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CXXII., No. 3180 LUMINESCENCE IN INORGANIC BODIES. (A CRITICAL SURVEY). By J. FREDERICK CORRIGAN. appear to be the chief point of distinction between phosphorescence and fluorescence. Phosphorescence which is caused by light-stimulus may conveniently be named photo-luminescence, or the term phosphorescence itself may be retained for that particular phenomena. Photo-luminescent or phosphorescent compounds were known long before they were "discovered" by the medieval alchemists. They were wondered at, and their preparation was very much sought after, but it was not until the close of the last century that a real enquiry into the nature and properties of these substances was begun. IT is a commonplace in science that many of the old, long-familiar, and most frequently occurring of the natural phenomena which everywhere beset us, often fall into the category of things unexplained. By some ironical humour of the Gods, Perfectly pure substances are considered to be the quibbles and uncertainties of scientific know incapable of phosphorescence, the phenomenon ledge often seem to centre around common occuronly being supposed to take place in a solid solurences, and to escape altogether the more comtion. If the supposition is correct, the amount of plicated points. There exists a most wonderful impurity required to produce phosphorescence and satisfactory way of accounting for the at one must be exceedingly minute, for in the phosphortime mysterious deviations of gases from their escent compounds prepared by H. Jackson (Phil. "laws." The phenomena accompanying the elec- Mag., 1898, xlvi., 402), the amount of impurity trolysis of solutions can be explained, and we can present was too small to be detected by ordinary even account for the tricky habit which some means. Again, it is considered that a certain proorganic compounds have of behaving as if they portion of the impurity corresponds to a maximum effect But where of possessed two alternative formulæ. phosphorescence, the effect being is the all-comprehensive diminished if the amount of the impurity is detheory which satiscreased or increased (Klatt and Lenard, Wied. factorily explains all the numerous facts connected with luminescence? It is not yet formulated. Ann., 1889, xxxviii., 90). And luminescence is a phenomenon which must have set the human mind wondering ages before even an explanation of the nature of light was considered necessary. Luminescence has long been observed. It occurs both in the organic and in the inorganic kingdoms. Deep-sea creatures depend on it for their light; it is as the lovetoken in the insect world; and under some circumstances it is even the sign of death and decay. However, it is beyond the scope of this paper to deal with the numerous forms of organic luminescence, our attention being given solely to the luminescence of the mineral world. Even here, the phenomena is very widespread and diverse, both in its mode of production and in its effects. Until recently, the literature on the subject was scant, contradictory, and unsatisfactory in the extreme. Even the very name of the phenomena has not yet been fully agreed upon, the terms "luminescence" and "phosphorescence" still being arbitrarily employed, although "fluorescence" is used to denote a modification of the phenomenon. On consideration, it is evident that fluorescence is a form of phosphorescence, and for these two phenomena the term luminescence has been proposed by Wiedemann. Wiedemann suggests that all phenomena which involve the evolution of light not due to ignition should be classed under the general terms luminescence. Immediately we have two divisions: In confirmation of this, it is interesting to note that Mourelo (Compt. Rend., cxxiv. (22), 1237) prepared pure strontium sulphide by reducing the sulphate with carbon, and also by treating the carbonate, heated to redness, with HS. The resulting purified product, he found, did not phosphoresce. If heated slightly, so that a little of the sulphide was oxidised to sulphate, the substance exhibited a brilliant phosphorescence. When, however, the substance strongly heated, so as to convert a larger percentage of sulphide into sulphate, no phosphorescence was apparent, but on subsequent reduction with charcoal, the phosphorescence was again regained. escence. was The nature of the impurity has apparently a great influence upon the colour of the phosphorThis may be strikingly demonstrated by placing in test tubes quantities of an alkalineearth sulphide to which traces of metallic salts have been added. On exposing to the light of a magnesium ribbon, the tubes will be seen to glow in different colours, according to the nature of the impurity present. The following table, constructed mainly on the observation of Mourelo and Becquerel, depicts the variation in colour of the phosphorescent alkaline-earth sulphides, due to the presence of a second substance in minute amounts. Impurity. Mn Zn Na Bi ΑΙ Rb Li (Mourelo, Compt. Rend., cxxvi. (21), 1508; cxxix. (26), 1336; Becquerel, Compt. Rend., cvii., 892). The temperature at which the phosphorescent substance is maintained has also an influence upon the colour of the resulting phosphorescence escence The work of Dewar (Proc. Roy. Inst., 1895, xiv., 115) on phosphorescence at low temperatures also supports this view. Most forms of energy disappear at low temperatures, in addition to being made greater by increase in temperature, and the phenomenon of phosphorescence is no exception to the rule.