52 On Citric Add. 



the mixed liquor is afterwards to be concentrated by evaporation. 

 Wliere the citric acid is made as an article of manufacture, a 

 boiler of lead* will be the inost convenient for this purpose. 

 In this the acid nv-\\ be kept continually steaming without any 

 dan/j-er, initil it acquire tlie speciiic gravity of about 1*13(). It 

 will then be prudent to withdraw the firef entirely, and proceed 

 to remove the acid into a smaller vessel of lead, which should be 

 so fixed within an iron pan of boiling water, as to form a com- 

 plete Inlnettm. marice. In this bath the fluid may be further 

 concentrated by steaming, until it become so much reduced in 

 bulk as to render it necessary to remove it into another water- 

 bath still smaller, but constructed in the same manner as the 

 former. In this last vessel the liquor is to be further evaporated 

 until it ac(.|uire the consistence of very thin molaescs, and then 

 it should be watched with the greatest attention for the pellicle 

 to appear, wliicli may now be expected to arise on the surface. 

 This pellicle will first show itself in small detached spots, and 

 by degrees will increase in "ize and number till the whole of the 

 liquor is nearly covered with them. It should then be imme- 

 diately taken out of the bath, and laid aside to cool and crystal- 

 lize J. It is of the utmost consequence to attend to this circum- 

 stance ; for so much of the fiuid is evaporated before the pellicle 

 appears, that, if it were to remain half an hour longer, the whole 

 might be carbonized, and reduced to a dry mass. 



It is like'.vise })roper to reniark, that however carefully the 

 process may have l)ecn conducted, the whole of the calcareous 

 earth will not precipitate totally while the supernatant liquor is 

 loaded with the citric acid ; for we know that this is capable of 

 holding the citrate of lime in solution §. To remedy this, there^ 

 fore, it will be requisite, as the acid becomes concentrated, to 



* The bottom of this boiler should be made of one continiicd sheet of lead, 

 not less tlinii ten pounds to the fou:, ihough five-pound lead will be sufficient 

 for the sides. But wliere saving; of e^ipense is not the first object, it is best to 

 have the vessel of one entire sheet of lead, by which all solder mav be avoided. 



f In building- the fire-place, it shoulil be entirely covered vvith a continued 

 row of iron jilates, which are to I>e sr.pporrcd at each end by tlie walls of the 

 fire-pbce. These should not be le s than one inch thick; and if they are cast, 

 only six inches broad : it will be less enpense to repair them whenever any of 

 them are worn oi'.t by the action of the fire. It is by these, plates the leaden 

 boiler i. to be .supported. 



J When reniov,.-d, it should remain undisturbed for three or four days, that 

 tht'largest possible quantity of crystals may be obtained fiom the liquor. After 

 this time has elapsed, the mother water may be drained from them, as, after 

 that period, it wiP be in Arain to expect a lui ther increase without a further'abs- 

 traciioi) of water iVoiu t'ie mothers by a fresh evaporation. 



§ Dize having dissolved 100 parts of crystallized citric acid in pure water at 

 212", found that it would take up 50 parts of calcareous citrate. Joiirn.de Phys. 

 1794, torn. ii. page i!31. Kence some nicety is required in separating the lime, 

 without adding an unnecessary and injurious quantity of sulphuric acid. 



add 



