Boiling under Diminished Pressure. 475 that it comes in contact with air or other bodies colder than itself, the cooled part becomes water. A similar phenomenon is seen when a person directs his warm breath (which has always some vapour of water invisibly mixed with it) against a window-pane or looking-glass, or any polished metallic surface, colder than the breath; a cloud or dimness immediately covers the surface, because the water of the breath is condensed upon it. Light traversing this film of vapour condensed on the glass is refracted and split into circles of prismatic colours. 683. In order to exhibit the effect of diminished pressure, water several degrees below the boiling point of low situations, but which would be boiling at the top of Mont Blanc, is caused to boil instantly by placing it under the receiver of an air-pump, and making a few strokes of the piston. Water may be thus made to boil at 70°. If the exhaustion be rendered nearly complete, the water will rise, even when colder than the blood of animals; and at degrees of temperature still much lower, it will at the surface be assuming the form of air, although not with a force sufficient to produce the visible agitation of boiling. The following experiment will show the boiling of water under diminished pressure in the absence of an air-pump. Boil water in a clean Florence oil-flask half full, until all the air has been expelled from the flask. Remove the flask and immediately close the mouth of it with a very accurately fitting cork. This should be so well fitted that no water can escape on inverting the flask. Place it inverted on a ringstand, and now pour over it carefully some cold water. The steam in the body of the flask is immediately condensed-the pressure on the water is thereby diminished, and the water begins again to boil -the boiling continuing for some time.* Other liquids, as alcohol, * The following is a still more remarkable experiment, as it illustrates the force of atmospheric pressure when aqueous vapour enclosed thus undergoes sudden condensation. Place a thin stratum of water in a large cylinder made of the thinnest tin plate well-soldered, and having only a screw stop-cock at the top for pouring in the water and shutting off the stcam. Open the stop-cock and boil the water by applying heat to the bottom of the cylinder. After boiling for some time, all the air is expelled by the escaping steam. When this has occured, withdraw the tin vessel from the fire, closing the stop-cock at the same time. In a few minutes, as a result of spontaneous cooling or by pouring over the vessel cold water, the steam is condensed in the interior, and the vessel is crumpled up by atmospheric pressure as if it were a scroll of paper. 476 Superheated Steam. ether, &c., from requiring inferior degrees of heat to separate their particles to aëriform distances, boil under the receiver of an airpump at very low temperatures; ether, for instance, when as cold as freezing water. As a rule liquids boil in vacuo from 60° to 140o below their ordinary boiling points. On the other hand, in order to exhibit the effect of increased pressure, if we confine the particles of a liquid still more than by a common atmospheric or equivalent pressure, degrees of heat higher than the common boiling point will be required to separate them. In a diving bell, or in a deep mine, like that of Monkwearmouth, near Sunderland, the boiling point of water is higher than 212° in proportion to the depth reached : and if, at the surface of the earth, we heat water in a close vessel into which air is forced, so as to press thirty pounds on the inch instead of fifteen, as the atmosphere does, or from which we prevent the steam's escaping until it has acquired the force of a double atmosphere, we shall, before making the liquid boil, have to raise the heat to 250°. The temperature of steam of twenty atmospheres is 418°, and is equivalent to a pressure of 300 pounds on the inch. Under a still stronger pressure, water may be rendered almost red-hot, but the force with which its particles are then tending to separate, is almost that of inflamed gunpowder. It is from the want of a proper estimate of this enormous rending force that fatal accidents so frequently occur by overweighting the valves of steam boilers. Even then, however, if a gradual issue were allowed, only a certain quantity of the water would absorb and render latent the existing excess of heat above 212° and would become common steam, leaving behind a considerable portion as boiling water of the ordinary temperature. The fact that liquids are driven off, or made to boil at lower degrees of heat when the atmospheric pressure is lessened or removed, has recently been applied to some very useful purposes. 684. The process for refining sugar is to dissolve the raw sugar in water, and after clarifying the solution by straining or otherwise, to boil off or evaporate the water again, that the dry crystallized mass may remain. Formerly this evaporation was performed under ordinary atmospheric pressure, and a heat of at least 220° was required to make the syrup boil; by which high temperature, however, a considerable portion of the sugar was discoloured and rendered uncrystallizable. I the beginning of this century, the valuable Boiling at Low Temperatures. 477 thought occurred to Mr. Howard, that the water of the solution might be drawn off or evaporated at a very low temperature by boiling the syrup in a vacuum, that is to say, in close pans, which would exclude the atmospheric pressure. This was accordingly done, and the value to the inventor of the patent right was said to have exceeded thirty thousand pounds a year. The syrup during the process is not heated to beyond 150°, and it yields either colourless crystals or loaf-sugar. The apparatus for evaporating and distilling in vacuo consists of vessels strong enough to bear, when quite empty, the external atmospheric pressure. They are generally of copper and are made of an arched form. The vacuum is produced and maintained by air-pumps worked by steam-engine or other power. By an ingenious arrangement, the state of the boiling syrup can be seen through a glass let into the upper part of the vacuum pan. Hundreds of gallons may be thus seen, under the agitation produced by the airpump and the heat, tossing about like the waves of a turbulent sea. In the preparation of many medicinal substances the process of boiling in vacuo is equally important. Many extracts from vegetables have their virtues impaired, and some medicinal principles (alkaloids) are even destroyed, by a heat of 212°; but when the water used in making the extract is driven off in vacuo, the activity of the fresh plant remains in the product. 