COLLOIDS 57 



and a negative colloid are mixed in such proportions that the electric 

 charges are neutralized, precipitation usually occurs. When it does so, 

 we can tell the nature of the electric charge of an unknown colloid by 

 its behavior when a colloid of known electric sign is added, to it. For 

 example, if ferric hydroxide (positive) causes a precipitate to form 

 when it is added to an unknown colloidal solution, the electric charge 

 of the latter must be negative; if it does not precipitate with ferric 

 hydroxide, but does so with arsenious sulphide (negative), it must be 

 positive. 



5. Brownian Movement. Like the particles in fine mechanical suspen- 

 sions, those of colloidal solutions, especially when examined ultra- 

 microscopically, exhibit the so-called Brownian movements, which have 

 been described as "dancing, hopping and skipping." These movements 

 occur in straight lines, which are suddenly changed in direction and 

 are quite independent of external sources of energy, such as change in 

 temperature (although they become quicker as the temperature of the 

 solution is raised), earth vibrations, chemical changes, or the electric 

 charge of the colloid. The movements become more rapid the smaller the 

 particles, and they become sluggish as the viscosity of the solution in- 

 creases. Addition of electrolytes decreases the movement by causing the 

 particles to clump together. The density and viscosity of the disper- 

 sion medium, the electric charge of the dispersoid and the presence of 

 Brownian movements, are the forces which operate together to prevent 

 sedimentation of the particles in a colloidal solution. 



6. Osmotic Pressure. As one of the distinguishing properties of col- 

 loids we have seen that their diffusibility, as into gelatin or agar jel- 

 lies, is extremely slow when compared with that of a molecular solution. 

 This does not mean, however, that colloids are possessed of no power of 

 diffusibility if left long enough. Indeed the existence of the Brownian 

 movement indicates that such diffusion must occur, and therefore it 

 should be possible, by the application of the same principles as those 

 which govern molecular solutions (e. g., by using a semipermeable mem- 

 brane), to measure the osmotic pressure. 



Many studies of the osmotic properties of colloidal solutions have been 

 undertaken, especially by those who are interested in the possibility 

 that the colloids of blood serum (serum albumin and globulin) may cre- 

 ate an osmotic pressure. If this should prove to be the case, it would 

 be necessary for the osmotic pressure to be overcome by mechanical 

 pressure such as that supplied by the heart (i. e., the blood pressure) in 

 the various physiologic processes of filtration and diffusion taking place 

 through cell membranes (as in the formation of urine in the kidney). 



For measuring the osmotic pressure of colloids, osmometers similar 



