ALUMINA. 



liquid, if the alumina is in sufficient quantity. The pigments 

 called lakes are such aluminous compounds. The fibre of 

 cotton, when charged with this earth, attracts and retains with 

 force the same colouring matters. Hence, the great application 

 of aluminous salts in dyeing, to impregnate cloth or yarn with 

 alumina, and thus enable it to fix the colouring matter, and 

 produce a fast colour. 



Alumina, it will be observed, is not a protoxide, and is greatly 

 inferior to the preceding earths in basic power. It is dissolved 

 by acids, but never neutralises them completely. Hence, alum 

 and all the salts of alumina have an acid reaction. Their so- 

 lutions have an astringent and sweetish taste which is peculiar 

 to them. Alumina dissolves, to the extent of several equi- 

 valents, in some acids, particularly hydrochloric acid, forming 

 feeble compounds, which are even deprived of a portion of their 

 alumina, by filtering them through paper. It does not combine 

 with some of the weaker acids, such as carbonic acid. Hence 

 an alkaline carbonate throws down alumina from alum, and not 

 a carbonate of that earth. Alumina dissolves readily in solu- 

 tions of potash or soda, forming compounds in which it must 

 play the part of an acid. Such combinations occur in nature, 

 spinell being an aluminate of magnesia (MgO, A1 2 O 3 ), and 

 gahnite an aluminate of zinc (Zn O, A1 2 O 3 ). 



All the known compounds of aluminum correspond with alu- 

 mina, in the ratio of their equivalents, that is, to two atoms of 

 aluminum they contain three atoms of another body. 



Sulphuret of aluminum is formed by burning the metal in the 

 vapour of sulphur. It is a black semi-metallic mass, which is 

 rapidly transformed, by contact of water, into alumina and sul- 

 phuretted hydrogen. Hydrosulphuret of ammonia has the same 

 effect upon the solution of a salt of alumina, as ammonia itself, 

 neutralising the acid of the salt, and throwing down alumina, 

 while sulphuretted hydrogen escapes. 



Chloride of aluminum; A1 2 C1 3 ; 1670.3 or 133.84. When 

 alumina is dissolved in hydrochloric acid, it is to be supposed 

 that water and a chloride of the metal are formed (3 II Cl and 

 A1 2 O 3 =A1 2 C1 3 and 3HO). The solution, when concentrated 

 by spontaneous evaporation in a very dry atmosphere, yields 

 crystals, which Bonsdorff found to contain 44.7 per cent, or 

 12 eq. of water. But it generally forms a saline mass, which 



