Qualities. Fused nitrate of silver is in small cylinders of a dark grey colour, and presenting, when broken across, a crystalline structure. Odour, none; Taste, intensely bitter, austere and metallic; it tinges the skin indelibly black; when perfectly free from copper, it is not deliquescent. Chemical Composition; oxide of silver 70, nitric acid 30, or one atom of oxide and one atom of acid. Solubility. In an equal weight of water, at 60°; it is also soluble in alcohol. The solution readily yields transparent colourless crystals, the primary form of which is a right rhombic prism. Incompatible Substances. Fixed alkalies and alkaline earths, the muriatic, sulphuric, and tartaric acids, and all the salts which contain them; Soaps, arsenic, hydro-sulphurets, astringent vegetable infusions, undistilled waters. The solutions of nitrate of silver are not disturbed by ammonia, the ammoniuret being very soluble; the carbonate of ammonia, however, produces a precipitation. Nitrate of silver tinges the skin and hair black, and has been frequently employed for the latter purpose;^[394] it likewise forms the basis of permanent ink.^[395] Medicinal Uses. Tonic, antispasmodic, and escharotic; it is said to prove efficacious in epilepsy, but during a trial for several years in the Westminster hospital, I never could discover its virtues; many of the cases in which it has been supposed to have been successful, probably derived advantage from the purgative medicines which were simultaneously administered. It possesses a bitter taste, and it has been said to act like vegetable bitters upon the digestive organs, and to offer a resource in dyspeptic complaints. It is principally useful as an external application, and may be considered as the strongest and most manageable caustic that we possess;^[396] whilst in solution it acts as a useful stimulant in indolent ulcers; and being possessed of the power of coagulating animal matter, it does not spread to any extent, and is therefore extremely convenient where a large eschar is to be avoided. A weak solution of this metallic salt has lately been strongly recommended by a French surgeon, as a remedy for piles of long standing; it also forms an excellent lotion to excite the weak granulations of fungous ulcers. It is, moreover, said to be highly useful as an injection in cases of puriform discharges from the ear; before we direct however such an application, it is highly necessary that we should ascertain the tympanum to be entire, or the liquid may escape into the internal ear, and occasion very alarming irritation; an event which unhappily occurred, not long since, in the case of a noble duke of high military renown. Forms of Exhibition. For internal use, in pills made with crumb of bread, with the addition of some sugar, to prevent the mass from being too hard. Dose, gr. 1/8, gradually increased to gr. j. Adulterations. Copper may be always suspected when it deliquesces, and is to be immediately detected by its solution assuming a blue colour, when supersaturated with ammonia. The sticks should be preserved in closely stopped phials, and covered with soft and dry paper. Antidote. When this substance has been taken in excess, muriate of soda is its true antidote; indeed, so completely does it decompose, and separate it from water, that if a saturated solution of nitrate of silver be filtered through common salt, it may be afterwards drunk with impunity. This circumstance alone, would of necessity render nitrate of silver a very uncertain remedy; and yet it is evident that the basis of this salt is occasionally absorbed, for there are several cases upon record, in which the oxyd of silver has been deposited in the rete mucosum, and given a purple hue of a very singular appearance to the patient; I have lately witnessed an instance of this kind in a lady who had taken large doses of the nitrate, for the purpose of curing a dyspeptic complaint; and several other similar cases stand recorded in different works.

Qualities. Taste, hot and acrid; Odour, pungent. Chemical Composition. All its virtues depend upon an essential oil. Solubility. Both water and alcohol extract its active principles, but they are dissipated by decoction. Medicinal Uses. As a stimulant in paralysis it is often useful; Sydenham found it successful in dropsies which were consequent on intermittent fevers; Cullen recommends a syrup made with the infusion of horse radish, to remove that species of hoarseness which depends upon local relaxation; Dr. Withering extols an infusion of this root in milk as a cosmetic both safe and effectual. Incompatible Substances. Alkaline Carbonates; Oxy-muriate of Mercury; Nitrate of Silver; the Infusion of Galls and of Yellow Cinchona Bark, produce precipitates with the infusion of this root. Forms of Exhibition. In substance, scraped or swallowed whole, or in infusion.^[398] Dose of the substance ʒj, of an infusion f℥ij. See Infus Armoraciæ comp. Officinal Preparations. Infusum Armoraciæ comp: L. Spiritus Armoraciæ comp: L.D.

