86 



NATURE 



{Nov. 24, 1887 



in round numbers that the meteorites are distributed each 250 

 miles away from its neighbours.^ 



If, then, these observations may be accepted to be good for 

 any part of space, we may, and indeed must, expect celestial 

 phenomenon which can be traced to meteorites in all parts of 

 space. 



Further, we 'have the experience of our own system that these 

 meteors are apt to collect in groups. 



A comet, it is now generally accepted, is a swarm of meteors 

 in company. Such a swarm finally makes a continuous orbit by 

 virtue of arrested velocities ; impacts will break up large stones 

 and will produce new vapours in some cases, which will con- 

 dense into small meteoroids. 



A meteorite in space under any of the conditions indicated by 

 the comets, new star>:, and such first-magnitude stars as a Orionis, 

 will evidently be subject to collisions, but only to a greater 

 number of collisions than those which must ordinarily occur if 

 space is as full of meteorites as Prof. Newton's calculations, from 

 observations made on the earth, would naturally seem to indicate. 



The Velocity of Luminous Meteors. 



In spite of the difficulties which attend the observations 

 necessary to determine the velocity of meteors entering our 

 atmosphere, many observations have been made from which it 

 may be gathered that the velocity is rarely under 10 miles a 

 second or over 40 or 50. It is known that the velocities of some 

 meteor-swarms are very different from those of others. Prof 

 Newton, our highest authority on this subject, is prepared to 

 consider that the average velocity may be taken to be 30 miles a 

 second. 



Result of Collisions, 



If we take these velocities as representing what happens in 

 other regions of space, and assume the specific heat of the 

 meteorites to be 'lo, the increase in their temperature when 

 their mo'ions are arrested by impacts will be roughly as 

 follows : — 



Velocity i mile per second 

 ,. 10 ,, ,, 



20 ,, 



60 ,, 



3,000° C. 



300,000° 



1,200,000° 



2,700,000° 



10,800,000° 



It is 'clear, however, that we should under the conditions 

 slated be more frequently dealing with grazes than collisions. 



Comets due to Collisions of Meteorites. 



The fact that comets are due to swarms of meteorites was first 

 established by Schiaparelli in 1866, when he demonstrated that 

 the orbit of the August meteors was identical with that of the 

 bright comet of 1862.^ 



Nebula due to Collisions of Meteorites. 



So far as I know the first suggestion that nebulae were really 

 in some manner associated with meteorites and not with masses 

 of gas was made by Prof. Tait in 1871.' I have used the sug- 

 gestion in my lectures ever since, and it is now some years ago 

 since I put it to an experimental test by showing that both the 

 spectra of comets and nebulas, so far as carbon and hydrogen 

 were concerned, could be produced from a vessel containing the 

 vapours produced by meteorites. More recently, M. Faye has 

 stated in his works on the nebular hypothesis that the solar nebula 

 may as probably have consisted of a cloud of stones as of a mass of 

 gas. This view, however, has not been favoured by Dr. Huggins, 

 who in his observations both on nebulae and comets has inferred 

 from the near coincidence of the line of 500 with the strong air 

 line that we are probably in presence of nitrogen, or of a form of 

 matter more elementary than nitrogen ; the line at 373 being 



' Article on " Meteorites," Prof. Newton, " Encyclopsedia Britannica," 

 vol. xvi. 



^ Letters to Father Secchi, printed in the Bollettino of the Collegio 

 Romano, and reproduced in Les Mondes, t. xiii. 



3 " It seems to me that we have a series of indications of what (for want of 

 a better phrase) may be called the period of life oi 3.?Xa.x or group, beginning 

 with the glowing gases developed by impacts of agglomerating cold masses 

 (planetary nebulae and others irresolvable, such as thcs« of Orion, Lyra, 

 &o., where the spectrum consists of a very few bright lines only)" (Prof. 

