Dec] 6. r888] 



NATURE 



139 



NOTES ON METEORITES} 

 VI. 



\ Comets are Meteor-Swarms which have entered the 

 ,SoLAR System some time or other. 



"T^HESE swarms, then, are comets. The final demonstration, as 

 ■*■ we have seen, we owe to the labours of Newton, Adams, and 

 Schiaparelli chiefly. But long before their time the connection 

 between shooting-stars (and even meteorites) and comets had been 

 suspected on various grounds. - 



Many shooting-stars pass through the air with a trail. This 

 appearance is certainly suggestive of a very rapid comet. Hence, 

 perhaps, it \vas that such an appendage, often noticed in the case 

 of bright meteors, was sometimes in ancient records described as a 

 comet. •' It is known that Cardano described as a comet the great 

 meteor from which fell i2CO stones on the territory of Crema on 

 September 4, 1511.^ 



Not only, as we have seen, Kepler (1600) regarded shooting- 

 stars as akii) in nature to meteorites, but he held that both had 

 the same origin as comets : — " Falling stars are compo^ed of 

 inflammatory viscous materials. .Some of them disappear during 

 their fall, while others indeed jjfill to the earth, drawn by their 

 own weight. Nor, indeed, is it improbable that tKey have been 

 formed into globes from feculent tnaterfals mixed with the ethereal 

 .air itself,.and thrown from the ethereal region in a straight Hue 

 ^trough the air like very small comets, the cause of the motion 

 .of both being hidden." * 



Ilalley (ijco) though he thought that the phenomenon of 

 shooting-stars » was produced by a material disseminated 

 through celestial space falling upon the sun and meeting the 

 earth in its passage, did not associate it with cometary phenomena; 

 but Maskelyne (1765) held that meteors were of celestial origin," 

 apd was inclined; to assimilate them to comets. He wrote as 

 follows in a letter to the Abbe Cesaris, the astronomer at Milan, 

 about December 12, 1783: — "Freely accept, I pray you, this 

 map, which I have lately published in order to stir up learned 

 men rather than the unlearned, to observe more keenly the 

 phenomena called fire-balls. In all probability they will turn out 

 to be comets. . . ."^ 



To Chladni belongs the credit of having broached the theory 

 which modern observations have established. 



We have already seen that Chladni formulated the view, in 

 1794, that space is filled with matter. In 1819 he extended it 

 by stating that both shooting-stars, meteorites, and comets were 

 but different manifestations of it.' 



Chladni made a step in this matter of which, as pointed out 

 by Schiaparelli, only to-day are we able to appreciate the im- 

 portance. In suggesting the cosmical hypothesis, he regarded 

 two possible cases : either the meteors were formed of masses of 

 independent materials which had never formed part of the 

 Lvgcr celestial bodies, or they are the result of the destruction 

 of a celestial body previously existing. Chladni held the second 

 hypothesis as possible, but held to the first as more probable. 

 He stated that we could not doubt the existence in the celestial 

 space of small botlies endowed with movement, which are now 

 and then visible by passing before the sun. 



He held, therefore, that the small masses which appear under 

 the forms of bolides and falling stars do. not differ essentially 

 from cornels. It is also probable, he say.s, that comets consist 

 of clouds composed in great part of masses of vapour and dust, 

 \yhich are kept together by mutual attraction. That this attrac- 

 tion is not enough to .'ensibly disturb the planetary movements 

 is a proof of the exceeding tenuity and dispersion of the materials 

 in such clouds, through which, however large, it is possible, to 

 observe the fixed stars."* 



In 1839 the Abbe Kaillard suggested a connection between 

 luminous metejrs and comets and the aurora," and Dr. Forster 



' Continued from vol. xxxviii. p. fo5. 



'■' For many references in what follows I am indebted to the hUtorlcal 

 notice in Schiaparelli 's " .Stelle Cadente." 



3 Humboldt, 'Cosmos," iv. p. 587 (OttO- Cardani, 'Opera." Lugduni, 

 1663, t. iii. p. 279. .See also Schiaparelli, "Stelle Cadente." 



■• Kepler, "Opera," ed Frisch, vol. vi. p. 157. 



5 Coulvier-Gravier et S.irgey, " Introd. Historiquc," p. 5. 



6 Metnorie ihtla Societa Italiana, vol. iii. p. 345, Verona, T786. 



' " Ueber Feuermeieore, und ueber die mit deiiselben herabgefaltenen 

 Massen" (Wien, 1819). See aho "Ueber den Ur..prung der von Pallas 

 gefundenen Eisenma-sen," p. 24. 



