Chap. IV Of Arsenic
Process I.
To extract Arsenic from its Matrices. Zaffre or Smalt.
Powder some Cobalt, white Pyrites, or other Arsenical matters. Put this powder into a retort with a short wide neck, leaving a full third thereof empty. Set your retort in a reverberating furnace; lute on a receiver; heat your vessel by degrees, and increase the fire till you see a powder sublime into the neck of the retort. Keep up the fire in this degree as long as the sublimation continues: when this begins to slacken, raise your fire, and make it as strong as the vessels will bear. When nothing more ascends, let it go out. On unluting the vessels, you will find in the receiver a little Arsenic in the form of a fine light farina. The neck of the retort will be full of white flowers, not quite so fine, some of which will appear like little crystals; and if a good deal of Arsenic be sublimed, a ponderous matter, like a white, semi-transparent glass, will be found adhering to that part of the neck of the retort which is next its body.
OBSERVATIONS.
Arsenic is a metallic substance still more volatile than Zinc; so that it cannot be separated from the matters with which it is mixed otherwise than by sublimation. It is proper, however, to take notice, that it is not naturally in a metallic form, and that, properly speaking, the whole Sublimate obtained from Cobalt, as above directed, is nothing but a metallic calx, that cannot be brought to the form and gloss of a metal, till it be worked up with fatty matters, as we shall shew in its place.
This calx is of a very singular nature, and differs from every other metallic calx, in that this is volatile, and all the rest extremely fixed; even those procured from the semi-metals: for the Flowers of Zinc, which are justly considered as a calcined Zinc, though obtained by a sort of sublimation, are not for all that of a volatile nature, but rather exceedingly fixed; seeing they are capable of sustaining the most violent fire, and melt instead of subliming. Arsenic, on the contrary, is not only extracted from its ore by sublimation, but when once sublimed continues to be volatile, and flies off in vapours as soon as it is exposed even to a moderate degree of heat.
This metallic matter, before it is combined with the phlogiston, is called White Arsenic, or plain Arsenic: it acquires the title of Regulus of Arsenic when it is united with the phlogiston, and glitters like a metal.
Though Arsenic be volatile, yet it requires a pretty strong fire to separate it from the minerals containing it, especially in close vessels; because it adheres very close to earthy and vitrifiable matters. This adhesion is so firm, that, when thus combined, it is capable of bearing a melting heat, and vitrifies with metallic calces, and other fusible matters. On this account it is impossible to extract from Cobalt, or other Arsenical matters, all the Arsenic they contain by working them only in close vessels. If such matters are to be freed from all their Arsenic, you must, after you have extracted all they will yield by distillation, put them into a crucible, and set it uncovered in the midst of a strong fire. Many Arsenical vapours will still rise; and care must be taken to stir the contents of the crucible frequently with an iron rod, to facilitate the discharge of the remaining Arsenic.
It often happens that the Arsenic, obtained from minerals by sublimation, is not very white, but of a lighter or darker grey colour. This is owing to some particles of inflammable matter, from which Arsenical minerals are seldom quite free. A very small quantity of phlogiston is sufficient to deprive much Arsenic of its whiteness, and to give it a grey colour. But when fouled in this manner, it may easily be brought to its due degree of whiteness: it need only be sublimed once more, after mixing it with some substance on which it doth not act; Sea-salt, for instance. If the matters from which Arsenic is extracted contain Sulphur also, as some pyrites do, the Arsenic sublimes with much less heat, than when it is united with earthy matters only; because it combines with the Sulphur, wherewith it hath a great affinity, and the Sulphur serves to separate the Arsenic, by this interposition, from the earth. In consequence hereof, Sulphur may be employed to extract Arsenic out of the earths in which it is fixed. In this case, the Sulphur changes the colour of the Arsenic, which it makes of a lighter or deeper yellow, or even red, in proportion to the quantity there is of it, and to the degree of fire that hath acted on both together.
The consistence of Arsenic is different, according to the degree of heat applied in subliming it. If the Arsenical vapour meet with a cold place, it gathers there in the form of a powder, as the Flowers of Sulphur do: this is the case with that which falls into the receiver in distilling it. But if it be stopped in a hot place, and cannot escape from that heat, it condenses into a heavy, compact, semi-transparent body, having undergone the first degree of fusion.
