383 



At first clear, it after a wliile becomes opaque and after an hour a 

 portion of the gold is already so stroiigh' coagulated that it is left 

 behind on filtration. Wiien shaken with water immediately after 

 being prepared, the gold passes with a yellowish-red colour into the 

 water; a small portion only deposited at the surface of contact in 

 the form of brown flakes ; the ethereal layer, however, was quite 

 decolorised. From the solution which had stood for a while, all the 

 gold disappeared also on shaking with water. The greater part 

 thereof arrived, however, at the layer of contact, whilst the 

 aqueous layer was coloured yellowish-brown and looked turbid 

 with incident light. 



An aqueous solution of AuCl, when shaken with ether to which 

 had been added a few drops of the solution of phosphorus was 

 coloured yellowish-brown whilst the ethereal layer remained 

 colourless. 



Hence, in regard to ether, the yellowish-brown moditication of 

 gold is stable in the aqueous phase (also see 8) whereas the reddish- 

 violet and blue gold is stable at the surface of contact. 



10. Addition of an impeding colloid such as gum arabic to the 

 gold exerts a twofold influence. First of all it may retard, or even 

 entirely prexent, the separation at the surface of contact. Even 

 0.001 7o of the gum caused the separation from a red solution 

 (prepared from AuClj and CO) to take place with great difliculty 

 and then only imperfectly ; with 0.005 "/« of the gum, a trace of 

 deposit could be observed only after prolonged shaking. The colour 

 of the gold in the contact layer was, however, blue as in the 

 solution free from gum. The separation at the surface of contact 

 with ether was entirely prevented by as little as O.OOI "/o ol* gn'^^ 

 arabic ; all the gold was retained in the aqueous solution with a 

 red colour. 



Secondly, without retarding the separation at the contact surface, 

 it may prevent the colour from changing from red to violet and 

 blue. This was observed in the case of C„Hg, CSj and CCl^, a very 

 small quantity of gum arabic (0.001 — 0.005 "/o) being sufflcient. 



11. Besides colloidal gold, the colloidal solutions of the following 

 substances were also investigated. 



Silver. (Oollargolum. Fabrik Von Heyden). With amylalcohol, 

 isobutylalcohol and carbon tetrachloride a portion passed with a 

 brown colour into the surface of contact, another portion remained 

 unchanged in the water. With benzene and with ether, nothing 

 was deposited at the contact surface and all was retained in the 

 water. 



