33G T>v Davy's j4gricultiiral Discourse. 



A soil fit for cultivation is never formed of any single earth — it 

 is more or less compounded, and the greater in degree, generally, the 

 better is its quality. In all good soils there is a certain proportion 

 of clay, and commonly of sand, either silicious or calcareous, or a 

 mixture of the two. What is designated clay, always consists of 

 many ingredients, — of which alumino and silica are the principal. 

 In three specimens from fertile soils in Flanders, carefully analysed, 

 besides alumine and silica, there were found present eighteen other 

 substances, — the most important of which were lime and magnesia, 

 the alkalies — including annnonia, certain acids — as the phosphoric, 

 sulphuric, and carbonic, and two or three kinds of vegetable matter. 



The peculiar quality of clay is, that it is retentive of moisture, 

 and of the most complete clay in its condensed state, using the term 

 in contradistinction to a loose state, — that it is an obstruction to 

 flowing water — a property of vast importance in the economy of 

 nature, — without which, the earth would be, in great measure desti- 

 tute of springs, — the gi'ound arid and unfit for vegetation, giving 

 rise to a universal desert-waste. This peculiar property is mainly 

 dependent on one earth, viz., alumine; and on the circumstance that 

 when separated in consequence of the decomposition of the mineral 

 compounds in which it e.\ists, it is, as when obtained by precipita- 

 tion, by the addition of an alkali to a salt of alumine in solution, in 

 a state of extraordinarily minute division, with the power of adhesion 

 particle to particle, and of becoming plastic from compression ; a 

 power this, of the first importance in the economy of soils, without 

 which, it is obvious the surface of the Earth would be in the state 

 of a moveable, drifting sand, such as we find where the binding ele- 

 ment of clay is deficient, as in the instance of the most remarkable 

 deserts. Of this state of minute division, you may satisfy yoursell" 

 most easily by a simple experiment — the precipitating of alumino 

 from a solution of alum by ammonia, and examining it under the Mi- 

 croscope. So minute are the particles of the precipitate, that even 

 when using one of the highest powers of a good instrument, a glass, 

 for instance, with a focal distance of one eighth of an inch, they arc 

 hardly distinguishable — indeed, I may say, they are not distinguish- 

 able individually, — only when connected one with another. This state 

 of minute division of the detached alumine is connected with, and 

 may be dependent on another property of this enrtli, its perfect in- 

 solubility in water, and in water impregnated with carbonic acid, 

 and owing to this insolubility, its inaptitude, when so detached, to 

 form crystals. 



To appreciate these peculiarities of alumine, let us consider for a 

 moment the qualities of the other earths, which are the other chief 

 ino-redients of soils, viz., silica, lime, and magnesia. 



Silica occurs in soils chiefly in the form of quartzose sand, derived 

 from the disintegration of certain compound crystalline rocks, espe- 

 cially granite, of which rock it is an ingredierit. It also occurs in 



