On the Physical Properties of Soil. 199 



between 59° and 65J° F., the earths absorb in a given time more 

 oxygen than in a temperature only a few degrees above the 

 freezing-point. 



5. When any fertile soils are entirely covered with water, and 

 exposed at a warm season to the influence of sun light, confervae 

 begin usually to form very soon on their surface, what has been 

 called the green matter of Priestley (Protococcus viridis, Agardh, 

 and Priestleya botryoides, Mejen) ; as soon as these are formed, 

 oxygen is developed through the influence of the sun-light on this 

 vegetable matter ; when this experiment is made in close vessels, a 

 distinct increase is perceived in the air standing over the water : the 

 oxygen contained in it was increased in some of my experiments to 

 25 and 27 per cent., though the atmospheric air of the vessels at 

 the beginning of the experiments had, as usual, only a proportion of 

 21 per cent, contained in it ; this phenomenon agrees with many 

 other observations in rendering it probable that a portion of the 

 oxygen gas, which during the warmer season suffers a diminution 

 from so many processes of animal life and vegetation, is again 

 compensated for by the action of sun-light on the products of the 

 vegetable kingdom. 



6. With regard to the reasons of this absorption of oxygen gas, 

 they are partly founded on the general property of many porous 

 bodies, in the damp state particularly, to absorb oxygen gas, with- 

 out any direct chemical combination being formed by them with 

 these bodies, as Ruhland has already accurately pointed out ; and 

 the principle of this absorption may partly be sought for in the 

 proportion of humus and oxide of iron, which arable soils always 

 contain in greater or less quantity; if the earths be previously 

 burnt, and their portion of humus thus volatilized, while the oxide 

 of iron is raised to a higher degree of oxidation, their power of 

 absorption of oxygen becomes considerably diminished thereby, 

 and in some instances disappears. 



Phenomena explained by this absorption of oxygen gas. — 

 Many phenomena prove that oxygen plays one of the most im- 

 portant parts in the economy of plants and animals ; that in par- 

 ticular it is highly necessary in the germination of seeds and for 

 the growth of plants ; by loosening, digging, ploughing, and work- 

 ing the soil in any way, fresh layers of earth are brought succes- 

 sively into contact with the air, and fertilized, as it were, by the 

 absorption of oxygen gas ; from the above experiments, however, 

 we infer that dryness influences this process unfavourably, and 

 that it is therefore desirable to keep the soil in a moderately damp 

 state, where that can be done. 



2. Soil freshly brought up from below is generally found less 

 fertile at first than it afterwards becomes when it has been ex- 

 posed to the air and worked for a longer period ; it seems by these 