* Dewar found that the sulphide of the alkaline-earth metals, whose phosphorescence is increased by rise in temperature, ceased to be phosphorescent when cooled to -80° C., but on being returned to ordinary temperatures, their phosphorwas regained. Although, at these low temperatures, the phosphorescent sulphides are unable to evolve light-energy, they are capable of absorbing it, and of releasing it at higher temlight-stimulated at a temperature of about -80° C. does not exhibit phosphorescence, but on warming up to ordinary temperatures, the usual phosphorescence is obtained. Thus light-energy, which is absorbed and stored in a potential form at a temperature of 80° C., can be completely evolved at higher temperatures. If the light-stimulation is carried out at still lower temperatures, the intensity of the resulting phosphorescence is increased. Another factor which determines the character of the phosphorescence is the intensity of the light-stimulus. J. R. Mourelo (Compt. Rend., 1899 cxxxviii., 557), showed that the phosphorescence of strontium sulphide is more intense and of a longer duration when the sulphide is light-peratures. For instance, calcium sulphide, when stimulated by diffused light. Light stimulation by bright sunlight shortens the duration of the phosphorescence and lessens its intensity. The sulphides of calcium, barium, and zinc also share this property. Diffused light also appears to increase the sensibilty of the substance to light-stimulation, and this sensibility may be further increased by repeated exposures to different light. Mourelo developed the sensibility of a specimen of strontium sulphide to such a degree that the light of a candle was sufficient to exert the necessary lightstimulus required for the phosphorescence. One specimen of strontium sulphide, when phosphorescing with sufficient intensity, can excite phosphorescence in another sample of the sulphide, although to a less degree. This phenomenon has been termed "auto-excitement", and it occurs when a flask containing a phosphorescent sulphide is exposed to light. Only the outside layer of the sulphide receives the direct light-stimulus, but it is found that the whole mass is phosphorescent, but in zones of decreasing intensity towards the centre of the flask. From the above, it would appear that phosphorescence is governed by a variety of conditions. Summarising these it will be seen that the phosphorescence of an alkaline-earth sulphide is dependent on : I. The amount of impurity present. 2. The nature of the impurity. 3. The temperature of the phosphorescent substance. 4. In the case of fluorescence, it has been shown that the phenomenon is often due to the presence of an impurity. It is really remarkable that the presence of a second substance in minute amount should impart such a striking property to the main substance. A theory which would attempt to satisfactorily explain this must be of a dual nature. It must account for the physical as well as the chemical action of the impurity. The intensity and duration of the lightstimulus. Photo-luminescence is really a case of lightstorage, or rather the storage of some form of light energy. If it were possible to measure the total amount of light-energy taken in by the phosphorescent substance, and also the total amount of light-energy subsequently evolved as phosphorescence, the two values would doubtless coincide. After all, the storage of energy in its various forms is not a novel idea. Mechanical energy, heat and electricity, can be stored in some potential form; why not also light? The duration of phosphorescence varies greatly. In some cases it may last for days, whilst in others it is almost instantaneous. Besides being dependent on the duration and intensity of the lightstimulus, the duration of the phosphorescence is to a large extent governed by the intrinsic physical, or chemical properties of the body itself. From time to time, suggestions have been put forward for the construction of an instrument which would measure the duration of the light evolved by a body possessing only a short-lived phosphorescence, but only two have really materialised; they are the phosphoroscopes of Becquerel and Lennard. The former deserves a brief description. of a Becquerel's consists Phosphoroscope shallow metal drum having two apertures cut in both ends, and exactly opposite each other. Inside the drum and near each end are place two discs which have been divided into sectors, and each alternative sector cut out. The discs are so arranged that when the sector at one end closes the aperture, light is freely admitted at the other end of the drum. Thus it is obvious that when light is admitted by a body placed in the drum, the aperture to which the eye is placed will be closed by one of the sectors, and will only be opened when a sector crosses the aperture at the opposite end of the drum. When the discs are rapidly rotated, the phosphorescent substance inside will be exposed to light and to the eye in rapid succession so as to produce a continuous vision. By this means, knowing the number of sectors and the rate of rotation, it is possible to estimate the duration of the phosphorescence of the experimental substance. Other Types of Luminescence. Light is not the only form of energy which can act upon bodies so as to cause them to exhibit luminescence. Luminescence due to other causes has long been known, and the phenomenon admits *Dewar observed that many non-phosphorescent bodies, such as ivory, rubber, cotton-wool, paper, gelatine, etc., become phosphorescent at the temperature of liquid air. This apparent contradiction deserves investigation. of certain more or less arbitrary divisions. Thus we can distinguish several classes of luminescence which are usually named according to their exciting cause. They are: I. 2. 3. 