CHAP. 2. 18. STRUCTURE OF CLOUDS. 97 



neously, and produce one kind of lightning 

 and thunder. This is, however, only vague 

 conjecture: nothing is certainly known about 

 the structure of clouds. It would make a very 

 good theory, with the solution of the following 

 questions. 1st. What are the circumstances 

 under which hydrogen could be contained in 

 a vesicle of water ? And, 2dly. What can 

 occasion such a separation of the gases on a 

 condensation of vapour into cloud? In short 

 the opinions entertained about vesicular vapour 

 seem in general to have been vague and ill- 

 founded. That the structure of different clouds 

 is very different, is manifest from their different 

 refracting and reflecting powers, producing the 

 various appearances of the Halo, Corona, Par- 

 helion, &c. on different occasions, as well as 

 from the very different appearance of the clouds 

 themselves. 



But there is, in fact, no proof that the 

 particles of water have any specific levity in 

 the air; they may, perhaps, only gravitate 

 very slowly to the earth, from their minuteness, 

 as soon as from any cause the elastic vapour is 

 condensed into a visible cloud. The manner 

 in which such aggregates may constitute a 

 visible and floating cloud, which preserves or 



H 



