The Effect of Rapid Cooling. 495 might be mixed. Until steam is cooled and condensed, it is of a nature to fill alone any appropriate vessel and powerfully to distend it, just as air fills and distends a bladder. Steam issuing from the spout of a tea-kettle is hardly seen near the mouth, but as its distance from the spout increases, it is cooled into a jet of thick cloud or vapour. 705. In a vessel from which air, and therefore atmospheric pressure, is excluded, even the temperature of freezing water is sufficient to maintain permanently in the state of invisible gas, many substances which exist as liquids under this pressure. A large mass of any such liquid when placed in a vacuum, is not instantly converted into gas, because the portion which first rises becomes an atmosphere compressing what remains, and because, moreover, that portion, by absorbing from the mass much heat in the latent state, cools for a time the remaining mass. We see, therefore, why liquids are so rapidly cooled when placed wherever a vacuum can be made and maintained; that is to say, where, after common air has been removed, the aëriform matter rising from the liquid and absorbing its sensible heat, is also promptly and continuously abstracted. It is thus that water placed in the exhausted receiver of an air-pump is so rapidly cooled, and that when there is placed beside the water, a vessel of concentrated sulphuric acid, or other substance capable of absorbing the watery vapour as it is formed, the water is soon reduced to the state of ice. The following experiment will furnish on a small scale an additional illustration. Blacken the interior of a watch-glass by holding it over the flame of burning camphor. Drop into this when cold one drop or more of water. Owing to the deposit of carbon, the water coheres in a globule, like the globules of dew on the hairy or downy leaves of plants. Place this under the receiver of a good air-pump and exhaust it rapidly. By the evaporation from the entire surface of the globule, conjoined with the cooling effect of radiation, the water is soon transformed into a solid pellet of ice. This represents the artificial production of a hailstone. Water placed in a thin glass vessel, surrounded by ether or sulphide of carbon, evaporating in a vacuum, is also quickly frozen. It has already been explained, that in a liquid there is the same tendency to evaporate, whether it be or be not exposed to the air, so we see the reason why all evaporation is a cooling process. The effect, however, in air is neither so rapid nor so great as in a 496 Climatic Influences. vacuum, first, because the presence of the air impedes the spreading of the newly-formed vapour from the liquid surface, and keeps it where its pressure resists the formation of more vapour; and, secondly, because the air in contact with the liquid gives a part of its heat to the liquid. 706. The conversion of sensible into latent heat by the evaporation which goes on from the sea and earth in all warm climates greatly tempers the heat of these climates, and the vapour afterwards spreading to the poles, as explained under the head of Winds (Art. 449), carries warmth thither to be given out as the vapour is condensed into the form of rain, or is solidified as snow. The formation anywhere of mist or rain warms the supporting air by the liberation of the latent heat from the precipitated vapour. Again, the water which, during winter, is converted into snow or ice, may be regarded as a reservoir of latent heat, which is given out and tempers the frosty air of the cold season. In the following spring such ice and snow may thus serve as receptacles in which the first violence of the returning sun may hide or expend itself. The vast masses of ice and snow among high mountains, as among the Alps and Pyrenees, serve often during the summer as stores of mild temperature to regions around: for, besides cooling the air near them, they are the never-failing sources of the cool rivers which run thence during the whole of summer, carrying freshness throughout distant lands. 707. In artificially raising temperature, we generally cause the liberation of heat which had been previously latent; and in lowering temperature or producing cold, we are generally rendering latent a quantity of heat which had previously been free. are When dwelling-houses, green-houses, or manufactories warmed by the admission of steam into systems of pipes which are distributed over them, the heat which they emit, is that which was previously latent in the steam, and which spreads around as soon as the steam, by touching pipes of lower temperature, is condensed to the state of water. 708. Again, for producing cold artificially, the processes generally involve the conversion either of a solid into a liquid, during which it takes in and hides in its new constitution, as latent heat, much of the heat previously sensible in it, and in the liquid which dissolves it; or of a liquid into vapour, during which heat equally becomes latent. Thus by dissolving a salt-nitrc, for instance—in water, we Freezing Mixtures. 497 obtain a solution which is very cold. In India a common mode of cooling wine for table is to surround the bottles with nitre dissolving in water; and the water of the solution being evaporated again before the next day, the salt is ready for use as before. If nitrate of ammonia is substituted for nitre, this salt, by reason of its greater and more rapid solubility in water, produces a much greater degree of cold. Equal parts of powdered nitrate of ammonia, crystallized carbonate of soda, and water, cause a reduction of temperature from 50° above, to 7° below the zero of Fahrenheit. Similar results are produced by the liquefaction of salts in acids. Thus eight ounces of sulphate of soda finely powdered, rapidly dissolved in five ounces of the strongest hydrochloric acid, form a convenient freezing mixture which may be obtained at all temperatures independently of ice and snow. In sufficient quantity it will lower the thermometer from 50° to near zero. 