OXYGEN IN THE SUN. 11 



it had been in that case. For the light which he would 

 receive from the earth would not in this case have passed 

 through the entire depth of moisture-laden air twice, but 

 twice only through that portion of the air which lay above 

 the clouds, at whose surface the sun's light would be reflected. 

 The whole of the moisture-laden layer of the air would be 

 snugly concealed under the cloud-layer, and would exercise 

 no absorptive action whatever on the light which the remote 

 observer would examine. If from the upper surface of the 

 layer of cumulus clouds aqueous vapour rose still higher, 

 and were converted in the cold upper regions of the 

 atmosphere into clouds of ice-crystals, the distant observer 

 would have still less chance of recognizing the presence of 

 moisture in our atmosphere. For the layer of air between 

 the cumulus clouds and the cirrus clouds would be unable to 

 exert any absorptive action on the light which reached the 

 observer. All such light would come to him after reflection 

 from the layer of cirrus clouds. He would be apt to infer 

 that there was no moisture at all in the air of our planet, at 

 the very time when in fact there was so much moisture that 

 not one layer only, but two layers of clouds enveloped the 

 earth, the innermost layer consisting of particles of liquid 

 water, the outermost of particles of frozen water. Using the 

 words ice, water, and steam, to represent the solid, liquid, 

 and vaporous states of water, we may fairly say that ice and 

 water, by hiding steam, would persuade the remote observer 

 that there was no water at all on the earth at least if he 

 trusted solely to the spectroscopic evidence then obtained.* 



* 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 



