It may be asked, How is the decay, or fermentation, of vegetable matter, such as farmyard manure, caused? or rather, To what is it due? Decay of any substance is just its slow combustion or burning. When a substance unites with the active chemical element in air--the oxygen gas--it is said to be oxidised. Now, this union of a substance with oxygen is the explanation of burning, and the phenomena of burning and decay are explained by the same chemical operation. When bodies decay, or when they burn, they unite with oxygen: when this union of a body and oxygen takes place very quickly, and the result is a flame and very great heat, then we call it burning; when, however, it takes place slowly, it is not called burning, but simply oxidation or decay. The ultimate products are the same, however, whether the body burns or decays; and the process of decay is always accompanied by heat, as well as the process of burning.[45] It is not, of course, only the vegetable or organic matter in a soil that decays, but also the mineral matter. The oxidation, however, of the mineral matter in the soil takes place so slowly, and the amount of heat generated by this oxidation is so slight, that the temperature of the soil can scarcely be said to be much affected by it. _Influence of Colour of a Soil._ There is still another quality of a soil on which its temperature depends, and that is its colour. This may seem at first sight to be scarcely worth taking into account, and yet it has been shown to have a very striking influence on the temperature of a soil. This naturally is best seen in climates where there is a good deal of sun. Dark-coloured soils have a greater heat-absorbing capacity than light-coloured soils; and experiments carried out for the purpose of determining the extent of this influence have shown that under certain conditions the difference between a soil covered with a black substance, and one covered with a white substance, amounted to from 13 deg. to 14 deg. Fahr. Other things being equal, a crop on a dark-coloured soil will be sooner ripened than one on a light-coloured soil. A soil covered by a crop is cooler than one without any crop. _The Power Soils have for absorbing Gases._ We have just seen that one cause of the heat of soils is the oxidation which is constantly going on in all soils, but more rapidly in soils containing a large quantity of vegetable matter. This suggests a word or two on the power soils have of absorbing gases.