liver of sulphur. Rouelle the younger likewise af firmed, that these fluids might be imitated by agitating water in contact with air, disengaged from an alkaline sulphure by an acid. Bergman carried this doctrine much farther; by examining the properties of sulphurated hydrogen gas, he has proved that this gas mineralizes sulphureous waters, which he therefore calls hepatic waters, and has directed methods of ascertaining the presence of sulphur. Notwithstanding these discoveries, Duchanoy, speaking of sulphureous waters, admits of sulphur, sometimes alkaline, sometimes calcareous, or aluminous. He follows the opinion of Le Roy of Montpellier, who proposed a sulphure with base of magnesia in imitating these waters. It appears in fact to be true, that there are waters which contain a small quantity of sulphur, while there are others which are mineralized only by sulphurated hydrogen gas. In this case it will be necessary to distinguish sulphureous waters into two orders: 1. Those which contain a small quantity of alkaline or calcareous sulphur; and, 2. Those which are only impregnated with sulphuric hydrogen gas. The waters of Bareges and Cauterets, and the Bonnes waters, appear to belong to this first order; and those of St. Amant, Aix la Chapelle, and Montmorency, appear to belong to the second. Most of these waters are thermal, but that of Montmorency is cold. CLASS IV.-Ferruginous Waters. Iron being the most abundant of metals, and the most susceptible of alteration, it is not to be wondered at that water easily becomes charged with it, and consequently that the ferruginous waters are the most abundant and most common of all mineral waters. Modern chemistry has thrown great light on this class of waters; they were formerly supposed to be all impregnated with sulphat of iron. Monnet has ascertained that most of them do not contain this salt, and be supposed that the iron is dissolved without the intermedium of an acid. It is at present known, that the iron is not in the state of sulphat, but is dissolved by means of the carbonic acid, and forms the salt which we have called the carbonat of iron. Lane, Rouelle, Bergman, and many other chemists, have put this out of doubt. The greater or less quantity of carbonic acid, and the state of the iron in waters of this kind, render it necessary to distinguish the present class into three orders. The first order comprehends martial acidulous waters, in which the iron is held in solution by the carbonic acid, whose superabundance renders them brisk and subacid. The waters of Bussang, Spa, Pyrmont, Pouhon, and La Dominique de Vals, are of this first order. The second contains simple martial waters in which the iron is dissolved by the carbonic acid, without excess of the latter. These waters consequently are not acidulous. The water of Forges, Aumale, and Conde, as well as the greater number of ferruginous waters, are of this order; this distinction of ferruginous waters was made by Duchanoy. But we add a third order, after Monnet, which is that of waters containing sulphat of iron. Though these are extremely rare, yet some of them are found. Monnet has placed the waters of Passy in this order. Opoix admits the sulphat of iron, even in a considerable quantity, in the waters of Provins. It is true, that De Fourcy denies its existence, and considers the iron of these waters as dissolved by carbonic acid. But no decision can be made respecting this subject, because the results of these chemists entirely disagree, and require new experiments to be made. It must be added, that the iron is not found alone in these waters, but is mixed with chalk, sulphat of lime, various muriatic salts, &c. However, as the metal they contain is the principal basis of their properties, they must be called ferruginous, in conformity with the principles we have laid down. As to the saponaceous waters admitted by Duchanoy, we must wait till chemical and medical experiments have ascertained the cause of their saponaceous property, which this physician attributes to alumine; as well as of the effects they may produce in the animal economy, as medicines, by virtue of this property. From these details we find, that all mineral and medicinal waters are divided into nine orders, viz. Cold acidulous waters. Hot or thermal acidulous waters. Examination of Mineral Waters according to their Physical Properties.-After having shown the different matters which may be found in waters, and exhibited a slight sketch of the method in which they may be divided into classes and orders, according to their principles, it will be necessary to mention the methods of analysing them, and discovering with the greatest possible degree of accuracy the substances they hold in solution. This analysis has been justly considered as the most difficult part of chemistry, since it requires a perfect knowledge of all chemical phenomena, joined to the habit of making experiments. To obtain an accurate knowledge of the nature of any water proposed to be examined, 1. The situation of the spring, and the nature of the soil, more especially with respect to mineral strata, must be carefully observed; for this purpose, cavities may be dug to different depths, in order to discover, by inspection, the substances with which the water may be charged. 2. The physical properties of the water itself, such as its taste, smell, colour, transparence, weight and temperature, must next be examined; for this purpose, two thermometers, which perfectly agree, and a good hydrometer, must be provided. nary experiments require likewise to be made in the different seasons, different times of the day, and especially in different states of the atmosphere; for a continuance of dry weather, or of abundant rain, has a singular influence on waters. These first trials usually show the class to which the water under examination may be referred, and direct the method of analysis. 