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Comets and Their Trails.—In discussing these erratic bodies, Professor Zollner starts with the fact that fluids as water, mercury, and solids of nearly all kinds, give off vapor of low tension though in too small quantity to be recognized by any tests with which we are now acquainted. It therefore follows that the masses scattered throughout space are ultimately surrounded by an atmosphere of their own vapor. If the volume of such masses is too small to exert sufficient attractive force to retain this vapor, the whole mass ultimately assumes the vaporous state. Professor Zollner thinks that many of the small comets are such masses of vapor, while others are fluid, consisting of water or perhaps of liquid hydrocarbons, an idea which is fortified by the character of the spectra of certain nebulae as well as of some of the smaller comets.

Regarding the self-luminosity of comets and the formation of their trains, Professor Zollner says, there are but two causes which can produce the first of these results, viz: elevation of temperature and electric action. Setting the first aside as being utterly inadequate under the circumstances, the author thinks the electricity developed by the solar rays, either in process of evaporation or by the mechanical or molecular disturbance they produce, is amply sufficient to cause the luminosity, and also to form the train. The explanation here given of the formation of the tails or trains of comets, is exceedingly ingenious, for it not only applies in those instances in which the train is directed from the sun, acting under these circumstances by repulsion, but it also accounts for the fact that in some instances the tail is directed toward the sun, there being under these circumstances electrical attraction instead of repulsion.