f "basic slag."
In the original process the sides of the "converter" were lined with
fire-bricks, consisting largely of silica. This process was known as the
"acid" process. In the "Thomas-Gilchrist" process, however, the sides of
the "converter" are lined with _lime_ (dolomitic limestone being largely
used), lime being also added to the pig-iron. An air-blast is injected
through the molten mass, and the impurities are burnt, or oxidised as it
is chemically termed. The phosphorus in the iron is thus converted into
phosphoric acid, and, uniting with the lime, forms phosphate of lime,
which rises, as we have already said, to the surface in the form of a
scum, and is separated from the steel by being poured off.

_Not at first used._

This, then, is how the _Thomas-slag_ is obtained. It did not seem,
however, for some years after the introduction of this ingenious
process, to have struck any one that this rich phosphatic bye-product
might prove a valuable addition to our artificial fertilisers. The
result was, that the Thomas-slag was treated as another of the only too
numerous valueless bye-products which seem to be necessarily incidental
to most of our chemical and other manufactures, and was allowed to
accumulate in large quantities without being used for any purpose.

_Discovery of its Value._

In 1883 some short articles published in Germany on the subject were the
means of first drawing the attention of the public to its importance as
a manure. During the years 1884 and 1885 numerous experiments were
carried out on the subject in the same country; and from then up till
the present hour it has become more and more extensively used in
Germany, till in 1887, as already stated, its consumption amounted to
nearly 300,000 tons.

_Composition._

It consists mainly of phosphate of lime, silicate of lime, free lime,
free magnesia, and oxides of iron and manganese. Its composition, of
course, naturally varies; but the following may be taken as an average
analysis:[233]--

Per cent.
*Phosphoric acid 17
Lime in combination with phosphoric, silicic,
sulphuric, and carbonic acids 40
Free lime 15
Oxides of iron 12
*Equal to tricalcic phosphate 37

As a rule, the phosphoric acid varies considerably, ranging from