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.