66 EFFECTS OF FOEBBTS ON HUMIBITT. 



of a forest, sheltered and shaded by the trees, and expose it for some 

 hours to the bright sunshine ; after this weigh it, and most probably 

 it will be found to have lost weight. This loss will have been 

 occasioned by the sunshine and open air having caused an evapora- 

 tion of moisture from the soil, though it appeared to be dry, and the 

 quantity of moisture so evaporated may be determined by the 

 weight lost. 



Take again the same weight of such forest soil, so dried in the air 

 and sunshine, and of such dry gravelly saudy soil as you may find on 

 the road, similarly exposed to the drying influeaoes of the air and sun- 

 shine ; expose them together on papers for an hour in an oven heated 

 to such a degree that the papers will be browned, but not burned ; 

 weigh them again, and probably both will be found to have lost 

 weight, but the loss of weight sustained by the soil taken from the 

 forest will probably greatly exceed the loss of weight sustained by 

 the soil taken from the open field. The loss of weight is occasioned 

 in this experiment, as in the first, by loss of moisture contained in 

 the soil, notwithstanding that this was sun-baked and apparently dry. 



Had the papers been burned there might also have been burned 

 organic, vegetable, or animal remains in the soil, occasioning a 

 loss of weight in addition to that occasioned solely by the loss of 

 moisture, for which allowance must have been made ; but this has 

 been avoided by the precaution adopted. And the heat prescribed 

 was to be such as would brown the paper without burning it, to 

 secure as great a heat as possible short of what would have done 

 more than merely drive oif the moisture contained in the soil. And 

 if the loss of weight sustained by the soil from the forest so exposed 

 be in excess of that sustained by the soil taken from the road, this 

 indicates that it, when apparently dry, contained more moisture than 

 did that other, which excess was attributable to moisture held fast 

 by products of vegetable decomposition in the soil. 



But this is not all. If equal portions of the two parcels of 

 thoroughly desiccated soil be spread out on glass, and exposed for a 

 day or two to the atmosphere, protected from rain or dew (within 

 doors or without, in a dry room or in a cellar), and these be then 

 again weighed, they will be found to have gained weight, but in 

 different degrees, much more having been gained by the forest soil 

 than by the sandy gravelly soil of the road. This will be done by 

 their imbibing moisture from the atmosphere, dry as that atmosphere 

 seemed to be ; and the proof is forthcoming : expose them again in 

 an oven as before, and the weight gained will again be lost, 



