SPRINGS, RIVERS, CANALS, LAKES, 



quisite to perform this operation with several hundred pounds. The 

 nature and form of the vessels in which waters are exposed for 

 evaporation, i.s not a matter of indifference . those of metal, ex- 

 cepting silver, are altered by water; vessels of glass, of a certain 

 magnitude, are very subject to be broken ; but those of glazed 

 smooth pottery are the moist convenient, though the cracks in the 

 glaze sometimes cause an absorption of saline matter ; vessels of 

 unglazed porcelain, called biscuit., would doubtless be the most 

 convenient, but their price is a considerable obstacle. Chemists 

 have proposed different methods of evaporating mineral waters ; 

 some have directed distillation to dryness, in close vessels, in order 

 to prevent foreign substances, which float in the atmosphere, from 

 mixing with the residue; but this method is excessively tedious: 

 others have advised evaporation by a gentle heat, never carried to 

 ebullition, because they supposed that this last heat alters the fixed 

 principles, and carries up a portion of them. This was the opinion 

 of Venal and Bergman. Monnet, oi> the contrary, directs the wa. 

 ter to be boiled, because this motion prevents the reception of 

 foreign matters contained in the atmosphere. Bergman avoids this 

 inconvenience, by directing the vessel to be covered, and a hole left 

 in the middle of the cover for the vapours to pass out this last me- 

 thod greatly retards the evaporation, because it diminishes the sur- 

 face of the fluid. At the commencement, the heat used must be 

 sufficient to repel the dust; but the greatest difference in the mani- 

 pulation of this experiment consists in some writers directing that 

 the substances deposited should be separated, as the evapora- 

 tion proceeds, in order to obtain each pure and by itself ; others, on 

 the contrary, direct the operation to be carried on to dryness. We 

 are of the opinion of Bergman, that this last method is tiie most ex. 

 pedilious and certain; because, notwithstanding the care which may 

 be taken, in the fir>t method, to separate the different substances 

 which are deposited or crystallized, they are never obtained pure, 

 and must alwass be examined by a subsequent analysis; and the 

 method is besides inaccurate, on account of the frequent till rations, 

 and the loss it occasions. Lastly, it is very embarrassing, and ten- 

 ders the evaporation much longer. Mineral waters may therefore 

 be evaporated to dryne.is, in open glass vessels, on the wateubath, 

 or still more advantageously in glass retorts, on a sand-bath. 



Various phenomena are observed during this operation; if th* 



