Bit temperature. 55 



heated air at the surface of the earth (which 

 is condensed to a certain extent) rises to the 

 higher regions of the atmosphere. There is a 

 gradual expansion as it ascends, and conse- 

 quently a gradual cooling, because a given 

 amount of heat is being constantly distributed 

 over a greater amount of space. At an alti- 

 tude of forty-five miles it will have expanded 

 about 25,000 times, which will bring the tem- 

 perature down to between 200 and 300 degrees 

 below zero. 



When we get beyond the limits of the at- 

 mosphere we get into the region of absolute 

 cold, because heat is atomic motion, and there 

 can be no atomic motion where there are no 

 atoms. 



We have now traced the atmosphere up to 

 the point where it shades off into the ether 

 that is supposed to fill all interplanetary space. 

 As Dryden says : 



There fields of light and liquid ether flow, 

 Purg'd from the pond'rous dregs of earth below. 



By interplanetary space we mean all space 

 between the planets not occupied by sensible 

 material. It is the same as interatomic space, 

 or the space between atoms, except in degree, 

 as the same substance that fills interplanetary 

 space also fills interatomic space, so that all 

 the atoms of matter float in it and are held 

 together from flying off into space by the at- 



