90 



NATURE 



\_May 26, 1 88 1 



in intensity that of sulphide of calcium. The colour is pale 

 bluish green, becoming whiter as the intensity .of the discharge 

 increases : no lines are seen in its spectrum. 



Lanthana precipitated as hydrate and ignited shows no phos- 

 phorescence. After it has been heated for sometime before the 

 blowpipe it phosphoresces of a rich brown. 



Didymia, from the ignition of the hydrate, has scarcely any 

 phosphorescence ; what little there is appears to give a con- 

 tinuous spectrum with a broad black band in the yellow-green. 

 On examining the light reflected from this earth when illu- 

 minated by day or artificial light, the same black band is seen, 

 and with a narrow slit and sunlight the band is resolved into a 

 series of fine lines, occupying the position of the broadest group 

 of absorption lines in the transmission spectrum of didymium 

 salts. 



Yttria shows a dull greenish light, giving a continuous 

 spectrum. 



Erbia phosphoresces with a yellowish colour, and gives a 

 continuous spectrum, with the two sharp black bands so cha- 

 racteristic of this earth cutting through the green at \ 520 and 

 523. These lines are easily seen in the light reflected from erbia 

 when illuminated by daylight. It is well known that solid erbia 

 heated in a flame glows with a green light, and gives a spectrum 

 which chiefly consists of two bright green lints in the same 

 place as the dark lines seen by reflected light. 



A curicus phenomenon is presented by erbia when the spark 

 passes over it at a high exhaustion. The particles of earth which 

 have accidentally covered the poles are shot off with great 

 velocity, forming brightly luminous lines, and, striking on the 

 sides of the tube, rebound, remaining red hot for an appreciable 

 time after they have lost their velocity. They form a very good 

 visible illustration of radiant matter. 



Titanic acid phosphoresces dark brown, with gold spots in 

 places. 



Stannic acid gives no phosphorescence. 



Chromic, ferric, and eerie oxides do not appreciably phos- 

 phoresce. 



Magnesia phosphoresces with a pink opalescent colour, and 

 shows no spectrum lines. 



Baryta (anhydrous) scarcely phosphoresces at all. Hydrated 

 baryta, on the contrary, shines with a bright orange-yellow light, 

 but shows no discontinuity of spectrum ; only a concentration in 

 the yello^^•-orange. 



Strontia (hydrated) phosphoresces with a beautiful deep blue 

 colour, and when examined in the spectroscope the emitted 

 light shows a greatly nicreased intensity at the blue and violet 

 end, without any lines or bands. 



Lime phosphoresces of a bright orange-yellow colour, changing 

 to opal blue in patches where the molecular discharge raises the 

 temperature. In the focus of a concave pole the lime becomes 

 red- and white-hot, giving out much light. This earth com- 

 mences to phosphoresce more than 5 millims. below the vacuum, 

 and continues to grow brighter as long as the electricity is able 

 to pass through the tube. On stopping the discharge there is a 

 decided residual glow. No lines are seen in the spectrum of the 

 light. 



Calcium caibonate (calcite) shows a strong phosphorescence, 

 which begins to appear at a comparatively low exhaustion 

 (Sm.m.). The interior of the crystal shines of a bright straw 

 colour, and the ordinary and extraordinary rays are luminous 

 with oppositely polarised light. Calcite shows the residual 

 glow longer than any substance I have as yet experimented with. 

 After the current has been turned off", the crystals shine in the 

 dark with a yellow light for more than a minute. 



Calcium phosphate gener.ally gives an orange-yellow phos- 

 phorescence and a continuous spectrum. Sometimes, however, 

 a yellow-green band is seen superposed on the spectrum. 



Potash phosphoresces faintly of a blue colour. The spectrum 

 shows a concentration at the l)lue end, but the light is too faint 

 to enable lines, if any, to be detected. 



Soda phosphoresces faintly yellow-, and gives the yellow line 

 in the spectrum. 



Lithium carbonate gives a faint red phosphorescence. Exa- 

 mined in the spectroscope, the red, orange, and blue lithium 

 lines are seen. 



I have already said that the diamond phosphoresces with great 

 brilliancy. In this respect perfectly clear and colourless stones 

 " of the first water " are not the most striking, and they gene- 

 rally glow of a blue colour. Diamonds which in sunlight have 

 a slight fluorescence, disappearing when yellow glass is inter- 



posed, generally phosphoresce stronger than others, and the 

 emitted light is of a pale yellowish green colour. 



