ed injurious action which would be exerted on
plant-life by the protoxide of iron it contained. Accordingly, a large
number of patents were taken out, "covering almost every conceivable
method for treating the slag, whether practicable or not. They all in
the main are combinations or variations of the following processes:--

"1. _Preliminary preparation of the Slag._

(_a_) By treating molten, or otherwise, with superheated steam, or
cooling when hot with water, to reduce it to small pieces or
to a fragile state.

(_b_) Grinding.

(_c_) Treating with water to wash out free lime, or with sugar
solution.

(_d_) Roasting in the air, or with some oxidising agent.

"2. _Solution of the Slag._

(_a_) _Completely_ in weak or strong acids (hydrochloric, sulphuric,
&c.)

(_b_) _Partially_, so as to dissolve the phosphates and silicates of
lime, and leave most of the iron and manganese oxides.

"3. _Precipitation_ of the phosphoric acid, with lime
or iron salts: or,

"Processes in which the slag is smelted with charcoal,
to reduce phosphates to phosphides, treated with
acid, and the phosphuretted hydrogen burnt to phosphoric
acid; and,

"Processes in which the slag is fused with soda or
potash salts,--caustic, chlorides, sulphates, carbonates,--with
or without steam being forced through, to form soluble alkaline
phosphates."[234]

Many of these processes were tried; but it was found by experiment that
the best and most economical way was by applying the slag direct to the
ground in a state of very fine powder. Experiments further showed that
it had _not_ the injurious effect on vegetation which it was feared it
would have from the protoxide of iron it contained. The discovery that
its phosphoric acid existed, as has been already explained, as a
tetrabasic phosphate of lime, has strengthened the opinion that this is
the best method of application.

A good deal has been found to depend upon the fineness of the ground
slag, with the result that it is now commonly sold on a mechanical as
well as a chemical analysis--_i.e._, the slag is guaranteed to pass
through a sieve of a certain fineness.

_Solubility of Slag._

Professor Wagner of Darmstadt has carried out some extremely interesting
experiments on the solubility of slag. He found that very finely
powdered slag was dissolved in carbonic acid water to the extent of 36
per cent, while,