THE COLLOIDAL STATE 89 



tobacco smoke or pale forest-fire smoke, of mist, blue eyes, 

 feathers, and skimmed milk is due to the presence of tiny par- 

 ticles in permanent suspension, in other words, to matter in the 

 colloidal state. Metals may be so finely dispersed in water as to 

 remain in permanent suspension. Gold so dispersed forms a 

 classical colloidal suspension. Where dispersed particles settle, 

 as does sand in water, or rise, as does cream in milk, the system 

 is a coarse suspension. Only the smaller particles which remain 

 behind in permanent suspension are colloidal. Minuteness in 

 size of particles and (relative) permanency in suspension char- 

 acterize the colloidal state. 



The medium in which the particles of a colloidal system are 

 scattered is termed the dispersion medium, or continuous phase; 

 and the scattered particles are the dispersed, or discontinuous 

 phase; thus, the air of clouds is the dispersion medium, and the 

 droplets of water are the dispersed phase. 



Matter finely divided and in permanent suspension is said to 

 be colloidally dispersed rather than in solution, because the 

 particles are above the molecule in size, though one may speak 

 of colloidal solutions ; furthermore, a molecular dispersion may be 

 colloidal if the molecules are exceedingly large (as in the case of 

 proteins). 



As particle size is characteristic of the colloidal state, the latter 

 may be (somewhat arbitrarily) defined in terms of the former. 

 The maximum size of colloidal particles is conveniently placed 

 at the limit of microscopic visibility. The minimum size is 

 above that of the (average) molecule. This means that the 

 largest colloidal particles are below 0.1 /i or 0.0001 mm. in diam- 

 eter and therefore invisible and above 1 m/x or 0.000001 mm. 

 The lower limit is often placed at 10 m^t, though Zsigmondy 

 claims to have detected colloidal gold particles as small as 3 m^u. 



This is the world of colloidal dimensions, the world which 

 Findlay has picturesquely called "the twilight zone of matter." 



Dialysis. — Colloid chemistry is said to have had its beginning 

 in 1861 with the discovery by the English chemist Thomas 

 Graham that certain substances in solution will pass through a 

 parchment-paper membrane while others will not. Those that 

 pass through Graham called crystalloids, because crystalline sub- 

 stances, such as salt and sugar, are typical of them. Those 

 that will not pass through he called colloids (Gr. kolla, glue; 



