Abbildungen der Seite
PDF
EPUB

precipitate, though it was the most saturated solution he could make in the cold, and presented the greatest degree of commotion, effervescence, and red vapours, during the combination of the mercury and acids. As it had deposited crystals, he added two drachms of distilled water, which dissolved the whole without any appearance of precipitation. With much greater safety, therefore, may such solutions as have been made in the cold with common nitric acid, and half their weight of mercury, be used in the analysis of mineral waters, for they will never afford a precipitate by the addition of

mere water.

2. The weakest nitric acid strongly heated on mercury will dissolve a larger quantity than the strongest acid in the cold. The solution, which is of a light yellow colour, will appear thick and oily, and will afford by standing an irregular yellowish mass, which may be changed into a beautiful turbith by the addition of boiling water; this solution poured into distilled water, forms a very abundant precipitate of a yellow colour, similar to turbith. A solution made in the cold exhibits the same result, if it be strongly heated, so as to disengage a large quantity of nitrous gas. The solutions made with heat ought therefore to be excluded from the analysis of mineral waters, because they are decomposable by distilled water.

3. The two solutions appear to differ from each other in the quantity of oxyde of mercury, which is much greater in that which is precipitated by the water than in that which is not decomposable by that fluid. M. Fourcroy has proved this, by evaporating equal quantities of both these solutions in an apothe. cary's phial, to reduce them into red precipitate, and he obtained one fourth more of this precipitate from the solution which is decomposed by water than from that which is not rendered turbid. The specific gravity likewise appeared to me to be a good method of ascertaining the relative quantities of oxyd of mercury contained in these different fluids. He compared weights of equal masses of three mercurial nitrous solutions: the one, which was not at all precipitated by distilled water, and was the result of the first mentioned experiment, weighed one ounce, one drachm, and sixty seven grains, in a bottle which contained exactly an ounce of distilled water. The second solution was made by a very gentle heat, and produced a slight opal colour with distilled water, and scarcely any sensible quantity of precipitate. The same bottle contained one ounce six drachms twenty-four grains. Lastly, a third mercurial solution considerably heated, and which precipitated a true turbith mineral of a dirty yellow, by distilled water, weighed in the same bottle one ounce seven drachms twenty-five grains. A decisive experiment remained to be made to confirm this opinion still more perfectly. If the solution precipitated by water owed this property to a quantity of mercurial oxyd too large with respect to the acid, it would of course lose that property by the addition of acid; this accordingly happened. Aquafortis was poured on a solution which was decomposed by water, and it soon acquired the property of no longer being precipitated, and was absolutely in the same state as that which had been made slowly at first, by the mere heat of the atmosphere. Monnet has mentioned this process, as a means of preventing crystals or mercurial nitrat from becoming converted into oxyd by the contact of the air. It is by a contrary process, and by evaporating a portion of the acid of a good solution, which is

not precipitated by water, that it is converted into a solution much more strongly charged with mercurial oxyd, and consequently capable of being decomposed by water; its original property may be restored by the addition of a quantity of acid, equal to that which it lost by evaporation.

Such are the different considerations M. Fourcroy has thought necessary to exhibit, that the effects of re-agents on waters may be better ascertained: but whatever may be the degree of precision to which researches of this nature may be carried; however extensive the knowledge we may have acquired concerning the degrees of purity, and the different states of such substances as are combined with mineral waters, for the purpose of discovering their principles; if it still remains a fact, that each of these re-agents is capable of indicating two or three different substances dissolved in these waters, the result of their action will always be subject to uncertainty. Lime, for example, seizes the carbonic acid, and precipitates salts with the base of alumine, and of magnesia, as well as the metallic salts. Ammoniac produces the same effect. Fixed alkalis, besides the above mentioned salts, precipitate those with base of lime. The calcareous prussiat, the prussiat of potash, and gallic alkohol, precipitate the sulphat and carbonat of iron. The nitric solutions of silver and of mercury decompose all the sulphuric and muriatic salts, which may be various both in quantity and in kind, in the same water, and are themselves decomposable by alkalis, chalk, and magnesia. Among this great number of complicated effects, how shall we distinguish that which takes place in the water under examination, or by what means shall we ascertain whether it is simple or compounded?

