in solution by such waters, from the phænomena they present, are called re-agents. The best chemists have always considered the use of re-agents as a very uncertain method of discovering the principles of mineral waters. This opinion is founded on the considerations that their effects do not determine in an accurate manner the nature of the substances held in solutions in waters; that the cause of the changes which happen in fluids by their addition is often unknown; and that in fact the saline matters usually applied in this analysis are capable of producing a great number of phænomena, respecting which it is often difficult to form any decision. For these reasons, most chemists who have undertaken this analysis have placed little dependence on the application of re-agents. They have concluded, that evaporation affords a much surer method of ascertaining the nature and quantity of the principles of mineral waters: and it is taken for granted, in the best works on the analysis of these fluids, that re-agents are only to be used as secondary means, which at most serve to indicate or afford a probable guess of the nature of the principles contained in waters; and for this reason, modern analysts have admitted no more than a certain number of re-agents, and have greatly diminished the list of those used by the earlier chemists. But it cannot be doubted at present, that the heat required to evaporate the water, however gentle it may be, must produce sensible alterations in its principles, and change them in such a manner, as that their residues, examined by the different methods of chemistry, shall afford compounds differing from those which were originally held in solution in the water. The loss of the gaseous substances, which frequently are the principal agents in mineral waters, singularly changes their nature, and besides causing a precipitation of many substances, which owe their solubility to the presence of these volatile matters, likewise produces a re-action among the other fixed matters, whose properties are accordingly changed. The phænomena of double decompositions, which heat is capable of producing between compounds that remain unchanged in cold water, cannot be estimated and allowed for, but in consequence of a long series of experiments not yet made. Without entering, therefore, more fully into these considerations, it will be enough to observe, that this assertion, whose truth is admitted by every chemist, sufficiently shows that evaporation is not entirely to be depended on. Hence it becomes a question, whether there be any method of ascertaining the peculiar nature of substances dissolved in water without having recourse to heat; and whether the accurate results of the numerous experiments of modern writers afford any process for correcting the error which might arise from evaporation. The following pages, extracted from a memoir communicated by M. Pourcroy to the Royal Society of Medicine, will shew, that very pure re-agents used in a peculiar manner may be of much greater use in the analysis of mineral waters than has hitherto been thought. Among the considerable number of re-agents proposed for the analysis of mineral waters, those which promise the most useful results are tincture of turnsole, syrup of violets, lime-water, pure and caustic potash, caustic ammoniac, concentrated sulphuric acid, nitrous acid, prussiat of lime, gallic alkohol, or spirituous tincture of nutgalls, the uitric solutions of mercury and of silver, paper coloured by the aqueous tincture of fernambouc, which becomes blue by means of alkalis, the aqueous tincture of terra merita, which the same salts convert to a brown red, the oxalic acid to exhibit the smallest quantity of line, and the muriat of barytes to ascertain the smallest possible quantity of sulphuric acid. The effects and use of these principal re-agents have been explained by all chemists, but they have not insisted on the necessity of their state of purity. Before they are employed it is of the utmost importance perfectly to ascertain their nature, in order to avoid fallacious effects. Bergman has treated very amply of the alterations they are capable of producing. This celebrated chemist affirms, that paper coloured with the tincture of turnsole becomes of a deeper blue by alkalis; but that it is not altered by the carbonic acid. But as this colouring matter is useful chiefly to ascertain the presence of this acid, he directs its tineture in water to be used, sufficiently diluted, till it has a blue colour. He absolutely rejects syrup of violets, because it is subject to ferment, and because it is scarcely ever obtained without adulteration in Sweden. Morveau adds in a note, that it is easy to distinguish a syrup coloured by turnsole, by the application of corrosive sublimate, which gives it a red colour, while it converts the true syrup of violets to a green. Lime-water is one of the most useful agents in the analysis of mineral waters, though few chemists have expressly mentioned it in their works. This fluid decomposes metallic salts, especially sulphat of iron, whose metallic oxyd it precipitates; it separates alumine and magnesia from the sulphuric and muriatic acids, to which these substances are frequently united in waters. It likewise indicates the presence of carbonic acid, by its precipitation. M. Gioanetti, a physician of Turin, has very ingeniously applied it to ascertain the quantity of carbonic acid contained in the water of St. Vincent. This chemist, after having observed that the volume or bulk of this acid, from which its quantity has always been estimated, must vary, according to the temperature of the atmosphere, mixed nine parts of lime-water with two