194 



NATURE 



\yan. 8, 1874 



the hydrocarbon spectrum ; while Young and I observed no such 

 coincidence. 



9, CoD'Ct IV. 1S71 (Tuttle), examined only by me, gave a 

 spectrum of three bands. Accurate measurement of their posi- 

 tion showed 710 coincidence with the hydrocarbon spectrum. 



Of these nine comets, there is only one (1. 1870) for which we 

 have no observations as to the position of the bright bands. Of 

 the remaining eight, the spectra of five (i, 2, 4, 7 and g) have 

 shown no agreement with the hydrocarbon spectrum. As re- 

 gards the Comet IT. 1S67 the supposition is offered that its 

 spectrum was similar to ihe spectrum named ; as to Encke's 

 Comet 111. 1871, it remains uncertain in which class it is to be 

 reckoned (Huggins' ob.'-ervations being at variance with those of 

 Young and myself). There remains only the Comet II. 1868, 

 for which Hufjgins' and Secchi's observations assert a proba- 

 bility of coincidence of the lines in its spectmm with those in the 

 spectra of volatile hydrocarbons. 



It thus appears a somewhat questionable view, that the comets 

 consist of such matter ; and we should, I think, content our- 

 selves with the deduction, that a portion of the light emitted by 

 the comet is its own light, and very probably from glowing gas. 

 Perhaps a brighter comet may enable us to find out their nature 

 more exactly, yet it seems to me extremely difficult to determine 

 the nature of tlie glowing gas of the comet through a comparison 

 of spectra from the electric spark in Geissler tubes; since there 

 must be, in the comet, circumstances of pressure and temperature, 

 which it is impossible for us to imitate, and through which, it is 

 known, the spectrum undergoes great modifications. 



Dr. Zenker has further asserted (Astr. jYach. loc. cit.) that " in 

 the spectrum of Brorsen's comet, Huggins has recognised the 

 bright line cf nitrogen." This statement is incorrect ; the ob- 

 servation having been, that the bright band situated in the 

 green of the spectrum, had nearly the same position as the 

 brightest line of the nebula, which, it is known, coincides with 

 the double line of nitrogen. The band in the comet spectrum 

 is a litile displaced towards the red end ; and this displacement 

 could not be due to the motion of the comet, for, as Huggins 

 pointed out, the latter was moving towards the earth, and the 

 line would have been displaced towards the rioh't. At an 

 earlier date, Huggins, observing the Comet I. 1S66, gave out 

 the opinion that the material forming it might be nitrogen ; the 

 spectrum appeared to consist of only one band of light, which 

 nearly coincided with the brightest niirogen line. But Secchi 

 disproved this view, having observed three bands, and the 

 weaker bands showing no coincidence with those of the nitrogen 

 spectrum. The accurate measurements alterwards made by 

 Huggins with the bright Brorsen comet, are of interest specially 

 because they put it beyond doubt, that there is no connection 

 between the spectrum of nitrogen and that of the comet. 



Again, Dr. Zenker arrives at the conclusion that there must 

 be water-vapour in the comets ; since they have, according to 

 Schmidt, a yellowish-red colour', and the sun's rays, when they 

 pass through a considerable thickness of aqueous vapour, are 

 coloured thus. But apart from the consideration that sunlight 

 has a yellowish-red colour on passing through other vapours, as 

 well as aqueous, I would remark, that we must take the proper 

 light of the comet, which appears from spectral analytic observa- 

 tions, to be generally more intense than the reflected light, as 

 determining its colour. According to the observations made, 

 we should expect that the comet is, on the whole, of greenish or 

 greenish-blue colour', since all the spectra consist, as we have 

 seen, of two or three bands of light, of whicli one is in the yellow, 

 the second and brightest in the green, and the weakest in the 

 beginning of the blue. Of the (generally very faint) continuous 

 spectrum, only the brightest part — yellow, green, and com- 

 mencement of blue— is visible. The entire image, therefore, 

 even where the weak continuous spectrum appeals, will seem of 

 greenish colour. Colour-data have been furnished by other ob- 

 servers besides Schmidt ; and the head ol the Comet 181 1, e.g. 

 had, according to Herschel, a greenish or bluish-green colour ; 

 the nucleus was slightly red. The colour of Halley's comet, at 

 its return in 1825, was a bluish-green (Struve). Winnecke says 

 of the comet of 1S62, " The colour of the neck appears to me 

 yellowish ; the coma has bluish light." 