These questions, though very difficult, for the time when the expedients of chemistry were little known, are nevertheless capable of being discussed in the present state of our knowledge. It must first be observed, that the nature of re-agents being much better known at present than it was some years ago, and their re-action on the principles of water better ascertained, it may, therefore, be strongly presumed that their application may be much more advantageously made than has hitherto been supposed; nevertheless, among the great number of excellent chemists who have attended to the analysis of waters, Messrs. Baumé, Bergman, and Gioanetti are almost the only persons who have been aware of this great advantage. We have been long in the habit of examining mineral waters by re-agents, in very small doses, and often in glasses; the phenomena of the precipitations observed have been noted down, and the experiment carried no further. Baumé advises, in his chemistry, that a considerable quantity of the mineral water under examination should be saturated with fixed alkalis and with acids, that the precipitates be collected, and their nature examined. Bergman apprehended that the quantity of the principles contained in waters might be judged of from the weight of the precipitates obtained in these mixtures. Several other chemists have likewise employed this method, but always with a view to certain particular circumstances; and no one has hitherto proposed to make a connected analysis of mineral waters by this means. To succeed in this analysis, it would be proper to mix several pounds of the mineral water with each re-agent, till the latter ceases to produce any precipitate: the precipitate should then be suffered to subside during the

time of twenty-four hours, in a vessel accurately closed; after which the mixture being filtered, and the precipitate dried and weighed, the operator may proceed to examine it by the known methods. In this manner the nature of the substance will be clearly ascertained, on which the re-agent has acted, and the cause of the decomposition may consequently be inferred. A certain order may be followed in these operations, by mixing the waters first with such substances as stand least capable of altering them, and afterwards passing to other substances capable of producing changes more varied and difficult to explain. The following method is that which Fourcroy commonly uses in this kind of analysis. After having examined the taste, the colour, the weight, and all the other physical properties of a mineral water, he pours four pounds of lime water on an equal quantity of the fluid; if no precipitate is made in twenty-four hours, he is sure that the water contains neither disengaged carbonic acid nor alkaline carbonat, nor earthy salts with the base of aluminous earth or magnesia, nor metallic salts. But if a precipitate be formed, he filters the mixture, and examines the chemical properties of the deposited substance; if it has no taste, if it be insoluble in water, or effervesces with acids, or forms an insipid and almost insoluble salt by the addition of sulphuric acid, he concludes that it is chalk, and that the lime water has acted only on the carbonic acid dissolved in the water. If, on the contrary, it is small in quantity, and subsides very slowly; if it do not effervesce, and affords with the sulphuric acid a styptic salt, or a bitter and very soluble salt, it is formed by magnesia or aluminous earth, and often by both.

After the examination by lime water, Fourcroy pours on four other pounds of the same mineral water, a drachm or two of ammoniac perfectly caustic, or causes ammoniacal gas, disengaged by heat from the alkali, to pass into the water. When the water is saturated, it is left at rest in a close vessel for twentyfour hours; if a precipitate be afforded, it can onty consist of ferruginous, or magnesian, or aluminous salts, whose nature is examined by the different methods mentioned in the foregoing paragraph. But the action of ammoniacal gas being more fallacious than that of lime water, which produces the same decompositions, it must be observed that this last should only be used as an assistant means, which does not afford results equally accurate with those produced by the former re-agent.

When salts with base of aluminous earth, or magnesia, have been discovered by lime water, or by ammoniacal gas, the caustic fixed alkalis may be used, to distinguish those with base of lime, such as sulphat and muriat of lime. For this purpose Fourcroy precipitates some pounds of the water, which is examined by either of these liquid alkalis, till it no longer produces any turbidness. As this alkali decomposes salts with base of aluminous earth, as well as those composed of lime; if the precipitate resembles in its form, colour, and quantity, that which lime water has afforded, it may be presumed that the water does not contain calcareous salt; and the chemical examinations of the precipitate usually confirms this suspicion: but if the mixture is much more turbid than that made with hime water; if the deposition be much heavier, more abundant, and more readily afforded, the lime is mixed with magnesia or alumine. This is ascertained by treating the precipitate after the different methods before explained. It may easily be concluded, that iron precipitated by re-agents, at the same time as the saVOL VIII.

lino-terrestrial substances, is easily known by its colour and its taste; and that the small quantity of this metal separated in these processes, is not sufficient to affect the results.

