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Variable Capacity for Heat.

raised one degree, takes thirty times as much heat as a pound of mer. cury. This may be proved in various ways. First, if the heat be derived from any uniform source, the water must remain exposed to it thirty times as long as the mercury. Secondly, if both substances, after being equaliy heated, are placed in ice until cooled to the freezing point, the heat which escapes from the water will melt thirty imes as much ice as that which escapes from the mercury. Third, when a pound of hot water is placed with a pound of cold mercury, instead of the two becoming of a middle temperature, as is the case when equal quantities of hot and cold water are mixed, and every degree of heat lost by the one quantity becomes just a degree gained by the other-the pound of hot water, by giving up one degree to the pound of cold mercury, raises the temperature of the latter thirty degrees; and in the same proportion for other differences :— or on reversing the experiment, a pound of hot mercury will be cooled thirty degrees by warming a pound of water one degree.

To put this in a practical shape, if equal measures of water at 70° and of mercury at 130° are mixed, the resulting temperature will not be the mean (100°), but only 90°. In this case, therefore, the mercury loses 40°, while the water gains only 20°. This refers to equal bulks of the two liquids. When we make the comparison by weight, which is more convenient, we find that a pound of water absorbs thirty times more heat than the same weight of mercury. The capacity of water for heat is therefore to that of mercury, as 30 to I, or 1000 to 33, and it is usual thus to express the capacities of bodies for heat by a series of numbers having reference to water, as 1000, such numbers representing what are called specific heats.

634. Each particular substance in nature has, like water or mercury, its peculiar capacity for heat or its specific heat; and experiments, made by such modes of mixture and of melting ice as above described, have led to the construction of tables which exhibit these relations. The following table shows the comparative capacities of equal weights of some common substances. Water, of which the capacity is greater than that of any other substance except hydrogen, for reasons of convenience, has been chosen as the standard of comparison. It appears that a pound of hydrogen gas takes about three and a half times more heat to produce in it a given change of temperature than a pound of water, while a pound of mercury or gold takes about thirty times less.

The comparative quantities of heat required to raise equal weights of different substances through the same range of temperature, are

Table of Specific Heats.

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commonly expressed in a tabular form. Water is taken as the

[These numbers are given differently by some authorities.]

635. Specific Heats.-If we seek a reason or reasons why there should be among bodies the differences of capacity here stated, the circumstances chiefly attracting attention are the following: Ist. Equal weights of the various substances have very different bulks or volumes, and therefore seem to have different room in which (if there be a compressible elastic medium concerned, Art. 555) the heat may be received or oscillations of particles may play ;—as a pound of mercury, for instance, is only one-thirteenth and a half as bulky as a pound of water. That the bulk, however, is not the only influencing circumstance appears in the fact that mercury has but one-thirtieth of the capacity of water. 2nd. In equal bulks of different substances, the space may be more completely occupied by the particles of one than of another—as is probably true of the particles of mercury compared with those of water. The influence of bulk or volume, in determining the capacity for heat, is shown in many other facts. In the table, for instance, it is seen that hydrogen and the gases generally, with their great comparative bulk, have also great capacity; that liquids have less capacity than gases; that solids have less than liquids. Yet the capacity is not in strict proportion to bulk; for hydrogen, which is many thousand times more bulky than an equal weight of water, has only three and a half times the capacity. Then, if any body whatever be suddenly com pressed into less bulk, heat escapes from it as if it were squeezed out. Thus iron or other metal suddenly condensed by the heavy blow of a hammer is thereby rendered hotter. Water and alcohol on being mixed, occupy less space than when separate, and there is from the mixture a corresponding discharge of heat. This truth is most remarkably exemplified in gases, owing to their great range of

142

Capacity of Gases for Heat.

elasticity. They may be condensed or dilated a hundredfold or more, and there will be a simultaneous concentration or diffusion of their heat; that is to say, the production, in the space occupied by them, of intense heat or cold.

636. Many mineral waters contain carbonic acid, which remains in tranquil combination while the water is bearing a certain pressure underground, but which in part escapes as soon as the water issues to the air and has only the atmospheric pressure to bear: such waters are called sparkling waters. The reason that champagne and the aërated waters are so cool when first decanted is, that their carbonic acid, in assuming its gaseous form, absorbs, as latent heat, a proportion of the sensible heat which was previously existing in the liquid.

If a gallon of air at the surface of the earth contain a certain quantity of heat, that heat is diffused equally through the space of the gallon; and if the air be then compressed into one-tenth of the bulk, there will be ten times as much heat in that tenth as there was before; an increase affecting the thermometer. In like manner, if by taking off pressure the gallon be made to dilate to ten gallons, the heat will be in the same degree diffused, and any one part will be colder than before.

