ON THE FOOD OF PLANTS. 1 1 



harsh when rubbed between the finger and thumb, and it cuts 

 glass if rubbed agaiust it. 



Alumnia, so called, as constituting the base of alum, occurs 

 generally iu the form of stiff retentive clay; without a certain 

 proportion of sand, it will scarcely admit water, and conse- 

 quently an unfit medium for vegetables ; but it does not occur 

 in a state of absolute purity and minute division, and though it 

 frequently requires an additional portion of sand, to render it 

 a proper stage for vegetables, but I am not aware of its being 

 absolutely barren. The agricultural character given of this clay, 

 in Conybeare and Phillips's invaluable work on the Geology of 

 of England and Wales, is, that " it chokes the plough, and it rolls 

 before it, in a broken and muddy state ; after rain, it is not slip- 

 pery, but adheres to the shoes ; after drought, it presents cracks 

 nearly a yard in depth and several inches in breadth. According 

 to Townsend, it is sometimes called wood grower's land, because, 

 although it is productive of the finest elm, oak, and ash timbers, 

 it requires chalk before it can produce good corn ; yet on Ep- 

 ping forest, Windsor forest, and much of the New Forest, the oaks 

 are finest where clay is mixed with sand. " It does not effervesce 

 in acids, and when in a state of minute division, is unctuous and 

 impalpable to the touch. It is known by the terms, argilaceous, 

 clay, stiff retentive clay, &c. 



Calcareous earth results from limestone or chalk; in the former 

 case, the soil is always mixed with other ingredients, and is na- 

 turally suited to agricultural purposes, hence the extraordinary fer- 

 tility of many of the Irish counties. In the latter, it is occasion- 

 ally very indifferent even in England, but on the continent, ac- 

 cording to Cuvier and Erongniart, " sterility is one of its most 

 decided characters, and Champagne is mentioned, as beino-, in 

 some cases, absolutely uninhabitable." It is easily distinguished 

 from the last, by its effervescing in acids. Besides these, there 

 are six other enumerated by chemists, only one, (magnesia) is 

 found in sufficient quantity to modify in any considerable degree 

 the general nature of the soil, 



It therefore appears that pure silica, alumina, or lime, are not 



capable of supporting vegetation. It is the opinion of an eminent 



French chemist, that the most fertile soils will be generally 



found to consist, as Dearly as possible of four parts of clay, three 



and, two of calcareous earth, and one of magnesia. 



Teuua. 



