MANUFACTURE OF SALT. 115 



on the surface. By this time nearly all the less soluble impurities have subsided, and are to 

 be found at the bottom of the vats in beautiful crystals. The brine, thus reduced to satura- 

 tion, is drawn otT into the lower tier of vats, where the formation of salt goes on. It is sub. 

 sequently taken out and dried, as before, by the heat of the sun ; when it consists of large 

 crystals, hard and dry, and of a beautiful white colour. It may be observed, that the evapora- 

 tion of the brine depends greatly upon the state of the atmosphere, and that the process is of 

 course retarded when there is much humidity. But to prevent the embarrassment which 

 would ensue from the access of rain, by the dilution of the brine, covers are constructed for 

 the vats, which run upon rollers, and may be easily moved when occasion requires. 



This, then, is the simple process of nature ; and much may be learned by noticing the dif- 

 ferent steps of the operation. And if, as there can be no doubt, the purest salt may thus be 

 obtained, the difEculties which attend the other modes of manufacture may all be referred to 

 some departure from the principles which are here unfolded. 



2. Evaporation ivith artificial heat. — When this mode is pursued, large iron v'ats are em- 

 ployed, into which the brine is conducted, and afterwards subjected to a moderate degree of 

 heat ; the impurities having been previously in a great measure removed, by exposure in the 

 cistern, or by the addition of lime. The heat is either applied directly to the vats, or by 

 means of tubes through which steam is made to pass. In this way, salt of great purity may 

 be obtained, of the crystallized variety. In Great Britain, a similar process has long been 

 advantageously pursued ; and the only reason why it has not been more generally introduced 

 here, seems to be the outlay of capital which is required. 



3. By boiling. — The mode pursued in the manufacture of salt by boiling, is as follows : 

 From twenty to forty iron kettles, such as are used in the manufacture of potashes, are ar- 

 ranged in two parallel rows, and firmly fixed in brick work over a furnace, constituting what is 

 known as a block. These kettles, being of a capacity of about one hundred -gallons, are filled 

 with brine, which is made to boil. As soon as ebullition commences, the water becomes very 

 turbid, and the calcareous salts begin to precipitate. These are repeatedly removed by large 

 iron ladles, which are placed for that purpose at the bottom of the kettles, and are called bit- 

 tern ladles. A proportion, however, adheres to the bottom and sides of the kettles, and after 

 a few days forms a solid coating, called pan scale. When the process is properly conducted, 

 this is frequently removed, as its accumulation retards the boiling and impairs the purity of 

 the salt. Soon after this calcareous matter is deposited, crystals of salt begin to shoot out 

 and sink to the bottom, and this continues until nearly all the brine is evaporated. The salt 

 is now reinoved to proper places for the purpose of draming off the brine, and allowing it to 

 become dry. Tliis completes the work of the manufacturer. The salt made in this way 

 consists of fine grains, more or less hard and pure, according to the care which has been taken 

 in conducting the process. This is the mode in which by far the largest proportion of the 

 Onondaga salt is still marmfactured. 



The princijud difficulty in the manufacture of salt by rapid boiling is, that the natural pro- 



