May 23, 1878] 



NATURE 



109 



SOCIETIES AND ACADEMIES 

 London 



Royal Society, May 2. — " On the Reversal of the 

 Lines of Metallic Vapour?," by G. D. Liveing, M.A,, Pro- 

 fessor of Chemistry, and J. Dewar, M.A., Jacksonian Professor 

 University of Cambridge, No. II. 



Since their last commmiication to the Society the authors have 

 succeeded in reversing characteristic lines of the vapours of 

 rubidium and caesium. They operated in glass tubes into which 

 some dry rubidium or ccesium chloride was introduced, and a 

 fragment of fresh cut sodium, and afterwards either dry hydrogen 

 or dry nitrogen admitted, and the end of the tube sealed off at 

 nearly the atmospheric pressure. Through these tubes placed 

 lengthways in fi-ont of a spectroscope a lime-light was viewed. 

 On warming the bulb of a tube in which rubidium chloride had 

 been sealed up with sodium, very soon there appeared two 

 dark lines near the extremity of the violet light identical in 

 position with the well-known violet lines of rubidimn. Next 

 appeared faintly the channelled spectrum of sodium in the 

 green, and then a dark line in the blue, very sharp and decided, 

 in the place of the more refrangible of the characteristic lines of 

 caesium in the flame spectrum. As the temperature rose these 

 dark lines, especially those in the violet, became sensibly 

 broader ; and then another fine dark line appeared in the blue in 

 the place of the less refrangible of the caesium blue lines. During 

 this time no dark line could be observed in the red, but as the tem- 

 perature rose a broad absorption band appeared in the red with its 

 centre about midway between B and C, ill-defined at the edges, 

 and though plainly visible not very dark. The lines in the violet 

 had now become so broad as to touch each other and form one 

 dark band. On cooling the absorption band in the red became 

 gradually lighter without becoming defined, and was finally 

 overpowered by the channelled spectrum of sodium in that 

 region. The double dark line in the violet became sharply 

 defined again as the temperature fell. There are two blue lines 

 in the spectinim of rubidium taken with an induction-coil very 

 near the two blue lines of caesium, but they are comparatively 

 feeble, and the two dark lines in the blue which the authors 

 observed in the places of the characteristic blue lines of caesium 

 they believe must have been due to a small quantity of caesium 

 chloi-ide in the sample of rubidmm chloride. 



When a tube containing caesium chloride and sodium was 

 observed, in the same way as the former, the two dark lines in 

 the blue were seen very soon after the heating began, and the 

 more refrangible of them broadened out very sensibly as the 

 temperature increased. The usual channelled spectrum of sodium 

 was seen in the green, and an additional channelling appeared in 

 the yellow, which may be due to caesium or to the mixture of 

 the two metals. They have at present no metallic ccesium where- 

 with to decide this question. Indeed the cajsium chloride used 

 was not free from rabidium, and the dark lines of rubidium 

 were distinctly seen in the violet. 



It is remarkable that these absorption lines of caesium coincide 

 with the blue lines of ccesium as seen in the flame, or in the 

 spark of an induction-coil without a jar, not with the green line 

 which that metal shows when heated in an electric spark of high 

 density. In like manner both the violet lines of rubidium are 

 reversed in the tubes, and both these violet lines are seen when 

 the spark of an induction-coil, without jar, is passed between 

 beads of rubidium chloride fused on platinum wire, though only 

 one of them appears when a Leyden jar is used. 



The authors have extended their observations on the absorp- 

 tion of magnesium and of mixtures of magnesium with potassiimi 

 and sodium, using iron tubes placed vertically in a small furnace 

 fed with Welsh^coal, as described in their former communication. 



The result of several observations, when commercial mag- 

 nesium {i.e., magnesium with only a small percentage of sodium 

 in it) was used, is that the absorption produced by magnesium 

 consists of — 



1. Two sharp lines in the green, of which one, which is 

 broader than the other, and appears to broaden as the tempera- 

 ture increases, coincides in position with the least refrangible of 

 the b group, while the other is less refrangible and has a wave- 

 length very nearly 5,213. These lines are the first and the last 

 to be seen and very constant, and they at first took them for the 

 extreme lines of the b group, 



2. A dark line in the blue, always more or less broad, difficult 

 to measure exactly, but very near the place of the brightest blue 

 line of magnesium. This line was not always visible, indeed 



rarely when magnesium alone was placed in the tube. It was 

 better seen when a small quantity of potassium was added. The 

 measure of the less refrangible edge of this band then gave a 

 wave length of very nearly 4,615, 



3, A third line or band in the green, rather more refrangible 

 than the b group. This is best seen when potassium as well as 

 magnesium is introduced into the tube, but it may also be seen 

 with sodium and magnesium. The less refrangible edge of this 

 band is sharply defined, and has a wave-length about 5, 140, and 

 it fades away towards the blue. 