3. The depositions formed at the bottom of the basons, the substances. which float on the water, and the matters which rise by sublimation, form likewise an object of important research, which must not be neglected. After this preliminary examination, the proper analysis may be proceeded on, which is made after three methods, by re-agents, by distillation, and by evaporation. These prelimi Examination of Mineral Waters, by Re-agents.— Those substances which are mixed with waters in order to discover the nature of the bodies held 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 reagents 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. With out 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 commuricated by M. Foureroy to the Royal Society of Medicine, will show, 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 nitric solutions of mercury and of silver, paper coloured by the aqueous tincture of feruamboue, 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 lime, 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 carbonie acid. But as this colouring matter is useful chiefly to ascertain the presence of this acid, he directs its tincture 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 distingush 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 esti mated, 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 combi nation of the carbonic acid of the mineral water with lime, and found, according to the calcula tion 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. Gioannetti, 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 se veral 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 filtered 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 hes 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, Foureroy 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 ammoniae, 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 car. bonat 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 gas, disen gaged 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 sul phat 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 men. tioned 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 ex. pedient, which may be applied in this case, 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 ammoniacal gas 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 gas 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. phenomenon took place with the nitrats and muriats of magnesia and alumine, dissolved in equal quantities of distilled water, and treated in the The quickness or slowness of the same manner. The same precipitation of a mineral water, by the addition of ammoniacal gas, therefore affords the means of ascertaining the nature of the earthy salt decomposed by this gas. In general, salts, with base of magnesia, are much more usually met with than those with base of aluminous earth. Bergman has observed, that am. moniac 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 ammoniac sulphat, a mixed neutral salt, similar to the ammoniac muriat, or sal alembroth. The ammoniac does not, therefore, precipitate the whole of the magnesia, and consequently does not accurately exhibit the mercurial 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 gas. 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 sul phuric 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 sul phurated hydrogen gas 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, Baunie 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 Baume, 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 on sulphuric. In fact the carbonie gas, 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 nut-galls, 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 nut-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, relatively 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 me. tallic oxyds. Nut-galls in powder, the infnsion 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 ethicacious a reagent, 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 fith part of a grain of sulphat of iron. All these phenomena proceed from the great facility with which the matter of nut-galls 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 sul phuric 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 atrice 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; but they do not in the same manner indicate the presence of the sulphuric acid, since, according to Bergman's estimate, at least thirty grains of sulphat of soda must exist in the pint of water, in order to produce an immediate sensible effect. To this we may 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. The solution of mercury by the nitric acid is still more productive of error: it not only indicates the presence of the sulphuric and muriatic acids in waters, but it is likewise precipitated by the earthy and alkaline carbonats, in a yellowish powder, which might be mistaken for an effect of the sulphuric acid. It has been commonly supposed, that the very abundant white precipitate which it forms in water is owing to the presence of a muriatic salt; yet mucilaginous and extractive substances 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. He 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. 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 gas 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 striæ were formed, which were dissolved by agitation, and afforded no |