OF VITAL PHENOMENA 71 



see particles .004 p in diameter. Since a molecule of soluble 

 starch is calculated to be .005 /* in diameter, the limit of vision 

 is within the size of the largest molecules. The colloidal particles 

 visible with the ultramicroscope are called submicrons, but 

 particles too small to see singly, amicrons, may cause a diffuse 

 light. Since a diffuse light is caused also by submicrons which 

 are out of focus, their presence must be excluded in order to 

 be sure that the light comes from amicrons. Submicrons show 

 the Brownian movement that is sometimes seen in the ordinary 

 microscope when small suspended particles are present. The 

 amplitude of the vibrations is inversely proportional to their size 

 and the viscosity of the medium. 



Suspensoid particles bear electric charges in the same way that 

 the particles of a suspension do. The origin of the charges 

 is different in different cases. It may be due to electrolytic 

 solution tension, as in case of the negative sols (colloid solu- 

 tions) of platinum, gold or iridium. It may be due to electrolytic 

 dissociation as in case of positive sols of metallic hydroxides or 

 basic dyes or negative sols of silicic, stannic or tannic acids, 

 mastix or sulphonic acid dyes. Or it may be due to adsorption 

 of ions as in case of silver iodide sol. 



Colloidal particles of different sizes may sometimes be sep- 

 arated by means of the ultrafilter (Bechhold, 1908). 



Suspensoids are precipitated by ions or suspensoids of oppo- 

 site charge. The isoelectric point, the point at which the particles 

 migrate neither to the cathode nor anode, is the point of com- 

 plete precipitation. Only a small amount of the precipitant is 

 necessary and if more is added the charge on the particles may 

 be reversed and hold them in suspension. The precipitation 

 follows the adsorption curve (Fig. 24). Hence, bivalent ions are 

 more than twice and trivalent much more than three times as 

 effective as monovalent ions, but this seems not to explain en- 

 tirely the difference. Among ions of the same valence the less 

 the electrolytic solution tension and the greater the ionic speed 

 the greater is the precipitating power. Thus negative sols are 

 more easily precipitated by H and heavy metal ions. Monovalent 

 and bivalent ions may antagonize one another in precipitation. 



A precipitating electrolyte is more effective the faster it is 

 added to the sol. The probable explanation is that when the 



