October 1, 1H97.] 



KNOWLEDGE. 



233 



indeed, are spectral pbenomena the only gronnds for 

 holding chromospberic and coronal materials to be in a 

 high degree subtle. The inappreciably resisted passage 

 through them of sundry comets has time after time demon- 

 strated the fact. What is new is the discovery that precisely 

 this subtlety prescribes the character of theu- spectrum, so 

 far at least as calcium is concerned. For heat influences, 

 ■we must remember, were, in the Tulse Hill experiments, 

 virtually eliminated by being kept moderate and uniform. 

 T.'ie spark employed to produce luminosity was of unusually 

 small intensity — was, indeed, " purposely made as little hot 

 as possible in order to emphasize the important fact that 



SPARK SPECTRA SHEWING EFFECT OF DENSITY ON THE 

 RELATIVE INTENSITIES OP THE LINES OF CALCIUM- 



^ipb^m 



K ^;^ 



^27 



MUGGINS 



IS97 



the determining condition of the spectral changes under 

 discussion was not one of increase of temperature." Our 

 authors add the following remarks : — 



" In the modifications of the calcmm spectrum arising 

 from variations in the relative intensities of the lines 

 which have been discussed in this paper, and which 

 correspoad to those observed in the celestial bodies, there 

 does not appear to us any reason for assuming, much less 

 any direct evidence in favour of, a true dissociation of 

 calcium — that is, its resolution into chemically different 

 kinds of matter. It would be remarkable if, by decom- 

 position through increase of temperature, a large number 

 of lines of a spectrum should become relatively enfeebled, 

 and that as the result of decomposition a spectrum should 



become simpler, and not, as analogy would suggest, more 

 complex." 



While the chromospberic spectrum of calcium, as at 

 present known, consists of H and K only, it may eventually 

 be found to include a second pair of lines too highly refran- 

 gible for registration by the methods available at Chicago 

 or Paris. For in Sir William and Lady Huggins's 

 photographs, radiations at A3159 and AB179 showed 

 persistently, imitating the behaviour of H and K, and 

 belonging presumably to the same immediate vibratory 

 system. They are too remote for convenient representa- 

 tion in the plate. Their importance to spectral theory 

 may yet prove very great. 



The Fraunhofer lines due to calcium are numerous, a^^d 

 in several instances conspicuous. But H and K, dusky 

 and diffuse, markedly dominate over the rest. A curious 

 circumstance regarding the couple as thus presented in 

 the ordinary solar spectrum has lately been brought into 

 evidence. It appears that they integrate the absorption 

 exercised at two different levels in the sun's atmosphere. 

 The central bands are produced in the reversing layer ; 

 the " wings ' are appended lower down near the photo- 

 sphere in a region of greater pressure. This dual origin of 

 Fraunhofer's H and K was inferred a couple of years ago by 

 Mr. Jewell of Baltimore, on the ground of certain spectro- 

 graphic measurements ; it was amply confirmed by Mr. 

 Shackleton's Novaya Zemlya photograph of the prismatic 

 " flash " at the edge of the eclipsed sun, in which the 

 famous calciiim pair appear strong, yet sharply terminated. 

 This state of things indicates an unexpected degree of 

 rarity in the calcium ingredient of the reversing layer — a 

 degree of rarity which we are no longer entirely without 

 the means of gauging. One of the most significant 

 results of Sir William and Lady Huggins's latest investi- 

 gation is to afford a standard of comparison on this 

 point ; and out of the series of their photographs 

 we should be inclined to select No. 4, marked " D," 

 as the best match for the eclipse record. It shows, we 

 may remind our readers, the spectrum of a spark passing 

 between platinum terminals, from which a solution of 

 calcium chloride had been washed off with water. The 

 density of the ignited calcium vapour must then have been 

 slight indetd, and it can scarcely, in the reversing layer, 

 exceed a fraction of an atmosphere, even allowing for the 

 fining-off effect upon spectral rays of production in a 

 heterogeneous mixture. True, Messrs. Jewell, Mohler, and 

 Humphreys last year estimated the pressure of calcium 

 in that stratum at from three to six atmospheres,'^ but on 

 grounds as yet imperfectly understood and highly pre- 

 carious. Most noteworthy is the fact that it remains 

 sensibly uniform through a depth of at least two hundred 

 and fifty miles. Yet, under the influence of solar gravity, 

 the density of a gaseous substance in atmospheric equili- 

 brium should increase twofold for each furlong of descent. 

 In a single mile there would be eight such doublings ; 

 between the top and bottom of ihe reversing layer 

 the gain in compactness should be at least 2-'"'"-fold — a 

 number meaningless from its immensity. Since no sign 

 of the exertion of this enormous power is traceable 

 until the low-l)ing stratum is reached, where H and K 

 acquire the shadowy borders displayed in the Fraunhofer 

 spectrum, gravity must be in some way counteracted, 

 la u-Iiat way remains problematical. 



Anomalies in the distribution of materials near the sun 

 may accordingly be looked for, and the vast heights to 

 which sparsely scattered metallic molecules apparently 

 ascend cease to be incredible. Much of its force thus 



* Astrophysical Journal, \o\. III., p. 138. 



