FUNCTION.] ALUMINA. 279 



kinds, or when they themselves are not both in the same 

 stages of development at the same time. On a soil, for 

 example, which contains potash, both Wheat and Tobacco 

 may be reared in succession, because the latter plant does 

 riot require phosphates, salts which are invariably present in 

 Wheat, but requires only alkalies and food containing nitro- 

 gen." This seems to be the true explanation of the rotation 

 of crops. 



" In order to apply these remarks," says Dr. Liebig, " let 

 us compare two kinds of tree, the woods of which contain 

 unequal quantities of alkaline bases, and we shall find that 

 one of these grows luxuriantly in several soils upon which 

 the others are scarcely able to vegetate. For example, 10,000 

 parts of Oak-wood yield 250 parts of ashes; the same quan- 

 tity of Fir-wood only 83, of Lime-wood 500, of Eye 440, and 

 of the herb of the Potato-plant 1500 parts. Firs and Pines 

 find a sufficient quantity of alkalies in granitic and barren 

 sandy soils, in which Oaks will not grow ; and Wheat thrives 

 in soils favourable for the Lime-tree, because the bases which 

 are necessary to bring it to complete maturity exist there in 

 sufficient quantity. A harvest of grain is obtained every 

 thirty or forty years from the soil of the Luneburg heath, by 

 strewing it with the ashes of the Heath-plants (Erica vulgaris) 

 which grow on it. These plants, during the long period just 

 mentioned, collect the potash and soda, which are conveyed 

 to them by rain-water ; and it is by means of these alkalies 

 that Oats, Barley, and Rye, to which they are indispensable, 

 are enabled to grow on this sandy heath." 



Alumina. The minute quantity of this substance which 

 occurs in plants, renders it doubtful whether its presence is 

 more than accidental ; and its importance to soil is usually 

 regarded as dependent upon its mechanical action. In some 

 plants it does not occur at all, as in the Scotch fir, according 

 to the Prince of Salm Hostmar; this chemist ascertained, 

 however, that the statement of Berzelius is correct, that 

 alumina is found in Lycopodium complanatum and Helleborus 

 niger. In the former he found more than thirty-eight per cent., 

 although a mere trace was detected in the ashes of a Juniper 

 bush growing close to the Lycopod. The Prince supposes 



