10 to
20 per cent--that is, from 22 to 44 per cent tricalcic phosphate. This
is owing to the difference in the percentage of phosphorus in the raw
material and the quantity of lime added. Attempts have been made in
Germany during the last two or three years to obtain a slag richer in
phosphoric acid than that obtained heretofore, and a process for this
purpose has been patented by Professor Scheibler. This consists of a
slight modification in the ordinary process. Instead of treating the
pig-iron with an excessive quantity of lime, the amount added is not
sufficient to effect the complete dephosphorisation of the iron. The
resulting slag is very rich in phosphoric acid, and is correspondingly
poor in iron. The iron is then again treated with fresh lime, and the
phosphorus completely removed, while the same lime may be used over
again. Such slag forms a very much more concentrated phosphatic manure
than the ordinary slag, and is known as _patent phosphate meal_.

A point which not only renders the slag a product of peculiar interest
from a chemical point of view, but has a most important bearing on its
value as a manure, is the nature of the compound formed by the union of
the lime with the phosphoric acid.

In the ordinary so-called raw phosphates, such as bone-meal, bone-ash,
coprolites, &c., the lime and phosphoric acid are combined in the form
of what is known, in chemical phraseology, as _tribasic phosphate of
lime_. That is to say, that for every equivalent of phosphoric acid
there are three equivalents of lime. Now it was naturally concluded at
first that the tribasic phosphate was the form in which these two
substances existed in the slag. This, however, was found out not to be
the case, in the following way. On allowing the slag to cool, it was
found that small but perfectly defined crystals were formed. These
crystals, by careful analysis, were shown, first by Hilgenstock, to
consist of a form of phosphate of lime hitherto unknown, in which four
equivalents of lime were combined with one equivalent of phosphoric
acid, and which was therefore called "tetrabasic phosphate."

_Processes for preparing Slag._

As soon as the idea of utilising the slag as a manure was suggested,
various plans for extracting its phosphoric acid, and rendering it
available as plant-food, were devised. These were deemed necessary, it
was thought, by the very insoluble nature of the phosphates in the slag,
as well as by the suppos