562 



NA TURE 



[October 6, 1898 



colour in a low vacuum to a green or blue glow in a high vacuum 

 is to be attributed to shorter oscillations in the exciting cause, 

 and how far the change is connected with a dissociation of com- 

 plex groupings into simpler ones ; a dissociation which may be 

 considered to be brought about by the rapid oscillations breaking 

 up the lime groups into two or more smaller groups. Con- 

 nected with this is also the question dealing with the possibility 

 of phosphorescence being coincident with the recombination of 

 the separated smaller grouping ; but this part of the subject can 

 only be illustrated by experiments of too minute a character to 

 be suitable to a lecture, and involves besides the study of too 

 many details. One other thing which must be taken into 

 account in drawing any deductions from the change in the 

 colour of the glow as the temperature rises is that in some cases 

 the effect of heat is to discharge some colours in a complicated 

 substance, and so leave visible others which were before 

 masked. . 



The whole question of the inter-relations of the molecular 

 weights of the phosphorescent substances, of the wave-lengths 

 of the exciting undulations, and of the wave-lengths of the re- 

 sulting glows is an important and interesting one ; but it must 

 be left alone in the present lecture with the statement, some- 

 what unsatisfactory it is feared, that, while there is no doubt 

 that special undulations of measurable wave-length are most 

 efficient in exciting phosphorescence in some substances, the 

 same effects can be produced, though to a less degree, by 

 vibrations which can perhaps be best described as undifferen- 

 tiated and irregular pulses. 



Returning to the sources of oscillations, there is one other 

 source which has yet to be considered, and that is chemical 

 ■combination. The fact that many substances will phosphoresce 

 ■during and after exposure to the flame of hydrogen has already 



■teen alluded to. The flame of coal-gas burnt in a Bunsen 



^burner will excite phosphorescence in many specimens of lime ; 



'but the effect is not strong enough to be shown to an audience. 



Naturally this effect would be stronger the nearer the lime 



was placed to the source of light. Inside the flame itself would 



*be the nearest attainable position, but then the heating effect 

 practically masks or destroys all others. In phenomena'such as 

 the glow of phosphorus the temperature does not rise to any 

 very marked extent. It is possible to obtain chemical combin- 

 ation in the presence of many bodies of a porous nature without, 

 during the early stages of the action, getting very marked heat- 

 ing effects. The action of spongy platinum in inducing the 

 oxidation of coal-gas or alcohol vapour may be taken as a 

 familiar illustration of the use of a porous material for this 

 purpose. 



In the case of a conducting metal it could not be expected 

 that the oscillations arising from the chemical combination 

 would cause phosphorescence even in the early stages, when the 

 temperature has not risen to any extent ; but if such a body as 

 lime could be obtained in a very porous condition it might, 

 while acting as an inducer of chemical combination, itself 



-respond to the oscillations arising out of that combination. 



This is found to be the case. A jet of unlighted coal-gas 



- allowed to play over warm porous lime produces a slight phos- 

 phorescence, very faint, but quite visible in a dark room. 



By dusting easily volatile substances, such as finely powdered 

 resin, over slightly heated lime, the oxidisable vapour is brought 

 more closely into contact with the lime, and the phenomenon 



- of phosphorescence is made more visible. So far, however, it 

 has not been obtained with sufficient brilliancy to be shown to 



-more than a few people at a time. When the different limes 

 that have already been experimented with are subjected to oscil- 

 lations from this chemical source, they yield their respective 



-.colours in the same way as before. The lime, which showed a 

 green glow in the vacuum tube, or when dusted on to a hot 

 plate after exposure to the jar-spark, gives a green glow with the 

 powdered resin. So also in the cases of the orange and blue 

 yielding limes. The possibility of the phosphorescence being 

 due to the resin vapour itself is excluded by control experiments 

 with other porous bodies which do not phosphoresce, but yet are 

 equally active in bringing about oxidation. 