It were useless to explain at large the effects of sulphuric acid, nitrous acid, gall-nuts, or of the calcareous and alkaline prussiats, employed as re-agents on mineral waters. The general account of these effects which has already been given may suffice; it need therefore only be noticed, that when they are mixed in large doses with these waters, and the precipitates collected, the nature and quantity of their principles may be more accurately ascertained, as has been done by Messrs. Bergman and Gioanetti. The products which the nitric solutions of silver or mercury afford when mixed with mineral waters, deserve particular attention. It is more particularly necessary to ope rate with large quantities of water, when these reagents are used, in order to determine the nature of the acids contained in the waters. The analysis of these fluids will be complete when their acids are known, because these are often combined with the bases exhibited by the re-agents before-mentioned. The colour, the form, and the abundance of the precipitates afforded by the nitric solutions of mercury and silver, have hitherto exhibited to chemists the nature of the acids which caused them. A thick and ponderous deposition immediately formed by these solutions, denotes the muriatic acid: if it is small in quantity, white, and crystallized with the nitrat of silver, or yellowish, and yellow and irregular when formed with that of mercury, and if it subside but slowly, it is attributed to the sulphuric acid. But as these two acids are often met with in the same water, and as alkali and chalk likewise decompose the solutions, the results or deductions made from the physical properties of the precipitates must be uncertain. It is therefore necessary to examine them more effectually for this purpose, solutions of silver or of mercury may be mixed with five or six pounds of the water intended to be analysed; the mixtures being filtered, twenty-four hours after the precipitates must be dried, and treated according to the methods of chemistry. If the precipitate afforded by the nitric solution of mercury be heated in a retort, the portion of metal united with the muriatic acid of the waters will be volatilized into mercurius dulcis, and that which is combined with the sulphuric acid will remain at the bottom of the vessel, and exhibit a reddish colour. These two salts may likewise be distinguished by putting them on a hot coal; the sulphat of mercury, if present, emits a sulphureous acid, and assumes a red colour; the mercuria! muriat remains white, and is volatilized without exhibiting any smell of sulphur. These phenomena likewise serve to distinguish the precipitates which may be formed by the alkaline substances contained in water, since the latter do not emit the sulphureous smell, and are not volatile without decomposition.

The precipitates produced by the combination of mineral waters with the nitric solution of silver may be as easily examined as the foregoing. Sulphat of silver being more soluble than the muriat of the same metal, distilled water may be successfully used to separate these salts. Muriat of silver is known by its fixity, its fusibility, and especially in its being less easily decomposed than sulphat of silver. This last, placed on hot coals, emits a sulphureous smell, and leaves an oxyd of silver, which may be fused without addition.

The Examination of the Mineral Waters by Distillation.-Distillation is used in the analysis of wa

D

ters, to ascertain the gaseous substances they may be united to. These substances are either air, more or less pure, or carbonic acid, or sulphurated hydrogen gas. To ascertain their nature and quantity, some pounds of the mineral water must be poured into a retort, sufficiently large to contain it, without being filled more than half or twothirds of its capacity to this vessel a recurved tube is to be adapted, which passes beneath an inverted vessel filled with mercury. In this disposition of the apparatus, the retort must be heated till the water perfectly boils, or till no more elastic fluid passes over. When the operation is finished, the quantity of air contained in the empty space of the retort must be subtracted from the bulk of the gas obtained; the rest consists of aeriform fluid, which was contained in the mineral water, whose properties may quickly be known by the proofs of a lighted taper, tincture of turnsole, and lime water; if it catches fire, and has a fœtid smell, it is sulphurated hydrogen gas; if it extinguishes the taper, reddens turnsole, and precipitates lime-water, it is the carbonic acid; lastly, if it maintains combustion without taking fire, is without smell, and alters neither turnsole nor lime water, it is atmospheric air. It may happen that this fluid may be purer than the air of the atmosphere in this case its degrees of purity may be judged by the manner in which it maintains combustion, or by mixing it with nitrous or hydrogen gas, in the eudiometers of Fontana and Volta. The process used in obtaining gaseous matters contained in waters is entirely modern. A moistened bladder was formerly used, which was adapted to the neck of a bottle filled with mineral water: the fluid was agitated, and by the swelling of the bladder, an estimate was made of the quantity of gas contained in the water. This method is now known to be fallacious, because water cannot give out all its gas but by ebullition, and because the sides of the moistened bladder alter and decompose the elastic fluid obtained. It is scarcely necessary to remark, that the phenomena exhibited by the water, during the escape of the gas, must be carefully examined, and that a less quantity of water may be exposed to distillation, in proportion as its taste and sparkling indicate that it contains a larger quantity of gas.

