'94 



NATURE 



\yuly 5, 1877 



near the temperature at which it condenses, its molecules 

 as a rule aggregate before they finally pass into the liquid 

 state. Each molecular aggregation has its own spectrum, 

 and thus a change of temperature producing a difference in 

 the relative number of molecules in these different states 

 will increase the absorption of one spectrum, while it 

 weakens some other absorption spectrum. If the spectrum 

 which gets weaker lies in the invisible part, the only visible 

 effect of temperature is to increase the darkness of the 

 whole observed spectrum. In many cases, however, we 

 can follow out not only the increased absorption of one 

 spectrum, but at the same time the gradual breaking up 

 of another spectrum, and this affords a beautiful illustra- 

 tion of the gradual dissociation which we can now trace 

 step by step. 



In order to make my meaning clearer I shall trace in 

 detail the effect of an mcrease of temperature on the ab- 

 sorption of sodium vapour. If we heat up sodium in an 

 iron tube we observe at first a continuous spectrum 

 stretching from the red and blue end of the spectrum 

 towards the green. If the mass of sodium vapour is suf- 

 ficiently great the two banks will join, and the whole 

 spectrum will be absorbed. If we now raise the tempera- 

 ture the light will force its way through the bank in the 

 green, and then, almost as suddenly as if a curtain was 

 withdrawn, the spectrum will open from the green towards 

 both sides. The continuous absorption now has entirely 

 disappeared, but a series of finely-shaded bands ate seen. 

 The D-line will soon make its appearance, and if we now 

 continue to increase the temperature it will increase in 

 darkness, and we shall soon arrive at a point where the 

 bands quickly fade away, and as we see them disappearing, 

 we see the whole absorption thrown into the Dline, which 

 gets thick and of an intense black colour. 



No one who has ever actually observed this fact can 

 for a moment doubt that we have here to do with a real 

 breaking up of the band-molecule, as we may call it, into the 

 simpler molecular state in which it gives the line absorp- 

 tion. Similarly at lower temperatures the band-molecules 

 aggregate and give the continuous spectrum of which we 

 have spoken. 



The three vapours which Mr. Moser has examined are : 

 Iodine, bromine, and the oxides of nitrogen. That the 

 darkening of the absorption bands of iodme is due to a 

 breaking up of a more comolex molecule is shown by the 

 fact, discovered by Lockyer, that there exists a molecule 

 of iodine which gives a continuous absorption, and which 

 is more complex than the bmd-niolecule. There can be 

 little doubt that this also is the explanation of the analo- 

 gous phenomenon in bromine vapour. The brown vapour 

 of the oxides of nitrogen are known to be a mixture, and 

 it is, t erefore, not surprising that the relative quantity of 

 the different oxides changes with the temperature. 



It is, indeed, the second effect of temperature observed 

 by Mr. Moser, that some of the bands disappear as the 

 temperature is raised. There can be little doubt that in 

 the case of the oxides of nitrogen, the three disappearing 

 bands are due to a compound which is broken up as the 

 temperature is raised. 



A curious alteration is noticed in the case of iodine and 

 bromine vapour. In each case one band was observed to 

 disappear and to be replaced by a number of fine equi- 

 distant lines. I noticed some time ago a similar re- 

 placement of a fluted band by a number of fine lines 

 approximately in the same place in the emission spectrum 

 of carbonic oxide. The change requires a more careful 

 study before any decided opinion can be given as to its 

 cause. Arthur Schuster 



OUR ASTRONOMICAL COLUMN 

 The late Prof. Santini.— Giovanni Santini, Pro- 

 fessor of Astronomy and Director of the Observatory in 

 the University of Padua, whose decease has been an- 



nounced in the daily journals during the last week, had 

 nearly completed his 91st year, having been born at Borgo 

 S. Sepolcro, in Tuscany, on the 30th of June, 17S6. 

 Educated at the University of Pisa, he applied liimself 

 especially to the study of the exact sciences, and in 1814 

 was appointed successor to Vincenzo Cheminello at the 

 Observatory of Padua. He subsequently became Rector 

 of the University and Director of Mathematical Studies, 

 to which position was attached the Professorship of 

 Astronomy. 



Santini was the last of a phalanx of distinguished prac- 

 tical astronomers among whom were Argelander, Bessel, 

 Carlini, Encke, Miidler, Struve, and others, which will 

 long live in the annals of the science. The work with 

 which he has been more particularly associated and 

 brought into prominence in the astronomical world, 

 relates to the celebrated comet of Biela, the perturbations 

 of which body were calculated by him upon a uniform 

 system from 1826, when the comet's period of revolution 

 was first determined, to 1859, for which year he prepared 

 an ephemeris, though the track in the heavens was too 

 unfavourable to permit any hope of observations. San- 

 tini's memoirs on Biela's comet appear partly in AFcmorie 

 licit' L R. Istituto Veneto di Scienzc, Sr'c., earlier ones in 

 the Transactions of the Academies of Padua and Modena. 

 His colleague. Dr. Michez, took up his work for 1S59 and 

 continued the calculation of perturbations to 1866, in 

 which year the comet was sought in vain, and we have no 

 further computation of the effect of planetary attraction 

 upon its motion. 



Amongst the other astronomical works of Prof. Santini 

 are several catalogues of stars in the neighbourhood of 

 the equator, or from declination -j- 10° to - 12', for the 

 epoch 1840, in which the differences from the positions in 

 Bessel's zones are exhibited. Also an investigation of the 

 mass of the planet Jupiter, from observations of the elonga- 

 tions of the fourth satellite, made at Padua in the first four 

 months of 1835, which gave a result confirming the value 

 deduced by Sir George Airy by similar observations at Cam- 

 bridge a short time previous, Airy's figure being 



I048'69 



and Santini's . He was the author of a valuable 



1049-2 

 work for the student, " Eilementi di Astronomia," which 

 contains a great amount of information relating to the 

 practice of astronomy, that can hardly yet be said to have 

 become antiquated, though the second and last edition 

 appeared in 1830 ; the writer of these lines gladly ac- 

 knowledges his indebtedness to this work, when a student 

 of astronomical methods of observation and calculation, 

 some thirty years since. Santini was elected a corre- 

 spondent of the Institute of France (Academy of Sciences) 

 in 1845. 



The Double-star 72 Ophiuchi.— The close com- 

 panion to this star was detected by M. Otto Struve, with 

 the Pulkowa 15-inch refractor, in 1842. In a note to his 

 catalogue of 1850, he remarks: "I have looked at this 

 star very frequently, and have noted it many times as a 

 single star. On three occasions, however, I have seen it 

 double, always very nearly in the same direction, and at a 

 distance of I '"5. I do not know how to explain these dis- 

 cordances, except by supposing that the light of the 

 satellite is very variable." In 1S47 the angle was mea- 

 sured i66"'3, and the distance i'"59. Dawes states he 

 had examined the star with different telescopes, including 

 Mr. Lassell's 20-feet reflector, but had never obtained a 

 glimpse of the companion. Secchi thought he saw it 

 double in July, 1S57, but placed small reliance upon the 

 observation ; in August, 1859, he obtained undoubted 

 evidence of duplicity ; the angle was 3'75, and the 

 distance o"'6o4 for 1859-61, magnitudes 4 and 7 ; he 

 remarks on this occasion : " E certamente doppia, e ben 

 separata." The suspicion of the discoverer is, therefore. 



