progress. He accumulates his knowledge, and each generation, as it passes away, leaves the world richer in facts, thoughts, and ideas, than it found it: thus, the habits of man become changed as successive generations of his race pass away. But this is not the case with any of the lower tribes of animals; they are probably the same to-day as they were when their first progenitors appeared upon the earth. The dog, the pig, the monkey, the elephant, are not conscious of a history, and we have every reason to believe that they are guided by the same instincts, and ruled by the same laws now as they ever have been. The slow growth of man, and the great age he attains, in comparison with his size, are both points in his economy which are He remarkably characteristic of his race. is not able to procure food for himself until he is at least more than three years old: but long after this period he requires the constant attention of his parents. This long dependence of the offspring upon the care of its parents gives rise to those social relations which last through life, and which afford so much happiness to man, but little of which exists amongst any of the brute creation. As far as can be ascertained, man lives much longer than any of the kinds of animals with which we are at present acquainted; for, although his average period of existence does not exceed between thirty and forty years, and many instances are known of animals living so long; yet there is abundant evidence to prove that man lives sometimes to the age of one hundred years, and that, were his attention more directed to the removal of the causes of death, and the abolition of the practice of war, he might extend the average of his life to sixty or seventy years. We shall not enter here into any detail of the contrast between the mental powers of man and the lower animals, but merely refer to man's capacity of reasoning from effect to cause, and his powers of proceed ing to recognise, in all that exists around him, a great First Cause-omnipotent, omniscient, and omnipresent-whom he worships as his Creator, from whom he has proceeded, and to whom he hopes in spirit to return-for the most convincing argument of man's essential distinction from the rest of the animal kingdom, and his dignified position in the world in which he is placed. FAMILIAR LECTURES ON CHEMISTRY. LECTURE VI.—WATER-continued. I Now proceed to speak of the properties of water. Of its ordinary characters it is not necessary to say much, as they are familiar to every one. When pure, it is transparent, and totally destitute of taste and smell. In comparatively small quantities, it is colourless; but, in very large masses, it exhibits a deep blue tint, as may be seen in the colour of the sea at great distances from land, frequently, also, in the water of large lakes, and in the pure ice of glaciers. Of the manner in which water is affected by heat-of its expansion and contraction by change of temperature-of the phenomena attending its passage from the solid to the liquid, from the liquid to the gaseous state, and the contrary-I do not intend to treat in detail at present. These matters are too important and interesting to be passed over slightly; and to treat them in detail would take more time than we can devote to them here. There are, however, one or two parts of the subject which it will be proper to touch upon, as we shall have to refer to them hereafter. The scale of the ordinary thermometer is constructed with reference to the tenperature at which water passes from the solid to the liquid, and from the liquid to the gaseous state. 210 180 150 120 90 The thermometer, or But to fit the instrument must have a scale; and to Fig. 15. 60 30 0 30 render different thermometers comparable See p. 43. Terrest. Phenomena.-March. one with the other, the scales must be constructed with reference to certain fixed points. Now, it is found that if a thermometer be immersed in melting ice, and a mark made on the stem at the top of the mercurial column, the mercury will always sink to the same point, whenever the instrument is again plunged into melting ice. This, then, is one fixed point on the scale. It is called the freezing point, because the temperature at which water freezes is, under ordinary circumstances, the same as that at which ice melts. Another fixed point is obtained by immersing the instrument in boiling water. This point, however, is not so completely fixed as the other, because the temperature at which a liquid boils, varies with the atmospheric pressure. It must be understood, that the point on the scale marked boiling point indicates the temperature at which water boils when the height of the barometer is thirty inches. Two fixed points on the scale having been determined, the interval between them is divided into a certain number of equal parts, and the same division continued above and below. In Fahrenheit's scale, which is that used in England, the interval is divided into 180 equal parts, or degrees, and the zero, or commencement of the scale, is placed at 32 of these degrees below the freezing point-this point being marked 32°, and the boiling point 212°. In the Centigrade scale, chiefly used on the continent, the interval between the freezing and boiling points is divided into 100 equal parts, the freezing point being marked 0°, and the boiling point 100°. Reaumur's scale, also used on the continent, differs from the Centigrade only in having a division of 80 instead of 100. In all three scales, degrees below the zero are distinguished by prefixing the negative sign; thus -15° means 15 degrees below zero. The Centigrade scale is by far the most simple of the three, and much good would result from its adoption in this country instead of Fahrenheit's division, which is very clumsy and inconvenient; but it is difficult to break through established usage. Latent heat of water.