a well known fluid, diffused through the atmosphere, and over the surface of the globe, and abounding in a certain proportion in animals, vegetables, and minerals.

The uses of water are so universally known, that it would be superfluous to enumerate them in this article. It is essential to animal and vegetable life; it makes easy the intercourse between the most distant regions of the world; and it is one of the most useful powers in the mechanic arts. It is often found combined with various substances, and is then frequently beneficial in curing or alleviating diseases.

Those properties of water which fit it for answering mechanical purposes are explained in other articles of this Work (see Hydrostatics, Pneumatics, no. 3. Resistance, and Rivers); but it still remains for us to give an account of the late celebrated discovery of the composition of water, and the various substances which are often found chemically united with it.

The ancient philosophers considered water as one of the four elements. During the age of the alchemists, when it was believed that different substances could be converted into gold, it was also an opinion, adopted by many, that water could be changed into earth. Even so late as the time of Mr Boyle

Lance by a silk fibre rolled round the cylindric axis of the balance. Mr Hooke, long after this, complained to the Society of Mr Oldenburgh's communicating this and other things to Huyghens, with whom he had an intimate correspondence. In 1665 Sir Robert Moray wrote a letter to Mr Oldenburgh, presuming, from his intimacy with Mr Huyghens, that he would know how soon his watches would be ready, and defined him to ask Mr Huyghens, "Whether he did not apply a spring to the axis of the balance?" and if he should say anything to that purpose, then to tell him what Hooke had done in that way, and that he intended more. N.B. Before this time the treaty had been dropped, and there appeared to Sir Robert no farther need of concealment.

From these and other facts that might be produced, we think it most evident that Mr Hooke invented the regulating spring of a watch, by which it is made perfectly adequate to the purpose of finding the longitude at sea; that he invented it eight or ten years before Mr Huyghens thought of such a thing, and fifteen years before he published it in the Journal des Savans in 1674.

Our readers cannot fail of making some remarks on this anecdote, which will perhaps extenuate a little Mr Hooke's morose behaviour, and explain, and perhaps excuse, his disposition to boast of his own inventions and arrogate those of others. If any of the expressions in the article allotted to his name should have made too unfavourable an impression, this note may help to soften it. We do not think that it can be inferred from those facts that either Hautefeuille or Huyghens purloined Hooke's invention. The one might fall upon it in the course of his many experiments; and the other, from his mathematical discoveries of the requisites for isochronous vibrations, might be induced to try whether springs afforded such a force. But there can remain no doubt but that Hooke made the discovery like a philosopher. If to this Work any Supplement shall be given by the present Editor, he will endeavour still farther to wipe away the obloquy which has been cast upon the memory of Dr Hooke for his arrogance in claiming the merit of inventions supposed to be the property of others. Boyle this sentiment was not laid aside. He relates, that a friend of his, by distilling a quantity of water an hundred times, found at length that he had got fix-tenths of the first quantity in earth: whence he concludes, that the whole water, by further protecting the operation, might be converted into earth. Others have made experiments to the same purpose, and seemingly with the same success; but the deception is now found out. Water has the power of corroding the hardest bodies, even glass itself, by long digestion, especially when assisted by heat; and hence those who have made the experiments just mentioned have been themselves deceived, by supposing the earth which really came from the containing vessel to come from the water.

Margraaf made several experiments to determine whether water be transmutable into earth, and found that after every distillation a sediment was left. Lavoisier repeated Margraaf's experiments, and gave the explanation which we alluded to, that the sediment consisted of portions of the glass separated by the water. Dr Black, in the valuable course of lectures which he has for many years delivered, with so much honour to himself, and so much to the advancement of the science of chemistry, goes still farther: he ingeniously supposes, that the alkali, which is an essential ingredient in the composition of glass, unites with the water, and makes the glass swell, and thus occasions small portions of it to be detached.

Historical Account of the Discovery of the Composition of Water.

That water is not a simple but a compound substance, consisting of a mixture of vital and inflammable air, is one of the most astonishing and important discoveries which has been made since the origin of chemistry, or indeed since the origin of science. The history of this curious and interesting discovery we shall trace back with as much precision and impartiality as possible to the first hints which were thrown out upon the subject, and endeavour at the same time to assign to all who have contributed to the discovery the merit to which they are respectively entitled.

The first thing that led chemists to make experiments concerning the composition of water, was a letter which Mr John Warltire, lecturer in natural philosophy, wrote to Dr Priestley, dated Birmingham 18th April 1781, and published in the Appendix to the 5th volume of Dr Priestley's Experiments and Observations. This gentleman had long entertained an opinion that the question "whether heat be a heavy body," might be determined by burning inflammable air mixed with atmospheric air. For some time he was deterred from trying the experiment, from an apprehension that the consequences of passing the electrical spark through so combustible a mixture might be attended with danger; but at length, being encouraged by Dr Priestley, he prepared an apparatus for the purpose. He got a copper ball weighing 14 oz. and sufficient to contain three wine pints, with a screw stopper adapted to it, so that no air could escape. When he filled this ball with inflammable and common air, and made the electric spark to pass through it, a loss of weight was observed, upon an average, about two grains. When the same experiment was made in close glass vessels, the inside of the glass, though clean and dry before the operation, became immediately wet with dew, and was lined with a foamy substance. When Mr Warltire saw the moisture, he said to Dr Priestley, that it confirmed an opinion which he had long entertained, that common water profits its moisture when it is phlogisticated. After this experiment had been repeated by Dr Priestley and Mr Warltire in company, they next fired a mixture of vital and inflammable air; but the only effects which they observed were, that the light was much more intense, and the heat much greater.

During the same year, and after the publication of the volume of Dr Priestley's works, referred to above, Mr Cavendish repeated the experiments of Mr Warltire; but, though the vessel which he used held 24,000 grains of water, and though the experiment was repeated several times with common and inflammable air, he could never perceive any effects of weight of more than one-fifth of a grain, and commonly none at all. In all these experiments Mr Cavendish did not perceive the least foamy matter; but the inside of the glass globe became dewy, as Mr Warltire had observed. The inflammable air was procured from zinc.

That he might examine the nature of the dew, he burned 500,000 grain measures of inflammable air with two and a half times that quantity of common air, and the burned air was made to pass through a glass cylinder eight feet long, and three quarters of an inch diameter, in order to deposit the dew. These two kinds of air were mixed and set on fire by a lighted candle. In a short time 135 grains of water were condensed in the cylinder, which had no taste nor smell, and which left no sensible sediment when evaporated to dryness; neither did it yield any pungent smell during the evaporation: in short, it seemed pure water. From this experiment Mr Cavendish concluded, that when inflammable and common air are exploded in a proper proportion, almost all the inflammable air, and near one fifth of the common air, lose their elasticity, and are condensed into dew; which, when examined, is found to be pure water.

He wished next to examine the effect produced by firing a mixture of vital and inflammable air. He took a glass globe holding 8800 grain measures, furnished with a brass cock, and an apparatus for firing air by electricity. The globe was exhausted of its air by an air-pump, and then a mixture of 19,500 grain measures of dephlogisticated air, and 37,000 of inflammable air, was conveyed successively from a glass jar, inverted in water, into the globe, and there fired by electricity. At the end of the experiment, when the whole air was consumed, a condensed liquor was found in the globe, weighing about 30 grains, which was sensibly acid to the taste; and, by saturation with fixed alkali and evaporation, yielded near two grains of nitre. This product of nitre must have been occasioned by a mixture of azotic gas, which had combined with part of the oxygen, or dephlogisticated air; which are now well known to be the component parts of the nitric acid. These experiments, Mr Cavendish informs us, were made in 1781.

Mr Cavendish having mentioned these experiments to Dr Priestley, that gentleman made a course of experiments in order to investigate the same subject; an account of which is published in the Philosophical Transactions for 1783, and by Dr Priestley in the last volume of his Experiments. Having formerly observed several remarkable changes in fluid substances, in consequence of long exposure to heat in glass vessels hermetically sealed, Dr Priestley formed a design of exposing all kinds of solid substances to great heats in close vessels. As many substances consist of parts so volatile as to fly off before attaining any considerable degree of heat in the usual pressure of the atmosphere, he imagined that if the same substances were compelled to bear great heats under a greater pressure, they might assume new forms, and undergo remarkable changes. Happening to mention these ideas to Mr Watt, the ingenious improver of the steam-engine, Mr Watt mentioned a similar idea of his, that it might be possible to convert water or steam into permanent air. Water. For many years before this period, Mr Watt tells us he had entertained an opinion, that air was a modification of water, which was originally founded on the facts, that in Mr Watt's most cases wherein air was actually made (which should be distinguished from those wherein it is only extricated from substances containing it in their pores, or otherwise united to them in the state of air), the substances were such as were known to contain water as one of their constituent parts; yet no water was obtained in the processes, except what was known only to be loosely connected with them, such as the water of the crystallization of salts. This opinion arose from a discovery, that the latent heat contained in steam diminished in proportion as the sensible heat of the water from which it was produced increased. In other words, the denser the steam was, the less latent heat it contained.

