162 NOTES ON MARS. 



surrounded by air ma}^ at one epoch of its career have free ox^ygen as 

 an ingredient in its atmosphere, while at other epochs free oxygen 

 may be absent. This may arise from another cause besides the pos- 

 sible loss of the gas by diffusion into space from small globes in the 

 manner already explained. Indeed, it seems quite probable that the 

 oxygen in our own air is not destined forever to remain there. It 

 passes through various vicissitudes by being absorbed by animals and 

 then restored again in a free state under the influence of vegetation. 

 But there is an appetite for oxygen among the inorganic materials of 

 our globe which seems capable of using up all the oxygen on the globe 

 and still remain unsatisfied. We have excellent grounds for believing 

 that there is in the interior of the earth a quantity of metallic iron 

 quite sufficient to unite with all the free oxj^gen of the air, so as to form 

 oxide. In view of the eagerness with which oxygen and iron unite 

 and the permanence of the compound which they form, it is impos- 

 sible for us to regard the presence of oxygen in the air as represent- 

 ing a stable condition of things. It follows that, even though there 

 may be no free oxygen in the atmosphere of Mars, it is by no means 

 certain that this element has always been absent. It is, however, not 

 at all beyond the reach of scientific resources to determine what the 

 actual composition and extent of the atmosphere of Mars may be, 

 though it can hardly be said that as yet we are in full possession of the 

 truth. 



An almost equally important question is as to the telescopic evidence 

 of the presence of water on Mars. Here again we have to be reminded 

 of the fact that even at present, when the planet is relativel}^ so near us, 

 it is still actually a very long way off. It would be impossible for us 

 to sa}^ with certainty that an extent which by its color and general ap- 

 pearance looked like an ocean of water was reall}^ water, or was even 

 a fluid at all. It is so easy to exaggerate the capabilities of our great 

 telescopes that it maj^ be well to recount what is the very utmost that 

 could be expected from even our greatest instrument when applied to 

 the study of Mars. Let us consider, for example, the capabilities of 

 the Lick telescope in aiding such an inquiry as that before us. This 

 instrument, both from its position and its optical excellence, offers a 

 better view of Mars at the present time than can be obtained else- 

 where. But the utmost that this telescope could perform in the way 

 of i-endering remote objects visible is to reduce the apparent distance 

 of the object to about one-thousandth part of its actual amount. Some, 

 indeed, might consider that even the Lick instrument would not be 

 capable of giving so great an accession to our powers as this statement 

 expresses. However, I am willing to leave the figure at this amount, 

 only remembering that if I estimated the powers of the telescope less 

 highly than these facts convey, the arguments on which I am entering 

 would be correspondingl}^ strengthened. 



