On the Physical Properties of Soil. 



207 



The reason why the temperature observed at Geneva on the 

 several days, in contact with the surface of the earthy rises to a less 

 degree than at Tubingen, depends perhaps on the higher and 

 probably more windy situation in which the thermometer was 

 placed, — Geneva lying 1334 and Tubingen only 1076 English 

 feet above the level of the sea ; nor is it unlikely that the bulb of 

 the thermometer at Geneva was rather deeper in the earth, and 

 in a situation proportionally less warm, namely, exposed to a 

 northern aspect. 



X. Capacity of Soils to develope Heat ivithin themselves on being 

 moistened. — It has already been mentioned (in a former part of 

 my Agricultural Chemistry,') that powdery substances in general, 

 and consequently the earths, possess the property of developing 

 warmth when moistened while in a dry state ; and the results ob- 

 tained on this subject with different bodies have been already 

 communicated in a tabular form. We might suppose that this 

 property in the case of the earths of the soil would be of important 

 influence on the fertility of the land ; this does not, however, 

 appear to be the case. The earths develope warmth in this 

 manner only when moistened after a previous state of perfect dry- 

 ness ; but, in nature, they are scarcely ever found in this perfectly 

 ^dry condition ; and even when dried artificially, the development 

 of heat in the case of ordinary earths is always very inconsider- 

 able, amounting in general to only J° or 1° F. : even with dry 

 humic acid and artificial turf-earth, I could detect no greater a 

 development of heat. The falling rain in warm seasons is many 

 degrees colder than the lower stratum of the atmosphere and the 

 upper surface of the earth, which it immediately moistens ; so 

 that the earth in hot weather becomes rather cooled than other- 

 wise by the rain ; this property of the earths at the utmost can 

 therefore have, perhaps, the effect of diminishing the cooling of 

 the earth by rain some half or whole degree of Fahrenheit, when 

 the earth previously has been yery dry : such a result can have 

 but a very inconsiderable influence on vegetation; and in the 

 colder seasons, when the earth is already damp, so slight a deve- 

 lopment of heat must be inappreciable. 



XI. Galvanic and Electrical Relations of the Earths. — The 

 electrical relations of bodies stand in such manifold relations to 

 chemical and organic processes, that the properties of the earths, 

 even in this respect, deserve consideration. 



Electric Conducting Power. — The pure earths, as sand, lime, 

 magnesia, gypsum, in their dry state, are non-conductors; the 

 clays, on the contrary, are imperfect conductors ; and the compound 

 clayey earths are weak imperfect conductors. The presence of the 



