490 Work of the Modern Engine. be remarked that, in the present more perfect state of the engine, it appears a thing almost endowed with intelligence. It regulates with perfect accuracy and uniformity the number of its strokes in a given time, counting or recording them moreover, to tell how much work it has done, as a clock records the beats of its pendulum ;—it regulates the quantity of steam admitted to work ;—the briskness of the fire;—the supply of water to the boiler; the supply of coals to the fire;-it opens and shuts its valves with absolute precision as to time and manner;-it oils its joints; it takes out any air which may accidentally enter into parts which should be vacuous ;—and when anything goes wrong which it cannot of itself rectify, it warns its attendants by ringing a bell :—yet with all these talents and qualities, and even when exerting the force of hundreds of horses, it is obedient to the touch of a hand ;—its aliment is coal, wood, charcoal, or other combustible ;—it consumes none while idle ;—it never tires, and wants no sleep ;—it is not subject to malady when originally well made; and only refuses to work when worn out with age; it is equally active in all climates, and will do work of any kind;—it is a water-pumper, a miner, a sailor, a cotton-spinner, a weaver, a blacksmith, a miller, &c., &c.; and a small engine in the character of a steam-horse may be seen dragging after it on a railroad a hundred tons of merchandize, or a regiment of soldiers, with four times the speed of the fleetest coaches of old. It is the king of machines, and a permanent realization of the Genii of Eastern fable, performing supernatural feats at the command of man. We need not wonder that the inventor of an engine having such qualities should be deemed deserving of the highest honours which his fellow-men could bestow. In November, 1825, a public meeting was held, to vote a monument to WATT, then recently deceased; and the most distinguished men of the empire, of all parties, philosophers, and statesmen, met to vie with one another in speaking his praise. Eloquent indeed were the discourses pronounced; but perhaps in the progress of civilization, there can rarely be offered such motive and occasion. The common voice of that assembly scarcely exaggerated, when attributing to Watt's genius and perseverance that increase of our national commerce, and riches, and power which had enabled free Britain, almost single-handed, at an extraordinary crisis of human affairs, to contend with Europe combined against her, and at last to triumph, securing thereby her own well-being, and probably advancing that of the human race. Papin's Digester.—The Cryophorus. 491 699. Papin's Digester.-The vessel so called is a very strong iron pot or boiler, which can be kept closed by a regulating valve against the force of the steam formed within it; and in such a vessel, water can be heated considerably beyond the ordinary boiling point,—sufficiently, for instance, to dissolve and extract all the gelatin or the animal matter of bones immersed, and so to form from them a rich soup where ordinary boiling would get nothing. The cook who increases the fire under a boiling pot not closed, with the hope of making the water hotter, is foolishly wasting the fuel, for the water can only boil, and it does boil at 212° of the thermometer, the surplus heat being carried away in the steam. As different substances, under any given pressure, become aëriform at different temperatures, mixtures of such may be decomposed by heat. If a mixture of alcohol and water, for instance, be placed over a fire, the alcohol will boil off long before the water; and if the vapour be directed into a cold tube or vessel, it takes the liquid condition, and the operation is called Distillation. In a mixture of ether, alcohol, and water, by using a cooling apparatus for the vapour and taking care that the temperature does not exceed 96°, the greater part of the distillate, or liquid distilled, will be ether; if under 174°, alcohol; and at 212°, water. Fig. 175. 700. The Cryophorus.-The little instrument called the Cryephorus (or carrier of cold) is represented in fig. 175. It serves to illustrate some of the principles above set forth. It consists of a bent glass tube blown into bulbs at the ends, a and b. Some water is introduced so as partly to fill one bulb. This is boiled, and the steam expels the air. It is then hermetically sealed. There will always be in the apparently empty part a quantity of watery vapour of a density depending on the temperature. If one of the bulbs be heated more than the other, the steam over water in that one will, for the reason stated above, be denser and stronger than in the other, and will, therefore, be forcing its way into the other; where, owing to the lower temperature, a part of it will be constantly relapsing into the state of water, and making room for more. Hence, if the difference of temperature between the bulbs be long maintained the whole water will, by a sort of distillation, gradually 492 Cold produced by Evaporation. pass into the colder bulb. Two other remarkable facts which this instrument can exhibit are-1st, that if held with the tube nearly level and undermost, the warmth of a hand grasping one bulb will form steam, causing all the water to pass rapidly to the other, and to boil there; and 2nd, that by dipping one of the bulbs, nearly empty, into a freezing mixture, the continued condensation of the vapour within will so cool the water in the other bulb, as to cause it to freeze or become ice. 701. The cold produced by evaporation may be demonstrated by other experiments. Water dropped on the hand produces a sense of coldness while evaporating. Alcohol, ether, and chloroform, by their greater volatility, produce a more marked sensation; while the sulphide of carbon, by its rapid evaporation, produces a painful degree of cold. If a small quantity of this liquid is placed in a saucer, and some slips of filtering paper are introduced, the sulphide is rapidly absorbed and conveyed by capillary attraction to the fibres at the extreme end of the paper. The cold resulting from the evaporation is here so great that the vapour, with the moisture of the surrounding air, is deposited in white snowy-looking crystals on the edges of the paper. 