GARDENING AS A SCIENCE. 
227 
In the first place, whether the Italian compost be rigidly followed, or fibrous 
heath-mould, leaf-soil, decayed cow-manure, &c. &c, be substituted ; certain it is 
that the iron filings, however introduced, could not act directly upon the organism 
of the roots ; but would be gradually converted to an oxide, by the decomposition 
of water — directly, or secondarily, by that of the vegetable matter contained in the 
compost. In either case, we do not expect that any of the iron would be dissolved 
and conveyed through the sap vessels into the floral organs ; unless, indeed, some 
vegetable acid were produced capable of acting upon the iron at the moment of its 
oxidation, and thus forming a soluble chalybeate salt. 
To ascertain with any kind of precision the operation of the iron, it would be 
needful strictly to analyze the earth prior to the addition of the iron, and thus to 
note the quantity of iron already present in it. Then to mix a given weight of 
filings with any required quantity of soil or compost, and set the mixture aside in 
a garden-pot, keeping it just so moist with rain-water, as would enable a plant 
to grow and thrive, and placing by its side another pot of the same soil con- 
taining a young Hydrangea, just raised from a cutting. 
If the blue tint were obtained in the first head of flowers, a small portion of the 
soil should be taken from the immediate vicinity of the roots, and a similar portion 
from the centre of the pot without any plant. Both portions being dried at the 
heat of boiling water, an equal quantity (say one drachm of each) should be 
accurately weighed, and minutely analyzed, so as to determine the quantity and 
precise condition of the iron. The result, though it might not prove demon- 
strative, would, we apprehend, lead to a shrewd conjecture how far the iron had 
acted as a primary or secondary agent. 
There can be no doubt that, in either case, the filings would be converted to 
rust, or oxide of iron ; but were the soil in which the plant had grown, found by 
comparison to contain a diminished proportion of iron, it might be inferred that the 
plant had absorbed a certain quantity, w T hich being laborated in appropriate vessels, 
had thus become the direct agent in effecting the change of colour. 
The extraordinary change of colour effected in the foliage of many plants by 
the free use of leaf-mould, leads to the inference, that by the decomposition of 
vegetable matter certain hydro-carbons are produced, which are taken up by the 
absorbents of the roots. But iron, however fine its particles, cannot be absorbed ; 
therefore, the filings or dust must of necessity be so changed as to become soluble ; 
or, by attracting oxygen from the moisture of the soil, and also from the decom- 
posing leafy remains, to be converted into an oxide. In the former case, a salt of 
iron, soluble in water, may combine with the crude sap ; but in the latter, the iron 
will remain, as rust, in the body of the soil. Yet, though inactive in itself, the 
new products, or some one of them, resulting from combinations of the elements of 
the leaf-soil, will become the colouring agent. 
The infinite play of affinities between oxygen, hydrogen, and carbon, when 
excited by galvanic electricity, must induce phenomena altogether beyond the ken 