-The thermometer indicates temperature, that is to say, the tendency which a body has to make those in contact with it hotter or colder. But it gives little or no direct information as to the quantity of heat contained in a body; for heat may be present in a substance in such a state as to produce no effect whatever upon those which surround it. For example,—if a quantity of pounded ice, at a temperature considerably below the freezing point-say at 22°-be placed ove; a lamp, or a fire, which yields a regular sup ply of heat, a thermometer immersed in the ice will rise till it marks 32°. The ice will then begin to melt; but, what is very remarkable, the thermometer will remain stationary till the whole is completely liquefied, after which it will again begin to rise. Moreover, it will be found by careful observation, that the time occupied in the liquefaction of the ice is fourteen times as long as that which elapses while the thermometer immersed in the ice is rising from 22° to 32°-that is to say, through an interval of 10 degrees. Hence, the source of heat being supposed uniform, it follows that the quantity of heat required to liquefy the ice is fourteen times as great as that which raises its temperature 10 degrees, and, consequently, would be sufficient to raise the temperature of an equal bulk of water 140 degrees, or, what comes to the same thing, to produce a rise of one degree in a quantity of water 140 times as great. The heat thus absorbed in the liquefaction of the ice is quite insensible to the thermometer and to the feelings; water at 32° feels quite as cold as ice at 32°. Heat in this state is said to be LATENT; and the fact just stated with regard to water is expressed by saying, that the latent heat of water is equal to 140 degrees. A similar absorption and apparent destruction of heat takes place when water is converted into vapour. Place a vessel of cold water over a fire, and immerse a thermometer in the water. You will see the mercury rise till the water boils, when it will mark 212°; but at that point it will remain stationary as long as the boiling continues. Here again the heat is employed in changing the state of the water, and does not make it hotter; the thermometer indicates the same temperature, whether it be immersed in the boiling water or in the steam. It is found, that the quantity of heat rendered latent in the conversion of water into vapour is sufficient to produce a rise of temperature of 972° in an equal quantity of water (sup posing that the water could sustain such a change of temperature and still retain its liquid form), or a rise of temperature of one degree in 972 times that quantity of water. This is expressed by saying that the latent heat of steam is 972°. The practical method of determining the latent heat of liquids and vapours will be considered hereafter. Water considered as a standard of specific gravity. Water is adopted as the standard of specific gravity of solids and liquids: it is peculiarly adapted for this purpose, by the facility with which it may be obtained in a state of purity,-an essential quality in a standard. When, therefore, we say that the specific gravity of silver is 10, we mean that any given bulk of silver -say a cubic inch-weighs 10 times as much as an equal bulk of water. The process of taking the specific gravity of a liquid is extremely simple. A small bottle (Fig. 16) having a narrow neck, is carefully weighed; then filled with pure water up to a certain mark (a) on the neck; weighed a second time; emptied and carefully dried; filled up to the same mark with the liquid to be tried; and, lastly, weighed a third time. The difference between the first and second weighings gives the weight of the water; the difference between the first and third, the weight of the other liquid; and this, divided by the weight of the water, gives the specific gravity. Thus, suppose the liquid to be strong oil of vitriol, and that the water weighs 300 grains; you will find that an equal bulk of oil of vitriol weighs 555 grains; and 555300 1.85, the specific gravity of the oil of vitriol. Since the volume of a body varies with its temperature, it is essential that the temperature of the water and the other liquid be the same; it is usual to bring them both to 60° Fahrenheit. The method of determining the specific gravities of solids will be considered in a future part of the course. Fig. 16. A cubic inch of water at 62° Fahrenheit weighs 252-45 grains; a cubic foot weighs 1000 ounces, or 62 pounds avoirdupois; an imperial gallon contains 70,000 grains, or 10 pounds avoirdupois. Compounds of water.