_a._ Good benzoin, crushed small or in the state of coarse powder, is
placed in a cylindrical iron pot with a flat bottom, and from 8 to 9
inches in diameter, so as to form a layer of from 1 to 2 inches deep. The
open end of the pot is next covered with a sheet of soft and loose
blotting-paper,[146] which is attached to the rim with paste. A cone, cap,
or cylinder formed of strong thick paper (cartridge paper), open at its
lower end, is then placed over the top of the pot, including the
blotting-paper; and this is also attached with paste and string. The
apparatus, thus prepared, is then placed on a sand bath,[147] and exposed
for 4 to 6 hours to a gentle and uniform heat. It is next removed from the
sand bath, and, when it has sufficiently cooled, inverted, and the string
detached, when crystals of benzoic acid are found in the paper cone. If,
owing to want of care in manipulating, the product is either coloured or
empyreumatic, it must be enveloped in several folds of bibulous paper,
then submitted to powerful pressure, and afterwards resublimed. The simple
form of apparatus figured in the engraving answers well on the small
scale, and is that recommended by Dr Mohr.
[Footnote 146: Felt--Liebig.]
[Footnote 147: On an iron plate on which sand has been spread--Ph. Bor.]
[Illustration]
_b._ (Ph. D. 1850.) The subliming pot is ordered to be of sheet-iron. It
is to be fitted into a circular hole in a sheet of pasteboard, and a
collar of tow interposed between it and the flange, so as to produce a
nearly air-tight junction. The paper receiver or cap is to be cylindrical,
open at one end and about 18 inches high, with a diameter at least twice
that of the pot; and it is to be secured in an inverted position on the
pasteboard, and fastened to it by slips of paper and flour-paste. A couple
of inches of the pot is to be passed through a corresponding hole in a
plate of sheet-tin, which is to be kept from contact with the pasteboard
by the interposition of a few corks; and a heat[148] only just sufficient
to melt the benzoin is to be applied for at least six hours.
[Footnote 148: That of a gas-flame is recommended. A ring of very small
gas-jets answers better.]
_c._ (Process adopted at Apothecaries' Hall, London.) The best gum-benzoin
is put into an iron pot, set in brickwork over a suitable small fire-place
(or flue),[149] and communicating by a conical metal neck, with a wooden
box (technically termed a 'house') lined with white blotting-paper, as a
receiver for the flowers. A piece of fine muslin, or of bibulous paper, is
interposed between the top of the subliming-pot and the receiver, to
prevent the sublimate falling back into the former. The sublimation is
conducted rather rapidly, and the acid condenses in beautiful white, soft,
flexible crystals, which are at once ready for the market. When the
process is conducted more slowly, the product is proportionately scaly.
[Footnote 149: A pan with a steam-jacket answers well, and is very
manageable.]
_Obs._ Good samples of benzoin yield from 10 to 12%, or even 12-1/2%, of
'flowers' or 'acid of the first sublimation.' This, after being pressed in
blotting-paper and again sublimed, gives 8-1/2 to 10% of nearly pure
benzoic acid. The loss arising from a second sublimation is thus so great
that the utmost care should be taken to avoid its necessity.