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 //; 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. 