Yet it cannot be perfectly melted, so as to flow like other fused matters: not that it is refractory; for, on the contrary, the degree of heat in which it begins to melt is very moderate, and it is in its own nature very fit to promote the fusion of refractory matters: but the reason is this; it is necessarily converted into vapours by the degree of heat necessary to fuse it, and these vapours burst the vessels, if they find no vent.
Arsenic made yellow by a mixture of Sulphur, which is also called Orpiment, is reducible to the form of a solid Sublimate with more ease; because it is alloyed with a twentieth, or perhaps a tenth part, of its weight of Sulphur, which renders it more fusible.
Red Arsenic, which contains still more Sulphur, melts also more easily. It then becomes of a transparent red, like a ruby: and hence, when it is in this form, it is called Ruby of Arsenic.
When a combination of Sulphur and Arsenic is wanted, it is better to mingle and distil together such minerals as contain Sulphur and Arsenic, the white and the yellow pyrites, for instance, than to mingle pure Arsenic with pure Sulphur: for the great volatility of these two substances is a hindrance to their uniting; whereas, when combined with other matters, they are capable of sustaining a much greater degree of heat, which favours and promotes their union.
Those who work by the grate do not extract Arsenic out of Cobalt by distillation: they throw the ore mixed promiscuously with wood and charcoal into a great furnace, from whence a flue carries the vapours into a long winding passage, across which beams of wood are fixed at proper distances from each other. The Arsenical vapours being conducted into this passage, adhere both to the sides thereof and to the joists that lye across it. The fuliginous parts of the combustible matters being lighter ascend higher, and go out through a chimney at the farther end of this passage.
The Arsenic sublimed by this method is not white, but of a grey colour; owing to the inflammable matter of the wood and charcoal with which the ore is torrefied.
When all the Arsenic the Cobalt will yield is thus separated, the earthy fixed matter left behind is mixed with divers fusible matters and vitrified, and produces a glass of a beautiful blue colour. It is called Smalt. This glass is to be prepared in the following manner.
Take four parts of fine fusible sand, an equal quantity of any Fixed Alkali perfectly depurated, and one part of Cobalt from which the Arsenic hath been sublimed by torrefaction. Pulverize these different substances very finely, and mix them thoroughly together; put the mixture into a good crucible, cover it, and set it in a melting furnace. Make a strong fire, and keep it up constantly in the same degree for some hours. Then dip an iron wire into the crucible; to the end of which a glassy matter will stick, in the form of threads, if the fusion and vitrification be perfect. In this case take the crucible out of the fire; cool it by throwing water on it, and then break it. You will find in it a glass, which will be of an exceeding deep blue, and almost black, if the operation hath succeeded. This glass, when reduced to a fine powder, acquires a much brighter and more lively blue colour.
If you find after the operation that the glass hath too little colour, the fusion must be repeated a second time, with twice or thrice the quantity of Cobalt. If, on the contrary, the glass be too dark, less Cobalt must be used.
Instead of the mixture here prescribed you may employ a ready-made glass, providing it be white and fusible. But as glass is always hard to melt, and as the mixing Cobalt with it renders it still more refractory, therefore though an Alkaline Salt be one of the ingredients in its composition, it is proper to promote the fusion, by mixing therewith calcined wine-lees, in the quantity of one third part of the weight of the Cobalt.
In order to make the assay of a particular Cobalt, with a view to know what quantity of blue glass it will yield, it is necessary to perform the operation in the manner here set down; a great deal of time and trouble may be saved by melting one part of Cobalt with two or three parts of Borax. This Salt is very fusible, and turns, when melted, into a substance which, for a time, possesses all the properties of glass. In this trial the glass of Borax will be nearly of the same colour as the true glass, or Smalt, made with the same Cobalt.
The ores of Bismuth, as well as Cobalt, yield a matter that colours glass blue; nay, the Smalt made with these ores is more beautiful than that procured from the ore of pure Arsenic. Some Cobalts yield both Arsenic and Bismuth. When such Cobalts are used, it is common to find at the bottom of the crucible a little button of metallic matter, which is called Regulus of Cobalt. This Regulus is a sort of Bismuth, generally adulterated with a mixture of ferruginous and arsenical parts.
The heaviest and most fixed Flowers of Arsenic, procured from Cobalt, have likewise the property of giving a blue colour to glass. But this colour is faint: it is owing to a portion of the colouring matter carried up along with the Arsenic. These Flowers may be made an ingredient in the composition of blue glass, not only because of the colouring principle they contain, but also because they greatly promote fusion; Arsenic being one of the most efficacious fluxes known.