Having been informed by Dr Priestley of the result of the experiment of firing a mixture of dephlogisticated and inflammable air, Mr Watt was enabled to form the very theory which has been since demonstrated to be true. "Let us consider (says he) what obviously happens in the case of the deflagration of the inflammable and dephlogisticated air. These two kinds of air unite with violence, they become red hot, and upon cooling totally disappear. When the vessel is cooled, a quantity of water is found in it equal to the weight of the air employed. The water is then the only remaining product of the process; and water, light, and heat, are all the products, unless there be some other matter free which escapes our senses. Are we not then authorized to conclude, that water is composed of dephlogisticated air and phlogiston deprived of part of their latent or elementary heat; that dephlogisticated or pure air is composed of water deprived of its phlogiston and united to elementary heat and light; and that the latter are contained in it in a latent state, so as not to be sensible to the thermometer or to the eye; and if light be only a modification of heat, or a circumstance attending it, or a component part of the inflammable air, then pure or dephlogisticated air is composed of water deprived of its phlogiston and united to elementary heat."

We have said that the theory of Mr Watt is now demonstrated to be true. To this assertion an objection may be raised from the language in which he states his theory; for he explains it by using the word phlogiston, a word which is now exploded from philosophy as the name of an imaginary substance. But it is sufficient to reply, that Mr Watt uses the word phlogiston as synonymous with inflammable air. It may be proper also to add, that the passage quoted above was contained in a letter from Mr Watt to Dr Priestley, dated the 26th of April 1783.

Most of the experiments hitherto made favoured the conclusion which Mr Watt had drawn; but so many difficulties occurred to Mr Cavendish and Dr Priestley, that they seemed to hesitate about the theory. Dr Priestley in particular, after consideration, declared against it; while Mr Cavendish only waited till the difficulties should be removed. In the meantime experiments were made in a different quarter, which gave the most incontestable proofs of the truth of the theory.

M. de Luc had gone to Paris in January 1783. During his residence there, he received a letter from Dr Priestley, announcing the result of his experiments concerning the conversion of water into air. M. de Luc immediately communicated the contents of this letter to several members of the Academy of Sciences. But the difficulties which had occurred to Dr Priestley prevented them from acquiescing in Mr Watt's theory. In the month of June following, Dr Blagden, who was well acquainted with all the experiments both of Mr Cavendish and of Dr Priestley, and of the opinions of Mr Watt, made a journey to Paris, in which he had an opportunity of conversing on this subject with the same gentlemen of the Academy to whom M. de Luc had formerly imparted the experiments of Dr Priestley. Notwithstanding the additional facts which he was enabled to lay before them, he found them averse from admitting the theory. They supposed that the water collected after the combustion of the two kinds of air had been dissolved in them before. As the question depended upon the proof of a fact, they resolved however to make the proper experiments for examining it. The celebrated Lavoisier took this experiment upon himself. It was made on the 24th of June in the presence of Dr Blagden and many gentlemen of the academy; and the success was as complete as the most sanguine imagination could have conceived. It was repeated by Mefris Monge and Meunier, and the same result was found. The composition of water was now therefore put beyond doubt, and is now almost universally received as an unquestionable fact.

As we wish upon all occasions to ascribe to all eminent men the honour which they deserve, we should willingly estimate the comparative merit of those philosophers who were the most active in this discovery; but though we feel ourselves concerned disposed to be altogether impartial, it is attended with so many difficulties, that we will not presume to affirm that our opinions are formed with perfect accuracy. With respect to Mr Watt, we think it appears that he was the first person who formed the true theory. He had for many years before thought it probable, that if the latent heat of steam could be wholly converted into sensible heat by a great increase of heat, the steam might suffer some remarkable change, such as into permanent air. And no sooner had he heard of the deflagration of oxygenous and hydrogenous gas by Dr Priestley, than he formed this theory.

Mr Cavendish had the merit of making a proper use of Dr Priestley's account of Mr Warltire's experiment, from which Dr Priestley had been able to draw no conclusions, but had considered it merely as a curious fact. Without knowing anything of Mr Watt's ideas, as far as appears to us, he made a number of ingenious experiments, which led him to conclude, that it was highly probable that water was a composition of air. The air which he employed seems not to have been pure; so that besides the water he procured a quantity of nitrous acid. He however acted like an able and candid philosopher; he went as far as his experiments would permit him, and he went no farther. In one point he continued to differ from Mr Watt after his theory was made public. Mr Watt supposed that water consisted of dephlogisticated air (oxygenous gas) and phlogiston (hydrogenous gas according to him), deprived of part of their latent heat; whereas Mr Cavendish thought there was no such thing as elementary heat. We must further add, that it was Mr Cavendish who taught Dr Priestley to turn to a proper account the experiment of Mr Warltire; and therefore, that it was in fact from Mr Cavendish's experiments ultimately that Mr Watt was enabled to establish his theory.

The merit of Dr Priestley lies wholly in his being the instrument of promoting this discovery. He first published Priestley's experiment of Mr Warltire; and when Mr Cavendish had informed him of the success he had met with in repeating that experiment, he began also to study the same subject. His discoveries were more useful to Mr Watt than to the author himself; for Mr Watt formed the theory which he had formerly been meditating; but Dr Priestley never came to a steady conclusion on the subject. We have read over carefully all his papers concerning the conversion of water. Water into air, but cannot help saying, that we went along with the bewildered author weary and fatigued. His experiments are very often made at random, almost always founded on false principles, and seldom lead to anything but to doubt and perplexity. M. Lavoisier sent him a copy of his ingenious paper on the composition of water; he repeated some of the experiments of that illustrious chemist, but he only involved himself in numberless difficulties. We are now no longer surprised at the singularity of Dr Priestley's opinions in religion; either at his incredulity in some things, or at his licentious sentiments in others. He that can doubt of the conclusive evidence which M. Lavoisier has given of the composition of water, must either have received less understanding than the bulk of mankind, or his mind must be warped with inextricable prejudices. With peculiar pleasure we mention Dr Black on this occasion. That gentleman, no less conspicuous for his candour and modesty than for his ingenuity, had, along with all other chemists of the time, believed the doctrine of phlogiston, and taught it in his public lectures; but, upon examining the Lavoisierian system, he was convinced of its truth, and had the honesty to confess it, though he was thus obliged to acknowledge to his students, that he had for many years been teaching errors. This acknowledgment does much honour to Dr Black, and proves that he is well entitled to the high character which he has so long held.

The merit of M. Lavoisier was great upon the present occasion. From England indeed he received the theory and the first experiments on the composition of water; but he was the first person who demonstrated the theory, and put it beyond doubt. His knowledge of the distinction between carbure and hydrogene, as well as the perfect accuracy with which his experiments were made, enabled him to prove, with as much certainty as physical science generally admits, that water is composed of vital and inflammable air. We will now give some account of the proofs of this fact; and, as we have never seen them stated with more clearness and precision than by M. Lavoisier himself in his Elements of Chemistry, we shall take our account of them from him.

Proofs of the Composition of Water,

Exper. 1. Take a glass tube from 8 to 12 lines diameter, and place it across the furnace EFCD, with a gentle inclination from E to F (a). The higher extremity of the tube is then fitted to the glass retort A, containing a known quantity of distilled water. To the lower extremity F is fitted the worm SS, the lower end of which is fixed in the neck of the bottle H, which bottle has the bent tube KK fixed to a second opening. This bent tube is intended to carry off any elastic fluids which may escape into the bottle H. A fire is then lighted in the furnace EFCD, sufficient to keep the tube EF red hot, but not to melt it. The water in the retort A is kept boiling by a fire in the furnace VVXX. The water is gradually changed into steam by the heat of the two furnaces. It passes through the glass tube EF into the worm SS, where it is condensed, and then drops into the bottle H. When the whole water is evaporated, and all the communicating vessels are emptied into the bottle H, it is found to contain exactly the same quantity which was put into the retort. This experiment therefore is a simple distillation.

Exper. 2. Everything being disposed as in the last experiment, let 28 grains of pure charcoal, broken into small parts, and which has been exposed to a red heat in a clove vessel, be introduced into the tube EF. The experiment is then performed in the same manner as the former. The water is evaporated, and a portion of it is again condensed in the worm SS, and then falls into the bottle H; but at the same time a considerable quantity of an elastic fluid escapes through the tube KK, which is received in vessels. When the water is entirely evaporated, and the tube examined, the 28 grains of charcoal have wholly disappeared.

