October 6, 1898] 



NATURE 



employed, it is conceivable that the oscillations would appear as 

 short ethereal waves or, in other words, Rontgen rays. In the 

 case of a low vacuum, or of no vacuum at all, the charged 

 particles would discharge themselves against the intervening 

 gas, which would in its turn respond to the rapid oscillation 

 and give out its own particular coloured light. The expression 

 " short ethereal waves " is used intentionally, for if there should 

 be forthcoming experimental evidence of the complex molecular 

 structure of a gas, it is reasonable to suppose that in a high 

 vacuum, with consequently a high potential at the electrode, 

 the internal electrical repulsion in a group would tend to a 

 dissociation resulting finally in the simplest form of system 

 capable of separate existence in those conditions. It might be 

 expected that the oscillation frequency of so simple a system 

 would be very high. 



Here it may be stated that this comes to practically the same 

 thing as Sir William Crookes' original conception of radiant 

 matter. 



Leaving the method of electrical excitation in vacuo for obtain- 

 ing phosphorescence we may now turn to light as a source of 

 oscillations. For the sake of simplicity it will be best to 

 continue the experiments with the same substance, viz. lime. 

 If this body be exposed to the light of the sun, of the electric 

 arc, of a hydrogen flame, and of a great many other substances 

 in a state of vigorous combustion, a phosphorescent effect is 

 obtained, feeble in comparison with the results in vacuo, but 

 apparently similar in kind. The best light for inducing the 

 phosphorescence is the spark from a fairly powerful coil with a 

 Leyden jar in circuit. Many specimens of lime go on giving 

 out light for a considerable time after exposure. A cylinder ot 

 lime such as is used in the production of the lime-light glows 

 quite visibly when it is rotated before a jar-spark. 



The light from the sun is not so active in inducing this glow ; 

 but with suitable arrangements a fairly visible result can be 

 obtained. The colour of the glow from most lime made from 

 limestones is an orange-red becoming a golden orange when the 

 lime is heated. The introduction of glass, mica or Iceland spar 

 between the spark and the lime, cuts off the glow at once ; 

 since these bodies are opaque to the undulations to which lime 

 of this kind responds. Quartz, rock salt, and selenite are quite 

 transparent. 



It is found that the different forms of lime which have 

 already been exhibited in vacuum tubes yield when exposed to 

 the jar-spark their specially coloured phosphorescent glows. 

 But these are difficult to see ; they are very faint when pure 

 specimens of lime are used. However, there is a way out of 

 the difficulty. The faint light scarcely visible at the ordinary 

 temperature may be increased very considerably by raising the 

 temperature. As an extreme instance of this a specimen of 

 calcium sulphide may be taken. After exposure to almost any 

 source of white light this glows with a bluish phosphorescence 

 which becomes quite brilliant when the sulphide is heated. A 

 similar change is noticeable in the case of the different limes. The 

 orange, green and blue varieties exposed to a series of jar-sparks, 

 and subsequently dusted over hot plates, give with easy visibility 

 the colours which they exhibited in the vacuum tubes and which 

 may, for the present, be considered as sensible indications of 

 their molecular constitutions. 



Two important considerations have to be dealt with at this 

 point. In the first place the question arises how far one and 

 the same light, i.e. one and the same oscillation frequency, will 

 excite the different specimens of lime. Without entering into 

 dry numerical details, it is not possible to give a complete 

 answer to this question. In a general sense, however, it is 

 apparently true that, although the range of frequency is large, 

 the red and orange varieties of lime respond to oscillations less 

 rapid than those which readily affect the varieties giving a green 

 or blue phosphorescence. It is possible to obtain a form of lime 

 which illustrates this experimentally. It is not easy to make. 

 It is prepared from calcium urate by heating this for many hours 

 to a dull red heat, and afterwards raising the temperature of the 

 blackened mass sufficiently to burn off all the organic matter 

 and leave only lime. The residue on analysis was shown to be 

 really lime. Such a specimen exposed freely to jar-sparks, and 

 afterwards heated, shows mainly an orange phosphorescence ; 

 but if glass or mica or Iceland spar be placed between the lime 

 and the source of light, then the effect of heat is to intensify 

 greatly a phosphorescence of a blue colour. It must be clearly 

 understood that this blue was there before, only masked by the 

 superior brilliancy of the orange colour ; the undulations which 



would otherwise have affected the molecular groupings capable 

 of giving out the orange light being cut off by the glass or mica. 

