68-(38) ARE OTHER WORLDS INHABITED ? 



causes the intensity of the solar rays to be five times as 

 great on Mercury as on the earth. Water could exist 

 there only as superheated vapor, and all the more fusible 

 metals, as tin and lead, only in the fluid form. Life 

 would be impossible. 



Those who with Prof. Proctor believe that Mercury has 

 an atmosphere, will be likely to say that the solar heat 

 is very probably warded off by a dense covering of 

 clouds. But even Prof. Proctor, eager as he is to prove 

 the inner planets inhabited, (he gives up the others) 

 finds in the comjiaratively small amount of light radi- 

 ated from Mercury a strong argument against his 

 theory, and admits that it is "therefore difficult to 

 believe that the light of that planet comes from a cloudy 

 envelope." Besides, it must be remembered that vapor 

 increases the temperature derived from the sun, and 

 never diminishes it. This is according to the well 

 known law, first I think established by Prof. Tyndall, 

 that vapors permit the solar heat to pass freely through 

 them, while they refuse to permit the surface heat to 

 escape with equal freedom into space. If the clouds 

 let any heat enter, and reach the body of the planet, it 

 would accumulate. If they are totally impervious to 

 the solar heat, then the planet is uninhabitable on 

 account of the cold. Nor could this result be avoided 

 by the internal heat of the planet, for it has been shown 

 that the heat which comes through an earth crust of any 

 considerable thickness, is too small to produce any sen- 

 sible effect. In either case, therefore, life would be 

 impossible. 



In order to escape the argument, one may be tempted 

 to say that in other planets the laws governing the trans- 

 mission of heat are different from those on the earth. 

 But this would be unphilosophical. The tendency of 

 science is towards oneness of law everywhere. That 

 like causes produce like effects in all parts of the uni- 

 verse, is the foundation belief on which rest all other 

 beliefs in matters of science. Admitting this, we must 

 also admit that life is impossible in the planet Mercury. 



Very little is known about Venus. It is so enveloped 

 in clouds, that there is great doubt whether any portion 

 of its solid surface has been seen. 



