162 METALS AND THEIR COMBINATIONS. 



tically this is accomplished by precipitating aluminum hydroxide from solutions 

 containing coloring matter, which latter is carried down and precipitated upon 

 the fibre by the aluminum hydroxide ; or by impregnating the articles to be 

 dyed with this compound and placing them in the colored solutions. 



Experiment 25. Dissolve 10 grammes of sodium carbonate in 100 c.c. of 

 water, heat it to boiling, and add to it, with constant stirring, a hot solution, 

 made by dissolving 10 grammes of alum in 100 c.c. of water. Wash the pre- 

 cipitate first by decantation, and then upon a filter, until the washings are not 

 rendered turbid by barium chloride. Dry a portion of the precipitate at a low 

 temperature, and use as aluminum hydroxide. Mix a small quantity of the 

 wet precipitate with a decoction of logwood (made by boiling about 0.2 

 grammes of logwood with 50 c.c. of water), agitate for a few minutes, and 

 filter. Notice that the red color of the solution has entirely disappeared, or 

 nearly so, in consequence of the great surface-attraction of the aluminum 

 hydroxide for coloring matter. 



Aluminum oxide, A1 2 O 3 (Alumina), is obtained as a white, 

 tasteless powder either by burning the metal or by expelling the 

 water from the hydroxide by heat : 



A1 2 (OH) 6 = A1 2 O 3 + 3H 2 O. 



Aluminum sulphate, Alumini sulphas, A1 2 (SO 4 ) 3 .16H 2 O=630. 

 A white crystalline powder, soluble in about its weight of water, 

 obtained by dissolving the oxide or hydroxide in sulphuric acid and 

 evaporating the solution to dryness over a water-bath. 

 A1 2 (OH) 6 + 3H 2 S0 4 == A1 2 (S0 4 ) 3 + 6H 2 O. 



Aluminum chloride, A1 2 C1 6 . This compound is of interest on 

 account of being the salt from which the metal was formerly obtained. 

 Most chlorides may be obtained by dissolving the metal, its oxide, 

 hydroxide, or carbonate in hydrochloric acid. Accordingly aluminum 

 chloride may be obtained in solution : 



A1 2 (OH) 6 + 6HC1 = A1 2 C1 6 + 6H 2 O. 



On evaporating the solution to dryness, however, and heating the 

 dry mass further with the view of expelling all water, decomposition 

 takes place, hydrochloric acid escapes, and aluminum oxide is left : 



A1 2 C1 6 -f 3H 2 O = A1 2 3 -f 6HC1. 



Aluminum chloride, consequently, cannot be obtained in a pure 

 state (free from water) by this process, but it may be made by expos- 

 ing to the action of chlorine a heated mixture of aluminum oxide 

 and carbon. Neither carbon nor chlorine alone causes any decompo- 