parts of the water of St. Vincent: he weighed the calcareous earth formed by the combination of the carbonic acid of the mineral water with lime, and found, according to the calculation of Jaquin, who proves the existence of thirteen ounces of this acid in thirty-two ounces of chalk, that the water of St. Vincent contained somewhat more than fifteen grains. But as the lime-water may seize the carbonic acid united with fixed alkali, as well as that which is at liberty, M. Gioanetti, to ascertain more exactly the quantity of this last, made the same experiment with water deprived of its disengaged acid by ebullition. This process may therefore be employed to determine, in an easy and accurate manner, the weight of disengaged carbonic acid, contained in a gaseous mineral water. One of the principal reasons which have induced chemists to consider the action of re-agents in the analysis of mineral waters as very fallacious, is, that they are capable of indicating several different substances held in solution in waters, and that it is then very difficult to know exactly the effects they will produce. This observation relates more especially to potash, considered as a re-agent, because it decomposes all the salts which are formed by the union of acids with alumine, magnesia, lime, and metals. When this alkali precipitates a mineral water, it cannot, therefore, be known by simple inspection of the precipitate of what nature the earthy salt decomposed in the experiment may be. Its effect is still more uncertain, when the alkali made use of is saturated with carbonic acid, as is most commonly the case, since the acid to which it is united augments the confusion of effects: for this reason, the use of very pure caustic potash is proposed, which likewise possesses an advantage over the effervescent alkali, viz. that of indicating the presence of chalk dissolved in a gaseous water, by virtue of the superabundant carbonic acid: for it seizes this acid, and the chalk falls down of course. This fact is established by pouring soap lees newly made into an artificial gaseous water, which holds chalk in solution. The latter substance is precipitated in proportion as the caustic fixed alkali seizes the carbonic acid which held it in solution. By evaporating the filtrated water to dryness, carbonat of soda is obtained, strongly effervescent with acids. The caustic fixed alkali likewise occasions a precipitate in mineral waters, though they do not contain earthy salts; for if they contain an alkaline neutral salt, of a less soluble nature, the additional alkali will precipitate it by uniting with the water, nearly in the same manner as alkohol does. M. Gioanetti has observed this phenomenon in the waters of St. Vincent; and it may easily be seen by pouring caustic alkali into a solution of sulphat of potash, or muriat of soda; these two salts being quickly precipitated. Caustic ammoniac is in general less productive of error when mixed with mineral waters; because it decomposes only salts, with base of alumine or magnesia, and does not precipitate the calcareous salts. It is necessary, however, to make two observations respecting this salt: the first is, that it must be exceedingly caustic, or totally deprived of carbonic acid; without this precaution, it decomposes calcareous salts by double affinity: the second is, that the mixture must not be left exposed to air, when the effect of its action is required to be inspected several hours after it is added; because, as M. Gioanetti has well observed, this salt in a very short time seizes the carbonic acid of the atmosphere, and becomes capable of decomposing calcareous salts. To put this important fact out of doubt, Fourcroy made three decisive experiments; some grains of sulphat of lime, formed of transparent calcareous spar, because chalk, or Spanish white, contain magnesia and river water: he divided this solution into two parts; into the first he poured a few drops of very pure sulphuric acid, recently made, and very caustic; this he put into a well-closed bottle: at the end of twenty-four and forty-eight hours it was clear and transparent, without any precipitate, and therefore no decomposition had taken place. The second portion was treated in the same manner with ammoniac, but placed in a vessel which communicated with the air by a large aperture: at the end of a few hours a cloud was formed near the upper surface, which continually increased, and was at last precipitated to the bottom. This deposition effervesced strongly with sulphuric acid, and formed sulphat of lime. The carbonic acid contained in this precipitate was therefore afforded by the ammoniac which had attracted it from the atmosphere. This combination of carbonic acid and ammoniac forms ammoniacal carbonat, capable of decomposing calcareous salts by double affinity, as Black, Jacquin, and many other chemists have shown, and as may be easily. proved by pouring a solution of ammoniacal carbonat into a solution of sulphat of lime, which is not rendered turbid by caustic ammoniac. Lastly, to render the theory of this second experiment clearer, Fourcroy took the first portion to which the caustic ammoniac had been added, and which, having been kept in a close vessel, had lost no part of its transparency. He reversed the bottle which contained it, over the funnel of a very small pneumato-chemical apparatus, and by the assistance of a syphon, passed into it carbonic acid gass, disengaged from the effervescent fixed alkali by sulphuric acid. In proportion as the bubbles of this acid passed through the mixture, it became turbid in the same manner as lime-water; by filtration a precipitate was separated, which was found to be chalk, and