Qualities. Form, shining semivitreous lumps, breaking with a conchoidal fracture, and when reduced to powder, bearing some resemblance to white sugar; Taste, acrid and corrosive, but not in any degree corresponding with its virulence, leaving an impression of sweetness. Specific gravity 3·7; it is volatilized at the temperature of 383° Fah: and by a strong heat is vitrified into a transparent glass capable of crystallizing in tetrahedra with truncated angles, or rather in octohedra. In the state of vapour it is quite inodorous, although it is asserted in many chemical works of authority to yield a smell like that of garlic; the fact is that the alliaceous or garlic-like smell is wholly confined to metallic arsenic in a state of vapour, and whenever the arsenious acid seems to yield this odour, we may infer that its decomposition has taken place; this happens when it is projected upon ignited charcoal, or when heated in contact with those metallic bodies which readily unite with oxygen, as Antimony and Tin. It is stated by Orfila and other chemists, that if it be projected upon heated copper the alliaceous odour is evolved. This assertion is undoubtedly true, but the fact requires to be explained with more precision, or we may fall into an important error respecting it. The phenomenon takes place only when the copper is in a state of ignition, at which temperature its affinity for oxygen enables it to reduce the arsenious acid; for I find by experiment that if a few grains of this substance be heated on a plate of copper, by means of a spirit lamp or blow-pipe, no odour is perceptible, for the whole of the acid is dissipated before the copper can acquire a sufficiently exalted temperature to deoxidize it. If the arsenious acid be heated on a plate of zinc, the smell is not evolved until the metal is in the state of fusion; if instead of these metals we employ in our experiments those of gold, silver, or platinum, no alliaceous smell whatever is produced, at any temperature. It however deserves particular notice, that the flame of the spirit lamp is itself capable of decomposing the oxyd, in consequence of the operation of its hydrogen: a fact which is very likely to betray the chemist into the fallacious belief that the oxyd does yield the odour in question.^[399] It is probable that arsenical vapours which yield this peculiar odour are less noxious than those which are inodorous, but I am not aware that the knowledge of this fact can be applied to any purpose of practical importance.^[400] Chemical Composition. This substance possesses many of the essential habitudes of an acid, as for instance, that of combining with the pure alkalies to saturation; it is therefore very properly denominated Arsenious Acid. It may be farther acidified by distilling it with nitrous acid, and the compound which results is a white concrete substance termed Arsenic Acid; from experiments on the quantity of oxygen absorbed by metallic arsenic, during its conversion into these two compounds, instituted by Proust and Davy, it appears that the arsenious acid consists of about 25 of oxygen and 75 of metal, and the arsenic acid of 33 of oxygen and 67 of metal; or, the quantity of metal being the same, that the oxygen in the latter compound is to that in the former nearly as three to two. Solubility. We have but lately been set right upon this point; Klaproth has shewn that it requires for its solution 400 parts of water at 60° and only 13 at 212°, and moreover, that if 100 parts of water be boiled on the arsenious acid, and suffered to cool, it will retain three grains in solution, and deposit the remainder in tetrahedral crystals; this fact shews the importance of employing boiling water in every chemical examination of substances supposed to contain arsenic. It is soluble in alcohol and oils, the former taking up two per cent.; with lime water it produces a white precipitate of arsenite of lime, which is soluble in an excess of arsenious acid; with magnesia it forms a soluble arsenite, which proves very virulent. The poisonous effects of arsenious acid are so amply detailed in medical works,^[401] that it would be superfluous to dwell upon them in this place; it may however be interesting and useful to record an account of the pernicious influence of arsenical fumes upon organized beings, as I have been enabled to ascertain in the copper smelting works, and tin burning-houses of Cornwall. This influence is very apparent in the condition both of the animals and vegetables in the vicinity; horses and cows commonly lose their hoofs, and the latter are often to be seen in the neighbouring pastures crawling on their knees, and not unfrequently suffering from a cancerous affection in their rumps, whilst the milch cows, in addition to these miseries, are soon deprived of their milk; the men employed in the works are more healthy than we could a priori have supposed possible; the antidote upon which they all rely with confidence, whenever they are infested with more than an ordinary portion of arsenical vapour, is sweet oil, and an annual sum is allowed by the proprietors in order that it may be constantly supplied; this opinion is not solitary, for Tachenius relates that the poisonous effects, such as convulsions, gripes, and bloody stools, with which he was seized from exposure to the fumes of arsenic, were relieved by milk and oil.

