50 



KNOWLEDGE 



[March 1, 1894. 



througli a depth of sixty thousand miles. When we pass 

 beyond the limb we look' through one hundred and twenty 

 thousand miles of chromosphere, and every minutest point 

 of light from the chromospheric region just outside the 

 Hmb is the summation of every variety of density, from the 

 rarest upper layers to the densest which rest on the 

 photosphere itself. Indeed, since the chromospheric light 

 which comes to us from close to the limb comes not only 

 from the lowest but also fi'om the highest strata, we might 

 even assume that only the highest stratum had any real 

 existence. A thin hollow shell, everywhere removed three 

 thousand six hundred miles from the photosphere, would 

 still appear to spring from the limb. 



This fact, that it is the sum of a great number of super- 

 posed strata which are represented by a single point at the 

 sun's limb, has a yet wider appHcation. At the lowest 

 stratum of the chromosphere we get the region of metallic 

 vapours, which by their absorption give rise to the Fraun- 

 hofer lines of the solar spectrum, " the reversing layer," 

 as it is commonly called. Above the chromosphere we 

 get a region yielding faint lines of hydrogen, seen only 

 during total eclipses. Above this we get the corona, 

 properly so called, and above the corona we get those vast 

 filmy extensions which Newcomb was able to trace to 

 twelve solar diameters from the limb. We look through 

 all of these whenever we scrutinize the sun's surface, and we 

 see, not only that surface itself, but the integration of 

 everything between us and it — reversing layer, chromo- 

 sphere, prominences, cool hydrogen, corona, and streamers, 

 and if there be anything beyond them, it too adds its 

 quota to the general total. Yet, so little do these appear 

 to be present, so little do they affect the distinctness of 

 our view, that it requires a very distinct mental effort to 

 realize, as we look at the chromosphere beyond the limb, 

 that precisely the same layer lies over every point of the 

 general disc. We have only to bear in mind the dimming 

 and distorting effect of our own atmosphere, which, after 

 all, in its total depth is only equal to five miles of air at 

 the standard sea-level pressure, to be quite sure that all 

 these various appendages have very little substance in 

 them after all, or it would be far more difficult to photo- 

 graph the solar spots and mottling than it is to map out 

 the seas and continents of Venus. 



Indeed, it is not impossible that the amount of 

 atmosphere above every square foot of surface of that 

 planet may be actually greater than over a similar area of 

 the sun. Glancing down the solar spectrum, we have no 

 difficulty at all in picking out which are the densest and 

 blackest groups of lines. Except for the two giants H and 

 K— which stand, hke Jachin and Boaz, at the gate from 

 the visible to the invisible spectrum — and the probably 

 fortuitous crowding of lines at G, the most prominent 

 groups are those of oxygen, A, B, and a, or, in wet weather, 

 the aqueous bands. In other words, the bands due to the 

 influence of our own atmosphere rival or exceed those 

 proper to the sun. 



Just above I suggested, by way of illustration, a density 

 for the chromosphere at its base equal to that of mercury, 

 and at its summit of one hundred millionth of an atmos- 

 phere. Both these are inadmissible. The Fraunhofer 

 lines of hydrogen are so narrow and sharp — F showing 

 only a slight fringe — that we are certain they can only be 

 produced at low pressure. One hundredth of an atmosphere 

 would probably not be far fi-om the mark, that is to say, 

 about one millionth the density of mercury. But, on the 

 other hand, the upper surface of the chromosphere is far 

 too distinct, definite, and brilliant for so great a rarity as 

 the hundred millionth of an atmosphere to represent its 

 condition there. There are, then, only two alternatives ; 



the one to assume a far higher temperature for the 

 chromosphere than 35,000° C. — an assumption quite 

 inadmissible on our present knowledge, for as we regard 

 the photosphere as a condensation surface we are unable 

 to adopt a temperature for it that would render carbon and 

 silicon permanent gases — the other to adopt Mr. Eanyard's 

 contention that we have not to do with a statical atmos- 

 phere in the chromosphere, but with molecules of hydrogen 

 moving in free paths. We should probably require a 

 temperature of fully 1-10,000' C. to give us a statical atmos- 

 phere, the base and surface densities of which would, in 

 the least, fit in with the observed facts of the case. And 

 the total amount of gas above every square foot of the 

 solar surface would only be about five and a half times 

 that above the same unit on the earth. 



Outside the chromosphere we meet with a region of less 

 brilliant hydrogen some seven minutes in depth. If the idea 

 of a chromospheric atmosphere must be rejected, much more 

 must we reject that of a coronal atmosphere. In any case 

 even two hundred thousand miles depth of hydrogen fails 

 to produce absorption lines in the solar atmosphere to 

 surpass those which our own shallow envelope can produce. 

 ^^'e may accept it as certain that the extensions which lie 

 above this inner corona are much more attenuated, and 

 that to find an adequate comparison for them we must not 

 refer to mists or fogs, but to the unsubstantial phenomena 

 of cometary tails and streamers. To speak, as some have 

 done, of the corona as if it were an important part of the 

 sun, and therefore to regard the solar diameter as amount- 

 ing in reality to so many millions of miles, is about as 

 absurd as it would be to regard the edge of a Channel fog 

 as the veritable coast-line of England. 



Apart from the failure of the corona to give any sub- 

 stantial evidence of its existence by selective absorption, 

 or by any markings which we can distinctly recognize as 

 coronal when seen projected on the disc of the sun, the 

 fact of the low density of the sun as a whole is a strong 

 argument against ascribing any appreciable density to its 

 surroundings. Its mean density is but one quarter of that 

 of the earth, even if we suppose that it is entirely com- 

 prised within the photosphere ; or to put it another 

 and better way, only about one thousand one hundred 

 times that of air. If, then, we imagine the corona and 

 chromosphere to contain any great amount of matter — 

 above all, if we imagine a real solar atmosphere extending 

 some considerable distance above the photosphere — we 

 lower the mean density of the part of the sun below that 

 level, and at the same time, by thus setting up an appreci- 

 able surface pressure, we render it more difficult to under- 

 stand how it is that, with the enormous solar gravity and 

 a radius of four hundred and thirty thousand miles, the 

 mean density does not exceed what we are able very easily 

 to produce m our terrestrial laboratories. ■ 



If the amount of matter above every square foot of the 

 solar photosphere were equal to that above the same unit 

 of the earth's surface, then the total mass of the sun's 

 surroundings would bear one twenty-seventh of the 

 ratio to the mass of the sun that our atmosphere 

 does to the earth. Probably this is under the mark, but 

 on the other hand, we can scarcely suppose there is 

 twenty-seven times the amount of matter to the unit of 

 surface, as this would render the reversing layer and the 

 chromosphere far denser than we can imagine. On the 

 whole, then, the entire system of the sun's appendages, 

 chromosphere, prominences, corona, and streamers, prob- 

 ably do not form so important a part of the sun, so 

 far as their total mass is concerned, as our atmosphere 

 does of our own world; whilst they are, relatively, dis- 

 tributed over a much greater extent of space. 