These absorptions are all seen both, when potassium and 

 when sodium are used along with magnesium, and may be fairly 

 ascribed to magnesium, or to magnesiiun together with 

 hydrogen. 



But besides these other absorptions are seen which appear to 

 be due to mixed vapours, 



4, When sodium and magnesium are used together, a dark 

 line, with ill-defined edges, is seen in the green, with a wave- 

 length about 5, 300. This is the characteristic absorption of the 

 mixed vapours of sodium and magnesium, it is not seen w ith 

 either vapour separately, nor is it seen when potassium is used 

 instead of sodium. 



5. When potassium and magnesium are used together, a pair 

 of dark lines are seen in the red. The less refrangible of these 

 sometimes broadens into a band vnih. ill-defined edges, and has 

 a mean wave length of about 6,580, The other is always a fine 

 sharp line, with a-wave-length of about 6,475. These lines are aa 

 regularly seen with the mixture of ix)tassium and magnesium as 

 the above-mentioned line (5,300) is seen with the mixture of 

 sodium and magnesium, but are not seen except with that 

 mixture. 



6. On one occasion, with a mixture of potassium and mag- 

 nesium, another dark line was seen in the blue, with a wave- 

 length nearly 4,820. This line is very near one of the bright 

 lines, seen when sparks from an induction-coil, without a 

 Leyden jar, are taken between electrodes of magnesium, and 

 may very likely be due to magnesium alone, and not to the 

 mixture of vapours, as we only observed it on one occasion. 



There is a certain resemblance between the absorptions above 

 ascribed to magnesium and the emission spectrum seen when 

 the sparks of a small induction-coil, without Leyden jar, are 

 taken between electrodes of magnesium. This emission spec 

 trum is the same, with the addition of some blue lines, as that 

 seen when the sparks are taken from a solution of magnesium 

 chloride, as accurately described by Lecocq de Boisbaudran, and 

 as that seen in burning magnesium (Dr. Watts, P/iil. Mag., 1875). 



The pair of lines (l) correspond nearly with the b group, but 

 slightly displaced towards the red ; the shaded band (3) corre- 

 sponds less closely to the series of seven lines 5,000 to 4,930, 

 which pro^essively decrease in brightness towards the blue, 

 and is also a little less refrangible than that series; the 

 broad line in the blue (2) corresponds to the pair of lines 4,570 

 and 4,590, and the remaining line (6) to the line 4,797; also 

 both displaced a little towards the red. No absorption corre- 

 sponding to the extreme lines 4,481 and 5,528 was observed. 

 There is plainly no exact reversal except of the line b^, and even 

 in that case it may be an accident if we suppose the two dark 

 lines (i) to represent the extreme lines of the group b. It may 

 be noted in connection with this that the absorption lines de- 

 scribed by the authors in their former communication as seen 

 with sodium and potassium (wave-lengths 5)5^° ^^^ Sj73o) ^^^ 

 near to, but more refrangible than, well-known emission lines of 

 those elements. 



They observe that there is in the solar spectrum an absorption 

 line, hitherto unaccounted for, closely corresponding to each of 

 the above-described absorption lines. Thus, on Angstrom and 

 Thalen's map there are dark lines at 6,580 and 6,585, with more 

 or less continuous absorption between them, a broad dark line 

 between 6,474 ^^^^ 6,475, and a dark line at S>300. There are 

 also dark lines nearly, if not exactly, coincident with the series 

 of seven bright lines of magnesium above described, which they 

 have not seen strictly reversed. The coincidences of the series 

 of the solar spectrum hitherto observed have, for the most part, 

 been with lines given by dense electric sparks ; while it is not 

 improbable that the conditions of temperature, and the admix- 

 tures of vapours in the upper part of the solar atmosphere, may 

 resemble much more nearly those in their tubes. 



They intend to pursue their observations, using higher tem- 

 peratures, if they can obtain tubes which will stand under those 

 circumstances. 



