352 DECREASE OF TEMPERATURE WITH ALTITUDE. [169 a. lowers its temperature. That this does take place on a large scale is well shown in the formation of cumulus cloud during a calm day after a cloudless night; the sun heating the surface of the ground evaporates the dew, and the warm and moist air rises. At a certain distance above the earth's surface the temperature of the air is lowered by expansion to the dew point, and the aqueous vapour is condensed in the form of a cloud with a nearly horizontal lower surface and a rounded top. The action of wind also causes an elevation of portions of the air which are thus cooled. If no circulation took place in the air, the temperature of the lower strata would be very much raised above that of the upper in consequence of their contact with the earth; and the actual temperatures of the different layers are probably intermediate between those which would be produced under this condition and those caused by the sudden exexpansion of the air, as described above. A progressive diminution of the temperature is experienced, as the altitude of the observer above the surface of the earth increases; and this depression of temperature is such, that even in tropical climates, the summits of lofty mountains are always crowned with snow. At the equator the snow line occurs at an altitude of about 5000 metres, but in England the line of perpetual snow occurs at about 2000 metres, since the limit of perpetual snow gradually descends (subject, however, to irregularities, from local causes) towards the level of the sea, according as the place of observation approaches towards either pole. A blast of cold air, therefore, in descending from a lofty height would have its temperature elevated by the mere condensation which it experiences as it approaches the surface of the globe, without any supply of heat from extraneous sources; and the danger arising from its chilling influences would be thus simply and effectually averted. Observations have shown that the average depression of temperature in ascending from the sea level amounts to 1° F. for every 300 feet, or 1° C. for every 165 metres. The observations made by Glaisher in his balloon ascents have, however, shown that in our own latitude this regularity of progression is liable to considerable disturbance from currents which are variable in direction and in force. The following table is given by Daniell (Meteorology, 1845, i. 41) as an approximate estimate of the distribution of heat in the atmosphere due to this cause, sup posing, as indicated in the second column, that the initial temperature of 80° is that of the surface of the earth near the equator, and that the initial temperature of o° F. indicated in 170.] VARIATION OF SPECIFIC HEAT. 353 the third column is that towards the poles: but from what has been stated above, the vertical distribution of temperature in the air is much more complex than was formerly supposed. In proportion as the temperature of a substance rises, its specific heat gradually increases: owing, probably, to the increase in the volume of the body with the rise of temperature, and to the augmentation of the space between the molecules of the heated substance. This increase in the specific heat with the rise of temperature may be seen by examining the following table compiled from the experiments of Dulong and Petit: Rise of Specific Heat with Rise of Temperature. (170) Variation of Specific Heat with Change of Physical State.-A body in the liquid state has a higher specific heat than the same substance when it is in the solid form. It is lower in the gaseous than in the liquid condition. This is remarkably shown in the case of water, in which the specific heat is double that of ice, and also more than double that of steam. Contrasting together the specific heats, as obtained for the following solids by Regnault, with the numbers obtained by Person (Ann. Chim. Phys. 1847 [3], xxi. 295, and 1848, xxiv. 129) for the same bodies when liquefied, the amount of this difference will be seen to be liable to great variation : Specific Heat of the same Substance, both in the Solid and Of all solids and liquids water is that which possesses the highest specific heat. This circumstance contributes in no small degree towards moderating the rapidity of transitions from heat to cold, or from cold to heat, owing to the large quantity of heat which the ocean absorbs or emits in accommodating itself to the variations of external temperature. Mercury, on the other hand, has a very low specific heat, which much enhances its sensibility to changes of temperature, and increases its fitness for thermometric purposes. Specific Heat of Gases and Vapours. 171.] SPECIFIC HEAT OF GASES AND VAPOURS. 355 (171) Specific Heat of Gases and Vapours.-The determination of the specific heats of gases and vapours is attended with unusual difficulties; and the earlier researches on the subject, though conducted by many philosophers distinguished for experimental skill, gave discordant and unsatisfactory results. The subject has been submitted to a very elaborate and rigorous investigation by Regnault, who, taking the specific heat of water as the unit of comparison, finds that of air to be =02375, and he gives the foregoing numbers as representing the specific heat of the various gases and vapours upon which he made his experiments. As the result of a numerous and elaborate series of experiments, Regnault concludes, contrary to the statement of Delaroche and Bérard, that the specific heat of air does not increase with rise of temperature, at any rate between the temperatures of -30° and 200° C. The same result holds good for gases which, like hydrogen, are not readily liquefiable. Condensible gases like carbonic anhydride exhibit a variation which, on the contrary, is quite perceptible: thus, the specific heat of carbonic anhydrideBetween -30° and 10° C. was found =0*18427. +10° and 100° =0 20246. Or the specific heat of carbonic anhydride is as follows: : A similar variation, though probably to a still greater extent, occurs with vapours generally. Another remarkable experimental result obtained by Regnault, indicates that for pressures ranging between 1 and 12 atmospheres, the specific heat of equal weights of a non-condensible gas, such as atmospheric air or hydrogen, is uniformly the same, and is independent of the density; consequently, that the specific heat of a given volume of a gas increases directly as its density is increased. The specific heats of the simple gases for equal volume, are nearly the same in the case of the incondensible gases-oxygen, nitrogen, and hydrogen—and appear to follow the law of Dulong and Petit (172); but for condensible gases and vapours, such as chlorine and bromine, it is far from being true. Compound gases which are formed without undergoing condensation, such as hydrochloric acid and nitric oxide, also obey the law of Dulong 356 SPECIFIC HEATS OF LIQUIDS AND VAPOURS COMPARED. [171. and Petit. When a body can be obtained in the solid, liquid, and gaseous states, it is found to have the highest specific heat when in the liquid form, and much less in the aëriform state. In determining the specific heats of gases and vapours, after a trial of various methods, Regnault ultimately adopted a modification of that employed by Delaroche and Bérard:-The gas under trial was first condensed into a strong receiver, and then by means of apparatus especially contrived for the purpose, a known quantity of this gas was allowed to escape at a perfectly constant rate, into a long spiral tube plunged into a vessel of hot oil, which was maintained at a fixed temperature; the gas was in this way during its passage through the spiral, raised to a known temperature, equal to that of the oil in the bath; the heated gas was then transmitted through a metallic vessel, surrounded by a known quantity of water; finally, the gas was allowed to escape into the atmosphere, care being taken that no sensible difference in temperature existed between the issuing gas and the water of the calorimeter. In this way Regnault ascertained the rise of temperature experienced by a known quantity of water, when a given |