Water, a liquid composed of oxygen and hydrogen. The earlier chemists supposed it to be an element, and it was only about a century ago that the researches of Cavendish and Lavoisier established its compound nature, which has since been abundantly verified both by analysis and by synthesis. By the action of an intense heat or by the electric current it is resolved into its constituents and yields one volume of oxygen gas and two volumes of hydrogen gas; or, as the former has 16 times the density of the latter, eight parts by weight of oxygen to one of hydrogen. These two gases when mingled in these proportions unite with explosive violence by the contact of flame, and reproduce water, the union being attended with great elevation of temperature. Pure water at ordinary temperatures is a liquid devoid of taste and smell, transparent, and nearly colorless, but when viewed in mass is found to possess a faint blue color. A cubic inch of pure water, at a temperature of 62° F. and a barometric pressure equal to 30 in. of mercury, weighs 252.458 grains troy; and for the purposes of ordinary calculation a cubic foot of water may be reckoned at 1,000 oz. avoirdupois, or 62½ lbs.

In France water at its point of maximum density, or 4° C. (39.2° F.), is taken as the standard, and the weight of a cubic centimetre at this temperature, and under a pressure equal to 760 millimetres of mercury, is one gramme, which is the unit of weight in the metric system. Its density at this temperature is about 770 times that of atmospheric air, and is taken as the standard of comparison for the density of all liquid and solid bodies. Hence its specific gravity is said to be unity or 1.000. (See Gravity, Specific) Water is slightly elastic, and by the increased pressure of one atmosphere has its volume diminished to the extent of about 0-000047, its compressibility increasing with the augmentation of temperature. The density of water below 39.2° F. is diminished by cooling, it being 0-999877 at 32° (0° C). One hundred parts of water at 32° expand to 104.29 when heated to 212°, and to 110.16 at 314.24°. At the ternperature of 32° liquid water under the ordinary conditions of pressure becomes changed into ice, with a considerable augmentation of volume.

The specific gravity of ice is -920, water at its greatest density (at 39.2°) being 1.000. The expansion of nearly 1/11- which takes place in the freezing of water suffices to break very strong vessels; but when so confined that its expansion is prevented, it can be cooled to very low temperatures without losing its liquid form. Pressure thus reduces the freezing point and prevents the congelation of water, and in like manner lowers the melting point of ice. Ice is a crystalline solid which assumes the forms of the hexagonal system of crystallization, as is well seen in snow flakes. Its color when in large masses is like that of liquid water, slightly blue. Much heat is liberated in the formation of ice, so that water cooled to 32° requires a prolonged exposure to a temperature below this point for its solidification; and conversely the melting of ice is attended with a great absorption of heat. When a pound of water at 174-56° F. is mixed with a pound of ice at 32°, the latter in melting reduces the temperature of the whole to 32°. Water is volatile at all temperatures, a portion of watery vapor being given off from ice below the freezing point.

As the temperature is raised, the tension of the vapor disengaged from the surface of the liquid augments, until it equals the atmospheric pressure, beyond which the liquid enters into ebullition. The boiling point of water in metallic vessels is 212° when the barometric column is 29.922 in., but varies with the pressure, so that on mountains, where the weight of the barometric column is reduced, the boiling point is proportionally lowered, while under increased pressure it is augmented. Thus, with a pressure equal to two atmospheres water boils at 260.52°; with four atmospheres, at 293.72°; with ten atmospheres, at 358.88°; and with 30 atmospheres, at 457.2, a temperature above the melting point of tin. The nature of the inner surface of the containing vessel affects somewhat the point of ebullition, so that in smooth glass or porcelain it is one or two degrees above that in a rough metallic vessel. The evaporation of water at temperatures below boiling takes place only from the surface, and in a confined space ceases after the surrounding air has become saturated with the watery vapor.

The process is therefore accelerated by a free circulation, which removes the saturated atmosphere. "Water boiling at the ordinary pressure is converted into more than 1,600 times its volume of vapor, which at the temperature of ebullition (212° F.) has a specific gravity of 0'622 as compared with air at that temperature, or of 0.455 as compared with air at 82°. The conversion of water into vapor is attended with absorption of heat. One pound of water at 212°, in becoming vapor of the same temperature, consumes as much heat as would raise 5.37 lbs. of water from 32° to 212°; hence one pound of steam, at 212°, will raise 5.37 lbs. of water to 212°, being itself condensed, so that the result is 6.37 lbs. of water of 212°. Aqueous vapor is colorless and transparent, and only becomes visible in the air when partially condensed, as in the case of escaping steam. Watery vapor is precipitated from air upon cold surfaces in the form of dew, but occasionally also as hoar frost, thus changing from gas to solid without passing through the intermediate condition of liquidity.