It deserves notice that the smelters are occasionally affected with a cancerous disease in the scrotum, similar to that which infests chimney-sweepers, and it is singular that Stahl in describing the putrescent tendency in the bodies of those who die from this poison, mentions in particular the gangrenous appearance of these parts. It is a very extraordinary fact that previous to the establishment of the copper works in Cornwall, the marshes in their vicinity were continually exciting intermittent fever, whereas since that period a case of ague has not occurred in the neighbourhood; I have heard it remarked by the men in the works, that the smoke kills all fevers. The fact is here stated without any other comment than that the agricultural improvements which have taken place in the district, are not sufficient to afford any clue to the explanation of the circumstance. Medicinal Uses. Much has been said upon this subject, and the propriety and safety of its exhibition has been often questioned; there can be no doubt but that the greatest circumspection is required in the practitioner who administers it, and it ought not, in my opinion, to be employed until other remedies have failed; that it is capable of accumulating in the system is very evident, and this, in certain habits, may predispose the patient to serious diseases; the form in which it is most manageable and least dangerous, is that of solution. See Liquor Arsenicalis. Some practitioners have exhibited it in substance, made into pills, by rubbing one grain with ten of sugar, and then beating the mixture with a sufficient quantity of crumb of bread to form ten pills, one of which is a dose. The Chinese and other oriental nations form the sulphuret of arsenic (realgar) into medical cups, and use lemon juice, after it has stood some hours in them, by way of cathartic. As an external application, arsenic has long been extolled in the cure of cancers; the caustic so extensively used under the sanction of the late Mr. Justamond in cases of open cancer, consisted of two parts of Antimony, and one of Arsenious acid, fluxed together in a crucible, and afterwards levigated, and reduced to the requisite degree of mildness by the addition of powdered Opium.^[402] But it deserves notice in this place, that repeated experiments have proved that arsenic kills^[403] more rapidly when applied externally to an abraded part, than when internally administered. See page 132. Lionardo di Capoa relates the case of a child killed by the violent vomiting and purging arising from a slight wound made in the head by a comb, wet with oil, in which Arsenic had been infused for the purpose of killing vermin; and we have numerous instances on record, where the application of arsenical cerates and ointments has been followed by violent and dangerous symptoms. We also learn from the different historians of the Plague of London, that the arsenical amulets which were worn, as preservatives, on that occasion, were sometimes attended with deleterious consequences; Crato (Epist. 168.) observed an ulcer of the breast produced by them. Vernascha, violent pains and syncope. Amongst the foreign authors who have related cases of poisoning by the external application of Arsenic we may mention Desgranges (Recueil Period: de la Société de Med: de Paris, T. vi. p. 22.) who records the history of a chambermaid, poisoned by having rubbed her head with an arsenical ointment for the purpose of destroying vermin; and Roux (Nouveaux Elemens de Med: Operat. par. J. P. Roux,) who confessed to have killed a young girl of eighteen by an application of the “Pate Arsenicale” to a cancerous breast. To the Empirics of our own times we are indebted for many fatal illustrations of the subject. Since the last edition of this work, a Lady applied to a well known Quack, distinguished for his impudent pretensions in the treatment of cancer, and submitted to a caustic application to the breast. In a short time paralysis ensued, and the application was discovered to contain a large proportion of Arsenic, and that the disease, for the cure of which it had been applied, was not cancer. A somewhat analogous case occurred under the care of a bold empiric in the neighbourhood of St. George’s Fields, who undertook to remove the deformity of bow legs in a dandy drawing-master! by rasping the shin bones, and applying arsenic to the surface of the wound; in consequence of which, in addition to extensive local mischief, the unhappy dupe became paralytic. It is also necessary to inform the practitioner that Arsenious acid has been known to produce poisonous effects when applied to the unbroken skin; a case of this nature is related by Desgranges, in the sixth volume of the Recueil Periodique de la Soc: de Med: another may be found in the 22d volume of the Acta Germanica (1730); and Renault obtained similar results in his experiments on animals. When the system is under the influence of arsenic, the following symptoms will appear, viz. thickness, redness, and stiffness of the palpebræ, soreness of the gums, ptyalism, itching over the surface of the body, restlessness, cough, pain in the stomach and bowels, head-ache, and I have also occasionally noticed paucity of urine, and even stranguary, a fact of which I find no mention in other authors. Strange as it may appear, Arsenic has been inhaled, together with the vapours of frankincense, myrrh, and those of other gums during a paroxysm of asthma! This extraordinary practice arose from the practitioner mistaking the gum juniper, or Vernix of the Arabians, which by their medical authors was prescribed in fumigations under the name of Sandarach, for the Σανδαρακη of Aristotle, which was a sulphured of arsenic.

