May 26, j88i] 



NA TURE 



habitants of the Nicobar Islands, a subject to which he has paid 

 mucli attention. As the result of his visit and investigations 

 last year he concludes that there is an element of Papuan origin 

 among the people of the interior, and that this is strongly 

 mixed with another not curly-haired race. It is tiue, then, as 

 has been suggested, that there is a curly-haired race in the 

 interior of Great Nicobar, but whether the Andaman Negrito 

 and this tribe are related is very doubtful. Mr. Koepstorff 

 hopes that future researches may enable him to settle the 

 matter. 



It is asserted in Algiers that a letter from Itarem, Chief of 

 the Hoggar Tuaregs, has been intercepted, taking credit to 

 himself for the massacre of Col. Flatters' expedition. This is 

 the man who sent two of his relatives to Col. Flatters as a sign 

 of goodwill, with which that officer professed himself so well 

 satisfied. 



The Times correspondent states that it is announced by Dr. 

 Nachtigal that the first diet of German geographers will be held 

 at Berlin on June 7 and 8. 



r 



ON DISCONTINUOUS PHOSPHORESCENT 

 SPECTRA IN HIGH VACUA'- 



TN a paper which I had the honour of presenting to the Royal 

 Society in March, 1879,- I drew attention to the fact that 

 many substances when in high vacua and submitted to the 

 molecular discharge by means of an induction coil, emitted 

 phosphorescent light ; and I especially mentioned the phosphor, 

 escent sulphides, the diamond, the ruby, and various other forms 

 of alumina, crystalline and amorphous. 



Pure alumina chemically prepared has very strong phosphor 

 escence. Sulphate of aUimina is dissolved in water, and to it is 

 added an excess of solution of ammonia. The precipitated 

 hydrate of alumina is filtered, washed, ignited, and tested in the 

 molecular stream. It phosphoresces of the same crimson colour, 

 and gives the same spectrum as the ruby. 



Alumina in the form of ruby glows vnA a full rich red colour, 

 and when examined in the spectroscope the emitted light is seen 

 to be discontinuous. 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 tlie intensity of the coloured glow is due. The wave- 

 length of this red line, which appears characteristic of this form 

 of alumina is, as near as I can measuie, \ 6S95 m.m.m. 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 phosphoro- 

 scope. 



This coincidence is of considerable interest, as it shows a 

 relation between the action of molecular impact and of sunlight 

 in producing luminosity. The phosphorescence induced in a 

 crystal of ruby by the molecular discharge is not superficial, but 

 the light comes from the interior of the crystal, and is profoundly 

 modified according as its direction of vibration corresponds or 

 makes an angle with the axis of the crystal, being quenched in 

 certain directions by a Nicol prism. 



Sunlight falling on the ruby crystal produces the same optical 

 phenomena. The light is internally emitted, and on analysis by 

 a prism is seen to consist essentially of the one brilliant crimson 

 line, \ 689'5- This fact may account for the extraordinary 

 brilliancy of the ruby, which makes it so highly prized as a gem. 

 The sun not merely renders the red-coloured stone visible, as it 

 would a piece of coral, but it excites the crystal to phosphor- 

 escence, and causes it to glow with a luminous internal light, the 

 energy of which is not diffused over a broad portion of the 

 spectrum, but is chiefly concentrated into one wave-length. 



The crimson glow of alumina remains visible some time after 

 the current ceases to pass. When the residual glow has ceased, 

 it can be revived by heating slightly with a spirit-lamp. 



After long experimenting with chemically pure alumina pre- 

 cipitated from the sulphate as above described, a curious pheno- 

 menon takes place. When sealed up in the vacuum tv.- j years 

 ago it was snow white ; but after being frequently submitted to 

 the molecular discharge for the purpose of exhibiting its brilliant 

 phosphorescence, it gradually assumes a pink tinge, and on 

 examination in sunlight a trace of the alumina line can be 



* Paper read before the Roya! Society, May ig, by William Crookes, 

 F.R.S. 

 ' Phil. Tratis. Part 2, 1S79, p. 660. 



detected. The repeated molecular excitation is .slowly causing 

 the amorphous powder to assume a crystalline form. 



Under some circumstances alumina glows with a green colour. 

 Ammonia in large excess was added to a dilute solution of alum. 