the water, by evaporation, afforded ammoniacal sulphat: gaseous water, or the liquid carbonic acid, produced the same composition in another mixture of sulphat of lime, and caustic ammoniac. This de-, cisive experiment clearly shows, that ammoniac decomposes sulphat of lime by double affinity, and by means of the carbonic acid. Hence we see, that when it is required to preserve a mixture of the mineral water with ammoniac for several hours (which is sometimes necessary, because it does not decompose certain earthy salts, but very slowly), the experiment must be made in a vessel which can be accurately closed, in order to prevent the contact of air, which would falsify the result. This precaution, which is of great importance in the use of all re-agents, is likewise mentioned by Bergman and Gioanetti. To these may be added another observation concerning the use of ammoniac. As it is a matter of considerable difficulty to preserve ammoniac in the state of perfect causticity, though it is necessary to be had in such a state, for the analysis of mineral waters, a very simple expedient, which may be applied in this case. It is to pour a small quantity of ammoniac into a retort, whose neck is. plunged in the mineral water: when the retort is slightly heated, the ammonical gass becomes disengaged, and passes highly caustic into the water. If it occasions a precipitate, it may be concluded that the mineral water contains sulphat of iron, which may be known by the colour of the precipitate, or otherwise that it contains salts, with base of aluminous or magnesian earth. Generally this precipitate is formed by the chalk which was held in solution in the water, by means of the carbonic acid; ammoniac absorbs this acid, and the chalk is deposited. It is difficult to determine from the physical properties of the earthy precipitate formed in waters by caustic ammoniac, to which of the two last bases it is to be attributed; yet the manner in which it is formed may serve to decide. Six grains of sulphat of magnesia were dissolved in four ounces of distilled water, and six grains of alum in an equal quantity of the same fluid: through each of these solutions a small quantity of ammoniacal gass was passed: the first solution immediately became turbid, while the latter did not begin to exhibit a precipitate till twenty minutes after. These mixtures were carefully included in well-closed bottles. The same phenomenon took place with the nitrats and muriats of magnesia and alumine, dissolved in equal quantities of distilled water, and treated in the same manner. The quickness or slowness of the precipitation of a mineral water, by the addition of ammoniacal gass, therefore affords the means of ascertaining the nature of the earthy salt decomposed by this gass. In general, salts, with base of magnesia, are much more usually met with than those with base of aluminous earth. Bergman has observed, that ammoniac is capable of forming with sulphat of magnesia a compound, in which a portion of this neutral salt is combined, without decomposition, with a portion of ammoniacal sulphat. This non-decomposed portion of sulphat of magnesia may probably form, with the ammoniacal sulphat, a mixed neutral salt, similar to the ammoniaco-mercurial muriat, or sal alembroth. The ammoniac does not, therefore, precipitate the whole of the magnesia, and consequently does not accurately exhibit the quantity of Epsom salt, of which that earth is the base. For this reason lime water is preferable for ascertaining the nature and quantity of salts with base of magnesia contained in mineral waters. It has likewise the property of precipitating the salts with aluminous base much more abundantly and readily than ammoniacal gass. The concentrated sulphuric acid precipitates a white powder from water which contains barytes, according to Bergman; but, as the same chemist observes, that this earth is seldom found in mineral waters, it will not be necessary to enlarge on the effects of this re-agent. When it produces an effervescence, or bubbles in water, it indicates the presence of chalk, carbonat of soda, or pure carbonic acid; each of these substances may be distinguished by certain peculiar phenomena. If water containing chalk be heated after the addition of sulphuric acid, a pellicle and deposition of sulphat of lime are soon formed, which does not happen with waters which are simply alkaline. At first consideration it may seem that the sulphat of lime ought to be precipitated as soon as the sulphuric acid is poured into water containing chalk; this, however, very seldom happens without the assistance of heat, because these waters most commonly contain a superabundance of carbonic acid, which favours the solution of the sulphat of lime, and of which it is necessary to deprive them before the salt can be precipitated. This fact may be shown in the clearest manner, by pouring a few drops of concentrated sulphuric acid into a certain quantity of lime water which has been precipitated, and afterwards rendered clear by the addition of carbonic acid: if the lime water be highly charged with regenerated calcareous earth, a precipitate of sulphat of lime is thrown down in a few minutes, or more slowly in proportion as the carbonic acid is set at liberty. If no precipitate be afforded by standing, as will be the case when the quantity of sulphat of lime is very small, and the superabundant carbonic acid considerable, the application of a slight degree of heat will cause a pellicle of calcareous sulphat, and a precipitate of the same nature to be formed. The nitrous acid is recommended by Bergman to precipitate sulphur from hepatized waters. The experiment may be made by pouring a few drops