WEEKLY EVENING MEETING, Friday, January 18, 1895. SIR FREDERICK BRAMWELL, BART., D.C.L. LL.D. F.R.S., Honorary Secretary and Vice-President, in the Chair. PROFESSOR DEWAR, M.A. LL.D. F.R.S. M.R.I. Phosphorescence and Photographic Action at the Temperature of Boiling Liquid Air. [Abstract.] CONTINUED investigation of the properties of matter at extremely low temperatures has resulted in a considerable addition to our knowledge on this subject, more especially in regard to phosphorescence and photographic action. Phosphorescence and fluorescence are terms applied to similar phenomena which apparently differ only in degree, the first being practically an instantaneous effect, while the other lasts for a relatively long period after the withdrawal of the light stimulus. In all cases the luminous effects called phosphorescence and fluorescence belong to a less refrangible part of the spectrum than the exciting rays. Professor Stokes has shown that the singular surface appearance observed in fluorescent liquids is due to a change of refrangibility of the light absorbed, and again given off by their upper layers. Phosphorescence may be regarded as a kind of fluorescence which lasts a long time after the excitation has ceased, and may be briefly defined as the phenomena observed when certain substances give out light through the transformation of absorbed vibrations of shorter period. This must not be confused with the luminosity due to the slow oxidation of phosphorus; nor with the "phosphorescent" appearance accompanying the slow combustion of decaying animal and vegetable matter; nor with the more or less voluntary display of light by fireflies, glow-worms and small marine animals. The researches of Becquerel showed that the intensity of phosphorescence depended directly on the product of the intensity of the stimulating light, and a factor of absorption, and inversely, as some coefficient representing molecular friction or damping. When phosphorescing sulphides of calcium are heated they increase in their light emission, whereas if cooled to 80° they cease altogether to be luminous, and if maintained at this low temperature for hours, keep a latent store of light energy that may be again evolved on allowing the sulphide to rise to the ordinary temperature. But while the temperature of -80° is sufficient to stop all sensible emission from proviously-excited sulphide, it does not prevent an unexcited sulphide from "absorbing" light energy that can be evolved at higher temperature. By means of liquid air we can now cool substances to temperatures ranging from 180° to 200°. Under such conditions all known organic compounds are solids, and this condition of matter is specially favourable to phosphorescent phe nomena. Moreover, the list of truly phosphorescent bodies has been greatly extended, and knowledge of their peculiarities in this direction increased. The effect of temperature on phosphorescence is easy to observe by taking two portions of the same substance placed in similar very thin test tubes, cooling one of the specimens in liquid air, and then quickly exposing both samples side by side to the same light stimulation. The form of apparatus used is shown in Fig. 1. A is a powerful electric lamp in a lantern, the latter carrying a fitting whereby the light is screened from the eye of the observer. E is The a double vacuum vessel containing liquid air or oxygen. substance to be examined is plunged below the surface of the liquid air, as at F. When it is thoroughly cooled, it is withdrawn from the liquid and exposed for a few seconds to the full light of the are in the horizontal part of the tube. It is then quickly withdrawn and examined for phosphorescence in the darkened room. The fitting B is another modification of the apparatus for experimental purposes, and consists of a slit and suitable lens and prism, whereby the spectrum can be thrown on to a table, as at D C, and cooled bodies examined in various parts of the spectrum. If, during the light excitation caused by burning magnesium or a flash of the electric light, the eyes are carefully covered, then the comparative phosphorescence, if any, of the cooled and uncooled substance can be observed. In this mode of working the action of the very short wave-lengths of light are stopped by the opacity of glass, but the solid condition of all substances at the low temperature enables the use of glass to be abandoned when necessary. As a general rule it may be stated that the great majority of substances exhibiting feeble phosphorescence at ordinary temperature, become markedly more active at these very low temperatures. Thus gelatin, celluloid, paraffine, ivory, horn, and india-rubber become distinctly luminous, with a bluish or greenish phosphorescence, after cooling to 180° and being stimulated by the electric light. Hydroquinone was more luminous than the isomeric resorcinol or pyrocatechol, and in the same way pyrogallol was faint compared with phloroglucol. All alkaloids forming fluorescent solutions become phosphorescent at low temperatures. The hydrocarbons, alcohols, acids and ethers of the fatty series are all more or less active, and glycerin, sulphuric and nitric acids are all very bright, so also are concentrated hydrochloric acid and strong ammonia solution. Coloured salts generally show little activity, but a large number of colourless salts are