384 



Ferric hydroxide {1^!^ years old). This was i-etained in the water 

 with every liquid used. 



Arsenic trisiilphide (1'/. year old). With paraflui oil, carbon 

 tetrachloride, benzene and ether the liquid remained unchanged; 

 with amylalcohol and with isobutylaicohol, the sulphide was depo- 

 sited quantitatively at the surface of contact'). 



Selenium (prepared from H^SeO^ -\- SO,J. With amylalcohol a 

 portion is deposited at the contact surface and another portion in 

 the alcohol hxyer. Witii butylalcohol, carbon tetrachloride, benzene 

 and etiier all the sulphide collects at tiie surface of contact. 



12. From the above it appears that case h — separation at the 

 surface of contact — is a very commoji one. A few instances, 

 however, could also be found in the other two cases. 



In fact, case /; is so common that it will often be possible to 

 determine the colloidal nature of a solution by exhibiting the sepa- 

 ration at the contact surface on shaking with a second liquid. 



The above cases rehile to non — or only partially — i-eversible 

 colloids. 



With typically reversible colloids such as gelatin, the separation 

 at the contact surface has, however, also been obserxed and this 

 reaction has even been utilised for demonstrating the presence of these 

 substances in a solution"). In most cases, however, the total quantity 

 that deposits at the contact surface appears to be small. Jonker ^) 

 found for tannin and alcohol a distribution over the two layers ; 

 these solutions so far as the size of the colloidal particles is concerned 

 will have to be classed very closely with the molecular solutions 

 and be considered as homogeneous phases. 



The adhesion to the surface of contact of a second liquid is, 

 moreover, quite comparable to the surfaca adhesion of an added 

 solid substance, a phenomenon that has been observed with different 

 colloidal solutions. 



Colloidal gold, for instance, is taken up by carbon powder, barium- 

 sulphate and various fibrous substances ; colloidal arsenic trisulphide 

 by carbon and by barium sulphate ; carbon by paper ; selenium, 



-) In this case also it was very evident that the separation is due to the action 

 of the surface of contact. An aqueous soluüon of butyldcohol left the solution 

 entirely unchanged ; the instant, however, that an excess of the alcohol was added, 

 thus forming a separate layer, all the AS2S3 soon deposited at the contact surface. 



2) k. WiNKELBLECH, .Zeltschr. f. angew. Ghem. 19, 1953 (1906). The phenomenon 

 is, hov;ever, somewhat different as the gelatin arrives at the contact surface as a 

 thin, white lather. Hence, besides the liquid phases the air-phase also cooperates. 



■5) Zeitschr. f. Koiloidchemie 10, 126. 