1. By its reduction to a metallic state. Mix a portion of the suspected powder with three times its weight of black flux;^[404] put the mixture into a thin glass tube, hermetically closed^[405] at one end, about eight inches in length, and one-fourth of an inch in diameter; should any of the powder adhere to the sides of the tube, it must be carefully brushed off with a feather, so that the inner surface of its upper part may be perfectly clean and dry; the closed end of the tube, by way of security, may be thinly coated with a mixture of pipe-clay and sand,^[406] but this operation is not absolutely necessary; the open extremity is to be loosely plugged with a piece of paper; the coated end must be now heated on a chaffing dish of red hot coals, when the arsenic, if present, will sublime, and be found lining with a brilliant metallic crust the upper part of the tube; a portion of this reduced metal, if it be arsenic, will, when laid on heated iron, exhale in dense fumes, which are characterised by a strong smell of garlic. Mr. Phillips has lately stated that the tube may be sufficiently heated, for the purpose of metallization, by means of a spirit lamp.^[407]

It merits particular notice, that in reducing by the above process the arsenious acid to the state of metal, the presence of potass in the flux is very essential, since it forms immediately an arsenite of potass, and thereby fixes the arsenious acid, and prevents it from being volatilized before the temperature is sufficiently high to enable the charcoal to decompose it; an ignorance of this fact has not unfrequently proved a source of disappointment and fallacy.

Another method of identifying white arsenic by metallization, is to form at the moment of its reduction, an alloy with copper, which is easily effected in the following manner,—Mix the suspected powder with black flux, as in the former experiment, and place the mixture between two polished plates of copper, bind them tight together by iron wire, and expose them to a low read heat; if the included substance contained arsenic, a silvery white stain will be left on the surface of the copper, which is an alloy of the two metals. If in this, as in the former experiment, charcoal be employed without the addition of a fixed alkali, the result may, for the same reason, prove unsatisfactory. But, with whatever care this experiment is conducted, it is, to say the least, a clumsy and unsatisfactory test, and ought never to be relied upon.