Such is the method recommended by modern chemists to obtain the elastic fluids combined with waters it must be observed, 1. That this process cannot be depended on, with regard to acid ulous waters, unless the pressure of the atmosphere, and the state of compression of the elastic Huid under the glass vessels, be more accurately accounted for and as this is not easily done, the absorption of carbonic acid by lime water, proposed by Gioanetti, appears to be preferable. 2. Though it has been recommended by Bergman to obtain sulphurated hydrogen gas from sulphureous waters, it does not answer, because the heat of ebullition decomposes the gas, and it is likewise decomposed by the mercury, which is converted into ethiops as soon as it comes in contact with this elastic fluid: for this reason, litharge should be used to absorb this gas in the cold, and to deprive sulphureous waters of their sulphur.

Examination of Mineral Waters by Evaporation.-Evaporation is generally considered as the most certain method of obtaining all the principles of mineral waters. We have before observed, and

here repeat, that the experiments of Venel and Cornette show, that long continued ebullition may decompose saline matters dissolved in water, and for that reason we have advised the examination of them by re-agents, employed in greater proportions; yet evaporation may afford much information, when used, together with the analysis by re-agents, which ought always to be considered as one of the principal methods of examining

waters.

The intention of evaporation being to collect the fixed principles contained in a mineral water, it is obvious, that in order to know the nature and proportion of these principles, a considerable quantity of the water must be evaporated, and so much the more, in proportion as the principles appear to exist in smaller quantities. When the water is thought to contain a large quantity of saline matter, about twenty pounds must be evaporated: if, on the contrary, it appears to hold but a very small quantity in solution, it will be necessary to eva porate a much larger quantity. It is sometimes requisite to perform this operation with several hundred pounds. The nature and form of the vessels in which waters are exposed for evaporation, is not a matter of indifference: those of metal, excepting silver, are altered by water; vessels of glass, of a certain magnitude, are very subject to be broken; but those of glazed smooth pottery are the most convenient, though the cracks in the glaze sometimes cause an absorption of saline matter; vessels of unglazed porcelain, called biscuit, would doubtless be the most convenient, but their price is a considerable obstacle. Chemists have proposed different methods of evaporating mineral wa-> ters; some have directed distillation to dryness, in close vessels, in order to prevent foreign substances, which float in the atmosphere, from mixing with the residue; but this method is excessively tedious; others have advised evaporation by a gentle heat, never carried to ebullition, because they supposed that this last heat alters the fixed principles, and carries a portion of them. This was the opinion of Venel and Bergman. Monnet, on the contrary, directs the water to be boiled, because this motion prevents the reception of foreign matters contained in the atmosphere. Bergman avoids this inconvenience, by directing the vessel to be covered, and a hole left in the middle of the cover for the vapours to pass out this last method greatly retards the evaporation, because it diminishes the surface of the fluid. At the commencement, the heat used must be sufficient to repel the dust; but the greatest difference in the manipulation of this experiment consists in some writers directing that the substances deposited should be separated, as the evaporation proceeds, in order to obtain each pure and by itself; others, on the contrary, direct the operation to be carried on to dryness. We are of the opinion of Bergman, that this last method is the most expeditious and certain; because, notwithstanding the care which may be taken, in the first method, to separate the different substances which are deposited or crystallized, they are never obtained pure, and must always be examined by a subsequent analysis; and the method is besides inaccurate, on account of the frequent filtrations, and the loss it occasions. Lastly, it is very embarrassing, and renders the evaporation much longer. Mineral waters may therefore be evaporated to dryness, in open glass vessels, on the water-bath, or still more advantageously in glass retorts, on a sand bath.

