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affinity is extremely ftrong and fcarcely to bebroken but by the combined effect of the moftpowerful agents. It appears probable too fromScheele ’s refearches, that the malic acid is inmoft inftances a neceffary confequence of theproduction of the oxalic, but not being cryf-tallizable, nor very readily detedted, it hasprobably been moft commonly overlooked.

The oxalic acid is a valuable teft in chemicalexperiments to determine the prefence of limein liquids, in whatever combination this earthexifts. The precautions required in its ufe willbe mentioned under the article Oxalat of lime.

The affinities of oxalic acid are in the follow-ing order: lime, barytes, ftrontian, magn&fia,potafh, foda, ammonia, alumine, and metallicoxyds.

OXALATS.

The oxalic acid unites with all the alkalies,earths, and moft metallic oxyds, forming falts•of which a very few only are of importance.This acid like the tartareous, combines with thealkalies in two different proportions, in each ofwhich it forms cryftallizable- falts. One ofthefe ftates is with the acid in excefs, formingtherefore falts known by the general term ofacidula, the other is when the two ingredientsare in perfeft faturation. Of thefe the Acidu-lous oxalat of potnfa, or according to Dr.Thompfon’s ufeful nomenclature, the Sttper-txalat of potafh, has been examined with thegreateft care.

All the oxalats are readily decompofed byheat, the acid burning off, and leaving the bafe.carbonated.

Oxalic Acidulum. Super-oxalat of Potafh.Salt of Soi rel.

A fait has long been prepared in feveral partsof Europe from the juice of the forrel plant,for medicinal and other purpofes, particularlyin Swabia, Thuringia, and fome parts of Swit-zerland . The method of making it has beendefcribed by Savary, Cartheufer, Weigleb, andOthers, and is very fimple. Large quantities ofthe plant are collected and expofed to confi-derable preffure In a mill, (or in any other way)and the turbid juice thus expreffed is allowedto ftand for fome days till much of the fecu-lence has fubfided. The clear liquor is thenevaporated to one half, and fet by in a coolplace, and after fome days a quantity of fmall-grained cryftals are depofited. Thefe are re-moved, and the liquor further evaporated, bywhich a fecond crop of cryftals is collected,and fometimes a third or a fourth is got by theiame means. The cryftals are at firft very

mucilaginous and impure, but by re-folution andevaporation they become clean, and then appearin the form of fmall white needles, in whichftate they are fold at . a high price for takingout iron-mould fpots from linen, and for medi-cinal ufes. According to Savary, 50 lbs. ofthe frefh plant yield about 25 lb. of juice, fromwhich 2j oz. of the eflential fait may be ex-tracted. Cartheufer 3 obtained much more,one pound of the plant giving as much as1 y dram, which would be in the proportionof nearly 9^ oz. from 50 lbs. of the plant, butprobably this was lefs pure.

The tafte of this fait is very four, fettingthe teeth on edge, but has nothing bitter orunpleafant. The fait is hard and brittle, anddecrepitates in the fire. It is very commonlyadulterated by fulphat of potafh,

It was Scheele that firft difcovered the truecompofition of this fait, and fhewed it to becompofed of potafh with excefs of an acid,which by every teft was identical with the acidof fugar which had been lately difcovered byBergman. The method purfued by Scheele was the following: fome warm water wasfaturated with fait of forrel, and a folution ofacetite of lead was then added as long as anyprecipitate appeared, which laft was edulcoratedwith warm water. This confifted of the oxalicacid united to the oxyd of lead, whilft thefupernatant liquor contained acetite of potafhproduced by this double decompofition. Thisprecipitate was then decompofed by digeflionwith fulphuric acid, by which the oxalat oflead was converted into an infoluble fulphat,and the clear liquor contained the pure oxalicacid, which was obtained in its cryftalline formby due evaporation and cooling. The propor-tion of fulphuric acid required was determinedby decompofing by this acid feparately as muchof acetite of lead as had been ufed, and notingthe quantity of acid employed. The acid thusobtained from the fait of forrel was then exam-ined chemically, and found to agree exactly inall its properties with that obtained from fugarby the nitric acid. A fait precifely refemblingthe fait of forrel was then reproduced byadding gradually to a folution of the acid fomeliquid carbonat of potafh, much lefs than fuf-ficient for faturation, and during the effervef-cence occafioned by the mixture, a number offmall faline fpiculac fell down, which whenexamined were found to be exaftly the famewith the native fait of forrel.

The oxalic acidulum may alfo be produced byadding pure oxalic acid to nitre, ^is die fame

* Mat-cay, Appar. Med. vol. J.