CONDITIONS OF HABITABILITY OF A PLANET. 



81 



leaf. Between the heaven and man came the cloud. His life 

 being partly as the falling leaf and partly as the Hying 

 vapour." 



The leaf and the cloud are the signs of a habitable world. 

 The leaf, that is to say, plant life, vegetation, is necessary 

 because animal life is not capable of building itself up from 

 inorganic material. This step must have been previously taken 

 by the plant. The cloud, that is to say water- vapour, is neces- 

 sary because the plant in its turn cannot directly assimilate to 

 itself the nitrogen from the atmosphere. The food for the plant 

 is largely brought to it by water, and it assimilates it by the 

 help of water. Life on a planet therefore turns upon the 

 presence of water, the great neutral liquid and general solvent, 

 the compound of the two most abundant elements, hydrogen 

 and oxygen. There is no other compound of like properties 

 and simplicity of constitution that could take its place, or that 

 the elements could supply in sucli abundance. We cannot 

 imagine a world wherein bisulphide of carbon or hydrochloric 

 acid or any other such compound could discharge the functions 

 which water fulfils here. It is, therefore, upon the question of 

 the presence of water that the question of the habitability of 

 a given world chiefly turns. In the physical sense man is 

 " born of water," and any world fitted for his habitation must 

 " stand out of the water and in the water." 



Where shall we find such another world ? There were two 

 bodies whose surfaces men could study to some extent, even 

 before the invention of the telescope — the sun and the moon. 

 But we are able now to determine the temperature of the sun 

 with some approach to precision, and we know that not only is 

 it far too hot for the presence of vegetation, but it is so hot that 

 oxygen and hydrogen would usually refuse to combine there. 

 The components of the molecules of water would be driven 

 asunder ; water would be dissociated. And as with the sun so 

 with all the stars, for they, in various measures and degrees, are 

 all suns. The moon also is without the leaf and the cloud ; its 

 surface has been drawn, photographed and measured over every 

 ^ square mile, until the side visible to us has been more thoroughly 

 surveyed than our earth, but it shows us only bare unchanging 

 rock. A man placed there could draw no nutriment from the 

 atmosphere around him, or the soil beneath ; no vapour would 

 ever soften the hardness of the heaven above, no leaf the 

 hardness of the rock below. 



But what of planets ? There may be planets circling round 

 the stars, or there may not be ; we have no means of knowing, 



G 