 Tait, Proc. R.S. Edin., 1871). 



attributed by him also to some unknown form of hydrogen on 

 account of its coincidence with one of the series of hydrogen lines 

 in the ultra-violet observed in the spectra of stars of the first 

 class. 



"New Stars " due to Collisions of Meteorites. 



The idea that the Novas which appear from time to time are 

 due to collisions of meteorites was, I think, first advanced by 

 myself in 1877, when I wrote in connection with Nova Cygni : — 



" The very rapid reduction of light in the case of the new star 

 in Cygnus was so striking that I at once wrote to Mr. Hind to 

 ask if any change of place was observable, because it seemed 

 obvious that, if the b^dy which thus put on so suddenly the 

 chromospheric spectrum were single, it might only weigh a few 

 tons, or even hundredweights, and, being so small, might be 

 very near us. Mr. Hind's telescope was dismounted, and I 

 have not yet got any information as to change of position ; and 

 as I am now writing in the Highlands, away from all books, I 

 have no opportunity of comparing the position now given by 

 Lord Lindsay in R. A. 2ih. 36m. 52s., Decl. -1- 42° 16' 53", with 

 those given on its first appearance by Winnecke and others. 



" We seem driven, then, from the idea that these phenomena 

 are produced by the incandescence of large masses of matter, 

 because if they were so produced, the running diwn of brilliancy 

 would be exceeding slow. 



"Let us consider the case, then, on the supposition of small 

 masses of matter. Where are we to find them ? The answer is 

 easy : in those small meteoric masses which, an ever-increasing 

 mass of evidence tends to show, occupy all the realms of 

 space." ■* 



The Ejects of Collisions. 



The question of what must happen to the meteorites them- 

 selves in consequence of this system of collisions is worth going 

 into thoroughly. A very cursory examination seems to indicate 

 that much light is thrown on the condition of meteorites as we 

 know them, and their division into iron and stony. 



As 30 miles per second is a very frequent value obtained f jr 

 the velocity of meteorites when they enter our atmosphere, it is 

 possible to compare temperatures brought about by collisions 

 with those produced by passage through our atmosphere. Two 

 masses of meteoric iron meeting each other in space would pro- 

 bably, if moving with a certain velocity, be formed into a pasty 

 conjoined mass, and this process might go on until an iron of 

 large dimensions was formed, and the various meteorites thus 

 welded together would present in time a very fragmentary 

 appearance. While irons were thus increasing in size, collisions 

 with smaller meteorites would be attended with very local in- 

 creases of temperature, perhaps sufficient to volatilize the surface 

 or allow it to be indented, and in this manner the well-known 

 " thumb-marks " receive explanation. 



The masses of iron, when in a state of fusion, whatever 

 their size, would be able to include stony meteorites in their 

 vicinity. In the case of stones it is easy to see that the result 

 would be very different. Their collisions would have, most pro- 

 bably, the effect of reducing large pre-existing masses to smaller 

 ones, and the collision of a large stone with a large iron would 

 probably effect the driving of the stone irto fragments, while the 

 iron would be liquefied so as to inclose some of the fragments in 

 its mass. 



These operations of Nature might go on either in free space, 

 or in the head of a comet, or in meteor-swarms. They probably 

 cause the appearance of the so-called new stars, and in these 

 various circumstances the rate of subsequent cooling would of 

 course be very different, so that the results would be very 

 different indeed. 



Large masses on collision probably destroy each other, produce 

 fragments and vapour, which aj;ain condense. The heterogeneous 

 structure is thus to a certain extent explained. On collision the 

 part of the substance of the meteorite given up will depend on the 

 temperature, and thus a mass of metallic iron mixed with silicates 

 at low temperature will get rid of the iron at once, which must 

 then perforce condense in a separate swarm ; therefore under low 

 temperature conditions, say at aphelion, irons alone will be formed 

 and the stones will become spongy. The stones will absorb the 

 C and H vapours. 



' Natvrf, vol. xvi. p. 4-3. 



