We are indebted to the illustrious Scheele for the discovery of this acid, though Macquer had before noticed its combinations. It may be obtained by various methods. If six parts of nitric acid be poured on one of the concrete arsenious acid, or white arsenic of the shops, in the pneumato-chemical apparatus, and heat be applied, nitrous gas will be evolved, and a white concrete substance, differing in its properties from the arsenious acid, will remain in the retort. This is the arsenic acid. It may equally be procured by means of aqueous chlorine, or by heating concentrated nitric acid with twice its weight of the solution of the arsenious acid in muriatic acid. The concrete acid should be exposed to a dull red heat for a few minutes. În either case an acid is obtained, that does not crystallize, but attracts the moisture of the air, has a sharp caustic taste, reddens blue vegetable colours, is fixed in the fire, and of the specific gravity of 3.391.

If the arsenic acid be exposed to a red heat in a glass retort, it melts and becomes transparent, but assumes a milky hue on cooling. If the heat be increased, so that the retort begins to melt, the acid boils, and sublimes into the neck of the retort.

If a

covered crucible be used instead of the glass retort, and a violent heat applied, the acid boils strongly, and in a quarter of an hour begins to emit fumes. These, on being received in a glass bell, are found to be arsenious acid; and a small quantity of a transparent glass, difficult to fuse, will be found lining the sides of the crucible. This is arseniate of alumina.

Combustible substances decompose this acid. If two parts of arsenic acid be mixed with about one of charcoal, the mixture introduced into a glass retort, coated, and a matrass adapted to it; and the retort then gradually heated in a reverberatory furnace, till the bottom is red; the mass will be inflamed violently, and the acid reduced, and rise to the neck of the retort in the metallic state mixed with a little oxide and charcoal powder. A few drops of water, devoid of acidity, will be found in the receiv

er.

With sulphur the phenomena are different. If a mixture of six parts of arsenic acid, and one of powdered sulphur, be digested together, no change will take place; but on evaporating to dryness, and distill. ing in a glass retort, fitted with a receiver, a violent combination will ensue, as soon as the mixture is sufficiently heated to melt the sulphur. The whole mass rises almost at once, forming a red sublimate, and sulphurous acid passes over into the receiver. If pure arsenic acid be diluted with a small quantity of water, and hydrogen gas, as it is evolved by the action of sulphuric acid on iron, be received into this transpa

rent solution, the liquor grows turbid, and a blackish precipitate is formed, which, being well washed with distilled water, exhibits all the phenomena of arsenic. Sometimes, too, a blackish gray oxide of arsenic is found in this process.

If sulphuretted hydrogen gas be employed instead of simple hydrogen gas, water and a sulphuret of arsenic are obtained.

With phosphorus, phosphoric acid is obtained, and a phosphuret of arsenic, which sublimes.

The arsenic acid is much more soluble than the arsenious. According to Lagrange, two parts of water are sufficient for this pur pose. It cannot be crystallized by any means; but, on evaporation, assumes a thick honey-like consistence.

No acid has any action upon it: if some of them dissolve it by means of the water that renders them fluid, they do not produce any alteration in it. The boracic and phosphoric are vitrifiable with it by means of heat, but without any material alteration in their natures. If phosphorous acid be heated upon it for some time, it saturates itself with oxygen, and becomes phosphoric acid.

The arsenic acid combines with the earthy and alkaline bases, and forms salts very different from those furnished by the arsenious acid.

All these arseniates are decomposable by charcoal, which separates arsenic from them by means of heat.

* Berzelius, from the result of accurate experiments on the arseniates of lead and barytes, infers the prime equivalent of arsenic acid to be 7.25, oxygen being 1.0; but Dr. Thomson, from his experiments on the arseniates of potash and soda, conceives that the double of the above number ought to be preferred, viz. 14.5. Ann. of Phil. vol. xv.

On the latter supposition, Berzelius's insoluble salts will consist of two primes of base and one of acid; and the acid itself will be a compound of 5 of oxygen = 5, + 9.5. of the metallic base = 14.5; for direct experiments have shown it to consist of 100 metal, and from 52 to 53 oxygen. But 152.5: 100 :: 14.5 : 9.5 nearly.

