PHENOUBNA OF DBLIQUESOBNOS. 67 



In this eflfect of vegetable mould we see how forests may 

 exercise a third influence, over and above and distinct from both 

 shade and shelter, in maintaining a humidity of soil. 



This matter may deserve a little further consideration. 



In treating of Rehoisement in France, I had occasion (p. 95) to cite 

 certain experiments made by Thurmann, made with a view to ascertain 

 to what extent different minerals could absorb and retain moisture, 

 and to state results obtained by him showing that cubes of diflFerent 

 minerals thoroughly dried, weighing each 100 grammes, immersed in 

 water for five minutes, varied in the quantities absoi-bed by them 

 from half-a-gramme to from 10 to 30 grammes. 



These results have been considered indicative of the absorption of 

 water being proportional to the state of molecular sub-division of the 

 material comprising the rock, and they have been resolved into dis- 

 tinct phenomena — hydroscopicity and capillarity : the former, the 

 power of each molecule of the rock to retain around it a layer of 

 moisture difficult to withdraw ; the latter, the power of structures of 

 molecules of earth to retain in interstices by which they are separated 

 small globules of water. And thus may the results obtained by the 

 first-mentioned experiments be accounted for. They indicate greater 

 hydroscopicity and capillarity to be possessed by the soil of the forest 

 than is possessed by gravelly and sandy soil of the road — this being, 

 as may afterwards be proved, an eflfect or consequence of forest pro- 

 ducts embodied in that soil. 



But besides substances which manifest capillarity and hydroscopicity 

 there are substances which attract and abstract moisture from the 

 air, some of them doing so to such an extent that they deliquesce or 

 dissolve in their own brine ; and unless the air be kept excluded from 

 them, or they be kept in a warm place, where the heat will either 

 keep the air comparatively diy, or evaporate the moisture attracted 

 and absorbed by them, you cannot keep them dry. This is signally 

 the case with the carbonate of potash — the potash of commerce ; it 

 is manifestly the case with chloride of sodium, or table salt ; it is 

 also the case, though less manifestly so, with clay ; and to a very 

 marked degree is it the case with humus, the product of the decay 

 and decomposition of vegetable matter, and une which abounds in 

 forest soil from the fall of leaves and twigs. 



To this humus mainly may be attributed the great attraction of 

 the forest soil for moisture, indicated by the gain of weight when, 

 after being thoroughly desiccated, it is exposed to a dry atmosphere ; 



