Professor Orfila has advanced another objection,—that the process will yield all the indications mentioned above, if binoxalate of potash be present, or sorrel-soup, which contains a little of that salt.[398] The objection is valid, were these substances apt to come in the way. But the binoxalate of potash is not put to any medicinal use in Britain, and English cookery does not acknowledge the “soupe à l’oseille.” The process he recommends to meet the difficulty, an important one in France, is the following: 1. Having made a watery solution as above, evaporate nearly to dryness, agitate the residue with cold pure alcohol, repeatedly during a period of several hours; decant the tincture, and repeat this step with more alcohol; evaporate to obtain crystals, if possible; dissolve these again in cold pure alcohol, and crystallize a second time by evaporation. If crystals do not form on first concentrating the alcoholic solution, evaporate it till a pellicle begins to form, agitate the residue with cold pure alcohol, and concentrate again to obtain crystals. Lastly, examine the crystals by the tests for pure oxalic acid. The object of these steps in the process is to separate binoxalate of potass, oxalate of magnesia and oxalate of lime, which, he says, are all either not soluble, or very sparingly so, in absolute alcohol. 2. More oxalic acid may be got by acting with distilled water on the matter left by the action of alcohol, evaporating this watery solution nearly to dryness, agitating the residuum with cold alcohol as before, and so on. 3. The preceding operations may have left oxalate of magnesia and oxalate of lime unacted on by the water among the solids remaining on the filter. The former compound may be dissolved out by cold hydrochloric acid diluted with four times its volume of water; and by an excess of pure carbonate of potass, the oxalate of magnesia in the solution is converted into insoluble carbonate of magnesia and soluble oxalate of potass, from which oxalic acid is to be obtained by a salt of lead and sulphuretted-hydrogen, as explained in my own process. 4. Oxalate of lime, which may still remain, is to be sought for by boiling the residuum of the action of hydrochloric acid with solution of bicarbonate of potash, so as to obtain here also an oxalate of potass in solution. I have not had an opportunity of trying this method. But I find, that, contrary to Orfila’s statement, binoxolate of potass, though sparingly soluble in cold alcohol of the density of 800, is sufficiently so to vitiate the principle on which the process is founded. Caustic potass must not be used for decomposing oxalate of lime or magnesia, because the pure alkali, as Gay-Lussac has shown, produces oxalic acid in acting on animal substances at a boiling temperature. Carbonate of potass has no such effect.