2 THE PROPERTIES OF SOLUTIONS. [CH. I. 



other hand, we have two-phase liquids, each phase con- 

 taining both components in different concentrations. The 

 solute is able to combine to a certain extent with the solvent. 

 They are intermediate between suspensions and true solu- 

 tions. In certain cases there is evidence to show that the 

 solute may be partially ionised. Solutions of native pro- 

 teins, starch, dextrins, etc., are emulsoids, whereas the 

 denaturised proteins behave more like suspensoids. 



Sorensen's view as to the main differences between the two types of 

 colloids is as follows : 



" The suspensoids show a viscosity differing but little from that of the 

 pure external phase. There is generally a well-marked difference in electrical 

 charge between the two phases. Only comparatively small concentrations 

 of electrolytes are required to bring about coagulation, which is in most cases 

 irreversible. 



"The emulsoids show a great viscosity and power of foam formation; 

 the system commonly does not show any marked difference in electrical charge 

 between the two phases. Great concentrations of electrolytes are commonly 

 necessary to bring about coagulation, which is in most cases reversible." 



B. Diffusion and Dialysis. 



The difference between crystalloids and colloids that 

 has been most emphasised is the disparity in the rate of 

 diffusion of the two substances. If a solution of sodium 

 chloride or glucose be separated from distilled water by 

 means of a film of collodion, parchment paper, or gold 

 beater's skin, the dissolved substance is found to pass 

 through the membrane, the process being known as 

 diffusion. If a colloidal solution be tested in the same way, 

 it will be found to pass through either very slowly or not at 

 all. In other words, the colloids are relatively indiffusible. 

 This is sometimes employed as a convenient method of 

 separating crystalloids from colloids, and is known as 

 dialysis. It should be noted, however, that abrupt 

 transitions are not common in nature, and that all emul- 

 soids do diffuse through such membranes, though extremely 

 slowly as compared to crystalloids. 



i. Preparation of collodion sacs for dialysis. A convenient 

 size is made by use of a large boiling tube (200 x 15 mm.). Into a 

 clean, dry tube pour about 10 cc. of the collodion solution described 



