himself into a
cloud of smoke and went into the box again. Sinbad fastened the lid, and
the _genie_ must have remained there until the box was destroyed.

Now man is very much like Sinbad, he lets the carbonic acid out from the
limestone (when it expands and becomes a gas); and then he raises a
crop, the leaves of which drink it in and pack the carbon away in a very
small compass as vegetable matter. Here it must remain until the plant
is destroyed, when it becomes carbonic acid again, and occupies just as
much space as ever.

The burning of limestone is a very prolific source of carbonic acid.

MAGNESIA.

[What do you know about magnesia?

What is phosphoric acid composed of?

With what substance does it form its most important compound?]

_Magnesia_ is the remaining alkali of vegetable ashes. It is well known
as a medicine, both in the form of calcined magnesia, and, when mixed
with sulphuric acid, as epsom salts.

Magnesia is necessary to nearly all plants, but too much of it is
poisonous, and it should be used with much care, as many soils already
contain a sufficient quantity. It is often found in limestone rocks
(that class called _dolomites_), and the injurious effects of some kinds
of lime, as well as the barrenness of soils made from dolomites, may be
attributed entirely to the fact that they contain too much magnesia.

ACIDS.

PHOSPHORIC ACID.

_Phosphoric acid._--This subject is one of the greatest interest to the
farmer. Phosphoric acid is composed of phosphorus and oxygen. The end
of a loco-foco match contains phosphorus, and when it is lighted it
unites with the oxygen of the atmosphere and forms phosphoric acid; this
constitutes the white smoke which is seen for a moment before the
sulphur commences burning. Being an acid, this substance has the power
of combining with any of the alkalies. Its most important compound is
with lime.

[Will soils, deficient in phosphate of lime, produce good
crops?

From what source do plants obtain their phosphorus?]

_Phosphate of lime_ forms about 65 per cent. of the dry weight of the
bones of all animals, and it is all derived from the soil through the
medium of plants. As plants are intended as food for animals, nature has
provided that they shall not attain their perfection without taking up a
supply of phosphate of lime as well as of the other earthy matters;
consequently, there are many soils which will not produce good crops,
simply because t