OBSERVATIONS. 75 



though samples collected in the town, precisely 

 at the same time, on being subjected to the test, 

 generally have their transparency more or less 

 impaired. This fact seems to prove, that, not- 

 virithstanding muriate of soda is never raised into 

 the atmosphere by evaporation, yet the air over 

 large towns usually contains a very minute por- 

 tion of muriatic acid, which, as Mr. Dalton ob- 

 serves,* is probably supplied by the sublimation 

 of muriate of ammonia during the combustion 

 of fuel. A considerable increase of muriatic 

 acid takes place in the rain which falls in Man- 

 chester, when accompanied with a brisk breeze 

 from the west, of several hours duration ; as is 

 evident from the greater degree of opacity ob- 

 served in samples caught under such circum- 

 stances, when treated with a few drops of the 

 solution of nitrate of silver ; and that which falls 

 in the adjacent country, then manifests a sensible 

 trace also : indeed, the direction of the wind 

 remaining the same, its force and duration seem 

 almost entirely to regulate the quantity of muri- 

 atic acid in the atmosphere ; which completely 

 establishes the fact, that it is brought from the 

 sea by the mechanical action of powerful currents 

 of ain 



• Society's Memoirs, New Series, vol. IV. p. 370. 

 k2 



