Chap. II.

Acids.

329

paper. Evaporate the filtered liquor with a gentle heat,until it form crystals on cooling.In order to render the crystals pure, they must be dissolvedtwice, or oftener, in water, filtered each time, evaporatedand crystallized.

This process was contrived by Scheele, and was reducedto determinate quantities by Proust, as follows : To 94 partsof lemon juice, 4 parts of carbonate of lime are to be added ;the carbonic acid is separated by effervescence, and a quan-tity of insoluble citrate of lime is precipitated. By evaporat-ing the supernatant liquor, another portion of citrate of limeis obtained. These added together amount to about 7-J parts,and require 20 parts of sulphuric acid, of the specific gravityof 1.15, to decompose them. The sulphate of lime, beingnearly insoluble, is precipitated, while the citric acid remainsin solution, and is to be separated by washing, and crystalliz-ed by evaporation. If too much sulphuric acid be added,when the liquor is much concentrated, the citric acid is re-acted upon, and part of it is charred. In this case a littlechalk must be added, to saturate the excess of sulphuric acid.Mr Parker, Tilloch's Journal, vol. xlvi p. 60, on the autho-rity of a manufacturer of citrate of lime in Sicily, has givensome curious details on the subject. 74.964 gallons of lemonjuice were used, which, with 35.017 pounds of chalk gave

f902 pounds avoirdupoise of citrate. The quantity ot ci-;e produced by every pound of chalk varied from 19 toounces, and from every gallon of juice from 8^ to 1 2\ounces. This disparity arose from inequality in the acidityof the juice, and from the same quantity of chalk being usedwith every kind of juice. The chief difficulties of the manu-facture consisted in the drying properly the citrate. In some re-spects this concrete acid is superior, and in others greatly in-ferior to lemon juice. It has not the flavour; and, what is ofmore consequence, it has not the freshness or antiscorbuticpowers of the fruit; but from its solid form and gradual solu-tion it is convenient, and is excellently adapted for effer-vescing mixtures. Dr Haygarth found that 26 parts of thesolid acid saturates 61 of subcarbonate of potash, 42 subcar-bonate of ammonia, and 40 of carbonate of magnesia.

The crystals are permanent, and dissolve in three-fourthsof their weight ol'crtld, and half their weight of boiling water.Dissolved in eight waters, it is said to be equal in strength tolemon juice.