RAINBOW PHOSPHORESCENCE.

UNDER this title we propose to describe a series of curious preparations made in France with great perfection, and capable, after exposure to sunshine, electric light, or magnesium light, of emitting in a dark chamber, very beautiful luminous effects, red, blue, yellow, and green. The substances sold for this purpose are enclosed in long flat bottles, and placed side by side in a box, the lid of which is removed when the experiments are made. The origin of all such preparations may be found in the "Bologna phosphorus," well known to all chemical students. It appears that in 1602 Vincenzo Cascirclo, an artisan of Bologna, engaged in alchemical pursuits, accidentally made a phosphorescent sulphide of barium by calcining sulphate of baryta in contact with charcoal, and this material, which attracted the greatest interest amongst philosophers, was subsequently named the "Bologna stone," or "Bologna phosphorus." In 1675 Baudouin described a similar preparation made by calcining nitrate of lime, and named it "hermetic phosphorus." To prepare Bologna phosphorus, sulphate of baryta is reduced to powder, moistened with water or white of egg, and made into cakes, which are placed in layers of braise (charcoal ashes), and calcined in a furnace.

In his recent work on light, M. Becquerel* states that Margraf, who published an account of his experiments in 1862, by pulverizing and calcining these substances two or three times. in succession (as recommended by Pothier), obtained a mixture of phosphorescent tints, and thus to some extent anticipated the more recent preparations we shall proceed to describe. At the end of the sixteenth century, Homberg introduced his phosphorus made from chloride of calcium; and in 1730, Dufay published a memoir, showing that many minerals, shells, and calcareous concretions exhibited similar properties after calcination; and Beccaria soon afterwards demonstrated that insolation, or exposure to sun-light, imparted the property of phosphorescence to a considerable number of dried organic substances, and other materials. In the course of his experiments Beccaria devised a phosphoroscope, or rotating apparatus, for exposing a substance to the light, and then exhibiting it to an observer situated in the dark. By these means, which have been improved in later times, a very slight degree of phosphorescence may be rendered visible. Zanotti, secretary of the Bologna Academy, about the same time observed that

"La Lunière ses Causes et ses Effets," par M. Edmund Becquerel, de l'Academie des Sciences, de l'Institut de France, Professeur au Conservatoire des Arts et Métiers, etc. Paris: Firmin Didot. 1867.

the Bologna phosphorus emitted one shade of colour without reference to the part of the solar spectrum to which it was exposed.

In 1764, Canton introduced the so-called phosphorus bearing his name, and as his method was easy to follow, it was very generally adopted. He calcined oyster-shells in a crucible, powdered the resulting substance, mixed it with one-fourth its weight of sulphur, and kept it a red heat for one hour. The result of this process is a sulphide of calcium, which, after exposure to light, shines in the dark with a green or yellow lustre, according to the details of its preparation. Canton," observes M. Becquerel, "showed that the light of a candle, of the moon, or of electric sparks, rendered this substance active. He also showed that heating it in the dark did not render it luminous unless it had been previously exposed to the light, and that if heated immediately after such exposure, its action was more energetic than when it was heated a few days afterwards. Thus, heat occasions a rapid emission of light that would have been very slowly given forth at ordinary temperatures. Canton also showed that if at the end of several months this substance, previously insolated, was heated with boiling water, it gave no result; but when the heat was carried to a temperature of about 500° (C.), it became luminous, and then fell back to its inactive state until revived by a fresh insolation.”

In 1780 Wilson published his experiments on the prismatic colours exhibited by phosphorus. He noticed that different portions of calcined oyster-shells exhibited different phosphorescent tints, red, yellow, green, or blue, and he separated them accordingly. Wilson also confirmed the statements of Dufay and Zanotti, that each portion of his calcined shells emitted their peculiar light, whatever might be the colour of the exciting rays; thus his red luminous fragments, he said, emitted a red light in the dark, whether placed in the violet, the blue, or the red of the spectrum, and the luminous portions exhibited a green phosphorescence after the action of violet, blue, or red rays. M. Becquerel remarks that these experiments were partly vitiated by the employment of an imperfect spectrum. His own researches show that the blue and violet rays act more energetically than the red, and that with Canton's phosphorus the red not only give no illumination, but exert a destructive power. A similar observation had been previously made by Goethe and Seebeck, and M. Becquerel cites the following passage from the work of the former on colours :-"The Bologna phosphorus becomes luminous under the influence of blue and violet glass, and never under yellow or orange glass; and it may be remarked that this phosphorus rendered luminous by blue or violet colours becomes extin

guished sooner in yellow or orange rays than if placed nearly in a dark chamber. If these experiments are repeated with a prismatic spectrum the same results are always obtained."

The Newtonian theory that light consisted in an emission of minute particles shot forth with great velocity, led the older philosophers to conceive that these phosphorescent bodies had bottled up the light, something like the cucumber of Laputa, and gave it back again, as the celebrated sage of that island desired to make the cucumbers do. Such notions, however, and the somewhat similar one, that they absorb light, as sponges absorb water, are quite inconsistent with some important facts, and with the undulatory theory of light, which may now be considered as well established. M. Becquerel observes that such comparisons are incorrect, inasmuch as the light emitted usually differs from that which excites the phosphorescent body. Phosphorescence is generally an emission of light-waves of less velocity than those of the rays which excited it.

