60 Influence of the Physical properties of Soils on Vegetation. 



crystal have the very similar specific heats O20 and 0*19, and that, 

 moreover, most of the other silicates have values very near 0*19, 

 according to Neumann and Kopp's determinations. Magne- 

 sian spar also agrees completely with calcareous spar. 



We shall not be far out in ascribing to all dry soils free from 

 humus a common thermal capacity of about i. The highest 

 thermal capacity I observed was that of turf (0-507). Next to 

 it stands the soil which contains most humus of all which I in- 

 vestigated (from the top of the Schneeberg), with 0*4143; and 

 next to this is the soil from the granite plateau, in Muhlviertel, 

 with 0*3489, which is likewise very rich in humus. 



The rest of the earths lie between these extremes, according to 

 their proportion of humus. Most of them have a specific heat 

 of 025 to 0*28. 



Besides the greater proportion of organic remains, a high per- 

 centage of water must materially raise the thermal capacity. 

 Hence clayey soils more especially, which absorb and retain 

 water, must come near the soils richest in humus. 



Hence we now know that the geognostic character of the sub- 

 stratum is of inconsiderable importance as concerns the relation 

 to heat ; and in its place we must put proportion of humus and 

 power of retaining water. To these, therefore, most attention 

 must be paid in future investigations. 



It has been observed that plants w r hich occur usually on schists 

 can grow also upon a lime substratum, if this is covered with a 

 strong cushion of humus. This is explained by saying that 

 this layer of humus keeps the lime from the plant ; but may it 

 not also be that this humus z acts, in a certain sense, as a substitute 

 for those very moist loamy soils so frequently formed by the 

 disintegration of the schist ? for both substrata belong to the same 

 class as regards their capacity for heat. 



A complete solution of the question can only be obtained with 

 a better knowledge of the conducting-power of soils ; but mean- 

 while some indications may be given of the influence which a 

 difference in the thermal capacity of earths exerts on the tempe- 

 rature, even with equal conducting and absorptive power. 



If equal quantities of earth with equal surfaces, one with the 

 specific heat =0*2 and the other =0*4, both at the same tem- 

 perature, be exposed to the heating action of the sun and warm 

 air, the first warms sooner than the second. Hence if, for in- 

 stance, the quantity of heat which the first receives is sufficient 

 for that increase of temperature which a plant needs, that received 

 by the second is insufficient. 



Again, the greater capacity for heat delays the diminution of 

 temperature. If in any night the temperature of the first-men- 

 tioned earth sinks just so far that a plant freezes, it does not 



