LIFE IN SPACE AND TIME 1 5 



relatively high temperature of the dark side, and it is 

 probable that the rotation period is a few weeks in length. 



All these indications are favorable; but the crucial test 

 remains, i.e. the composition of the atmosphere. Fortunately 

 oxygen and water vapor show numerous absorption lines 

 in the observable region, mainly in the red, and can be 

 detected by spectroscopic means. The application of this 

 test to the planets is hindered by the fact that we have 

 to observe through the Earth's atmosphere, which is rich 

 in both substances. But, if the planet is observed when 

 it is rapidly approaching the Earth, or receding from it, 

 the "Doppler shift" due to this motion sets the hnes pro- 

 duced in the planet's atmosphere a little to one side of 

 the stronger ones arising in that of the Earth. Even if 

 the two are not clearly resolved, measures of the position 

 of the resulting blend will show what the relative intensities 

 of the two absorptions are. 



This ingenious method, devised independently by Lowell, 

 Campbell, and St. John at the three great observatories 

 of the Western United States, has been applied to Venus 

 by the last-named astronomer. Not the slightest effect 

 due to the presence of oxygen on the planet could be detected, 

 though the lines should have been clearly separated. By 

 comparison with laboratory measures, he concludes that 

 the quantity of oxygen above the visible surface of Venus 

 must be less than Kooo of that in the Earth's atmosphere. 

 This test is apparently decisive. The spectroscopic test 

 for water vapor also gave a negative result; but it is much 

 less delicate, and a small quantity might escape detection. 



It would seem decidedly improbable, on general principles, 

 that Venus, which is quite as much like the Earth in size 

 and mass as one of a pair of twins is usually like the other, 

 should have been initially entirely devoid of water while the 

 earth had so much. Escape of water vapor against 

 the planet's gravitation appears to be out of the question. 

 The cloud-like forms observed with ultraviolet light suggest 

 condensations of vapor of some sort, and nothing else than 

 water appears to be at all plausible in the connection. 

 But how so little vapor can exist above the clouds, if they 

 are really composed of water, at a relatively high tem- 