very luminous. Water when pure is only feebly phosphorescent, but remarkably so when impure. Acetic acid and acetamide appeared fairly equal in luminosity; hippuric acid was very fine, as were most substances containing a ketone group. Lithium platinocyanide changed from white to red on cooling, and was excelled in phosphorescing power by yellow ammonium platinocyanide, which was exceedingly bright. Definite organic substances possessing exceptional powers of phosphorescence when stimulated at 180° C., are acetophenone, benzophenone, asparagin, hippuric acid, phthalic anhydride, urea, creatine, urethane, succinimide, triphenyl methane, diphenyl, salicylic acid, glycogen, aldehyde-ammonia, &c. It will require long and laborious experiments, however, to measure the relative brightness of the phosphorescence of bodies belonging to definite series. Remarkable results were obtained with an egg-shell and a feather respectively. The egg shone brilliantly as a globe of blue light, and the feather was equally brilliant, its outline showing clearly in the darkened room. Other organic substances giving good results were cotton-wool, paper, leather, linen, tortoiseshell, and sponge, all phosphorescing brightly, as did also a white flower, a cultivated species of Dianthus. Coloured glasses and papers as a rule exhibit no phosphorescence, and when the alcohols are coloured by the addition of a trace of iodine, the luminous effect is destroyed. Milk was shown to be highly phosphorescent and much brighter than water. The white of egg has greater phosphorescing power than the yolk, white substances generally being superior in this respect to coloured ones. On cooling a layer of white of egg on the outside of a test tube to -190°, and then exposing it to a flash of the electric arc, the brilliancy of the phosphorescent light is very striking. The chloro-, bromo-, iodo-, sulpho-, and nitro-compounds, as a rule, show nothing, or are but faintly luminous. Amongst basic bodies nicotine is more luminous than quinoline or pyridine. Metals also phosphoresce, but in this case the action is due to some organic film deposited from the air, because it disappears on ignition. If the metal is subsequently touched, the phosphorescence re-appears. So far as the examination has been carried, the two most remarkable classes of substance for phosphorescence are the platinocyanides amongst inorganic compounds, and the ketonic compounds, like acetophenone and ethyl phenyl ketone, and others of the same type, amongst organic. When ammonium platinocyanide is cooled with liquid air and maintained at this temperature by being immersed in the liquid while stimulated by exposure to a beam of the electric arc, it continues to glow in the dark with a feeble emission as long as the temperature is kept about -180°. On pouring off, however, the liquid air from the crystals so that the temperature may rise, then the interior of the test tube glows like a lamp from the sudden increase of light emissivity as the temperature rises. It seems clear from this experiment that similar initial light intensities being used for stimulating, the substance at this low temperature must have acquired increased power of absorption, and it may be that at the same time the factor of molecular friction or damping may have diminished. That the absorptive power of substances for light is greatly changed at low temperatures is proved by the change of colour in substances like oxide, iodide, and sulphide of mercury, chromic acid, &c., when cooled. Many quantitative photometric measurements must be made before the actual changes taking place in the conditions governing the phenomena can be definitely stated. Along with these experiments on phosphorescence, a number of photographs have been taken at 180°, using various sensitive plates and films, and these have been compared with similar photographs taken at the same time under similar conditions at the ordi nary temperature. The first plan (Fig. 2) was simply to immerse a strip of sensitive bromide paper A B, in one of the vacuum vessels containing liquid oxygen, and when the part immersed had been thoroughly cooled down, exposing it to the light of a piece of burning magnesium. The paper was then developed, when a result something resembling C was obtained. The part which had been cooled by the liquid oxygen, as at A, was untouched by the light, whereas the portion of paper above the liquid at B developed up quite black. Further modifications were made in Fig. 3, where the strip of sensitive film E was enclosed in a cover of sheet lead B, having two small discs cut away as at C and D. The strip was then cooled in liquid oxygen A, and then exposed to a flash of burning magnesium. After development, the strip appeared something like E, when again the action of the light was considerably diminished on the part of the film which had been cooled. In Fig. 4 a form of apparatus was adopted whereby the exposures were made without the disadvantage of the light passing through the glass sides of the vacuum vessels. B and E were vacuum vessels enclosed in a blackened box; into B a quantity of liquid oxygen was poured. The sensitive plate or film was then lowered so as just to touch the surface of the liquid as at C. D was a comparison plate exposed at the same time and at the same distance from the source of light A, only the comparison plate |