Various phenomena are observed during this eperation; if the water be acidulous, it emits bub bles, as soon as the heat first begins to act; in proportion as the carbonic acid is disengaged, a pellicle is formed, with a deposition of calcareous earth, and carbonat of iron. These first pellicles are succeeded by the crystallization of sulphat of lime; and lastly, the muriats of potash and soda crystallize in tubes at the surface, but the deliquescent are not obtained but by evaporation to dryness.

The residue must then be weighed, and put into a small phial, with three or four times its weight of alkohol: the whole being agitated, and suffered to subside for some hours, must be filtrated, and the alkohol preserved separate. The residue on which the spirit has not acted must be dried in a gentle beat, or in the open air; when perfectly dry it must be weighed, and the loss of weight will show what quantity of calcareous or magnesian muriat was contained, because these salts are very soluble in alkohol. We shall presently speak of the method of ascertaining the presence of these two salts in the spirituous fluid.

The residue, after treatment with alkohol; and drying, must be agitated with eight times its weight of cold distilled water, and filtered. After some hours standing, the residue is to be dried a second time, and boiled half an hour in four or five bundred times its weight of distilled water; this last residue, after filtration, consists of that which coid or boiling water is insufficient to dissolve. The first water contains neutral salts, such as sulphat of soda, or of magnesia; the muriat of soda, or potash and the fixed alkalis, especially soda united with carbonic acid: the large quantity of boiling water scarcely contains any sub stance but sulphat of lime. There are therefore four substances to be examined, after these diftlerent operations on the matter obtained by evaporation. 1. The residue insoluble in alkohol, and in water of different temperatures. 2. The salts dissolved in alkohol. 3. The salts dissolved in cold water. 4, and lastly, Those dissolved in boiling water. We shall now proceed to the experiments necessary to ascertain the nature of these different substances.

1. The residue which has resisted the action of the alkohol and water may be composed of calcareous earth, of carbonat of magnesia and iron, of alumine, and of quartz. These two last substances are seldom found in waters, but the three first are very common; the brown, or more or less deep yellow colour, indicates the presence of iron. If the residue be of a white grey, it does not contain this metal. When iron is present, Bergman directs it to be moistened, and exposed to the air till it rusts; in which state vinegar does not act on it. In order to explain the methods of separating these different substances, we will suppose an in. soluble residue to consist of the five substances here mentioned; it must first be moistened, and exposed to the rays of the sun; and when the iron is perfectly rusted, the residue must be digested in distilled vinegar. This acid dissolves the lime and magnesia, and by evaporation affords the calcareous acetit, distinguishable from the acetit of magnesia, by its not attracting the humidity of the air. They may consequently be separated by deliquescence, or by pouring sulphuric acid into their solution. The latter forms sulphat of lime, which precipitates; but if the magnesian acetit be present, the sulphat of magnesia, composed of mag

nesia united with the sulphuric acid, will remain in solution, and may be contained by a well conducted evaporation. To ascertain the quantity of magnesia and calcareous earths contained in this resi

due, sulphat of lime is first to be precipitated: and the sulphat of magnesia, formed by the sulphuric acid poured into the acetous solution, must then be precipitated by carbonat of potash. The quantities of these precipitates are known by weighing. When the chalk and magnesia of the residue are thus separated, the iron, the alumine, and the quartz remain. The iron and the alumine are dissolved by pure muriatic acid, from which the former is precipitated from prussiat of lime, and the latter by carbonat of potash. These precipitates must likewise be weighed. The matter which remains after the separation of the alumine and iron is usually quartorse; its quantity may be known by weighing, and its habitudes by fusion of the blow-pipe with carbonat of soda. Such are the most accurate processes, recommended by Bergman, for examining the insoluble residue of

waters.

2. The alkohol used in washing the solid residue of mineral waters must be evaporated to dryness. Bergman advises treating it with sulphuric acid diluted with water, in the same manner as the acetous solution before spoken of; but it must be observed, that this process serves only to exhibit the bases of these salts. To determine the acid, which is ordinarily united with magnesia or lime, and sometimes with both, a few drops of concentrated sulphuric acid must be poured on, which excites an effervescence, and disengages the muriatic gas, known by its smell and white vapour, when the salt under examination contains that acid. This may likewise be known by dis. solving the whole residue in water, and adding a few drops of the nitric solution of silver. The nature of the base, which, as we have observed, is either lime, magnesia, or both together, is known by the name of the sulphuric acid, by a similar process with that already explained respecting the acetous solution.

