May 2, 1895 J 



XATURE 



tlii: nchula ordinarily seen is hul the brightest part of a nebula 

 . sterling over a space in the surrounding; neij;hl>ourho<id, which 

 lecem research shows is scarcely limited to the whole con- 

 ^ellatlon. 



Now, it so happens that the spectrum o the nebula of Urion 

 has recently been very carefully studied from the point of view of 

 the cheiaical substances which may be buildiii}; up this special 

 s]K-clroscopic tyjie. Here is a photograph of a pait of the 

 spectrum of the nebula of Orion ; and I may tell you ;hat it is a 

 very ilifiicilt thing to obtain a photograph of such a very feeble 

 light sourc". It is a copy of a photogra])h which was exposed 

 for sonicthjig like three hours at the focus of a 30-inch mirror 

 of short fuels ; and in copying it we lose a great deal of the 

 detail, a grtat many of the lines which are recorded by Dame 

 ;.ature hersdf in what we call the negative. The negative 



Fig, 23, — The Great Nebuln in Orion, from a long exposure photograph 

 by Dr. Kot)erl>. 



contains something like fifty lines, which have already been 

 measured ; but in the attempt to enlarge, a gieat many of these 

 have been left behind. 



Vou will see, however, without any difficulty, that the spec- 

 irum shows many bright lines; that being so. an attimpl 

 lias been made to deteimine the positions of all ol thini. The 

 result is really extremely interesting. \Vc find, in fact, that 

 there may in all probability be three perfectly different i-ources 

 of the btij;ht lines which, taken together, build up the so-called 

 spectrum of the nebula. In the first place, I showed 50U that 

 when we experiment with meteoritic dust in our laboratories, 

 it has not been subjected to a low presiuie very U.ng before it 

 begins to give out certain compounds of carbon, mingled 

 "i'h hydrogen gas, \\\A we find that in the nebula of Orion 



NO. 1331, VOL. 52] 



we really do get indications of gaseous comixiunds of carbon, 

 and also of the gas hydrogen. In order to make the distinction 

 perfectly clear between the two other possible sources of 

 nebula lines, let me ask you for one moment to conceive 

 yourselves in the middle of the gigantic battle which is going on 

 between meteoritic particles in such a nebula as that of Orion. 

 You have particles rushing together in all possible directions, 

 particles, no doubt, difterent in origin. You will expect, among 

 those millions and billions and trillions of collisions, to get a 

 very considerable number of grazes; and the whole point of 

 collisions among physical particles is that, if two things go 

 straight at each other, you get what you may call an end-on 

 collision, which may be bad for one or both of the Ixidies con- 

 cerned : physically we may say the teinpcraturc under 

 these circumstances is at a ma.xinnnn. Kut you will under- 

 stand that the number of grazes, or near misses, must be 

 \er\- much greater than the mmiber of end-on collisions ; in 

 such a case as we are imagining, there will be an immense 

 number of grazes. What will a graze do? It is simply a 

 slight collision : the amount of temperature developed b> 

 it will be small: we .shall therefore get the production 

 of vapours at a low temperature, and if we get any luminous 

 effect at all, it will be (me proper to the vapours at low tempera- 

 ture. So that on first principles we should expect in such a 

 nebula as the one we are discussing to get a very large number 

 of grazes, giving us low temperature eflects, and a very much 

 smaller luimber of end-on collisions, giving us very high tem- 

 persiture effects. Now, what are the facts ? We .say the inosl 

 numerous collisions are partial ones, grazes. Well, there is the 

 fluting, most probably due to magnesium atA5oo, and that fluting. 

 of magnesium is the lowest temperature indication of the exist- 

 rnce of magnesium ; if magnebium becomes luminous at all by 

 virtue of its temperature, one of the first things revealed to us 

 spectroscopically is the particular fiuting in question. We 

 may also note the longest lines seen in the oxy-hydrogen flame 

 of iron, calcium and magnesium as well. Those lines we are 

 justified in considering as indications of an enormous number of 

 grazes among these meteoritic particles. liut that is not all. 

 Going further, we find that there will be a small number of end- 

 on collisions giving us the highest possible temperature.. Being 

 students of science, we are of course anxious to know what con- 

 <litioiisare present in a case of that kind ; that is, we want to 

 know what the possible results of the highest temperature will 

 be. The natural thing, I think, is to go to the sun, which Ls 

 pretty hot. and then find out the very hottest place, which we 

 can do by means of our spectroscojjes, and then study very care- 

 killv. for vears even, the s]iectroscopic indications in that parti- 

 i ularly hiittcst place of the nearest star which we can get at. I 

 hope you will acknowledge that that is a ])hilosophic way of 

 going to work. Thus we are landed in what is called the 

 chromosphere of the sun. The upper atmosphere of the sun 

 must be rapidly cooling, but the chromosphere is a thin envelope 

 some 5000 or icooo -miles thick, just outside the photosphere, 

 agreed to be the hottest part of the sun w ithin our ken, and there- 

 fore anv lines which we see special to that region are called 

 chromo'spheric bnes, and they sh<iuld be proper lo high temix-ra. 

 u.\s 



■ The chromospheric line Dj represents a line near the sodium 

 line I) in the solar s|)ectrum, which with a few others has the 

 proud pre-eminence of nearly always being bright ; hence we 

 supp.ise that we have something hotter than .anything else 

 which exists at the exterior level of the solar photosphere. 

 Running in couples with this line \\ there is another in the blue 

 part or the spectrum, represented by a certain wave-length (447 1) 

 which behaves alwavs in the same way, /.(•. it is almost always 

 seen very bright, and it is never seen dark among the Fraunhofer 

 lines in the solar spectrum. Krom the solar point of view then, 

 as the sun is a thing that we can get at better than any of the 

 other stars, because it is so near to us, a mere trifle of 90 millions 

 of miles or so, we are justified in saying that these two lines 

 repre.sent. in fact, the spectrum of the hottest part of space 

 about which we can be alxsolutely certain. Hence it is very 

 interesting to inquire whether or not these two lines exist in the 

 nebuhv as representing the high temjierature results of end-on 

 collisions. 



They do exist in nebula:, and in some of them thev are among 

 the most striking indications in the spectrum. 



So that we find in the spectrum of the nebula of OriiW, when 

 it is carefully .studied, indications of the gases which are known 

 to be occluded in meteorites, and which are perfectly prepared 

 to come out of them the moment you give them the least 



