Phosphorescent action.
Becquerel[[184]] has studied the action of radium rays in producing phosphorescence in various bodies. The substance to be tested was placed above the radium in the form of powder on a very thin mica plate. Examination was made of the sulphides of calcium and strontium, ruby, diamond, varieties of spar, phosphorus and hexagonal blende. Substances like the ruby and spar, which phosphoresce under luminous rays, did not phosphoresce under the radium rays. On the other hand, those which were made luminous by ultra-violet light were also luminous under the action of radium rays. The radium rays show distinct differences from X rays. For example, a diamond which was very luminous with radium rays was unaffected by X rays. The double sulphate of uranium and potassium is more luminous than hexagonal blende under X rays, but the reverse is true for radium rays; under the influence of these rays, sulphide of calcium gave a blue luminosity but was hardly affected by X rays.
The following table shows the relative phosphorescence excited in various bodies.
| Substance | Without screen. Intensity | Across screen of black paper |
|---|---|---|
| Hexagonal blende | 13·36 | ·04 |
| Platino-cyanide of barium | 1·99 | ·05 |
| Diamond | 1·14 | ·01 |
| Double sulphate of Uranium and Potassium | 1·00 | ·31 |
| Calcium Fluoride | ·30 | ·02 |
In the last column the intensity without the screen is in each case taken as unity. The great diminution of intensity after the rays have passed through black paper shows that most of the phosphorescence developed without the screen is, in the majority of cases, due to the α rays.
Bary[[185]] has made a very complete examination of the class of substances which become luminous under radium rays. He found that the great majority of substances belong to the alkali metals and alkaline earths. All these substances were also phosphorescent under the action of X rays.
Crystalline zinc sulphide (Sidot’s blende) phosphoresces very brightly under the influence of the rays from radium and other very active substances. This was observed by Curie and Debierne in their study of the radium emanation and the excited activity produced by it. It has also been largely used by Giesel as an optical means of detecting the presence of emanations from very active substances. It is an especially sensitive means of detecting the presence of α rays, when it exhibits the “scintillating” property already discussed in section 96. In order to show the luminosity due to the α rays, the screen should be held close to the active substance, as the rays are absorbed in their passage through a few centimetres of air. Zinc sulphide is also luminous under the action of the β rays, but the phosphorescence is far more persistent than when produced by the α rays.
Very beautiful luminous effects are produced by large crystals of the platinocyanides exposed to the radium rays. Those containing lithium give a brilliant pink colour. The calcium and barium salts fluoresce with a deep green light, and the sodium compound with a lemon yellow. The mineral willemite (zinc silicate) was recently found by Kunz to be an even more sensitive means of detecting the presence of the radiations than platinocyanide of barium. It fluoresces showing a beautiful greenish colour, and a piece of mineral exposed to the action of the rays appears quite translucent. The crystals of the platinocyanides of barium and lithium are especially suited for showing the action of the γ rays, and, in this respect, are superior to willemite.
A very striking effect is shown by the mineral kunzite—a new variety of spodumene discovered by Kunz[[186]]. This is a transparent gem like crystal, often of very large size, which glows with a beautiful reddish colour under the action of the β or γ rays, but does not appear to be sensitive to the α rays. The luminosity extends throughout the crystal, but is not so marked as in the platinocyanides or willemite. The mineral sparteite[[187]], a form of calcite containing a few per cent. of manganese, has been found by Ambrecht to fluoresce with a very deep orange light under the β and γ rays. The colour appears to depend on the intensity of the rays, and is deeper close to the radium than at some distance away.
If kunzite and sparteite are exposed to the action of the cathode rays in a vacuum tube, the colour is different from that produced by the radium rays. The former appears a deep yellow, instead of the deep red observed with the radium rays.
The different actions of the radium rays on these fluorescent substances can be illustrated very simply and beautifully by the following experiment. A small U tube is filled with fragments of the fluorescent substance arranged in layers. The U tube is immersed in liquid air and the emanation from about 30 mgrs. of radium bromide is condensed in the tube. On closing the tube and removing it from the liquid air, the emanation distributes itself uniformly in the tube. The shades of colour produced in the different substances are clearly seen.
It is observed that all the crystals increase in luminosity for several hours, on account of the excited activity produced by the emanation. This effect is especially observed in kunzite, which at first hardly responds to the rays, since the β and γ rays, which causes it to fluoresce, are not given out by the emanation itself but by one of its later products. The intensity of the β and γ rays is, in consequence, small at first but rises to a maximum after several hours; the luminosity observed varies in a corresponding manner.
Sir William Crookes[[188]] has made an examination of the effect of continued exposure of a diamond to the radium rays. An “off-colour” diamond, of a pale yellow colour, was placed inside a tube with radium bromide. After 78 days’ exposure, the diamond had darkened and become bluish green in tint; when heated at 50° in a mixture of potassium chlorate for ten days, the diamond lost its dull surface colour and was bright and transparent, and its tint had changed to a pale bluish green. The rays have thus a double action on the diamond; the less penetrating β rays produce a superficial darkening due to the change of the surface into graphite, while the more penetrating β rays and the γ rays produce a change of colour throughout its mass. The diamond phosphoresced brightly during the whole course of its exposure to the rays. Crookes also observed that the diamond still retained enough activity to affect a photographic plate 35 days after removal, although, during the period of 10 days, it was heated in a mixture sufficiently powerful to remove the outer skin of graphite. This residual activity may possibly be due to a slow transformation product of the emanation which is deposited on the surface of bodies (see [chapter XI]).