4. Tribo-luminescence ("Luminosity by Attri- Thermo-luminescence, 5. Electro-luminescence. These subdivisions of luminescent phenomena, together with photo-luminescence (phosphorescence), and fluorescence, are all included in the generic term Luminescence (Wiedeman). As their cause is probably fundamentally the same, it will be desirable to deal with each class in turn. Tribo-luminescence. Thomas Wedgwood is generally considered to be the discoverer of "Luminosity by Attrition," although it was .observed some years before by Du Fay (1735). In a paper on "The Production of Light from Different Bodies by Heat and Attrition" (Phil. Trans., 1792, 17, cxxviii., 215), Wedgwood put forward the view that almost all substances possessed some latent "phosphorism" which might be made apparent by heat or by attrition (rubbing). His experiments extended over a wide range of minerals obtained from different localities, and even to this day his paper has not been surpassed for painstaking care and thoroughness by any other workers on the subject. Wedgwood observed that the colour of the light varies according to the minerals used. He also noticed that the "bodies give out their light the instant they are rubbed on each other, and cease to be luminous when the attrition is discontinued." He paid attention to the "fœtid smell" which invariably accompanies the phenomenon of triboluminescence, although he offered no explanation of the peculiar odour. The cause and nature of this odour is still unknown. Quartz exhibits triboluminescence even under water, alcohol, or ether. This fact has prompted a writer in Nature (Nov. 4, 1920) to suggest that the apparently volatile odoriferous matter could probably be obtained in solution by grinding quartz under a suitable liquid. However, at present, speculation is of little use until more experimental evidence has been accumulated, for with regard to the latter, our knowledge of the subject is little more than it was in Wedgwood's time. Thermo-luminescence, as its name implies, is the phenomenon which accompanies the heating of of certain substances to temperature below red heat. Wedgwood carried out experiments on thermo-luminescence by dropping substances on to a heated iron plate. He found them to emit "considerable light." Many bodies emit light when heated. In the case of chlorophane, a variety of fluorspar, the heat of the hand is sufficient to cause the mineral to luminesce vividly. But here again our knowledge of the subject is almost zero. Ordinary phosporescence or photoluminescence might very well be considered to 'come under the heading of thermo-luminescence, for below a certain temperature no amount of light-stimulation will cause luminescence, but on warming up to ordinary temperatures, the luminescence becomes apparent. One might postulate and determine a critical temperature of luminescence, and thus absorb the category of thermo luminescent bodies into the larger photo-luminescent classification. Wedgwood noticed the fact that the luminescence decreased with subsequent heating, although after many periods of heating he was never able to drive the luminescence off, as it were. However, modern investigation would probably be able to determine whether repeated periods of heating are able to rid a body of its luminescence, and whether after light-stimulus at ordinary temperatures, the body again becomes luminescent on heating. This piece of simple experimental evidence would enable us to definitely fix our views with regard to photo and thermo-luminescence, and most probably the two divisions would become absorbed in one larger category. Chemi-luminescence includes all cases of chemical action which give rise to luminescence without any apparent rise in temperature. The luminescent properties of phosphorous in contact with air and moisture are well known and have been carefully studied. The luminescence of decaying vegetable matter, due to the presence of luminous fungi and bacteria, have also rather prematurely been classed under this heading. Crystalo-luminescence.-Certain salts, on crystallising from suitable solvents, give out a luminescence of short duration at the moment of crystallisation. The luminescence of arsenious acid crystallising from HCI has long been known. Berzelius noticed that saturated solution of sodium fluoxide evolved a momentary yellow-ligh when crystallising. Bandrowski (CHEMICAL NEWS, 1894, lxx., 313) studied the luminous phenomena attending the crystallisation of NaCl, KCl, KBг, K2SO., KNO, from aqueous solutions. He also observed a luminescence which occurred when salts were precipitated from the aqueous solutions by means of HCl. Sodium chloride, he states, gives a striking result, the light evolved being of a greenish-blue colour, and of considerable intensity. Bandrowski considers that luminescence is brought about by the union of oppositelycharged ions from the solution. This statement he regards as confirmed by the fact that only salts which are electrolytically dissociated exhibit the phenomenon. Farnau (J. Phys. Chem., 1913, xvii., 637) has investigated the crystalo-luminescence of the haloids of Na, K, Cd, Zn, Hg, Li, Rb, and Cs. He extends his observations to luminescence of all kinds, which he considers to be due to obscure chemical actions. If the rate of the reaction is increased by means of catalytic agencies, or by temperature, the intensity of the luminescence is increased in proportion. According to Farnau, the nature of the luminescence depends only on the metallic content of the compound, and is the same howsoever produced. Electro-luminescence.