The most singular changes of colour are shown by flowers which are composed of different tints, their red streaks turning green, the white yellow, etc. Another curious example is that of red and white fuchsias, which, through the action of ammonia, turn yellow, blue, and green. When flowers have been subjected to these changes of colour, and afterwards plunged into pure water, they preserve their new tint for several hours, after which they gradually return to their natural colour. Another interesting observation, due to M. Gabba, is that asters, which are naturally inodorous, acquire an agreeable aromatic odour under the influence of ammonia. Asters of a violet colour become red when wetted with nitric acid mixed with water. On the other hand, if these same flowers are enclosed in a wooden box, where they are exposed to the fumes of hydrochloric acid, they become, in six hours’ time, a beautiful red colour, which they preserve when placed in a dry, shady place, after having been properly dried. Hydrochloric acid has the effect of making flowers red that have been rendered green by the action of ammonia, and also alters their appearance very sensibly. We may also mention, in conclusion, that ammonia, combined with ether, acts much more promptly than when employed alone.
Phosphorescence.
Artificial flowers are frequently to be seen prepared in a particular manner, which have the property of becoming phosphorescent in darkness, when they have been exposed to the action of a ray of light, solar or electric. These curious chemical objects are connected with some very interesting phenomena and remarkable experiments but little known at the present time, to which we will now draw the reader’s attention.
The faculty possessed by certain bodies of emitting light when placed in certain conditions, is much more general than is usually supposed.
M. Edmond Becquerel, to whom we owe a remarkable work on this subject, divides the phenomena of phosphorescence into five distinct classes:
1. Phosphorescence through elevation of temperature. Among the substances which exhibit this phenomenon in a high degree we may mention certain diamonds, coloured varieties of fluoride of calcium, some minerals; and sulphur, known under the name of artificial phosphorus, when it has previously been exposed to the action of the light.
2. Phosphorescence through mechanical action. This is to be observed when we rub certain bodies together, or against a hard substance. If we rub together two quartz crystals in the dark, we perceive red sparks; and when pounding chalk or sugar, there is also an emission of sparks.
3. Phosphorescence through electricity. This is manifested by the light accompanying disengagement of electricity, and when gases and rarefied vapours transmit electric discharges.
4. Spontaneous Phosphorescence is observed, as every one knows, in connection with several kinds of living creatures,—glow-worms, noctilucids, etc., and similar phosphorescent effects are produced also with organic substances, animal or vegetable, before putrefaction sets in. It is manifested also at the flowering time of certain plants, etc.
5. Phosphorescence through insolation and the action of light. “It consists,” says M. Edmond Becquerel, “in exposing for some instants to the action of the sun, or to that of rays emanating from a powerful luminous source, certain mineral or organic substances, which immediately become luminous, and shine in the dark with a light, the colour and brilliancy of which depend on their nature and physical character; the light gradually diminishes in intensity during a period varying from some seconds to several hours. When these substances are exposed anew to the action of light, the same effect is reproduced. The intensity of the light emitted after insolation is always much less than that of the incidental light.” These phenomena appear to have been first observed with precious stones; then, in 1604, in calcined Bologna stone, and later, in a diamond by Boyle, in 1663; in 1675 it was noticed in Baudoin phosphorus (residuum of the calcination of nitrate of lime), and more recently still in connection with other substances which we will mention. The substances most powerfully influenced by the action of light are sulphates of calcium and barium, sulphate of strontium, certain kinds of diamonds, and that variety of fluoride of calcium, which has received the name of chlorophane.