At the heat of melting platinum it is separated into its constituent gases. - Water is widely distributed in nature: in the form of ice and snow in the polar regions, in the condition of aqueous vapor, which forms a constant ingredient in the atmosphere, and in the liquid form not only in oceans, lakes, and rivers, but permeating the soil and most of the known rocks. It is the predominant element in the sap and juices of plants, and in the blood and flesh fluids of animals, and constitutes about five sixths of the weight of the human body. Water, or at least the elements of which it consists, hydrogen and oxygen, moreover exist in a great many bodies in such a state of combination that these are generally described as compounds of water, and are known as hydrates. Thus gypsum when exposed to heat gives off 20 per cent, of water, which it will again absorb if brought in contact with it at ordinary temperatures. Serpentine rock contains 12 per cent., brown iron ore 15 per cent., and alum 45 per cent. From all of these bodies it is given off by heat. Some of these hydrates retain the water much more strongly than others, and it is a question with chemists whether the water is to be regarded as existing ready formed in these and similar compounds.

Besides, there are numerous bodies, such as starch, sugar, and woody fibre, which are often spoken of as hydrates of carbon, and may be represented as compounds of carbon with water. But they are rather to be looked upon as triple compounds of carbon, hydrogen, and oxygen, in which water as such does not exist; and in philosophical exactness, the same view should be extended to the mineral hydrates just mentioned. - Water is remarkable for its solvent power, by which we understand its capacity to unite with or take up into itself various solid, liquid, and gaseous matters, forming with them homogeneous liquid compounds called solutions. Familiar examples of this are seen in its action on salt and sugar. Different bodies have very different degrees of solubility, and many are soluble in water to so slight an extent that they are generally classed as insoluble. Thus, while one part of common salt requires about three parts by weight of water to dissolve it, one part of gypsum requires about 400 parts, and one part of carbonate of lime under ordinary conditions 10,000 parts of water; while sulphate of baryta is very much less soluble, and for all practical purposes may be regarded as insoluble in pure water, though somewhat soluble in saline solutions.

Certain bodies, such as the metals, resins, carbon, sulphur, and oils, are regarded as wholly insoluble; but all of these bodies form chemical compounds which are soluble in water. With regard to a great many substances it is known that they occur in two conditions, the one soluble and the other insoluble. Thus silica, which in the form of flint or quartz appears wholly insoluble in water, not only forms a soluble compound with the alkalies, the socalled soluble glass, but when separated from this compound is itself soluble in water to the extent of 14 per cent. In like manner the sesquioxides of iron, chrome, and aluminum, though completely insoluble in water in their ordinary artificial forms, and constituting moreover some of the most insoluble minerals in nature, may be by chemical means obtained in eminently soluble forms. In a great many other cases it can be shown that bodies when generated in the presence of water by chemical reactions are soluble for a time, though when they have once passed into insoluble forms it is not easy to restore the condition of solubility. Solution is a process of condensation, in which the volume of the body dissolving is more or less completely lost in that of the solvent.

Hence pressure, which favors condensation, augments the solvent power of water; experiments have shown that the solubility of certain salts in water is notably increased under a pressure of many atmospheres. Heat exercises an important influence on the solvent power of water; thus, while gypsum and lime are much more soluble in cold than in hot water, and sea salt has about the same solubility in both, the greater number of salts are much more soluble in hot than in cold water. Some bodies nearly insoluble in cold water possess a considerable degree of solubility at 212°, while others apparently insoluble at this point enter into solution in water when heated under pressure to temperatures considerably higher. The presence of carbonic acid, which is found in most natural waters, greatly augments the solvent power of this liquid for many substances. As a result of this wide range of solubilites, it follows that pure water, except as an artificial product, is unknown, and that all natural waters have their characters modified by the presence of foreign matters.

That which falls from the clouds as rain or snow water holds in solution, besides the gases nitrogen, oxygen, and carbonic acid dissolved from the atmosphere, small portions of ammonia and nitrous compounds, and a minute but variable amount of mineral matters which were previously suspended in the air. After falling on the earth these same waters become further impregnated with foreign ingredients. From decaying vegetation they take up two classes of substances: first, the organic products of decomposition, the so-called soluble organic matters, which give to the waters of marshy districts their brownish color; and second, the mineral matters which form an essential part of all vegetation and constitute its ash, but are for the most part liberated in soluble forms during its slow decay. These consist chiefly of salts of potash, lime, and magnesia, with phosphates and silica. At the same time the free oxygen of the atmospheric waters is absorbed by the organic matter and replaced by carbonic acid derived therefrom.