702. When a liquid has reached the temperature at which it boils, that is to say, at which its power of emitting vapour becomes rather more than a balance to the atmospheric pressure, its dilating force is very strong. A weight of 15 lbs. on the square inch, which is the ordinary atmospheric pressure, is equal to 2160 lbs., or nearly a ton, on a square foot; and such is the power with which the vapour of all boiling substances open to the air, rises from them, that, in a large steam-engine this is sufficient to urge the piston with a force of 600 horses! But at temperatures far below boiling, the tendency to expand, as already stated, is still very great, and although not attracting common attention, is silently working most important ends in the economy of nature. As freezing water, or even solid ice, emits in a perfect vacuum, a steam or vapour which can lift an opposing weight with a force of 1 ounce per inch, or 16 lbs. on a square foot, so also do many other liquids and solids. Thus, in the apparently empty space called the Torricellian vacuum, existing over the mercury in a barometer tube, there is always the vapour of mercury, more or less dense in proportior to the temperature; and around camphor, the essential or volatile oils, and other volatile liquids, there is always an atmosphere of the substance, in the form of gas Vapours emitted in a Vacuum. 493 or vapour, whether in otherwise empty space, or mixed with common air. This is demonstrable in camphor by throwing freshly broken fragments of that substance on perfectly clean water. It is light and floats, and the vapour constantly given off from the surface causes it to move about with great rapidity, and to perform a number of remarkable gyrations. If some portions are ignited, they will move about like miniature fire-ships. 703. It had long been known that solids and liquids placed in a vacuum emitted quickly, in the form of vapour, a quantity of their substance proportioned to their temperature; but Dalton made the important discovery that in a space containing air, these vapours arise in quantity the same as if air were not present-the two fluids seeming to exist there independently of each other, with the exception, however, that in a vacuum, the equable diffusion of a vapour takes place at once, while in a space already occupied by air, it proceeds more slowly, as the vapour has to force its way through the particles of the air, and it then takes place by a tranquil evaporation from the surface of the liquid, instead of by the agitation of ebullition. In an apartment with an open vessel of water in it there is soon mingled with the air, although invisible, a watery vapour, as dense as if the room had been a vacuum at the same temperature. The following are consequences of this important truth :— That it is only the atmosphere of any volatile substance, which by pressing on the mass of it can prevent its further dissipation by heat. Thus camphor, musk, essential oil, spirits, water, &c., can be preserved only by placing them in closed bottles or vessels, in which, in addition to the air present, an atmosphere of their own vapour is formed, by which further evaporation is checked. The important process of drying any moist object is merely the placing it under an elevated temperature if attainable, and in an atmosphere not containing so much moisture as to be saturated at the temperature. The effect of wind or motion of the air in quickening evaporation is owing to its removing air charged with the moisture, and substituting fresh air which is not so charged. Damp clothes cannot be dried in a close room; for so soon as the air is saturated with moisture, no more vapour is emitted. If air at a certain temperature contain mixed with it as much water as can be sustained in the form of invisible vapour at that temperature, and if then, by any cause, as rising in the atmosphere, the air be cooled, it will abstract heat from the vapour, and cause a 494 Condensation of Vapours. portion to be precipitated or visibly condensed into a fog or rain. Water rising as invisible vapour from the surfaces of lakes or rivers, often, when it has reached a certain height and is thereby cooled, condenses into the stratum of mist or clouds which there appears, and which for a time may be usefully protecting the fields from the intense meridian sun, or may fall again as refreshing showers over the country. It is the tranquil and invisible evaporation of which we are now speaking, which lifts from the surface of the globe all the water which, after condensation, returns to the ocean in the form of the myriads of river streams that give life and beauty to the face of nature. 704. There are other instances of vapour which is invisible while at a certain moderate temperature, but is copiously precipitated when the air with which it is mixed is cooled, or when it touches a colder solid body; among these may be mentioned the steam observed at night and morning hovering over brooks and marshes heated by the sun during the day :— the frost-smoke, as it is called, which lies on the whole face of the Greenland seas in the beginning of winter, where the water, warmed by the long day of the polar summer, continues to emit its vapour for a considerable time after the summer is past, into an atmosphere become too cold to preserve it in an invisible form :-the breath or perspiration of animals, of horses in particular after strong exertion, becoming so strikingly visible in cold and damp weather, or even in warm weather, when the air is already charged with much moisture :-in cities where there are deep drains communicating with kitchens, manufactories, &c., and constantly filled with moist and warm air; the vapour-loaded air, although clear or transparent in the drain, immediately on escaping into a frosty atmosphere, lets go its moisture, with the appearance of steam issuing from a great subterranean cauldron. Steam over water in any boiler is transparent or perfectly aëriform-as may be seen when water is made to boil in a glass flask with a long neck; but as soon as it is cooled, by contact or admixture of air colder than it, it ceases to be true steam, and is condensed into small particles of water, visible spherules suspended in the air. Many persons, while thinking of steam, figure it only in this last-mentioned visible condition, of particles of water mixed with air nearly as a subtle powder |