-The circumstances under which bodies can be made luminescent by electrical means are well known, and it is in this branch of the study that most of the work connected with luminescence has been performed. Many substances, especially diamonds and rubies, become luminescent more or less readily when subjected to the action of cathode rays, X-rays, radium emanation, ultra-violet light, and other forms of radiant energy. Rarified gases also become luminescent when subjected to the action of the cathode rays. These appear to be the only known conditions under which gases become luminescent, although Newall (Nature, 1897, Ivi.) has apparently observed luminescent phenomena attending the compression of a gas from low pressures up to ordinary pressure. Obviously, the phenomenon of luminescence in rarified gases is due to the operation of some kind of radiant energy, and Jackson (Trans. Chem. Soc., 1894, cxxxiv.) put forward the suggestion that the colour of the sky might in reality be of a luminescent nature, the highly rarified gases existing on the fringe of the earth's atmosphere becoming luminescent under the influence of the solar radiant energy. Such are the main departments into which all luminescent phenomena are divided. Although luminescence is the study in question, we for the most part still grope in darkness when considering it. In every branch of the study the possibilities of research are enormous, and long vistas of the unknown await the scientific man who undertakes a close and persistent investigation. But our article would be incomplete without a short reference to the Theories of Luminescence. Under this heading we shall confine ourselves to a rapid survey of the theories of photo-luminescence (phosphorescence) only. As they are very numerous, and as each investigator appears to have his own particular theory, it will be convenient here to gather them up into classes or types, for each theory conforms more or less to a standard type. Actual Absorption of a Material Light.-All early theories are based on this conception, and only differ in the explanation of the nature of the light emitted (Boyle, Canton, &c.). Theories Involving Ionic Changes. These theories which attempt to explain the apparently necessary presence of minute traces of impurities in phosphorescent substances, regard the phosphorescent body (alkaline-earth sulphide) as containing in solid solution a heavy-metal sulphide. By the act of solution the latter substance is decomposed into ions, which are further split up by light into electron-like particles. When these latter unite to re-form ions, light is emitted, and thus a phosphorescent state is set up (De Visser, Chem. Centr., 1902, 1, X., 583; Belby, Nature, vol. 1904-5). The are emitted from the phosphorescent body. electrons, however, are only evolved from certain "centres," and not uniformly from the whole area of the alkaline-earth sulphide exposed to light. The areas or localities from which the electrons have been liberated now acquire a positive charge of electricity, whilst the electrons themselves become attached to the sulphur atoms. It is in the return of the electrons from the sulphur atoms to their original "centres," and in the subsequent neutralisation of charges that the phenomena of phosphoresana has its rise (Lennard, Ann. der Physik, 1910, xxxi., 675; Stark, Prinzipien der Atomdynamik, 1911; Allen, "Photo-Electricity," 1913). Although most of the theories may be referred to the above types, the list is by no means exhausted. However, space doos not admit of any one of them being treated in detail. The most rational and far-reaching explanation of phosphorescence will probably be based on the belief held towards the end of the last century, that in some manner, light or some accompanying-light energy is held by the substance in the form of a strain of energy, and that during a recovery from the strain, light is emitted, in much the same way as energy can be stored in the form of a strain, by twisted elastic cords. Klatt and Lennard added substance to this view by showing that the alkaline-earth sulphide entirely lose their property of phosphorescence when subjected to high pressures. But here we must leave the subject as it stands. A complete understanding of the phenomena involves a knowledge of the play and interplay of atomic forces; a knowledge of which is yet lacking, but towards which the physicist is slowly but surely groping his way. Meanwhile, much remains to be done in revising and placing on a firm scientific basis many observations, the accuracy of which is to be doubted. The varied phenomena of luminescence should cease to be regarded as scientific curiosities, but should more properly be looked upon as manifestations of some unknown natural law. SOME TECHNICAL APPLICATIONS OF SPECTROSCOPY DURING THE WAR. By A. DE GRAMONT. SOME years ago, the author showed that the spark discharge of an electrostatic condenser when it illuminates the surface of a solid or fused com Theories Involving Chemical Changes.-This type of theory may be stated as follows: When a substance A becomes luminescent under the influence of light, it is converted into a new sub-pound gives a complex spectrum in which all the stance B. This latter is destroyed by heat, being converted back again into A. Thus luminescence is the energy which is evolved as a result of the transformations of the substances A and B. When thermo-luminescence does not occur, the compound B is probably of an unstable nature, and is decomposed at the instant of its formation. constituent elements of the compounds exhibit their individual line spectra. The spectrum thus produced may be considered to be obtained by the super-position of the spectra of the compounds, in which, of course, the stronger rays of one element may eclipse the fainter lines of another element in their immediate neighbourhood. The spectra of compounds thus give time dissociation spectra in which all the substances present can be recognised, the sensibilities being different according to the element under consideration. apparatus used by the author consisted either of a spectrograph with two crown glass "uviol” prisms, or one with a single quartz prism, the former. giving the best results. When the substance under examination consisted of a non-conducting The |