In rivers and waters exposed to the further action of the air, oxygen is again absorbed, and a slow oxidation of the dissolved organic matters is effected. "When the atmospheric waters sink into the soil, either directly or after being thus impregnated with the products of organic decay, they undergo still further changes, dependent upon the nature of the strata through which they pass. Ordinary soils contain no matters soluble in pure water, yet they are not without action on the infiltrating waters whose composition has juskbeen described, especially if the soils are more or less clayey. In this case the silica and phosphates, together with much of the organic matter, are retained, while the potash salts are exchanged for those of soda and of lime. Carbonates of lime and magnesia, when present in the soil, are moreover taken into solution by the carbonic acid present. Hence the ordinary waters of wells and springs, supplied by this filtration, differ very much in their composition from the superficial waters.

These reactions, in virtue of which the foreign matters derived by the superficial waters from the decay of plants are absorbed by the soil, are important alike for the nutrition of subsequent generations of plants and for the purification of the waters, which are thus rendered potable. Besides these reactions, which depend upon the mineral matters previously dissolved by the atmospheric waters, there are others not less important due to the direct action of the water and its dissolved gases on the solid rocks, in virtue of which the silicated minerals of these are decomposed with the liberation in a soluble form of certain of their elements. In this way large quantities of alkalies, lime, and magnesia are set free and are dissolved in the form of carbonates, together with a considerable proportion of silica, which is liberated in a soluble condition. This process of decay has been going on from remote ages, and has effected the decomposition and disintegration of vast portions of the crystalline rocks, which have thereby been reduced to the condition of clays, while immense amounts of soluble matter have been added to the waters of the earth.

The rivers which drain regions of crystalline rocks are, as a result of this process, remarkable for containing in solution carbonates of soda and potash, together with a large relative proportion of silica, as may be seen in the waters of the Ottawa, the Loire, and the Garonne. Waters of a like origin are deprived by filtration through the soil of their silica and potash. A process similar in its results probably takes place at great depths under the influence of carbonic acid from subterranean sources, from which result waters more highly impregnated with alkaline salts, constituting some of the best known mineral springs. In addition to these various impregnating matters should be noticed those derived from the oxidation of metallic sulphurets, chiefly pyrites, giving rise to sulphates of iron and alumina, and indirectly to sulphates of lime and magnesia. Another and an important supply of foreign matters is derived from the soluble salts, such as chlorides and sulphates, which are enclosed in many stratified rocks, while the carbonates of lime and magnesia of these rocks are themselves dissolved by carbonic acid, and with the iron oxide taken into solution through the intervention of the organic matters contribute to the complex constitution of the water of springs, rivers, and lakes.

The quality called hardness in such waters depends upon the power they possess of decomposing soap, and is due to the salts of lime and magnesia, whether present as carbonates or in the form of sulphates and chlorides. Boiling, by expelling the excess of carbonic acid,, precipitates the carbonates in an insoluble condition, and thus gives rise to incrustations. The sulphate of lime, from its sparing solubility at elevated temperatures, is deposited in like manner in high-pressure boilers. - The ocean is the great receptacle of all soluble matters from the land, and from its waters have been deposited the greater part of the stratified rocks of the earth's crust. The waters of the ocean vary somewhat in composition, but contain on an average about 34 parts of solid matter in 1,000, though in the Mediterranean, where evaporation is rapid, this rises to 36 or even 40 parts, and in other regions falls very much lower, owing to the admixture of fresh water from the land. The saline matters of the sea may be regarded as consisting of from 78 to 80 per cent, of chloride of sodium or sea salt, with 2 per cent, of chloride of potassium, 7 or 8 per cent, of chloride of magnesium, about 7 per cent, of sulphate of magnesia, and from 3 to 4 per cant, of sulphate of lime.