685. In the same manner, in the process of distillation of the essential oils of vegetables, which is merely the receiving and condensing again in appropriate vessels an aëriform matter raised by heat from the vegetable mass, those which are changed and injured by an elevated temperature, may be obtained of perfect quality by carrying on the operation in a vacuum. The essential oils of lavender, peppermint, and others are said to have their natural odour and flavour better preserved, since this pian has been adopted. 686. If the valuable apparatus invented by Howard, could be used generally in the countries where sugar is produced from the juice of the sugar-cane, more sugar than now, and of superior quality, would be obtained from a given quantity of juice. The complete apparatus, however, is costly at first, is of complex construction, and requires delicate management where skilled labour is difficult to obtain, and if damage occur, engineers capable of repairing it are far away. Complete interruption of the work from any cause would bring heavy loss to the proprietor who had trusted to the superior apparatus. Under these circumstances, it has appeared 478 Vacuum Process for Heating. to the writer that the simple plan now to be described would in many places render service. It is merely to establish a communication betwcen a close boiler, as a, and the vacuum at the top of a V....... Fig. 171. a water-barometer, as b (fig. 171). To produce that vacuum, the strong vessel, b, forming the top of the barometer, and thirty-six feet of tube below, reaching to d, are first filled with water through a cock, c, at the top; this cock being then shut, and another cock, d, at the bottom, which had been shut, being opened, the water sinks down out of the vessel, b, until the column in the tube is only thirty-four feet high, as at ƒ, that being the height which the atmosphere will support. On then opening a communication between the boiler, a, and the vacuum in b, the operation will go on as desired, and the steam rising from a may be condensed in by a little stream of cold water allowed constantly to run through and be scattered from above. This water, it is evident, will always pass downwards, becoming part of the barometer column below, without filling up or impairing the vacuum. If air should find admittance in any way, the original degree of vacuum can always be easily reproduced as at first; and to prevent interruption from this cause, it might be convenient to have two vessels like ỏ, of which one could always be in action while the other was being emptied of air: On many sugar estates there is a fall of water, fit to supply the barometer without the trouble of pumping; but even the expense of pumping by hand or horse-power would not be deemed waste for the end here sought. The tube, d c, needs not to be perpendicular, provided it be longer in proportion to its obliquity; and it may be very small: some yards of common lead-pipe would answer. It will be observed that the principle here suggested for producing a vacuum is similar to that described at p. 303 (Art. 470, Sprengel's vacuum), except that mercury is there employed instead of water. The very great difficulty of erecting and preserving a water barometer is an obstacle to its employment for any industrial purpose. 687. When it was understood that, at common temperatures, water and many other liquids would be existing in the form of air or Pressure of Steam at Different Temperatures. 479 gas, but for an atmospheric pressure opposing the separation of their particles, it became of great importance in many of the arts, and for comprehending certain phenomena of nature, to ascertain exactly, with respect to some of these liquids, particularly water, the degrees of expansive force belonging to them at different degrees of temperature. The subject, as far as water is concerned, has been investigated with great care, and the following table shows part of the results. The left-hand column marks temperatures rising from 32° of Fahrenheit's thermometer, or the freezing point of water, to 418. and the right-hand column marks the corresponding degrees of force with which the water tends to expand into the state of steam, and therefore also, the force and density in any vessel of the steam confined above the water which it contains. One ounce and a half per square inch is the expansive force exerted on the sides of any containing vessel by the steam rising from freezing water, that is to say, the force with which freezing water seeks to dilate into steam or air; and sixty pounds per inch is the force of the water heated to 290°. To many readers the idea will be quite new and surprising, that if some freezing water, or even ice, be inclosed in a bladder or bag of caoutchouc containing nothing else, and the bladder or bag be lodged in the exhausted receiver of an air-pump or other vacuum, the bladder will quickly be distended with steam strong enough to support a weight of one ounce and a half on a square inch of its surface. 688. From this table we perceive how much more rapidly the tendency to dilate into steam increases, than the temperature of the water. A rise of eighteen degrees, viz., from 32° to 50°, at the beginning of the scale, only increases the dilating force one ounce and a quarter on the inch, while an equal rise of 18° at the end of |