WHAT ARE COLLOIDS 7 



and emulsions, for within certain limits, we are able to suspend as 

 much clay or emulsify as much fat as we wish; the "finer" the clay 

 or the fat is subdivided, the more "dissolves." The same thing 

 holds for colloids, which are characteristically different in this re- 

 spect from crystalloids, the latter having a sharply defined solubility. 



As a matter of fact we can get "supersaturated solutions" of 

 crystalloids, and certain small additions increase the solubility dis- 

 proportionately. Such additions (e.g., albumin, albumoses, gelatose, 

 dextrin) when employed in the case of suspensions and colloids, are 

 called protective colloids (schutz-kolloide) because they protect the 

 lixiviated clay or finely dispersed silver from separating out. 



As indicated, many of the pure, inorganic sols, especially the metal 

 sols obtained by electric pulverization, are very sensitive to elec- 

 trolytes by which they are easily precipitated, whereas on the con- 

 trary natural colloids are relatively insensitive. It has been shown 

 that the addition of certain natural sols acting as protective colloids 

 gives metal sols, etc., properties which cause them to approach the 

 natural sols in stability. The inorganic colloids employed in medi- 

 cine, such as colloidal silver (collargol, lysargin), colloidal calomel 

 (kalomelol), colloidal bismuth, etc., are all stabilized by protective 

 colloids. 



Thus we see a complete transition from the suspension and emul- 

 sion of insoluble substances, to the true solution of crystalloids, where 

 there occurs a disintegration by the solvent, which is so profound in 

 the case of electrolytes, that they separate into their electrically 

 charged atoms (ions). As everywhere in nature, here too there are 

 no sharp lines of demarcation. We cannot deny that at a certain 

 size the particles possess the maximum colloidal properties, especially 

 those conditioned by surface phenomena. These properties decrease 

 when the particles are larger, i.e., if they approach those of true sus- 

 pensions or emulsions; or when they become smaller, i.e., if they 

 approach the molecular condition. 



TH. SVEDBERG* has shown that the light absorption of colloidal 

 gold and selenium increases as the particles become smaller, reaches 

 a maximum in the amicroscopic field, and again decreases as the 

 particles approach molecular dimensions. It is noteworthy also 

 that at a certain degree of dispersion the tinctorial power reaches a 

 maximum which in the case of gold is forty times stronger than the 

 powerful color fuchsin. The color of colloidal gold having a particle 

 size of 10 to 20 MM is ruby red; when the particles are smaller it is 

 fuchsin red; but when the particles are still smaller the color becomes 

 yellowish red. In other words it approaches the color of gold salts 

 (auric chlorid) in which the gold is molecularly dispersed. 