In short, all those blue glasses, or Smalts, contain a certain quantity of Arsenic; for a portion of this semi-metal always remains united with the fixed matter of the Cobalt, though roasted for a long time, and in a very hot fire. The portion of Arsenic that is thus fixed vitrifies with the colouring matter, and enters into the composition of the Smalt.
The blue glass made with the fixed part of Cobalt hath several names, according to the condition in which it is. When it hath undergone the first imperfect degree of fusion only it is called Zaffre. It takes the name of Smalt when perfectly vitrified: and this again being pulverized is called Powder-blue, or, if finely levigated, Blue Enamel; because it is used in enamelling, as well as in painting earthen ware and porcelain.
Process II
To separate Arsenic from Sulphur.
Powder the yellow or red Arsenic which you intend to separate from its Sulphur. Moisten this powder with a Fixed Alkali resolved into a liquor. Dry the mixture gently; put it into a very tall glass cucurbit, and fit on a blind-head. Set this cucurbit in a sand-bath; warm the vessels gently, and increase the fire by degrees, till you perceive that no more Arsenic sublimes. The Arsenic, which before was yellow or red, rises into the head partly in white flowers, and partly in a compact, white, semi-transparent matter, which looks as if it were vitrified. The Sulphur combined with the Fixed Alkali remains at the bottom of the cucurbit.
OBSERVATIONS.
A Fixed Alkali hath more affinity than any metallic substance with Sulphur: so that it is not surprising Sulphur should be separated from Arsenic by its interposition. Yet there is an inconvenience attends the use of it: for it hath a great affinity with the Arsenic also, and so always retains some part thereof, which continues fixed with it. For this reason care should be taken not to mix, with sulphurated Arsenic, a greater quantity of Alkali than is necessary to absorb the Sulphur it contains. Nothing, however, but experience and repeated trials can teach us the exact quantity of Alkali that ought to be employed; because the quantity of Sulphur that may be contained in yellow or red Arsenic is indefinite.
The vessel ought to be tall, that the upper part of the head, where the Arsenical particles condense, may be the less exposed to heat. Towards the end of the operation the fire must be strongly excited, so as to make the sand red-hot; because the last portions of Arsenic that rise are strongly retained by the Fixed Alkali.
Arsenic that is grey or blackish may be depurated and whitened by the same means; because a Fixed Alkali absorbs the phlogiston likewise with great avidity. Mercury, as well as a Fixed Alkali, is an excellent additament for separating Arsenic from Sulphur. If you will use it for that purpose, reduce the sulphurated Arsenic to a very fine powder, by rubbing it a long time in a glass mortar; when it is well pulverized, let a few drops of Mercury fall upon it, by squeezing it through chamoy, and continue the trituration. The yellow or red colour of the Arsenic will insensibly change, and gradually grow darker as the Mercury incorporates with it. When the Mercury is perfectly killed, add a little more of it than you did the first time, and in the same manner: continue to triturate till it disappear; and thus go on adding more and more till the Mercury you add remain quick, and you can kill no more of it. Neither the red nor the yellow colour will then appear in the mixture; which will be grey, if it contain but a little Sulphur, and black, if a great deal.
Put this mixture into a very tall glass cucurbit; fit on a blind-head; set it in a sand-bath, and bury it in the sand as far as the contained mixture reaches. Heat the vessels, and, during the whole operation, keep up a degree of fire a little weaker than that required for subliming Cinabar. White Arsenical Flowers will adhere to the upper part of the head, amongst which will be some beautiful crystals of Arsenic; and underneath them you will find some Cinabar sublimed, but not entirely free from Arsenic. If you desire to have your Cinabar and your Arsenic purer, and more unmixed with each other, separate the upper sublimate, which is Arsenical, from the lower, which consists chiefly of Cinabar. Powder each of them coarsely, and sublime them separately each in a different cucurbit.
On this occasion the Mercury separates the Sulphur from the Arsenic, because it hath a greater affinity than Arsenic with that mineral. It is not the only metallic substance of this character: for, as hath been shewn, there are several others that have a greater affinity than Mercury with Sulphur, being able to decompose Cinabar by their interposition. Yet those metallic substances must not be substituted for Mercury in the present operation: because there is none of them but hath at the same time a very great affinity with Arsenic, or even as strong an one as they have with Sulphur; whereas Mercury will by no means unite with Arsenic.
