274 POPULAR SCIENCE MONTHLY. 



monly employed than that just described, consists in the dialysis of a 

 salt solution in which a colloidal base or acid is present, either owing 

 to natural hydrolysis or to the previous addition of an alkali or acid. 

 Thus colloidal silicic acid may be prepared by dialyzing either a solu- 

 tion of sodium silicate alone, or one to which hydrochloric acid has 

 been previously added. A dark red but perfectly clear colloidal sus- 

 pension of ferric hydroxide is obtained by the dialysis of a ferric chlo- 

 ride solution which has been treated with ammonium carbonate until 

 a permanent precipitate begins to form. This process of dialysis is 

 commonly resorted to also for freeing colloidal solutions or suspen- 

 sions prepared in other ways from mineral impurities. It is most 

 conveniently carried out in parchment tubes, which are now an article 

 of commerce. As the surface exposed by these is large, the process 

 is a comparatively rapid one. The solution to be dialyzed is placed 

 within such tubes, and these are immersed first in running tap water 

 and afterwards in distilled water which is frequently renewed. 



There is one other method of sufficient importance to deserve men- 

 tion, and this is the process recently described of preparing colloidal 

 suspensions of metals by producing an electric arc under water between 

 electrodes of the metal in question. This is most readily carried out 

 with the non-oxidizable metals, such as gold or platinum. When gold 

 is used, red clouds of colloidal gold are formed near the arc, and in half 

 a minute the whole liquid assumes a red color. The method depends 

 on the fact that the metal is volatilized into the arc or spattered into 

 it in an extremely finely divided form, and is then condensed or ab- 

 sorbed by the water, which, owing to the absence of electrolytes, has 

 little tendency to cause aggregation of the particles. 



Besides these colloidal suspensions artificially prepared from min- 

 eral substances, others can be obtained by dialysis and other treatments 

 from animal and vegetable sources. Among the most fully investigated 

 of these are heated albumen and gum mastic. 



Properties indicating Heterogeneity. — Turning now to the proper- 

 ties of such colloidal suspensions, it seems appropriate first to refer 

 to those which indicate that these mixtures really are suspensions of 

 minute particles and not true solutions. The fact that the components 

 of the mixture are separated by filtration through animal membranes 

 or close-grained porcelain filters is not of itself an evidence of physical 

 heterogeneity; for by copper ferrocyanide membranes, prepared by de- 

 positing a precipitate of this substance in an unglazed porcelain cyl- 

 inder, sugar and even salts can be separated from true solutions. In 

 some cases, the presence of particles in suspension in so-called colloidal 

 mixtures has been proved directly by microscopic observation; thus 

 this is the case with the colloidal mercuric sulphide and with colloidal 

 arsenious sulphide when prepared under certain conditions, but not 



