296 VEGETABLE SUBSTANCES. 



To obtain the citric acid this calcareous salt is now 

 treated with dilute sulphuric acid. The quantity of 

 carbonate of lime used in the previous process should 

 have been accurately noted, and for every ten pounds 

 employed, nine and a half pounds sulphuric acid of 

 the specific gravity of 1'84, or 1*845, diluted with 

 about fifty-six pounds of water, are added, the whole 

 being poured gradually on the citrate of lime : the 

 mixture is then well stirred for a considerable time 

 with a strong wooden spatula that there may be no 

 chance of any of the citrate remaining unbroken, and 

 in consequence unacted upon by the stronger acid. 

 Lime, having a greater affinity to the sulphuric than 

 to the citric acid, immediately quits the latter and 

 combines itself with the former, leaving the citric acid 

 disengaged in a state of solution in the supernatant 

 fluid. The insoluble part of the mixture is now sul- 

 phate of lime, and after allowing a sufficient time for 

 its precipitation, the liquor is drawn off from it, and it 

 is repeatedly washed in order that none of the citric 

 acid may be left among it : the water in which it has 

 been thus washed is mixed with the liquor, and the 

 whole is concentrated by evaporation. This is per- 

 formed in leaden vessels which are cased with wood, 

 and heated by means of steam. When evaporated to 

 about one-eighth in bulk, the solution is transferred 

 to a vessel of a smaller size, wherein it is farther 

 evaporated until it is reduced to the consistency of 

 thin molasses. When a pellicle is seen gradually to 

 spread itself over the surface, the mixture is imme- 

 diately removed to other vessels and left to crystal- 

 lize : there it remains undisturbed for three or four 

 days, at the end of which time the mother-water 

 is drained from the crystals. The crystals thus 

 produced are most generally needle-formed, and 

 nearly as dark as the brownest sugar, in conse- 

 quence of the presence of mucilage ; but if this have 



