Chapter LX: Section I: Of the Chemical History and Tests of the Preparations of

Copper._

Metallic copper has a special red colour, to which it gives its own name. Its specific gravity is nearly 9, its hardness considerable, its tenacity great, its point of fusion about 27° W. or at a full white heat.

It unites with oxygen in two proportions, forming a yellowish-red protoxide, and a peroxide, which, when dry, is brownish-black—when hydrated, azure-blue. It unites also with sulphur in two corresponding proportions, forming a gold-yellow proto-sulphuret, the natural copper-pyrites, and a black bisulphuret, which is formed by sulphuretted-hydrogen in all the solutions of this metal. The peroxide unites with ammonia. The acids all unite with the oxide and form blue or green salts, some of which are soluble, some insoluble. The oxide is frequently mixed with other matters to form various pigments; but in such compounds the union is generally mechanical, not chemical. Of the substances thus formed and existing in nature and the arts the following only require notice here. 1. _Mineral green_, and other pigments formed with the hydrated oxide. 2. _Natural verdigris_, or the carbonate. 3. _Blue vitriol_, or the sulphate. 4. _Artificial verdigris_, or the mixed acetates.

1. _Mineral Green._

The description of this substance and its chemical properties must be introduced with a short account of the tests for the unmixed _peroxide_. When free of water the peroxide is a brownish-black powder or granular mass, which is usually procured by decomposing nitrate of copper at a low red heat. It is easily known by the solvent power of nitric acid, the blue colour of the filtered solution, and the beautiful deep violet tint communicated to the solution by an excess of ammonia. The last property is considered by chemists the most satisfactory proof of the presence of oxide of copper in a fluid. It is alone quite free of fallacy, and may be applied to all the soluble and also many insoluble compounds of copper, provided they are not mixed with a large proportion of vegetable or animal fluids, in which case the colour is often greenish.

In the case of the peroxide and of copper poisons generally, the process of reduction, which has been applied with such delicacy and precision to arsenical and mercurial poisons, loses all its advantages. The metal remains in the flux, and intimately diffused; so that of its physical qualities the colour only can be estimated, and even that but inaccurately, except in the instance of one compound, verdigris.

The _hydrated peroxide of copper_, when newly formed and well prepared, has a fine azure-blue colour; but on exposure to a gentle heat, it parts with its water, and becomes the anhydrous peroxide. It is procured by precipitating any of the soluble salts of copper by means of caustic potass. It is at once known by the action of ammonia, which immediately forms with it a deep violet-blue solution.

_Mineral green_, as already mentioned under the head of Arsenic (p. 223), was originally an arsenical pigment introduced into the art of colour-making by Scheele, and now sometimes sold in this country by the name of emerald-green. But the mineral green of the colourist now contains no arsenic, being a hydrate of peroxide of copper intimately mixed with a little lime, which is generally carbonated. This variety of mineral-green probably varies a little in composition. Some parcels I have found to contain the lime in the state of carbonate; in others the lime was chiefly caustic.

The best method of determining its nature is to dissolve it in diluted hydrochloric acid, which leaves only a slight cloudiness from accidental impurities; and then to transmit through the filtered solution a stream of sulphuretted hydrogen gas. The copper on boiling is all thrown down in the form of a black bisulphuret, and hydrochlorate of lime remains in solution. The lime is then to be detected by its proper tests, after the solution has been filtered and neutralized (see p. 192). In general this long process is unnecessary, as the medical jurist may be simply required to say whether the suspected substance contains copper. In that case it is only requisite to subject the substance to the action of ammonia, as if it was hydrated peroxide.

_Verditer_, another green pigment, the basis of which is always oxide of copper, does not appear to differ essentially in composition from mineral green. The samples I have examined consist of a large proportion of hydrated oxide of copper, and a small proportion of carbonate of lime.