 _ This phosphorescence was often well seen when some of the 

 limes were being prepared in a furnace. (It has been already 

 mentioned that many substances retain the power of phos- 

 phorescing at a high temperature, especially if they are in a 

 very fine state of division or not quite pure.) Most of the limes 

 were made from organic salts of calcium, and as the organic 

 matter burnt away, a thin and scarcely visible flame played 



NO. I 5 10, VOL. 58] 



over the surface of the lime at the top of the crucible in which 

 the calcination was carried out. It was frequently quite possible 

 to predict by watching the glow which was developed in the 

 lime what colour would be given when the phosphorescence 

 was brought about by oscillations from the other sources, such 

 as the jar-spark or the discharge in vacuo. 



No one who has spent much time in experimenting with 

 various substitutes for lime in lantern work can have failed to 

 be struck by the very different appearances of the light on the 

 screen given by such bodies as magnesia and zirconia in com- 

 parison with lime ; but, perhaps, the best examples are the two 

 mantles in use at the present day for incandescent gas lights. 

 One of them, the Welsbach mantle, gives a light of almost a 

 white colour. The other, or Sunlight mantle, shows a much 

 pinker colour to the eye. 



Experiments with many substances used in a similar way to 

 the mantles seem to indicate that, in addition to the ordinary 

 heating effect of the gas flame, there is another and a phos- 

 phorescent effect which probably, so far as observation can tell, 

 precedes the ordinary hot stage. It is not usual to find any 

 pure substances capable of showing this phenomenon to any 

 marked extent unless, as mentioned just now, they are in an 

 extremely fine state of division ; a condition which, like the 

 presence of impurities, may be considered to be unfavourable 

 to the too rapid discharge of the strained particles ; thus giving 

 them the opportunity of becoming fully enough charged to make 

 their oscillations, when they are discharged, of sufficient vigour 

 to be sensibly visible. 



If either of the mantles mentioned be introduced into a tube 

 and treated with an electric discharge in a high vacuum, the 

 phosphorescent glow can be studied either with or without the 

 heating effect. The glow of the Welsbach mantle is a greenish 

 white, but not very marked. The Sunlight mantle gives a fine 

 red glow. It is interesting to note that the glow shows great 

 persistence even when the temperature of the substance has 

 been raised very considerably by the vigour of the bombardment. 



Having now dealt with the last source of oscillation which it 

 was proposed to consider, it may be as well to summarise the 

 conclusions which for the present seem to be the least open to 

 objection so far as experimental evidence goes. The attempt 

 has been made to connect together all the phenomena of phos- 

 phorescence with a view of showing between them a likeness in 

 kind. Any theoretical suggestions should be taken only as 

 hypotheses for assisting this attempt and for pointing the direc- 

 tion of further experiments. It is believed, then, that the 

 following typical examples of the various phenomena which are 

 described as phosphorescent phenomena are similar in kind and 

 can be related to one another by the application of slight 

 modifications of the same general principle — the glow of 

 phosphorus, the fluorescence of quinine, the sparkling of heated 

 chlorophane, the luminosity of Balmain's paint, the light from 

 lime in a vacuum tube, and the glowing of barium platino- 

 cyanide under the influence of X-rays. To these it is proposed 

 to add coloured flames and the spectral light of glowing gases. 

 It is suggested that all these phenomena may be looked upon 

 as outward evidences of response on the part of the substances 

 to rapid oscillations, whether these oscillations have their origin 

 in chemical combination in what is commonly spoken of as 

 light, or in electrical discharge. The nature of that response 

 may in some cases be of a direct character ; but, when account 

 is taken of the many degrees of persistence of phosphorescence 

 and of potential phosphorescence, it seems in many cases first to 

 assume the form of something which, to avoid circumlocution, 

 may be called a statical charge. The release of this condition 

 of strain is accompanied by oscillations which give rise to the 

 visible undulations of the phosphorescent light. 



One final suggestion may perhaps be made, though it is 

 mentioned with diffidence, as many may consider it outside of 

 the subject. 



If it be accepted that the light of the sun has its immediate 

 origin mainly in the masses of luminous clouds floating in the 

 photosphere, and if these clouds be considered as condensations 

 into material of greater molecular complexity than that from 

 which they were condensed, then it may be not altogether out 

 of place in the present lecture to speculate on the relation 

 between the actual light from the glowing clouds and possible 

 oscillations of the particles of the medium in which they exist. 

 There is no need to emphasise the idea that the oscillations of 

 very simple molecular systems give rise to undulations which 

 can only be perceived when, by their action upon something 