2. By the application of certain Reagents, or Tests, to its Solutions.

A great and important question has arisen in medical jurisprudence, whether any chemical proof of the presence of white arsenic, short of its actual reduction to the state of metal, can be depended upon, or ought to be received as evidence in the courts of criminal law. After a full experimental investigation of the subject, and an impartial review of all the facts which bear upon the question, I feel no hesitation in declaring it to be my conviction, that white arsenic may be detected without any fear of fallacy, by a proper application of certain tests, and that the contrary opinion is entirely founded in error, and unsupported by experiment, as will more fully appear in the sequel.^[408]

(A) Fused Nitrate of Silver, or Lunar Caustic—For this test we are indebted to Mr. Hume of London, who first gave it to the public in the Philosophical Magazine for May 1809, vol. xxxiii. His method of applying it is as follows: into a clean Florence flask introduce two or three grains of the suspected powder, to which add about eight ounces of rain or distilled water, and heat the solution until it begins to boil, then while it boils frequently shake the flask, and add to the hot solution a grain or two of sub-carbonate of potass, agitating the whole to make the mixture uniform. Pour into a wine glass about two table spoonsful of the solution, and touch the surface of the fluid with a stick of lunar caustic. If arsenic be present, a beautiful yellow precipitate will instantly proceed from the point of contact, and settle towards the bottom of the glass as a flocculent and copious precipitate.

By this test the 60th part of a grain may be satisfactorily recognised in two ounces of water. The presence of some alkali is essential to the success of the experiment, since arsenious acid is unable, by the operation of simple affinity, to decompose the nitrate of silver.^[409] The validity of this test has been questioned on the following grounds, which shall be fairly examined in order.

Objection 1. The alkaline phosphates are found to produce precipitates with silver, analogous in colour and appearance to the arsenite of silver. This is undoubtedly the case when the experiment is performed in the manner just stated, but there are other reagents which will immediately distinguish these bodies, as will be seen under the history of the Ammoniuret of silver; I have also shewn that there is a mode of so modifying the application of the silver test itself, that no error or doubt can arise in the use of it from the presence of phosphoric salts.^[410] My method consists in conducting the trial on writing paper, instead of in glasses, thus—drop the suspected fluid on a piece of white paper, making with it a broad line; along this line a stick of lunar caustic is to be slowly drawn several times successively, when a streak is produced of a colour resembling that known by the name of Indian Yellow; and this is equally produced by the presence of arsenic and that of an alkaline phosphate, but the one from arsenic is rough, curdy, and flocculent, as if effected by a crayon, that from a phosphate homogeneous and uniform, resembling a water-colour laid smoothly on with a brush; but a most important and distinctive peculiarity soon succeeds, for in less than two minutes the phosphoric yellow fades into a sad green, and becomes gradually darker, and ultimately quite black; while, on the other hand, the arsenical yellow remains permanent, or nearly so, for some time, when it becomes brown. In performing this experiment the sun-shine should be avoided, or the transitions of the colour will take place too rapidly. It would be prudent also for the inexperienced operator to perform a similar experiment on a fluid known to contain arsenic, and on another with a phosphoric salt, as a standard of comparison. In this way the nitrate of silver, without the intervention of any other test, is fully capable of removing every ambiguity, and of furnishing a distinguishing mark of difference between the chemical action of arsenic and that of the phosphates. Mr. Hume states that he has repeated this experiment to his entire satisfaction,^[411] and that, in a late unfortunate case of poisoning, he derived considerable information by its application. The laborious author of the London Dispensatory accepts it as an excellent test, but observes that it is rendered more luminous by brushing the streak lightly over with liquid ammonia immediately after the application of the caustic, when, if the arsenic be present, a bright queen’s yellow is produced which remains permanent for nearly an hour; but that when the lunar caustic produces a white-yellow before the ammonia is applied, we may infer the presence of some alkaline phosphate, rather than that of arsenic. One of the great advantages of this test is the very small quantity that is required for examination; it would be well therefore for the operator to perform the experiment in both ways on a separate paper.

