428 Action of the Atmosphere upon newly-deepened Soil. 



mena of combustion, or it may occur slowly without sensible 

 light and heat, as in the rusting of iron and the gradual oxidation 

 and decay of organic matters, Avhether animal or vegetable, 

 which are thus resolved into forms of combination suitable for the 

 food of plants. 



As gradual oxidation is but a sort of slow combustion, there is 

 probably also a small but continual evolution of heat during the 

 process, the totality of which is equal to that resulting from 

 rapid combustion, just as it is found that the same amount of 

 heat is required to cause Avater to pass into the state of vapour, 

 whether this takes place by rapid ebullition or by insensible 

 evaporation. The oxidation, therefore, of the matters of the soil, 

 especially where it contains much organic substances, may not 

 be without some beneficial effect on the temperature of the 

 ground. 



Oxygen is the most abundant and important of the elementary 

 bodies, 8-9ths of the water of the globe being composed of it, 

 and about l-5th of the air, besides a large amount of all the rocks, 

 earths, and animal and vegetable substances. It is indispensable 

 to the existence of all animated nature, and to the decay of dead 

 organic matter. 100 cubic inches of water, from which the air 

 has been expelled by boiling, dissolve, according to Henry, o"55 

 cubic inches of oxygen. According to Anderson it is found in 

 plants in quantities ranging from 30 to 36 per cent. 



So strong is the affinity of oxygen for organic matter in a state 

 of putrefaction, that the latter is one of the most powerful deoxi- 

 dizing agents known ; so much so as to be capable of reducing 

 even sulphate of lime in the state of gypsum, and of converting 

 sulphate of iron into sulphuret of that metal ; and in the neigh- 

 bourhood of iron exposed in these conditions organic matters are 

 frequently found coated or penetrated with crystals of common 

 pyrites, or bisulphuret of iron, the oxygen going to form carbonic 

 acid and sulphuric acid, while from solutions of gypsum carbonate 

 of lime is a resulting product. It will afterwards be shown that 

 water and dew are powerful oxidising agents, from the large 

 amount of oxygen they hold within them. 



Ozone is the name given by Dr. Schonbein, of Basle, on ac- 

 count of its pungent smell, to a substance produced by passing 

 electricity through dry oxygen, or when water is decomposed by 

 the galvanic current. Its odour is similar to what is sometimes 

 perceived in thunderstorms ; and as electricity may be shown to 

 be more or less generally present in the air, it is supposed that 

 ozone is very frequently developed. Faraday found at Brighton 

 that the pure air from the ocean abounds with it ; and Schonbein 

 met with it plentifully during a storm on the Jura. Its compo- 

 sition is not as yet clearly understood, and it has been thought 



