250 
CULTURAL CHEMISTRY. 
the land, and by its adhesive power the roots of plants are secured and firmly 
retained in their exact position ; but chemically, it is nearly inert, and rarely is 
traceable in the ashes of any plant. 
Chalk. — Carbonate of lime is found in extremely varying proportions. 
Chemically, it consists of pure lime neutralised by about 44 parts in 100 of carbonic 
acid, or as 56 of lime to 44 of the acid. Lime, pure, is never found in soils ; it 
is an oxide of the metal calcium, discovered by Davy, anno 1808. Some earths 
contain scarcely a trace of chalk ; such are the sandy heath-soils and others, wherein 
silicious sand is the chief ingredient. In the best loams we frequently detect more 
than one-twelfth part of their weight ; and in districts where chalk rock is found 
near the surface, a far greater proportion is brought up and mixed with the earth. 
Chalk is slightly soluble in water, and plants absorb more or less of it, a proof of 
which is given by analysis of their ashes. 
Oxide of iron confers colour according to the condition in which it is blended 
with the several earths. When 28 parts of metallic iron combine with 8 parts of 
oxygen, a black oxide is the result. As such, it gives a black or dark grey tint to 
soils. But there is another condition — that of the p^r-oxide, wherein 12 pro- 
portionals of oxygen are combined with 28 of the metal; in this state the colour 
varies according to the method by which it has been obtained, from that of a pale 
ochreous yellow to a full Homan red. Hence may, to a certain extent, be traced 
the varied colours of the soil, though it must be admitted that much will depend 
upon the nature of the vegetable substances that are combined with the earths. 
The quantity of iron is small — seldom more than 2 per cent., yet it is found in 
the ashes of many plants, and therefore must be essential to them. On a former 
occasion it was stated that the writer experimented freely with a solution of sulphate 
of iron poured over the soil of a small pot, wherein a white-stemmed Balsam was 
growing. It was employed so long and so freely, that at length the simple loam 
of the pot was, in appearance, converted to a rust of iron. The object of the 
experiment was to determine the channels of the ascending sap. That was not 
however, attained, for the plant refused to imbibe the ferruginous liquid ; but it 
became crippled, though not killed, the stem assuming a contorted figure. Not a 
trace of colouring matter was discoverable in the tissues. It were well to repeat 
such experiments under several modifications, and the balsam is peculiarly favour- 
able to them ; so would be the Oleander, as its proper sap contains, if we mistake 
not, gallic acid, and hydrocyanic (Prussic) acid. 
Magnesia is found in soils, and also in the ashes of some plants, as is shown 
where the delicate tests of ammonia and phosphoric acid are sldlfully applied. 
The mention of phosphoric acid leads to the consideration of the great importance 
which is now attached to it by modern chemical research. It is produced by the 
combustion of that singular combustible Phosphorus in oxygen gas. In nature, 
phosphoric acid is never found alone ; it is combined to a great extent with lime 
in animal bones, and in some native phosphates, of which the Coproliths, and 
