OF SOIL AND MANUKE. 337 



Mustard ; copper in Coffee,* Wheat, and many other 

 plants (it is believed in the state of a phosphate) ; 

 iron, as a peroxide, in Tobacco. John, in his experi- 

 ments upon these matters, found that the Eamalina 

 fraxinea and Borrera ciliaris, two lichens, contained 

 a great quantity of the last metal, although he could 

 not find a trace of it in the Fir tree, on the topmost 

 branches of which the lichens grew. "We cannot sup- 

 pose i/Iiat such things are the result of accident, and 

 that it is unimportant to the plants containing mine- 

 rals thus constantly, whether such substances are pre- 

 sent in their soil or not.f 



* Seventy millions of kilogrammes of coffee arrive annually in 

 Europe ; of these, 560 kilogrammes consist of copper, according tc M. 

 Sarzeau. The weight of copper consumed in bread in France is 3660 

 kilogrammes annually. (Z)e Candolle, Physiologie Vegetate, p. 389.) 



f [The quantity of earthy or saline matters which different plants 

 absorb 's probably nearly uniform, under the same circumstances, 

 since their roots possess little if any power of selection. But the 

 quantities which are retained vary according to the constitution of 

 each species, and the chemical composition of its products. The 

 ashes of Pine and Fir trees, no matter on what soil they may have 

 grown, contain a much smaller quantity of alkalies than the ashes 

 of the Oak, Beech, or Maple. Hence the former are found to thrive 

 upon sandy or sterile soils, which do not furnish sufficient alkali for 

 the latter. All plants of the grass kind require a considerable por- 

 tion of silicate of potash, which is deposited in their stems ; and of 

 phosphate of magnesia, which is a constituent of their seeds. The 

 quantity of both varies in different species. ' One hundred parts of 

 the stalks of Wlieat yield 16'5 parts of ashes ; the same quantity of 

 the dry stalks of Barley 8-54 parts ; and one hundred parts of the 

 stallcs of Oats only 4'42 ; the ashes of all these are of the same com- 

 position. " These several crojjs, therefore, extract the potash of the 

 soil in different degrees ; and upon a field which will yield but one 

 harvest of Wheat, two crops of Barley or three of Oats may be 



15 



