NOTES AND OBSERVATIONS.
740
LNote 18. p. 478.1
Dr. Black having only given the general conclusions which have been drawn by these
eminent chemists concerning this very singular substance, it seems necessary to mention
the chief and moſt simple facts by which theſe opinions are supported.
If we boil the prepared alkali, called the lixivium sanguinis, or Prussian alkali, with a
diluted sulphuric acid, vapours come off which are extremely volatile. When the distil
lation is skilfully conducted, the first vapours are found much more volatile than water,
and cannot be condensed, unless water be mixed with them, or presented to them in the
receiver. This vapour has a strong smell, not disagreeable, and is extremely inflam
mable.
This ſubſtance contains the colouring principle, and the lixivium now possesses none.
Its taſte is ſweetiſh and aſtringent. But as this watery solution of it unites with all alka
line substances, and with metallic oxyds, and may be detached from one of them by
means of another, in the same manner as acids, it is considered, chemically, as an acid,
and has been called the PRUSSIAN, or PRUSSIC ACID, and its compounds have been called
PRUSSIATES. That compound called the lixivium sanguinis, prepared, or Prussian alkali,
&c. is the PRUSSIATE OF POTASH.
The pruſsic acid, prepared in the way juſt now mentioned, is but impure. Mr.
Scheele procured it in the greatest purity, by first forming of this impure acid a prussiate
of mercury, and then putting some filings of iron into it and a small quantity of sulphu
ric acid. This instantly decomposed the mercurial prussiate, and even greatly weakened
the attraction of the Prussic acid for the iron, by the excess of sulphuric acid. The mix
ture being now distilled with an extremely gentle heat, and the first vapours only pre
ferved, the result was a prufsic acid perfectly pure. With this, or such as this, all the ex
periments for ascertaining its properties are tried.
It combines, as I have faid, with all the substances which combine with acids, with
various degrees of elective attraction. Lime water digested on Prussian blue combines
with this matter alone, without taking up any iron, as the solution of alkali does. It ac
quires a yellow or straw colour, and no longer manifests its alkaline qualities. It does
not affect the test paper. It has lost its alkaline taste, and its attraction for carbonic acid.
This compound, called the prussiate of lime, is a much fitter test of th prel nee of iron,
than the lixivium sanguinis, or prussiate of potash, because it contains no iron.
The union of this colouring matter with the various bases, is exceedingly weak; for
every acid, even the carbonic, detaches it from them all. Nay, Scheele found that car
bonic acid rendered blood incapable of imparting the colouring quality to the lixivium
fanguinis. But, as observed by Dr. Black, the addition of a small quantity of iron, either
pure or very flightly calcined, so fixes this acid in the lixivium fanguinis, that it now re
siſts the action of any diluted acid. It is found, however, that strong sulphuric acid dis¬