If we know but little about the actual surface of Uranus, we know now a good deal about its atmosphere. And this partly because atmosphere is almost all that it is. The satellites are the only solid thing in the system. If we needed a telltale that the solar system had evolved, the gaseous constitution of its primaries and the condensed state of their attendants would sufficiently inform us. Probably all the major planets are nothing but gas. It has been debated whether Jupiter be almost all vapor with a solid kernel beneath, or vapor entirely. That he grows denser toward the core is doubtless the case, but that he is anywhere other than a gaseous fluid is very unlikely. For if he had really begun to condense, he must have contracted to far within his present dimensions. The same is true of Uranus. The surprising thing about Uranus is the enormous extent of his atmosphere. The earliest spectroscopists perceived this, but the more spectroscopy advances, the greater and more interesting it proves to be. By pushing inquiry into the red end of the spectrum, hitherto a terra incognita, Dr. Slipher has uncovered a mass of as yet unexplained revelation. Of these remarkable spectrograms we shall speak later. Here it is sufficient to say that so great is the absorption in the red that only the blue and green in anything like their entirety get through; which accounts for the well-known sea-green look of the planet. Furthermore, the spectroscope shows that this atmosphere, or the great bulk of it, must lie above what we see as the contour of the disk. For the spectroscope is as incapable of seeing through opacity as the eye, though it distances the eye in seeing the invisible. It is not what is condensed into cloud, but what is not, of which it reveals the presence. We are thus made aware of a great shell of air enveloping the planet. In Uranus, then, we see a body in an early amorphous state, before the solid, the liquid, and the gaseous conditions of matter have become differentiate and settled each into distinctive place. Without even an embryo core its substance passes from viscosity to cloud. Neptune has proved a planet of surprises. Though its orbital revolution is performed direct, its rotation apparently takes place backward, in a plane tilted about 35° to its orbital course. Its satellite certainly travels in this retrograde manner. Then its appearance is unexpectedly bright, while its spectrum shows bands which as yet, for the most part, defy explanation, though they state positively the vast amount of its atmosphere and its very peculiar constitution. But first and not least of its surprises was its discovery,—a set of surprises, in fact. For after owing recognition to one of the most brilliant mathematical triumphs, it turned out not to be the planet expected.