All its salts, with the exception of those of potash, soda, and ammonia, are insoluble in water; but except arseniate of bismuth, and one or two more, very soluble in an excess of arsenic acid. Hence, after barytes or oxide of lead has been precipitated by this acid, its farther addition redissolves the precipitate. This is a useful criterion of the acid, joined to its reduction to the metallic state by charcoal, and the other characters already detailed. Sulphuric acid decomposes the arseniates at a low temperature, but the sulphates are'decomposed by arsenic acid at a red heat, owing to the greater fixity of the latter. Phosphoric, nitric, muriatic, and fluoric acids, dissolve, and probably convert into subsalts all the arseniates. The whole of them, as well as arsenic acid itself when decomposed at a red heat by charcoal, yield the characteristic garlic smell of the metallic vapour. Nitrate of silver gives a pulverulent brick-coloured precipitate, or, according to Dr. Thomson, a flesh red, with arsenic acid. The acid itself does not disturb the transparency of a solution of sulphate of copper; but a neutral arseniate gives with it a bluish green precipitate; with sulphate of cobalt, a dirty red, and

with sulphate of nickel, an apple green precipitate. Th These precipitates redissolve, on adding a small quantity of the acid which previously held them in solution. Orfila says, that arsenic acid gives, with acetate of copper, a bluish white precipitate, but that it exercises no action either on the muriate or acetate of cobalt; but with the ammonia-muriate it gives a rosecoloured precipitate. Arsenic acid ought to be accounted a more violent poison than even the arsenious. According to Mr. Brodie, it is absorbed, and occasions death by acting on the brain and the heart. *

The arseniate of barytes is insoluble, uncrystallizable, soluble in an excess of its acid, and decomposable by sulphuric acid, which precipitates a sulphate of barytes. Of the arseniate of strontian nothing is known, but no doubt it resembles that of barytes.

With lime-water this acid forms a precipitate of arseniate of lime, soluble in an excess of its base, or in an excess of its acid, though insoluble alone. The acidulous arseniate of lime affords on evaporation little crystals, decomposable by sulphuric acid. The same salt may be formed by adding carbonate of lime to the solution of arsenic acid. This acid does not decompose the nitrate or muriate of lime; but the saturated alkaline arseniates decompose them by double affinity, precipitating the insoluble calcareous arseniate.

If arsenic acid be saturated with magnesia, a thick substance is formed near the point of saturation. This arseniate of magnesia is soluble in an excess of acid; and on being evaporated takes the form of a jelly, without crystallizing. Neither the sulphate, ate, nitrate, nor muriate of magnesia is decomposed by arsenic acid, though they are by the saturated alkaline arseni

ates.

Arsenic acid saturated with potash does not easily crystallize. This arseniate, being evaporated to dryness, attracts the humidity of the air, and turns the sirup of violets green, without altering the solution of litmus. It fuses into a white glass, and with a strong fire is converted into an

acidule, part of the alkali being abstracted by the silex and alumina of the crucible. If exposed to a red heat with charcoal in close vessels it swells up very much, and arsenic is sublimed. It is decomposed by sulphuric acid; but in the humid way the decomposition is not obvious, as the arsenic acid remains in solution. On evaporation, however, this acid and sulphate of potash are obtained.

on the

ter

If arsenic acid be added to the preceding salt, till it ceases to have any effect on sirup of violets, it will redden the solution of litmus; and in this state it affords very regular and very transparent crystals, of the figure of quadrangular prisms, minated by two tetraedral pyramids, the angles of which answer to those of the prisms. These crystals are the arsenical neutral salt of Macquer. As this salt differs from the preceding arseniate by its crystallizability, its reddening solution of litmus, its not decomposing the calcareous and magnesian salts like it, and its capability of absorbing an additional portion of potash, so as to become neutral, it ought to be distinguished from it by the term of acidulous arseniate of potash.

With soda in sufficient quantity to saturate it, arsenic acid forms a salt crystallizable like the acidulous arseniate of potash. Pelletier says, that the crystals are hexaedral prisms terminated by planes perpendicular to their axis. This neutral arseniate of soda, however, while it differs completely from that of potash in this respect, and in becoming deliquescent instead of crystallizable on the addition of a surplus portion of arsenic acid, resembles the arseniate of potash in its decomposition by charcoal, by acids, and by the earths.

Combined with ammonia, arsenic acid forms a salt affording rhomboidal crystals analogous to those of the nitrate of soda. The arseniate of ammonia, which is produced likewise in the decomposition of nitrate of ammonia by arsenious acid, is decomposable in two ways by the action of heat. If it be gently heated, the ammonia is evolved, and the arsenic acid is left pure. If it be exposed to a violent and rapid heat, part of the ammonia and part of the acid reciprocally decompose each other; water is formed; azotic gas is given out; and the arsenic sublimes in a shining metallic form. Magnesia partly decompo ses the arseniate of ammonia, and forms a triple salt with a portion of it.