2. _Natural Verdigris._

This is a compound of no great importance in a medico-legal point of view. Nevertheless an instance has been lately published in which it was taken for the purpose of committing suicide, and was found abundantly in the stomach.[1049] The carbonate of copper exists naturally in two states. In one form it constitutes the rust of copper, or natural verdigris, and is produced as a powdery crust on metallic copper by long exposure to moist air. It is insipid and insoluble, so that pure water left in vessels incrusted with it does not become poisonous. It dissolves with effervescence in sulphuric acid, and without effervescence in ammonia, forming the usual violet solution. In another form it exists in the mineral kingdom, constituting the chief part of a beautiful ore, malachite, and also a considerable proportion of some blue-copper ores.

3. _Blue Vitriol._

Blue vitriol, blue copperas, blue stone, vitriol of copper, as it is variously called in common speech, is the sulphate of copper. In the solid form it constitutes large crystals of a deep blue colour, and an acrid, astringent, metallic taste, efflorescent in dry air, and very soluble in water. Under the action of heat it first loses its water of crystallization without undergoing the watery effusion; then its sulphuric acid is driven off partly unchanged, partly decomposed; and at last the brown peroxide is left behind in a state of considerable purity. If carbonaceous matter be previously mixed with the sulphate, the oxide is decomposed at a low red heat, so that the process of reduction may be performed in a glass tube. For the reasons formerly stated, this process does not constitute a convenient or characteristic test for sulphate of copper. The best mode of ascertaining its nature is to dissolve it, and then to apply the tests for the solution.

There are many excellent tests for copper in solution. But the four following are the most delicate and characteristic,—ammonia, sulphuretted hydrogen, ferro-cyanate of potass, and metallic iron.

1. _Ammonia_ causes a pale azure precipitate, which is redissolved by an excess of the test, forming a deep violet-blue transparent fluid. If the solution is very diluted, there is no previous precipitation; the fluid becomes violet without its transparency being disturbed. This is a perfectly characteristic test of copper, and one of great delicacy.

2. _Sulphuretted hydrogen gas_ causes a dark brownish-black precipitate, the sulphuret of copper. This test is one of very great delicacy; but it is not alone decisive of the presence of copper, since lead, bismuth, mercury, and silver, are similarly affected by it. A method, however, will be presently described, by which the precise nature of the sulphuret may be determined.

The alkaline hydrosulphates, for example the hydrosulphate of ammonia, answer equally well with sulphuretted-hydrogen. The solution of the common liver of sulphur throws down, not a black, but a chestnut precipitate.

3. _Ferro-cyanate of potass_ causes a fine hair-brown precipitate, the ferro-cyanide of copper. This test is also exceedingly delicate and characteristic.

4. A polished rod or plate of _metallic iron_, held in a solution of sulphate of copper, soon becomes covered with a red powdery crust, which is metallic copper; and ere long the solution is changed in colour from blue to greenish-yellow. The action is simple; the iron merely displaces the copper in the solution, in which a sulphate of iron is consequently formed. This test is characteristic, and even of considerable delicacy. At the same time other substances may cause a reddish encrustation on iron by simply rusting it, so that the test cannot be relied on alone.

The four preceding reagents taken together are amply sufficient to prove the existence of copper in a solution. Three other tests, however, may be here briefly alluded to.

Caustic potass in a solution not too diluted causes a fine azure-blue precipitate, the hydrated peroxide of copper.

Oxide of arsenic, with the previous addition of a few drops of ammonia, causes a fine apple-green or grass-green precipitate, the arsenite of copper. This test, which is both delicate and characteristic, has been already fully considered under the head of Arsenic.

The process by fluid reagents, as hitherto laid down, merely proves the presence of copper, but does not indicate the acid with which the oxide is combined. In order to determine whether it is sulphuric acid, the fluid must also be tested with nitrate of baryta followed by nitric acid: a heavy white precipitate is thus produced, which the excess of nitric acid does not redissolve.

