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Arsenic. The vapour of this substance glows distinctly when heated in nitrogen. In hydrogen also the glow is distinct, but fainter than in nitrogen.

To sum up, then, it appears that besides iodine, the vapours of bromine, chlorine, sulphur, selenium, and arsenic can all be made more or less incandescent by heating to the temperature at which glass combustion-tube softens, and the light emitted by each of these glowing vapours appears to give a perfectly continuous spectrum; whilst the corresponding absorption spectra are selective. Thus there is no such close relation between emission and absorption as is implied by Kirchhoff's law of radiating bodies. There seems, however, to be a general relation between the total absorbing and radiating power for the visible rays: those vapours which are highly coloured and absorb strongly in the visible spectrum also radiate conspicuously in that part of the spectrum, whilst colourless non-absorbing vapours, such as phosphorus, emit no perceptible light when heated.

That the glowing in these cases in no way differs from the glowing of heated solids seems, to say the least, extremely probable, for there is no evidence whatever that chemical changes accompany the luminosity; and there is besides the fact that when direct combination does occur between the vapour and the gas in which it is heated, as in the case of iodine in hydrogen, and possibly also arsenic in hydrogen, there is no luminous effect at all.

It may be questioned, however, whether molecular dissociation may not be concerned in the radiation, or alternate dissociation and reaggregation of the atoms of the molecules. For, according to the kinetic theory, at a given temperature and pressure the vapours may contain a certain proportion of free atoms distributed among the more complex molecular groups, but the individuality of these uncombined atoms will continually change whilst the proportion remains the same, for there will be a constant reaction or interchange going on between the atoms and the molecules. The emission of light may be supposed to depend on this act of union or disunion. of the atoms, the radiant energy being indirectly derived from the heat supplied to the system to maintain the temperature.

Thus in the case of the diatomic gases iodine, bromine, and chlorine, a proportion of the molecules I2, Br2, Cl2 may dissociate into 21, 2Br, 2C1, and sulphur vapour may similarly dissociate from Se to 3S2, and so on. From recent determinations of the vapour densities of the halogens, it appears that iodine begins to dissociate between 600° and 700° C., at a pressure of 1 atmosphere. Chlorine, on the other hand, * Crafts and Meier, Ber. deut. chem. Ges. xiii.

remains at a normal density corresponding to Cl, between about 200° and 1200° C. With regard to the former element, the temperature at which dissociation commences (say 600°) is not much above that at which the glowing is first seen, and as in most of the experiments the iodine or bromine vapour is largely diluted with a neutral gas, so that the partial pressure is a good deal less than one atmosphere, it might well be supposed that dissociation was going on even at the lowest temperature at which the glow can be seen. But in the case of chlorine dissociation begins at some 500° above the temperature of my experiments (assumed at about 700°): moreover, there is no dilution of gas, which is observed at the atmospheric pressure, so there can be no question of dissociation here; or at any rate, as there is no independent evidence of it, we have no more right to assume it as a cause of the luminosity than we have in the case of glowing solids.

But, apart from the fact that chlorine can be made incandescent although it is not dissociating, it appears to me that the general relation mentioned above between radiation and absorption of the visible rays, and the fact that the intensity of the glowing of the more absorptive vapours (the others being too difficult to observe) appears to closely follow that of a solid raised simultaneously through the same range of temperature, gives strong support to the view that there is no essential difference between gases and solids in the manner in which they radiate, at any rate under the conditions of the foregoing experiments. If dissociation were concerned, say, in the case of glowing iodine, one would expect the intensity of the light to rapidly increase when the temperature is made to approach the actual temperature of dissociation, where the maximum interaction of the atoms occurs. It should in fact increase in a much greater ratio than in the case of a glowing solid. But I have failed to detect any evidence of such relative increase on the part of either iodine or any other glowing gas. Further, a decrease of density (by exhaustion or dilution) will facilitate dissociation, and thus should tend to counteract the reduction of luminosity due to a smaller number of molecules concerned. But no such effect is in fact to be seen under these conditions.

* J. M. Crafts, ibid. xvi.; also Jahn, ibid. xv.

†The radiation from iodine may be easily compared with that of a solid at the same temperature, by placing a small piece of carbon inside the heated portion of the glass tube described in exp. I. Also when the glass contains opaque particles, these are seen to glow with the same intensity as the iodine, whatever the temperature, when the vapour is of sufficient density to give the maximum luminosity.

The Sodium Radiation.

Having thus far failed to produce discontinuous spectra by external heating, I next tried what could be done with metallic vapours. Sodium was the metal chosen for the initial experiments, the powerful absorption produced by the vapour of this element on "D" light seeming, on Kirchhoff's hypothesis, to give the best chance of success at the very moderate temperatures I could command with a single large Bunsen flame.

