LIFE IN OTHER WORLDS. 29 



that we know, the following conditions, among others, are necessary, 

 namely, (1) A supply of radiant energy which is intermittent or 

 variable, (2) one or more elements which (like nitrogen) are very 

 sensitive to changes of energy, and (3) one or more elements which 

 (like carbon and hydrogen) are capable of alternately accumulating 

 and dispersing energy by means of opposite chemical changes. 



Since popular ideas as to the physical conditions of other worlds 

 are generally hazy and often far from correct, it may not be amiss 

 to recount such particulars as astronomical research has revealed in 

 the members of the solar system. 



In looking for a world where the conditions are most like our 

 own, we naturally turn first to Venus. Her size and gravitation are 

 nearly the same as those of the earth, and her atmosphere may there- 

 fore be expected to have a similar density and to hold approximately 

 the same gases. As she is nearer the sun, the general temperature 

 would be considerably higher, and the equatorial region might be 

 too hot for our life; but there might nevertheless be a suitable tem- 

 perature nearer the poles. But our speculations are damped by a 

 suspicion, strong but not fully confirmed, that Venus has no day and 

 night, but always keeps the same side toward the sun. If this is 

 really the case, then the sunny side must be always burning hot and 

 quite dry, while the opposite side must be always encased in ice — nay 

 more, in a mixture of ice and solidified atmospheric gases. The 

 life of such a world must be very different from any that we know. 



After Venus, Mars is the planet whose conditions seem most to re- 

 semble those of our world. But there are far greater differences than 

 are generally supposed. Mars is so small that he can not provide much 

 heat from within, and so far from the sun that he receives com- 

 paratively little heat from without. His gravitation is so slight that 

 the atmosphere is rare and nearly cloudless, and therefore heat must 

 be readily lost by radiation. Thus on theoretical grounds Mars should 

 be intensely cold: in fact his surface should be constantly in the 

 condition of the highest mountain-tops of our world, only receiving 

 less heat than they do from the sun. At such low temperature and 

 pressure, water could never exist in the liquid form, though it might be 

 solid or gaseous. But water is very possibly absent from Mars. Dr. 

 Johnstone Stoney has calculated, by application of the dynamic theory 

 of gases, that any water vapor introduced into the atmosphere of that 

 •planet would escape into space, the gravitation being there insufficient to 

 retain it. Professor G. H. Bryan, calculating from slightly different 

 data, questions Dr. Stoney 's conclusions; but at all events Mars's 

 gravitation is very near the dividing line between the ability and in- 

 ability to retain water. If water is absent from Mars, then the polar 

 caps and other seeming evidence of its presence must be due to some 

 other fluid or gas, having heavier molecules and lower freezing and 



