Nov. 24, 1887] 



NATURE 



85 



we find ourselves in a position of some difficulty, but it would 

 appear that future work may enable us really to divide stars of 

 the second class into two series, and if we can do so there is 

 very little doubt that one series will represent the phenomenon 

 of decreasing temperature of the absorbing layers, while the 

 other series will represent the phenomenon of increasing 

 temperature. 



What considerations are likely to help us in such an inquiry 

 as this ? The atmosphere of a star built up by meteorites should 

 resemble in its constitution the totality of the chemical constitu- 

 tion of meteorites, and therefore it might be inferred that the 

 spectroscopic phenomena presented by such an atmosphere 

 would not be widely different from the spectroscopic pheno- 

 mena presented by the vapours of many meteorites volatilized 

 together. 



To investigate this question I have obtained composite photo- 

 graphs of the spectra of several meteorites, with a solar spectrum 

 for purposes of comparison. I find that, while, on the one 

 hand, the composite photograph giving us the spectrum of 

 the meteorites greatly resembles that of the sun, as it should 

 do, there are some variations which suggest the line of separa- 

 tion to which I have before alluded. From Dr. Huggins's mag- 

 nificent photographs of the stars we have learned that, as I 

 had predicted years before the photographs were taken, the 

 thickness of H and K varies very greatly in different stellar 

 spectra. In those stars, presumably the hottest ones, in which 

 we get the series of hydrogen lines almost alone as great ab- 

 sorbers, K is almost absent ; it finally comes in, however, and 

 after a certain stage has been reached it is the most important 

 line in the spectrum. But there are stars in which the lines h 

 and G of hydrogen are not very much more developed than they 

 are in the case of our own sun, in which K is much thinner than 

 in the solar spectrum ; and associated with this condition of K 

 there is the absorption of a hydrogen line more refrangible 

 than K at wave-length 3800, which is not represented in the 

 solar spectrum with anything like the intensity. The question 

 arises, therefore, whether the enormous thickening of K ob- 

 served in the sun and some other stars may not be limited to 

 those stars which, like our sun, are reducing their temperature ; 

 for we certainly are justified in assuming that the temperature of 

 the sun now is not so high as it was in an earlier stage of the 

 development of the system. Such a difference as that, if it is 

 subsequently established, can only come from the atmo- 

 sphere, as an effect of cooling, becoming richer in those sub- 

 stances the lines of which get broader as the star cools down. 

 We can easily imagine that during the process of cooling the 

 relative quantities of the vapours should not always remain 

 constant, although it is impossible in the present state of our 

 knowledge to give any particular reason why such and such 

 vapours should disappear from the spectrum in consequence of 

 chemical combination, while others should develop apparently in 

 consequence of their retirement. 



Hydrogen phis Carbon indicates Mixed Swarms. 



If we assume a brightening of the meteor-swarm due to 

 collision as tlie cause of the so-called new stars, we have good 

 grouiids for supposing that in these bodies the phenomena should 

 be mixed, for the reason that we should have in one part of the 

 swarm a number of collisions probably of close meteorites, 

 while among the out-liers the collisions would be few. We 

 shall in fact have in one jiart the conditions represented in 

 Class III. a, and in the other siich a condition as we get in y 

 Cassiopeiae. I have in another part of this paper discussed the 

 flutings observed in Nova Orionis, and have shown that so far 

 as they were concerned we have the radiation of carbon and the 

 absorption of manganese ; but there is evidence to show that 



with these fluted appearances bright lines were observed D, 



and F, although no mention is made of C,^ 



We have here, there is little doubt, the vera causa of stellar 

 long-period variability. 12 per cent, of stars of Class Ill.a are 

 variable, and 9 per cent, of Class 1 1 1. 3. In the one case, meteor- 

 swarms produce the increased brightness by colliding with those 

 of the condensing one. In the other, they do so by their peri- 

 astron passage round the dim condensed one. There is no 

 variability, in the usual sense of the word, in stars like the sun 

 and a Lyrse, and the reason is now obvious. 



' Konkoly, Astr. Nach. 2712, D3 and F; Ricci indicates D3 in Astr. Nach 

 2707, 



The Conditions of Collisions of Meteorites, 



The Chemical Elements most frequently determined in Meteorites. 



I think it well to give here as a reminder a short table showing 

 the chief substances met with in meteorites. It will indicate the 

 cause of the continued reference to the spectra of Mg, Fe, and 

 Mn in what follows. 



SIDERITES. 



Nickel-iron, copper, manganese. 



Troilite = FeS. 



Graphite. 



Schreibersite = iron and nickel phos- 

 phide. 



Daubreeite = iron and chromium sul- 

 phide. 



SIDEROLITES, 



CHONDRITIC— 



(a) Non-carbonaceous = Olivine = chrysolite = peridot = 



(MgFe)204Si = SiOa 4I -3, MgO 



50*9, FeO 77. 

 Enstatite MgOgSi = SiOj 60, MgO 



40. 

 Bronzite = enstatite in which some 



Mg is replaced by Fe. 

 Nickel-iron, manganese, 

 Troilite. 



Chromite = iron protoxide 32, chro- 

 mium sesquioxide 68, -f -Al and 



Mg. 

 Augite = pyroxene, SiOjSS, Ca023, 



MgO 16, MnO O'S, FeO 4. 

 Silicate of calcium, sodium, and 

 aluminium. 



(5) Carbonaceous .. 



NON-CHONDRITIC 



Carbon in combination with H and O. 

 Sulphates of Mg, Ca, Na, and K. 



Anorthite. 



Enstatite. 



Bronzite. 



Olivine. 



Augite. 



Troilite. 



The Numbers oj Meteorites in Space, 



It is well known that observations of falling-stars have been 

 used to determine roughly the average number of meteorites 

 which fall on the earth each twenty-four hours ; and having this 

 datum to determine the average distance apart between the 

 meteorites in those parts of space which are traversed by the 

 earth as a member of the solar system, Dr. Schmidt, of Athens, 

 from observations made during seventeen years found that the 

 mean hourly number of luminous meteors visible on a clear 

 moonless night by one observer was fourteen, taking the time of 

 observation from midnight to i a.m. 



It has been further experimentally shown that a large group of 

 observers who might include the whole hemisphere in their ob- 

 servations would see about six times as many as are visible to 

 one eye. Prof. H. A. Newton and others have calculated that 

 making all proper corrections the number which might be visible 

 over the whole earth would be a little greater than 10,000 times 

 as many as could be seen at one place. From this we gather 

 that not less than twenty millions of luminous meteors fall upon 

 our planet daily, each of which in a dar!< clear night would pre- 

 sent us with the well-known phenomenon of a shooting-star. 



This number, however, by no means represents the total 

 number of minute meteorites that enter our atmosphere, because 

 many entirely invisible to the naked eye are often .seen in tele- 

 scopes. It has been suggested that the number of meteorites if 

 these were included would be increased at least twenty-fold : this 

 would give us 400 millions of meteorites falling on the earth's 

 surface daily. If we consider, however, only those visible to 

 the naked eye, and if we assume that the absolute velocity of the 

 meteors in space is equal to that of comets moving in parabolic 

 orbits. Prof. H. A, Newton has shown that the average number 

 of meteorites in the space that the earth traverses is in each 

 volume equal to the earth about 30,000. This gives us a result 



