35 



and those plants, particularly clovers, which are impatient of 

 sudden change of temperature, are readily destroyed by the 

 frost.* And soils by the growth of white clover, red clover, 

 and tares, become more pulverulent, puffy, and less cohesive 

 in proportion to the frequency of the growth of these crops, 

 and this explains why these lands tire of clover. The several 

 particles of earth, by the long, deep, and numerous radicles of 

 these plants, become mechanically forced from their position, 

 and their points of contact thus rendered fewer in number ; 

 and such soils comparatively between themselves become spe- 

 cifically lighter in proportion to the weight of a determinate 

 volume. That the soils upon which the clover is destroyed 

 do lose the cohesiveness of their particles, and, when compared 

 with those of the same chemical constitution which preserve 

 the plant, are specifically lighter in proportion to the weight 

 of a determinate volume, is proved by the following trials, 

 in which a solid foot of each was dug from the ground and 

 weighed upon the spot. 



In the two portions of the field whose soil was analysed, 

 several trials of the weight of the two soils gave a difference 

 of 6 lbs. to 7 J lbs. per solid foot (of 17 x 17 x 6 inches, or 6 

 inches in depth) or from 117 to 140 tons weight per acre over 

 and above the failing or sick portion of the field : and this 

 difference of the weight does not arise from a difference in 



* " The power of retaining heat is nearly in proportion to the weight of a deter- 

 " minate volume of soil, i. e., to absolute weight; the greater mass an earth 

 *' possesses in the same volume, the greater will be in general its power of retaining 

 " heat — we may, therefore, from the absolute weight of an earth, conclude with a 

 *' tolerable degree of probability, as to its greater or less power of retaining heat." 

 f Schuhler on the Physical Properties of Soil, p. m, vol. I. part 2, Journal of Royal 

 Agricultural Society of England. J " When different earths are mixed together, a 

 *' cubic inch of the mixture obtained gives a greater weight than the arithmetical 

 " mean (or common average) of the earths, entering into the mixture, whether 

 " mixed in equal portions, according to weight or volume, or in other quantities." 

 f Ibid, p. 181.; 



e. g. 2 parts of sand and 1 part of clay, by volume, have an arithmetical mean 

 weight of 2.566, while the actual weight is 2.825, or an increase of 259. 



D 2 



