Oxalic acid exists, ready formed, in many vegetable and in some animal bodies. Combined with potassa, it is found in the leaves of Oxalis acetosella and corniculata, Rumex acetosa, and Geranium acidum; and with lime, in the roots of Rhubarb, Valerian, several lichens, and of many other plants. Berthollet procured it from honey, hair, tendons, albumen, and some other animal matters; but that which is found in the shops, manufactured for the purposes of art, is produced artificially by the action of nitric acid on sugar. The following process, which was first described by Bergman, is still adopted for the production of oxalic acid.

Into a tubulated retort, put one ounce of white sugar, and pour over it three ounces of strong nitric acid of sp. gr. 1.567. When the whole is dissolved, boil the liquor until it cease to afford nitrous fumes, and acquire a reddish-brown colour; then add three ounces more of nitric acid, and continue the boiling until the fumes cease, and the colour of the fluid disappear. Empty the contents of the retort into a wide vessel, and, upon cooling, a crystallisation will take place, which is oxalic acid. On boiling the lixivium with two ounces of nitric acid in the retort, until the red fumes almost disappear, a second supply of crystals will be obtained. One hundred grains of sugar, when properly treated, yield fifty-eight grains of crystallised oxalic acid. The rationale of the process is very obvious; the nitric acid is partially decomposed, and yields up a portion of its oxygen, which is one of its components, to the sugar, which is a compound of oxygen, carbon, and hydrogen; and, by this addition of oxygen, the carbon is converted into oxalic acid, and the hydrogen is wholly carried off in the form of water.

The relative proportion of these components of sugar, is oxygen 49.4, carbon 44.5, and hydrogen 6.1, in 100 parts; that of oxalic acid, oxygen 66.6, carbon 33.3, in 100 parts, or two proportions of carbon = 12.24 + 3 of oxygen = 24, making the equivalent 36.24; but the crystals contain three equivalents of water = 27, so that the equivalent of the crystallised acid is 63.24.

The crystals should be purified by repeated solution in distilled water and recrystallisation.

Qualities. - The crystals of oxalic acid, when they are properly prepared, are flat four-sided prisms, with the sides alternately larger. They are white, transparent, inodorous, have a very acrid, sour taste, and redden all the vegetable blues except indigo. One grain of oxalic acid communicates a sensible acidity to 2633 grains of water. The crystals dissolve in twice their weight of water at 65°, and in less than their own weight of boiling water. Alcohol at a mean temperature dissolves 40 parts of them, and boiling alcohol 56 parts. They are sparingly soluble in ether. The crystals are in the state of a hydrate, 100 grains containing 58 only of acid, and 42 of water.

Oxalic acid combines with alkalies, earths, and metallic oxides, forming oxalates. Hydrochloric and acetic acids dissolve it without alteration; but it is decomposed both by sulphuric and nitric acids aided by heat. It is readily distinguished by its affording a white precipitate with lime water, or the salts of lime, which is insoluble in an excess of the acid.

Medical properties and uses, - Oxalic acid, in small doses, when it is dissolved in a very large quantity of water sweetened with sugar, forms an agreeable cooling beverage, which may be used in febrile diseases, in the same manner and with the same intention as lemonade. It may also be employed to check an external haemorrhagy, which it appears to do by charring the blood as it issues from the wound, and thereby mechanically stopping its flow. It is a virulent poison when swallowed in large doses; and from the resemblance of its crystals to those of sulphate of magnesia, many fatal accidents have occurred from mistaking oxalic acid for that purgative salt. The acid taste of the salt and the bitter of the acid would always prevent such accidents, were individuals to taste their medicines before swallowing them: but besides the occurrence of accidents, oxalic acid has been too frequently employed by the wretched suicide for the purposes of self-destruction. It is, therefore, important, that every medical practitioner should be acquainted with the qualities of oxalic acid, its effects on the animal economy, and the means of counteracting these, when it has been taken in a sufficient dose to operate as a poison.

The exact extent of the dose of oxalic acid which may be taken with impunity has not been determined; but its poisonous properties are more or less virulent according to the degree of dilution of the dose. From some experiments on animals, which I made ten years since, and the published details of the appearances in dissection of several fatal cases of poisoning by oxalic acid, I was led to form an opinion that "the primary morbid action of this poison is on the stomach itself, on the coats of which its chymical action occasions the organized animal solid to enter into new combinations, and thence produces a decomposition both of the acid and the part to which it is applied:" that the acid, however, also enters into the circulation by absorption; but "that the proximate cause of death from oxalic acid is the suspension of the functions of the heart and brain, which are sympathetically affected by the local injury done to the stomach." l The subject has been, since then, investigated with much care and great ingenuity by Dr. Christison and Dr. Coindet, and their observations published in the Edinburgh Medical Journal.2 From the labours of these gentlemen, I am induced to change my opinion as to the extent of the injury done to the living stomach, and to believe that the pultaceous state of that organ, which was found by me in my experiments on dogs, is to be attributed to the action of the acid on its coats after death.