8 " Feueruieteore,"p. 395 ; see Kaemtz, " Meteor jlogie," vjl. iii. p. 316. 



^ Les Mondes, t. xii. p 649, et t. xiii. p. 606. 



noted that the years marked by the appearance of a laq»e comet 

 are remarkable also for the abundance of falling stars, especially 

 of white ones.^ 



Perhaps the first to give a more solid support to the cometary 

 theory of falling stars on geometric grounds was Boguslawski,. 

 who conceived the idea of representing by means of parabolas 

 the apparent orbits observed in some of the August meteors of 



For the next important advance in thought upon this subject 

 we have to come down to 1858, in which year Baron Reichen- 

 bach published a most important memoir* attacking the 

 question from an entirely new point of view. Reichenbach, 

 accepting as proven by the then knowledge the most intimate 

 connection between meteorites and falling stars, reasoned in 

 the following manner, that both were connected with comets. 

 He first recapitulated the facts then accepted with regard to 

 comets : — 



(i) Comets, both lail and nucleus are transparent. 



(2). Light is transmitted through comets without refraction; 

 hence the cometary substance can be neither gaseous nor liquid. 



(3) The light is polarized, and therefore borrowed from the 

 sun. 



(4) Comets have no phases like those of moon and planets. 



(5) They exercise no perturbing influences. 



(6) Donati's comet (which was then visible) in its details and 

 its contour is changing everyday — according Iq Tiazzi, almost 

 hourly. 



(7) The density of a comet is extremely small. 



(8) The absolute mass is sometimes small (von Littrow having 

 calculated very small comets, tail and all, as scarcely reaching 

 8 pounds). 



From these data the following conclusions might be drawn : — 

 (i) That a comet's tail must consist of a swarm of extremely 

 small but sjlid particles, therefore granules. 



(2) That every granule is far away from its neighbour — in 

 feet, so far that a ray of light may have «tn uninterrupted course 

 through the swarm. 



(3) That these granules, suspended in space, move freely and 

 yield to outer and inner agencies — agglomerate, condense, or 

 expand ; that a comet's nucleus, where one is present, is nothing 

 else than such an agglomeration of loose substances consisting 

 of particles. 



Hence we must picture a comet as a loose, transparent, illa- 

 minated, free-moving swarm of small solid granules suspended 

 in empty space. 



The next step in Reichenbach's reasoning was to show that 

 meteorites (of which he had a profound knowledge) were really 

 composed of granules. 



He pointed out that these granules (since called chondroi> 

 formed really the characteristic structure both of irons and stones, 

 so that both orders were chiefly aggregates of chondroi — stony 

 ones in iron meteorites, iron ones in stony meteorites. 



In some irons, such as Zacatecas, they exist as big as walnuts,, 

 firmly adherent, but they can be separated ; inside these are balls 

 oftroilite, oftenfirmlyembedded,sothat on breaking the meteorite 

 they will divide, but in other cases so loose that they fall out, 

 and they are smooth enough to roll off a table. 



Sometimes chondroi have smaller ones sprinkled in them, 

 sometimes dark chondroi have white earthy kernels. 



In some cases these chondroi are so plentiful as to form nearly 

 the whole mass of the meteorite. They are often perfectly round, 

 but not always, and they are often so loose that they tumble out 

 and leave an empty smooth spherical cavity. 



The stones chiefly consist of such chondroi and their debris. 



He adds that each magnesic chondros "is an independent 

 crystallized individual — it is a stranger in the meteorite. Every 

 chondros was once a complete, independent, though minute 

 meteorite. It is embedded like a shell in limestone. Millions 

 of years may have passed between the formation of the spherule 

 andits embeddal." 



He then goes on to remark that the chondroi of meteorites 

 indicate a condensation of innumerable bodies such as we see 

 must exist in the case of comets ; further, that they have been 

 formed in a state of unrest and impact from all sides. Many 

 meteorites are true breccias ; they have many times suffered' 

 mechanical violence. He then shows that in comets we have- 

 precisely the conditions where such forces could operate, and 



' " Es-ai sur rinfluence des Comctes," &c. (Bruges,' 1843). 

 ^ Coulvier-Gravier et Sargey, " Introd. Hisitrique," p. 103, 

 3 Poiixenc'orff's At^naiin, vol. cv. p. 438. 



