56o 



NATURE 



[October 6, 1898 



light is acting. Balmain's luminous paint is an illustration of 

 the persistence of the phosphorescent light. With many 

 minerals, notably some fluorspars and felspars, light is given 

 out when they are slightly heated, or in some cases only 

 crushed. 



The most brilliant phenomena are those which can be studied 

 when many bodies are excited with electric discharges inside a 

 Crookes' vacuum tube, while outside of a slight modification of 

 his focus tube fairly brilliant phosphorescence can be obtained by 

 the action of Rontgen rays upon several substances, notably 

 upon some of the platinocyanides. 



In dealing with the whole subject of phosphorescence with 

 the view of attempting to connect all the various phenomena 

 together, it is convenient to divide it into — the nature of the 

 substance giving out the light, the nature of the light given out, 

 and the nature of the exciting causes. 



With regard to the nature of the substance, either very much 

 or little might be said ; very much from the details of numerous 

 experiments with a great number of compounds, but little from 

 the point of view of general principle. The most important 

 question in this respect is probably the question of the relation 

 of phosphorescence to the purity of the substance giving out the 

 light. Experiments with carefully prepared compounds of many 

 metals make it clear that not a few substances can be made to 

 exhibit phosphorescence when they are so free from impurities 

 that none can be detected by any analytical methods. In some 

 cases, however, there is either no light given out under any of 

 the conditions for exciting phosphorescence, or the light is so 

 feeble that it is necessary to add impurities so as to obtain a 

 suitable molecular condition for rendering a substance responsive 

 to excitement. That the light given out is not to be ascribed to 

 the impurity has been determined by many experiments with 

 varying impurities and careful examination with the spectro- 

 scope. The further consideration of these physical and chemical 

 conditions is better left until the other two aspects of the subject 

 have been dealt with. 



If a large number of observations be made of the phosphor- 

 escent lights given out by compounds of such metals, for 

 example, as sodium, potassium, calcium, strontium and barium, 

 magnesium and aluminium, it is hardly possible to avoid coming 

 to the conclusion that the colours of these lights have a close 

 resemblance to the colours of the lines and bands seen in the 

 various spectra of the different metals and some of their com- 

 pounds. Examination by the spectroscope confirms this con- 

 clusion in several instances. It is not suggested that the lines 

 of the metals and the bands of their compounds are reproduced 

 in the spectra of the phosphorescent lights. What is noticeable 

 is that the maxima of light are grouped about these bands and 

 hnes, fading away from them and extending to other parts, so 

 that a more or less continuous spectrum is seen with positions 

 of greatest brilliancy. In the case of some specimens of lime 

 these positions are well defined, and in some kinds of fluorspar 

 the green and some red bands are well seen, either when the 

 fluorspar is heated or when it is excited by discharge in vacuo. 

 The questions of exact coincidence and of the shifting of the 

 positions of the maxima of brightness seen with different com- 

 pounds of the same metal need not be considered here. The 

 intention is only to emphasise the similarity between the phos- 

 phorescent spectra of several metallic compounds and the spectra 

 of these compounds, or of the metals in them, obtained in other 

 ways. 



In experimenting with phosphorescent compounds it is fre- 

 quently noticed that specimens of the same substance in appar- 

 ently the same state of purity give different colours. Confining 

 attention for the present to lime, as a very infusible substance 

 easily obtained in a state of purity, what follows will be made 

 clearer by a brief consideration of the spectrum of the coloured 

 flame produced by holding some compound of calcium, e.g. 

 calcium chloride, in the flame of a bunsen burner. 



