14 HUMAN BIOLOGY 



as to be gaseous, or else the core of the planet is surrounded 

 by an ocean thousands of miles deep. 



These four planets, therefore, are in all probabiHty 

 hfeless. 



The case of Mercury is still worse. All the tests show 

 it to be devoid of atmosphere and water. One face, turned 

 continually towards the Sun, reaches a temperature of 

 400°c. (as shown by the radiometric measurements). The 

 other, in permanent darkness, must be exceedingly cold. 

 No more inhospitable world could be imagined, it is a real 

 Inferno. 



Two worlds only, out of a billion, remain, Venus and 

 Mars, and in these cases the conditions are more favorable. 

 Both are large enough to retain an atmosphere, and both 

 actually possess one; one is hotter than the Earth, and 

 the other colder, but neither is outside the limits of tolerance. 

 They and they alone, need discussion in detail. 



Venus, until a few years ago, appeared the most favorable 

 known habitat for life outside the Earth. Though she 

 receives twice the intensity of solar radiation that we do, 

 the reflecting power of her surface is high, and less heat 

 in proportion remains upon the planet. The radiometric 

 measures indicate a temperature of about 6o°c. for the 

 sunlit side (which, though rather high, is not out of the 

 question), and — 20°c. for the dark side (not impossibly 

 low). The existence of an atmosphere is proved conclusively 

 by the appearance of twilight when we see the planet as a 

 thin crescent. Above the visible surface, this atmosphere 

 appears to be less extensive than the Earth's; but this 

 surface may well be composed of clouds. The uniform white- 

 ness of the surface, and the absence of definite markings 

 have long been recognized as favorable to this view; and the 

 remarkable photographs of Ross in 1927, which show darkish 

 spots visible by ultraviolet light only, and changing from 

 night to night, go far to settle the matter. 



After three hundred years of observation, the planet's 

 rotation period is still unknown. There is no doubt that 

 it is much longer than the Earth's, and it was at one time 

 supposed that, like Mercury, Venus kept one face always 

 toward the Sun; but this is hardly reconcilable with the 