Most diamonds which emit a very strong yellow ish light in the 

 molecular discharge give a continuous spectrum, having bright 

 lines across it in the green and blue. A faint green line is seen 

 at about A 537 ; at A 513 a bright greenish blue line is seen, and 

 a bright blue line at A 503, a darkish space separating the last 

 two lines. 



Diamonds which phosphoresce red generally show the yellow 

 sodium line superposed on a continuous spectrum. 



There is great difference in the degree of exhaustion at which 

 various substances begin to phosphoresce. Some refuse to glow 

 until the exhaustion is so great that the vacuum is nearly non- 

 conducting, whilst others commence to become luminous when 

 the gauge is 5 or 10 millimetres below the barometric level. 

 The majority of todies, however, do not phosphoresce till they 

 are well within the negative dark space. 



During the analysis of some minerals containing the rarer 

 earths experimented on, certain anomalies have been met with, 

 which seem to indicate the possible presence of other unknown 

 elements awaiting detection. On several occasions an earthy 

 precipitate has come down where, chemically speaking, no such 

 body was expected ; or, by fractional precipitation and solution 

 from a supposed simple earth something has separated which in 

 its chemical characters was not quite identical with the larger 

 portion; or, the chemical characteristics of an earth have agreed 

 fairly well with those assigned to it in books, but it deviated in 

 some physical peculiarity. It has been my practice to submit 

 all these anomalous bodies to molecular bombardment, and I 

 have»;had the satisfaction of discovering a class of earthy bodies 

 which, whilst they phosphoresce strongly, also give spectra of 

 remarkable beauty. 



The spectrum seen most frequently is given by a pale yellow- 

 ish coloured earth. It consists of a red, orange, citron, and 

 green band, nearly equidistant, the citron being broader than 

 the others and very bright. Then comes a faint blue, and lastly 

 two very strong blue violet bands. These bands, when seen at 

 their best, are on a perfectly black V)ack ground ; but the parent 

 earth gives a continuous spectrum, and it is only occasionally, 

 and as it were by accident, that I have so entirely separated it 

 from the anomalous earth as to see the bands in their full purity. 

 Another earthy body gives a spectrum similar to that just 

 described, but wanting the red, and having a double orange and 

 double citron band. A third gives a similar spectrum, but with 

 a yellow line interposed between the double orange and the 

 double citron, and having two naiTow green lines. 



At present I do not wish to say more than that I have strong 

 indications that one, or perhaps several, new elements are here 

 giving signs of their existence. The quantities I have to work 

 upon are very small, and when each step in the chemical opera- 

 tion has to be checked by an appeal to the vacuum-tube and 

 to the incuciion-coil the progress is tediously slow. In the 

 thallium research it only occupied a fen minutes to take a portion 

 of a precipitate on a platinum loop, introduce it into a spirit- 

 flame, and look in the spectro;cope for the green line. In that 

 way the chemical behaviour of the new element with reagents 

 could be ascertained with rapidity, and a scheme could be 

 promptly devised for its separation from accompanying impuri- 

 ties. Here however the case is different : to perform a spectrum 

 test, the body under examination must be put in a tube and 

 exhausted to a very high point before the spectroscope can be 

 brought to bear on it. Instead of two minutes, half a day is 

 occupied in each operation, and the tentative gropings in the 

 dark, unavoidable in such researches, must be extended over a 

 long period of time. 



The chemist must also be on his guard against certain pitfalls 

 which catch the unw ary. I allude to the profound modification 

 which the presence of fluorine, pihosphoru=, boron, &c., causes 

 in the chemical reactions of many elements, and to the inter- 

 fering action of a large quantity of one body on the chemical 

 properties of another which may be present in small quantities. 



The fact of giving a discontinuous phosphorescent spectrum is 

 in itself quite insufficient to [establish the existence of a new 

 body. At present it can only be employed as a useful test to 

 supplement chemical research. "When, however, I find that the 

 same spectrum-forming earthy body can always be obtained by 

 submitting the mineral to a certain chemical treatment ; when 

 the chemical actions w hich have separated this anomalous earth 

 are such that only a limited number of elements can possibly be 

 present ; w hen I find it impossible to produce a substance giving 



