Of the detection of copper in organic mixtures.—As in the instance of arsenic and mercury, so in that of copper the presence of vegetable and animal principles interposes material obstacles in the application of the ordinary tests and methods of analysis. Some substances, such as albumen, milk, tea, coffee, and the like, decompose the solutions of the salts of copper, throwing down the oxide of copper in union with various proximate principles. Others, such as red wine, bile, vomited matter, and the tissues composing the stomach, although they do not decompose the soluble copper salts, alter materially the action of reagents on them. These facts were established long ago by Professor Orfila;[[1080]] and various processes were suggested by him, by myself in former editions of this work, and by various other authors, with the view of overcoming the difficulties in question.
More lately a fresh difficulty has been started, which has been thought to render every prior process fallacious, including that which I have proposed. For it is alleged that copper exists naturally as a constituent part of many vegetable and animal substances, and more especially in the organs of the human body. This statement is so important as to deserve attentive consideration before fixing on a method of analysis for medico-legal cases.
Some time ago Meissner pointed out the existence of a trace of copper in some vegetable substances;[[1081]] and more recently M. Sarzeau alleged that a minute quantity of this metal, sometimes not above a 1,500,000th and never exceeding a 120,000th part, may be detected not only in all vegetable substances, but likewise in the blood, as well as other fluids and solids of the animal body. Among vegetable substances he examined with great care cinchona-bark, madder, coffee, wheat and flour; and he succeeded in separating metallic copper from them all.[[1082]]
The accuracy of these researches was called in question. By some chemists the discoveries of Meissner and Sarzeau were confirmed so far as they relate to vegetable substances. By others the confirmation was extended to the animal body, and more especially to the human organs and secretions. Thus M. Devergie says, that, having been struck with the singular circumstance of two cases occurring to him in a single year, where analysis indicated copper in the tissues of the alimentary canal of persons suspected of having died of poison, he was led to inquire, along with M. O. Henry, whether the metal was contained naturally in the textures of the human body; and that in the course of many experiments, although unable to detect any in a solution made by means of weak acetic acid, he could always find it by the process of incineration.[[1083]] Orfila has also repeatedly detected traces of copper in the bodies of animals not poisoned with the preparations of that metal.[[1084]]
By other experimentalists opposite results have been obtained, more especially in regard to animal solids and fluids. In the course of an inquiry relative to the question, whether poisons pass into the blood, I failed to detect copper in the blood, muscles, or spinal marrow of animals, although the method of analysis must have enabled me to discover extremely minute quantities of that metal. Afterwards M. Chevreul was unable to detect the slightest trace of copper in beef, veal, or mutton; nor was he more successful in the case of wheat, provided care was taken to keep the sample clean.[[1085]] And more recently MM. Flandin and Danger have denied that there is any copper ever found naturally in the body.[[1086]]
These discrepant results appear to be in a great measure reconciled in an extensive inquiry into the subject by M. Boutigny; who found that wheat, wine, cider, and some other substances of a vegetable nature, do frequently present minute traces of copper, but only when copper is contained in the manure used in raising the grain, apples, and the like; that manure from the streets of great towns always contains copper, and introduces it into vegetable articles grown where such manure is used; and that the occasional presence of the same metal in animal substances may be traced either to copper vessels having been employed in preparing or preserving them, or to the animals producing them having been fed on vegetables presenting from the causes mentioned above a faint cupreous impregnation.[[1087]]—Another fallacy, which may account for the alleged invariable success of some chemists, has been pointed out by M. Hiers-Reynaert of Bruges. Having once obtained copper in a specimen of suspected bread, when he used paper for a filter, but none when he used linen, he was led to examine various filtering papers, and found that some kinds contain an appreciable trace of copper.[[1088]] This important fact must be attended to in all medico-legal investigations.
On the whole, whatever may be thought of the physiological question, whether copper forms a constituent of the textures and fluids of vegetables and animals, it seems well established that this metal is often present there in minute proportion; and consequently its possible presence must not be overlooked in medico-legal researches. Fortunately methods of analysis are known which this source of fallacy does not affect.
Process. The following method embraces all possible cases; and it is exempt, so far as yet appears, from every source of error.
1. Should the subject of analysis not be a liquid, render it such by dividing it into small fragments, and boiling it gently for an hour in distilled water acidulated with acetic acid, which must previously be ascertained not to contain any copper. If the liquid be not viscid, filter it at once; but if it be too viscid for filtration, pass it through a muslin sieve, add two volumes of rectified spirit to it when cool, and then filter it. Transmit through a small portion of it a stream of hydrosulphuric acid gas; and if a brownish-black precipitate or cloud form, subject the whole liquid to the gas. A brown precipitate, which is sulphuret of copper, will separate either immediately, or after ebullition and repose for an hour. Collect the precipitate, if abundant, by filtration, if scanty, by repeated subsidence and affusion. Dry it, subject it to a low red heat, and then heat it with a little strong nitric acid, which will convert the sulphuret into the sulphate of copper. This salt, dissolved out by boiling distilled water, may be subjected to the tests described above, and especially to ammonia.
2. If the copper be extremely minute in quantity, sulphuretted hydrogen will not act upon it in a fluid much charged with organic matter. To meet this possible case, which may occur when the subject of analysis is an organ of the human body into which the poison has been conveyed by absorption,—let the liquid be evaporated to dryness, and charred in the following manner. Heat in a porcelain basin a quantity of nitric acid equal in weight to the residuum, together with a fifteenth of chlorate of potash. Add the dry residuum in successive portions of such magnitude as not to occasion too great effervescence. When it has been all added, heat the product till it become dark-red and thick. It will then, or soon afterwards, begin suddenly to char, and at length a thick vapour will arise in dense clouds; upon which, the charring being complete, the heat must be withdrawn. Pulverise the carbonaceous mass; boil it with nitric acid diluted with its own volume of water; and evaporate the filtered fluid to dryness, so as to expel any excess of acid. Dissolve the saline residuum, and test the solution with the usual reagents.