Oct. 9, 1879] 



NATURE 



565 



the complete band-spectrum (at a pressure of about 5-10 mm.) 

 in which the variability of the spectrum is most conspicuousi 

 viz., in the green and blue (from wave-lengths 563 to 449); this 

 is followed by an account • of the gradual changes taking place 

 in two different places when the density of the gas is decreased. 

 It would lead us too far to enter into the details here ; suffice it 

 to .say that the richly-shaded band-spectrum changes quite 

 gradually into a line-spectrum, and that most of the lines of the 

 latter are in places which show no maximum of brightness in the 

 band-spectrum. It results from these observations that the 

 positions of the maxima of emission-power do not remain at all 

 the same in all temperatures, but that in consequence of the 

 changes of temperature accompanying the decrease of pressure 

 they may be considerably displaced. 



Prof. Wiillner gives the reasons why the wider tubes are 

 unsuitable for the gradual conversion of band-spectrum into line- 

 spectnim, and eventually describes the differences between the 

 line-spectrum obtained from the band-spectrum in the manner 

 described, and the line-spectrum obtained by the spark. Upon 

 comiiarison of those regions in the two spectra which were 

 examined more closely (beginning at wave-length 572), it was 

 found that in the spark-s]3ectmm there are about forty lines in 

 this region. Of these eight correspond very closely to maxima 

 or lines in the fully developed band-spectrum. The number of 

 coincidences mth the lines of the low density line spectrum i-, 

 however, much greater ; perfect, or very nearly perfect coin - 

 cidence "occurs in nineteen lines, i.e., about half the number, 

 and amongst these therb are four which are the same in the 

 three different forms of the nitrogen spectrum. The brightest 

 lines of both line-spectra are, indeed, perfectly coincident. These 

 are the yellowish-green lines 568,4 and 567,1, the green lines 

 5007 and 500*4 (which result from the green-channelling 

 described in detail) and the blue line 463'2 (developed from the 

 blue channelling). Another interesting similarity exists between 

 the two line-spectra, however differently about half of their lines 

 may be situated. 



Plucker and Hittorf distinguish five principal groups in 

 the line-spectrum of nitrogen, between which there are other 

 single lines. Of these five groups Nos. II. to V. belong to the 

 region si^ecially studied by Prof. Wiillner. These groups are : — 

 Group II. between wave-lengths 577 — 567 

 Group III. „ „ 555—545 



Between groups III. and IV. there are first three lines : 535'6, 

 5344, and 532'3 ; and then two lines : 5i8*i, and 5i7'6. 



Group IV. between wave-lengths 508 — 499 

 Between groups IV. and V. there are first fonr lines : from 

 489'5, to 484*8, and further on three lines r 4fio'5, 479'0, and 

 478-1. 



Group V. between wave-lengths 464*5 — 46o*2. 



All these groups are situated in such parts of the spectrum 

 where also the line-sjiectrum developed from the band-spectrum 

 is very rich in lines. It follows, therefore, that the spark-line 

 spectrum is developed, on the whole, in such places which show 

 the greatest variability, even in a gradual decrease of the density 

 of the gas, and which are richest in lines in the low-density line- 

 spectrum. 



Prof. Wiillner recapitulates his interesting treatise in the fol- 

 lowing manner : — The course of spectral phenomena of nitrogen, 

 which takes place w hen the gas is gradually reduced in density 

 in tubes of sufficiently small diameter, shows exactly those 

 changes which may be deduced from Kirchhoff's maxim, that 

 the smaller the number of incandescent molecules, the more the 

 spectrum contracts into a number of bright lines. 



At the same time it may be directly observed when the density 

 of the gas is decreasing, how, in consequence of the temperature 

 rising through increasing resistance in the tube, the maxima of 

 brightness change their position, how the maxima of the fully- 

 developed band-spectrum fade away, and how the lines become 

 prominent in places which are either secondary or tertiary 

 maxima or uniformly illuminated regions in channelled spaces. 

 Now, if wc further consider that the lines of the spark-spectrum, 

 compared to those of the former line-spectrum, are not more consi- 

 derably displaced than the latter are with regard to the maxima of 

 the band-spectrum, then we can hardly doubt tliat in the different 

 forms of the nitrogen-spectrum we see nothing else but the light 

 emitted each time in accordance with the different temperature, 

 density, and thickness of the radiating layer of gas, and that a 

 new hypothesis for the explanation of these spectral phenomena 

 is unnecessary and superfluous. 



A HISTORICAL SKETCH OF THE VARIOUS 

 VAPOUR-DENSITY METHODS^ 



A LTIIOUGH Southern," in 1803, made some very careful 

 ■'*■ experiments to determine how much water was required to 

 furnish \ cubic foot of steam at various pressures, still the 

 foundation of vapour-density methods was laid by Gay-Lussac. 



