72 Life and Matter [chap. v. 



a planet only in size, but the difference in 

 size involves also many other differences, 

 notably the fact that the larger body can 

 attract and hold to itself an atmosphere 

 a circumstance of the utmost import- 

 ance to the existence of life on its sur- 

 face. In order, however, that a planet 

 may by gravitative attraction control the 

 roving atoms of gas, and confine their 

 excursions to within a certain range of 

 itself, it must have a very considerable 

 mass. 



The earth is big enough to do it ; the 

 moon is not. By simply piling atoms or 

 stones together into a mighty mass there 

 comes a critical point at which an atmosphere 

 becomes possible ; and directly an atmosphere 

 exists, all manner of phenomena may spring 

 into existence, which without it were 

 quite impossible. 



So, also, it may be said that a sun differs 

 from a dark planet only in size ; for it is just 

 the fact of great size which enables its 

 gravitative-shrinkage and earthquake-sub- 



