WHAT ARE COLLOIDS 5 



unchanged for months provided they are preserved in Jena glass, 

 which yields no electrolytes to water. These colloidal gold, silver or 

 platinum solutions consist of more or less fine metal particles, each 

 of which often comprises thousands of metal molecules. By varying 

 the strength and tension of the current, finer or coarser particles 

 may be obtained. 



Gold, silver and other sols have been prepared from gold, silver 

 and other salts by chemically liberating the metal. According to 

 the method of preparation, the metal is obtained in a more or less 

 fine state of subdivision. If the metal sol is once produced it is 

 impossible by the solvent alone to make the particles still smaller 

 (without using chemical means). Unlike albumin they do not have 

 the tendency to disintegrate of themselves in the solvent. We can 

 accordingly call them artificial colloids, because they can be brought 

 into such fine subdivision only by artificial means. If one were to 

 further subdivide the molecules of such artificial colloids, the mole- 

 cule would remain intact; gold would remain gold, and silver, silver. 

 R. ZSIGMONDY * has prepared gold solutions so finely subdivided 

 that they approach molecular dimensions, and TH. SVEDBERG has 

 shown that, with these gold sols, and also with selenium sols, the 

 finer the subdivision, the nearer these substances approach in color 

 and light absorption their respective molecular solutions. In general, 

 however, the artificial sols which can be seen in the ultramicroscope 

 consist of much coarser particles than the natural sols. 



While the chemical constitution of inorganic colloids is revealed 

 by their method of preparation, nothing was known concerning the 

 constitution of natural organic colloids until 1913, when EMIL FISCHER 

 succeeded in synthetically preparing organic colloids resembling 

 tannin and having molecular weights above 4000. Exact knowledge 

 of the chemical constitution of these substances will reveal much to 

 colloid research. 



Suspension, Emulsion, Solution. 



By suspension we mean the floating of a powder in a fluid, e.g., 

 clay in water. An emulsion is the minute division of one fluid in 

 another with which it does not mix, e.g., oil in milk or water. The 

 smaller the particles of the "dispersed phase" 1 (cf. p. 11) of the clay 

 or the fat, the longer it takes for them to separate. Such a suspen- 

 sion or emulsion, in which the dispersed phase is easily distinguished 



1 Portions of a structure separated from each other by physical surfaces are 

 called phases (WiLH. OSTWALD). A mixture of oil and water contains two phases. 

 Oil is one phase and water the other. Dispersed means scattered, distributed. 

 In the above examples oil or clay is the "dispersed phase." 



