Note 12 — Front matter (continued 3)

According to Bredikhine, the long straight tails, as seen in the comet of 1861, are composed of hydrogen; the long curved tails, like the principal tail of Donati's Comet, consist largely of hydrocarbon vapors; while the somnolent, rare, short tails of violent curvature are made up of mixed iron, sodium and other metallic vapors. This classification has received support from spectroscopic evidence. In 1882, Fitzgerald first propounded the theory that the tail was due to the pressure of light upon the gaseous matter composing it. In 1900 Arrhenius revived the theory, but modified it to the extent of supposing the tail to consist, not of gaseous matter, but of fine particles produced by condensation from the emanations of the comet.—New International Encyclopædia.