And then, what enormous periods of time must have been required to distribute the fragments of a single comet so widely that one would be found returning to its perihelion more than two centuries after another! When I spoke of one member of the Biela group being in aphelion when another would be in perihelion, I was speaking of a difference of only three and one-third years in time; and even that would require thousands of years. But the scattered cometic bodies which returned to the sun's neighborhood in 1668 and 1887 speak probably of millions of years which have passed since first this comet was formed. It would be a matter of curious inquiry to determine what may have been the condition of our sun, what even his volume, at that remote epoch in history. * * * * * THE ISOLATION OF FLUORINE. The element fluorine has at last been successfully isolated, and its chief chemical and physical properties determined. Many chemists, notably Faraday, Gore, Pflaunder, and Brauner, have endeavored to prepare this element in the free state, but all attempts have hitherto proved futile. M. Moissau, after a long series of researches with the fluorides of phosphorus, and the highly poisonous arsenic trifluoride, has finally been able to liberate fluorine in the gaseous state from anhydrous hydrofluoric acid by electrolysis. This acid in the pure state is not an electrolyte, but when potassium fluoride is dissolved in it, a current from ninety Bunsen elements decomposes it, evolving hydrogen from the negative and fluoride from the positive electrode. [Illustration: (+) (-) | | | | __/_|_\_A __/_|_\_A | | | | | | |____|____| |____|____| | | | | | | _____| | | | | |_____ / ---- | | | | -----\ // | | | | | | \\ || F |===|===| |===|===| H || || |- -|- -| |- -|- -| || || | - | - | | - | - | || || |- -|- - \_________/ - -|- -| || || | - | - - - - - - - - - | - | || // \___________________________/ \\ ]