in its Relations to Chemistry and Geolor/y. 
231 
proportion of mineral acids and alkalies in plants varies. But 
they contain organic acids * and alkalies. Have these mineral 
and organic substances any relation to each other in quantity ? 
Do they perform the same general functions ? Can they take the 
place of each other in whole or in part ? When mineral acids or 
bases are scarce, can organic acids and bases be produced or taken 
up in their stead, so as to form a healthy plant? If so, what in- 
fluence has soil or culture upon this, and how far does this replace- 
ment of mineral by organic matter affect the nutritive qualities of 
the produce? 
h. Again, in what states of chemical combination the food enters 
into the roots of plants is a question the immediate bearing of 
which upon practice you can readily perceive. In what states can 
the different kinds of food enter ? — in what states is it best for the 
plant that they should enter ? — and what is the influence of cir- 
cumstances in bringing them into these states? Of all these we 
know as yet almost nothing but what rests on bare analogy. j 
The results of direct experiment are almost entirely wanting. 
* Lime is combined with quinic acid in the China b.ark, with oxalic acid 
in the lichens and cacti, with tartaric acid in the grape, and with citric acid 
in the lemon. Potash is combined with citric acid in the lemon, tartaric 
acid in the grape, malic acid in the gooseberry, oxalic acid in the sorrel, 
and so on. 
+ It is not easy for the common order of practical men at once to see 
how refined analytical investigations or apparently abstruse theoretical 
speculations can have any direct application to practical agriculture : 
and yet there is hardly a new speculation in organic chemistry which has 
not some bearing upon the physiology of plants and a connection therefore 
more or less remote with their profitable culture. I will give an illustra- 
tion. 
A very ingenious, and as it appears to me a not improbable, view of the 
constitution of those organic acids which occur in plants, has recently been 
advanced by Frankland and Kolbe, that they consist, namely, of oxalic 
acid in combination with a radical consisting of carbon and hydrogen only, 
and having the form generally of the radicals of the alcohols. 
Now plants absorb carbonic acid by their leaves, and give off in return 
nearly as much oxygen as the carbonic acid contains. But the nature of 
the decomposition by which this evolution of oxygen is effected is per- 
fectly unknown to us. 
We know that plants coiitain oxalic acid, and we can understand that if 
carbonic acid lose one-fourth of its oxygen it is converted into oxalic acid. 
The production of this acid in this way, in the leaf, would require the exer- 
tion of less energy, if I may so speak, on the part of the chemical rays in 
the sunbeam, and consequently less time than the separation of the whole 
of the oxygen from the carbonic acid. 
Now if this acid when formed unite in the sap with some radical, con- 
sisting only of carbon and hydrogen, to form either the other acids which 
occur in plants or the indifferent substances, cellulose, dextrin, starch, and 
sugar, which are always present in them, then this carbo-hydrogen com- 
pound may have its origin either from the root or from the leaf. 
. If it be formed by or in the leaf, then the burden is laid upon the leaf and 
upon the rays of light which fall upon it, of decomposing both carbonic acid 
and water to produce this radical, in addition to that of producing the 
