On the Physical Properties of Soil 197 



sorbedj in 12 hours, 37 grains of moisture, and consequently more 

 than a very fertile garden-mouhl, which in the same time had 

 absorbed only 35 grains. The assumption of Davy,* that this 

 capacity of absorption possessed by a soil was to be received as a 

 conclusive proof of its fertility, is liable, therefore, to many ex- 

 ceptions : and, if applied without modification, might easily mis- 

 lead. 



VII. Property of earths to absorb oxygen gas from the atmos- 

 phere. — The earths possess the remarkable property of absorbing 

 oxygen gas from the atmospheric air, a phenomenon pointed out 

 many years ago by Alexander von Humboldt ;f this fact has 

 indeed been subsequently called in question by some philosophers, 

 but a more recent and extensive series of observations which I 

 have myself made on this subject, and communicated in the eighth 

 volume^ pages 141, Sec, of the new series of Schweigger's 

 *^ Journal of Chemistry,' shows that this property of the earths is 

 confirmed almost without an exception, provided they be em- 

 ployed for this purpose in a moist state ; the capability, therefore^ 

 of the earths to absorb moisture from the atmosphere appears to 

 be of great importance, in dry seasons,, as a preparation for this 

 further process of absorbing oxygen, which we have now to 

 examine. 



In order to examine this property, introduce determinate quan- 

 tities of the several earths, about 200 grains of each, in their 

 moistened state into glass vessels (flasks) of equal size, and con- 

 taining each about three or four cubic- inches of atmospheric air ; 

 make them air-tight by means of stoppers, surrounded at the edge 

 with resinous cement ; and, after several days have elapsed, test 

 this included air for the quantity of oxygen it may contain by 

 means of an accurate eudiometer, and thus ascertain the amount of 

 oxygen gas absorbed, by the diminution which is found to have 

 resulted in the proportion of that gas contained in the remaining 

 air. The following table contains the results I obtained from my 

 experiments on this point with the several earths; the experiments 

 w ere made in glass vessels of 15 cubic-inches' contents, and with 

 1000 grains, in each case, of the several earths in a moderately 

 moistened state, and in a temperature varying from 59° to 65J° F. ; 

 excepting in the case of magnesia, of which, on account of its 

 levity, only half that quantity was taken ; the air remaining was 

 afterwards analyzed by the voltaic eudiometer, and from the 

 volume of the air absorbed its quantity was calculated by weight ; 

 for the sake of comparison, other earths of the same kind were 

 likewise exposed in a state of perfect dryness. 



* Davy's 'Agricultural Chemistry,' 4to. 1813, pages 159, 160. 

 t Gilbert's * Annals of Philosophy,' vol. i. p, 512. 