4. _Artificial Verdigris._

_Artificial verdigris_ is a common pigment, which is met with in the form either of earth-like masses, or of a light powder of a greenish-blue colour and peculiar disagreeable smell, approaching that of vinegar. Like blue vitriol it has a strong metallic, astringent taste. The effect of heat is peculiar. Some acetic acid is in the first place distilled over; a portion of the acid, however, is decomposed and reduces the oxide; and a low red heat is sufficient to make the outer crust of the verdigris distinctly copper-red, when the material is contained in a glass tube.

Artificial verdigris varies somewhat in composition. Foreign verdigris contains chiefly the hydrated diacetate, with a little carbonate, oxide, and even metallic copper, along with particles of the fruit and fruit-stalks of the grape. British verdigris consists of little else than the hydrated diacetate. It is known by the following characters. Ammonia dissolves it almost entirely, forming a deep violet solution. Diluted sulphuric acid dissolves it, evolving an odour of acetic acid, and forming a solution of sulphate of copper, which may be known by the tests for that salt. Boiling water converts it partly into an insoluble brown powder, which is oxide of copper in union with a small proportion of acetic acid, and partly into a greenish-blue neutral acetate, which is dissolved, and may be known by the four tests for sulphate of copper, and the want of action of nitrate of baryta.

It may be right to notice shortly three other salts of copper, the nitrate, the ammoniacal sulphate, and the muriate. The _nitrate_ forms a violet solution, which is acted on by reagents in the same way as the dissolved acetate, but has not any odour of vinegar. The _ammoniacal sulphate_ [ammoniated copper—ammoniuret of copper], has been occasionally used in medicine. It forms, when solid, small scaly crystals, of an intense violet colour and strong ammoniacal odour; and when dissolved it retains its peculiar colour even though very much diluted.—The _muriate_ of copper has a lively grass-green colour, and is acted on by reagents in the same way as the solution of verdigris.

_Of the corrosion of copper by articles of food and drink._—To these observations on the chemical history of copper a few remarks must be added relative to the action of various articles of food or drink upon the metal. Unpleasant accidents have often happened from the use of copper vessels in the preparation of food; and it is therefore necessary for the medical jurist to know the circumstances, so far as they have been investigated, under which the poison may be dissolved.

Dr. Falconer found, that distilled water kept several weeks on a polished plate of copper, neither injured its lustre, nor acquired any taste, nor become coloured with ammonia;[1050] and Drouard afterwards observed, that distilled water, kept for a month on copper filings, did not contain any of the metal.[1051] Eller of Berlin, however, remarked, that water, if it contain a considerable quantity of common salt, as four ounces in five pounds, or a twentieth part, will give slight traces of copper after being boiled in a brass pan; and that if the pan be made of copper, a powder is procured by evaporation, which when treated with acetic acid yields so much as 20 grains of acetate of copper.[1052] But it is a singular circumstance, also observed by the same experimentalist, that if beef of fish be boiled with the usual allowance of salt, and with the addition also of various vegetable substances, the liquid does not yield any copper. This observation has been lately denied by Professor Orfila; who says he found copper deposited on a plate of iron in salt water in which beef had been boiled, and that he also obtained copper from the beef itself.[1053] The quantity thus dissolved, however, must be exceedingly small, if the copper be kept clean and free of oxide; for copper vessels, although they have often been the source of fatal accidents, if carelessly used in the preparation of food, have appeared under careful management to be quite harmless. An excellent practical confirmation of this will be found in Michaelis’s Commentaries. He states, that in the Orphan Hospital of Hallé, the food was in his time prepared in large copper vessels, which were kept remarkably clean; and that out of a population of eight or nine hundred he never heard of any one having suffered from symptoms of poisoning with copper.[1054] Several other saline matters promote the solution of copper in water. Thus Dr. Falconer found that alum has this effect when aided by heat; and probably nitre and Epsom salt possess the same quality.[1055] Their mode of action is not very well known.

