HUMUS AS A GEOGRAPHICAL AGENCY. 259 



hand, the amount of water influences conduction and calorific 

 capacity. For example, during the cold season, a peaty soil 

 is the warmest, then loam, limestone, and lastly sand. In 

 spring these temperature relations are altered, so that in summer 

 the sand is wai-mest, then peaty soil, limestone, and loam. 

 Thus if we ai'e considering the warm period only, the thermal 

 characters of clay or loam are the least favourable for decom- 

 position, those of sand the most favourable. Hence it will be 

 seen why practical men call clayey grounds cold and sandy 

 soils warm. 



The chemical nature of the soil may be of great importance. 

 For example, lime, as carbonate, furthers the formation of 

 humic acids ; as sulphate, it favours nitrification. 



The carpet of living plants and the decaying deposits are 

 at work hastening or abating the above-mentioned influences. 

 A living covering retards the decomposition of organic matter 

 just in proportion to the vigour and density of the ])lants 

 composing it, while in a soil coated with dead material the 

 eremacausis is slower than in bare earth, but more rapid than 

 in ground clothed with growing plants. The result of the 

 decomposition of organic matter is thus the production of a 

 number of elementary volatile substances with a solid organic 

 residue containing also mineral salts. It is to this organic 

 residue that the name humus applies. 



Wherever a moderate amount of moisture, and a fairly high 

 temperature, along with enough air, produce a strong ere- 

 macausis, humus is only formed in trifling quantity. When 

 one of the factors falls to minimum, the gasification of the 

 organic matter correspondingly decreases, balanced by an 

 increase of solid residues. Climate and soil thus both have 

 their influence upon the production of humus. Humus is 

 comparatively thin in a moist and moderate climate, as in the 

 greater part of France, where peat-bogs are rare, occurring only 

 in the mountains. In tropical regions, whei-e the insolation 

 is continuous, fermentation may proceed so rapidly that 

 practically no humus residue is left. With the lowering of 

 temperature at higher latitudes or altitudes, the thickness of 

 organic residues increases, as we see in the zone of lowland 

 peat-bogs or moi^asses of Ireland, Holland, Hanover, North 

 Germany, and Russia, or in the mountain peat-bogs. 



