n impure variety from Idria in Carniola, in which
the cinnabar is mixed with bituminous and earthy matter.

Metacinnabarite is a cubic form of mercuric sulphide, this compound
being dimorphous.

For a general description of cinnabar, see G.F. Becker's _Geology of
the Quicksilver Deposits of the Pacific Slope_, U.S. Geol. Surv.
Monographs, No. xiii. (1888). (F. W. R.*)

CINNAMIC ACID, or PHENYLACRYLIC ACID, C9H8O2 or C6H6.CH:CH.COOH, an acid
found in the form of its benzyl ester in Peru and Tolu balsams, in
storax and in some gum-benzoins. It can be prepared by the reduction of
phenyl propiolic acid with zinc and acetic acid, by heating benzal
malonic acid, by the condensation of ethyl acetate with benzaldehyde in
the presence of sodium ethylate or by the so-called "Perkin reaction";
the latter being the method commonly employed. In making the acid by
this process benzaldehyde, acetic anhydride and anhydrous sodium acetate
are heated for some hours to about 1800 C, the resulting product is made
alkaline with sodium carbonate, and any excess of benzaldehyde removed
by a current of steam. The residual liquor is filtered and acidified
with hydrochloric acid, when cinnamic acid is precipitated,
C6H5CHO+CH3COONa = C6H5CH:CH.COONa+H2O. It may be purified by
recrystallization from hot water. Considerable controversy has taken
place as to the course pursued by this reaction, but the matter has been
definitely settled by the work of R. Fittig and his pupils (_Annalen_,
1883, 216, pp. 100, 115; 1885, 227, pp. 55, 119), in which it was shown
that the aldehyde forms an addition compound with the sodium salt of the
fatty acid, and that the acetic anhydride plays the part of a
dehydrating agent. Cinnamic acid crystallizes in needles or prisms,
melting at 133 deg. C; on reduction it gives _phenyl propionic acid_,
C6H5.CH2.CH2.COOH. Nitric acid oxidizes it to benzoic acid and acetic
acid. Potash fusion decomposes it into benzoic and acetic acids. Being
an unsaturated acid it combines directly with hydrochloric acid,
hydrobromic acid, bromine, &c. On nitration it gives a mixture of ortho
and para nitrocinnamic acids, the former of which is of historical
importance, as by converting it into orthonitrophenyl propiolic acid A.
Baeyer was enabled to carry out the complete synthesis of indigo
(_q.v._). Reduction of orthonitrocinnamic acid gives orthoaminocinnamic
acid, C6H4(NH2)CH:CH.COOH, which is of theoretical importance,