It is a common though erroneous idea, that milk, heated or allowed to stand in a copper vessel, becomes impregnated with the metal. Eller has shown, that, on the contrary, if the vessel be well cleaned, milk, tea, coffee, beer, and rain-water, kept in a state of ebullition for two hours, do not contract the slightest impurity from copper;[1056] and the same remark has been also made by Dr. Falconer with respect to cabbage, potatoes, turnips, carrots, onions, rice, and barley.[1057]

But Eller farther remarked, that, if the vessel is not thoroughly clean, then all acid substances dissolve the carbonate that encrusts it, especially if left in it for some time. Nay, it appears that some acid matters, though they do not dissolve clean copper by being merely boiled in it a few minutes, nevertheless, if allowed to cool and stand some time in it, will acquire a sensible impregnation.[1058] Dr. Falconer also observed that syrup of lemons, boiled fifteen minutes in copper or brass pans, did not acquire a sensible impregnation; but if it was allowed to cool and remain in the pans for twenty-four hours, the impregnation was perceptible even to the taste, and was discovered by the test of metallic iron.[1059] This fact has been farther confirmed by the researches of Proust,[1060] who states, that, in preparing food or preserves in copper, it is not till the fluid ceases to cover the metal, and is reduced in temperature, that solution of the metal begins. Inattention to this difference has been the cause of fatal accidents, of which the following case from Wildberg’s Practical Manual will serve as a good example. A servant left some sour-krout for only a couple of hours in a copper pan which had lost the tinning. Her mistress and a daughter, who took the cabbage to dinner, died after twelve hours illness; and Wildberg found the cabbage so strongly impregnated with copper, that it was detected by the test of metallic iron.[1061]

Some wines have the same power, by reason of the acid they contain. Hence Eller found twenty-one grains of the acetate in five pounds of French white wine, after being boiled in a copper vessel. An epidemic disease, mentioned by Fabricius, which broke out in 1592 among the senators of Bern, and a number of their guests who had been invited to a great entertainment, was supposed to have arisen from a poisonous impregnation of this kind. The wine used at the feast had been kept cool in copper vessels immersed in a very cold well. Many of the company were attacked with dysenteric symptoms, and some died.[1062]

Vinegar also dissolves metallic copper. Dupuytren observed that the vinegar sold by hawkers in the streets of Paris generally contained copper from the action of the acetic acid on the stop-cocks of the little vessels used in retailing it.[1063] Others in like manner have found copper in vinegar pickles prepared in copper vessels. Thus Dr. Percival found a strong impregnation of copper in pickled samphire, of which a young lady ate one morning two breakfast platefuls, and which proved fatal to her in nine days.[1064] And Dr. Falconer once detected so large a quantity in some pickled cucumbers bought at a great London grocer’s, that it was deposited on a plate of iron, and imparted its peculiar taste and smell to the pickles.[1065] It seems indeed to have been at one time the custom to make a point of adulterating pickles with copper; for in many old cookery-books the cook is told to make her pickles in a copper pan, or to put some halfpence among the pickles to give them a fine green colour.[1066]

The action of the vegetable acids, and more particularly of vinegar on copper, depends on the co-operation of the atmospheric air held in solution by the fluid, and in contact with its surface. Without such co-operation the copper cannot be oxidated. This fact, which was determined experimentally by Proust,[1067] will explain the observations of Eller and Falconer,—that it is not dangerous to boil acidulous liquids in copper vessels, while it is very unsafe to keep these fluids cold in the same vessels. In the latter instance the liquid is impregnated with atmospheric air, while in the former the usual aëriform contents are driven off by the heat. I must observe, however, in limitation of Proust’s statement, that strong vinegar, such as the pyroligneous acetic acid, will become impregnated to a certain extent if boiled in copper vessels. The action which takes place is the same as that remarked by him in the case of cold vinegar:[1067] the copper where it is always covered remains quite bright; but at the edge of the fluid it becomes oxidated, and the oxide is dissolved by the occasional bubbling up of the acid.