When the water in the bottle H is examined, it is found to have lost 85.7 grains of its weight; and when the elastic fluid which passed off by the tube KK is weighed, it is found to weigh 113.7 grains, which is exactly the weight which the water has lost, added to the 28 grains of charcoal which had disappeared. The elastic fluid, on examination, is discovered to be of two kinds; namely, 144 cubic inches of carbonic acid gas weighing 100 grains, and 380 cubic inches of a very light gas weighing only 13.7 grains. Now 100 grains of carbonic acid gas consist of 72 grains of oxygen, combined with 28 grains of carbure. It is therefore evident, that the 28 grains of charcoal must have acquired 72 grains of oxygen from the water. It is also evident, that 85.7 grains of water are composed of 72 grains of oxygen, combined with 13.7 grains of a gas capable of being burned.

Exper. 3. Everything being put in the same order as in the two former experiments, with this difference, that instead of the 28 grains of charcoal, 274 grains of soft iron, in thin plates rolled up spirally, are introduced into the tube EF. The tube is kept red hot while the water is evaporating from the retort. After the water has been distilled, it is found to have lost 100 grains. The gas or elastic fluid weighs 15 grains, and the iron has gained 85 grains additional weight, which put together make up 100 grains, the weight which the water has lost. The iron has all the qualities which it would have received by being burned in oxygen gas. It is a true oxyd (or calx) of iron. We have the same result as in the last experiment, and have therefore another proof for concluding, that 100 grains of water consist of 85 grains of oxygen, and 15 of the base of inflammable gas (b).

We have now exhibited two sufficient proofs, that water is composed of oxygen and hydrogen; but as the composition of water is so interesting and important a subject, M. Lavoisier was not satisfied with these proofs alone. He justly concluded, that if water be a compound of two substances, it ought to follow, that by uniting these two substances, water would be produced. He accordingly proved the truth of this conclusion by the following experiment.

(a) The tube EF should be made of glass which can bear a strong heat without melting. It should also be coated over with a lute composed of clay and powdered stone-ware; and to prevent it from bending during the experiment, it must be supported about the middle by an iron bar.

(b) This elementary substance Mr Lavoisier has denominated hydrogen, which signifies "the generative principle of water;" from "aqua" "water," and "producere" "I produce." When this substance is combined with caloric, it is called hydrogenous gas. It is the lightest substance yet known, being 1/16th of the weight of an equal bulk of atmospheric air. It is very combustible, for it has so great an attraction for oxygen, that it attracts it from caloric; so that its inflammable property is merely its power of decomposing oxygenous gas, for it will not burn by itself. When drawn into the lungs, it produces instant death. See Alchemy. Exper. 4. He took a large crystal balloon A, fig. 2, containing about 30 pints, and having a large mouth; round which was cemented the plate of copper BC, pierced with four holes, through which four tubes pass. The first tube Hb is intended to exhaust the balloon of its air, by adapting it to an air pump. The second tube gg communicates with a reservoir of oxygenous gas placed at MM. The third tube d D s is connected with a reservoir of hydrogenous gas at NN. The fourth tube contains a metallic wire GL, having a knob at its lower extremity L, from which an electric spark is passed to it; in order to set fire to the hydrogenous gas. The metallic wire is moveable in the tube, that the knob L may be either turned towards it, or away from it, as there is occasion. We must also add, that the three tubes H b, g g, d D s are furnished with stop-cocks.

It is necessary that the oxygenous gas, before being put into the reservoir, should be completely purified from carbonic acid. This may be done by keeping it for a long time in contact with a solution of caustic potash. The hydrogenous gas ought to be purified in the same manner. The quantity employed ought to be double the bulk of the oxygenous gas. It is best procured from water by means of iron, as was described in Experiment Third.

Great care must also be taken to deprive the oxygenous and hydrogenous gas of every particle of water. For this purpose they are made to pass in their way to the balloon A, through salts which have a strong attraction for water; as the acetate of potash (a compound of vinegar and vegetable alkali), or the muriate or nitrate of lime (the muriatic or nitric acid combined with lime). These salts are disposed in the tubes MM and NN of one inch diameter, and are reduced only to a coarse powder, that they may not unite into lumps, and interrupt the passage of the gases.

Every thing being thus prepared for the experiment, the balloon is exhausted of its air by the tube H b, and is filled with oxygenous gas. The hydrogenous gas is also pressed in through the tube d D s by a weight of one or two inches of water. As soon as the hydrogenous gas enters the balloon, it is set fire to by an electric spark. The combustion can be kept up as long as we please, by supplying the balloon with fresh quantities of these two gases. As the combustion advances, a quantity of water is collected on the sides of the balloon, and trickles down in drops to the bottom of it. By knowing the weight of the gases consumed, and the weight of the water produced, we shall find that they are precisely equal. M. Lavoisier and M. Meunier found that it required 85 parts by weight of oxygenous gas and 15 parts of hydrogenous gas to produce 100 parts of water.

Thus we have complete proofs, both analytical and synthetical, that water is not a simple elementary substance, as it has been long supposed, but is compounded of two elements, oxygen and hydrogen. We must add, that M. Lavoisier used the most scrupulous accuracy in making the experiments which we have described; and that he is of opinion that the proportions given above cannot be far from the real truth. Such then is the history and proof of the composition of water. We come next to consider what substances are chemically united or dissolved in it.

Analysis of the different Substances contained in Water.

Since it is made certain by observation and experiment, that water contains many different kinds of substances; and as its qualities, and consequently its uses, differ much according to the nature of the substances combined with it—the knowledge of an easy and accurate method of analyzing waters is become a matter of the utmost importance. By such an analysis we shall be enabled to select the purest water for the purposes of life, and to avoid water which might be improper and hurtful; or, when good water cannot be had, to separate those substances from it which render it impure. By the same important art we shall find it easy to distinguish those waters which are best adapted to the arts and manufactures; we shall also be able to compare different mineral waters, to explain the causes of their effects in medicine, and to imitate those by art which are most efficacious.

All natural waters are more or less impure; for water has so strong an attraction for different substances, that it imbibes part of them in every situation in which it is found, not only when it flows over beds of earth, but when it filters through strata of metals, and even when it is dissolved in the atmosphere. Water cannot be procured in a pure state without undergoing the process of distillation.

Before we proceed to state the methods by which the different substances found in water may be detected, it will be proper to point out to the reader such sensible qualities of particular waters as may enable him to institute the process by which the analysis ought to be conducted. In every course of experiments, that order ought to be followed which will lead with most ease and certainty to the end which is in view; but unless a man from general knowledge be able to conjecture with some degree of accuracy what are the results to be expected in particular cases, he cannot be able to determine what experiments he ought to make.

The general circumstances which are first to be attended to in the examination of waters, are their colour, smell, taste, specific gravity, temperature, and local situation.

1. The first thing to be attended to in water is its colour. Pure water is transparent like crystal. Muddiness or a brown colour is a certain proof that some extraneous substance is diffused through the water. A green colour indicates the presence of iron, and a blue that of copper. If upon agitation airy bubbles appear in the water, we are sure that it contains carbonic acid or fixed air. The water which is to be examined with respect to colour should be put into a deep glass, that we may look down into a considerable body of it; for we shall thus discover any muddiness much better than by viewing the water horizontally through the glass.

2. We are next to observe whether the water has any smell. If it be pure, it will have no smell; if it diffuse a subtile penetrating odour, we have reason to conclude that it contains carbonic acid; if the smell of putrid eggs or of the scourings of a gun arise from it, we infer that it is impregnated with hepatic sulphur, or sulphur combined with an alkali.

3. Pure water has no taste. Water containing carbonic acid has a mild sour taste. If it have a bitter taste, it may contain sulphate of soda or Glauber's salt, nitre or the phosphate, nitrate or muriate of magnesia, or lime combined with the nitric or muriatic acid. If the water has a slight astringency of taste, we may expect that it contains lime or gypsum; if it be saltish, it contains common salt; if the taste be lixivious, alkali is present; if seruginous, there is copper; if ferruginous or inky, we have reason to suppose that it contains iron.

4. The specific gravity of water can enable us to discover whether it contains some extraneous matter, but does not point out what sort of matter it is. We are always sure that the lightest waters are the purest. The standard to be employed for comparing the specific gravity of water to be examined is distilled water.

5. Another circumstance to be considered is the temperature of the water, whether it be hot, cold, or tepid. We must determine whether the temperature be the same during the whole year, or whether it depends on the weather; whether whether it freezes in winter; if hot, whether, when allowed to cool, it deposits any sediment, and loses its taste and smell.

