66 PHYSICAL SCIENCE BK. n 



in all probability there are cases of ignition from 

 the heat of the superposed ether. In nature there 

 is never a sudden transition from one element to a 

 totally different one. Hence there must be some 

 congruity between lowest ether and highest atmo- 

 sphere ; conversely highest atmosphere cannot be 

 wholly dissimilar to lowest ether On the confines 

 the two elements pass so imperceptibly into one 

 another that at a particular point there might 

 well be doubt whether one is in atmosphere or in 

 ether. 



XV 



SOME of the Stoics believe that air, being inter- 

 changeable with other elements such as fire and 

 water, does not derive from without a fresh cause 

 of fire; it kindles itself by its internal motion. 

 Then in dissipating masses of thick, compact clouds 

 it necessarily emits a loud noise from the bursting 

 of such large bodies. Besides, the very conflict 

 of the resisting clouds contributes to the energy of 

 the fire. In the same way the hand contributes to 

 the cutting power of an instrument, but the actual 

 cutting is done by the steel. 



XVI 



LET me now explain the difference between the 

 flash and the bolt of lightning which you naturally 

 wish to know. The flash is the fire widely spread 

 out, the bolt is the condensed fire hurled with 

 violence. Let me use a homely illustration. We 

 sometimes join our two hands in order to take up 