A different arrangement of these elements in the water is conceivable, and is even probable; but when exposed to spontaneous evaporation, as in the manufacture of salt, the whole of the lime separates as gypsum or hydrated sulphate of lime, after which the sea salt is in great part deposited in a pure state, followed by a mixture of it with sulphate of magnesia, until there remains in the dense solution or bittern little more than chloride of magnesium. In addition to the above named compounds, the sea water contains in the form of bromides an amount of bromine equal to about To.f 1/10000 weight, besides appreciable quantities of iodine, fluorine, phosphates, and borates. Moreover, it holds in solution a small and variable amount of carbonate of lime, some silica, and traces of various metals, including, besides iron and manganese, arsenic, copper, lead, silver, and gold. It may in fact be expected that the sea will contain all the mineral elements which are capable of being held in solution in its waters, so that the progress of chemical investigation may greatly add to the preceding list.

The waters of the ocean are subject to constant changes from several different causes which have been active from remote ages, first among which may be placed the action of the alkaline waters, the origin of which has already been described, and which sooner or later find their way to the ocean. These, by the carbonate of soda which they contain, decompose the lime salts in the sea water, generating carbonate of lime and soda salts, of which the former remains in solution until it is taken up by growing plants and animals, including the coccoliths and nullipores among the former, and the foraminifera, radiates, and mollusca among the latter, from the remains of all which organisms the limestones are at last built up. To this, however, the carbonate of lime directly brought down from the land contributes, and also the carbonate of magnesia, which reacts upon the lime salts in the sea water precisely like carbonate of soda, giving rise by double decomposition to carbonate of lime. The water of the ocean in early times contained a large proportion of chloride of calcium, which in the course of ages has been decomposed with the formation of carbonate of lime and chloride of sodium, until at present the lime in the sea water is insufficient to form gypsum with more than a small proportion of the sulphate.

The ultimate result of this process will be the elimination from the ocean's waters of the whole of its soluble lime salt, and its replacement by salts of soda and of magnesia. In these changes in the ocean waters it is evident that the intervention of vegetable and animal life in the formation of limestone is but incidental, since the insolubility of the chemically formed carbonate of lime would eventually lead to its separation in a solid form, independent of organic beings. But there are other changes in the composition of the ocean's waters which are directly dependent upon the agency of life. The ash of marine plants, like that of those of the land, contains large quantities of potash salts and phosphates which have been abstracted from the waters of the sea, besides portions of iodine and of the rarer metals. These same elements are not confined to plants, but enter into the composition of the marine animals. The phosphate of lime of the sea weeds passes into the bones of fishes; copper is found in the fluids of certain mollusks and crustaceans, and iodine is concentrated in sponges as well as in sea weeds.

Thus these various elements pass from the waters into animal and vegetable tissues, by the decay of which on land or in the ooze at the bottom of the sea they become fixed in insoluble forms, being thus removed from the oceanic circulation and restored to the solid earth. Through these reactions the sea has doubtless suffered great modifications in composition, and but. for them its waters would become charged with phosphates and the various mineral matters mentioned above. The composition of the waters of the sea in past ages has also been profoundly modified by evaporation and by processes connected therewith. So long as the waters of the open ocean receive again the whole amount of water raised by evaporation, the only change which can result from this process is that already explained as effected by the soluble matters brought down from the land; but the results are very different in the case of basins cut off from the ocean, like the Dead sea, which is a type of a vast number of much larger areas existing in former geological periods under similar climatic conditions.

These conditions are, an amount of evaporation exceeding the rainfall of the enclosed sea and its geographical basin, from which results a gradual diminution of the volume of water and its consequent concentration, causing the precipitation from the water of beds of sulphate of lime or gypsum and of rock salt, often of great thickness and extent, and more rarely of soluble salts of potash and magnesia, the results of a still further evaporation, all of which are today found imbedded in the rocky strata of past geological ages, and represent large amounts of saline matter removed from the ocean's waters. Into these restricted and cut-off basins, moreover, the limited rainfall brings the soluble salts from the land, which by their reaction on the salts of the sea water effect, in addition to the changes already described, others peculiar and not less remarkable. These waters generally contain, as already explained, carbonates of lime, soda, and magnesia, of which the latter two decompose the" chloride of calcium and the sulphate of lime present, with separation of carbonate of lime, until at length the whole of these more soluble lime salts are converted into carbonate.

