344 0?i the Modifications of Clouds, and 



If the proportions of this natural carbonate of soda be 

 compared witji that of the recently prepared crystals of ar- 

 tificial carbonate of soda, which contain 



Soda - - - - 23 



Carbonic acid - - - 16 



Water of crystallization - 62 



100 



a striking difference will be found in the proportion of 

 the carbonic acid ; for that of the carbonic acid is at most 

 73 parts in a hundred in the common carbonate, while it is 

 nearly 103 in the natrum of Tripoli, which makes 30 more. 



It is this more complete saturation of soda by the car- 

 bonic acid which gives to trona the property of not efflo- 

 rescing. There are certainly some local circumstances 

 which tend to favour it. But these must be determined by 

 future naturalists who may visit the places where it is found. 



Common carbonate of soda is not at its maximum of sa- 

 turation ; consequently it is susceptible of combining with 

 a larger quantity of carbonic acid. This may be effected in 

 the same manner as the complete saturation of the carbo- 

 nate of potash. That of artificial soda - onipletely saturated 

 approaches, therefore, to trona, both externally in regard to 

 the foliated form of its crystals, and the property which it 

 acquires at the same time of presenting greater resistance 

 to efflorescence, 



[To be continued.] ' 



LXI V. On the Modifications of Clovds, and on the Princi- 

 ples of their Froiduction, Suspension, and Destruction ; 

 being the Substance of an Essay read before the Askesian 

 Society in the Session 1802-3. By Luke Howard, 

 TLsq, 



[Continued from p. 107.] 



Explanation of the Plates. 



JTlate V. a, a, represents different appearances of the cir- 

 rus : h, a regular cumulus : c, a stratus occupying a valley 

 at sun-set, in the midst of which is supposed a spot of 

 higher ground, with trees, See, 



Plate VI. ::,n, exhibits the character of the cirro-cumulus, 

 as also its appearance in the distance. /', /■, a light and a 

 dark cirro-stratus } the former taken just before the com- 

 mencement 



