Sept. 27, 1888] 



NATURE 



53i 



thrown off when striking a flint. It is known that such particles 

 in burning take a spherical form, and are surrounded by a layer 

 of black magnetic oxide. 



How are we to trace this dust in the air ? It is well known 

 that at times the air is electrically illuminated, not only by the 

 flashes of lightning which pass along its lower levels, but by 

 so-called "auroral " displays in its higher reaches. 



It is now many years since the idea was first thrown out that 

 the aurora was in some way connected with shoooting-stars. 



M. Zenger has prepared a catalogue of aurora observed from 

 1800 to 1877, in which he shows an apparent connection between 

 the brightest displays and the appearance of large numbers of 

 shooting-stars. 



M. Denza noted the same connection on November 27, 

 1872, and remarked that he had noticed it before. 



Admiral Wrangel, as quoted by Humboldt, observed that in 

 the auroras so constantly seen on the Siberian coast, the passage 

 of a meteor never failed to extend the luminosity to parts of the 

 sky previously dark. 1 



It is clear that in such a case as this the spectroscope is the 

 only chemical aid applicable, and it has long been recognized 

 that the spectrum observed is not the spectrum of the con- 

 stituents of the atmosphere, as we can study it in our 

 laboratories. 



The spectrum, however, strictly resembles that seen in the 

 "glows," to which reference has been made ; if the factors 

 present in both cases are meteor dust, low pressure, and feeble 

 electric currents, the resulting phenomena should not be dis- 

 similar. 



The results of recent inquiries certainly justify us, therefore, 

 in concluding that the upper reaches of the atmosphere contain 



particles giving us the spectra of magnesium, manganese, iron, 

 and carbon. 



The natural origin is the dust of those bodies which are con- 

 tinually entering those regions, and hence the proof afforded by the 

 spectroscopic observation of shooting-stars, that they are identical 

 in chemical composition with meteorites, is strengthened by these 

 auroral observations, while, on the other hand, the origin of tfc< 

 auroral spectrum is placed beyond all doubt. 



Proof from the Fallen Dust. 



It is universally recognized that the atmosphere holds in 

 suspension an immense number of very minute particles of 

 organic and inorganic origin. These must be either dust taken 

 up by aerial currents from the ground — the re;ult of volcanic 

 action — or extra-terrestrial bodies. Many scientiric men, among 

 whom we may mention Ehrenberg. Daubree, Reichenbach, 

 Nordenskjokl, Tissandier, Murray, and Renard, have studied 

 this problem. Dust collected in various places at different times 

 has been examined with a view of determining whether its origin 

 was meteoric. In many cases, in which chiefly definite iron 

 chondroi have been observed, the evidence has seemed very 

 strong in favour of the view. 



It is at once obvious that the detection of such dust which 

 falls on the general surface of the land is hopeless, and that that 

 which is collected on snow in inhabited countries containing 

 foundries and the like is doubtful. 



But a considerable advance of this question has recently been 

 made in studying the deep-sea deposits collected by the 

 Challenger Expedition. Messrs. Murray and Renard, 1 in giving 

 the results of their researches, point out that at the greatest 



sea level 



~oced clay 



Fig. 3.— Section of ccean showing red clays at depths of 3000 fathoms (18,000 feet). 



depths of the ocean furthest from land, the sea bottom is very 

 different from that nearer the coast lines. 



Under these necessary conditions of exceeding slow deposition 

 and absence from ordinary sources of contamination, it is clear 

 that the problem can be attacked under the best conditions. 



We read : — "The considerable distance from land at which 

 we find cosmic particles in greatest abundance in deep-sea 

 deposits, eliminates at once objections which might be raised 

 with respect to metallic particles found in the neighbourhood of 

 inhabited countries. On the other hand, the form and character 

 of the spherules of extra-terrestrial origin are essentially different 

 from those collected near manufacturing centres. These mag- 

 netic spherules have never elongated necks or a cracked surface, 

 like tho^e derived from furnaces, with which we have carefully 

 compared them. Neither are the magnetic spherules with a 

 metallic centre comparable either in their form or structure to 

 those particles of native iron which have been described in the 

 eruptive rocks, especially in the basaltic rocks of the north of 

 Ireland, of Iceland, &c." 1 



Messrs. Murray and Renard then state on what they rely in 

 support of their view that many of the particles thus obtained 

 from great depths are of cosmic origin : — 



" If we plunge a magnet into an oceanic deposit, especially a 

 red clay from the central parts of the Pacific, we extract par- 

 ticles, some of which are magnet ie from volcanic rocks, and to 

 which vitreous matters are often attached ; others again are 

 quite isolated, and differ in most of their properties from the 

 former. The latter are generally round, measuring hardly o '2 mm., 

 generally they are smaller, their surface is quite covered with a 

 brilliant black coating, having all the properties of magnetic 

 oxide of iron ; often there may be noticed upon them cup- like 

 depressions clearly marked. Jf we break down these spherules 

 in an agate mortar, the brilliant black coating easily falls away, 

 and reveals white or gray metallic malleable nuclei, which may 



1 "Cosmos" (OtteO, vol. i. p. 114. 



be beaten out by the pestle into thin lamellae. This metallic 

 centre, when treated with an acidulated solution of sulphate of 

 copper, immediately assumes a coppery coat, thus showing that 

 it consists of native iron. But there are some malleable metallic 

 nuclei extracted from the spherules which do not give this 

 reaction, they do not take the copper coating. Chemical 



V~**" 



Fig. 



Fig. 



4- *ig. 5- 



Fig. 4. — Black spherule with metallic nucleus (60 : 1). This spherule, 

 covered with a coaling of black shining magnetite, represents the most 

 frequent shape. The depression here shown is often found at the surface 

 of these spherules. From 2375 fathoms. South Pacific. 

 Fig. 5. — black spherule with metallic nucleus (60 : 1). The black external 

 coating of magnetic oxide has been broken away to show the metallic 

 centre, represented by the clear part at the centre. From 31^0 fathoms, 

 Atlantic. 



reaction shows that they contain cobalt and nickel ; very prob- 

 ably they constitute an alloy of iron and these two metals, such 

 as is often found in meteorites, and whose presence in large 

 quantities hinders the production of the coppery coating on the 

 iron. G. Rose has shown that this coating of black oxide of 



' " On the Microscopic Characters of Volcanic Ashes and Cosmic Dust 

 and their Distribution in Deep-sea Deposits," Proc. R.S.K., and Nature, 

 vol. xxix. p. 585. 



