Dec. ii, 1879] 



NATURE 



129 



may be got rid of, or at least, so much paled as to betray 

 their character as interlopers. This, however, is an 

 amount of labour hardly to be expected of those who 

 make tubes in a commercial way, and it is to be regretted 

 that in Mr. Capron's painstaking research, he was com- 

 pelled to employ such tubes instead of preparing them 

 for himself. In a future research we would suggest the 

 employment of tubes thoroughly heated and washed out 

 with air in the first instance, and then worked with a 

 blowpipe fed with pure hydrogen. 



Unfortunately throughout, the tubes employed both by 

 Mr. Capron and by Dr. Vogel seem to have been of 

 doubtful purity. That figured on plate xiv. as hydrogen, 

 contains bands of most suspicious resemblance to those 

 of nitrogen, while the oxygen tubes, beside the one or two 

 lines which seemed peculiar to themselves, gave others 

 which were proved by direct comparison, to coincide with 

 those of carbon and hydrogen, though the relative 

 intensities seemed somewhat altered. 



Supposed coincidences have been pointed out by 

 Angstrom, Vogel, and others, between the auroral spec- 

 trum and those of the various gases, such as oxygen, 

 nitrogen, and hydrogen, which are present in the atmo- 

 sphere. Unfortunately these coincidences do not extend 

 to the one bright line which has been accurately 

 me.sured, but only to the fainter ones, the positions of 

 which are so doubtful that they might be made to corre- 

 spond with any spectrum the lines of which were toler- 

 ably numerous, so that, intrinsically probable as they 

 may be, we cannot regard them as positively established. 



Absolutely no coincidence has been made out between 

 the bright yellow-green line of the aurora and a principal 

 one of any other known spectrum, and the same may be 

 said of the sharp red line which occasionally flashes out 

 in the spectrum, of red aurora;. Mr. Capron, however, 

 points out that the green line coincides with a faint atmo- 

 spheric absorption band, while the red line seems to 

 occupy the position of the well-known a line of the solar 

 spectrum, which Prof. Smyth has shown to be due to dry 

 air. 



It would not be fair to conclude our notice of " Aurora;" 

 without a few words of praise to the admirable illustra- 

 tions, several of which are chromolithographs. Of these 

 perhaps the best inartistic effect is a facsimile of a water- 

 colour drawing of a white aurora seen by the author at 

 Kyle Akin in Skye. But in fact the whole appearance of 

 the book suggests at first glance art rather than science 

 and we should suppose it is but rarely that a purely 

 scientific treatise has appeared in so ornamental a dress. 



OUR BOOK SHELF 



A Treatise on Metalliferous Mines and Mining. By D. 



C. Davies, F.G.S. Svo. (London: Crosby Lockwood 



and Co., 1880.) 

 The objects of this book, as stated in the preface, are 

 " to describe in a concise and systematic manner the 

 conditions under which metallic ores are found in different 

 countries in the world," and further, "by defining the 

 zones occupied by the various metallic ores to lessen 

 somewhat the amount of unsuccessful search for them." 

 For the first purpose the author notices a large number 

 of mines in various parts of the world, partly from his 

 own observations and partly from accounts published in 

 special journals and in the transactions of scientific 



societies ; and for the second, he deduces from such 

 descriptive matter certain general conclusions, which, in 

 their more important points, are as follows : — 



" Gold and silver never occur in strata newer than the 

 carboniferous period." 



" Copper ores with trifling exceptions are only found in 

 the lower Cambrian carboniferous and new red sand- 

 stone formations." 



" The highest horizon of lead ores is in the carboniferous 

 limestone." 



The conclusions are apparently derived from the study 

 of phenomena in Wales, and to render them universally 

 applicable all that is necessary is to reconstruct the 

 geology of the rest of the world to suit them, which the 

 author does in a thorough-going fashion. Thus the 

 system requires for Cornwall that the age of the granites 

 should be Laurentian, and the killas and other schistose 

 rocks Cambrian, Silurian, Devonian, &c, in regular suc- 

 cession ; and therefore the author concludes that the 

 received view which makes the granite post-carboniferous 

 is a mistake, and corrects his authorities accordingly, 

 even when quoting their observations. Thus in repro- 

 ducing Dr. Foster's account of the Hay Tor iron ores he 

 disputes their probable carboniferous age, and states that 

 they may belong to an older group, and that possibly of a 

 still older age are the deposits of the West of Ireland, 

 which are found interstratiried on the basaltic and por- 

 phyritic rocks that skirt the west coast. It appears from 

 a preceding page that by these are meant the iron ores of 

 Antrim, which occur in miocene basalts on the north-east 

 coast between Lame and the Giant's Causeway, and 

 about whose age no question can possibly be raised by 

 any one with the smallest geological knowledge. 



Much of the information concerning foreign mines is 

 exceedingly inaccurate, indeed it is difficult to see whence 

 some of it is derived. For example, on p. 240, in a para- 

 graph describing the zinc ores of Silesia, it is stated that 

 the calamine of that country averages 20 to 30 per cent, 

 of metallic zinc, which by selection and dressing is 

 brought up to 70 per cent. ; that in 1876 sixty-four mines 

 produced 31,315 tons of zinc ore, and a reference to a 

 paper by Huene in the Journal of the German Geological 

 Society is given as an authority. As these statements are 

 contrary to what is generally known upon these subjects, 

 an attempt has been made to verify them ; and it appears 

 that (1) the average yield of the Silesian zinc ores in 

 1876 as smelted was 11 84 per cent. ; (2) the production of 

 zinc ores in Silesia in 1876 was 449>374 tons; (3) the 

 paper by Huene, published in 1S51, has nothing whatever 

 to do with Silesia, as it describes some zinc mines at 

 Bergisch-Gladbach near Cologne. 



The above examples taken quite at random will be suf- 

 ficient to show the generally untrustworthy character of 

 the book. H. B. 



LETTERS TO THE EDITOR 

 [The Editor does not hold himself responsible for opinions expressed 

 by his correspondents. Neither can he undertake to return, or 

 to correspond with the writers of, rejected manuscripts. No 

 notice is taken of anonymous communications. 

 [The Editor urgently requests correspondents to keep their letters as 

 short as possible. The pressure on his space is so great that it 

 is impossible otherwise to ensure the appearance even of com- 

 munications containing interesting and novel facts.'] 



Why the Air at the Equator is not Hotter in January 

 than in July 



The following, I think, is the explanation of Mr. Fisher's 

 difficulty (Nature, vol. xx. p. 577), why the January tempera- 

 at the equator when the earth is in perihelion is not much higher 

 than in July when in aphelion. The temperature to which Mr. 

 Fisher refers is the ordinary temperature as indicated by the 

 shade thermometer, which of course is simply that of the air. 

 The difficulty is more apparent than real, for if we examine the 

 indirect results which follow from the present distribution of land 



