156 THE ART OF CULTURE. 



into plants, although we cannot affirm that they 

 are necessary to them. 



It appears that, in certain cases, fluoride of cal- 

 cium 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 

 distinguished 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 

 interior 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, (Berzelius). 



De Saussure remarked, that plants require 

 unequal quantities of the component parts of soils 

 in different stages of their development ; an obser- 

 vation of much importance in considering the 

 growth of plants. Thus, wheat yielded y^fo of 

 ashes a month before blossoming, TtHhr while in blos- 

 som, and TO oo~ after the ripening of the seeds. It is 

 therefore, evident, that wheat from the time of its 

 flowering restores a part of its organic constituents 

 to the soil, although the phosphate of magnesia 

 remains in the seeds. 



The fallow-time, as we have already shown, is 

 that period of culture, during which land is exposed 



