Note 2 — OXYGEN IN THE SUN.

The case here imagined is not entirely hypothetical. We examine Mercury and Venus very nearly under the conditions here imagined; for we can obtain only spectroscopic evidence respecting the existence of water on either planet. In the case of Mars we have telescopic evidence, and no one now doubts that the greenish parts of the planet are seas and oceans. But Venus and Mercury are never seen under conditions enabling the observer to determine the colour of various parts of their discs.

I may add that a mistake, somewhat analogous to that which I have described in the cases of an imagined observer of our earth, has been made by some spectroscopists in the case of the planets Jupiter and Saturn. In considering the spectroscopic evidence respecting the condition of these planets’ atmospheres, they have overlooked the circumstance that we can judge only of the condition of the outermost and coolest layers, for the lower layers are concealed from view by the enormous cloud masses, floating, as the telescope shows, in the atmospheric envelopes of the giant planets. Thus the German spectroscopist Vögel argues that because in the spectrum of Jupiter dark lines are seen which are known to belong to the absorption-spectrum of aqueous vapour, the planet’s surface cannot be intensely hot. But Jupiter’s absorption-spectrum belongs to layers of his atmosphere lying far above his surface. We can no more infer the actual temperature of Jupiter’s surface from the temperature of the layers which produce his absorption-spectrum, than a visitor who should view our earth from outer space, observing the low temperature of the air ten or twelve miles above the sea-level, could infer thence the actual temperature of the earth’s surface.