66 THE SOIL. 



animal wool and vegetable fibre share in common Avith 

 charcoal, is perceptible in those kinds of charcoal only 

 which possess a certain degree of porosity. 



Powdered pit coal, and the shining, smooth, bhstered 

 charcoal from sugar or blood, have hardly any decolorising 

 action ; whereas porous blood-charcoal and bone-charcoal 

 with its fine pores exceed all other varieties 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 colour- 

 ing 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 properties. Wlien 

 dissolved in water, a salt simply assumes the fluid state, 

 and its particles acquire mobihty; but in all other 

 respects it retains its characteristic properties, 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 that 

 of the water, then the colouring matter is completely 

 Avithdrawn fi^om the water ; if the attraction of both is 

 equal, a division takes place, and the extraction is only 

 partial. 



The materials attracted by the charcoal retain all their 

 chemical properties, and continue unaltered, merely losing 

 theii' solubihty in water ; yet very shght circumstances, 

 increasing in the least degTce the attractive force of the 

 water, are sufficient again to withdraw fi'om the charcoal 

 the materials absorbed by it, and which simply coat its 



