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Page 91
Bodies absorbing oxygen, as sulphurous acid, phosphorous acid, and
alcohols, decolorize it; under the influence of oxygen its color is
restored.
We must here mention that Moroz has prepared a beautiful blue substance
from _Centauria cyanus_ by treatment with absolute alcohol.
_Rose-red Coloring Matter._--We have employed alcohol to extract the
substance which colors rose-red certain dahlias, roses, poeonias, &c.
For the procuration of this coloring matter the method pursued is
exactly as that for the preparation of cyanine.
By an attentive comparison of the properties of this coloring matter
with those of cyanine, we have found that the rose-red coloring matter
is the same as the blue, or at least results from a modification of the
same independent principle. It appears in the rose-red modification,
when the juice of the plant, with which it exists in contact, possesses
an acid reaction. We have always observed this acid reaction in the
juices of plants with red or rose-red coloration, while the blue juices
of plants have always exhibited an alkaline reaction.
We have exposed most of the rose-red or red-colored flowers which are
cultivated in the Paris Museum to the influence of alkalies, and have
seen that they first become blue and then green by their action.
It is often perceived that certain rose-red flowers, as those of the
_Mallow_, and in particular those of the _Hibiscus Syriacus_, acquire by
fading a blue and then a green coloration, which change, as we have
found, depends on the decomposition of an organic nitrogenous substance,
which is found very frequently in the petals. This body generates as it
decomposes ammonia, which communicates to the flowers the blue or green
color. By action of weak acids, the petals can be restored to their
rose-red color.
The alteration of color of certain rose-red flowers can also be
observed when the petals are very rapidly dried, for example, in
_vacuo_, by which it cannot be easily assumed that a nitrogenous body
has undergone decomposition to the evolution of ammonia. But, before all
things, it must be mentioned that in this case the modification of color
passes into violet, and never arrives at green; and, further, that it is
always accompanied with the evolution of carbonic acid, which we have
detected by a direct experiment. Petals which were before rose-red, and
have become violet by slight drying, evolve carbonic acid, and on that
account it may be assumed that the rose-red color is produced in the
petals by this carbonic acid, and that by its expulsion the petals
assume the blue color, by which the flowers with neutral juices are
characterized.
We believe that we are able to speak with certainty that flowers with a
rose-red, violet, or blue color, owe their coloration to one and the
same substance, but which is modified in various ways by the influence
of the juices of plants.
Scarlet-red flowers also contain cyanine reddened by an acid, but in
such cases this substance is mixed with a yellow coloring matter which
we will now describe.
_Yellow Coloring Matter._--The simplest experiments show that no analogy
exists between the substance which colors flowers yellow and that of
which we have already spoken. The agents which generate so easily with
cyanine, the rose-red, violet, or green coloration, cannot in any case
impart these colors to the yellow substance obtained from flowers.
By the examination of the various yellow-colored flowers, we have
ascertained that they owe their coloration to two substances, which
differ from one another in their properties, and appear not to be
derived from the same independent principle. One is completely insoluble
in water, which we have termed xanthine, a name which Runge has given
to a yellow matter from madder. As this name has not been accepted in
science, we have employed it to denote one of the coloring matters of
yellow flowers. The other substance is very soluble in water, and is by
us termed xantheine.
_Xanthine, or the Yellow Coloring Matter insoluble in water._--We have
prepared this coloring matter from many yellow flowers, but chiefly from
_Helianthus annuus_.
To obtain it we treat the flowers with boiling absolute alcohol, which
dissolves the coloring matter in the heat, and by cooling almost
completely allows it again to precipitate. The yellow deposit which is
obtained in this way, is not pure xanthine, as it contains a rather
considerable quantity of oil. To separate this oil we have recourse to a
moderate saponification; thus, we heat the yellow precipitate with a
small quantity of alkali to saponify the fatty body mixed with the
xanthine, which even contains the xanthine dissolved. As the coloring
matter is soluble in the soap solution, we do not treat the mass with
water, but decompose it with an acid which isolates the xanthine and the
fatty acids resulting from the saponification. This precipitate we treat
with cold alcohol, which leaves behind the fatty acids, and dissolves
the xanthine. This substance is a fine yellow color, insoluble in water,
but soluble in alcohol and ether, which are thereby colored golden
yellow. It appears to be uncrystallizable, and possesses the general
properties of resins.
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