FAMILIAR LETTERS ON CHEMISTRY. 37 



upon the same soil for centuries, the land never receiving back any of those mineral 

 elements which were withdrawn in the grain and straw. On the other hand, there 

 are fields in which the necessary amount of soluble silicate of potash for a single 

 crop of wheat is not separated from the insoluble masses in the soil in less than 

 two, three, or even more years. 



The term fallow, in Agriculture, designates that period in which the soil, left to 

 the influence of the atmosphere, becomes enriched with those soluble mineral con- 

 stituents. Fallow, however, does not generally imply an entire cessation of cultiva- 

 tion, but only an interval in the growth of the ceralia. That store of silicates and 

 alkalies which is the principal condition of their success is obtained, if potatoes or 

 turnips are grown upon the same fields in the intermediate periods, since these crops 

 do not abstract a particle of silica, and therefore leave the field equally fertile for 

 the following crop of wheat. 



The preceding remarks will render it obvious to you, that the mechanical work- 

 ing of the soil is the simplest and cheapest method of rendering the elements of 

 nutrition contained in it accessible to plants. 



But it may be asked, Are there not other means of decomposing the soil besides 

 its mechanical subdivision ? are there not substances, which, by their chemical 

 operation, shall equally well or better render its constituents suitable for entering 

 into vegetable organisms? Yes: we certainly possess such substances, and one of 

 them, namely, quick-lime, has been employed for the last century past in England 

 for this purpose ; and it would be difficult to find a substance better adapted to this 

 service, as it is simple, and in almost all localities cheap and easily accessible. 



In order to obtain correct views respecting the effect of quick-lime upon the soil, 

 let me remind you of the first process employed by the chemist when he is desirous 

 of analysing a mineral, and for this purpose wishes to bring its elements into a 

 soluble state. Let the mineral to be examined be, for instance, feldspar ; this sab- 

 stance, taken alone, even when reduced to the finest powder, requires for its solution 

 to be treated with an acid for weeks or months ; but if we first mix it with quick-lime, 

 and expose the mixture to a moderately strong heat, the lime enters into chemical 

 combination with certain elements of the feldspar, and its alkali (potass) is set free. 

 And now, the acid, even without heat, dissolves not only the lime, but also so much 

 of the silica of the feldspar as to form a transparent jelly. The same effect which 

 the lime in this process, with the acid of heat, exerts upon the feldspar, it produces 

 when it is mixed with the alkaline argillaceous silicates, and they are for a long 

 time kept together in a moist state. 



Common potters' clay, or pipe-clay, diffused through water, and added to milk of 

 lime, thickens immediately upon mixing; and if the mixture is kept for some months, 

 and then treated with acid, the clay becomes gelatinous, which it would not have 

 done without the admixture with the lime. The lime, in combining with the elements 

 of the clay liquifies it ; and, what is more remarkable, liberates the greater part of 

 its alkalies. These interesting facts were first observed by Fuchs, at Munich : they 

 have not only led to a more intimate knowledge of the nature and properties of the 

 hydraulic cements, but, what is far more important, they explain the effects of 

 caustic lime upon the soil, and guide the agriculturist in the application of an 

 invaluable means of opening it, and setting free its alkalies substances so im- 

 portant, nay, so indispensable to his crop. 



In the month of October the fields of Yorkshire and Oxfordshire look as if they 

 were covered with snow. Whole square miles are seen whitened over with quick- 

 lime, which, during the moist winter months, exercises its beneficial influence upon 

 the stiff, clayey soil, of those counties. 



According to the humus theory, quicklime ought to exert the most noxious 

 influence upon the soil, because all organic matters contained in it are destroyed 

 by it, and rendered incapable of yielding their humus to a new vegetation. The 

 facts are indeed directly contrary to this now abandoned theory : the fertility of the 

 soil is increased by the lime. 



The ceralia require the alkalies and alkaline silicates, which the action of the 

 lime renders fit for assimilation by the plants. If, in addition to these, there is any 

 decaying organic matter present in the soil supplying carbonic acid, it may facili- 

 tate their development ; but it is not essential to their growth. If we furnish the 

 soil with ammonia, and the phosphates, which are indispensable to the ceralia, with 

 the alkaline silicates, we have all the conditions necessary to ensure an abundant 

 harvest. The atmosphere is an inexhaustible store of carbonic acid. 



A no less favorable influence than that of lime is exercised upon the soil of peaty 

 land by the mere act of burning it: this greatly enhances its fertility. We have 

 not long been acquainted with the remarkable change which the properties of clay 

 undergo by burning. The observation was first made in the process of analysing 



