poisoning with oxalic acid in its medico-legal relations was examined by Dr. Coindet of Geneva and myself;[3] and in 1828, another experimental inquiry, which confirms most of the results we obtained, was published by Dr. Pommer of Heilbronn.[4]
Section I.—Of the Tests for Oxalic Acid.
Oxalic acid is commonly in small crystals of the form of flattened six-sided striated prisms, transparent, colourless, free of odour, very acid to the taste, and permanent in the air. Two other common vegetable acids, the citric and tartaric acids, present a totally different crystalline form. In general appearance it greatly resembles the sulphate of magnesia, for which it has been so often and so fatally mistaken. So close, indeed, is the resemblance, that repeatedly, on desiring several persons to point out which was the poison and which the laxative, I have found as many fix on the wrong as on the right parcel. The sulphate of magnesia has of course a very different taste, being strongly bitter. Various plans have been devised for preventing the accident to which this unlucky resemblance has given rise. The best of them imply the use of a safeguard by the patient before he takes his laxative draught. It seems to have escaped the notice of those who have proposed the plans in question, that, if accidents are to be prevented in this manner, by far the simplest and most effectual security will be to let the public know, that a laxative salt ought always to be tasted before being swallowed. Its solubility has been much overrated by some chemists. It does not appear to me soluble in less than eleven parts of water.
In determining the medico-legal tests for oxalic acid, it will be sufficient to consider it in two states,—dissolved in water,—and mixed with the contents of the stomach and intestines or vomited matter. If the substance submitted to examination is in the solid state, the first step is to convert it into a solution.
1. In the form of a pure solution, its nature may be satisfactorily determined by the following process.
The acidity of the fluid is first to be established by its effect on litmus paper.—A small portion is next to be tested with ammonia, which, if the solution of the acid be sufficiently concentrated, will produce a radiated crystallization, as the oxalate of ammonia formed is much less soluble than oxalic acid itself. This property, according to Dr. O'Shaughnessey, distinguishes it from every other acid.[5] The remainder of the fluid is next to be subjected to the following reagents.
Hydrochlorate of lime causes a white precipitate, the oxalate of lime; which is dissolved on the addition of a drop or two of nitric