Arsenic acid saturated with alumina forms a thick solution, which, being evaporated to dryness, yields a salt insoluble in water, and decomposable by the sulphuric, nitric and muriatic acids, as well as by all the other earthy and alkaline bases. The arsenic acid readily dissolves the alumina of the crucibles in which it is reduced to a state of fusion; and thus it attacks silex also, on which it has no effect in the humid way.

We know nothing of the combination of this acid with zircone.

By the assistance of a strong fire, as Fourcroy asserts, arsenic acid decomposes the alkaline and earthy sulphates, even that of barytes; the sulphuric acid flying off in vapour, and the arseniate remaining in the retort. It acts in the same manner on the nitrate, from which it expels the pure acid. It likewise decomposes the muriates at a high temperature, the muriatic acid being evolved in the form of gas, and the arsenic acid combining with their bases, which it saturates; while the arsenious acid is too volatile to have this effect. It acts in the same manner on the fluates, and still more easily on the carbonates, with which, by the assistance of heat, it excites a brisk effervescence. Lagrange, however, denies that it acts on any of the neutral salts, except the sulphate of potash, and soda, the nitrate of potash, and the muriates of soda and ammonia, and this by means of heat. It does not act on the phosphates, but precipitates the boracic acid from solutions of borates when heated.

Arsenic acid does not act on gold or platina; neither does it on mercury or silver without the aid of a strong heat; but it oxidizes copper, iron, lead, tin, zinc, bismuth, antimony, cobalt, nickel, manganese, and arsenic.

This acid is not used in the arts, at least directly, though indirectly it forms a part of some composition used in dyeing. It is likewise one of the mineralizing acids combined by nature with some of the metallic oxides.

ACID (ARSENIOUS). Fourcroy was the first who distinguished by this name the white arsenic of the shops, which Scheele had proved to be a compound of the metal arsenic with oxygen, and which the authors of the new chemical nomenclature had consequently termed oxide of arsenic. As, however, it manifestly exhibits the an acid, though in a slight

degree, it has a fair claim to the title, for

many oxides and acids are similar in this, that both consist of a base united with oxygen, and the only difference between them is, that the compound in which the acid properties are manifest is termed an acid, and that in which they are not is called an oxide.

This acid, which is one of the most virulent poisons known, frequently occurs in a native state, if not very abundantly; and it is obtained in roasting several ores, particularly those of cobalt. In the chimneys of the furnaces where this operation is conducted, it generally condenses in

thick semi-transparent masses; though sometimes it assumes the form of a powder or of little needles, in which state it was formerly called flowers of arsenic.

The arsenious acid reddens the most sensible blue vegetable colours, though it turns the sirup of violets green. On exposure to the air it becomes opaque, and covered with a slight efflorescence. Thrown on incandescent coals, it evaporates in white fumes, with a strong smell of garlic. In close vessels it is volatilized; and, if the heat be strong, vitrified. The result of this vitrification is a transparent glass, capable of crystallizing in tetraëdra, the angles of which are truncated. It is easily altered by hydrogen and carbon, which deprive it of its oxygen at a red heat, and reduce the metal, the one forming water, the other carbonic acid, with the oxygen taken from it: as it is by phosphorus, and by sulphur, which are in part converted into acids by its oxygen, and in part form an arsenical phosphuret or sulphuret with the arsenic reduced to the metallic state. Hence Margraaf and Pelletier, who particularly examined the phosphurets of metals, have asserted they might be formed with arsenious acid. Its specific gravity is 3.7.

It is soluble in thirteen times its weight of boiling water, but requires eighty times its weight of cold. The solution crystallizes, and the acid assumes the form of regular tetraëdrons according to Fourcroy; but, according to Lagrange, of octaëdrons, and these frequently varying in figure by different laws of decrement. It crystallizes much better by slow evaporation than by simple cooling.

* The solution is very acrid, reddens blue colours, unites with the earthy bases, and decomposes the alkaline sulphurets. Arsenious acid is also soluble in oils, spirits, and alcohol; the last taking up from 1 to 2 per cent. It is composed of 9.5 of metal + 3 oxygen; and its prime equivalent is therefore 12.5. Dr. Wollaston first observed, that when a mixture of it with quick-lime quick-lin is heated in a glass tube, at certain temperature, ignition suddenly per

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a

limes. As arseniate of lime is found at the bottom of the tube, we perceive that a portion of the arsenious acid is robbed of its oxygen, to complete the acidification of the rest.*

There are even some metals, which act upon the solution, and have a tendency to decompose the acid, so as to form a blackish precipitate, in which the arsenicis very slightly oxidized.