Objection 2. The Muriates produce precipitates with silver so copious and flocculent as to overcome every indication which the presence of arsenic would otherwise afford. Dr. Marcet proposes to obviate this difficulty, by adding to the fluid to be examined dilute nitric acid, and then cautiously applying the nitrate of silver until the precipitation ceases; in this way the muriatic acid will be entirely removed, whilst the arsenic, if it be present, will remain in solution, and may be rendered evident by the affusion of ammonia, which will instantly produce the yellow precipitate in its characteristic form. This mode however it must be confessed appears complicated, and requires some chemical address for its accomplishment; it should be also known that the yellow precipitate thus produced is not always permanent, for it is soluble in the nitrate of ammonia formed during the process. Under these circumstances, it is surely preferable to precipitate at once from the suspected fluid all the substances which nitrate of silver can effect, and then to expose the mixed and ambiguous precipitate so obtained, to a low heat in a glass tube, when the arsenious acid will be immediately separated by sublimation. In this way the presence of muriates may even in certain cases be serviceable, especially if the quantity of arsenic be minute; for by increasing the bulk of the precipitate we shall decrease the difficulty of its examination. By this process, also, I should propose to meet the embarrassments which arise from the influence of various animal and vegetable substances, as milk, broth, wine, &c. so frequently present in the suspected liquid, and which are known to alter the character of the arsenical indications. In this case, however, we must not rely upon any single precipitant; after having thrown down all that is precipitable by the silver test, the supernatant liquid should be decanted, slightly acidified by acetic acid, and submitted to the action of Sulphuretted Hydrogen; when, should any precipitate occur, it must be separated and added to the former. Dr. Christison has demonstrated the importance of this proceeding by shewing that the precipitates, occasioned by the Ammoniurets of silver and copper, are soluble in certain vegetable infusions.^[412] M. Orfila proposes to remove the difficulties and embarrassments, occasioned by the colouring matter of different media, by the application of Chlorine, so as to change the colour to a shade that will not offer any optical impediment to the characteristic indications of the different tests. I am ready to admit that such a mode of proceeding may, on certain occasions, assist the accomplished chemist in his analysis, but in the hands of a person less accustomed to chemical manipulation, I hesitate not to declare that it is subject to fatal fallacies;^[413] whereas, by precipitating the whole, and submitting the precipitate to the process of sublimation, we shall avoid every source of error. Why then should we attempt to pursue our game through the windings of a labyrinth, when a direct road lies before us, by which we may at once drive it into the open plain? Mr. Phillips has recently proposed the addition of animal charcoal (Ivory Black) for the purpose of destroying the colouring matter. He found that by mixing this substance with the Liquor Arsenicalis, that the colouring matter was so completely destroyed in a few minutes, that the test of nitrate of silver, or any other might be readily applied. This experiment was repeated with Port wine, gravy soup, and a strong infusion of onions, and he succeeded in these cases in procuring a solution sufficiently colourless for the application of the most delicate reagents. It might be supposed, adds Mr. Phillips, that the Phosphoric acid which the animal charcoal contains, might have some share in the production of the yellow precipitate with silver; he found, however, that water, or wine, which was merely digested on the animal charcoal, produced no effect with the nitrate of silver, except a slight precipitate of chloride; and this even, was prevented by lixiviation. I have, however, a serious objection to offer to this proposal. Animal charcoal, by some mode of operation not understood, possesses the property of removing certain substances from their solution in water; I have already noticed this effect with respect to Lime water, (see p. 247, note), and I have lately found that it takes place with very dilute solutions of Arsenic.^[414] Hence charcoal, as we shall presently find, may be employed for the purpose of detecting minute portions of arsenic.

Objection 3. Chromate of potass produces, with Nitrate of Silver, a yellow precipitate which, when placed side by side with one produced by Arsenious acid, cannot be distinguished by colour or appearance from it. This fact has lately been announced by Dr. Porter of the University of South Carolina. (Silliman’s Journal, iii, 355.) But as the presence of chromate of potass can never be suspected in any research after arsenic, in cases of forensic interest, the fact is of no importance to the physician.