This method of separating Arsenic from Sulphur hath two advantages over that in which a Fixed Alkali is the medium. The first is, that by this means all the Arsenic contained in the mixture is extracted out of it; and the second, that, as Mercury doth not absorb Arsenic, we are not put to the trouble of groping out, as it were, by trials the quantity necessary to be added; and that, though more be added than is necessary to absorb all the Sulphur, it will be of no prejudice to the operation. But then it is attended with the inconvenience of being much more tedious and more laborious than the other. For, in the first place, it requires previously a very tiresome trituration, in order to procure an union between the Sulphur and the Mercury, and so to form an Æthiops; without which the Mercury and the sulphurated Arsenic will sublime separately, so that no decomposition will be effected. Secondly, though the Mercury be sufficiently united with the Sulphur of the Arsenic by the long trituration that precedes the sublimation, this doth not prevent, as we took notice above, the sublimed Arsenic and Cinabar from being in some measure blended together; so that each requires a second separate sublimation to render it very pure.
These inconveniencies cause a Fixed Alkali to be used preferably to Mercury; the loss of a small quantity of the Arsenic, which remains united with the Alkali, being little regarded; as that metallic substance is neither scarce nor precious.
When Arsenic is united with a great quantity of Sulphur, it may be freed from a part thereof without the intervention of any third body: it is sufficient for the purpose to sublime it with a very gentle fire, increased by insensible degrees. The most sulphureous part ascends first; what rises afterwards is more Arsenical, and less sulphureous; and the last flowers of all are pure Arsenic, or at least nearly so.
Process III
To give Arsenic the Metalline Form. Regulus of Arsenic.
Take two parts of white Arsenic in fine powder, one part of the black flux, half a part of Borax, and as much clean iron filings. Rub the whole together, in order to mix them thoroughly. Put this mixture into a good crucible, and over it put Sea-salt three fingers thick. Cover the crucible; set it in a melting furnace; and begin with a gentle fire to heat the crucible equally.
When arsenical vapours begin to ascend from the crucible, raise the fire immediately so as to melt the mixture. Examine whether or no the matter be thoroughly melted, by introducing an iron wire into the crucible; and if the fusion be perfect, take the crucible out of the furnace. Let it cool; break it; and you will find in it a Regulus of a white and livid metallic colour, very brittle, scarcely hard, but rather friable.
OBSERVATIONS.
White Arsenic is, as hath been said, a metallic calx; and consequently wants no more, in order to its acquiring the metalline properties, than to be combined with the phlogiston: this is effected by the operation before us.
The Iron added doth not serve here, as in making the Regulus of Antimony, to precipitate the Regulus of Arsenic, by separating it from some other substance with which it was united: on this occasion it does nothing but join the Regulus of Arsenic, to which it gives solidity and consistence. This is the only reason of its being made an ingredient in the mixture; as the Regulus of Arsenic, without it, would have such a tender consistence, that it could scarce be handled without falling asunder into little bits. The Iron procures a further advantage in this process; which is, that it prevents a great quantity of Arsenic from being lost in vapours: for the Arsenic, with which it combines, is restrained, and, in some measure, fixed by it.
Copper may be substituted for Iron, and procures the same advantages.
It is very necessary to remove the crucible from the fire as soon as the matter is melted, and indeed to cool it as expeditiously as possible, to prevent the Arsenic from flying off in vapours: for, when once the Regulus is formed, the proportion of Arsenic, with respect to that of the metal mixed with it, is continually lessening while it stays in the fire; so that, after some time, there will be left in the crucible, not a Regulus of Arsenic, but only Iron or Copper, alloyed with a little Arsenic. On this occasion the Copper turns white, and assumes the colour of Silver; but it soon tarnishes in the air.
It is easy to perceive, by what hath been said, that the Regulus of Arsenic made according to this process is never pure, but contains always a considerable quantity of Iron or Copper, whatever precautions be used: but it is difficult to avoid this inconvenience, for the reasons above assigned; and if we attempt to fuse Arsenic alone, with reducing fluxes, the greatest part thereof is dissipated in vapours, long before the very flux begins to melt: and that part of it, which is found metallized, is not collected in one mass at the bottom of the crucible, as in other metallic reductions; but in small particles, dispersed and mixed among the scoriæ. There are nevertheless several expedients for obtaining a Regulus of Arsenic absolutely pure, and unalloyed with any metallic substance.
