212 



On the Physical Properties of Soil. 



Comparative Review of these Results. — In the preceding Table 

 I have brought the principal results obtained from the several 

 earths into a comparative view, in order to show at once those 

 different properties, the joint operation of which exercises so 

 great an influence over the processes of vegetation. With respect 

 to the different variations of these properties in the earths, I must 

 refer back to the paragraphs in which they have been severally 

 treated. This tabular arrangement will much facilitate the cri- 

 tical examination of the physical properties of the several kinds 

 of soil, and may relieve others from the necessity of undertaking 

 anew, in every examination they may wish to institute, those re- 

 searches which T have myself often found both troublesome and 

 tedious. 



In the examination of soils, the determination of their power 

 of containing water, and of their weight, consistency, and colour, 

 in connexion with their chemical analysis, will, in the majority of 

 cases, be sufficient to enable us to conclude, with great probability, 

 as to their remaining physical properties. The more an earth 

 weighs, the greater also in general is its power of retaining 

 heat ; the darker its colour and (at the same time) the smaller 

 its power of containing water, the more quickly and strongly will 

 it be heated by the sun's rays ; the greater its power of con- 

 taining water, the more has it in general the power also of ab- 

 sorbing moisture when in a dry, and oxygen when in a damp, 

 state from the atmosphere, and the slower it usually is to be- 

 come dry, especially when it is endued at the same time with a 

 high degree of consistency ; lastly, the greater the power of con- 

 taining water and (at the same time) the consistency of a soil, the 

 colder and wetter of course will that soil be, as well as the stiffer 

 to work either in a wet or dry state, and the more judicious there- 

 fore will it be to break it up before the setting in of the frost, 

 in order that its consistency may be improved by the due penetra- 

 tion of the frost during the winter ; and, for the cultivation of many 

 plants, the more requisite will it be found for the permanent im- 

 provement of such a soil, to counteract its too great consistency and 

 power of containing water, by mixing with it looser earths, as 

 lime, marl, and sand. 



