Violets are divided into two groups: those affected by potassa, and
those upon which it has no action. The first group embraces logwood,
orchil, alkanet, and aniline violets, including under the latter term
Perkin's violet, (probably the original "mauve"), dahlia, Parme or
magenta violet, methyl, and Hofmann's violets. The action of potassa
gives indications for each of these violets. Logwood violet is browned;
that of orchil, if slightly reddish, is turned to a blue-violet; that of
alkanet is modified to a fine blue. Lastly, Perkin's mauve, dahlia, and
methyl violet become of a grayish brown, which may be re-converted into
a fine violet by washing in abundance of water. When the shades are
very heavy, this grayish brown is almost of a violet-brown, so that the
violets might seem to be unaltered.
The action of hydrochloric acid distinguishes these colors better still
if the aid of ammonia is called in for two cases.
The acid turns logwood violet to a fine red, and equally reddens orchil
violet. But the two colors cannot be confounded, first, because the two
violet shades are very distinct, that of orchil being much the brighter;
and secondly, because ammonia has no action on logwood violet, while it
turns orchil violet, if at all reddish, to a blue shade. Hydrochloric
acid, whether dilute or concentrated, is without action on alkanet
violet. If the acid is dilute, it is equally without action on Perkin's
violet and dahlia. If it is strong, it turns them blue, and even green
if in excess. Hofmann's violet turns green even with dilute acid, but
prolonged washing restores the original violet shade. Dahlia gives a
more blue shade than Perkin's mauve. The action of acid is equally
characteristic for methyl violet. It becomes green, then yellow. Washing
in water re-converts it first to a green, and then to a violet.
The second group includes madder violet, cochineal violet, and the
compound violet of cochineal and extract of indigo. These three dyes are
thus distinguished: Hydrochloric acid turns the madder violet-orange,
slightly brownish, or a light brown, and it affords the characteristic
reaction of the madder colors described above under reds. Cochineal
violets are reddened. Sometimes they are decolorized, and become finally
yellow, but do not pass through a brown stage.
The compound violet of cochineal and extract of indigo presents this
characteristic reaction, that if boiled with very weak solution of
sodium carbonate the liquid becomes blue, rather greenish, while the
cloth becomes a vinous-red--_Moniteur Scientifique.--Chem. News._
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