indicates only 212° F. (100° C.). All saline solutions boil and freeze with more difficulty than water. All bodies soluble in water behave in a similar manner-that is, they are soluble in it only in definite quantities, and in most cases hot water dissolves more of them than cold. Experiment 6. If the solution obtained in the last experiment be poured into a porcelain dish, previously heated, and be suffered to remain quiet until cold, then the three ounces of saltpetre which were last added separate again, not as a powder, but as regularly formed prisms. These prisms are six-sided, and are surmounted by two faces similar to a roof; they are called crystals of saltpetre. (Fig. 27.) All crystals are characterised by having planes, edges and angles, constructed, as it were, of simple triangular, quadrangular, or poly-angular pieces, with polished surfaces. This symmetry is found even in the interior of them, as can easily be Fig. 27. seen by holding a transparent crystal towards the light, and turning it slowly round; or breaking it, when the fragments will often exhibit the same regular form which characterised the whole crystal. Thus, in inanimate nature, a mysterious power exists, similar to that which compels the bees to construct six-cornered cells, and the potato to produce its five-angled corolla and five stamens, and by which the smallest particles of bodies, called molecules, are forced to arrange themselves in a fixed order, assuming a regular shape. But this can in general only be accomplished by a body in its fluid or aeriform state, since free motion of the molecules is essential. Time also is required for this operation; hence crystals are always more regular the more slowly they are formed. Many of the splendid crystals which are dug from the depths of the earth were, perhaps, thousands of years in forming. Experiment 7.-Evaporate the liquid remaining above the crystals in the former experiment at a gentle heat, until a scum is formed on the surface, then remove it from the fire and let the solution cool, stirring it constantly with a wooden stick. In this way, instead of crystals, a powder of saltpetre will be obtained consisting of minute crystals. The liquid just alluded to may be regarded as a cold saturated solution, containing about a quarter of an ounce of saltpetre. If, by evaporation only, so much water is left as is sufficient when hot to keep in solution but a quarter of an ounce of the saltpetre, then crystals begin to appear in the form of a film on the cooler surface, indicating the saturation of the liquid. If this again is allowed to cool quietly, a second crop of crystals will be obtained; but, by continual stirring, they are broken at the moment of their formationby slow movement into a coarse, and by rapid movement into a fine powder. This may be called interrupted crystallization. Sugar presents a similar example; the same syrup, which, if cooled quietly, yields sugar-candy, when stirred, yields common loaf-sugar. Experiment 8.-Put into boiling water as much common salt as will dissolve, and let the solution cool; no crystals will form, because salt is as soluble in cold as in hot water. Now evaporate one-half of the solution over a spirit-lamp, and set aside the other half in a warm place; in the first case, mere irregular grains of salt will be obtained, but in the latter case, after some days, regular cubes of salt will be deposited. Experiment 9.-Dissolve a spoonful of salt and one of saltpetre in lukewarm water, and put the solution in a warm place, that the water may gradually evaporate; the two salts, which are intimately mixed in the solution, will upon crystallization separate completely from each other. The saltpetre separates into long prisms, containing no trace of the common salt, and the latter separates into cubes, entirely free from saltpetre. Thus the particles of salt and saltpetre did not attract each other; but, upon crystallizing out of the solution, the homogeneous salts assumed separately a regular form, precisely as if one only of these two substances had been dissolved. PRESSURE. The earth is surrounded by air, as by a mantle; this mantle is called the atmosphere, and is supposed to extend about forty-five miles above the solid earth. The air possesses no colour, and is transparent; hence it is invisible, and its particles are so easily movable upon each other that it cannot be grasped by the hand. But it is rendered obvious that the air is material, and fills every space commonly called empty, by wrapping strips of moistened paper round a funnel, so that it may fit exactly into the mouth of a bottle; if the funnel be now filled with water, the fluid will not Fig. 28. run into the bottle, as the air contained in the latter will not let it enter; but if the funnel be raised a little, the air escapes, and the water immediately rushes into the bottle. We learn also by the balance that a vessel apparently empty, i.e., containing atmospheric air, weighs more than one which is really empty, as when the air has been exhausted from it. But