In the last place, the property of oxidating and uniting with copper is likewise possessed by fatty matters and oils. According to Falconer, fatty substances do not act on metallic copper unless they are rancid.[1068] But Proust is probably more correct when he states, that they will act, though fresh, provided they are aided by the co-operation of atmospheric air.[1069] I have found, that, if a plate of copper be thrust into a mass of fresh butter, its surface becomes dark in twenty-four hours, and the butter becomes green wherever it is in contact both with the copper and the air, but not where it covers the metal closely. In fresh hog’s lard, however, I have found that the whole lard in contact with the copper becomes blue even at a depth to which the air can scarcely reach. The action of oils is similar. It is even probable that they act when hot; for Mr. Travis found that hot oil became green when kept for only four or five minutes in a copper vessel.[1070] Dr. Falconer mentions that the property of acting on copper is possessed in an eminent degree by volatile oils, and especially by oil of cloves and oil of cinnamon.[1071]

The general result of the preceding observations is, that there is hardly any article of food or drink which may not become impregnated with copper if kept in copper vessels, as there are few articles which do not contain either an acid or some fatty matter; and it farther appears, that the impregnation will scarcely ever take place during the boiling of such articles, but only during the preservation of them in a cold state. It must also be considered, that, independently of these chemical impregnations, articles of food may be mixed mechanically with copper, in consequence of the vessels being allowed, through the carelessness of the cook, to become covered with rust or carbonate, which is subsequently removed by the friction of the solid parts of any article that is boiled in them.

In order to prevent accidental impregnations, copper vessels are usually tinned. The tinning consists of an alloy of tin and lead, which is much less easily attacked than the copper, and the safety of which is farther insured by the circumstance, that the substances endowed with the property of dissolving lead, cannot attack that metal before the whole tin of the alloy is oxidated.[1072] The tinning of copper, however, has been found to be but a partial protection, as the tinning is apt to be worn away without attracting the attention of servants. Hence the use of copper in the fabrication of kitchen utensils is becoming every day more and more limited, especially since the manufacture of cast-iron vessels was brought to perfection in this country.

Many instances might be adduced of the ignorance and carelessness which prevailed, even not far back in the last century, as to the employment of copper vessels for culinary purposes. In addition to the instances already quoted, the following are well deserving of notice. Gmelin was consulted by the abbot of a monastery, on account of a violent disease which prevailed throughout the whole brotherhood of monks. The symptoms were obstinate and severe colic, retching and bilious vomiting, costiveness, flatus, burning pain in the pit of the stomach, under the sternum, in the kidneys and extremities, and paralytic weakness in the arms. On inquiring into the cause of this singular combination of symptoms, Gmelin found that every vessel in the kitchen, the pots and pans, and even the milk pails and butter dishes for storing the butter, were made of copper.[1073] In 1781 an establishment of Jacobin monks at Paris were all violently affected from a similar error. The cook on a Friday and the subsequent Saturday, after boiling fish for the dinner of the monks in a copper pan, and drawing off the water, poured vinegar over the fish, and left it thus in the pan for a considerable time. On the evening of Friday several of them were taken severely ill with headache, acute pain in the stomach and bowels, precordial anxiety, purging, great feebleness, and cramps in the legs. The rest of them, to the number of twenty-one in all, were similarly attacked next morning; and the symptoms continued in most of them for five or six days.[1074]