709. By taking advantage of these principles, the same substances may be made to produce great heat or an intense cold, according to the proportions in which they are used. If one part of broken ice by weight be rapidly mixed with four parts of strong oil of vitriol, the ice instantly disappears, and the water formed combines with the acid, producing a heat of 170°. Phosphorus takes fire on being brought in contact with the glass containing the mixture, and if ether in a tube is plunged into it, the ether boils, and a large quantity of inflammable vapour is evolved. On the other hand, if four parts of ice are rapidly mixed with one part of oil of vitriol, the ice is rapidly liquefied, and intense cold is produced, so that there is at first a deposit of dew and then of frozen water like snow, on the outside of the vessel. Certain saline compounds readily combine with water in the solid form of ice, and cause it to pass rapidly to the liquid state; and as in this case both the water and the salt render heat latent, the fall of temperature is doubly great. Thus, common salt at 50° and snow (or powdered ice) at 32°, when mixed, dissolve into liquid brine 37° colder than freezing water, or 5° below the zero of Fahrenheit. This action of salt on ice leads to the common practice of sprinkling salt on an ice-covered pavement before a street door, or in the roads to clear away the ice. The salt and ice quickly combine and form liquid brine, which either of itself runs off into the gutter and disappears, or is easily swept off. Salt has been largely used in some of the London parishes during 498 The Measurement of Heat. the winter season for the purpose of removing ice and snow from the public roads. According to Dr. Whitmore, in one London parish alone, seventy-seven tons of salt were used to remove the snow of two snowstorms occurring in December, 1875, and January, 1876. It produced 2234 loads of liquefied matter, which were carted away at a cost of £620. While salt is thus useful in liquefying the snow and ice, it has the effect of lowering the temperature of the air around, and it produces a liquid mixture, many degrees below the freezing point. This is injurious to pedestrians as well as horses. It penetrates boots and shoes, renders them cold to the feet, and prevents them from becoming dry. There is less objection to the use of crude chloride of calcium in the summer. This cools the road, and maintains a degree of moisture which keeps down the dust. "For any given substance, the changes of state from solid to liquid, and from liquid to air, happen, under similar circumstances, so precisely at the same temperature, that they mark fixed points in a general scale of temperature, and enable us to regulate and compare our various Thermometers." 710. As we can neither weigh heat, nor measure its bulk, nor see it, and as, even if our sense of touch were a correct judge in the matter, which it is not,-we dare not touch things that are very hot or very cold, some other expedient is required for estimating the presence in bodies of this very subtle agency; and that has been found in the measuring of its most obvious and constant effect, namely, that dilatation or expansion of bodies, which again ceases when the heat is withdrawn. Any substance, solid or liquid, which will allow this expansion to be accurately measured, becomes to us a Thermo · meter,* or measurer of heat. In solid substances, the direct expansion by heat, is so small as to be seen or measured with difficulty. In air, again, the expansion is very great; but there is the objection that in any apparatus yet contrived, which will allow this completely to appear, the air cannot be protected from the varying pressure of the atmosphere— an influence which affects its volume as much as changes of temperature. 711. The first thermometers constructed were of this kind. The *From the Greek, Oépun, heat, and μérpov, a measure. Invention of the Thermometer. 499 A invention of the instrument is ascribed to Sanctorio, of Padua, about the year 1600. It is represented in fig. 176. Coloured water was introduced so as to fill one-half of the bulb, A, and the liquid rose or fell according to whether the air in the bulb was heated or cooled. Very slight changes of temperature are indicated by this instrument. The hand applied to A rapidly expands the air, and causes the coloured liquid, which in the drawing half fills the bulb, to descend in the tube, B, and ascend in C D. A paper scale marked in divisions may be fixed to the tube, C D. As the tube is open at the top, the liquid rises and falls by changes in barometrical pressure. Observations of very slight 3 changes of temperature may, however, be rapidly made with this instrument. It is so sensitive that a number of persons entering a large room will soon cause the liquid to rise, by the heat radiated from their bodies. A Liquids appear to have been first used for measuring temperature by the Florentine Academicians about 1650. They enclosed spirits of wine in sealed tubes, and constructed a scale which was divided into 100 parts, but as there were no fixed points, it was practically useless. It was discovered by Hook that ice B b Fig. 176. always melted at the same temperature, and that, under a given atmospheric pressure, water, boiled in a metallic vessel, always indicated the same fixed temperature. Making use of these facts, Sir Isaac Newton proposed as fixed points of comparison, the points of freezing and boiling water. Bearing these facts in mind, the construction of the thermometer will be easily understood. 712. A small quantity of the liquid selected, alcohol or mercury, is placed in a glass bulb, as a (fig. 177), having a long neck or tube, a b, Finto which it may rise to be measured when expanded by heat. Among liquids, mercury is, on several accounts, peculiarly suitable. The use of this was first suggested by Fahrenheit, ɔf Dantzic, in 1720. In it, the range of temperature between freezing and boiling, is greater than in any other liquid, and it reaches a Fig. 177. |