IV. THE KEACTIONS OCCUBEING IN SOILS 77 



extent, ammonium compounds and organic nitrogenous substances are 

 carried away. 



(3) By volatilisation of ammonia or ammonium carbonate resulting 

 from the decomposition, under the action of organisms, of nitrogenous 

 organic matter. Owing to the solubility of ammonia in water and the 

 ease with which it is absorbed by humus and other ingredients, this 

 loss is probably ok small consequence. It is, doubtless, greatest in hot 

 weather and in dry soils. 



APPENDIX TO CHAPTER IV. 



The chemical changes occurring in a soil take place between sub- 

 stances in highly dilute solution, and it is probable that considerable 

 help might be afforded by the application of Arrhenius's electrolytic 

 dissociation theory of solution to such problems. 



It would be out of place to attempt complete exposition of this 

 theory here, but it may perhaps be advisable to very briefly mention 

 the chief conceptions contained in it. 



When a compound capable of undergoing electrolysis in aqueous 

 solution is dissolved in water, the mere act of solution is accompanied 

 by its partial or complete dissociation into ions, as the metal and acid 

 radical are called. Thus in a solution of common salt there are a 

 number of molecules of NaCl, but if the solution be dilute the greater 

 portion of the salt exists as free ions of sodium and chlorine, the ions 

 consisting of atoms (in this case) each carrying a charge of electricity, 

 positive in the case of the metal, negative in the case of the halogen. 

 The degree of dissociation, i.e., the proportion of the total quantity of 

 salt which thus undergoes " ionisation, " increases with the dilution. 



The chemical changes which may ensue when two solutions are 

 mixed are dependent upon the action of the ions upon each other. 

 Thus, if to a dilute solution of NaCl a solution of silver nitrate be 

 added, we get a precipitate of silver chloride, because this substance, 

 being insoluble in water, cannot undergo ionisation. The action 

 which occurs is to be regarded thus : Both solutions contain chiefly 



+ + 



ions, the one Na and Cl, the other Ag and NO 3 . On mixing the 

 solution a precipitate of insoluble AgCl separates owing to the union 

 of the Ag and Cl ions, and the solution now contains not NaNO 3 , as 



+ 



is usually taught, but really ions Na and N0 3 . On evaporation these 

 ions unite and form NaNO 3 , which may be obtained in crystals. All 

 the changes which take place when aqueous solutions of salts are. 

 mixed are to be thus represented. The most difficult conception in 

 connection with the theory is that the ions have not the usual proper- 

 ties of the free substances themselves (which in many cases could not 

 exist in contact with water), but, because of their electrical charges, are 

 utterly different. When the electrical charge is removed, as by elec- 

 trolysis, the ions disappear and the material of which they are 

 composed takes its usual properties. 



Thus, to consider common salt solution, the Na ions possess none 



