PROTOPLASM 35 



eye down to single molecules and ions. In the latter case we have a true 

 solution: between colloid and crystalloid the line of demarcation is thus a 

 purely arbitrary one. In many cases the suspended particles are too 

 small to be seen with the ordinary microscope, which will not render 

 visible a body with a diameter less than about 0.20 to 0.25 jw; but with 

 the ultramicroscope, which will reveal particles about one-fortieth of 

 this size, they may be clearly seen. Again, the ultramicroscope is insuffi- 

 cient in the case of certain colloids, in which the presence of suspended 

 particles can still be shown, however, by the Tyndall effect (a milky 

 appearance when a beam of light is passed through them). Most proto- 

 plasmic colloids are of this last type. 



In a colloidal solution the particles are separate from one another, 

 (sol), whereas in the denser "set" condition (gel) they are more closely 

 aggregated and hence not free to move upon one another. A colloid 

 may be made to pass from the sol to the gel state or vice versa; in some 

 cases this change is reversible, but in others it is not. 



Colloids are usually classified as suspensoids and emulsoids. Sus- 

 pensoids, in which the particles are solid, are comparatively unstable; 

 are readily precipitated or coagulated by salts; carry a constant electric 

 charge of definite sign; are not viscous; do not show a lower surface 

 tension than that of the medium of dispersion alone; and are mostly 

 only slightly reversible. Emulsoids, in which the suspended particles 

 are fluid, are comparatively stable; are less readily coagulated by salts; 

 are either positively or negatively charged; are usually viscous; have a 

 lower surface tension than the medium of dispersion; form surface mem- 

 branes; and are highly reversible. Most organic colloids are emulsoids, 

 and there can be no doubt that many of the characteristics of living or- 

 ganisms are due to their presence. 



In an emulsion each physically homogenous constituent is known as 

 a phase. In mayonnaise dressing, to cite a familiar example, there are 

 three phases: a water phase, consisting of water and substances dissolved 

 in it; an oil phase; and a protein phase (egg). These three physically 

 diverse substances are brought into the emulsified state by beating; one 

 of them is the medium of dispersion (external phase) and the others 

 (internal phases) are suspended in it as liquid particles or droplets. In 

 such an emulsion a given phase usually consists of more than one chemi- 

 cal substance: the water phase, for example, is not pure water, butjan 

 aqueous solution of salts and other water-soluble substances. These dif- 

 ferent chemical substances, including the solvent, which make up a single 

 phase, are known as components. 



It is shown by certain investigators (Bancroft, Clowes) that the drop- 

 lets of a suspended phase in a stable emulsion are bounded by films of 

 different constitution: between the phases of an alkaline water-oil emul- 

 sion, for example, there appear to be delicate films of a soapy nature. 