The action of the other acids upon the arsenious is very different from that which they exert on the metal arsenic. By boiling, sulphuric acid dissolves a small por

tion of it, which is precipitated as the solution cools. The nitric acid does not dissolve it, but by the help of heat converts it into arsenic acid. Neither the phosphoric nor the carbonic acid acts upon it; yet it enters into a vitreous combination with the phosphoric and boracic acids. The muriatic acid dissolves it by means of heat, and forms with it a volatile compound, which water precipitates; and aqueous chlorine acidifies it completely, so as to convert it into arsenic acid.

The arsenious acid combines with the earthy and alkaline bases. The earthy arseniates possess little solubility, and hence the solutions of barytes, strontian, and lime, form precipitates with that of arsenious acid.

con

The acid enters into another kind of combination with the earths, that formed by vitrification. Though a part of this volatile acid sublimes before the glass enters into fusion, part remains fixed in the vitrified substance, to which it imparts transparency, a homogeneous density, density, and siderable gravity. The arsenical glasses appear to contain a kind of triple salt, since the salt and alkalis enter into an intimate combination at the instant of fusion, and remain afterwards perfectly mixed. All of them have the inconvenience of quickly growing dull by exposure to the air.

With the fixed alkalis the arsenious acid forms thick arsenites, which do not crystallize; which are decomposable by fire, the arsenious acid being volatilized by the heat; and from which all the other acids precipitate this in powder. These saline compounds were formerly termed livers, because they were supposed to be analogous to the combinations of sulphur with the alkalis.

With ammonia it forms a salt capable of crystallization. If this be heated a little, the ammonia is decomposed, the nitrogen is evolved, while the hydrogen, uniting with part of the oxygen of the acid, forms water. Neither the earthy nor alkaline arsenites have yet been much examined; what is known of them being only sufficient to distinguish them from the arseniates.

The nitrates act on the arsenious acid in a very remarkable manner. On treating the nitrates and arsenious acid together, the nitrous acid, or nitrous vapour, is extricated in a state very difficult to be confined, as Kunckel long ago observed; part of its oxygen is absorbed by the arsenious acid; it is thus converted into arsenic acid, and an arseniate is left in the retort. The same phenomena take place on detonating nitrates with arsenious acid; for it is still sufficiently combustible to produce a detonation, in which no sparks are seen, it is true, but with commotion and efferves cence; and a true arseniate remains at the bottom of the crucible. It was in this VOL. I.

[3]

way chemists formerly prepared their fixed arsenic, which was the acidulous arseniate of potash. The nitrate of ammonia exhibits different phenomena in its decomposition by arsenious acid, and requires considerable precaution. Pelletier, having mixed equal quantities, introduced the mixture into a large retort of coated glass, placed in a reverberatory furnace, with a globular receiver. He began with a very slight fire; for the decomposition is so rapid, and the nitrous vapours issue with such force, that a portion of the arsenious acid is carried off undecomposed, unless you proceed very gently. If due care be taken that the decomposition proceeds more slowly, nitrous acid first comes over; if the fire be continued, or increased, ammonia is next evolved; and lastly, if the fire be urged, a portion of oxide of arsenic sublimes in the form of a white powder, and a vitreous mass remains in the retort, which powerfully attacks and cor. rodes it. This is arsenic acid. The chlo

rate of potash, too, by completely oxidizing the arsenious acid, converts it into arsenic acid, which by the assistance of heat, is capable of decomposing the muriate of potash that remains.

The arsenious acid is used in numerous instances in the arts, under the name of white arsenic, or of arsenic simply. In many cases it is reduced, and acts in its metallic state.

Many attempts have been made to introduce it into medicine; but as it is known to be one of the most violent poisons, it is probable that the fear of its bad effects may deprive society of the advantages it might afford in this way. An ar senite of potash was extensively used by the late Dr. Fowler of York, who published a treatise on it, in intermittent and remittent fevers. He likewise assured the writer, that he had found it extremely efficacious in periodical headache, and as a tonic in nervous and other disorders; and that he never saw the least ill effect from its use, due precaution being employed in preparing and administering it. Externally it has been employed as a caustic to extirpate cancer, combined with sulphur, with bole, with antimony, and with the leaves of crowfoot; but it always gives great pain, and is not unattended with danger. Febure's remedy was water one pint, extract of hemlock 3j, Goulard's extract Ziij, tincture of opium Zj, arsenious acid gr. x. With this the cancer was wetted morning and evening; and at the same time a small quantity of a weak solution was administered internally. A still milder application of this kind has been made from a solution of one grain in a quart of water, formed into a poultice with crumb of bread.

