Page:The Scientific Monthly vol. 3.djvu/547

 WSAT WE KNOW ABOUT COMETS 541

with &e result that their tail Bpectro were proved to be very diSerent from the prevailing spectra of comets' heads and inner tails. Fowler has succeeded in duplicating the tail spectra of these two comets, in his laboratory, with remarkable agreement (Fig. 13), by photographing a cathode spectrum of carbon monoxide in a tube reduced to pressure not exceeding .01 mm. At higher pressures than this he obtained the so-called hydrocarbon spectrum, but it was not certain, and in fact it was improbable, that there was any hydrogen in the tube. The pres- ence of carbon and nitrogen in comets is certain, the presence of oxygen is probable, and the presence of hydrogen is doubtful.

��Fia. 18. (a) Ordinary pbatogTsph of Comet Horebouie. (b) Spectniin photo- sraph of Comet Morebouae made at aame time aa (a), (o) Fowler'a apectmin of carbon monoxide, whoae principal banda match the principal apectmm Images of the comet'a tall.

The comets which have approached very close to the sun turned to a yellowish orange in color and remained so while in the vicinity of the sun, because the yellow light of sodium then developed strongly in them, apparently by virtue of the intense heating of the cometary mat- ter by the sun's rays. This happened with the Wells comet of 1882, tbe great comet of September and October, 1882, the briUiant comet in January, 1910, and others. When the September, 1882, comet was only a few hundred thousand miles from the sun, Copeland and Lohee observed not only the sodium lines but half a dozen other bright lines which they concluded were well-known iron lines.

What ia the origin of the light which gives bright lines and bands? The sodium lines certainly, and the iron lines if actually observed, were no doubt due to incandescent vapors of those elements under the in- tense heat of the sun. Strangely enough, when the sodium comets ap- proached the sun, the carbon bands, which had previously been promi-

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