6. The local situation of the water must also be taken into review. We must consider the soil through which it flows, and inquire whether there be mines or veins of metals near, or any kind of substance which water can dissolve. We must also inquire whether the water flows in equal quantity during the whole year, or increases with rain, and decreases with dry weather; whether it is stagnant or flowing; if it flows, whether it flows swiftly or slowly; whether it deposits any sediment; and if it does, of what sort it is, whether a salt, earth, metal, or metallic ochre; whether it petrifies bodies thrown into it; and whether there be any sulphur to be found near it in a sublimed state.

It is also proper to observe whether it be hard or soft; whether any animalculae live or vegetables grow in it; and whether it has any reputation for its effects in medicine.

Water may be divided into two great divisions, fresh and salt water. — Fresh water may be divided into atmospheric, stagnant, and running.

Salt water comprehends most of the seas on the globe, but especially those of the torrid and the greater part of the temperate zones. It contains common salt in great quantity, sulphate or muriate of magnesia, and sulphate of lime, besides a great quantity of putrid matter brought into it by the rivers, or produced by the decomposition of the numerous tribes of animals which live and die in it. See Sea and Sea-Water.

Atmospheric water comprehends rain and snow water. Rain is the water which is evaporated from the sea and land, dissolved in the air, and afterwards discharged on the earth; it ought therefore to resemble distilled water in purity; and it would certainly do so, if the atmosphere did not abound with vapours and exhalations capable of being combined with it. It contains a small quantity of sulphate of lime, together with a very small portion of nitrous acid. The rain that drops from the tops of houses is always mixed with dust. Some showers have contained a quantity of the pollen of flowers, which has given rise to the stories of showers of sulphur. The rain which falls at a distance from towns, or after a long tract of wet weather, is purest; for the atmosphere is then in some measure washed, if we may use the expression, from all heterogeneous substances. — Snow water is contaminated with the same substances as rain water. When newly melted, it is destitute both of common air and of fixed air, or the carbonic acid. It is probably from the want of these that snow water is injurious to health.

Stagnant water forms a lake; and when a great quantity of earth is diffused through it, it forms a marsh. The water of lakes is generally very pure and transparent; for as they are not subject to so much agitation as streams, the substances that happen to fall into them are not much diffused, but soon subside to the bottom. Some lakes are salt. — Marshes are much more impure. They are generally contaminated with the putrid matter produced by the decomposition of animals and vegetables, and are often of a yellowish or brownish colour.

Running water comprehends spring and river water. — Spring water is the rain water, which, after discharging itself upon the earth, and being imbibed by it, again issues out. As it runs below the surface through different substances, it carries along with it such as it can dissolve, and is therefore not so pure as rain water. It often contains salts, earths, or metals. — Rivers consist of a collection of springs, and generally partake of the soil through which they pass. Rivers which run through great towns are loaded with animal and vegetable substances. But those which run at a distance from towns are purer than most springs; because, as they run with more rapidity, and to a greater distance, a great part of their impurities are thus eliminated. If the soil be soft through which a river runs, it will be full of earth; but if hard and rocky, the water is very clear and pure.

Water is called hard when it does not dissolve soap, or boil vegetables, or make an infusion of tea. It generally contains some acid combined with absorbent earth, for which it has less attraction than for the alkali of the soap. When soap is put into such water, its alkali is immediately attracted by the acid of the water, the soap is decomposed, and the oil of it swims on the surface of the water. Water is not reckoned hard if it contains less than 10 grains of extraneous substances in the pound weight.

If the acid with which the absorbent earth is united be the carbonic, the water may be purified by boiling. But in order to make it agreeable to the palate after the calcareous earth is deposited, it ought to be exposed in the open air in broad shallow vessels. It will thus recover a portion of the air which was expelled by the boiling. But if the earth be suspended by any other acid, the water can be corrected by the addition of some fixed alkali, which immediately joins itself to the acid, while the earth is deposited. A solution of potash, or of any other alkali, may be poured into the water till it cease to produce any turbid appearance, or till no more is precipitated. The water must then be decanted from the sediment, or filtered if necessary.

Having now mentioned the different kinds of waters, it will be next proper to describe the most accurate methods of analyzing them. There are two, by precipitation and analyzing by evaporation. Precipitants are substances which, being thrown into any impure water, separate the impurities, and throw them to the bottom of the vessel. Precipitation is the most expeditious method of examining waters; but it does not enable us to form so accurate an estimate as is often necessary of the precise quantity of extraneous substances contained in them.

The other method of analyzing water is by evaporation, and by converting the water into steam, and crystallizing the salts contained in it. Both these methods are often necessary to be employed, either of them separately being defective. As the precipitants indicate the proper method of conducting the evaporation, it will be proper, before we describe how to analyze water by evaporation, to describe particularly the effects produced on it by applying different precipitants.

Method of analyzing Water by Precipitation.

The substances hitherto found in water are, common atmospheric air, acids, alkalies, earths, sulphurs, and metals, contained in acids, when disengaged, may be discovered by turpentine water, or syrup of violets; and when combined with any base, they may be detected by the nitrate of silver, muriate of barytes, discovering and lime-water. Uncombined alkalies are ascertained by them. Brazil wood and turmeric; in combination with acids, they may be detected by spirit of wine. Earths are precipitated by the acid of sugar and the acetic acid. Sulphur is discovered by the mineral acids; and metals are precipitated by lime-water and tincture of galls.

Most waters contain common atmospheric air. Fixed Method of air, now called carbonic acid, is also found in all waters in analyzing quantity from 1/5th part of the bulk of the water to a containing bulk equal to the water itself. That some species of air is common air contained in water, is evident from the small bubbles which and carbo- may be often seen to rise in it when poured into a glass. These bubbles are still more distinguishable in water placed under the exhausted receiver of an air-pump; for the weight of the atmosphere being removed, the water expands; and the air contained in its interfaces is thus let loose, and rises to the surface. The air may also be separated from water by boiling, and may be easily collected by a proper apparatus. Experiments may then be made upon it to determine its species and quantity.

Carbonic acid is known to be contained in water by the following marks: The taste is somewhat pungent, acetic, cooling, and very agreeable. The smell is subtle and penetrating. When agitated, it emits a number of air-bubbles, which give it the appearance of briskness. These are the sensible appearances which aerated water exhibits; but there are tests which chemistry furnishes much more decisive.

From a pigment called litmus is obtained a tincture called the tincture of turpentine. The litmus is wrapped up in a clean linen cloth, and steeped in distilled water; the water soon assumes a blue or violet colour, and is then fit for use. The tincture enables the chemist to discover the smallest particle of disengaged acid; for a few drops of it poured into water containing an acid immediately communicates a red colour to the whole fluid.

There is a more convenient method of using the turpentine: The saturated tincture is boiled with a little starch, and then a piece of paper is dipped into it, so as to tinge it completely. Paper thus tinged, when dipped into water containing an acid, instantly receives a red colour. The tincture is, however, a more delicate and sensible test than the tinged paper; for water saturated with aerial acid does not make any change in the colour of the paper; yet one part of aerated water gives a distinct red to 50 parts of the tincture.

The method of collecting and ascertaining the elastic fluids contained in water was unknown till the present age. The earliest method is to fill a vessel terminating in a narrow neck with aerated water, then tie to the neck a bladder from which all the air has been carefully squeezed. Let the aerated water be boiled; the elastic fluid is then expelled, and ascends into the bladder, where it is collected. The bladder may then be removed from the vessel, and its mouth tied up.

There is another method, which is much more accurate, for determining the quantity contained in any quantity of water: Fill a bottle or retort with aerated water, and let a flopper be put into its mouth, with a hole in it. Let one end of a crooked tube be inserted into the hole of the stopper, so closely that no air may escape at the joining; and let the other end of the tube be bent upwards into an inverted vessel full of mercury. Fire is then applied to the bottle or retort, and continued till the water boils. The heat carries off the air which is conveyed through the crooked tube into the inverted vessel of mercury. If the water be kept boiling for a short time, the whole or greater part of the elastic fluid will be expelled, and its bulk is eliminated by the bulk of mercury which it has displaced. But it must be remembered, that the elastic fluid above the mercury is in a state of greater dilatation than the external air, for it is not pressed by the whole weight of the atmosphere; but, as M. Saussure observes, it is only charged with that weight diminished by the column of mercury.

When the aerial fluid is thus collected, if we wish to separate the carbonic acid from the common air, the process is easy: Let the aerial fluid be separated from the mercury, while the external air is carefully excluded; and let the vessel containing it be inverted into another vessel containing lime-water. The lime will immediately absorb the carbonic acid, and form calcareous earth, while the atmospheric air is left behind. The calcareous earth may then be weighed; and the carbonic acid being afterwards expelled, the loss of weight will give the quantity of carbonic acid.

