1911.] The Properties of Colloidal Systems. 



This being so, a proof of the actual existence of such a kind of compound is 

 of some interest. When the chemical reaction following the formation of the 

 adsorption compound takes place very slowly, it may also he possible to 

 obtain the latter in an isolated condition. I have, in the course of other 

 work, met with an instance where the adsorption compound and the true 

 chemical compound are unmistakably different bodies, both having a visible, 

 concrete existence. 



1. Adsorption Compounds of Colloidal Acid with Colloidal Bases. 



If to a solution of Congo red an excess of hydrochloric acid be added, the 

 blue free acid is precipitated ; but, if the precipitate be suspended in water 

 and dialysed, a deep blue colloidal solution is formed, as described in a 

 previous paper.* Freshly precipitated and well-washed aluminium hydroxide 

 is suspended in water, and a small quantity of the blue acid colloid is added. 

 A dark blue precipitate falls, which can be washed by decantation, best with 

 the aid of the centrifuge, and again suspended in water. It remains dark 

 blue, and might hastily be supposed to be merely an aggregated portion of 

 the acid colloid. That this is not so, and that the body contains also 

 aluminium hydroxide, is shown at once by its behaviour on warming. When 

 this is done, a red solution is rapidly formed, which, on cooling, deposits 

 flakes of a red substance, while the solution itself becomes pale in colour. 

 The same change occurs at room temperature, but very slowly. It is evident 

 that we have here, in the adsorption compound formed at first, acid and base 

 existing side by side but uncombined. On heating, chemical combination 

 takes place with the formation of the aluminium salt of Congo red, which, 

 like all the salts of this acid, is of a red colour. Congo red is a convenient 

 body for the present purpose, since the salts are of a colour which is so 

 different from that of the acid. 



The precise manner in which combination is caused to take place by the 

 action of heat does not immediately concern us ; the important fact is that 

 a body can be prepared containing acid and base uncombined. 



The mode of formation of the adsorption compound is, it will be noticed, 

 to all intents and purposes a case of the mutual precipitation of electro- 

 positive and electro-negative colloids, in this case, aluminium hydroxide and 

 Congo-red acid respectively. 



The dry preparation of aluminium hydroxide supplied by Kahlbaum can 

 be used, but, owing to the large size of the grains, it is not very effective. 

 It is important that, whatever preparation be used, no free caustic alkali 



* ' Eoy. Soc. Proc.,' B, 1909, vol. 81, p. 270. 



G 2 



