersed particles are packed 