3. The water used in washing the first residue of the mineral water, performed as before directed, with eight times its weight of cold distilled water, contains neutral alkaline salts, such as sulphat of soda, muriats, or marine salts, carbonat of potash, and of soda, and sulphat of magnesia: a small quantity of sulphat of iron is sometimes found. These salts never exist altogether in waters: the sulphat of soda, and the carbonat of potash, are very seldom found; but marine salt is frequently met with, together with carbonat of soda. The sulphat of magnesia is likewise frequently met with, and some waters even contain it in considerable quantities. When the first washing of the residue of a mineral water contains only one kind of neutral salt, it may easily be obtained by crystallization, and its nature ascertained from its form, taste, and the action of fire, as well as that of the re-agents: but this case is very rare, for it is much more usual to find many salts united in this lixivium. They must therefore be separated, if practicable, by slow evaporation; but as this method does not always perfectly succeed, however carefully this evaporation be conducted, it will be necessary to re-examine the salts obtained at the different periods of the evaporation. Carbonat of soda is usually deposited confusedly with the muriatic salts, but they may be separated by a process, pointed out by M. Gioanetti.

It consists in washing this mixed salt with distilled vinegar; for this acid dissolves the carbonat of soda. The mixture must then be dried, and washed a second time with alkohol, which takes up the acetit of soda, without acting on marine salt. The spirituous solution being evaporated to dryness, and the residue calcined, the vinegar becomes decomposed and burns. Soda alone remains, whose quantity may be then accurately determined.

4. The water used in the quantity of four or five hundred times the weight of the residuum of the mineral water contains only sulphat of lime. This may be ascertained by pure caustic ammoniac, which occasions no change, while caustic potash precipitates it abundantly. By evaporation to dryness, the quantity of earthy salt contained in the water may be accurately ascertained.

Artificial Mineral Waters.-The numerous processes we have prescribed for examining the residues of mineral waters by evaporation serve to ascertain, with the greatest precision, all the several matters held in solution in these fluids. Another process remains to be made to prove the success of the analysis, viz. That of imitating nature in the way of synthesis, by dissolving in pure water the different substances obtained by the analysis of mineral water which has been examined. If the artificial mineral water has the same taste, the same weight, and exhibits the same phenomena with re-agents as the natural mineral water, it is the most complete, and the most certain proof that the analysis has been well made. This artificial combination has likewise the advantage of being procured in all places at pleasure, and at a trifling expense; and is even in some cases superior to the natural mineral waters, for their whole properties may be changed by carriage, and other circumstances. The most celebrated chemists are of opinion, that it is possible to imitate mineral waters. Macquer has observed, that since the discovery of the carbonic

acid, and the property it is found to possess of rendering many substances soluble in water, it is much more easy to prepare artificial mineral waters. Bergman has described the method of composing waters which perfectly imitate that of Spa, Seltzer, Pyrmont, &c. He likewise informs us, that they are used with great success in Sweden, and that he himself has experienced their good effects. Duchanoy has published a work, in which he has given a number of processes for imitating all the mineral waters usually employed in medicine. We may therefore hope, that chemistry may render the most essential service to the art of healing, by affording valuable medicines, whose activity may be increased or diminished at pleasure.

In order to present the reader, under one point of view, with the most conspicuous features in the composition of the mineral waters of this and some other countries, the following synoptical table is subjoined, from Dr. Saunders' work on this subject.

The reader will please to observe, that under the head of Neutral Purging Salts are included the sulphats of soda and magnesia, and the muriats of lime, soda, and magnesia. The power which the earthy muriats may possess of acting on the intestinal canal is not quite ascertained, but from their great solubility, and from analogy with salts, with similar component parts, we may conclude that this forms a principal part of their operation.

The reader will likewise observe, that where the spaces are left blank, it signifies that we are ignorant whether any of the substance at the head of the column is contained in the water; that the word none implies a certainty of the absence of that substance; and the term uncertain means that the substance is contained, but that the quantity is not known.

For the several mineral waters, consult their respective heads, as MALVERN, MOFFAT, SPA WA• TERS, &c.

« ZurückWeiter »