The only other acids hitherto found in water besides the Method of carbonic acid, are the sulphuric and muriatic acids. The presence of the sulphuric acid is most accurately ascertained by the sulphate of barytes, which is a compound of the muriatic acid, with barytes or ponderous earth. Barytes has a strong attraction for the sulphuric acid, that it separates it from all other acids, and forms with it a compound called ponderous spar, which is insoluble in water. As the carbonate of alkali, or an aerated alkali, may produce a muddiness and precipitation resembling the effects of the sulphuric acid, it is necessary to add to it a few drops of the nitric acid, which will dissolve any portion of barytes precipitated by the aerated alkali.

The muriatic acid may be easily discovered, by throwing And mur into the water impregnated with it a little nitrate of silver (a compound of the nitric acid with silver). If there be the smallest portion of muriatic acid, it instantly seizes the silver, and is precipitated along with it in the appearance of a white mucilage. As the muriatic acid constitutes about one fourth of the nitrate of silver, we may easily determine its quantity, by subtracting one-fourth from the weight of the precipitate. Along with the nitrate of silver a little nitric acid should be added, for the reason mentioned in the last experiment.

Alkalis are known to exist in water by the lixivious or How alkali, faintly taste which they communicate, by their effervescence or detected.

There are three tests which may be employed for discovering the presence of alkalis. 1. Paper tinged blue by the tincture of turpentine, and made red by distilled vinegar, recovers its blue colour when dipped into water containing an alkali. 2. The watery tincture of Brazil wood also serves to discover alkalis. It may either be used in the state of tincture, or a piece of paper may be tinged with it after being boiled with a little starch. In both cases it receives a blue colour from the alkali. One grain of soda dissolved in 4295 grains of water changes the colour of the tinged paper to a blue, which, though delicate, may be easily distinguished. 3. Watery tincture of turmeric is changed to a brown colour by alkalis. Paper tinged with this tincture boiled with starch is also affected in the same way. A single grain of soda dissolved in 859 grams of distilled water will obscure the yellow colour of the tinged paper, and turn it into a brownish hue.

The tincture of Brazil wood is remarkable for its sensibility in discovering the presence of an alkali. The tincture of turmeric is much slower in its action; but this circumstance enables us, with some degree of accuracy, to estimate the quantity of alkali contained. The turmeric, too, answers best when there is occasion to examine an alkaline water by candle-light, as the change of colour which it produces is easily distinguishable.—Besides these tests now mentioned, any of the infusions of vegetables which are most easily affected by alkalis may be used with success, such as flowers of mallows and syrup of violets; but they are not on all occasions so decisive.

After being assured of the presence of an alkali, we must next determine what alkali it is. The alkalis most commonly found in water are the mineral and volatile, the vegetable seldom occurring. The mineral alkali is combined one with the carbonic, sulphuric, or muriatic acid; the volatile alkali is probably communicated by putrid animal or vegetable substances; and the vegetable is united with the sulphuric or Water.

or muriatic acid, but more frequently with the nitric acid. Bergman says, that mercury, dissolved in the nitric acid without heat, enables us to distinguish these alkalis. When a little of this solution is thrown into water, if a yellowish white substance is precipitated, we may conclude that a caustic vegetable alkali is present; if the precipitate be white, there is vegetable alkali saturated with the carbonic acid. If the precipitate be first yellow, and afterwards become white, mineral alkali is present; and if it be of a greyish black, we know that volatile alkali is present.

The species of alkali may be more easily ascertained, by pouring into the water a little sulphuric acid, or, what Morvan recommends as answering the purpose better, a little distilled vinegar, which with potash forms a deliquescent salt, and with soda a foliated crystallizable salt.

The earths which are mostly found in waters are lime and magnesia. If any other earth has been discovered, it has been by so few chemists, and in such small portions, that it has been little attended to (c). Lime and magnesia are always united with the carbonic or some of the fossil acids. The carbonic acid is easily expelled by boiling the water, and the earth falls to the bottom, and may then be easily examined by applying sulphuric acid. If the earth be calcareous, with sulphuric acid it forms gypsum; if it be magnesia, Epsom salt is produced; and if it be clay, the product is alum.

Scarce any water is entirely free from lime; even the purest water, after standing 24 hours, deposits some calcareated lime. The acid of sugar is one of the most sensible tests for discovering it. A small quantity of distilled water, in which there is dissolved a single grain of pure lime, will become muddy if the smallest quantity of the acid of sugar be thrown in. The presence of calcareous earth may also be discovered by employing the acetate of lead. It precipitates the earth in the form of a white powder. But as sulphuric acid also precipitates the acetate of lead, to make the experiment accurately, it is necessary to add a little distilled vinegar to the precipitate, and if it consist of calcareous earth, it will be immediately dissolved; but if it be a sulphate of lime, the vinegar will have no effect upon it.—When lime or magnesia is dissolved in any of the mineral acids, it may be detected by adding a little carbonate of potash. The nature of the earth may be afterwards easily determined.

Of the inflammable bodies, perhaps none has been found dissolved in water except sulphur. Sulphur is combined either with an alkali or with hydrogen, forming a sulphuret of hydrogen. Sulphuric or hepatic waters are easily known by the following marks: 1. A fetid smell, which is felt in approaching the spring. 2. The taste is strong, somewhat sweet, not unlike that of putrid eggs, but more disagreeable. 3. When a piece of silver is put into it, it becomes tarnished. 4. But the nicest test is a mark made on paper with the tartarite of bismuth or acetate of lead, which becomes black when exposed to the vapour of the hepatic water.

When we wish to discover the quantity of sulphur which is dissolved in an alkali, it may be precipitated by the sulphuric or muriatic acid, but much more plentifully by the nitric acid. To render the experiment successful, it is necessary that the mixture should be heated. When the nitric acid is dropped in, the sulphureous smell is instantly diffused, the water grows turbid, and a white sublimate slowly subsides. When dried, it is found to be genuine sulphur. When the water contains a fixed alkali, the acid has no effect in decomposing the sulphureous water till the alkali be saturated; but after the alkali is saturated, the hepatic air is then driven off by the acid, and the sulphur falls down.

Sulphureous water may easily be formed artificially: A Method of quantity of hepar sulphuris, consisting of equal parts of fulminating sulphur and potash, is to be put into a vessel which communicates by a crooked tube with an inverted glass filled with artificial water. Sulphuric acid is then poured into the vessel containing hepar sulphuris, a few drops at a time. The vessel containing the acid must communicate with the vessel containing the hepar sulphuris by a tube, that while the acid may be poured in at pleasure, the elastic gas which issues from the action of the acid on the hepar sulphuris may not be dissipated, but may pass into the inverted glass. This gas, if a candle be applied, will burn, and a residuum of sulphur of a whitish colour remains. The water in the inverted vessel must be frequently agitated, that the gas may be absorbed.

The metals hitherto found dissolved in waters are two, iron and copper. The former occurs often, the latter rarely. Iron is generally detected by a greenish or yellowish colour, by its inky taste, by an ochre which it deposits, by tincture of galls, and by the Prussian alkali. Only the two last of these methods require any description. Spirit of wine saturated with powdered galls precipitates iron slowly; the precipitate is purple when the quantity of iron is small; but when the quantity is large, it is black. In some cases indeed iron may be present in water without giving a dark colour to the galls. This is owing to a superfluity of acid. But if a sufficient quantity of alkali be added to saturate the acid, the black colour will then appear.—The Prussian alkali is prepared from four parts of Prussian blue, boiled with one part of alkali in a sufficient quantity of water. The clear liquor must then be saturated with an acid, and filtered, that it may be freed from the small portion of Prussian blue which is separated. A single drop of this alkali dropped into water containing the sulphate of iron immediately forms a Prussian blue. In making experiments with this alkali, it is proper to add a little muriatic acid.

The quantity of iron contained in water may be ascertained with considerable accuracy, by the colour communicated by the tincture of galls: for if the tincture be poured into distilled water, then small pieces of iron may be added, till the liquor has acquired the colour of the chalybeate water; and then we may conclude, that the quantity of iron contained in the chalybeate water is equal to the artificial mixture, if the colour be the same. There is also another way of estimating the quantity of iron. When precipitated, let the residuum be washed in pure water, then dried and weighed. Pour upon it one of the mineral acids, and digest them together, and after pouring it off, wash what remains undissolved; then dry and weigh it again, and from the diminution of weight collect that of the iron. In this experiment the acid employed ought not to be very strong nor great in quantity, nor ought the digestion to be continued long; for if the residuum should contain any selenite which is soluble by acids, the selenite might seize upon a considerable portion of the acid, and consequently the experiment be inaccurate.

Copper is sometimes united in water with the sulphuric acid. It is discovered by the blue colour which it imparts to it.

