state. This coincidence of lack of hydrogen with lack of liberty takes on yet more significance from the further fact that the same is not true of oxygen, water vapor, or indeed of any of the other gases we know. With them, freedom is not synonymous with absence. The Earth’s atmosphere contains plenty of free oxygen, nitrogen, and the like. But, as we have just seen, the maximum speed of all these gases falls short of the possibility of escape. This accounts for their presence. They have stayed with us solely because they must.
The appearance of the other heavenly bodies seems to confirm this conclusion. The Moon, for example, possesses no atmosphere, and calculation shows that the velocity it can control falls short of the maximum of any of our atmospheric gases, that velocity being but one and one half miles a second. All were, therefore, at liberty to leave it, and all have promptly done so. On the other hand, the giant planets give evidence of very dense atmospheres. They have kept all they ever had.
But the most striking confirmation of the theory comes from the cusps of Venus and Mercury; for an atmosphere would prolong, by its refraction, the cusps of a crescent beyond their true limits. Length of cusp becomes, consequently, a criterion of the presence of an at-
- See Appendix.