Sept. i7,i874j 



NATURE 



397 



bands are carefully represented, and a full description of 

 them is given in the body of the work. The whole is 

 designed to facilitate the application of spectrum analysis 

 to mineral chemistry ; and although some of the details 

 may hereafter require correction, the work is well exe- 

 cuted, and cannot fail to be of great value to the scientific 

 and practical chemist. The frequent reproduction of 

 the comparatively simple spectra of the metals obtained 

 at the low temperature of the gas flime in elemen- 

 tary works of chemistry, unaccompanied by sufficient 

 explanation, has tended to give rise to partial and even 

 incorrect conceptions of the grandeur and extent of this 

 subject. How many persons believe that the spectrum of 

 sodium consists solely of a pair of fine lines correspond- 

 ing to the double line D of the solar spectrum .'' How 

 few know that at the high temperature of the electrical 

 spark it exhibits three other pairs of well-defined lines, 

 one in the orange, another in the yellow, and another in 

 the green, together with a nebulous bind on the confines 

 of the blue.' (Huggins). All these lines may easily be 

 seen by passing the electrical spark in a non-luminous 

 flame between a fused bead of svilphate or chloride of 

 sodium and a platinum wire, together with a few other 

 feeble lines, especially in the violet (Lecoq de Bois- 

 baudran). The vivid line in the red, with its faint com- 

 panion in the orange, which forms the ordinary gpectrum 

 of the compounds of lithiun in the gas flame, gives place to 

 a very different spectrutn, when sparks are drawn from a 

 solution of the lithium salts. The red ray still continues 

 vivid, but it is surpassed in intensity by the orange, 

 which is now the most characteristic of the lithium rays, 

 while two new rays or lines come into view (X497'o, 

 46o'4). With a solution either of the ferrous or ferric 

 chloride, the electrical spark gives the numerous lines 

 with great sharpness and accuracy of detail, which con- 

 stitute the spectrum of metallic iron. 



M. Lecoq de Boisbiudran gives a delineation of what 

 he considers to be the spectrum of oxide of barium, as it 

 appears after a prolonged heating of the chloride in the 

 gas flame, and also of the spectra proper of the chloride, 

 bromide, and iodide of barium, as obtained by heating 

 those salts in the gas flame charged with hydrochloric 

 acid, bromine, and iodine vapours respectively. These 

 spectra are all different. Thus, in the case of the chlo- 

 ride, only slight traces of the lines and bands due to the 

 oxide are seen, while six new lines appear which are very 

 intense (A. Mitscherlich). On the interesting subject of 

 the bright lines which compose the spectrum of the earth 

 erbia and its phosphate, the following observations are 

 made in the work before us : — " According to Bunsen 

 and Bahr, the addition of a little phosphoric acid to 

 solid erbia gives to that earth a greater emissive power 

 and renders the lines sharper, without modifying their 

 number or position. On repeating this experiment, I find 

 that erbia alone and erbia to which phosphoric acid has 

 been added give very different spectra. On comparing 

 the spectra, the rod is more developed in the light of 

 the phosphate, whilst the green and the violet-tjlue are 

 more vivid in that of the oxide." 



The limits of this notice do not permit the discussion 

 of ciuestions of gi'cat interest in spectrum analysis, many of 

 which promise soonto be fully resolved. The observation 

 pf Roscoe and Upton, that the broad bands characteristic 



of certain metallic compounds at the low temperature of 

 the gas flame disappear at the higher temperature of 

 the electrical discharge, and the view they have set 

 forth, that in the former case the spectrum is that of 

 the compound, in the latter case that of the metal, have 

 received confirmation from later researches. Lockyer, in 

 his valuable contributions to spectrum analysis, has shown 

 that what he designates the shortest lines disappear first 

 on reducing the pressure, and that the difference between 

 the spectrum of the chloride and the spectrum of the 

 metal is that under the same spark condition all the 

 short lines are obliterated in the former case. The same 

 investigator has observed that metallic elements of low 

 specific gravity, such as sodium, calcium, magnesium, and 

 aluminium widen their lines by increase of vapour densily, 

 while metallic elements of high specific gravity, such as 

 iron, cobalt, and nickel, increase under the same condition 

 the number of their lines. 



Thomas Andrews 



OUR BOOK SHELF 



Comets and the Neia Comet of 1874. By the Author of 

 "Astronomy Simplified for General Reading." (Lon- 

 don : William Tegg and Co., 1874.) 

 This book purposes to be "a complete popular account 

 of all that is known of these wonderful bodies which 

 are so great a perplexity to science : " but the work con- 

 sists of only 56 pages, and it is needless to say that even 

 a popular account of these bodies to be complete must 

 extend over a much larger space. We think that a work 

 on any subject in science, to be popular, that is written to 

 be read by the public at large and not by persons who 

 are conversant with the subject only, should not refer to 

 explanations or theories that are not generally known, 

 without a very intelligible explanation ; theories of the 

 action of observed phenomena should not be given with- 

 out a very strong probability of their truth, or without 

 a caution against their acceptance ; and in dealing with a 

 subject like the present one, when our knowledge i-; 

 limited, and when there are so many different modes of 

 explaining appearances, it behoves an author to use more 

 than ordinary caution against the mention of anything 

 that is not strictly in accordance with ascertained 

 physical laws. On both these points the present bjok is 

 at fault. As an instance, the author mentions M. Faye's 

 theory of the repulsive power of the sun in virtue of its 

 heat, and then urges objections to the theory without a 

 word of explanation of it. Now to a person not conversant 

 with the experiments on the repulsion of gases and solids 

 by heat rays, the theoi-y would seem absurd and contrary 

 to experience ; and so the author carries the day with the 

 theory that the effect of solar heat upon the cometary 

 matter is electrical in its action. Again, he says : " For 

 example, the matter of comets is not possessed of con- 

 centric attraction even with reference to itself, neither is 

 it possessed of chemical affinity for itself. This is fully 

 established by the eccentric forms of comets and through 

 conspicuous variations of shape and size."' This is quite 

 new to us. Again, after mentioning that Lexell's comet 

 was entangled for about a month among the satellites of 

 Jupiter, he says : " Is there another instance — a single 

 analogy on record outside of cometary phenomena — of a 

 body of dead matter under great velocity being actually 

 barred and stopped in its path for four mouths, and then 

 suddenly starling off again .after being divested of its 

 force for so long a period ? What can the composition 

 and resolution of forces do for us here ? for here is the 

 most wonderful problem ever submitted to their laws. 

 What must be the amazing force of a body which, like an 



