TO OBTAIN PINE SALT. 21 



very small quantity of chloride of calcium and chloride of 

 magnesium, besides traces of chloride of potassium, bromide, 

 and iodide of magnesium, and some carbonate of protoxide of 

 iron, with free carbonic acid. The main object, under these 

 circumstances, is to remove the entire amount of iron, more 

 than one-half of the sulphate of lime, and as much as possible 

 of the very objectionable deliquescent compounds — chloride of 

 calcium and chloride of magnesium. To obtain these results, 

 the following mode of working is pursued : the brine — being 

 usually in its fresh state, colorless, and highly charged with 

 carbonic acid — is filled into shallow vats and kept there until 

 most of the carbonic acid has escaped and the protoxide has 

 been fully oxidized and settled as a brown-red, insoluble precip- 

 itate of hydrated peroxide of iron ; the saline liquid is then 

 drawn off to a lower series of vats, where it is left for evapor- 

 ation until crystals of salt appear ; during this period the excess 

 of sulphate of lime is separated. The brine being now com- 

 pletely saturated with salt is now called salt-pickle ; it is ready 

 for salt-making, and consequently removed from the separated 

 impurities to another set of vats, where, by mere solar heat, the 

 separation and accumulation of a coarsely crystallized salt takes 

 place. The salt is, from time to time, gathered, whilst the re- 

 maining mother liquor will be discharged as soon as it reaches 

 a concentration from 28°-30° Baume.* The less attention has 

 been paid to the separation of the iron and of the excess of 

 sulphate of lime (plaster or gypsum), the more concentrated 

 the mother liquor from which the salt has been gathered, the 

 less the salt-crystals themselves have been washed with new salt 

 pickle before their removal into perforated tops for drainage, 

 and the less chance they have enjoyed in rendering that last 

 process efficient, the more inferior is the quality of the coarse 

 salt produced. A good coarse or solar salt must be of a neu- 

 tral reaction, hard, large-sized, white or colorless, and produce 

 a clear solution. 



ON COMMON FINE SALT. 



The finer-grained qualities of salt are obtained by the direct 

 or indirect application of artificial heat to iron kettles, iron 



* Compare, for further details, my report "On the Manufacture of Solar Salt"; 

 Syracuse, N. Y. 1864. 