The heat of air just condensed, or the cold of that which has just expanded, is greater for the instant than the most quickly answering thermometer indicates, for there is so little heat, even in a considerable volume of air, that the mass of a mercurial thermometer absorbing a great part of it would be but little affected. The extent of the change of temperature, however, is seen in the facts, that by the sudden condensation of air we may produce a red heat (1000), and set fire to tinder immersed in it, and, conversely by allowing air suddenly to expand from a highly condensed state, we may convert any watery vapour diffused through it into ice or snow. It might be expected that air suddenly compressed into half its previous volume, should become just twice as hot as before, or if suddenly dilated to double volume, should be only half as hot; but the facts do not accord with this anticipation, as will be stated in a future page.

The different capacity of air (for heat) in different states of dilatation, produces effects of great importance in nature as well as in the arts-thus,

637. On the surface of the earth near the sea-shore, the air of the

Line of perpetual Snow.

443

atmosphere has a certain density—a cubic foot weighs about one ounce and a third-dependent on the weight and pressure of the superincumbent mass acting on its elasticity; but on a mountain top 15,000 feet high, where nearly half the mass of the atmosphere is below that level, the air is bearing but half the pressure, and consequently any quantity of it has nearly twice the volume of an equal quantity at the sea-side, with a temperature many degrees lower.

On the other hand, the air forming the bottom of the atmosphere, owing to its condensation by the weight of the air above it, is much warmer than if it were suddenly carried higher up, to where, from the pressure being less, it would be more expanded or thinner. Accordingly the height of mountains may be roughly estimated by the difference of temperature observed at the bottom and at the top. While a thermometer stands at 60° at the bottom of St. Paul's Cathedral in London, another marks only 58° at the top of the dome; and in the lofty ascent of a balloon, the thermometer soon falls to the freezing point and even below it, the cold to the aëronaut becoming almost insupportable. Alexander von Humboldt, from a number of observations made on the steep declivities of the Andes near the equator, concluded that the thermometer falls one degree Fahrenheit for every 343 feet of ascent. Dr. Joseph D. Hooker, from observations made in East Nepaul and at Calcutta, deduced a fall of one degree Fahrenheit for every 309 feet of elevation.

638. In every part of the earth, at a certain elevation in the atmosphere, differing according to the latitude or proximity to the equator, the thermometer is found to stand always below the freezing point. This limit in the atmosphere is called the line or level of perpetual congelation or of perpetual snow. This line in the equatorial regions of South America, is at the height of 18,300 feet. On the north side of the Himalaya Mountains it is found at the height of 16,625 feet, and on the south side at the height of 12,980 feet.* In Switzerland the snow line is at 8,900 feet, in Spain and Italy at 7,000 feet, and in Norway (lat. 71°) 2,400 feet. In the Alps it is 700 feet lower on the northern than on the southern

*On the north side of the Himalayas there is varied cultivation, with good crops, at the height of 13,000 feet, and Captain Gerard found vegetation in full activity at an elevation of 16,800 feet in lat. 32°, while on the southern side it hardly reached 10,000 feet. The birch tree grows at 14,000 feet on the north side, and the oak at 11,500 feet on the south side. (Berghaus.)

444

Climate affected by Altitude.

side. In the Himalayas, as above stated, this condition is reversed. In no part of Great Britain do the mountains reach the line of perpetual snow, which would correspond in this latitude to 4,500 feet. We see, therefore, that snow-capped mountains exist near the equator as well as near the pole. It is this effect of elevation which renders many of the tropical regions of the earth not only tolerable abodes for man, but as suitable as any others; contrary to the opinion of the ancient philosophers of Europe, who deemed them, by reason of the great heat, an everlasting barrier, as regarded man, between the northern and southern hemispheres.

639. Much of the tropical land of America is so raised, that, as to agreeable temperature, it rivals any European climate; while the lightness and purity of the air, and the brightness of the sun, add much to its charms. The vast expanse of the high table-land of Mexico is of this kind, enjoying the immediate proximity of the sun, and yet, by its elevation of seven thousand feet above the level of the sea, possessing the most healthful freshness. The land in many parts has the fertility of a cultivated garden, and can produce naturally nearly all that the powers of vegetation can bring forth over the diversified face of the globe. The plains of Columbia, in South America, and others along the ridge of the Andes, are similarly circumstanced. The contrast is very striking, after sailing a thousand miles up the gentle slope of the river Magdalena, in a heat scarcely equalled elsewhere on earth, and surrounded by the animal and vegetable forms which can exist only in such a climate, at once to climb to the table-land above, where Santa Fé de Bogota, the capital of the republic, commands a view of interminable plains, that bear the livery of the fairest fields of Europe!

640. Persons not understanding the law which we are now illustrating, will express surprise that wind or air blowing down upon them from a snow-clad mountain, should still be warm and temperate. The truth is, that there is just as much heat existing in an ounce of the air on the mountain-top as in the valley: but above, the heat is diffused through a space perhaps twice as great as when below, and therefore is less sensible. It may be the very same air which moves as a warm gale over a plain at the foot of a mountain, which then rises and freezes water on the summit-and which in an hour after, or less, is playing among the flowers of another valley, as warm and as genial as before.

As the temperature in different parts of the atmosphere is in