* It has been more lately used as an al

terative with advantage in chronic rheu matism. The symptoms which show the system to be arsenified are thickness, redness, and stiffness of the palpebra, soreness of the gums, ptyalism, itching over the surface of the body, restlessness, cough, pain at stomach, and headache. When the latter symptoms supervene, the administration of the medicine ought to be immediately suspended. It has also been recommended against chincough; and has been used in considerable doses with success, to counteract the poison of venemous serpents.

1

Since it acts on the animal economy as a deadly poison in quantities so minute as to be insensible to the taste when diffused in water or other vehicles, it has been often given with criminal intentions and fatal effects. It becomes therefore a matter of the utmost importance to present a systematic view of the phenomena characteristic of the poison, its operation, and consequenees. 1st, It is a dense substance, subsiding speedily after agitation in water. I find its sp. gr. to vary from 3.728 to 3.730, which is a little higher than the number given above; 72 parts dissolve in 1000 of boiling water, of which 30 remain in it, after it cools. Cold water dissolves, however, only 영양이 or to of the preceding quantity. This water makes the sirup of violets green, and reddens litmus paper. Lime water gives a fine white precipitate with it of arsenite of lime, soluble in an excess of the arsenious solution. Sulphuretted hydrogen gas, and hydrosulphuretted water precipitate a golden yellow sulphuret of arsenic. By this means 이이이이이 of arsenious acid may be detected in water. This sulphuret dried on a filter, and heated in a glass tube with a bit of caustic potash, is decomposed in a few minutes, and converted into sulphuret of potash, which reînains at the bottom, and metallic arsenic of a bright steel lustre, which sublimes, coating the sides of the tube. The hydrosulphurets of alkalis do not affect the arsenious solution, unless a drop or two of nitric or muriatic acid be poured in, when the characteristic golden yellow precipi tate falls. Nitrate of silver is decomposed by the arsenious acid, and a very peculiar yellow arsenite ofs silver precipitates, es, which however, is apt to be redissolved by nitric acid, and therefore a very minute addition of ammonia is requisite. Even this how ever, also, if in much excess, redissolves the silver precipitate.

1

omposed

As the nitrate of silver is justly regarded as one of the best precipitant tests of arsenic, the mode of using it has been a subject of much discussion. The presence of muriate of soda indeed, in the arsenical so

lution, obstructs, to a certain degree, the operation of this reagent. But, that salt is almost always present in the primæ viæ, and is a usual ingredient in soups, and other vehicles of the poison. If, after the water of ammonia has been added, by plunging the end of a glass rod dipped in it into the supposed poisonous liquid, we dip another rod into a solution of pure nitrate of silver, and transfer it into the arsenious solution, either a fine yellow cloud will be formed, or at first merely a white curdy precipitate. But at the second or third immersion of the nitrate rod, a central spot of yellow will be perceived surrounded with the white muriate of silver. At the next immersion this yellow cloud on the surface will become very conspicuous. Sulphate of soda does not interfere in the least with the silver test. The ammoniaco-sulphate, or rather ammoniaco-acetate of copper, added in a somewhat dilute state to an arsenious solution, gives a fine grass green and very characteristic precipitate. This green arsenite of copper, well washed, being acted on by an excess of sulphuretted hydrogen water, changes its colour and becomes of a brownish red. Ferro-Prussiate of of potash changes cha

it into a blood red. Nitrate of silver converts it into the yellow arsenite of silver.

1

Lastly, if the precipitate be dried on a filter, and placed on a bit of burning coal, it will diffuse a garlic odour. The cupreous test will detect ᅮᅮ이이이이 of the weight of the arsenic in water. The voltaic battery, made to act by two wires on a little arsenious solution placed on a bit of windowglass, developes metallic arsenic at the negative pole; and if this wire be copper, it will be whitened like tombac. We may here remark, however, that the most elegant mode of using all these precipitation reagents is upon a plane of glass, a mode practised by Dr. Wollaston in general chemical research, to an extent, and with a success, which would be incredible in other hands than his. Concentrate by heat in a capsule the suspected poisonous solution, having previously filtered it if necessary. Indeed, if it be very much disguised with animal or vegetable matters, it is better first of all to evaporate to dryness, and by a few drops of nitric acid to dissipate the organic products. The clear liquid being now placed in the middle of the bit of glass, lines are to be drawn out from it in different directions. To one of these a particle of weak ammoniacal water being applied, the weak nitrate of silver may then be brushed over it with a hair pencil. By placing the glass in different lights, either over white paper or obliquely before the eye, the slightest change of tint will be perceived. The ammoniaco