luminous, more so at some points than at others; and sometimes we can see on its borders bright white regions that remain in sight several days in succession, but are generally changeable, and show themselves sometimes in one place and sometimes in another. I attribute these phenomena to condensations going on in the atmosphere of Mercury, which reflects more light into space the more opaque it becomes. Similar white regions are also often seen in the interior of the disk, but they are not so brilliant there as on its border. Further, the dark spots of the planet, while they are permanent as to form and arrangement, are not always equally evident. They are sometimes more intense, at other times paler. Sometimes, also, one or another of them will become momentarily invisible. Such peculiarities can not be attributed to any other cause than atmospheric condensations similar to our clouds, which veil the ground of the planet in different degrees, sometimes in one region, sometimes in another. An observer, looking from the depths of space upon the countries of our earth covered with clouds, would perceive a like spectacle.
Very little can be said of the nature of the surface of Mercury. We must recollect that three eighths of it are inaccessible to the solar rays and to sight; on that side, therefore, we have but slight hopes of ever learning anything certainly. It will also be hard to gain a correct and sure knowledge of the part we can see. The dark spots, even when they are not clouded, usually appear under the form of extremely thin trails of shadow. In ordinary conditions they are distinguishable only at the expense of much attention and weariness. Under the best conditions they have a brown, warm tint, like that of sepia; of a tone very indistinct upon the general color of the planet, which is usually of a clear rose bordering on copper. Forms or bands so vague and diffuse, with indistinct borders, always leaving a place for arbitrary definition, are not easily represented in a satisfactory manner. Still, I believe the indeterminateness of outlines is, in the majority of cases, only apparent, and a result of the insufficient optical power of the instrument; for the more perfect the view and the finer the image we get of the shadows, the more do we find them disposed to break up into a multitude of smaller details. By employing more powerful telescopes, they could doubtless be resolved into more reduced forms.
While it is so hard to make a good study of the dark spots of Mercury, it is not easy to express a well-founded opinion upon their nature. They might be attributed to the different materials composing the solid surface of the planet or to its structure, as we know is the case with the moon. But if we are disposed to consider them as in some way resembling our seas, and to suppose the existence of an atmosphere around the planet, with condensa-
