J. H. Gilbert— Poin 



The subject of vegetation is such a very wide one, and might 

 be treated of in so many different ways, that it seems desirably 

 to state at the outset what is the scope, and what are the limits, 

 of the discussion which I propose to bring before you. I pro- 

 pose, then, to confine attention almost exclusively to the ques- 

 tion of the Sources <>t the iit'<-nf/i>n of vegetation in general, and of 

 a/jn'rii'lnrnt production in particular. I propose further to treat 

 of this subject mainly in the aspects in which it has forced 

 itself upon the attention of Mr. Lawes and myself during the 

 now thirty-three years of our a. itions; and, 



also, in so far as it, illustrates, and is illustrated by, the objects 

 contributed by Mr. Lawes to the Exhibition around us. 



Before entering upon the special subject matter of my dis- 

 course, I must claim the indulgence of those present, who are 

 already well acquainted with the main facts of the chemistry of 

 vegetation, while I call attention, very briefly, to some rather 

 elementary matters, with a view of rendering what has to follow 

 the more intelligible to an\ who may be less fully informed on 



When a vegetable substance is burnt — as a familiar instance, 

 let us say tobacco, for example — the greater part of it is dissi- 

 pated, but there remains a white ash. The ashes of crude or 

 unripe vegetable substances are found on analysis to contain 

 most, or all, of the foil* i namely : — 



Oxide of iron, oxide of manganese, lime, magnesia, potass, 

 soda, phosphoric acid, sulphuric acid, chlorine, and silica. 



Barer substances than these are also sometimes found. Now, 

 much lias of late years been established in regard to the occur- 

 rence, and the offices, of some of these substances in plants; 

 but I do not propose to touch upon the questions herein 

 involved. It will sulliee further to say in regard to these in- 

 combustible, or "mineral" constituents, that the ash of one and 

 the same description of plant, growing on different soils, may, 

 so long as it is in the growing or immature state, differ very 

 much in composition. Again, the ashes of different species, 

 growing on the same soil, will differ very widelv in the propor- 

 tion of their several constituents. But it is found that the nearer 

 we approach to the elaboration of the final products of the plant 

 — the seed, for example —the more fixed is the composition of 

 ash of such products of one and the same species. In other 



