204 ALUMINA THE PRINCIPAL CONSTITUENT OF CLAYS. 



IX. ALUMINIUM, ALUMINA, SULPHATE AND PHOSPHATE OF 



ALUMINA — ALUM. 



1°. Aluminium is another of those rare and Httle known metals, the 

 existence of which was established by Sir H. Davy. In combination 

 with oxygen it forms alumina, and in this state it exists in such abun- 

 dance in nature, as to form a large pt 'tion of the entire crust of the 

 globe. 



2°. Alumina, the earth of Alum. — When common alum is dissolved 

 in water, and a solution of carbonate of soda or of ammonia is added to 

 it, a bulky white powder falls, which, when collected on a filter, well 

 washed and dried, is nearly pure alumina. This substance occurs on 

 the surface of the earth in a pure state only in some rare minerals, such 

 as the corundum, the sapphire, and the ruby, — but it constitutes a large 

 proportion of all the slaty and shaley rocks. It is the principal ingre- 

 dient also of all clays (pipe-clay for example) and clayey soils, which 

 increase in tenacity in proportion to the quantity of alumina they contain. 



"When pure, it is a white tasteless earthy substance, which adheres to 

 the tongue, has a density of 2-00, and is insoluble in water, but dissolves 

 readily in caustic potash and soda and in most acids, at least when new- 

 ly thrown down from a solution of alum. When heated to redness, 

 however, it becomes hard and dense, as in burned clay and fire bricks, 

 and can then only be dissolved with extreme difficulty, even by the 

 strongest acids. Though it exists so largely in the soil, it contributes 

 but little in a direct manner to the nourishment of plants. The ash they 

 leave contains in general but a very small per-centage of alumina, as 

 will more clearly appear hereafter, — the principal agency, therefore, of 

 this ingredient of the soil is most probably of an indirect, perhaps of a 

 mechanical kind. 



It has been stated in a preceding Lecture (p. 23), that charcoal has 

 the property of absorbing gaseous substances, such as ammonia, from 

 the atmosphere, and that the action of charcoal powder, in promoting 

 vegetation, has been in a great measure ascribed to this property. The 

 same property, we have also seen (p. 136), is ascribed to gypsum, and 

 hence its fertilizing action has been explained in a similar way. Alum- 

 ina is said to be equally absorbent of ammonia ; and the use of burned 

 clay as a top-dressing, so strongly recommended by General Beatson, 

 [iVew System of Cultivation, London, 1820,] is ascribed to its power 

 of abstracting ammonia from the air, and fixing it in the soil ready to be 

 conveyed by the rains to the roots of the plants that grow upon it [Liebig, 

 p. 90.] It has been already shown (p. 136,) that this mode of ac- 

 counting for the action of gypsum is not satisfactory as a sole cause — in 

 the case of alumina, the fact of its absorbing ammonia is hypothetical,* 

 and therefore the explanation founded upon this fact is not to be impli- 

 citly relied upon. 



3°. Sulphate of Alumina. — When alumina is digested in diluted sul- 



Because clays of many varieties— pipe-clay for example— contain traces of ammonia, 

 which they evolve when moistened with a solution of caustic potash,— it is inferred that 

 they have absorbed this ammonia from the atmosphere. The same inference is drawn 

 from tlie fact of its presence in oxide of iron. — [Liebig's Organic Chemistry applied to Agri- 

 culture, p. 89.] — In neither case does the inference appear to me to be necessary. Much of 

 the ammonia may have been formed in the soil, during the oxidation of the iron itself, oi 

 during tlic decay of vegetable and animal substances.— See above. Lecture VIII., p. 153. 



