FERTILITY OF SOILS. 119 



also are absorbed into plants, although we cannot affirm that they 

 are necessary to them. 



It appears that in certain cases fluoride of calcium may take 

 the place of the phosphate of lime in the bones and teeth ; at 

 least it is impossible otherwise to explain its constant presence in 

 the bones of antediluvian animals, by which they are distin- 

 guished from those of a later period. The bones of human 

 skulls found at Pompeii contain as much fluoric acid as those of 

 animals of a former world ; for if they be placed in a state of 

 powder in glass vessels, and digested with sulphuric acid, the in- 

 terior of the vessel will, after twenty-four hours, be found 

 powerfully corroded (Liebig) ; whilst the bones and teeth of 

 animals of the present day contain only traces of it. (Ber- 

 zelius.)* 



In spring and in the first half of the summer, when the 

 earth is still moist with water, it is quite certain that a greater 

 quantity of alkaline bases and of salts must enter the organism 

 of a plant, than in the height of summer, when there is a 

 deficiency of water, this being the means of carrying the bases 

 to the plant. 



In many districts the crops of corn for the whole year depend 

 upon a single shower of rain ; for when water is deficient at a 

 certain period of the growth of plants, their future progress is- 

 retarded. The introduction of water to a soil is, properly speak- 

 ing, an introduction of alkalies and of certain salts, which, by 

 means of rain-water, become fit to be absorbed by plants. In 

 the middle of summer the air is much more charged with the 

 vapor of water than at other seasons of the year, and, therefore, 



* The researches of Daubeny, however, tend to show, not only that the 

 amount of fluoride of calcium in bones is larger than is commonly sup- 

 posed, reaching in some cases to 10 or 12 per cent, of the bone earth, but 

 that recent bones contain as much as fossil and ancient bones do. In 

 recent bones, however, it cannot be so easily detected, until they have 

 been burned, the presence of gelatine seeming to impede the detection of 

 fluorine by the usual tests. Dr. G. Wilson has very recently shown that 

 fluoride of calcium is soluble in water to an extent quite sufficient to ac- 

 count for its very general diffusion. He has found it in sea- water, and in 

 all the springs which he has examined. Daubeny suggests that the pre- 

 sence of fluoride of calcium in bones may prevent any tendency to crystal- 

 lization, and thu9 confer on the bone additional toughness. — W. G. 