Where the Arsenious acid is mixed with vegetable matter, and it becomes difficult to separate it by filtration, the whole may be evaporated to dryness, taking great care that the heat applied for such a purpose never exceeds 250° Fah: or we shall lose the arsenic by volatilization. The residue thus obtained may then be submitted to a higher temperature, in a subliming vessel, in order to procure the arsenious acid in its pure state. Should the arsenious acid have, in the first instance, been dissolved in oil, Dr. Ure proposes to boil the solution in distilled water, and to separate the oil afterwards by the capillary action of wick threads. If the arsenious acid be mixed with resinous bodies, oil of turpentine may be employed as their solvent, which will leave the arsenic untouched. Dr. Black directed the application of alcohol for this purpose, but this is obviously improper, since arsenious acid is soluble in that fluid.

It has been stated that, in consequence of the inability of arsenious acid to decompose nitrate of silver by simple elective attraction, the presence of an alkali becomes indispensable in the examination, for which purpose Dr. Marcet has suggested the superior advantages which will attend the use of ammonia, in cases where the arsenic has not been previously combined with a fixed alkali, since it does not, when added singly, decompose nitrate of silver; a circumstance which in using the fixed alkalies is very liable to occasion fallacy. This led Mr. Hume to improve his original plan, by forming at once a compound, which he calls the Ammoniaco-nitrate of silver, but which may with more propriety be designated as an Ammoniuret.^[415] This is a triumph in the art of analysis; for whilst it obviates the necessity of ascertaining the exact proportion of alkali required in each experiment,^[416] it possesses the valuable property of not in the least disturbing the phosphate of soda.

(B.) Sulphate of Copper. Like the preceding test, this also requires, for its success, that the arsenious acid should be combined with some alkali, in which case, by the operation of double elective attraction, an arsenite of copper is thrown down of a very striking and characteristic colour, being that of the well-known pigment called Scheele’s green; if arsenic be not present in the liquid so assayed, and a fixed alkali has been employed, the result will be a delicate sky-blue, instead of the grass-green precipitate.

Mr. Hume avails himself also of the peculiar property of ammonia to form a metallic salt, and has employed it with copper: he takes the sulphate or acetate of that metal, and by the same process as that described for the preparation of an ammoniuret of silver, forms another test. In using this, however, care must be taken that it be not too highly concentrated, for in that state it will not produce precipitation. Much controversy has taken place on the subject of sulphate of copper as a test for arsenic, and it has been stated, with more confidence than truth, that a decoction of onions has the property of imparting to the copper precipitate, which is produced by a fixed alkali, a colour and appearance analogous to that which is occasioned by arsenic. This opinion was boldly advanced and supported on a most important^[417] trial at the Lent assizes for Cornwall in 1817. Since this event an opportunity^[418] has occurred which has enabled me to examine this alleged fact, by a fair and appropriate series of experiments, the result of which satisfactorily proved that the opinion was grounded on an optical fallacy, arising from the blue precipitate assuming a green colour, in consequence of having been viewed through a yellow medium.^[419] The phosphoric salts may also, under similar circumstances, be mistaken for arsenic, for the intense blue colour of the phosphate of copper will thus necessarily appear green. This instance of optical fallacy is not solitary, for corrosive sublimate has been said to possess the character of an alkali, because it turns the syrup of violets green, whereas this change is to be attributed solely to the combination of the yellow hue of the sublimate with the blue colour of the violet.

Whenever therefore such a source of fallacy can be suspected, the operator would do well to repeat his experiment on white paper, in the manner I have before proposed, and the results which are obtained in glasses should always be examined by day-light, and viewed by reflected and not by transmitted light. The presence of Peroxide of Iron in the Cupreous salt will also impart a green colour to the precipitate produced by an alkali. To obviate any fallacy which might arise from this circumstance, Mr. Phillips proposes to add some pure Potass to the sulphate of copper; if pure, a fine blue precipitate will be thus obtained; to this may be then added the suspected solution, and if Arsenious acid be present, it will then convert this blue precipitate to a green one.

