On the Thermal Capacity of various kinds of Soils. 57 



with what has been said ; it only carries out the process more in 

 detail, and shows how the change of heat into work is effected. 

 The conclusion is, that the introduction of mineral constituents 

 into the plant is only possible by a consumption of heat. 



It may be shown that the absorption of the other groups of 

 constituents must have the same result. The formation of com- 

 bustible vegetable matter is, on the whole, a process of reduction. 

 Carbon, which here more especially comes into account, is changed 

 from its most highly oxidized to less highly oxidized forms. 

 The same quantity of heat which becomes free when wood is 

 burnt into carbonic acid and water, must be absorbed in the in- 

 verse process, or, to speak more precisely, used in increasing the 

 disgregation of the substances. Hence the process of the growth 

 of a plant is in all cases connected with a consumption of heat. 



The important part which heat plays among the conditions of 

 vegetation has long been recognized, though not considered from 

 the point of view of the mechanical theory of heat. 



In determining the influence of the composition of the soil 

 alone, it has been attempted to eliminate that of heat by inves- 

 tigating the growth of plants on different soils, either natural or 

 artificial, which were in an exactly identical condition as regards 

 the amount of heat which they received. Yet experiments of this 

 kind have not given definite results ; and it may be shown that 

 the condition of equal quantities of heat under these circumstances 

 is not fulfilled. For even if the temperature of the air were the 

 same, that of the ground is not necessarily so — because this de- 

 pends on the capacity for heat, on the internal and external con- 

 ducting-power, on the absorptive power of the surface as regards 

 radiant heat and light. And even were the temperature of the 

 soil the same, it by no means follows that the plant receives 

 equal quantities of heat. 



It has been observed that the growth of certain species de- 

 pends on the quantity of heat ; while in the case of others the 

 attainment of a certain maximum and the never falling below 

 a certain minimum, appears to be a condition of life. Now 

 upon both conditions the physical properties adduced exercise 

 the greatest influence, but more especially capacity for heat and 

 conducting-power. Their actions must have the greatest influ- 

 ence on those species which only attain a slight height ; because 

 in these the heat which they receive from the ground forms a 

 greater fraction of the total heat. 



If the conditions of vegetation are to be properly investi- 

 gated and reduced to an exact basis, this can only be done by 

 taking into account the physical properties. It is only when we 

 know how much the thermal capacities of various soils differ, that 



