68 THE POPULAR SCIENCE MONTHLY. 



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 at- 

 mosphere 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 mo- 

 mentarily 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 con- 

 ditions they are distinguishable only at the expense of much atten- 

 tion 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 bor- 

 dering 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 be-< 

 lieve 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 pow- 

 erful telescopes, they could doubtless be resolved into more re- 

 duced 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 con- 

 sider them as in some way resembling our seas, and to suppose 

 the existence of an atmosphere around the planet, with condensa- 



