CHEMISTEY OF HORTICULTURE. — NATIVE OR SPECIAL EARTHS. 
355 
of the stove at all times ; and, if judiciously treated, flowers several months in the year. 
The soil requisite for its successful cutivation, is a rich loam, well drained, and rendered 
open by the addition of a little turfy peat. Increase is effected by cuttings of the ripened 
wood planted in a mixture of sand and peat, and covered with a glass. 
Our drawing was prepared in October, 1849, from a plant blooming in one of the 
stoves at Chatsworth. 
The generic name was given by Plumier, in honour of Tabernaemontanus, a celebrated 
physician and botanist. 
CHEMISTRY OF HORTICULTURE.— NATIVE OR SPECIAL EARTHS. 
By John Towers^ Esq. 
(Continued from Page 326). 
These, to be understood, must be minutely investigated, and their qualities, if possible, 
accurately determined. We must carefully distinguish between Earths proper, and Soils 
rendered fertihsing by combination with substances capable of undergoing decomposition. 
The earthy bodies to which the reader's immediate and exclusive attention is directed, 
are sand, or silex — alumina the base or matter of pure clay — chalk, already treated on — 
as lime chemically united with carbonic acid, and iron united with oxygen, and thereby 
brought into the state of an oxide. These four earths — for so we conventionally call 
them — constitute the basal elements of fertile land, the two first, pre-eminently so. 
1. Sand, if pure, is identical with rock crystal. In chemical language it is called 
Silica, and is proved to be the oxide of a metallic body, Silicon, never found in nature. 
But this earth of silicon, silica, is found very extensively, and is the agent by which the 
texture of all clays and loams is modified. Silica is in itself insoluble in water, therefore 
it cannot enter the plant as a portion or component of the sap ; yet silica does exist in 
vegetable bodies, and very copiously in the cereals and fodder grasses, the outer coating 
(epidermis) of which is, as it were, a plate of glass. Silica consists of 
1 equivalent of Oxygen, numbered . . . . . 8 
1 equivalent of Silicon, also numbered . . . .8 
The particles or atoms, being in weight equivalent to . . . 16 
It is, I have said, insoluble ; it has no taste, and yet exerts an agency so remarkable 
in many instances, that many chemists have classed it with acids. Thus with potash 
and with soda, it unites and forms the silicate of potash and of soda ; with oxide of 
lead, silicate of lead : and these are constituents of glass. With silica, alumina, and 
magnesia, we obtain a china and porcelain body ; and in old mortar there is reason to 
believe the siliceous sand employed is chemically united with lime, as a true silicate of 
lime — an indestructible cement. From these data we arrive at the conclusion that silica, 
or the matter of flint and sand, is enabled to pass through the absorbents of roots as a 
soluble silicate of potash, and potass is proved to exist in many rocks. Hence the soils 
which are formed in process of ages by the action of air, water, and electricity upon their 
most exposed surfaces, has contained a proportional quantity of that mineral alkali. 
In further proof of the chemical affinity or combining energy, which exists between 
silica and certain bases, with which it forms neutral silicates, the following items are 
selected from a table given by Professor Johnson of the composition of a clay analysed by 
one of his assistants. In 100 parts of such clay there were found, of — 
Insoluble Silicate of Alumina ...... 11*32 
„ „ Protoxide of Iron . . . . . 2.74 
„ Peroxide of Manganese . . . .a trace 
„ Lime . . . . . • . 0-76 
„ „ Magnesia ...... 0*58 
„ „ Potash . . . . . . 0-96 
„ „ Soda. ...... 0-24 
16-60 
