76 THE SOIL. 



brought into contact with solutions of the articles of 

 food most essential for plants in pure water or in water 

 containing carbonic acid, it can withdraw these ele- 

 ments of food from such solutions. This power throws 

 light upon the form and condition in which these mate- 

 rials are contained or combined in the soil. 



To estimate this property correctly in its bearing 

 upon the life plants, we must call to mind a similar 

 property in charcoal, which, like arable soil, withdraws 

 from many fluids colouring matters, salts and gases. 



This power in charcoal depends upon a chemical 

 attraction proceeding from its surface, and the materials 

 withdrawn from the fluid adhere to the charcoal in 

 exactly the same way that the colouring matter adheres 

 to the fibre of coloured stuffs coated over with it. 



The property of decolorising coloured fluids, which 

 animal wood and vegetable fibre share in common with 

 charcoal, is perceptible in those kinds of charcoal only 

 which possess a certain degree of porosity. 



Powdered pit coal, and the shining, smooth, blis- 

 tered charcoal from sugar or blood, have hardly any 

 decolorising action ; whereas porous blood-charcoal and 

 bone-charcoal with its fine pores exceed all other varie- 

 ties in this property. 



Among the wood-charcoals, those made from poplar 

 or pine, having wide pores, are inferior to the charcoal 

 of the beech and box tree ; all these varieties decolorise 

 in proportion to the extent of surface which attracts 

 colouring matter. The attractive force which charcoal 

 exercises upon colouring matter is about on a par with 

 the feeble affinity of water for salts, which are dissolved 

 by it, but without alteration of their chemical proper- 

 ties. When dissolved in water, a salt simply assumes 

 the fluid state, and its particles acquire mobility ; but 

 in all other respects it retains its characteristic proper- 

 ties, which, as is well known, are completely destroyed 

 by the action of a stronger affinity than that of water. 



In this respect the attraction of charcoal resembles 

 that of water, for both attract the dissolved matter. If 

 the attraction of the charcoal is somewhat greater than 