 The strong ammonlacal solution filtered from the precipitated 

 alumina was now boiled. The alumina which the excess of 

 ammonia had dissolved was thereby precipitated. This was 

 filtered cff, ignited, and tested in the molecular discharge. It 

 gave no red light whatever, but phosphoresced of a pale green, 

 and on examination with a prism the light showed no lines, but 

 only a concentration of light in the green. 



Two earthen crucibles were tightly jmcked, tlie one with 

 sulphate of alumina, the other with acetate of alumina. They 

 were then expo ed, side by side, to the most intense heat of a 

 wind furnace — a heat little short of the melting-point of 

 platinum.' The resulting aluminas were then tested in the 

 molecular stream. 



The alumina from the sulphate gave the crimson glow and 

 spectrum line. 



The alumina from the acetate gave no red glow or line, but a 

 pale green phosphorescence. 



In my examination of rubies, many pounds of which have 

 passed through my apparatus, I have been fortunate enough to 

 meet with one solitary crystal, not to the eye different from 

 others, which emits a green light when tested in the molecular 

 stream. All others act as I may call normally. The spectrum 

 of this green-glowing crystal shows, however, a trace of the red 

 line, and on keeping the discharge acting on it for a few minntes 

 the green phosphorescence grows fainter and a red tinge is 

 developed, the spectrum line in the red becoming more distinct. 



Besides the ruby, other native forms of crystallised alumina 

 phosphoresce. Thus corundum glows with a pink colour. The 

 sapphire appears to be made up of the red-glow and the green- 

 glow alumina. Some fine crystals of sapphire shine with alter- 

 nate bands of red and green, arranged in layers perpendicular to 

 the axis. Unfortunately it is impossible to prepare a tube for 

 exhibition containing this variety of sapphire, .as it is constantly 

 evolving gas from the numerous fissures and cavities which 

 abound in this mineral. 



The red glow of alumina is chiefly characteristic of this earth 

 in a free state. Few of its compounds, except Spinel (aluminate 

 of magnesium), either natural or artificial, show it in any marked 

 degree. All the aitificially crystallised aluminas give a strong 

 red glow and spectrum line. An artificially crystallised alumi- 

 nium and barium fluoride ])hosphoresces with a blue colour, but 

 shows the red alumina line in the spectrum. Spinel glows red, 

 and gives the red Ime almost as strong as the ruby. 



The mineral Spodumene (an aluminium and lithium silicate) 

 phosphoresces very brilliantly with a rich golden yellow colour, 

 but shows no spectrum line, only a strong concentration of light 

 in the orange and yellow. A phosphorescing crystal of Spodu- 

 mene has ad the internal light cut off with a Nicol prism, when 

 the long axes of the Nicol and the crystal are parallel. 



It became of interest to see if the other earths would .show 

 phosphorescent properties similar to those of alu'uina, .and espe- 

 cially if any of them would give a discontinuous spectrum ; 

 considerable interest attaching to a solid body whose molecules 

 vibrate in a few directions only, giving rise to spei^tnun lines or 

 bands on a d.ark background. 



Glucina, prepared with great care, is found to phosphoresce 

 with a bright blue colour, but no lines can be detected in the 

 spectrum, only a concentration of light in the blue. 



The rare mineral phenakile (aluminate of glucinum), some- 

 times u-ed as a gem, phosphoresces blue like pure glucina, no 

 trace of the alumina line being found in its spectrum. This, 

 mineral shows a residual glow after the current is turned off. 



Thorina has very little, if any phosphorescence. This earth 

 is however remarkable for its very strong attraction for the 

 residual gas in the vacuum tube. On putting thorina in a tube 

 furnished with well-insulated poles whose ends are about a 

 millimetre apart in the centre, and heating strongly during 

 exhaustion, the earth on cooling absorbs the residual gas with 

 such avidity that the tube becomes non-conducting, the spark 

 preferring to pass several inches in air rather than strike across 

 the space of a millimetre separating the two poles. It is 

 probable that this strong attraction for gas is connected with the 

 great density of the earth thorina (sp. gr. = 9'4). 



Zirconia "ives a very brilliant phosphorescence, approaching 



^ This opera 

 Matthey. 



kindly performed for me by Me 



Johnson and 



