CITRIC ACID. 295 



fore left open for improvement. Soon after this a 

 French chemist published another process, the very 

 reverse of the former refrigerating method. The juice 

 was subjected to a slow degree of heat for a consider- 

 able time ; the heat caused the mucilaginous matter 

 gradually to thicken, and rise in a glutinous mass to 

 the surface of the liquor ; and this being removed, 

 the remaining portion continued for a long time in 

 an unaltered state. It was left however to Scheele, 

 the great Swedish chemist, to add this to his numer- 

 ous useful discoveries ; and he was the first who 

 obtained citric acid in a solid crystalline form, by the 

 same method which he had employed long before 

 for the purification and crystallization of tartaric 

 acid. He separated the real acid by carbonate of 

 lime, thus producing citrate of lime, which he decom- 

 posed by the intermedium of diluted sulphuric acid. 



To pursue this process the juice is drawn off into 

 a large open vessel, and neutralised by the gradual 

 addition of carbonate of lime. The lime immediately 

 combines with the citric acid in the juice, and disen- 

 gages the carbonic acid gas, causing great effer- 

 vescence : care is therefore necessary to prevent the 

 overflow and consequent waste of the liquor, by sup- 

 plying the carbonate of lime by very slow degrees : 

 this is added till no more effervescence ensues, and 

 until the liquor shows no signs of acidity on test 

 paper. The citric acid, thus combined with the 

 lime, produces an insoluble salt which precipitates at 

 the bottom of the vessel. When the whole of this 

 has fallen the supernatant liquor is drawn off, and 

 the precipitate preserved for use, after being passed 

 through a sieve, and frequently washed in warm 

 water till any remaining mucilage or soluble impu- 

 rity is entirely disengaged : this is ascertained to have 

 been effected when, on leaving the whole at rest for 

 some time, the water comes off clear and tasteless. 



