ces of salts cover the surfaces of
the walls, like a white nap, within a few days after they are
erected. If this saline incrustation is washed away by the rain, it
soon re-appears; and this is even observed on walls which, like the
gateway of Lisle, have been erected for centuries. These saline
incrustations consist of carbonates and sulphates, with alkaline
bases; and it is well known these act an important part in
vegetation. The influence of lime in their production is manifested
by their appearing first at the place where the mortar and brick
come into contact.

It will now be obvious to you, that in a mixture of clay with lime,
all the conditions exist for the solution of the silicated clay, and
the solubility of the alkaline silicates. The lime gradually
dissolving in water charged with carbonic acid, acts like milk of
lime upon the clay. This explains also the favourable influence
which marl (by which term all those varieties of clay rich in chalk
are designated) exerts upon most kinds of soil. There are marly
soils which surpass all others in fertility for all kinds of plants;
but I believe marl in a burnt state must be far more effective, as
well as other materials possessing a similar composition; as, for
instance, those species of limestone which are adapted to the
preparation of hydraulic cements,--for these carry to the soil not
only the alkaline bases useful to plants, but also silica in a state
capable of assimilation.

The ashes of coals and lignite are also excellent means of
ameliorating the soil, and they are used in many places for this
purpose. The most suitable may be readily known by their property of
forming a gelatinous mass when treated with acids, or by becoming,
when mixed with cream of lime, like hydraulic cement,--solid and
hard as stone.

I have now, I trust, explained to your satisfaction, that the
mechanical operations of agriculture--the application of lime and
chalk to lands, and the burning of clay--depend upon one and the
same scientific principle: they are means of accelerating the
decomposition of the alkaline clay silicates, in order to provide
plants, at the beginning of a new vegetation, with certain inorganic
matters indispensable for their nutrition.

LETTER XIV

My dear Sir,

I treated, in my last letter, of the means of improving the
condition of the soil for agricultural purposes by mechanical
operations and mineral agents. I have now to speak of the uses