4 THE ROYAL SOCIETY OF CANADA 



the precipitate as completely as possible, it was dissolved in nitric 

 acid and aluminium hydroxide again thrown down with a slight 

 excess of ammonia. After eight precipitations the sodium compounds 

 had completely disappeared. 



Ammonium Nitrate: — Five hundred c.c. of C. P. cone, nitric 

 acid were diluted to one liter and ammonia gas passed through this 

 until the acid was neutralized. 



Results. 



The following table contains the values for the solubility of the 

 aluminium hydroxide in solutions of ammonium hydroxide of different 

 concentration. These values indicate the number of grams of alum- 

 inium hydroxide dissolved in 100 c.c. of the solution at 20°C. The 

 time of rotation was between 12 and 24 hours; experiments having 

 shown that about 12 hours were required to bring the solution and 

 precipitate to an equilibrium. 



TABLE L 



These results show that the solubility of the aluminium hydroxide 

 increases with the concentration of the ammonia, until a concentra- 

 tion slightly exceeding 0-5 equivalent grams per liter is reached. 

 The solubility then decreases as the solutions become more concen- 

 trated. Undoubtedly associated with this decrease in solubility 

 for the more concentrated solutions is the fact that the aluminium 

 hydroxide gradually becomes more dense in appearance as the tubes 

 are rotated, assuming a more compact form. If the tubes were taken 

 out of the bath after one half hour rotation, and the solution 

 analyzed, it would be found that the amount in solution was consider- 

 ably greater than that given above; as, however, the rotation is con- 

 tinued, the solid hydroxide gradually changes, as noted above, and 

 the solubility diminishes, until after some 12 hours rotation, equil- 

 ibrium will have been reached. This precipitate was carefully analyzed 

 as it was thought it might contain an appreciable amount of silica as 

 aluminium silicate, but only a trace was found. The analysis proved 



