COPPER— IRON— SILICA. 561 



useless, and that the free sxdphurio acid attacks the straw and renders 

 it less capable of decay. 



It thus appears that the important substances which are 

 required to form an artificial food for plants are 1, Carbonic 

 acid; 3, Nitrogenous compounds; 3, "Water; 4, Lime; 5, Potash; 

 6, Soda ; 7, Phosphoric acid ; and 8, Sulphur. For practical 

 purposes other matters may be neglected. 



We are not, indeed, warranted in regarding the presence of iron, 

 copper, or other substances, in plants, ia minute quantities, as alto- 

 gether accidental and unimportant ; for I do not know what warrant 

 we have for saying that any of the constant phenomena of nature, 

 however minute they may seem to be, are accidental. It is certain, 

 that, where mineral substances occur abundantly in plants, they are 

 part and parcel of their nature, just as much as iron and phosphate 

 of Ume are of our own bodies ; and we must no more suppose that 

 grasses can dispense with silica in their food, or marine plants with 

 common salt, than that we ourselves could dispense with vegetable and 

 animal food. Copper occurs 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 experiments upon these matters, found that 

 the Ramalina fraxinea aiid Borrera oiliaris, two Uohens, 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 suppose that such things are the result of accident, and 

 that it is unimportant to the plants containing minerals thus con- 

 stantly, whether such substances are present in their soil or not ; but 

 we may be permitted to believe that all cultivated land contains as 

 much as plants require, and that they need not be supplied artifloiaUy. 



In order to ascertain which of these is most important to a 

 given plant, recourse has been had to the analysis of the ashes 

 of plants, and we have a large quantity of evidence upon the 

 subject; the value of that evidence has, however, been much 

 disputed. An eminent chemist speaks with great disrespect of 

 the laborious analyses of Sprengel, and those of others have 

 been objected to on various grounds. There are, however, 

 certain great facts which are admitted. AU Grasses and Horse- 

 tails must have silica in abundance ; lime is indispensable to 

 the Vine, to Peas, Clover, Saintfoia, and Tobacco ; soda-salts 

 to mariae plants ; alkaline matter, especially potash-salts, to 

 land-plants, and especially to Conifers and other trees ; phos- 