Then begins a reaction between the carbonate of soda and the soluble magnesian salts of the sea water, resulting in the production of carbonate of magnesia. Another and very different reaction gives rise to the same compound in waters which have lost their soluble lime salts either by the reaction just mentioned or by its separation in the form of gypsum. The carbonate of lime in the inflowing waters, which hold it as bicarbonate, yields by double decomposition with the sulphate of magnesia of the sea water sulphate of lime and bicarbonate of magnesia, the former of which salts, being very sparingly soluble, readily separates as gypsum, leaving the more soluble magnesian bicarbonate to be thrown down at a later period in the process of evaporation, in the condition of simple carbonate. From this, with the addition of carbonates of lime and magnesia brought by the inflowing waters, are formed the dolomites or magnesian limestones which abound in the rock formations of various geological periods, and appear to have been in all cases deposited in evaporating basins.

By virtue of the last described reaction, as will be seen, both of the constituents of the sulphate of magnesia are completely removed from the evaporating waters and fixed in insoluble forms, thus permanently modifying the composition of the water of enclosed basins, which by subsequent changes of sea and land become once more a part of the ocean, affecting the composition of its waters. In many cases the river or spring water flowing into closed basins contains neutral salts of lime and magnesia, so that we find in such lakes great variations in composition, from the bitter salines of the Dead sea, charged with chlorides of magnesium and calcium, to the alkaline waters containing carbonate and borate of soda which abound in central Asia, Egypt, and California. - The various stratified rock3, whether of mechanical or of chemical origin, have for the most part been deposited in the waters of the open sea or of enclosed basins. All of these rocks are more or less porous, and over great areas which have never been subjected to any considerable disturbance they are still impregnated with the saline waters in the midst of which they were deposited.

Leaving out of question the solid soluble salts enclosed in these strata, some notion of the amount of saline water which they contain may be had from a consideration of the degree of porosity of various rocks. Careful experiments upon the different rocks of the great American paleozoic basin show that various sandstones are capable of holding in their pores from 2 or 3 up to 10 and 20 volumes of water for 100 of rock, while the pure limestones generally hold not more than 1 or 2, and the dolomites from 5 to 10. The porosity of many rocks is even greater than any of these, and from a comparison of the above observations with others made in Europe, it is probably not an exaggeration to say that the stratified sedimentary rocks as a whole contain in their pores one tenth of their volume of water; so that the 40,000 or 50,000 ft. of 'palaeozoic strata in parts of North America and Great Britain would hold enclosed the equivalent of seas of nearly a mile in depth, and the volume of water enclosed in the rocky strata of the earth's crust bears a very considerable proportion to that of the ocean.

Many of the deposits of later times, it is true, are of fresh-water origin, and the older strata in regions where they have been much broken and disturbed have had their saline waters replaced by fresh waters from the atmosphere. Over very large areas, however, such strata are found to contain saline waters differing from those of the present ocean, and representing the ancient sea waters. Such an area is that of the great palaeozoic basin of the United States, including the valleys of the Mississippi, the Ohio, and the St. Lawrence, which by the evidence of numerous artesian wells and springs are shown to include saline waters with a predominance of salts of lime and magnesia such as should belong to the earlier sea, and by their great density, in many cases much exceeding that of the present ocean, indicate the former presence of partially dried-up seas, which is further shown by the interstratified deposits of rock salt at more than one geological horizon. These subterranean oceans are the source of the various saline mineral waters, in which however the ancient sea waters are found very much diluted or modified by the admixture of waters from superficial sources, or of the alkaline waters already mentioned. - By common usage the name of mineral waters is given to such as from the proportion or the nature of their mineral ingredients are unfitted for the ordinary uses of life.

They may be divided into several classes, of which the following are the most important: 1, those approaching sea water in composition, though more or less diluted, and holding variable proportions of salts of lime and magnesia; 2, waters holding chiefly carbonate of soda with variable proportions of carbonates of lime and magnesia. Between these two types are a large number of intermediate waters, such as would result from their intermingling, and presenting various gradations; the most marked being those which are at once alkaline and saline, like the springs of Saratoga, while the first type is represented by the bitter saline of St. Catharines, Ontario, and the second type by waters like Vichy and Carlsbad. In very many of these saline and alkaline waters are found small portions of the rarer elements, such as lithium, caesium, rubidium, strontium, and barium, with salts of iron, and occasionally arsenic, antimony, copper, lead, and many other metals in traces. The presence of phosphates, borates, and fluorides is also very frequent, and compounds of bromine and iodine are supposed to contribute very much to the value of certain mineral waters.

Besides these principal groups of saline and alkaline waters should be mentioned those which contain sulphates of aluminum and of iron, as the so-called alum springs, and others in which free sulphuric acid is the chief ingredient, as the acid springs of New York and Ontario. A sulphurous impregnation may belong to any of the classes noticed, and may be due either to the presence of free sulphide of hydrogen or to a soluble metallic sulphide.

