ALUMINIUM HYDROXIDE BY ELECTROLYTES. 5 



The coagulation of such a solution of colloidal aluminium 

 hydroxide by various agents was observed by Walter Crum and 

 described in the following terms : 



" A solution containing ^ percent of alumina is converted 

 into a transparent jelly, when mixed with half its bulk of water 

 acidulated with .^^ of sulphuric acid. The jelly has therefore 

 only göü of i^'^ weight of alumina, and j^^ of sulphuric acid. 



" By pressure in a bag the liquid part of this jelly was 

 readily separated, and the solid was reduced to -^ or ^~ of its 

 original volume. Pure water did not dissolve this residue, neither 

 did an excess of sulphuric acid, even at a boiling heat. On 

 examination it was found that the solid portion had imbibed 

 almost the whole of the sulphuric acid. It existed there in the 

 proportion of about 1 equivalent of acid to 15 of alumina. 



" The coagulating power of the various agents may be as- 

 certained with tolerable accuracy by employing an aluminous 

 solution so dilute as to contain not more than 1 part of alumina 

 in 800 of water, and shaking it in a phial with about half its 

 volume of a coagulating solution. In this manner it was found 

 that 1 atom of citric acid (tribasic) coagulates as powerfully as 

 3 atoms of sulphuric acid, and tartaric acid (bibasic) as much 

 as 2. 2 atoms of oxalic acid are required to produce the same 

 effect as 1 of sulphuric acid. Of muriatic and nitric acids, not 

 less than 300 equivalents must be employed to produce an effect 

 equal to that of 1 equivalent of sulphuric acid — the volume of 

 the acid being always -^ that of the aluminous solution. No acid 

 has the power of redissolving the coagulum. 



" Of the other acids which have been tried, the chromic, 

 molybdic, racemic, suberic, salicylic, benzoic, gallic, lactic, cin- 

 namic, butyric, valerianic, carbazotic, camphoric, uric, meconic, 