Marckwald observed that the α rays from radio-tellurium produced marked phosphorescence on some kinds of diamonds. An account of the various luminous effects produced on different gems by exposure to the radium and actinium rays has been given by Kunz and Baskerville[[189]].
Both zinc sulphide and platinocyanide of barium diminish in luminosity after exposure for some time to the action of the rays. To regenerate a screen of the latter, exposure to solar light is necessary. A similar phenomenon has been observed by Villard for a screen exposed to Röntgen rays. Giesel made a screen of platinocyanide of radio-active barium. The screen, very luminous at first, gradually turned brown in colour, and at the same time the crystals became dichroic. In this condition the luminosity was much less, although the active substance had increased in activity after preparation. Many of the substances which are luminous under the rays from active substances lose this property to a large extent at low temperatures[[190]].
116. Luminosity of radium compounds. All radium compounds are spontaneously luminous. This luminosity is especially brilliant in the dry haloid salts, and persists for long intervals of time. In damp air the salts lose a large amount of their luminosity, but they recover it on drying. With very active radium chloride, the Curies have observed that the light changes in colour and intensity with time. The original luminosity is recovered if the salt is dissolved and dried. Many inactive preparations of radiferous barium are strongly luminous. The writer has seen a preparation of impure radium bromide which gave out a light sufficient to read by in a dark room. The luminosity of radium persists over a wide range of temperature and is as bright at the temperature of liquid air as at ordinary temperatures. A slight luminosity is observed in a solution of radium, and if crystals are being formed in the solution, they can be clearly distinguished in the liquid by their greater luminosity.
117. Spectrum of the phosphorescent light of radium and actinium. Compounds of radium, with a large admixture of barium, are usually strongly self-luminous. This luminosity decreases with increasing purity, and pure radium bromide is only very feebly self-luminous. A spectroscopic examination of the slight phosphorescent light of pure radium bromide has been made by Sir William and Lady Huggins[[191]]. On viewing the light with a direct vision spectroscope, there were faint indications of a variation of luminosity at different points along the spectrum. In order to get a photograph of the spectrum within a reasonable time, they made use of a quartz spectroscope of special design which had been previously employed in a spectroscopic examination of faint celestial objects. After three days’ exposure with a slit of ¹⁄₄₅₀ of an inch in width, a negative was obtained which showed a number of bright lines. The magnified spectrum is shown in [Fig. 46 A]. The lines of this spectrum were found to agree not only in position but also in relative intensity with the band spectrum of nitrogen. The band spectrum of nitrogen and also the spark spectrum[[192]] of radium are shown in the same figure.
Some time afterwards Sir William Crookes and Prof. Dewar showed that this spectrum of nitrogen was not obtained if the radium was contained in a highly exhausted tube. Thus it appears that the spectrum is due to the action of the radium rays either on occluded nitrogen or the nitrogen in the atmosphere surrounding the radium.
It is very remarkable that a phosphorescent light, like that of radium bromide, should show a bright line spectrum of nitrogen. It shows that radium at ordinary temperatures is able to set up radiations which are produced only by the electric discharge under special conditions.
Sir William and Lady Huggins were led to examine the spectrum of the natural phosphorescent light of radium with the hope that some indications might be obtained thereby of the processes occurring in the radium atom. Since the main radiation from radium consists of positively charged atoms projected with great velocity, radiations must be set up both in the expelled body and in the system from which it escapes.
Fig. 46a.
Giesel[[193]] observed that the spectrum of the phosphorescent light of actinium consists of three bright lines. Measurements of the wave length were made by Hartmann[[194]]. The luminosity was very slight and a long exposure was required. The lines observed were in the red, blue and green. The wave length λ and velocity are shown below.
| Line | Intensity | λ |
| 1 | 10 | 4885·4 ± 0·1 Ångström units |
| 2 | 6 | 5300 ± 6 „ |
| 3 | 1 | 5909 ± 10 „ |
The line 4885 was very broad; the other two lines were so feeble that it was difficult to determine their wave length with accuracy. Hartmann suggests that these lines may be found in the spectrum of the new stars. The lines observed have no connection with radium or its emanation[[195]].
118. Thermo-luminescence. E. Wiedemann and Schmidt[[196]] have shown that certain bodies after exposure to the cathode rays or the electric spark become luminous when they are heated to a temperature much below that required to cause incandescence. This property of thermo-luminescence is most strikingly exhibited in certain cases where two salts, one of which is much in excess of the other, are precipitated together. It is to be expected that such bodies would also acquire the property when exposed to the β or cathodic rays of radium. This has been found to be the case by Wiedemann[[197]]. Becquerel showed that fluor-spar, exposed to the radium rays, was luminous when heated. The glass tubes in which radium is kept are rapidly blackened. On heating the tube, a strong luminosity is observed, and the coloration to a large extent disappears. The peculiarity of many of these bodies lies in the fact that the property of becoming luminous when heated is retained for a long interval of time after the body is removed from the influence of the exciting cause. It appears probable that the rays cause chemical changes in these bodies, which are permanent until heat is applied. A portion of the chemical energy is then released in the form of visible light.