ALUMINIUM. MAGNESIUM AND ZINC. 235 



The largest Oriental ruby yet found came from China, and 

 is now a jewel in the imperial crown of Russia. Emery 

 is nearly pure alumina. This, besides being used by the 

 ladies in their "emery-bags," is extensively employed in 

 polishing metals and precious stones. 



327. Common Alum. In this salt we have sulphuric acid 

 united with two bases, potassium and aluminium, forming 

 a sulphate. It is therefore said to be a double salt, and 

 has the composition Al 2 K 2 (SO 4 ) 4 -f-24H 2 O. It is not a mere 

 mixture of the two salts, but a chemical compound always 

 precisely the same in the proportions of its constituents. 

 The water of crystallization in this salt constitutes nearly 

 one half of it. If it be heated, the escape of this water 

 causes it to foam and melt, and swell up into a large porous 

 mass. This is what is called burnt alum. This salt, like 

 all the salts of aluminium, has an astringent taste. Warm 

 water will dissolve much more of it than cold. It is much 

 used in dyeing and calico-printing for the purpose of fasten- 

 ing the colors, or, in other words, making the colors unite 

 thoroughly with the fibre of the cloth. It is not, however, 

 the alum that does this, but the alumina which is in it. 

 The alum is decomposed in preparing the lakes, or fast col- 

 ors. Thus an infusion of Brazil-wood, with alum dissolved 

 in it, presents a brilliant red color. If now there be added 

 a solution of carbonate of potassium or sodium, a precipitate 

 is produced, which is the alumina of the alum united with 

 the red coloring matter. This dried is the Brazil-wood lake 

 of commerce. In like manner other lakes are prepared 

 from other vegetable coloring substances. The alumina is 

 said to act in these lakes as a mordant, a word which is de- 

 rived from the Latin verb meaning to bite. It is because 

 the compound which it forms with the coloring -matter 

 takes such strong hold of the cloth. Alumina is also em- 

 ployed in the production of those beautiful blue pigments 

 called stnalts and ultramarine. 