-Water combines with a great number of bodies, forming definite compounds called HYDRATES. 1. It combines with nearly all metallic oxides. The common process of slaking line affords an example of this kind of combination. Lime is the oxide of the metal calcium; in the pure, dry state, it constitutes quick-lime. Now, when this substance comes in contact with water, great heat is evolved; the water, if not added in too great quantity, disappears altogether; and the lime crumbles to powder. This powder, which is the hydrate of lime, or hydrate of calcium, is composed of 28 parts of lime and 9 of water. Bodies not in combination with water are said to be anhydrous. The heat evolved in the process is due, partly at least, to the passage of the water from the liquid to the solid state, whereby its latent heat is rendered sensible. Water combines in a similar manner with the anhydrous oxides of potassium, sodium, and barium. Common rust is an oxide of iron combined with water. 2. With acids.-When phosphorus is burned in oxygen gas, or in the air, a white flocculent substance is formed, which is pure anhydrous phosphoric acid; but this substance combines eagerly with water, forming a hydrate, which, when the excess of water is driven off by heat, assumes the appearance of a transparent and colourless glass. Common oil of vitriol is a compound of anhydrous sulphuric acid (a white crystalline substance) with water, in the proportion of 40 parts sulphuric acid and 9 water. This hydrate is itself very greedy of water, taking that compound from almost every hydrated substance with which it comes in contact. The product of this further combination is a second hydrate of sulphuric acid, containing 40 parts of the acid with 4 x 9 or 36 parts of water. Substances which, like oil of vitriol and anhydrous phosphoric acid, absorb water with great avidity, are often used for drying other bodies. A common mode of drying substances, which will not bear heat without decomposition, is to place them over a shallow vessel containing oil of vitriol, and cover the whole with a glass jar. The oil of vitriol dries the air in the jar, whereupon the water contained in the substance evapo rates, diffuses into the air, and is absorbed by the oil of vitriol; this action continuing till the substance is completely dried. Gases are frequently dried by passing them through bubes containing fragments of pumice stone soaked in oil of vitriol. A water refuses to take up any more. liquid which contains as much of a substance as it is capable of dissolving, is 3. With salts. If you take common said to be saturated with that substance, crystallized alum, put it into a porcelain and the limit of solubility is called the evaporating dish (Fig. 17), and heat it over point of saturation. Generally, speaking, a lamp, it will soon become liquid—after this limit rises with the temperature; thus which it will give off a large quantity of water at 60° dissolves only a quarter of its water in the state of vapour, and ultimately weight of nitre, whereas one part of boiling be reduced to a dry white porous mass, water dissolves four parts of that salt; again, commonly called burnt alum; this is the water at 70° dissolves one-eighth of its salt deprived of its water. Similar pheno- weight of alum; at 122°, half its weight; mena are exhibited by carbonate of soda, and at 167° ten times its weight; similarly sulphate of soda (Glauber's salt), and sul- with sulphate of magnesia, sulphate of copphate of magnesia (Epsom salt). Crys- per, &c. There are, however, some exceptallized sulphate of copper, which is a blue tions to this rule. Common salt is equally transparent substance (blue vitriol), like-soluble in water at all temperatures; and wise gives off a large quantity of water when heated, and is converted into a dry white powder. You will observe that all these salts, when they part with their water, also lose their crystalline form. Hence this combined water is called water of crystallization. Many salts, when exposed to a dry atmosphere, part with their water of crystallization, and crumble to powder, even at the ordinary temperature of the air. This change is called efflorescence; crystallized carbonate and sulphate of soda exhibit it in a remarkable degree. Other salts, on the contrary, quickly absorb water on exposure to the air, and dissolve in the water thus taken up. This change is called deliquescence; carbonate of potash and nitrate of soda afford striking examples of it. Aqueous solutions.-Besides the definite and generally solid compounds which water forms with other bodies, it is likewise capable of dissolving a vast number of substances. Thus, certain metallic oxides, those namely which are called alkaliespotash, soda, lime, baryta, &c.