With regard, lastly, to Dr. Zenker's proposition that " every 

 gas belonging to the solar system, as soon as it is visible on the 

 ilark ground of the heavens, must appear with the same lines of 

 the spectrum, as, according to its nature, it absorbs out of the 

 sunlight," I may be permitted to remark that I am not quite 

 convinced of this ; there is not yet furnished a satisfactory ex- 

 perimental basis for the assertion. But to seek to explain the 



line spectrum of a nebula thus, and by saying that the nebula is 

 shone upon by a fixed star in its "near neighbourhood," is 

 doubtful, inasmuch as it is a very rare ca.<-c that bright stars are 

 situated in such nearness to nebuloe (especially the planetary, 

 which best show the gas spectrum), that one can suppose a physi- 

 cal connection between them and the nebula;. 



I have been prompted to the foregoing remarks by the obser- 

 vation that in recent speculations on the constitution ol the 

 universe, the value of perceptions of sense, on which these 

 speculations rest, has been greatly over-cstimateil. The prin- 

 ciples on which the edifice of an hypothesis is raised must, above 

 all, be secure, and observations not sufficiently confirmed, or 

 even dt-noted as uncertain by those who have made them, should 

 preliminarily be disregarded, if it is desired that the hypo- 

 thesis have a stimulating and furthering influence on the progress 

 of scientific research. 



SCIENTIFIC SERIALS 



Jiisttis Licbig's Annahn dcr Chemie. Band 169, Heft 3. — 

 This number of the Annalen contains the following papers : — 

 On the decomposition of nitric acid by heat, by L. Cairus. This 

 paper, upwards of seventy pages in length, deals exhaustively 

 with the subject. Very numerous tables of the results of various 

 conditions of temperature, &c., are given, and the paper is il- 

 lustrated with two plates. — On the chlorides of molybdenum, by 

 Dr. L. P. Liechti and B. Kempe. — Chlorides of the formulae 

 MoCl„, M0CI3, MoClj, and MoClg are described. The authors 

 point out the parallelism shown by these bodies to the Tungsten 

 chlorides, where, however'. Tungsten wants the corresponding tri- 

 chloride, while molybdenum wants the hexachloride. In both 

 these series the colours of the salts become darker as the chlorine 

 increases in quantity. — On the atomic weight of molyt'denum, 

 by L. Meyer. The author from sixteen results deduces the 

 atomic weight 95 '86 for molybdenum, chlorine being taken as 

 35 '37 a' d srlver I07'66. This agrees very well with the result 

 obtained by Dumas 96, and by Debray 95 94. The author also 

 points out the following relaiions in three groups of ele- 

 ments : — 



V 51-2 Cr 52-4 Cu 63-3 



Plus 43 43-2 44-4 



Nb 94 Mo 95 '6 Ag 1077 



Plus 88 88-4 88-5 



Ta 182 W 184-0 Au 196-2 



On chromic dioxide, by E. Hintz. The author describes the 

 preparation, &c., of this body. — The number concludes with a 

 paper on sulpho-ortho-toluidrnic acid, by F. Gerver, and one on 

 the specific heat of zirconium siUcon, and boron, by W. G. Mixter 

 and E. S. Dand. 



The new number of the Quarterly Journal of Microscopical 

 Science contains many papers of interest. Prol. AUman com- 

 mences by giving an account of Kleinenberg's researches on the 

 anatomy and development of Hydra, in which, while he has 

 confirmed many of the statements of former observers, he has 

 shown the incorrectness of others, and has discovered several 

 important points in its anatomy, specially in connection with the 

 structure of the ectodermic layer, and the subject of develop- 

 ment. — Prof. Martin Duncan records some observations on the 

 method of development in Fiiciis I'esiciilosiis, in which, after 

 suggesting that they obtain their nutrition in part at least, from 

 the organic matter always present in sea-water, he describes the 

 growth of the terminal cells of two sets of finger-shaped pro- 

 cesses ; showing that by in-growths from the lateral walls, mem- 

 branous septa are formed at the apices of the processes, an active 

 mass of protoplasm occupying the extreme end. — Following this 

 is a translitton, with a plate illustrating it, of George O. Sars' 

 paper on the anatomy of that aberrant form Khal'iioflenra nnra- 

 hilis (M. .Sars), so peculiar in combining a crcepmg stem in 

 which is an axial cord ; lateral cells in which the somites are 

 free, except that a contractile cord binds each to the axial cord ; 

 a pair of tentacular arms; a differentiated alimentary canal, and 

 a loot-like process between the alimentary oi'ifices. Mr. E. R. 

 Lankester, in a separate paper, very clearly shows, with the aid 

 of some excellent diagrams, that this animal is a true molluscan 

 form, intermediate between the Polyzoa and Mollusca, and not 

 in reality related to the Hydrozoa as imagined by M. M. .Sai's. — 

 Mr. Tomes' observations on the development of the teeth 

 of the Armadillo are referred to in our Notes. — A translation 

 follows of the researches of Ph. van Pieghern and G. Le 

 Monnier, on the Miicorini, condensed from their memoii's in the 