(c) A small quantity of siliceous earth was found by Bergman in an acidulous spring, as also by Dr Black in the Geyser spring in Iceland. Clay may also be often found in waters; but it is probably only diffused, not chemically dissolved. Water.

to the water, by an acriduous taste, and by the ochre which it deposits. It may also be detected by throwing into the water a piece of polished iron; the copper will be precipitated upon the iron.

Method of analyzing Water by Evaporation.

Having now described the methods of detecting the various substances contained in water by precipitation, we come next to describe how they are discovered by evaporation.

The vessels employed in evaporating the water ought to be broad, for fluids evaporate more quickly in proportion to the extent of the surface. If earthen vessels can be found so close a texture as not to absorb any saline matter, they may be safely employed. Iron and copper vessels are improper, because they are liable to be corroded. The most convenient are thin glass vessels, which may without danger be exposed to a strong heat. The capacity of the vessels depends on the quantity of water which is necessary for the several experiments. The quantity of water may be small if it contain a large proportion of extraneous matter. The evaporation should be slow and gentle. The vessel employed ought to have a cover to keep out dust; but must have a hole several inches in diameter, that the vapors may issue out. The hole should not be opened till the vapors be so much condensed as to issue with such force as to keep the dust from falling in.

Some substances require more water to dissolve them than others. As the quantity of water is diminished by evaporation, they appear therefore in an order corresponding to their different degrees of solubility; those which are least soluble appearing first. The following is the order in which evaporating they are discovered: First carbonate of lime and carbonate of iron, then gypsum, then the sulphate of potash, then the sulphate of iron, then the nitrate of potash, and next in order the sulphate of copper; afterwards the muriate of potash, then soda, then the muriate of soda, then the sulphate of magnesia, and lastly the deliquescent salts. Aerated magnesia, or carbonate of magnesia, is not separated all at once, but continues to fall during the whole process. This order is often altered by the superabundance of any particular substance.

The different substances may be separated as they successively appear; but it is better to continue the evaporation to dryness. The residuum should be carefully collected and well dried. It is then put into a bottle, and alcohol poured on till it rise an inch above it. The bottle should then be closed and shaken. After standing for a few hours, the liquor may be filtered. What passes through the filter is preserved for a future analysis, and what remains behind has eight times its weight of cold distilled water poured upon it; the mixture is then shaken, allowed to stand for some time, and again filtered. What was dissolved by the water is preserved for future examination, and the residuum is then boiled for a quarter of an hour in somewhat more than four or five hundred times its weight of distilled water, and afterwards filtered.

Being now purified by alcohol, cold water and hot water, the residuum is no longer soluble in alcohol or water. If it show a brown colour, this is a mark that iron is contained in it. To ascertain this point, it may be exposed for some weeks in an open vessel to the rays of the sun, care being taken to moisten it from time to time. By the exposure to the air, the iron will imbibe oxygen, and is then no longer soluble in vinegar. The residuum may then be weighed; a quantity of acetic acid or distilled vinegar acid is then to be poured on it, and the mixture to be digested, to be poured. By the digestion the acid will dissolve the carbonate of lime and magnesia, if there be any in the residuum. What the acid has not dissolved may be washed, dried, and weighed, and by its loss of weight it may easily be determined what the acid has taken up.

The matter dissolved by the acetic acid is then to be evaporated to dryness. It may be determined whether it contains calcareous earth or magnesia by this circumstance; if it consist of calcareous earth, it continues dry in a moist air; but if it contain magnesia, it is deliquescent. The same point may also be ascertained by the sulphuric acid. This acid added to calcareous earth, forms gypsum, or the sulphate of lime; but when added to magnesia, it dissolves it, forming the sulphate of magnesia or Epsom salt; or if the residuum contain both lime and magnesia, there will be produced both sulphate of lime and sulphate of magnesia. The precise quantity of the simple substances contained in each may be known by weighing the compound, and remembering that 100 parts of the sulphate of lime contain about 32 of pure lime, 46 of sulphuric acid, and 22 of water (v); and 120 parts of the sulphate of magnesia contain 19 of pure magnesia, 33 of sulphuric acid, and 48 of water (v).

That matter which was not dissolved by the acetic acid is either iron or silica. The iron is soluble by muriatic acid or by an alkali. The portion which resists the action of the muriatic acid is siliceous earth, which may be farther examined by the blowpipe; for siliceous earth, when added to soda in a state of fusion, combines with it with a violent effervescence, and is thus changed into glass.

Having now shown how to examine the residue which was insoluble in alcohol and water, it will next be proper to describe how to analyze the solutions obtained by alcohol, cold water, and hot water.

1. The solution obtained by alcohol contains lime and magnesia, combined with the muriatic acid or with the nitric acid. To enable us to discover the nature and quantity of the ingredients, we evaporate them to dryness, and then pour sulphuric acid on the residue; the sulphuric immediately displaces the other acids, and unites with the base. If the base be lime, it forms a sulphate of lime; if it be magnesia, it produces the sulphate of magnesia.

2. The solution obtained by cold water must be examined by evaporation. The evaporation ought to be gentle, that the crystals may assume regular forms. The crystals, as obtained by the evaporation, are then to be placed on bibulous paper and dried; but not so much as to expel any of the water of crystallization. The species of the salt thus formed may be distinguished by the taste and shape of the crystals. But that they may be distinguished with accuracy, we shall mention other methods: The solution obtained by cold water may contain alkalies, neutral salts, salts united with earths, salts united with metals, and neutral salts combined with earths or metals.

The alkalies can easily be discovered by the methods mentioned.

(d) The proportions given above are Bergman's; but Dr Kirwan estimates them differently. According to him, 100 parts of the sulphate of lime contain 32 of earth, 29.44 of acid, and 38.56 of water. When well dried, it loses about 24 of water, and therefore contains 42 of earth, 39 of acid, and 19 of water.

(e) According to Dr Kirwan, 100 grains of the sulphate of magnesia perfectly dry contain 45.67 of sulphuric acid, 36.54 of pure earth, and 17.83 of water. In crystals they contain 23.75 of acid, 19 of earth, and 57.25 of water. mentioned above, but the neutral or compound salts will occasion more difficulty. We must first determine what the acid is, and with what base it is united. The sulphuric acid is detected by the muriate of barytes, as described above. The nitrous acid, when present, is expelled by the sulphuric acid, and may be easily distinguished by its smell and red fumes. It will be made still more evident by exposing its fumes to a paper moistened with ammonia or white alkali. The muriatic acid is easily detected by exposing the fumes of it to a paper moistened with water. This acid may also be discovered by the nitrate of silver.

It is more difficult to discover the bases of the neutral salts which are always alkalies. We formerly described the method of detecting them in water when detartrated, but we have now to separate them from an acid. Potash may be separated by barytes, soda is expelled by potash, and ammonia is expelled either by potash or soda.

We have mentioned already the method of discovering and distinguishing the earths and metals dissolved in water; but there is one compound which is extremely difficult to separate, viz. soda from common salt. The best method for effecting this is the process of M. Giaonetti: "It consists (says M. Fourcroy) in washing the mixed salt with distilled vinegar. The acid dissolves the mild soda; the mixture is dried, and washed afresh with spirit of wine, which is charged with the terra foliata mineralis, without touching the marine salt; the spirituous solution is evaporated to dryness, and the residuum calcined; the vinegar is decomposed and burned; we have then nothing but the mineral alkali, whose quantity is exactly found."

The solution obtained by boiling water contains only terebinth or gypsum. This may be separated in crystals by evaporation to dryness, or it may be decomposed by an alkali.

We have now said every thing that is necessary respecting the two modes of analyzing water by precipitation and evaporation; but as a difficulty may occur to the unexperienced chemist respecting the order in which he ought to proceed in making his experiments, we shall lay before our readers the method recommended by M. Fourcroy.

He first examines the sensible properties of the water, the taste, colour, weight, &c., and then pours upon four pounds of water the same weight of lime-water. If no precipitate falls in 24 hours, he concludes that the water contains no detartrated carbonic acid, nor mild fixed alkali, nor earthy salts with base of aluminous earth or magnesia, nor metallic salts. If a precipitate is instantly formed, he proceeds to filter the liquid, and to examine the chemical qualities of the precipitate. If it has no taste, it is insoluble in water, if it effervescences with acids, and if it forms with sulphuric acid an insipid salt almost insoluble in water, he concludes that it is chalk, and that the lime-water attracted only the aerial acid dissolved in the water. On the contrary, if the precipitate be not copious, if it collects slowly, if it excites no effervescence, if with the sulphuric acid it forms a bitter salt, it is magnesia; but if with the same acid it forms a sweetish effrangent salt, it is aluminous earth or clay. Sometimes it may be a compound of both.