of the brown and fuming acid on distilled water, in which the gas disengaged from caustic alkaline sulphure, heated in a retort, has been received. This artificial hepatic water, which does not considerably differ from natural sulphureous waters, except in the circumstance of its being more difficult to filter, and its always appearing somewhat turbid, affords a precipitate in a few seconds, by the addition of nitrous acid; the precipitate is of a yellowish white; when collected on a filter and dried, it burns with the flame and smell of sulphur, and in other respects has every character of that inflammable body. Nitrous acid seems to alter sulphurated hydrogen gass in the same manner as it does all other inflammable substances, by virtue of the great quantity of oxygen it contains. Scheele has recommended the oxygenated muriatic acid to precipitate the sulphur from waters of this nature only a very small quantity of it must be used, otherwise the sulphur will be burned and reduced to the state of sulphuric acid. Sulphureous acid precipitates the sulphur very readily from waters which contain it. There are few re-agents whose mode of action is less known than that of the alkaline lixivium of blood, which has been called phlogisticated alkali; it has been long since ascertained, that this liquor contains Prussian blue, or prussiat of iron, ready formed; it has been thought that this blue might be separated by the addition of an acid; and in this state it has been proposed as a substance capable of exhibiting iron existing in mineral waters. Nothing can be more uncertain than the complete separation of prussiat of iron from this prussiat of potash made of blood. This lixivium ought therefore to be no longer used as a re-agent. Macquer having discovered that Prussian blue is decomposed by alkalis, proposed potash saturated with the colouring matter of this blue, as a test to ascertain the presence of iron in mineral waters. But as the liquor itself likewise contains a small quantity of Prussian blue, which may be separated by means of an acid, as Macquer has shown, Baumé advises that two or three ounces of distilled vinegar be added to each pound of this Prussian alkali, and digested in a gentle heat, till the whole of the Prussian blue is precipitated; after which pure fixed alkali is to be added to saturate the acid of vinegar. Notwithstanding this ingenious process, Fourcroy has observed, that the Prussian alkali, purified by vinegar, deposits Prussian blue in process of time, more especially by evaporation. M. Gioanetti made the same observation by evaporating the Prussian alkali, purified, by the method of Baumé, to dryness: he has proposed two processes for obtaining this liquor in a state of purity, and totally exempt from iron; the one consists in supersaturating the Prussian alkali with distilled vinegar, evaporating it to dryness by a gentle heat, dissolving the remaining mass in distilled water, and filtrating the solution; all the Prussian blue remains on the filter, and the liquor which passes through contains none at all. The other process consists in neutralizing the alkali with a solution of alum, from which after filtrating the sulphat of potash is separated by evaporation. These two liquors do not afford a particle of Prussian blue with the pure acids, nor by evaporation to dryness. The lime water, saturated with the colouring matter of Prussian blue, mentioned by us in treating on iron, does not require these preliminary operations: when poured on a solution of sulphat of iron, it immediately forms pure Prussian blue, without any mixture of green. Acids only precipitate a few particles of Prussian blue from this re-agent; it therefore does not contain iron, and consequently is preferable to the Prussian alkalis, in the assay of mineral waters. This phenomenon doubtless depends on the action of the lime, which, when dissolved in water, is far from having the same efficacy on iron as alkalis have. This prussiat of lime seems to be exceedingly well adapted to distinguish ferruginous waters, whe ther they be gaseous or sulphuric. In fact the carbonic gass, which holds iron in solution in waters, being of an acid nature, decomposes Prussian lixiviums by the way of double affinity, as well as sulphat of iron. Fourcroy tried prussiat of lime on Spa waters, and those of Passy, and he immediately obtained a very perceptible blue in the former, and very abundant in the latter. This, therefore, is a liquor very easily prepared, which does not contain the smallest portion of Prussian blue, and is exceedingly well calculated to exhibit the presence of small quantities of iron in waters. It is a kind of neutral salt, formed by the prussic acid, or the colouring part of the blue and lime. Nut-galls, as well as all other bitter and astringent vegetables, such as oak bark, the fruit of the cypress tree, the husks of nuts, &c. have the property of precipitating solutions of iron, and exhibiting that metal of different colours, according to its quantity, its state, and that of the water in which it is dissolved. This colour in general is of all shades, from a pale rose to the deepest black. It is well known that the purple colour assumed by waters, with the tincture of nutgalls, is not a proof that they contain iron in its metallic state, since the sulphat and carbonat of iron likewise assumes a purple colour by the infusion of nat-galls. The differences of colour observed in these precipitations depend rather on the quantity of iron, its greater or less degree of adhesion to the water, and the more or less advanced state of decomposition of the solution, relauvely to the quantity of oxygen contained in the iron. The astringent