And now let us return to the mystery of comets’ tails. That we are fully justified in speaking of the tails of comets as mysterious is proved by the declaration of Sir John Herschel, who averred, in so many words, that “there is some profound secret and mystery of nature concerned in this phenomenon,” and this profound secret and mystery has not yet been altogether cleared up. Nevertheless, the all-explaining hypothesis of Arrhenius offers us once more a certain amount of aid. Comets’ tails, Arrhenius assures us, are but another result of the pressure of light. The reader will recall the applications of this theory to the Zodiacal Light and the Aurora. In the form in which we now have to deal with it, the supposition is made that as a comet approaches the sun eruptions of vapor, due to the solar heat, occur in its nucleus. These are naturally most active on the side which is directly exposed to the sun, whence the appearance of the immense glowing envelopes that surround the nucleus on the sunward side. Among the particles of hydro-carbon, and perhaps solid carbon in the state of fine dust, which are thus set free there will be many whose size is within the critical limit which enables the light-waves from the sun to drive them away. Clouds of such particles, then, will stream off behind the advancing comet, producing the appearance of a tail. This accounts for the fact that the tails of comets are always directed away from the sun, and it also explains the varying forms of the tails and the extraordinary changes that they undergo. The speed of the particles driven before the light-waves must depend upon their size and weight, the lightest of a given size traveling the most swiftly. By accretion certain particles might grow, thus losing velocity and producing the appearance of bunches in the tail, such as have been observed. The hypothesis also falls in with the researches of Bredichin, who has divided the tails of comets into three principal classes—_viz.:_ (1) Those which appear as long, straight rays; (2) Those which have the form of curved plumes or scimitars; (3) Those which are short, brushy, and curved sharply backward along the comet’s path. In the first type he calculates the repulsive force at from twelve to fifteen times the force of gravity; in the second at from two to four times; and in the third at about one and a half times. The straight tails he ascribes to hydrogen because the hydrogen atom is the lightest known; the sword-shaped tails to hydro-carbons; and the stumpy tails to vaporized iron. It will be seen that, if the force driving off the tails is that which Arrhenius assumes it to be, the forms of those appendages would accord with those that Bredichin’s theory calls for. At the same time we have an explanation of the multiple tails with which some comets have adorned themselves. The comet of 1744, for instance, had at one time no less than seven tails spread in a wide curved brush behind it.