232 



KNOWLEDGE. 



[October 1, 1897. 



motor periphery, where, through the muscles, the outer 

 world is acted on and other forms of energy are released, 

 all the processes vary only in place and in degree, and not 

 in dynamical character. We may see in the process of 

 cell formation something of chemical change, and an 

 influence by which latent energy is specially directed to 

 certain ends ; but how far physical energy enters into the 

 influence we have no perception. We have no evidence 

 that any one element that is produced in the body, that 

 any form of cell or similar organism, needs more energy 

 for its production than another. Indeed, we have no real 

 proof that any energy is needed. 



What we have said on this subject represents only a 

 peep through the keyhole into nature's laboratory, and the 

 inferences we have drawn from the observed phenomena 

 may be entirely wrong. However this may be, it would 

 seem that among the atoms and molecules, or foundation 

 stones of all hving things, there is a regular rhythm — 

 swing of the pendulum — from the atmospheric gases 

 through vegetable tissue to muscular iibre and back again ; 

 but we see it in outline only, as we see the horseman 

 riding swiftly past us in the twilight. 



CALCIUM IN THE SUN. 



By Miss Agnes M. Clerke, 



Author of " The System of the Stars," " A Popular History 

 of Astronomy during the Nineteenth Century," etc., etc. 



ONE of the most surprising facts in solar physics is 

 the wide diffusion of calcium vapour near the sun. 

 Difficult to explain, and impossible to have been 

 anticipated, it has, through the progress of photo- 

 graphicresearch, assumed fundamentalimportance. 

 A substance glowing with violet light of the well-known 

 H and K qualities mounts to the summits of the tallest 

 prominences, reaching, if not transcending, in spasmodic 

 outbursts, heights of some three hundred thousand 

 miles above the photosphere ; is wreathed into voluminous 

 spirals constituting faculse under a novel aspect, with 

 which the entire solar disc is found to be diversified ; 

 and radiates so profusely as to give rise, at certain epochs, 

 to double reversals in the general solar spectrum of such a 

 character as to transform the sun, yon tanto, into a bright- 

 line star. 



To designate this substance off-hand as calcium might, 

 indeed, until quite lately, have been deemed incautious, 

 since opinions were divided as to its true nature. And not 

 without some show of justification. The reasons that "gave 

 one pause" were twofold. They might be distinguished as 

 gravitational and spectroscopic. Was it conceivable, it 

 might be asked, that a vapour forty times more dense than 

 hydrogen should invariably be encountered at equal or 

 greater altitudes — at altitudes, moreover, far beyond the 

 apparent range of the lighter atoms of sodium and 

 magnesium, plentiful though they be lower down ? For 

 the high-level chemistry of prominences, so far as the 

 analysis of their light shows, is extremely simple. They are 

 composed of hydrogen, helium, and " calcium." It is true 

 that their ultra-violet spectrum includes a few unknown 

 lines, the future interpretation of which is, however, un- 

 likely to subvert the truth of the above statement. 



We have now to consider the vexed question of the 

 calcium spectrum. It comprises, when fully developed 

 in the electric arc, several scores of lines, some collected 

 into doublets and triplets, many others scattered singly 

 over the colour-scale. Among them a bright blue ray 

 of wive-length 4227 (ten-millionths of a millimetre) 



is conspicuous under ordinary conditions. Such, how- 

 ever, do not favour the development of H and K. At 

 the temperature of the Bunsen flame, these can barely 

 be got to show in long-exposure photographs. Electrical 

 excitement is needed to bring out their inherent strength, 

 which grows as if at the expense of their neighbour in the 

 blue. The progressive enfecblement of the latter, as the 

 arc is exchanged for the spark, suggests ultimate extinction 

 at a still higher pitch of molecular agitation, and the 

 reduction of the calcium spectrum — since all its visible 

 components throw in their lot with that at wave-length 

 4227 — to H and K, besides two similarly affected ultra- 

 violet pairs. These inverse symptoms of intensification 

 and fading in the violet pair and blue line respectively, 

 were connected by Sir J. Norman Lockyer with the 

 dissociative effects of high temperatures. He considered 

 that calcium, partially broken up in the laboratory, and 

 completely torn to pieces by the fierce solar heat, is 

 represented in the chromosphere and prominences by its 

 lightest constituent, a substance distinguished by the 

 emission of the H and K radiations dissevered from all 

 others. Now this hypothesis, plausible at first sight, 

 proves, on further inquiry, to be hampered by all manner 

 of difficulties, physical, chemical, and spectroscopic. But 

 their discussion has been rendered unnecessary by the 

 circumstance that the hypothesis has been shown to be 

 superfluous. 



In a paper read before the Royal Society, .June 17th, 

 1897,* Sir William and Lady Huggins announced their 

 success in simplifying the spectrum of calcium down to 

 the point exemplified in the solar prominences, by reducing 

 the density of the emitting vapour while its temperature 

 remained constant. The progression of their experiments 

 is shown in our plate. f The spark was, to begin with, 

 passed between calcium electrodes ; and the blazing aspect 

 (as it may be termed) of H and K, of the companion pair 

 in the ultra-violet, no less than of the blue line self-depicted 

 in Fig. A, corresponds to the considerable amount of the 

 metallic gas disengaged and illuminated. The marked 

 change produced by substituting a platinum electrode for 

 one of the calciums, comes out in Fig. B ; and it will 

 be noticed that it tells most on the lines of shortest wave- 

 length. Fig. C represents the effect of taking the spark 

 between platinum terminals moistened with a strong solu- 

 tion of calcium chloride. The fading of the blue Une is 

 obvious, its estimated intensity being now only one-fourth 

 that of H and K. Still more significant are the remaining 

 five photographs m the plate. They portray the spectra 

 obtained after successive washings of the platinum elec- 

 trodes, the arrangements remaining otherwise undisturbed ; 

 that is to say, the remnant of calcium vapour present was 

 rendered more and more tenuous, until its detection, even 

 by such an extraordinarily fine test as light-analysis 

 supplies, became a matter for surprise. At this final stage, 

 represented by the lowest figure of our plate, the condition 

 of the solar prominences was at last seemingly realized, 

 since H and K stood alone in the spectrum. This triumph 

 of artifice, thus for the first time achieved, led irresistibly 

 to the conclusion that the effect in the solar structures 

 was due to the same cause as in the laboratory — that the 

 irreducible minimum of radiation was arrived at, in the 

 one case as in the other, through the unimaginable 

 emaciation, so to speak, of the glowing substance. Nor, 



* Proceedings, Vol. LXI., p. 453. 



t Our illustration is reproduced from original photographs bv kiad 

 permissioi of Sir William and Lady Huggins, from the plate accom- 

 panying their paper in the Proceedings of the Soi/al Society, 

 Vol. LXI., No 377, p. 440. 