-nearly all acids, and great numbers of salts, and of animal and vegetable substances, dissolve in water with greater or less facility. Solutions are not exactly definite compounds like hydrates; the general law respecting them is, that a liquid will take up any portion of a solid up to a certain limit, but no further. Thus, if you add common salt to water, by small quantities at a time, you will find that the first portions dissolve very quickly; but as the quantity added increases, the subsequent portions dissolve more and more slowly, and at length the sulphate of soda is more soluble at 91° than at any other temperature, above or below. Since the solubility of most salts increases with the temperature, it follows that a solution saturated at a high temperature at the boiling point, for examplewill deposit a portion of the salt as it cools; this is, in fact, a very general method of obtaining salts in the crystallized state. It will be instructive to try a few experiments of this kind. You require for the purpose a number of evaporating basins of Berlin porcelain, of various sizes. Fig. 17. Fig. 17 represents a basin of this kind, supported on a retort stand, and heated over a lamp. If the premises on which you work are supplied with gas, by far the best and most economical source of heat that you can use is an Argand gas-burner, supported on a heavy foot, and connected with the gas-pipes by a flexible tube of vulcanized caoutchouc, as shown in the figure. Where gas cannot be obtained, an Argand oil-lamp is, perhaps, the best that can be used.-Another mode of crystallization is to leave the solution in a warm dry place, so that the liquid may evaporate gradually. The more slowly the solution is heated, or evaporated, the larger and better defined are the crystals obtained. Very common substances will serve to illustrate these principles; alum yields crystals having the form of the regular octohedron, a figure bounded by eight equilateral triangles; Epsom salts, nitre, and sulphate of soda, yield long, colour less, prismatic crystals; and sulphate of copper forms beautiful blue prisms. Water dissolves gases as well as solids. Some gases are but very slightly soluble in water-that liquid, at ordinary temperatures, dissolving only of its volume of oxygen, of nitrogen, and of hydrogen. Others, on the contrary, are dissolved in very large quantities; for example, of sulphurous acid, 30 times its volume, of hydrochloric acid 480, and of ammoniacal gas 670 times its volume. The solution of a gas in water is attended with rise of temperature, because the gas, in assuming the liquid form, parts with its latent heat. Hence the solution is facilitated by cooling the liquid; and, on the other hand, a gas absorbed by water may be expelled by the application of heat. The gases composing the atmosphere (oxygen, nitrogen, and carbonic acid) being soluble in water, it follows that ordinary water always contains a certain portion of air; and on applying heat, or placing the water under the receiver of an air-pump, the air may be seen to escape in bubbles. To purify water completely from air, it must be boiled for a long time, and then left to cool in vessels from which the air is completely excluded -a condition which may be ensured by filling them completely with the water, and then closing them with good corks tied round with sheet caoutchouc. Water being capable of dissolving so great a variety of substances, it is rarely found in nature in a state of purity. River-water dissolves a portion of the mineral substances with which it comes in contact, such as salts of potash, soda, lime, magnesia, iron, &c. The ocean, which is the common receptacle of all the matter carried down by rivers, and the water of which is continually carried off by evaporation, contains the same substances in larger quantity. Springs, which percolate through the various parts of the earth's crust, often contain large quantities of saline matter; those which are most highly charged with such ingredients, are commonly called mineral waters. Solid matter merely suspended in water, such as particles of fine sand or mud, may be separated by filtration; but those which are actually dissolved can only be got rid of by distillation, that is to say, by boiling the water, passing the vapour through a spiral tube surrounded with cold water, to reduce it again to the liquid form, and collecting the condensed water in a receiver. saline impurities are left behind in the boiler. Volatile matters, such as carbonic acid and ammonia, pass over with the first portions of the water, which should, therefore, be thrown away as impure. Rainwater which falls at a distance from large towns, is pure, with the exception of a small portion of the atmospheric gases. Water formed from fallen snow newly collected in the open country, is still purer. The The purity of water may be tested by its action on an alcoholic solution of soap. Such a solution mixes perfectly with pure water, and forms a clear liquid; but the slightest impurity produces a milky turbidity. Water which will not dissolve soap freely is said to be hard. Peroxide of Hydrogen.-Water is the only substance formed by the direct combination of oxygen and hydrogen; but, by a peculiar process, which will be explained hereafter, these elements may be made to unite in the proportion of 16 parts of oxygen to one of hydrogen. The compound, called peroxide of hydrogen, or oxygenated water, is a transparent and colourless liquid, which resembles water in appearance, has a sharp pungent taste, blisters the skin, and bleaches vegetable colours. It is very easily decomposed, the least elevation of temperature, or even agitation, causing it to give off half its oxygen, and pass to the state of ordinary water. |