258 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



formulate certain general conclusions, based upon the law of 

 maximum and minimum, which we have already mentioned. 



Among the different factors of climate, we must above all 

 consider temperatvire and moisture. ^ When we remember that 

 lowering of temperature is associated with a decrease of bio- 

 logical and chemical activities, we easily understand why 

 decomposition must be slower, and hence accuoaulation of 

 organic residues faster, in the north than in the south, in the 

 highlands than in the lowlands. Where moisture is relatively 

 constant, it is the temperature which controls the decomposition, 

 as in the Lombardo- Venetian plain. Conversely, when tempera- 

 ture is moderate and constant, moisture becomes the pre- 

 dominant agency, as in Sicily and Southern Italy. 



The influence of the soil vai-ies with its position, slope, and 

 physical and chemical constitution. In the dry season, or in 

 a dry country, for instance, a northern exposure will offer the 

 more favourable conditions for decomposition. Among the 

 mountains of the south of France the soil is deeper on the 

 northern slopes, because there the moisture necessary for 

 eremacausis is more abundant and remains longer than on the 

 southern slopes, showing that the greater moisture more than 

 compensates for the lower temperature. With these conditions 

 reversed, a southern exposure will produce the greater intensity 

 of eremacausis. As regards the slope of the ground, when there 

 is an average inclination of 20', the production of carbon dioxide 

 is at a maximum. It decreases when the slope is steeper or 

 more gentle. 



The physical constitution of the soil will determine its heat 

 and the quantity of air and water which it holds. Hence its 

 effects are very complicated. As the amount of water increases, 

 there is a corresponding decrease in the permeability to air. 

 The finer the grain of the soil, the less the permeability to 

 water. Soils which are rich in colloidal substances — humus, 

 cl*y, and iron — are imi)ermeable to air even when they contain 

 an amount of water still far from saturation. On the other 



^ Rcsearclies made by a number of scieutific workers liave shown beyond 

 doubt that ligbt, especially its blue and violet rays, has an injurious 

 i)illuence on bacteria. It is to be hoped that methodical observations and 

 experiments will bring into greater prominence the consequences of this 

 fact, which are, in all probability, of importance in the decomposition of 

 organic matter in nature. 



