408 Mr. H. Jackson on Phosphorescence. 



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 phos- 

 phorescence, 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 pre- 

 sent, 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 the 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 phospho- 

 rescence of a blue colour. It must be clearly understood 

 that this blue w T as 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 ^-raj'S may be taken as confirmatory; but to 

 follow the argument out from spectroscopic evidence would 

 involve a discussion unsuited to a lecture dealing with general 

 questions. Referring, however, to the suggested explanation 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 