The most luminous of these phosphori are the sulphides of alkaline earths, calcium, barium, and strontium. The first gives Canton's phosphorus emitting a yellow or a green light; the second, the Bologna phosphorus, for the most part orange. M. Becquerel remarks that "these substances, when well prepared, will shine in darkness for many hours after their exposure to solar action, decreasing, however, rapidly in lustre in the first moments, and then growing weaker more slowly. Their light is emitted in vacuum as well as in gases, and their action is not accompanied by any chemical effect; it is the result of a temporary physical modification. Amongst the metallic sulphides, those of strontium and barium exhibit the greatest vivacity of luminous emission, and those of calcium yield the greatest variety of colours."

A sulphide of zinc, formed in a particular way, is as phosphorescent as the sulphides of the alkaline earths. Other metallic sulphurets do not exhibit the property, not even those of the alkaline metals, and the other compounds of barium, strontium, and calcium, excepting their selenides, do not manifest energetic action of this nature. Following the sulphides just mentioned, come minerals, such as certain diamonds, especially those of a yellow tint, and most specimens of fluoride of calcium. The variety of calcic fluoride called chlorophane becomes very luminous by insolation, emitting a slightly bluish green tint of light. The reason why particular diamonds, or chlorophanes, are phosphorescent, while others do not exhibit that property, is at present unknown. M. Becquerel says, "the diamond and fluoride of calcium do not exhibit a vivacious lustre, but they remain luminous for a long time. Thus, I have

seen a fragment of green fluor spar and two white diamonds emit light for one hour after insolation."

In preparing these phosphori, it is noticed that the peculiarity of their luminous action depends on the primitive condition of the sulphates employed. "Thus the natural crystallized sulphate of baryta affords the orange yellow Bologna phosphorus; the natural sulphate of strontium from Sicily in rod-shaped (bacillary) crystals, yields a bluish green phosphorus, and if by the action of carbon different sulphates are reduced to the condition of sulphides, their luminous action will vary."

In preparing a phosphorescent sulphide with lime, or carbonate of lime, it is most convenient to add 85 parts of sulphur to 100 of the lime, or 48 to 100 of its carbonate. The materials are intimately mixed and placed in an earthen crucible in a charcoal furnace. M. Becquerel says it is necessary to pay attention to the temperature as well as to its duration. "Operating with fibrous arragonite, and heating the crucible to 500° (C.) for a time sufficient to allow the reaction between the lime and the sulphur to take place, and the excess of the latter to be eliminated, a feebly-luminous mass affording a bluish tint is obtained. If this mass is raised to a temperature of 800° or 900° (C.) and kept for five-and-twenty or thirty minutes at a point not exceeding the fusion of gold or silver, it yields a brilliant green light. The chemical composition is the same in both cases; but it is remarkable that if the process is conducted with carbonate of lime instead of with lime, the refrangibility of the light emitted does not vary with the temperature."

Too high, or too prolonged a temperature destroys the phosphorescence, and charcoal furnaces answer better than

coke.

Among the lime preparations, those made with pure Iceland spar give, after insolation, an orange yellow light, calc spar affords a less vivid tint, Carara marble a very weak yellow light, oyster-shells yellow, chalk a scarcely visible yellow. Arragonite of Vertaison in bacillary crystals, a green of medium intensity, fibrous arragonite a dominant violet tint, with some parts green, and lime obtained from fibrous arragonite a very vivid green.

If nitric acid is employed to dissolve the lime of these minerals, and it is then precipitated by carbonate of ammonia, the tints of the phosphorescence will vary according to the sources of the lime.

Phosphori composed of strontium sulphides usually require less heat in their preparation than the lime series, and an excess of heat destroys their luminosity. The barium phos

phori on the contrary are prepared with greater and more sustained heat. Chlorides of calcium and strontium tend to give blue and violet tints, those of barium yield green tints, while carbonates obtained from nitrates and acetates of baryta afford yellow orange phosphori, and analogous combinations of calcium and strontium give very luminous green ones.

A luminous orange phosphorus from barium is made by intimately mixing powdered crystalline sulphates with 12 to 15 per cent. of lamp black, moistened with a little alcohol. When the mass is dry it is calcined in a crucible for 45 to 60 minutes, at a temperature not exceeding cherry red, or the melting point of silver. The resulting mass is powdered and calcined a second time.

We have mentioned that, as a rule, these phosphori give the same tints whatever may be the colour of the light they are exposed to in order to excite them, but M. Becquerel cites three exceptional cases.

1. Sulphide of barium obtained by reducing the sulphate with lamp black, gives an orange yellow phosphorescence when illuminated by the action of the rays in the spectrum situated towards the end and beyond the violet (from lines H to P), while the effect of the rays from the blue to the violet (F to H) is to induce a redder phosphorescence.

2. The sulphide of calcium obtained from oyster-shells, which gives a red light when excited by rays from the blue (F) to the ultra-violet as far as O, has a green tint imparted to it when excited by the rays beyond O and P, which are nonluminous to human eyes.

3. A phosphorus obtained by the action of sulphide of potassium on oyster-shells, is excited to an indigo-violet luminosity by exposure to rays of that tint, while rays beyond the violet excite it to emit a blue colour.

It is interesting to observe from M. Becquerel's explanations and from a beautifully-coloured plate attached to his work, that while the most luminous parts of the spectrum, the yellow, actually exert a destructive effect on the light of these phosphori, they are all capable of excitation by non-luminous rays beyond its violet extremity.

We are afraid that experimenters will only succeed in making the more easy of their phosphorescent compounds, unless they possess a good deal of patience and considerable knowledge of chemical manipulation. When well prepared the varieties of colour are very distinct, and the luminous effects brilliant and pleasing. They not only afford an agreeable recreation, but they suggest curious speculations on the molecular condition of the several compounds. Light appears to excite a peculiar vibration of their particles without affecting