 It would be tedious to give all the reasons for assuming that the 

 oscillations exciting the blue phosphorescence are probably the 

 more rapid. To some extent the transparency of glass and mica 

 to X-rays may be taken as confirmatory ; but to follow the 

 argument out from spectroscopic evidence and measurements 

 would involve a discussion unsuited to a lecture dealing with 

 general questions. Referring, however, to the .suggested ex- 

 planation of the action taking place in a vacuum tube, it is not 

 inappropriate to mention now that it is possible to make a 

 specimen of lime give an orange glow in a moderate vacuum 

 while a portion of the same specimen is exhibiting a blue glow 

 in a high vacuum. The readiness with which this blue glow 

 appears, and the time which it takes to develop, must be taken 

 into account in dealing with its supposed origin, and with its 

 relevancy with the question of the relation of the rapidity of the 

 exciting undulation to the wave-length, i.e. to the colour, of the 

 phosphorescent light. Perhaps it is advisable to leave this 

 point for the moment, and to turn to the second consideration. 

 This deals with the question of the duration of the phos- 

 phorescence. 



At the beginning it was shown that some bodies glow only 

 while light is acting upon them, or while they are under the 

 direct influence of an electric discharge. In others there was a 

 marked after-glow ; vvhile still others required the application of 

 heat before any phosphorescence was visible, or, as in the case 

 of the limes, before the phosphorescence was easily visible. 

 With Balmain's luminous paint, or with any body which gives a 

 marked phosphorescence that lasts for some time after with- 

 drawal from the exciting influence, it can be readily shown that 

 lowering the temperature reduces the brilliancy of the glow, but 

 lengthens the time during which it lasts. The effect of heat has 

 already been mentioned as vastly increasing the brilliancy ; but 

 it greatly diminishes the duration of the light. On the other 

 hand, Prof. Dewar has shown that great reduction of the tem- 

 perature will cause the phosphorescence to linger for a consider- 

 able time in many substances which had hitherto been considered 

 as practically non-phosphorescent. The different behaviours of 

 substances in this respect can, perhaps, be best brought under 

 one explanation by applying the idea of a statical charge or a 

 condition of strain to the phosphorescent substances themselves. 

 Duration of phosphorescence would then be a measure of 

 rapidity of discharge. If it be supposed that, the strain having 

 been set up in the particles of a substance, these discharge them- 

 selves against one another, or rather against uncharged particles, 

 then a substance with great freedom of transference of move- 

 ment among its particles would fail to show any sign of phos- 

 phorescence ; since the strain would be released or conducted 

 away by rapid transference before a condition could be set up, 

 out of which oscillations of sufficient amplitude could arise. With 

 rather less freedom of movement among the particles the non- 

 conducting state might be reached by restricting the extent of 

 that movement by cold, as in Prof. Dewar's experiment. Still 

 less freedom of interchange may be considered to obtain in 

 Balmain's luminous paint, and even less in the limes, which 

 require heating to show up their phosphorescence ; while, in 

 the case of the chlorophane and many other minerals, the con- 

 dition of strain, however set up, can apparently be retained 

 indefinitely. Specimens of lime after exposure to the jar-spark 

 have been found to give out light when heated after being four 

 years in the dark. It seems not altogether improbable that the 

 influence of impurities in promoting phosphorescence may often 

 be attributed to their interfering with the freedom of movement, 

 and so permitting the groupings of the substance to be sufficiently 

 highly charged. The effect of heat in rendering a substance a 

 better conductor can be well studied with pure substances in 

 vacuo under the electric discharge. 



Under the vigorous bombardment of radiant matter the 

 temperature of the substance rises. In some substances this 

 leads to an increase in the brilliancy of the glow maintained 

 often even when the heating is very considerable ; in others the 

 hotter portions are marked out by a complete absence of phos- 

 phorescence. Observation seems to favour the conjecture that 

 this absence is in many cases to be explained on the hypothesis 

 that the heat endows the molecules with such freedom as to 

 practically render them uninsulated. To pursue this part of the 

 subject any further would lead to a discussion of a question that 

 can only be referred to. It is the consideration of how far the 

 change of glow in some specimens of lime from a red or orange 



NO. I 5 10, VOL. 58] 



