LIFE IN SPACE AND TIME 



and under terrestrial conditions a certain proportion of 

 the carbonaceous and other reduced compounds, which 

 represent the other side of the equation, are buried in 

 sediments, and so withdrawn from the cycle of change. 



If conditions on our own planet can be taken as a guide, 

 it would appear, therefore, that an oxygenated atmosphere 

 may be regarded, not as a prerequisite for hfe, but as its 

 result, and as strong evidence that hfe exists upon the 

 planet, or, at least, has existed in the past, within an interval 

 comparable with geological time. 



The oxygen of the Earth's atmosphere is of importance to 

 hfe in another fashion, less widely known. High in the 

 upper air, more than twenty miles from the surface, a 

 small proportion of it is transformed into ozone, probably 

 by the influence of short-wave radiation from the Sun. 

 Though transparent to visible hght, ozone has a remarkable 

 power of absorption for the ultraviolet rays; and, owing 

 to its presence, no radiations of wave-length shorter than 

 2900 Angstrom units reach the surface, though the Sun 

 doubtless emits them powerfully. This hmitation is a 

 great tribulation to the spectroscopist; but, were the ozone 

 removed, the short waves, whose injurious effects are 

 well known, would have a most disastrous effect upon 

 hfe at the surface. A very small quantity of oxygen in the 

 atmosphere would however probably suffice to produce 

 enough ozone to afford effective protection. 



One or two other factors may perhaps be mentioned. 

 The superficial gravity of the planet, though it would have 

 a great deal to do with the limiting size which could be 

 attained by land animals, appears to interpose no fatal 

 obstacle to the existence of life, even were it far greater 

 than on earth. But too small a force of gravity would permit 

 the escape of atmosphere, and make a world uninhabitable. 



Again a certain minimum atmospheric pressure is necessary 

 if liquid water is to exist at all, for if the pressure is less 

 than 3^f7o that which prevails at the Earth's surface, ice 

 would evaporate directly, at a temperature below its melting 

 point. A high atmospheric pressure and density would 

 presumably greatly influence the modes of respiration and 

 locomotion of animals; but the only hmit which can be set 



