On the Physical Properties of Soil. 195 



drying the greatest diminution of bulk, contracting one -fifth of its 

 volume on being dried, and again expanding in the same propor- 

 tion when moistened with water ; this explains the reason why 

 the upper surface of the earth in damp turf bottoms, containing 

 much humus, frequently rises or sinks several inches accordingly 

 as the soil is penetrated with more or less water, and why this 

 elevation of wet turf-soils becomes still more remarkable, when a 

 sharp frost sets in after wet weather, the freezing, by its expan- 

 sion, still further increasing the volume of the particles of water 

 which had previously penetrated the turf ; hence, too, the reason 

 why these turf-bottoms have in their wet state a remarkable elas- 

 ticity if heavily trodden upon, and often occasion, in consequence 

 of that yielding property, the feeling of fluctuation. 



VI. Property of the earths to absorb moisture from the atmos- 

 phere. — Most of the earths which are commonly found in soils 

 have the property in their dry state of absorbing moisture from 

 the atmosphere^ and this circumstance has a considerable influence 

 on their different degrees of fertility. 



The amount of this absorption may be found, by spreading a 

 given quantity of the fine and previously well-dried earth on a 

 plate, and placing it under a glass receiver, having its inverted 

 mouth closed underneath by immersion in water :— 



a 5 is the earth lying on the plate, which rests on a stand ; c is 

 the vessel below, containing the water into which the receiver is 

 inverted and thus closed from the external atmosphere. We allow 

 the earths to remain under this receiver the same time respectively 

 — 12, 24, or 48 hours — in a mean temperature, varying from 59° 

 to 65J° F. and then weigh them again; the increase of weight 

 corresponds to the quantity of water absorbed. The foUo^Aing 

 Table exhibits a statement of the results I obtained, in reference 

 to this inquiry, with the usual earths ; the whole of the experiments 

 were made in a temperature which varied between 59° and 65J° F., 

 and the amount of absorption is given in grains : — 



o 2 



