200 



On the Physical Properties of Soil 



means to become for tlie first time saturated with the quantity of 

 oxygen essential to vegetation^ while at the same time it becomes 

 looser, and enriched with greater portions of humus from 

 manures or decaying vegetables. 



3. Clay-soils containing humus exhibit a particularly strong 

 absorption of oxygen ; they maintain themselves also for a longer 

 time moist in dry weather than the sandy soils ; properties, both 

 of which must contribute to the fertility of these soils, especially 

 when they are at the same time sufficiently free. 



4. In subterraneous cavities excluded from the air, for instance 

 in mines, there occurs not unfrequently a production of suffocating 

 air, or choke-damp, as it is called, a phenomenon which appears 

 to be often a consequence of this absorption of oxygen gas. The 

 strata which enclose these places being frequently damp and 

 clayey, are consequently capable of absorbing easily the oxygen 

 gas from the air included within them, while the mephitic air is 

 thus left in their recesses: if these strata contain also humus, 

 or especially if carbon be found in them, as is the case with coal- 

 blende, pit-coal, &c., they will form carbonic-acid gas ; should 

 decomposition of water take place, in consequence of metallic 

 agency, as might so easily ensue with the sulphuretted pyrites, 

 hydrogen gas is set at liberty, and an explosive atmosphere be- 

 comes thus easily formed. 



5. In the clay-soils, nitric acid and the nitric salts are frequently 

 formed ; this occurs particularly during the artificial production of 

 saltpetre and in the slow process of saltpetre-beds ; and is also 

 found to take place spontaneously here and there in the upper 

 beds of soil, independently of the effect of artificial means ; the 

 absorption of oxygen gas induced by the soil, has probably a very 

 considerable influence in these formations of nitric acid. 



VIII. Power of the Earths to retain Heat. — The earths have the 

 property of giving out again to surrounding bodies, in different 

 lengths of time, the warmth communicated to them by the sun or the 

 temperature of the atmosphere, and of retaining, therefore, such 

 warmth within themselves for a longer or shorter space of time ; 

 this may be termed their power of retaining heat. It is not 

 identical with specific heat, as it does not depend merely on that 

 condition, but on the different capacity as well, which bodies pos- 

 sess of conducting heat. It is generally of a higher degree in pro- 

 portion as the specific heat of a body is greater, and its power of 

 conducting heat is less ; these two properties combined will con- 

 stitute the power of retaining heat. 



We may adopt the following process for examining the power 

 of retaining heat. We place equal quantities of the several earths 

 in the dry state in large vessels of similar size, made of thin 