A singular variety of adulteration with copper was brought not long ago into public notice on the continent,—namely, the impregnation of bread with the sulphate of copper, which was used in small quantity for promoting the fermentation of the dough. This practice was first detected in some of the towns of Flanders, but was afterwards found to prevail in France.[1075] Some chemists of reputation have indeed doubted altogether the existence of the practice; and M. Barruel in particular, who was consulted on the subject by the Prefecture of Paris, publicly declared his disbelief, because he remarked that, instead of favouring the panary fermentation, a very small proportion of sulphate of copper actually impeded it, and besides gave the bread a greenish colour of such depth that no customer would take it for a wholesome article.[1076] Subsequent inquiries, however, have shown that Barruel must have allowed himself to be misled, probably by using too much of the sulphate of copper. For the bakers of St. Omer admitted that they practised this ulceration for the sake of saving their yeast, the proportion required being an ounce of the salt in two pints of water, for every hundred weight (_quintal_) of dough, or about an 1800th part.[1077] And it appears from an interesting set of experiments by M. Meylink, a chemist of Deventer, that, contrary to the statements of Barruel, sulphate of copper not only possesses the property of promoting the panary fermentation, but likewise constitutes in several important respects a source of adulteration, which ought to be prohibited and strictly looked after. He found that when he added to half a Flemish pound of dough from one grain to eight grains of sulphate of copper, fermentation took place more quickly than in the same dough without such addition, and nearly in proportion to the quantity of the salt used;—that the adulterated loaves when taken out of the oven were much better raised, and the loaf with only one grain of the salt likewise much whiter, than those which were not adulterated;—that a slight increase, however, in the proportion rendered the loaf greenish, and gave it a peculiar taste; but especially that the employment of the salt of copper even in the small proportion of one grain had the singular effect of bringing about the complete fermentation of the dough with considerably less loss of weight than occurs in the common process of baking, the loss in the sound and in the adulterated loaves being in the proportion of 116 to 100.[1078] It certainly seems fully proved, then, that the adulteration of bread with sulphate of copper is an important fraud in more ways than one. Some doubt may be entertained whether any injury can result to the human body from even the habitual use of so small a quantity as that employed by the bakers; and at all events, we may be satisfied that if any bad effects do result, this can only happen from the continual use of the adulterated bread for a great length of time. But there can be no doubt that the practice is a fraud on the public, by enabling the baker to make his loaves of the standard weight with a less allowance of nutritive material.

Another important adulteration also indicated by foreign chemists, is that of syrup made with the coarsest kinds of sugar, and decolorized by means of sulphate of copper. The colour is removed by adding a solution of the sulphate to the syrup boiling hot, and decomposing the salt by lime; but a portion of the salt is often left behind, and in consequence accidents have arisen from such syrups being used in making various medicinal preparations.[1079]

_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.

The first branch of this process is nearly the same with the one adopted in the last edition of the present work. The second is derived from a process lately proposed by Orfila.[1089]

The principles on which it is founded are these. 1. Of the numerous organic compounds formed by vegetable and animal principles with the salts of copper, all either dissolve in very weak acetic acid, or part with their oxide of copper to it. This was pointed out by me in my last edition. 2, Weak acetic acid, as already mentioned (p. 356), has been shown by M. Devergie to be incapable of dissolving that copper which is contained naturally in the tissues, at least so as to render it discoverable by the subsequent steps of the process. 3, According to Orfila, copper naturally present in organic substances, is never indicated by the second branch of the process, provided the charred product of the action of nitric acid and chlorate of potash be not heated to incineration. It does not appear why the charring process, when so conducted, should separate adventitious copper, and not that which is present naturally. But the empirical fact may be accepted in the mean time, as it rests on apparently careful experiments.

Orfila does not use acetic acid in the first branch of his process, but merely infuses the suspected matter in cold water, and if copper be not thus found, he has recourse to boiling water. But this method introduces needless complexity; and besides neither maceration, nor boiling with mere water, will dissolve out the whole oxide of copper. Acidulation with acetic acid dissolves it all; and Devergie has shown that this advantage is gained without any additional fallacy arising from the possible presence of copper as a natural ingredient of the substance under examination (p. 356).