that the electro-magnetic power would be favored with a good conductor for extending its control to great distances, and its effects can be traced without having recourse to any unwarranted assumptions respecting the passage of electricity through an absolute vacuum or through interstellar space.
The operation of such an agency in the heavens is shown by researches of a different character. M. Gaston Plante, of Paris, has been led by experimental evidence alone to ascribe the form of the spiral nebulæ to electro-magnetic action; as their peculiar features correspond exactly to that which he produced by powerful electric currents under the controlling influence of a magnet. But the influence of the same forces is strongly impressed on the form of another class of nebulous objects. By investigations similar to those of Laplace in regard to the possible extent of the solar atmosphere, it may be proved that a rare gas surrounding a dense nucleus and with a uniform rotation could not be compressed in a greater degree than to show a thickness two thirds of its equatorial dimensions. Yet in many nebulæa with a central condensation the greater diameter is more than four times the less, and this would seem to indicate the operation of some force like dynamic electricity acting along the plane of the equator of these rarefied objects. The evidence on this point will seem stronger when we recollect that observation gives only an inadequate picture of the effects of this cause; as, in consequence of the position in which they are viewed, planetary nebulæ scarcely ever exhibit the full amount of their ellipticity or compression.
Other facts assist in revealing the nature of the forces at work in these celestial curiosities. Judging from peculiarities they present in the spectroscope, Lockyer and Frankland have concluded that several of the nebulæ must possess an exceedingly low temperature. Yet it is difficult to conceive that such cold, rarefied masses could be self-luminous, or that they could be visible to us even when surrounding a central sun, for gases have but a very feeble power of reflecting light. The difficulty, however, may be removed by supposing that the visibility of these nebulæ depends on the passage through them of electricity developed in some dark or bright binary system on the incorporation of the lesser with the greater orb. In this way an explanation may be found for the mysterious and unaccountable variations in the brightness of these objects. From the careful observations of Hind, D' Arrest, and other astronomers, it has been shown that, in a few cases, nebulæ have declined in light so as to become invisible, but reappeared after a time; thus exhibiting changes equally fatal to the ideas that they are congregations of stars or collections of fire-mist gradually cooling and condensing into planetary systems. But the mystery will be removed when we regard their light as dependent on the electro-magnetic action already described; for in its latter stages, especially when the tides on the smaller member of the binary were drawing to a close, there would be