First: into a little low cucurbit, covered with a blind-head, put Regulus of Arsenic made with Iron or Copper; set this cucurbit in a sand-bath; heat it till the sand begins to grow red, and you will see part of the Regulus sublime into the head, still retaining its metalline splendour. The portion of Regulus thus sublimed is pure Arsenic, or at least contains but a very small portion of the adventitious metal, which may have been carried up with it. What is left in the bottom of the cucurbit is the metal that was added, still containing a little Arsenic, which continues obstinately fixed with it, and which the violence of fire is unable to force away from it in close vessels.
Secondly: mix your Arsenic in equal parts with the black flux; put the mixture into such a cucurbit as that last mentioned; and apply to it the strongest degree of heat that can be procured by a sand-bath; arsenical flowers, of a blackish grey colour, will first sublime into the head, and after them a Regulus of Arsenic of a white metalline colour, which is pretty glossy, but tarnishes very soon in the air. This Regulus hath no solidity: it is exceedingly friable; but it is pure.
Thirdly: I have also made a Regulus of pure Arsenic by another method, which produces a much greater quantity thereof, with a much smaller degree of heat. For this purpose I powder the Arsenic, and mix with it any Fat Oil; so that the mixture may be like a liquid paste: this paste I put into a little phial of thin glass, like one of those used by apothecaries; I set this phial in a sand-bath, and gradually heat it, till the bottom of the pot containing the sand begin to be red. Part of the Oil first rises out of the phial in vapours, which must be suffered to pass off. After this the upper part of the phial is gradually lined, on the inside, with a glittering metallic crust, which makes it look like a quick-silvered glass. This crust is the Regulus of Arsenic. When it begins to sublime, the mouth of the phial must be slightly stopped with a bit of paper, and the heat increased a little, till you see that nothing more rises.
If you break the bottle after the operation, you will find its upper part crusted over with a coat of Regulus, thicker or thinner in proportion to the quantity of Arsenic employed. The Regulus is in a mass, of a beautiful brilliant colour, which to me seems to stand the air better than that of any Regulus made by other methods; probably because of the great quantity of fat matter with which it is united, and by which it is defended.
This Regulus of Arsenic is absolutely pure, and a much greater quantity thereof is obtained, by this method, than by treating it with the black flux; because the Arsenic is much sooner and more easily combined with the inflammable matter: and hence it comes to pass that part of the Arsenic doth not rise at first in grey flowers, as in operating with the black flux. Moreover, by our process, all the Arsenic is sublimed in Regulus: whereas, when the black flux is employed, a pretty considerable part of the Arsenic unites with the alkaline part of the flux, and remains fixed therewith. In our operation there is nothing left at the bottom of the phial, except an oily, light, but very fixed coal.
Regulus of Arsenic, in whatever manner made, may be easily reduced into white, crystalline Arsenic, by the means of a Fixed Alkali, or of Mercury, applied in the same manner as for separating Arsenic from Sulphur.
Process IV
To distil the Nitrous Acid by the interposition of Arsenic. Blue Aqua Fortis. A new Neutral Salt of Arsenic.
Pulverize finely any quantity you please of refined Salt-petre. Mix it accurately with an equal weight of white crystalline Arsenic, well pulverized, or else with very white and very fine flowers of Arsenic. Put this mixture into a glass retort, leaving one half of it empty. Set your retort in a reverberating furnace; apply a receiver having a small hole drilled in it, and containing a little filtered rain-water; lute the receiver to the retort with stiff lute. Begin with putting two or three small live coals in the ash-hole of the furnace, and replace them with others when they are ready to go out. Go on thus warming your vessels by insensible degrees, and put no coals in the fire-place, till the retort begin to be very warm. You will soon see the receiver filled with vapours of a dark-red, inclining to a russet colour. With a bit of lute stop the little hole of the receiver. The vapours will be condensed in the water of this vessel, and give it a very fine blue colour, that will grow deeper and deeper as the distillation advances. If your Salt-petre was not very dry, some drops of Acid will also come over, and falling from the nose of the retort mix with the water in the receiver. Continue your distillation, increasing the fire little by little as it advances, but exceeding slowly, till you see that when the retort is red-hot nothing more comes off; and then let your vessels cool.
