284 EARTHS. 



in slight excess, to prevent the formation of sub-sulphate ; digest a 

 little while, filter and wash the precipitate with boiling water, to re- 

 move the acid entirely ; but as some alkali may adhere to the earth, 

 re-dissolve it in dilute muriatic acid, and decompose by ammonia, or 

 its carbonate ; wash the precipitate thoroughly, and give it a white 

 heat, when it will be pure. 



(c.) When alum is composed of 'sulphuric acid and alumina, with 

 ammonia, and without any other alkali, the earth may be obtained by 

 heat alone, which expels the ammonia, and decomposes the acid. 



(d.) Galvanism discovers minute portions of the fixed alkalies and 

 acids in the alumina prepared as above, but not when it is dissolved 

 in muriatic acid, and precipitated by ammonia.* 



3. PROPERTIES. 



(a.) Tasteless, inodorous, insoluble in water ; no effect on test flu- 

 ids ; infusible in furnaces. Sp. gr. 2. No alkaline property, ex- 

 cept that of uniting with acids. 



(b.) Although insoluble in water, it attracts it powerfully ; when 

 dry, it adheres to the tongue ; when precipitated, and moderately 

 dried, it is a hydrate, half of whose weight is water, which cannot be 

 expelled except by a white heat. 



After ignition, it attracts water so fast from the air, that balances 

 show the increase of weight, f Henry. Dr. Thomson states that 

 there are two hydrates; the one composed of 1 equivalent of alu- 

 mina, and 2 of water, forming a bi-hydrate ; the other containing 

 one equivalent of each. 



(c.) Easily diffusible in water, and forms with it a plastic mass ; 

 and whether dry or moist, is impalpable between the fingers, or teeth, 

 not being harsh and gritty, like silica ; nor alkaline like lime, baryta, 

 and strontia ; nor rough, like magnesia. 



(d.) If precipitated from a concentrated solution of an alkali, it is 

 a light friable powder, and adhesive to the tongue. 



(e.) If from a dilute solution, the dried precipitate is transparent, 

 yellow, and brittle, compact, not earthy, and does not adhere to the 

 tongue; this retains water forcibly, and has .15 of it, even after in- 

 candescence. 



(/.) Infusible in furnaces, but fused by pure oxygen gas, on char- 

 coal, and by the flame of the compound blowpipe, into an enamel, or 

 a glass. 



(g.) Dissolved in the humid way by the fixed alkalies, but very 

 imperfectly by ammonia ; the earth precipitated from alum, potash 

 or soda, is re-dissolved by adding those alkalies in excess, and pre- 

 cipitated again by an acid. 



* Phil. Trans. 1800, Davy. 



t In Berzelius' hands, 15 1-2 per cent, were gained in a dry, and 33 in a humid air. 



