Tunc lo, iSS6] NATURE 



the characteristic lines appeared. It was found that how- 

 ever highly I heated the glass and then pumped the tube 

 free from hydrogen, I had only to heat the glass to a still 

 higher temperature to get a hydrogen spectrum in the 

 tube. I consider the hydrogen comes from vapour of 

 water, which is obstinately held in the superficial pores, 

 and which is not entirely driven off by anything short of 

 actual fusion of the glass. The bubbles noticed when the 

 disintegrated and fused surface of the tube was examiried 

 under the microscope are probably caused by escaping 

 vapour of water. 



When the negative discharge has been playing for 

 some time on German glass, so as to render it strongly 

 phosphorescent, the intensity of glow gradually dimi- 

 nishes. Some of this decline is due to the heating of the 

 glass or to some other temporary action, for the glass 

 partially recovers its property after rest ; some is due to a 

 superficial change of the surface of the glass ; but part of 

 the diminished sensitiveness is due to the surface of the 

 glass becoming coated with this brown stain. 



The luminous image of a hole in a plate of mica was 

 projected from a plalinum plate used as a negative pole, 

 to the side of a glass bulb. The coil was kept playing for 

 some time until the inside of the bulb was thoroughly 

 darkened by projected platinum. Although a bundle of 

 molecular rays could be seen all the time passing from the 

 platinum through the hole in the mica to the glass, where 

 it shone with a bright green light, I could detect no trace 

 of extra darkening when the part of the glass formerly 

 occupied by the green spot was carefully examined. 

 Platinum is a metal which flies off in a remarkable 

 manner when it forms the negative pole. It therefore 

 appears from this experiment that the molecular stream 

 does not consist of particles of the negative pole shot off 

 from it. 



One of the most striking of the phenomena attending 

 this research has been the remarkable power which the 

 molecular rays in a high vacuum possess of causing phos- 

 phorescence in bodies on which they fall. Substances 

 known to be phosphorescent under ordinary circumstances 

 shine w ith great splendour when subjected to the negative 

 discharge in a high vacuum. Thus, a preparation of 

 sulphide of calcium, much used now in Paris for coating 

 clock faces which remain luminous after dark, is invalu- 

 able in these researches for the preparation of phospho- 

 rescent screens whereon to trace the paths and trajectories 

 of the molecules. It shines with a bright blue-violet 

 light, and when on a surface of several square inches is 

 sufficient to light up a room. 1\I edifications of these phos- 

 phorescent sulphides shine with a yellow, orange, and 

 green light. 



The only body I have yet met with which surpasses 

 the luminous sulphides both in brilliancy and variety of 

 colour is the diamond. Most of these gems, whether cut 

 or in the rough, when coming from the South African fields, 

 phosphoresce of a brilliant light blue colour. Diamonds 

 from other localities shine with different colours, such as 

 bright blue, pale blue, apricot, red, yellowish-green, 

 orange, and bright green. Une beautiful green diamond 

 in my collection when phosphorescing in a good vacuum 

 gives almost as much light as a candle : the light is pale 

 green — almost white. A beautiful collection of diamond 

 crystals kindly lent me by Prof Maskclyne phosphoresce 

 with nearly all the colours of the rainbow, the different 

 faces glowing with different shades of colour. 



Next to the diamond, alumina in the form of ruby is 

 perhaps the most strikingly phosphorescent stone I have 

 examined. It glows with a rich, full red ; and a remark- 

 able feature is that it is of little consequence what degree 

 of colour the earth or stone possesses naturally, the 

 colour of the phosphorescence is nearly the same in all 

 cases ; chemically precipitated amorphous alumina, 

 rabies of a pale reddish-yellow, and gems of the prized 

 "pigeon's blood'' colour, glowing alike in the vacuum. 



127 



thus corroborating E. Becquerel's results on the action 

 of light on alumina and its compounds in the phospho- 

 roscope {Annales de Chhnie ct de Physique, %€x. 3, vol. 

 Ivii.). Nothing can be more beautiful than the effect 

 presented by a mass of rough rubies when the molecular 

 discharge plays on them in a high vacuum. They glow 

 as if they were red hot, and the illuminating effect is 

 almost equal to that of the diamond under similar 

 circumstances. 



By the kindness of M. Ch. Fell, who has placed large 

 masses of his artificial ruby crystals at my service, I have 

 been enabled to compare the behaviour of the artificially 

 formed crystals with that of the natural ruby. In the 

 vacuum there is no difference whatever ; the colour of 

 the phosphorescence emitted by M. Feil's crystals is of 

 just as an intense a colour, and quite as pure in character, 

 as that given by the natural stone. This affords another 

 proof, if one were needed, that Messrs. Fremy and Fell 

 have actually succeeded in the artificial formation of the 

 veritable ruby, and have not simply obtained crystals 

 which imitate it in hardness and colour. 



The appearance of the alumina glow in the spectroscope 

 is remarkable. There is a faint continuous spectrum 

 ending in the red somewhere near the line B ; then a 

 black space, and next an intensely brilliant and sharp red 

 line to which nearly the whole of the intensity of the 

 coloured glow is due. The wave-length of this red line, 

 which appears characteristic of this form of alumina, 

 is 689-5 m.m.m., as near as I can measure in my 

 spectroscope ; the maximum probable error being about 

 ± 'S- 



This line coincides with the one described by E. 

 Becquerel as being the most brilliant of the lines in the 

 spectrum of the light of alumina, in its various forms, 

 when glowing in the phosphoroscope. 



This coincidence affords a good proof of the identity of 

 the phosphorescent light, whether the phosphorescence 

 be produced by radiation, as in Becquerel's experiments, 

 or by molecular impact in a high vacuum. 



I have been favoured by my friend Prof Maskelyne 

 with the following notes of results obtained on submitting 

 to the molecular discharge various crystals which he lent 

 me for the purpose of these experiments : — 



" Diamond crystals. A very small crystal, exhibiting 

 large cube faces with the edges and angles truncated, was 

 of a rich apricot colour, the dodecahedral faces of a clear 

 yellow, and the octahedral of another yellow tint. No 

 polarisation of the light was detected. Some were opaque; 

 some gave a bluish hazy light. 



" Emerald. A small hexagonal prism gave out a fine 

 crimson-red colour. The light was polarised, apparently 

 completely, in a plane perpendicular to the axis ; this 

 would correspond therefore to extraordinary rays which 

 in emerald, as a negative crystal, represent the quicker 

 rays vibrating presumably parallel to the optic axis of the 

 crystal. 



" Other emeralds behaved in the same way, though the 

 illumination in two others experimented with appeared 

 conrtned more particularly to one end— the end opposite 

 to that at which the crystals presented some (in one 

 instance fine) terminal faces. 



" Beryls exhibited no corresponding phenomena. 



'•Sapphires gave out a bluish-grey light, distinctly 

 polarised in a "plane perpendicular to the axis. In this 

 case, again, the ray developed corresponds to the extra- 

 ordinary or quicker ray. 



" Ruby gives out a transcendently fine crimson colour, 

 exhibiting no marked distinction in the plane of its polar- 

 isation, though in one part of a stone the colour was 

 extinguished bv a Nicol prism with its long diagonal 

 paraltel to the axis of the crystal. Here, therefore, also 

 the light was that of the extraordinary ray. 



" It seemed desirable to determine the nature of the 

 phenomena in the case of positive crystals, and accord- 