Being now examined by lime-water, he pours upon it other four pounds of the same water, a gros or two (f) of volatile caustic alkali, or he passes it through some alkaline gas generated by means of heat. When the water is saturated, he leaves it in a close vessel for 24 hours; then if a precipitate be formed, as it must contain salts, with iron, magnesia, or aluminous earth for its base, he investigates the nature of it. It must be observed, that the alkaline gas is not to be depended upon alone, but may be used as an auxiliary.

M. Fourcroy next pours into a certain quantity of the water under examination a portion of caustic mineral alkali dissolved. He continues to pour it in till no farther mudiness is produced, as it decomposes the salts with a base of aluminous earth, or a base of lime. If the precipitate resembles in form, colour, and quantity, that which is yielded by lime-water, it may be presumed that the water contains no calcareous earth; but if it be more weighty, copious, and has formed more quickly than the precipitate formed by the lime-water, then it contains lime mixed with magnesia or aluminous earth. If the precipitate contain any iron, it is easily detected by its colour and taste.

These observations of M. Fourcroy will be of great use to the young chemist, in pointing out the order which he may follow with facility and advantage in the analysis of waters; and after he has formed his opinion concerning the ingredients contained in the water, he may examine the truth of it, by applying the particular tests which have already been described.

In the account which we have given of the method of analyzing waters, the chemical reader will observe, that we have chiefly followed Bergman. We have done so, because we reckon him the best writer on the subject, and because we have been more anxious to study truth and utility than novelty. We ardently wish that some able chemist would exhibit an accurate and easy mode of analyzing earths, which every farmer could practice without a deep knowledge of chemistry. Farmers would then be enabled to apply the manures proper to particular soils, in which they would be much assisted by Dr. Kirwan's valuable Treatise on Manures.

Under the title of Mineral Waters, we have given an analysis of the most remarkable waters in Europe. (See also Spa, Seltzer, Pyrmont, and the names of other celebrated waters.) Those who wish for more information concerning the mode of analyzing water, may consult Bergman's Chemical Essays, Fourcroy's Lectures on Chemistry, and the different books referred to by these authors.

Holy Water, which is made use of in the church of Rome, as also by the Greeks, and by the other Christians of the East of all denominations, is water with a mixture of salt, blessed by a priest according to a set form of benediction. It is used in the blessing of persons, things, and places; and is likewise considered as a ceremony to excite pious thoughts in the minds of the faithful.

The priest, in blessing it, first, in the name of God, commands the devils not to hurt the persons who shall be sprinkled with it; nor to abuse the things, nor disquiet the places, which shall likewise be so sprinkled. He then prays that health, safety, and the favour of heaven, may be enjoyed by such persons, and by those who shall use such things, or dwell in such places. Vessels, vessels, and other such things that are set apart for divine service, are sprinkled with it. It is sometimes sprinkled on cattle, with an intention to free or preserve them from diabolical enchantments; and in some ritual books there are prayers to be said on such occasions, by which the safety of such animals, as being a temporal blessing to the possessors, is begged of God, whose providential care is extended to all his creatures. The hope which Catholics entertain of obtaining such good effects from the devout use of holy water, is grounded

(f) A gros is equal to 59,0703 of English Troy grains. grounded on the promise made to believers by Christ (St Mark xvi. 17.), and on the general efficacy of the prayers of the church; the petition of which prayers God is often pleased to grant; though sometimes, in his Providence, he sees it not expedient to do so. That such effects have been produced by holy water in a remarkable manner, has been attested by many authors of no small weight; as, namely, by St Epiphanius, Haer. 30th; St Hierom, in the Life of St Hilarion; Theodore of Hipp., Eccl. lib. v. cap. 21.; Palladius, Hipp. Lauf.; Bede, lib. vii. cap. 4.

As a ceremony (says the Catholic), water brings to our remembrance our baptism; in which, by water, we were cleansed from original sin. It also puts us in mind of that purity of conscience which we ought to endeavour always to have, but especially when we are going to worship our God. The salt, which is put into the water to preserve it from corrupting, is also a figure of divine grace, which preserves our souls from the corruption of sin; and is likewise an emblem of that wisdom and discretion which ought to season every action that a Christian does, and every word that he says. It is wont to be blessed and sprinkled in churches on Sundays, in the beginning of the solemn office. It is kept in vessels at the doors of the same churches, that it may be taken by the faithful as they enter in. It is also often kept in private houses and chambers (a).

Putrid Water, is that which has acquired an offensive smell and taste by the putrefaction of animal or vegetable substances contained in it. It is in the highest degree pernicious to the human frame, and capable of bringing on mortal diseases even by its smell. It is not always from the apparent muddiness of waters that we can judge of their disposition to putrefy; some which are seemingly very pure being more apt to become putrid than others which appear much more mixed with heterogeneous matters. Under the article Animalcule, n° 33, is mentioned a species of insects which have the property of making water stink to an incredible degree, though their bulk in proportion to the fluid which surrounds them is less than that of one to a million. Other substances no doubt there are which have the same property; and hence almost all water which is confined from the air is apt to become offensive, even though kept in glass or stone-ware vessels. Indeed it is a common observation, that water keeps much longer sweet in glass vessels, or in those of earthen or stone-ware, than in those of wood, where it is exceedingly apt to putrefy. Hence, as ships can only be supplied with water kept in wooden casks, sailors are extremely liable to those diseases which arise from putrid water; and the discovery of a method by which water could easily be prevented from becoming putrid at sea would be exceedingly valuable. This may indeed be done by quicklime; for when water is impregnated with it, all putrefactive matters are either totally destroyed, or altered in such a manner as never to be capable of undergoing the putrefactive fermentation again. But a continued use of lime-water could not fail of being pernicious, and it is therefore necessary to throw down the lime; after which the water will have all the purity necessary for preserving it free from putrefaction. This can only be done by means of fixed air; and mere exposure in broad shallow vessels to the atmosphere would do it without anything else, only taking care to break the crust which formed upon it. Two methods, however, have been thought of for doing this with more expedition. The one, invented by Dr Alton, is, by throwing into the water impregnated with lime a quantity of magnesia. The lime attracts fixed air more powerfully than magnesia; in consequence of which the latter parts with it to the lime; and thus becoming insoluble, falls along with the caustic magnesia to the bottom, and thus leaves the water perfectly pure. Another method is that of Mr Henry, who proposes to throw down the lime by means of an effervescing mixture of oil of vitriol and chalk put down to the bottom of the water-casks. His apparatus for this purpose is as simple as it can well be made, though it is hardly probable that sailors will give themselves the trouble of using it; and Dr Alton's scheme would seem better calculated for them, were it not for the expense of the magnesia; which indeed is the only objection made to it by Mr Henry. Putrid water may be restored and made potable by a process of the same kind.

Of late it has been discovered that charcoal possesses many unexpected properties, and, among others, that of preserving water from corruption, and of purifying it after it has been corrupted. Mr Lowitz, whose experiments on charcoal have been published in Crell's Chemical Journal, has turned his attention to this subject in a memoir read to the Economical Society of Petersburg. He found that the effect of charcoal was rendered much more speedy by using along with it some sulphuric acid. One ounce and a half of charcoal in powder, and 24 drops of concentrated sulphuric acid (oil of vitriol), are sufficient to purify three pints and a half of corrupted water, and do not communicate to it any sensible acidity. This small quantity of acid renders it unnecessary to use more than a third part of the charcoal powder which would otherwise be wanted; and the loss of that powder is employed, the loss is the quantity of water lost by the operation, which, in sea-voyages, is an object worthy of consideration. In proportion to the quantity of acid made use of, the quantity of charcoal may be diminished or augmented. All acids produce nearly the same effects: neutral salts also, particularly nitre and soda-salt, may be used, but sulphuric acid is preferable to any of these; water which is purified by means of this acid and charcoal will keep a longer time than that which is purified by charcoal alone. When we mean to purify any given quantity of corrupted water, we should begin by adding to it as much powder of charcoal as is necessary to deprive it entirely of its bad smell. To ascertain whether that quantity of powdered charcoal was sufficient to effect the clarification or the faded water, a small quantity of it may be passed through a linen bag, two or three inches long; if the water, thus filtrated, still has a turbid appearance, a fresh quantity of powdered charcoal must be added, till it is become perfectly clear: the whole of the water may then be passed through a filtering bag, the size of which should be proportioned to the quantity of water. If sulphuric acid, or any other, can be procured, a small quantity of it should be added to the water, before the charcoal powder.

The cleaning of the casks in which water is to be kept in sea-voyages should never be neglected: they should be well washed with hot water and sand, or with any other substance capable of removing the mucilaginous particles, and afterwards a quantity of charcoal-dust should be employed, which will entirely deprive them of the musty or putrid smell they may have contracted.—The charcoal used for purifying water should be well burnt, and afterwards beat into a fine powder.