The form of apparatus used in the earlier experiments was designed with a view to excluding, as far as possible, from the tube in which the sodium was to be heated any gaseous substances that might be expected to react chemically with the vapour of the metal: the emission phenomena produced under these conditions being then compared with that produced when traces of oxygen or moisture were purposely allowed to remain in the neutral gas in which the sodium was volatilized. In the diagram (Plate VII. fig. 1) A and B are two similar gas-holders; a rubber tube leads from A to a couple of wash-bottles S', S", containing strong sulphuric acid; from S" a long tube of hard glass, P, containing a little phosphorus leads into the drying-tube C, which is packed with calcium oxide and calcium chloride-the former to remove carbonic acid, and the latter traces of water which may remain in the gas used after passing the sulphuric-acid bottles. The drying-tube connects on to the porcelain heatingtube H through a metal T-piece, one end of the T having a glass plate carefully cemented in so that one may look along the inside of the heating-tube, to which the T is connected by rubber tube tightened with wire, both connexions being also buried in sealing-wax. At the other end of H, which is covered in the centre by a fireclay arch, is a second T of glass, one limb connecting with a glass gland or stuffing-box, G, with pierced rubber ends, and filled with mercury. A long steel rod passes through the gland, the end being flattened to a spoon-shape; this can be pushed along to the centre of H, or drawn out past the entrance of the side tube of the T. This side tube is closed by a perforated rubber stopper, through which a small glass tube passes bearing a small reflecting prism cemented to the end, which is thus closed up; but in order to allow of the escape of the gases, so that a current may be set up in the apparatus, a hole is blown in the side of this tube near the prism. The outer end of the tube is connected by rubber tubing to another wash-bottle S"" containing sulphuric acid, and from this again a tube leads to the gasholder B. Thus the entire apparatus forms a closed circuit and has no inlet or outlet. The gas-holders have each a

Y-tube attached, one branch of the Y leading to the apparatus and the other connecting A with B by means of a rubber tube carrying a clip. The S at the foot of the diagram is a small direct-vision spectroscope, and L is a lens focussing the central parts of the tube H on to the slit of the instru ment; both are attached to a strip of hard wood movable horizontally about an axis placed between the lens and the slit. This enables one to instantly push aside the spectroscope into the position shown by dotted lines and observe the glowing tube directly.

To observe the sodium-spectrum, one fills the gas-holder A with some indifferent gas containing no oxygen, or only a trace of that element, such as nitrogen or hydrogen, or ordinary coal-gas. When full it is disconnected with the gas-generator or gas-main, as the case may be, and connexion is made with the apparatus. Next, weights are put on A until sufficient pressure is obtained to drive a current of gas through the wash-bottles, drying-tube, &c. into B. Then the Buusen is lighted under the porcelain tube, which it presently heats up to a bright incandescence for about two inches of its length. After sufficient dry gas has passed through, and all trace of moisture has gone, the current is stopped by closing the stopcock on A, and a small pellet of sodium is dropped into the steel spoon through the side tube of the glass T, the stopper with the inner tube and prism being removed for this purpose and quickly replaced. The current is then restarted, to drive away any oxygen that may have diffused in by the operation, and at the same time the tube P is gently heated by a spirit-lamp flame until a small faintly luminous flame is seen, indicating combination of the last traces of oxygen with the phosphorus. After this has gone on a sufficient length of time, and the apparatus may be considered to be free from oxygen, water, and carbonic acid, the stopcock on A is again closed and the steel spoon carrying the sodium is pushed into the hot part of the tube H, turned over, the sodium shaken out, and the spoon again withdrawn past the entrance of the side tube. Now the tube carrying the prism is pushed down into line with the tube H, and the white flame of a paraffin-lamp is placed close alongside the glass T, so that a ray of white light can be made to traverse the glowing vapour in H. One may now observe at will the absorption or emission spectrum of the glowing sodium by the simple operation of turning the lamp-flame up or down.

The experiments actually performed with this apparatus may be thus briefly described:

:

I. With the porcelain tube strongly heated, a slow current of coal-gas, not specially freed from oxygen, was allowed to

circulate in the apparatus, no sodium being admitted. A distinct and fine sodium line was visible in the spectroscope, which increased in brightness when a little air was mixed with the gas, but which gradually faded to invisibility when the phosphorus tube was heated so as to eliminate oxygen.

The explanation of this result appears to be simple enough. The trace of oxygen remaining in the coal-gas combines with the hydrogen when it reaches the hot part of the tube, and the 66 flame" so formed (which, however, is not visible as such, except when a large quantity of O is present) becomes tinted by the salts of sodium, which in excessively minute quantity are known to be driven off from the porcelain at a red heat, just in the same way as the Bunsen flame outside is tinted. This fine double D line, therefore, may not be the result of heat alone, since it is developed as a consequence of chemical reactions.

II. A pellet of clean sodium was placed in the steel spoon, and the gas-coal-gas-allowed to circulate, the phosphorus being heated. The line seen in Experiment I. was watched, and some time after it had quite disappeared the current was stopped and the sodium pushed into the hot part of the tube. Instantly the central bore of the porcelain was filled with light, which in the spectroscope was found to be perfectly continuous, but crossed by a very wide black line at D. Gradually the continuous spectrum faded, and as it became fainter the dark D line was seen to be bordered with a fringe of light on each side; and as the vapour became less dense, owing to the distillation of the sodium into cooler parts of the tube, the D line went through the changes represented in fig. 3, in the order a, b, c, d, finally persisting as a rather wide bright line in which a very fine dark line could usually be made out *. But at any stage of the experiment, the dark central line could easily be extinguished by allowing a gentle current of gas to push back the cooler absorbing layer into the hotter regions. Now the question to be decided was whether this broad bright, hazy D line was or was not the result of chemical activity.

III. In this experiment the phosphorus tube and the drying-tube were cut out of the circuit, the gas-holder A being connected directly with the heating-tube. With the current of gas stopped, the D line appeared as in the last experiment, but observations were somewhat impeded by opaque clouds of oxide which hung about the cooler parts

*The dispersion of the spectroscope employed in all the sodium experiments being insufficient to separate the two components of the D line, it is evident that when this line appeared widened the two were really fused into one broad band.

Phil. Mag. S. 5. Vol. 39. No. 240. May 1895. 2 I

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