When the vessels are cold, unlute the receiver, and, as expeditiously as you can, pour the blue aqua fortis it contains into a crystal bottle; which you must seal hermetically, because this colour disappears in a short time when the liquor takes air. You will find in the retort a white saline mass moulded in its bottom, and some flowers of Arsenic sublimed to its upper cavity, and into its neck.
Pulverize the saline mass, and dissolve it in warm water. Filter the solution, in order to separate some arsenical parts that will be left on the filter. Let the filtered liquor evaporate of itself in the open air; when it is sufficiently evaporated, crystals will shoot in it representing quadrangular prisms, terminated at each extremity by pyramids, that are also quadrangular. These crystals will be in confused heaps at the bottom of the vessel: over them will be other crystals in the form of needles; a saline vegetation creeping along the sides of the vessel; and the surface of the liquor will be obscured by a thin dusty pellicle.
OBSERVATIONS.
Arsenic, as we took notice in our Elements of the Theory, besides the properties it hath in common with metallic substances, possesses others also in common with saline substances. One of the most remarkable among the latter is that of decomposing Nitre; of expelling the Acid of that Salt from its Alkaline basis, assuming its place, and forming with that Alkali a Neutral Salt, which is very soluble in water, and shoots into regular crystals.
To inquire into what passes in the decomposition of Nitre by Arsenic, and into the new Salt resulting from thence, was the design of the first Memoir given in by me to the Academy of Sciences on this subject, and from that the present process is copied. Though the whole quantity of Arsenic prescribed in the process doth not enter into the composition of the new Neutral Salt, seeing some of it sublimes in flowers, that quantity must not therefore be thought too great: for we see, on the other hand, that part of the Nitre is not decomposed. The needle-like Salt is no other than Nitre that hath not suffered any decomposition, and actually deflagrates on live coals like common Nitre.
The precaution of putting some water in the receiver is absolutely necessary, to condense the nitrous vapours that rise in the distillation: for they are so elastic, so volatile, so dephlegmated, that a very small part of them will otherwise be condensed into a liquor, while the rest will remain in the form of vapours, to which vent must be given through the small hole in the receiver, as without that they will burst the vessels with impetuosity: and consequently scarce any Acid will be obtained; especially if the Nitre employed be very dry, as it must be to be reducible into a fine powder.
The blue colour communicated by the Nitrous Acid to the water is very remarkable. The cause that produces this colour is not yet known.
Though the Acid is, on this occasion, mortified by a great quantity of water, yet, when it rises out of the retort, it is so concentrated as to form, even with that water, if too much be not put in, a most active and even smoking aqua fortis.
It is necessary in this operation, and more so than in any other, to warm the vessels gradually, and to proceed exceeding slowly; otherwise the artist runs the risque of seeing his vessels burst to pieces with violence, and with great danger to his person: for Arsenic acts on Nitre with incredible vivacity; insomuch that, if a mixture of Nitre and Arsenic be heated to a certain degree, the Nitre is decomposed almost as rapidly, and with as great an explosion, as when it is made to fulminate with an inflammable matter. In short, the appearances are such, that one would be almost induced to think the Nitre really takes fire on this occasion: though it be only decomposed just as it is by the Vitriolic Acid.
The solution of the caput mortuum of this distillation contains, at the same time, several sorts of Salts: to wit, 1. the Neutral Salt of Arsenic, formed by the union of the Arsenic with the basis of the Nitre; this shoots into the prismatic crystals above-mentioned: 2. some Nitre that hath not been decomposed; this forms the needles and part of the vegetations: 3. a small portion of Arsenic, that is known to be soluble in water; this forms the thin dark pellicle that covers the surface of the liquor when it begins to evaporate.
For the properties of this new Neutral Salt of Arsenic you may consult what we have said thereupon in our Elements of the Theory, and in the Memoirs of the Academy of Sciences.
Process V.
To alkalizate Nitre by Arsenic.