(a) This article was furnished by an eminent divine of the church of Rome, to whom we are indebted for greater favours. Sea-Water. See Sea-Water.

Water-Carts, carriages constructed for the purpose of watering the roads for several miles round London; a precaution absolutely necessary near the metropolis, where, from such a vast daily influx of carriages and horses, the dust would otherwise become quite insufferable in hot dry weather. Pumps are placed at proper distances to supply these carts.

Water-Ordeal. See Ordeal.

Water, among jewellers, is properly the colour or lustre of diamonds and pearls. The term, though less properly, is sometimes used for the hue or colour of other stones.

Water-Bellows. See Machines for blowing Air into Furnaces.

Water Colours, in painting, are such colours as are only diluted and mixed up with gum-water, in contradistinction to oil-colours. See Colour-Making.

Water-Gong, a channel cut to drain a place by carrying off a stream of water.

Water-Hen. See Parra.

Water-Line of a Ship, certain horizontal lines supposed to be drawn about the outside of a ship's bottom, close to the surface of the water in which she floats. They are accordingly higher or lower upon the bottom, in proportion to the depth of the column of water required to float her.

Water-Lodged, the state of a ship when, by receiving a great quantity of water into the hold, by leaking, &c., she has become heavy and inactive upon the sea, so as to yield without resistance to the efforts of every wave rushing over her decks. As, in this dangerous situation, the centre of gravity is no longer fixed, but fluctuating from place to place, the stability of the ship is utterly lost: she is therefore almost totally deprived of the use of her sails, which would operate to overturn her, or press the head under water. Hence there is no resource for the crew, except to free her by the pumps, or to abandon her by the boats as soon as possible.

Water-Sail, a small sail spread occasionally under the lower jibbing-tail, or driver-boom, in a fair wind and smooth sea.

Water-Ouzel. See Turdus.

Water-Spout, an extraordinary meteor consisting of a large mass of water collected into a form of column, and moved with rapidity along the surface of the sea.

The best account of the water-spout which we have met with is in the Phil. Trans. Abridged, vol viii., as observed by Mr Joseph Harris, May 21, 1732, about sunset, lat. 32° N. long. 9° E. from Cape Florida.

"When first we saw the spout (says he), it was whole and entire, and much of the shape and proportion of a speaking trumpet; the small end being downwards, and reaching to the sea, and the big end terminated in a black thick cloud. The spout itself was very black, and the more so the higher up. It seemed to be exactly perpendicular to the horizon, and its sides perfectly smooth, without the least rugosities. Where it fell the spray of the sea rose to a considerable height, which made somewhat the appearance of a great smoke. From the first time we saw it it continued whole about a minute, and till it was quite dissipated about three minutes. It began to waste from below, and so gradually up, while the upper part remained entire, without any visible alteration, till at last it ended in the black cloud above: upon which there seemed to fall a very heavy rain in that neighbourhood.—There was but little wind, and the sky elsewhere was pretty serene."

Water spouts have been supposed to be merely electrical in their origin; particularly, by Signior Beccaria, Vol. XVIII. Part II.

who supported his opinion by some experiments. But if we attend to the successive phenomena necessary to constitute a complete water-spout through their various stages, we shall be convinced, that recourse must be had to some other principle in order to obtain a complete solution.

Dr Franklin, in his Physical and Meteorological Observations, supposes a water-spout and a whirlwind to proceed from the same cause; their only difference being, that the latter passes over the land, and the former over the water. This opinion is corroborated by M. de la Pryme, in the Philosophical Transactions, where he describes two spouts observed at different times in Yorkshire, whose appearances in the air were exactly like those of the spouts at sea, and their effects the same as those of real whirlwinds.

A fluid moving from all points horizontally towards a centre, must at that centre either mount or descend. If a hole be opened in the middle of the bottom of a tub filled with water, the water will flow from all sides to the centre, and there descend in a whirl: but air flowing on or near the surface of land or water, from all sides towards a centre, must at that centre ascend; because the land or water will hinder its descent.

The Doctor, in proceeding to explain his conceptions, begs to be allowed two or three positions, as a foundation for his hypothesis. 1. That the lower region of air is often more heated, and so more rarefied, than the upper, and by consequence specifically lighter. The coldness of the upper region is manifested by the hail, which sometimes falls from it in warm weather. 2. That heated air may be very moist, and yet the moisture so equally diffused and rarefied as not to be visible till colder air mixes with it; at which time it condenses and becomes visible. Thus our breath, although invisible in summer, becomes visible in winter.

These circumstances being granted, he supposes a tract of land or sea, of about 60 miles in extent, unsheltered by clouds and unrefreshed by the wind, during a summer's day, or perhaps for several days without intermission, till it becomes violently heated, together with the lower region of the air in contact with it; so that the latter becomes specifically lighter than the superincumbent higher region of the atmosphere, wherein the clouds are usually floated: he supposes also that the air surrounding this tract has not been so much heated during those days, and therefore remains heavier. The consequence of this, he conceives, should be, that the heated lighter air should ascend, and the heavier descend; and as this rising cannot operate throughout the whole tract at once, because that would leave too extensive a vacuum, the rising will begin precisely in that column which happens to be lightest or most rarefied; and the warm air will flow horizontally from all parts of this column, where the several currents meeting, and joining to rise, a whirl is naturally formed, in the same manner as a whirl is formed in a tub of water, by the descending fluid receding from all sides of the tub towards the hole in the centre.

And as the several currents arrive at this central rising column, with a considerable degree of horizontal motion, they cannot suddenly change it to a vertical motion; therefore as they gradually, in approaching the whirl, decline from right to curve or circular lines, so, having joined the whirl, they ascend by a spiral motion: in the same manner as the water descends spirally through the hole in the tub before mentioned.

Lastly, as the lower air nearest the surface is more rarefied by the heat of the sun, it is more impressed by the current of the surrounding cold and heavy air which is to assume its place, and consequently its motion towards the whirl is swifter, and so the force of the lower part of the whirl... whirl strongest, and the centrifugal force of its particles greatest. Hence the vacuum which incloses the axis of the whirl should be greatest near the earth or sea, and diminish gradually as it approaches the region of the clouds, till it ends in a point.

This circle is of various diameters, sometimes very large. If the vacuum passes over water, the water may rise in a body or column therein to the height of about 32 feet. This whirl of air may be as invisible as the air itself; though reaching in reality from the water to the region of cool air, in which our low summer thunder-clouds commonly float; but it will soon become visible at its extremities. The agitation of the water under the whirling of the circle, and the swelling and rising of the water in the commencement of the vacuum, renders it visible below. It is perceived above by the warm air being brought up to the cooler region, where its moisture begins to be condensed by the cold into thick vapour, and is then first discovered at the highest part, which being now cooled condenses what rises behind it, and this latter acts in the same manner on the succeeding body; where, by the contact of the vapours, the cold operates faster in a right line downwards, than the vapours themselves can climb in a spiral line upwards; they climb however; and as by continual addition they grow denser, and by consequence increase their centrifugal force, and being risen above the concentrating currents that compose the whirl, they fly off, and form a cloud.

It seems easy to conceive, how, by this successive condensation from above, the spout appears to drop or descend from the cloud, although the materials of which it is composed are all the while ascending. The condensation of the moisture contained in so great a quantity of warm air as may be supposed to rise in a short time in this prodigiously rapid whirl, is perhaps sufficient to form a great extent of cloud; and the friction of the whirling air on the sides of the column may detach great quantities of its water, disperse them into drops, and carry them up in the spiral whirl mixed with the air. The heavier drops may indeed fly off, and fall into a shower about the spout; but much of it will be broken into vapour, and yet remain visible.

As the whirl weakens, the tube may apparently separate in the middle; the column of water subsiding, the superior condensed part drawing up to the cloud. The tube or whirl of air may nevertheless remain entire, the middle only becoming invisible, as not containing any visible matter.

Dr Lindsay, however, in several letters published in the Gentleman's Magazine, has controverted this theory of Dr Franklin, and endeavoured to prove, that water-spouts and whirlwinds are distinct phenomena; and that the water which forms the water-spout, does not ascend from the sea, as Dr Franklin supposes, but descends from the atmosphere. Our limits do not permit us to insert his arguments here, but they may be seen in the Gentleman's Magazine, volume li. p. 559; vol. iii. p. 1025; and vol. iv. p. 594. We cannot avoid observing, however, that he treats Dr Franklin with a degree of severity to which he is by no means intitled, and that his arguments, even if conclusive, prove nothing more than that some water-spouts certainly do descend; which Dr Franklin hardly ever ventured to deny.

There are some very valuable dissertations on this subject by professor Willeke of Upsal.

Water-Works. See Water-Works (a).

Water-Works for entertainment. See Hydrostatics, sect. 6.