]52 I^R- SCHT.KIDEN'S THEORY OF AGRICULTURE. 



be rather more than two millions of pounds to the acre ; accord- 

 ing to Saussure, 8^ per cent, will be consumed by combustion 

 in a year, that is more than 160,000 lbs. ; a consumption capable 

 of raising the temperature of the water contained in the soil 

 3° of Reaumur. As regards moisture, the ground is less suscep- 

 tible of warmth the more evaporable water it contains, since the 

 evaporating fluid consumes the iieat which is present in the 

 process of tlie formation of steam. If, in addition, it has a pale 

 light colour, it is almost incapable of receiving heat. 



The actual temperature of the soil is in an inverse proportion 

 witli its susceptibility of Iieat. Since a body, on an average, 

 radiates heat the more readily it receives it, there may be soils 

 which attract heat slowly, and therefore are cooler in summer, 

 but which give off their heat also slowly, and therefore are 

 warmer in winter. We are only speaking however of heat 

 directly received from the sun. There is sometimes an accession 

 of heat independent of the sun, namely from the irrigation of 

 running water. Springs have generally a lower temperature in 

 summer, and a higher in winter, than the surrounding atmos- 

 phere. In consequence, the formation of seed in water-meadows 

 must be checked in summer, and vegetation kept up in winter, 

 on which principle their great produce is most easily explained. 



c. One of tlie most important properties of soils is the power 

 with which they imbibe gases. The question here relates prin- 

 cipally to those which are most important to vegetation, as car- 

 bonic acid and ammonia. Unfortunately, though tables exist 

 relative to the absorption of oxygen, there are none which show 

 how the matter stands with those just mentioned. Oxygen is 

 needed only during germination. 



Where, however, no actual chemical combination exists, it 

 appears that, when one kind of gas is powerfully absorbed, anotiier 

 is also powerfully absorbed by the same substance ; and, there- 

 fore, assuming this to be the case, we may use the existent table 

 for an approximation to the results which may be expected with 

 carbonic acid and ammonia. Now these gases are absorbed by 

 all absorbing substances, and particularly by water, which is of 

 especial consequence to our question, in incomparably greater 

 quantities than oxygen ; and we may therefore assume that the 

 same law liolds good with reference to the soil. With respect 

 to clay, for instance, it is known that, when heated, it begins 

 attain while cooling to receive ammonia from the atmosphere. 

 Many of the constituents of humus have so great an affinity for 

 ammonia that, as for instance crenic and apocrenic acid, they 

 were a long time considered as nitrogenous combinations when 

 we were not in a condition to separate them from ammonia. 

 According to some authorities, humus and humic acid, ulmin 