He, in 1811,^ started on the correct basis of accurate work 

 when he heated a weighed quantity of substance over mercury 

 in a graduated vessel. Muncke, in 1816,* heated the substances 

 in vacuo in elliptic glass balloons of 1550.1. capacity, closed 

 with a stop-cock and with thermometer and syphon barometer 

 suspended inside. In 1S22' Cagniard de la Tour determined 

 the combined effects of heat and pressure on certain volatile 

 liquids, but as his results were on the question of maximum 

 vapour-density, they hardly enter the domain of the present 

 sketch. In the same year Despretz," who gave no drawing, and 

 only a very imperfect description of his apparatus, published a 

 method in which he used a 9-litre exhausted globe, and made his 

 determinations at atmospheric temi^eratures, employing only a 

 small quantity of substance. 



In 1826 Dumas,' wishing to operate on substances which 

 attack mercury, worked out and published his well-known 

 method in which the volume is definite, but the amount of 

 substance required to fill that volume with vapour has to be 

 subsequently determined. 



In 1833* Mitscherlich proposed using tubes, sealed at one 

 end and drawn to a neck at the other, instead of bulbs, and gave 

 details and drawings of the apparatus for heating them ; but 

 Dumas, two years later, objected to the proposed alteration in 

 his method, for he wrote : — 



" We must then leave to this operation all its simplicity to 

 make it essentially practical, and such, in fact, that with an 

 ordinary cast-iron pot and some pieces of iron wire we can per- 

 form it. This is what I have done from the first, and what I 

 persist in doing, my aim never having been to make a piece of 

 apparatus for the cupboard of the physicist, but to give chemists 

 a simple and eminently practical and yet exact process. After all 

 IJtey are the only ones to be considered;" " 



beville and froost,'" however, in iS6o, in referring to that 

 same apparatus, called it "La methode si elegante de M. 

 Mitscherlich." 



Bineau, in 1838," published an elaborate paper, but unfor- 

 tunately without any drawings, for when we read the following 

 paragraph, " The bodies on which I have worked have been 

 volatilised sometimes by the aid of heat by following the pro- 

 cess of Dumas or that of Gay-Lussac, sometimes without eleva- 

 tion of temperature by working in the barometric vacuum or by 

 allowing the vaporisation to take place in dry air or hydrogen," 

 we cannot but feel that an enormous amount of valuable work 

 has been lost for want of details. In 1S44 we find Cahours,'" as 

 well as Bineau,'' at work at the same subject. In 1846 the 

 latter '* repeated the experiments of Despretz with slight modi- 

 fications, but called attention to the fact that the result was 

 seriously affected by very small errors in reading off the mercurial 

 column. 



In 1849 Regnault " described an apparatus very similar to 

 that of Mitscherlich, but arranged the tube supports so that the 

 two could be withdrawn simultaneously ; he also dispensed with 

 sealing the tube containing air by providing it with a stop-cock. 

 Bineau," in 1859, in order to operate at high temperatures, 

 coated the glass tubes w ith clay and heated them in a sand-bath. 



Kegnault,'' in 1861, to obtain the same result, used iron tubes, 

 and to ensure uniformity of temperature, heated them in a cast- 

 iron tube which was made to revolve over gas-burners. The 

 tube which served as air-thermometer was furnished with a stop- 

 cock, but that containing the substance only terminated in a 

 small aperture, and was not closed, as a sufficient quantity was 



' Paper read .it the British Association by Jas. T. Brown, F.C.S. 



" rv///. ;i/<i^ , 30, 113 (1847). 



3 Ann. cU Cliim., 80, 218 (1811); Gilbert Annal , 45, 333 (1813). 



*• Schiveigeer^s ymtrn. Chetn. Phys., 2j, i (i3i8). 



5 Ann. Cnim. Phys., 21, 127 and 178 (1822) ; 22, 410 (1823). 



« Ibid,, 21, 143 (1822) : Quart. Jonm. Sci., 15, 297 (1823). 



^ Ann. Cliim. Phys., 33, 337. 8 ibid., 55, 5-4" U^iV- 



9 " 'I'raitfS dc Chimie,* 5, 44. 



" Ann. Chim. Phys. [3 1. 58, 259. " IM.< 68. •♦■6- 



" Conift. Rend., 19, 771 (1844); PKgg. Annal., 63. 593 (>84-4)- 

 '5 Comft. Rend., 19, 768 (1844) : Licbig^s Ann. , 65. 424 (184 S)- 

 •■> Coin/il. Rend, 23. 414 (1846) ; Ann. Chim. Phys., 18, 326 (1846) ; Ann. 

 Chem. I'harm . 60, 158 (1846). '5 " Cours de Chimie," 4, «« («8,t9). . 



'<• Ctimpt. Rend.. 49. 799 (■859)- 

 ■? Ann. Chim. Phys. (3I, 63, 54 (l8«l). 