air is so light that 800 measures of it weigh rather less than one measure of water, yet the atmosphere presses with great weight on the earth, and upon everything thereon. But this pressure is only noticed when the air is removed from a place, thus leaving it without counter-pressure. The total weight of the atmosphere has been estimated to be equal to that of a globe of solid lead sixty miles in diameter. When a solid is immersed in a fluid (either a liquid or gas), it is pressed upon equally on all sides by the fluid with a force which depends partly upon the weight of the fluid and partly upon the depth to which the solid is immersed. A stone suspended one foot from the surface of water is only pressed upon with half the force that it would experience if sunk two feet; a stone sunk one foot in mercury would receive 13 times as much pressure as it would if sunk one foot in water, because mercury is 13 times as heavy as water. The same thing is true in regard to air and other gases, and bodies near the surface of the earth are pressed upon equally on all sides by the whole forty-five miles of air above. Pressure of the Atmosphere.-Experiment 1.-Wrap some tow round one end of a stick and grease it with tallow, thus forming a plug, which must fit tightly into a strong test-tube. Boil some water in the test-tube, and when the air has been expelled by the Fig. 29. steam, insert the plug; as the water cools the steam condenses, a vacuum is produced, and the plug will be pressed down upon the surface of the water; by heating, it is again forced up by the steam thus generated, and by immersing in cold water it is again forced down. In consequence of the cooling and condensation of the steam a vacuum is formed, and, therefore, the counter-pressure against the weight of the exterior air is removed; the pressure of the latter, accordingly, forces down the plug. On this principle the piston is forced up and down in the cylinder of many steamengines. This one-sided pressure often causes the ascent and reflux of liquids in tubes. Experiment 2.-If water is boiled as was directed at Fig. 30. LA page 39, by means of steam, and during the boiling the lamp is removed, then the pressure of the air acting on the surface of the water in the beaker-glass will very soon force the water contained in it through the tube back into the flask, which in a short time becomes quite filled with water. The counter-pressure of the steam must naturally decrease as it cools and condenses. As long as the lamp is under the flask, the pressure Fig. 31. of the steam is stronger than that of the air, and the steam, being continually generated, forces the air previously contained in the flask into the water of the beaker-glass. This reflux of liquids is particularly to be feared, when such kinds of gases are conducted into water as are absorbed by it readily, and in large quantities. This is prevented by passing through the cork a second glass tube open at both ends, and letting it reach nearly to the bottom of the flask, by which tube air can penetrate into the flask as the pressure of the steam diminishes. This contrivance is called a safety-tube. Fig. 32. Barometer. It can be proved by exact experiment that the atmosphere presses upon the earth with a force equal to that of a layer of mercury 30 inches deep, or a layer of water 13 times deeper (34 feet), water being 13 times lighter than mercury. The instrument by which the amount of atmospheric pressure can be observed and measured is called the barometer. Fill a glass Surface of the earth. Fig. 33. S tube, 32 inches in length, one end of which is closed, with mercury; close it with the finger, and invert it into a vessel of mercury; on removing the finger, the mercury will not run out, but will fall some inches, perhaps to s (Fig. 33). The height of the column of mercury, from a b to s, amounts to about 30 inches. The mercury does not fall lower, on account of the external pressure of the atmosphere, which is exerted on the mercury at a b, and not at 8, since this end is closed. The column of mercury in the tube may be regarded as the counterpoise to the atmospheric pressure, and it is hence concluded that the latter exerts just as much pressure upon the earth as a column of mercury 30 inches high. If the tube be opened at the top, the pressure of the air on both extremities being then made equal, the mercury will flow from the tube. The space above the mercury, at 8, is a vacuum, and is called the Torricellian vacuum, because it was first " observed by Torricelli. In common barometers the tube is curved at the bottom, and provided with a bulb. This bulb is open at the top, and supplies the place of the vessel filled with mercury in the preceding figure. Here also the pressure is only exerted at one end, for the atmosphere can only press on the mercury contained in the bulb. The height from o (Fig. 34) to the top of the mercury amounts to about 30 inches. If weights be placed in one pan of a balance, the opposite one will rise, but on their removal it will sink. The same |