principle is known to be a peculiar acid, since it unites with alkalis, converts blue vegetable colours to a red, decomposes alkaline sulphures, and combines with metallic oxyds. Nut-galls in powder, the infusion of this substance in water, made without heat, and the tincture by alkohol, are used to ascertain the presence of iron in mineral waters. The tincture is preferred, because it is not subject to become mouldy as the aqueous solution is. The distilled products of nut-galls likewise colour ferruginous solutions. The infusions in acids, alkalis, oils, and ether exhibit the same phenomenon. The iron precipitated by this matter from acids is in the state of gallat of iron, and forms a kind of neutral salt, which though very black, is not attracted by the magnet. It dissolves slowly, and without sensible effervescence in acids, but loses these properties by the action of fire, and is then attracted by the magnet. The nut-gall is so efficacious a re-agent, that a single drop of its tincture colours, in the space of five minutes, with a purple tinge, three pints of water, which contains only the twenty-fifth part of a grain of sulphat of iron. All these phenomena proceed from the great facility with which the matter of nutgalls burns, and from its readily absorbing from the iron a portion of the oxygen it contains, passing by this means to the state of a black oxyd, or Ethiops, the smallest quantity of which is very perceptible in transparent liquors. The two last re-agents we shall propose for the examination of waters, are solutions of silver and of mercury in the nitric acid. These have usually been employed to exhibit the presence of the sulphuric or muriatic acids in mineral waters; but many other substances, which do not contain the smallest portion of those, are likewise precipitated by these solutions. The white and heavy strie which the nitrat of silver exhibits in water, that contains no more than half a grain of muriat of soda in the pint, ascertains the presence of the muriatic acid with great certainty and facility; presence of the sulphuric acid, since, accordbut they do not in the same manner indicate the ing to Bergman's estimate, at least thirty grains water, in order to produce an immediate sensible of sulphat of soda must exist in the pint of effect. To this we inay add, that fixed. alkali, chalk, and magnesia, precipitate the nitric solution of silver in a much more evident manner, and consequently that the precipitation formed in a mineral water by this solution is insufficient to determine with precision the saline or earthy substances from which it arose. still more productive of error: it not only indicates The solution of mercury by the nitric acid is the presence of the sulphuric and muriatic acids in waters, but it is likewise precipitated by the earthy which might be mistaken for an effect of the suland alkaline carbonats, in a yellowish powder, phuric acid. It has been commonly supposed, that the very abundant white precipitate which it riatic salt; yet mucilaginous and extractive subforms in water is owing to the presence of a mustances exhibit the same phenomenon, as is now well known to all chemists. Besides these sources of error and uncertainty, dependent on the property which several substances have, of producing similar precipitates with the nitric solution of mercury, there are likewise others which depend on the state of this solution itself, and which it is of the utmost consequence to know, in order to avoid very considerable errors in the analysis of waters. Bergman has mentioned some of the remarkable differences observed in this solution, according to the manner in which it is made, either with or without heat, more particularly with respect to the colour of the precipitates it affords by different intermediums; but he does not say a word concerning the property this solution possesses of being precipitated by distilled water, when it is highly charged with the oxyd of mercury; though Monnet mentions this fact in his treatise on the dissolution of metals. As this subject is of great importance in the analysis of waters, Fourcroy endeavoured by a very minute investigation to arrive at some degree of certainty concerning it, and has succeeded, as shall presently appear, by very simple means. has made a great number of solutions of mercury, in very pure nitric acid, with different doses of these two substances, with heat and in the cold, and with acids of very different strengths. These experiments have afforded the following results. He 1. Solutions made in the cold became charged more or less readily with different quantities of mercury, according to the degree of concentration of the nitric acid; but whatever the quantity of mercury dissolved in the cold by the concentrated acid may be, no part of it will be precipitated by mere water. He dissolved in the cold two drachms and a half of mercury, in two drachms of nitrous acid red and fuming, weighing one ounce four drachms and five grains, in a bottle which contained an ounce of distilled water: the combination took place with the utmost rapidity; very dense nitrous gass escaped, together with aqueous vapours, dissipated by the heat of the mixture, amounting to more than one fourth of the acid. This solution was of a deep green, and very transparent: he poured a few drops into half an ounce of distilled water: some white stria were formed, which were dissolved by agitation, and afforded ne 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 gass. These 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 apothecary'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 acquried 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 |