I am, however, still of opinion, that death is less to be attributed to the sedative power of the poison acting on the brain and spine, to which it is carried by absorption, than to the sympathetic action on the nervous system from the local injury of the stomach; an opinion according with their first conclusions, that "oxalic acid, when introduced into the stomach in large doses, and highly concentrated, irritates it, or corrodes it by dissolving the gelatin of its coats; and death takes place by a sympathetic injury of the nervous system."1

1 London Med. Repos. vol. iii. p. 386.

2 See Edin. Med. and Surg. Journ, vol. xix. p. 163.

The general symptoms attending poisoning by oxalic acid are burning pain in the stomach, violent and incessant vomiting, the matter ejected being commonly dark-coloured and sometimes bloody; in some cases there have been violent gripings and purging; the pulse soon sinks and becomes almost imperceptible; and this state is followed by deadly coldness of the limbs, attended with lividity of the fingers and nails, and clammy sweats: convulsions, but not in every instance, and insensibility, precede death. With regard to the appearances after death, no particular change in the external state of the body has been noted. On opening the body, the stomach is found, generally, to contain a quantity of dark-coloured fluid, which is probably extravasated blood charred by the poison; in some instances, the coats of the stomach have been found greatly injured, presenting appearances of great vascularity, thickening of the mucous coat, the rugae pultaceous and easily wiped off, and, in some cases, the other membranes have been found tender, and even perforated, so that the contents of the organ have escaped into the cavity of the abdomen.

The lungs and heart have not been often examined; but in the lower animals killed by oxalic acid, both have presented indications of inflammation having existed in* them, particularly the lungs. The vessels of the brain have been found turgid.

The fatal effects of poisoning by oxalic acid are so rapid, that little opportunity is afforded for counteracting them by medical art. The first object certainly, in every case, is to evacuate the poison from the stomach, and when the stomach-pump is at hand, it should be instantly employed. The vomiting which usually supervenes precludes the necessity of employing emetics; and copious dilution, which in other cases of corrosive poison is advisable, is more likely to promote the absorption of the acid, and, consequently, increase l Toland, p. 185.

its powers. Before, therefore, emetics are employed, if they should be deemed necessary, the activity of the poison should be weakened by altering its nature by some substance with which, in chymically combining, its solubility is diminished. That chalk produces this effect, I discovered in making the experiments already alluded to: and many instances have since occurred in which its administration has saved the lives of individuals poisoned by oxalic acid. The lime of the chalk unites with the oxalic acid, and forms an oxalate, which is perfectly inert. Magnesia may be employed instead of chalk, and it has the advantage of not inconveniencing the patient by the extrication of carbonic acid gas, which is copiously evolved when the chalk unites with the acid; but, as the oxalate of lime is more insoluble than the oxalate of magnesia, and, consequently, more inert, it may be questioned whether the inconvenience of the gas be equivalent to the greater security from the employment of chalk. A mixture of chalk and water, or of magnesia and water, should, therefore, be instantly given when oxalic acid has been taken in a large dose; and, after the local effects of the poison have been counteracted, the system should be supported by cordials combined with opium, and the oxalate afterwards carried out of the system by the aid of purgatives.

To obtain legal evidence in cases of poisoning by oxalic acid, when none of the poison is found, we may be guided to suspect the nature of the poison by the symptoms and the post-mortem appearances; but a correct opinion can be formed only by an analysis of the vomited matter, the contents of the stomach and its coats. For this purpose the vomited matter and the contents of the stomach should be separately diluted with distilled water, and the coats of the organ itself boiled in distilled water. These solutions should then be separately filtered and decolorized with chlorine, and subjected to the following tests. If oxalic "acid be present, hydrochlorate of lime will occasion a precipitate which is soluble in a small quantity of nitric acid, but not in hydrochloric, unless a very large quantity of the acid be used. Sulphate of copper throws down a bluish white precipitate in any fluid containing free oxalic acid, which is insoluble in hydrochloric acid; and nitrate of silver occasions a heavy white precipitate, which, when dried and heated over the flame of a candle on the point of a spatula, becomes brown at the edge, then suddenly fulminates, and is all dissipated in white fumes.

This is a very delicate test; for Dr. Christison informs us that, from a quarter of a grain of oxalic acid dissolved in 4000 parts of water, he and Dr. Coindet procured enough of the oxalate of silver to show its fulmination twice.1