These various sulphides result from the reducing action of organic matters on sulphates, and the most strongly sulphuretted waters are generally gypseous and but feebly saline. An excess of carbonic acid is also frequently met with in saline and in alkaline waters, which are sometimes so highly charged with it as to be acidulous to the taste and sparkling, as is the case of the Saratoga waters. It seems however to be an accidental ingredient, absorbed by the waters at considerable depths under pressure, and is wanting in the greater number of these saline springs. Waters coming from considerable depths in the earth are found to have a more or less elevated temperature, and are designated thermal waters, a name which properly belongs to all such as are warmer than the mean annual surface temperature of the locality. The temperature of the solid crust of the earth increases on an average about 1° F. for each 60 ft. in depth, as has been shown in the water from deep artesian wells, and it is hence concluded that the waters of hot springs come from very considerable depths. Those of Virginia and of Arkansas are well known, but those of the Yellowstone park are still more remarkable.

Numerous springs of this region have temperatures varying from 160° to 200° F., a point above that of the ebullition of water in this elevated region. Hence the hotter of these waters on coming to the surface disengage vapor with explosive violence, giving rise to the phenomenon of geysers. These, like many other hot springs, hold in solution large quantities of silica, which they deposit at the surface, a fact which has been observed in similar waters in Nevada and in Iceland. The geological significance of such waters is very great, inasmuch as they give us some notion of the potent agencies which are at work in the deeper portions of the earth's crust, where the solvent action of the waters, exalted by heat and by pressure, is exerted upon alkaliferous rocks, giving rise to solutions which in their turn possess solvent powers far greater than those of pure water. Various experiments by recent investigators throw light on these actions of heated water and watery solutions. Thus it has been found that sulphate of baryta, when heated and cooled in presence of solutions of alkaline bicarbonates under pressure, is dissolved and redeposited in a crystalline form.

Silica under similar conditions dissolves and crystallizes again in the form of quartz, and various metallic sulphurets have in like manner been obtained in crystalline forms, like those found in nature, and by reactions between their constituent materials, crystalline feldspar, mica, and pyroxene have been produced. While these reactions take place rapidly at temperatures considerably above the boiling point of water, and in fact approaching a red heat, other observations have shown that very similar processes, resulting in the production of many of these mineral species, may take place more slowly at temperatures much lower. Examinations of the baths at Plombieres, Luxeuil, and Bourbonne-les-Bains in France have shown in the old constructions, which date from the Romans, the occurrence of crystalline calcite, fluor spar, and various silicates belonging to the class of zeolites, which have been generated by the long continued action of alkaline waters at temperatures from 140° to 160° F. upon the bricks and mortar; while coins and medals have given rise to well crystallized metallic sulphurets of various species identical in form and in composition with those met with in mineral veins.

These observations throw great light on the phenomena of metalliferous veins, in which all the various minerals named, together with many more, are met with, arranged in such a manner as to show that they have been deposited as incrustations on the walls of fissures, which doubtless served as channels for. the passage of heated waters. These, ascending toward the surface, where a diminished pressure, exists, have yielded up in crystalline forms their dissolved materials, thus in time filling up the fissures with a veinstone often charged with metallic ores. It has been shown that the hot springs in Nevada are even at the present time depositing silicious matters mingled with metallic sulphurets. A comparison between such lodes as those just described, in which the veinstone may be carbonate of lime, sulphate of baryta, or quartz, occasionally with silicates like mica and feldspar, and granitic veins, which are essentially composed of these latter mineral species with quartz, leads to the conclusion that these veins have been formed in a similar manner, and in fact that the elements of those granites which occur in veins have in like manner been at one time in solution.

It is difficult to draw the line between these and the larger masses of granitic rocks, in the production- of which water has doubtless intervened under conditions of which we have but an imperfect conception. The crystals of quartz and of various other minerals in granites are found to contain minute cavities wholly or partially filled with water, often holding saline matters in solution; and it is supposed that in the case of eruptive granites, as in lavas, water has played an important part in giving liquidity to the rock. (See Geology, Granite, and Volcano.) - For the natural history of water, see Bischof, Lehrouch der chemischen und physiTcaluchen Geologie (2 vols., 1847-54; English translation, 1854-'9); Lersch, Hydrochemie (Berlin, 1864); and Hunt, "Chemical and Geological Essays " (Boston, 1875).