(C.) Sulphuretted Hydrogen. This is a very delicate test for arsenic, producing with its solution a beautiful golden coloured liquor, which after a short time lets fall a precipitate. Mr. Phillips,^[420] in reviewing the third edition of the present work, has stated, that no such precipitate occurs, but I find that in close, as well as in vessels exposed to the air, it takes place by repose. By this reagent, so small a quantity as 1
000,000 may be detected in solution; and it may be also stated in farther proof of the utility of this test, that it is less affected than any other by the presence of animal or vegetable matter. The method of preparing a solution of sulphuretted hydrogen gas is extremely simple. Put into an oil flask about two ounces of undiluted muriatic acid and an ounce and a half of powdered Sulphuret of Antimony; fit a cork to the flask and pass through it the short leg of a small glass tube twice bent at right angles; pass the longer leg of the tube into a phial containing distilled water, and then by the heat of a spirit lamp applied to the flask, sulphuretted hydrogen gas will be abundantly liberated, and though much of it will escape, yet a sufficient quantity will be dissolved by the water. The annexed sketch represents the apparatus proposed by Mr. Phillips, and which will require but little practical skill, either for its construction or use.

There are several other tests by which arsenic may be identified. The process described in the Dublin Pharmacopœia for the preparation of Arsenias Kali, the arseniate, or rather super-arseniate of potass, which has been long known under the name of “the arsenical salt of Macquer,” has been strongly advised as a collateral proof; it consists in decomposing the nitrate of potass^[421] by the arsenious acid, but since this problem requires that the suspected poison should be in a solid and palpable form, it is impossible to examine its pretensions to our confidence, without being reminded of the story so often told to us in our infancy, of catching a bird by laying salt upon its tail.

It is necessary to observe in this place, that the arseniate, like the arsenite of potass, or that of ammonia, is obedient to the silver test, but that instead of the yellow precipitate which is produced by the latter salt, we obtain, by the former, a red or brick-coloured one.

If arsenious acid and quick-lime be heated together in a glass tube, a sudden ignition is occasioned at a certain temperature, when metallic arsenic will sublime, and an arseniate of lime be formed. In this case one portion of the arsenious acid is robbed of its oxygen to complete the acidification of the remainder.

In taking an impartial review of all the evidence which the investigation of this subject can furnish, it must appear to the most fastidious, that the silver and copper test above described are capable, under proper management, of furnishing striking and infallible indications, and that in most cases they will be equally conclusive, and in some even more satisfactory in their results, than the metallic reproduction upon which such stress has been laid, and for this obvious reason, that unless the quantity of metal be considerable,^[422] its metallic splendour and appearance is often very ambiguous and questionable. It has to my knowledge happened to a medical person, by no means deficient in chemical address, to ascribe to the presence of arsenic that which was no other than a film of very finely divided charcoal: in this state of doubt the last resource was to ascertain whether it yielded, or not, upon being heated, an alliaceous odour. Surely an unprejudiced judge would prefer the evidence of sight, as furnished by the arsenical tests, to that of smell, as afforded in the last experiment. No one will attempt to deny that it is the duty of the medical practitioner who is called upon to decide so important a question as the presence of arsenic, to prosecute by experiment every point which admits the least doubt; he should also remember that in a criminal case, he has not only to satisfy his own conscience, but that he is bound, as far as he is able, to convince the public mind of the accuracy and truth of his researches; and he fails in his duty if he omits, through any false principle of humanity, to express the strong conviction which the success of his experiments must necessarily have produced in his mind. Let it however be remembered, that the application of chemical reagents on solutions suspected to contain arsenic, so far from throwing any obstacle in the way of the metallic reduction of that body, are the very steps which should be adopted as preparatory to the “experimentum crucis,” since the precipitates which are thus produced may be collected, and easily decomposed, as before stated. Those who for judicial purposes may require farther information upon these subjects are referred to the second volume of our work on “Medical Jurisprudence.” Tit: Poisons.