106 
GARDENING AS A SCIENCE. 
by fire ; or, in chemical language, reducible at a red heat to carbonic acid and 
water, owing to the attraction of oxygen from the air during the act of combustion. 
From this analysis, we learn that the heaths and American hair-rooted plants 
which flourish in pure heath-soil, require no other manure than that very minute 
portion of black humus, almost reduced to the condition of charcoal, which is found 
in the mould of heathy districts. 
In the cultivation of the Gesneracew tribes that are usually furnished with a 
sort of " cormus," or bulbous depositary of their nutritive juices, besides the hairy 
processes which constitute the true roots, we appear authorized to employ a little 
loam, and decayed horse or cow-dung, in addition to the bulk of heath-soil that 
their delicate roots require. We have tried powdered charcoal pretty freely for 
these plants, according to the suggestion of Liebig, but without corresponding 
success. Charcoal cannot be decomposed by vegetable action : a /??/<iro-carbonate 
must be present in the soil, otherwise water cannot be formed, and without the 
presence of water, sap cannot be generated. 
Every fact yields some evidence of the identity that may be traced between 
vegetable vital action upon deposited manures, and the electrolyzation of the 
voltaic trough. 
The peculiar operation of manuring substances, and their specific adaptation to 
individual plants, may be in a degree ascertained by the colour of the foliage. 
Whenever the verdure acquires a tint of yellow, in lieu of the darker rich hue 
which is natural to it in a state of health, there can be little doubt that there is 
something wrong in the soil. It is true that the natives of warm climates, if kept 
too cool, will exhibit the yellow tint ; and most juicy plants, if droughted, will also 
be similarly affected : but setting aside these exceptions, a vitiated colour of the 
foliage generally proves that the soil is in fault. 
We are far too much in the dark respecting the operation of vegetable and 
animal matters upon the tints of flowering plants, and it would be a desideratum of 
very great interest to acquire some correct knowledge on the subject. In nine 
cases out of ten, we believe that the loam employed in different gardens is the cause jj 
of those variations in shade which the same individuals exhibit ; for as loam is 
liable to a thousand modifications, as respects the quantity and texture of its con- 
stituents, and moreover as all virgin loam contains one or other of the alkalies, 
(potash generally,) it is evident that the gardener ought to keep a jealous eye upon 
this staple material. 
In the kitchen-garden any tolerably good loam will do well enough with all the 
Brassica tribes ; but it is a widely different affair where we have to experimentize 
upon the delicate Gloxinias, Euphorbise, Russellia, &c., and particularly with that 
most beautiful but captious shrub, the Camellai, in all its varieties. 
Here again it becomes a duty to insist upon the rejection of all coarse spit 
loams whatever ; and, unless the purest turf or decayed couch-roots can be constantly 
obtained, to abandon common loams, and to depend upon leaf-mould, bog-earth, and 