Melt in a crucible the Nitre you intend to alkalizate. When it is melted, and moderately red, project upon it two or three pinches of pulverized Arsenic. A considerable effervescence and ebullition will immediately be produced in the crucible, attended with a noise like that which Nitre makes, when it detonates with an inflammable matter. At the same time a thick smoke will rise, which at first will smell like garlic, the odour peculiar to Arsenic; it will also smell afterwards like Spirit of Nitre. When the effervescence in the crucible is over, throw again upon the Nitre as much pulverized Arsenic as you did the first time; and all the same phenomena will be repeated. Continue thus throwing in Arsenic in small parcels, till it produce no more effervescence; taking care to stir the matter at every projection with an iron wire, the better to mix the whole together. Then increase your fire, and melt what remains. Keep it thus in fusion for a quarter of an hour, and then take the crucible out of the fire. It will contain a Nitre alkalizated by Arsenic.
OBSERVATIONS.
This operation, as well as the preceding one, is a decomposition of Nitre by Arsenic; yet the result is very different: for, instead of a Salt capable of crystallizing, and discovering no tokens either of Acid or Alkali, we obtain, on this occasion, only a Salt that runs into a liquor by the moisture of the air, doth not crystallize, and hath all the properties of an Alkali.
These differences arise only from the different manner in which the decomposition of the Nitre, and the union of the Arsenic with the basis of that Salt, is brought about. When the Nitrous Acid is distilled by the interposition of Arsenic, with a view to obtain the Arsenical Salt, the operation must be performed in close vessels; no greater degree of heat must be applied to the mixture than is necessary for enabling the Arsenic to act; and that heat must be administered very slowly and by insensible degrees. But, when the business is to alkalizate Nitre by the means of Arsenic, the operation is performed in a crucible, in a naked fire, with a strong degree of heat, and that suddenly applied. The violence of the heat, the suddenness with which it is applied, the vivacity wherewith the Arsenic unites with the basis of the Nitre; and, still more than all these, the free access of the air, occasion the greatest part of the Arsenic, which at first combines with the basis of the Nitre after having expelled its Acid, to be presently carried off and dissipated in vapours; and consequently the basis of the Nitre, not being sufficiently saturated, discovers its Alkaline properties.
I say, the concurrence of the air contributes, still more than all the rest, to separate the Arsenic from the Alkaline basis of the Nitre; experience having taught me that the Neutral Salt of Arsenic is not to be alkalizated by the most violent force of heat, as long as it continues in close vessels, and the external air hath no communication with it; but that some of the Arsenic contained in that Salt is dissipated, by exposing it to a strong heat in open vessels.
The tumult and effervescence that arise, when Arsenic is projected on Nitre fused in a crucible, are so considerable, and so nearly resemble the detonation of Nitre with an inflammable matter, that we should be tempted to think, if we trusted appearances only, that Arsenic furnishes a combustible matter, and that the Alkalization of the Nitre is effected, on this occasion, in the same manner as when it is fixed by charcoal: but, by examining attentively what passes, we easily discover that there is no inflammation at all, and that the Nitre is alkalizated in the manner and by the means above pointed out.
The first vapours that rise, when Arsenic is projected on Nitre, are purely arsenical; and, if any cold body be put in their way, they adhere to it in the form of flowers. These vapours are actual particles of Arsenic, carried up by the heat before they could come to act on the Nitre; but they are soon after mixed with Nitrous vapours, consisting of the Acid of the Nitre, which the Arsenic expels from its basis as fast as it comes to act on that Salt.
The nearer you come to the end of the operation, the more does the matter in the crucible lose of its fluidity, though an equal fire be constantly kept up in the furnace. At last it becomes quite like a paste, and the fire must be made much stronger to put it again in fusion. The reason of this is, that Nitre when alkalizated is much less fusible than when it is not so. The case is the same when this Salt is alkalizated by deflagration.
Though the Nitre, when alkalizated, makes no more effervescence with Arsenic, and though, when kept in fusion, it emits no more arsenical vapours, it doth not thence follow that it is a pure Alkali, and that it contains no Arsenic: it still contains a large quantity thereof, but so strongly united that the force of fire is not able to separate them; which hath led some authors to give this Salt the title of Fixed Arsenic.
The existence of Arsenic in this saline compound is easily discovered, by fusing it with metallic substances, on which it produces the same effects as Arsenic.
With solutions of metals in the Acids, it also presents almost the same phenomena as the Neutral Salt of Arsenic. Particularly it precipitates Silver dissolved by the Nitrous Acid in a red powder, as that Salt does; and the differences observed between the precipitations made by our new Neutral Salt of Arsenic, and those made by Nitre alkalizated with Arsenic, can be attributed only to the alkaline quality of the latter. See the Memoirs of the Academy for 1746.