COLLOIDS 559 



resultant arsenic svtlphid to appear in the flocculated or gel form. 

 If it is desired to prepare the substance in the sol form electrolytes 

 must be avoided. This may be done by using a solution of arsenic 

 trioxid instead of one of arsenic chlorid. 



Any reaction which is brought about under the above conditions 

 will result in the formation of a colloidal product. The dialysis of 

 salts which form insoluble hydroxids simply allows the normal 

 reaction of hydrolysis to complete itself. The resulting hydroxids 

 appear in such a way as to fulfill the above conditions, and conse- 

 quently appear in the colloidal state. In this connection note the 

 preparation of colloidal sodium chlorid (see ante). 



4. In an appropriate medium most if not all substances, crystal- 

 line or otherwise, may be brought directly, that is, without the inter- 

 vention of specific chemical reactions, into the colloidal state. In 

 some cases this may be accomplished by mechanical means. Thus 

 oils violently shaken with water disperse to some extent and form 

 emulsions of greater or less stability. By shaking glass, quartz, and 

 the like with various liquids in which they are virtually insoluble the 

 abrasion results in the formation of more or less dispersed systems, 

 usually not very stable. 



Many metals may be brought into the dispersed state by causing 

 an electric arc to pass between points held under a liquid. This 

 electrical dispersion method has been very considerably used, but is 

 obviously confined to substances which are conductors of electricity. 



On the other hand, many substances when merely brought into 

 contact with an appropriate medium spontaneously undergo disper- 

 sion. Thus gelatin, glue, tannin, and many other substances spon- 

 taneously disperse in water. Even crystalline substances frequently 

 do this. Thus soaps which have been crystallized from alcohol solu- 

 tions when brought into water disperse in the colloidal state. Crys- 

 tallized cuprous sulphid, the mineral known as chalcocyte, disperses 

 in the colloidal form in solutions of hydrogen sulphid. 



Substances which go spontaneously into the dispersed colloidal 

 state are usually spoken of as "lyophyllic/' while those that tend to 

 spontaneously leave the dispersed state are called "lyopkobic." It is 

 evident that a substance may be lyophyllic with respect to one me- 

 dium and lyophobic with respect to another. Furthermore, a very 

 small change in the nature of the medium may cause the change 

 from a lyophyllic to a lyophobic colloid. Thus oils are lyophobic 

 with respect to water, but lyophyllic with respect to even very dilute 

 soap solutions. 



Applications to Biology. 1. When one considers the relatively 

 infrequent occurrence in biological systems of either crystalline sub- 

 stances, or of substances that may be readily made to crystallize 

 from water (the universal biological dispersing medium), it imme- 

 diately becomes evident that the chemistry and physics of such 



