thing happens with the barometer. Any increase in the weight or density of the air presses the mercury up, and Fig. 34. the barometer rises; but any diminution of weight will make it fall. The height of the mercury may 29 1-28 27 26 25 be read off by fixing to the upper part of the tube a scale divided into inches and tenths of an inch. The mean state of the barometer is at 30 inches, and 31 is called a very high, and 29 a very low, 24 state of the barometer. In this country, as a general rule, the north and east winds cause the barometer to rise, and the south and west winds cause it to fall. The former winds, blowing chiefly from the land, are cooler, and at the same time drier, than the latter, which pass over the ocean, there becoming saturated with moisture; the former likewise come from colder into warmer, while the latter, on the contrary, proceed from warmer into colder regions; by which the capacity of saturation for vapour is increased in one case and diminished in the other. Hence it is natural that, when north and east winds prevail, it should rain less frequently than during south and west winds; and that the former winds are dry, while the latter are damp. Why water does not flow from a jar inverted over the pneumatic trough, why it continues to flow through a syphon after the air has been exhausted, why liquids will not run into a vessel when the air is confined, or why water will only rise to the height of 34 feet in a suction pump, are questions that scarcely require further explanation. Elasticity of Gases.-All gases are perfectly elastic; they all contract equally under an increase of pressure, and expand when it is diminished. When a gas is collected in a bottle or other vessel the weight of it that the bottle will hold depends upon the pressure of the air at the time, which, as we have seen, is measured by the barometer. It is therefore of the utmost importance in measuring a gas to take account of the state of the barometer at the time. One hundred grains of air, for example, will occupy 333 cubic inches when the barometer is at 29, 322 when it is at 30, and only 311 when it is at 31 inches. As the alterations in volume are exactly in inverse proportion to the alterations of pressure, it is easy, after noting the volume of the gas and observing the height of the mercury in the barometer, to calculate what the volume would be at some standard pressure. The standard pressure at which gases are always measured is represented by 30 inches (or 760 millimetres) of mercury. The calculation required for this purpose is called the "correction for pressure." It must be applied, in addition to the correction for temperature (page 34), whenever a gas is to be measured. It can always be made by the following formula : Vol. at observed pressure x Observed pressure Volume of gas at Standard pressure standard pressure. = For example, we have 333 cubic inches of air when the barometer stands at 29 inches. How many cubic inches would there be if the barometer stood at 30 inches? Here we must multiply by the observed pressure 29, and divide by the standard pressure 30: 333 × 29 = 322 cubic inches (very nearly). The ordinary air pump depends for its action in producing a vacuum upon the elasticity and pressure of the air. If the pressure or tension of a confined quantity of air be increased, by compressing it either directly or by the Fig. 35. addition of more air, it can be forced to stream out from a small opening with great rapidity, as is shown on a small scale in the common bellows, and on a larger scale in the blacksmith's bellows. Should there be water before this opening, the air will press it out in a jet or stream. Experiment 3.-Take a piece of a fine glass tube, drawn out to a point, and adapt it, by means of a perforated cork, to a bottle. Fill the bottle half full of water, and blow into it through the point of the tube; when the blowing ceases, the air will escape in a stream. But if the bottle be inverted as soon as the air is blown in, then the water will be spurted out by the compressed air above. Such an apparatus (the washing-bottle) is frequently employed for washing residues or precipitates remaining on filters, in order to free them from soluble matter. There is a similar contrivance connected with the common fire-engine, called the air-vessel, enabling it to throw an uninterrupted stream of water. The pressure of the atmosphere exerts a great influence on the boiling of water, and of other liquids. If water is made to boil when the mercury in the barometer is very low (in foul weather), brisk ebullition will take place at about 210° F. (99° C.); when the mercury is very high (in clear weather), boiling will not occur under 214° F. (101° C.). Experiment 4.-Heat a flask half filled with water till the water boils briskly; then remove it from the fire and quickly Fig. 36. cork it; the boiling immediately ceases, but will commence again if cold water be poured over the upper part of the flask. In this manner it can be made to bubble or boil, even though it be only lukewarm. There is no air in the flask, it having been expelled by the steam, and it could not re-enter it, on the cooling and condensation of the steam, on account of its having been closed, Consequently there is no pressure of air on the water, and it will boil even at a temperature of 68° F. (20° C.). The boiling ceased on account of the pressure of the steam upon the water; but the steam being condensed by the cold water, the pressure was so much diminished, that a portion of water again became aeriform with a boiling motion. In many manufactories, an appropriate apparatus (vacuum-pan) has been contrived for boiling and evaporating in a vacuum, as, for instance, in sugar-houses. The air is densest at the level of the sea, and thinner in proportion to its distance from the earth, as there is less air above it. Hence the mercury will stand lower, and water boil more easily, on the top of a mountain than in the valley below. On the top of Mont Blanc mercury rises only to the height of 16 inches in the barometer, and water boils at 183° F. (84° C.). Hence the barometer and the boiling point of water may be employed for calculating the heights of mountains. As water boils more easily under diminished pressure, so it boils with more difficulty when the pressure is increased. An increase of pressure can be produced, not only by the air, but by the steam of the water itself, if new steam be constantly generated, while the escape of that already formed is prevented. This is best done by heating water confined in a strong and firmly closed vessel. For this purpose a Papin's Digester may be used, in which water may be heated to the temperature of 392° F. (200° C.), and, indeed, still higher, whilst in open vessels it cannot be heated above 212° F. (100° C.). If the amount of steam in it is twice as much as in an uncovered vessel, the pressure is said to amount to two atmospheres; if there is 3, 4, 5, 10, 20 times the quantity, there is said to be a pressure of 3, 4, 5, 10, 20 atmospheres. Vessels of this kind are often employed to effect a complete penetration of the water into solid and hard substances. Thus, for example, water at 212° F. (100° C.) dissolves the gelatinous matter only on the surface of the bones, whilst water at a temperature ranging from 230° F. (110° C.) to 248° F. (120° C.) entirely penetrates the bones, and extracts the gelatine also from the interior of them. CHAPTER III. LAWS OF CHEMICAL FORCE. WE have already, in Chapter I., seen something of the operations of that form of force which is known as chemical force. We have seen that by its operation compounds are produced by the union of elements, the compounds being in all cases different from the elements which compose them. And we have seen that the science of chemistry is mainly concerned with the changes which are brought about by chemical force. It is now necessary, before entering on the systematic study of chemical facts, to consider the theories of chemistry-the laws according to which chemical actions take place. These laws have been discovered by thousands of difficult and laborious investigations. For the most part it will not be possible for the student to prove them to himself experimentally the processes necessary for this are too difficult, and require apparatus of too expensive a kind. He must be content in the outset of his study to take the laws upon trust. MODES IN WHICH CHEMICAL ACTION TAKES PLACE. Combination.-Experiment 1.-The simplest way in which chemical action can take place is when two substances, elements or compounds, combine directly together and produce a single new compound. Examples of this have already been given at page 13, but another may be added here. Take a globule of mercury about the size of a very small pea, heat it in a test-tube until it boils, and then throw in about an equal weight of iodine. Shake the tube slightly, so as to bring the two into contact. They will combine, and a beautiful scarlet compound called mercuric iodide will be found on the sides of the glass. |