GARDENING AS A SCIENCE. 
229 
acid, and the oxide of iron, which is abnost always in its least oxidised state, tends 
to combine with more oxygen ; and the consequence is, that the feldspar decom- 
poses, and likewise the mica ; but the first the most rapidly. The feldspar, which 
is as it were the cement of the stone, forms a fine clay ; and the mica, partially 
decomposed, mixes with it as sand : the undecomposed quartz appears as gravel, 
or sand of different degrees of fineness." 
Although it is impossible to enter into any particular details of such operations 
conducted in the vast laboratory of nature, yet in the foregoing statement we find 
general truths : these were fully appreciated by Liebig ; for we read in his preface, 
that " since the immortal author of the * Agricultural Chemistry,' no chemist has 
occupied himself in studying tlie applications of chemical principles to the growth 
of vegetables, and to organic processes. I have," he adds, " endeavoured to follow 
the path marked out by Sir Humphry Davy, who based his conclusions only on 
that which was capable of inquiry and proof." 
Heretofore we find almost all writers attempting to prove that plants derived 
their support from watery solutions of putrescent — i. e. of decomposable, 
matters^ contained in the ground, and conveyed through the roots into the 
organic structure. But Liebig—seizing the facts elicited by his great predecessor, 
examined by repeated and minute experiments the products of vegetables that 
had been submitted to the action of fire, and thence infers, that — " trustworthy 
examinations of the ashes of plants of the same kind, growing upon different soils, 
would be of the greatest importance to vegetable physiology," — and so it has 
happened ; — for now, instead of insisting any longer upon the old theory, we 
find all our best practical writers devoting their attention to the inorganic con- 
stituents of plants, which constituents are traceable in the ashes left after incinera- 
tion. By tliese inquiries we arrive at a simple and determinate system of 
manuring, and are enabled to reconcile apparent inconsistencies, for tliey instruct 
us that no substances whatsoever can enter the plant through the infinitely minute 
tissue of the roots, excepting those which are absolutely soluble in the w^ater of 
the soil, and such substances are potash, certain combinations of phosphoric acids, 
neutral salts, and electro-chemical combinations of carhon ; — but to recur to 
Davy. " As soon as the smallest layer of earth is formed on the surface of a rock, 
the seeds of lichens, mosses, and other imperfect vegetables which are constantly 
floating in the atmosphere, and which have made it their resting-place, begin to 
vegetate ; their death, decomposition, and decay, afford a certain quantity of 
organisable matter, which mixes with the earthy materials of the rock ; in this 
improved soil more perfect plants are capable of subsisting ; these, in their turn, 
absorb nutriment from water and the atmosphere, and after perishing afford new 
materials to those already provided ; the decomposition of the rock still continues, 
and at length, by slow and gradual processes, a soil is formed in which even 
forest trees can fix their roots, and which is fitted to rew^ard the labour of the 
cultivator," 