The spectroscope breaks this red flame up into red, orange 

 and green bands and a blue line. For the moment the sugges- 

 tion may be taken that these diff"erently coloured bands are 

 indications of the existence in the flame of groups of particles 

 of calcium compounds of varying degrees of complexity — the red 

 being related to more complex groups, the orange to less, and so 

 on. It seemed not unlikely that it might be possible by pre- 

 paring lime from a great many calcium salts to obtain separate 

 specimens which might preserve in the solid state some relation 

 in their own molecular complexity to that of the salts from 

 which they were obtained, or the conditions of decomposition 



of the different calcium salts might impress upon the residual 

 limes diff"erent characters of molecular structure. The pre- 

 paration of about 350 specimens of lime showed that it was 

 quite possible to get specimens some of which phosphoresced 

 red, some orange-red, some orange, others green, and some 

 blue. Examination of their phosphorescent lights with the 

 spectroscope showed, as referred to before, that the maxima of 

 brilliancy in their spectra were grouped about the bands and 

 lines of the usual spectrum of calcium oxide. The details of 

 the preparation of these specimens of lime are too elaborate to 

 enter into here, nor is it possible to do more than just to refer 

 to their varying densities and different rates of hydration. Out 

 of the number of specimens tried the most satisfactory were 

 analysed to make sure that it was really lime and only lime 

 which was being dealt with in each case. In general terms it 

 may be said that the most complicated organic sails of calcium 

 yielded the best attempts at Hme giving blue phosphorescence, 

 simpler bodies gave green, while the best orange was obtained 

 from Iceland spar, and the red from specially prepared calcium 

 carbonate. That lime yielding a blue colour was obtained 

 from highly complicated organic salts does not contradict the 

 former suggestion that perhaps it is really of simpler molecular 

 structure than the others. Chemists are familiar with the con- 

 ception that the complexity in structure arising from the massing 

 of many molecules together in groups is probably often greater 

 in bodies of apparently simple chemical composition than in 

 those of a much more highly complicated nature. 



The colours seen in the specimens of lime shown are not 

 pure. In each one the other colours are present ; thus the 

 orange contains also the red, green and blue, only these are 

 masked by the greater proportion of the one colour. Compare 

 for example the light obtained from a vacuum tube containing 

 the gas helium. In this case the colour is yellow, although the 

 spectrum contains beautiful red, green and blue lines. If the 

 different colours are related to varying molecular complexity in 

 the substances, then it might be said that the lime showing a 

 green light contains a large proportion of groupings of such a 

 nature as to be capable of oscillating in a way to give rise to 

 green light, and in like manner for the red, orange and blue 

 specimens. Whether it will be possible or is in the nature of 

 things to separate out the different kinds in a state of purity can 

 only be decided by further experiment. 



The examples of different forms of lime have been so far 

 exhibited only under the conditions obtaining in a high vacuum 

 with an electric discharge. Before trying to show the points in 

 common between these phenomena and the phenomena of phos- 

 phorescence in other conditions, it may be as well to consider 

 briefly the character of the action in a high vacuum. The 

 suggestion which follows is not intended to be anything but an 

 imperfect attempt to bring all the phenomena of phosphorescence 

 into line with one another. 



When a discharge passes through a vacuum there can be little 

 doubt that the transferring medium is the residuum of gas in 

 that partial vacuum. If the particles of this gas behave as 

 visible masses are seen to do, they are probably attracted or are 

 driven to the electrode, which is at high potential. Receiving 

 the same kind of charge as this electrode, they fly off" from it in 

 that charged condition. 



But if these particles consist of more than one unit, each 

 unit, after the group has travelled a certain distance from the 

 electrode, must repel each other unit in the same way as the 

 whole little group was repelled from the electrode. If, how- 

 ever, the units making up the group are held together by that 

 something which is called chemical attraction, a condition of 

 strain is set up in which the electrical repulsion is striving to 

 overcome the chemical attraction. Travelling unimpeded 

 through the high vacuum this condition of strain would be 

 maintained until the charged group met with something capable 

 of discharging it. At that moment of discharge the chemical 

 attraction would assert itself; there would be a rushing together 

 of the units composing the group, and an over-rushing, whereby 

 oscillations would be set up. These oscillations, considered as 

 blows or pulses, either directly or ethereally transferred to a 

 substance, would set it in turn oscillating in a manner fitted to 

 its own molecular structure, and its oscillations would in their 

 turn give rise to the undulations which appeal to our eyes as the 

 phosphorescent light. If instead of the discharge taking place 

 on a substance capable of responding to and absorbing most of 

 the energy of the consequent oscillations, it were to occur on 

 glass, platinum, or any of the materials which have been 



NO. 1510, VOL. 58] 



